U.S. patent application number 15/797481 was filed with the patent office on 2018-05-03 for parasitic channel mitigation using elemental diboride diffusion barrier regions.
This patent application is currently assigned to MACOM Technology Solutions Holdings, Inc.. The applicant listed for this patent is MACOM Technology Solutions Holdings, Inc.. Invention is credited to Kevin J. Linthicum.
Application Number | 20180122929 15/797481 |
Document ID | / |
Family ID | 58191166 |
Filed Date | 2018-05-03 |
United States Patent
Application |
20180122929 |
Kind Code |
A1 |
Linthicum; Kevin J. |
May 3, 2018 |
PARASITIC CHANNEL MITIGATION USING ELEMENTAL DIBORIDE DIFFUSION
BARRIER REGIONS
Abstract
III-nitride materials are generally described herein, including
material structures comprising III-nitride material regions and
silicon-containing substrates. Certain embodiments are related to
gallium nitride materials and material structures comprising
gallium nitride material regions and silicon-containing
substrates.
Inventors: |
Linthicum; Kevin J.; (Cary,
NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MACOM Technology Solutions Holdings, Inc. |
Lowell |
MA |
US |
|
|
Assignee: |
MACOM Technology Solutions
Holdings, Inc.
Lowell
MA
|
Family ID: |
58191166 |
Appl. No.: |
15/797481 |
Filed: |
October 30, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14847279 |
Sep 8, 2015 |
9806182 |
|
|
15797481 |
|
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/02381 20130101;
H01L 21/0254 20130101; H01L 29/2003 20130101; H01L 21/26553
20130101; H01L 29/41758 20130101; H01L 21/266 20130101; H01L
21/02488 20130101; H01L 29/7783 20130101; H01L 29/0623 20130101;
H01L 21/02658 20130101; H01L 21/0262 20130101; H01L 29/32
20130101 |
International
Class: |
H01L 29/778 20060101
H01L029/778; H01L 21/02 20060101 H01L021/02 |
Claims
1. A semiconductor structure, comprising: a substrate comprising
silicon and comprising at least a layer having a resistivity of
greater than 10.sup.2 Ohms-cm; a diffusion barrier region
comprising an elemental diboride located over a surface of the
substrate; and a III-nitride material region located over the
diffusion barrier region.
2. The semiconductor structure of claim 1, wherein the peak of the
sum of the concentrations of Group III species in the substrate is
less than about 10.sup.17/cm.sup.3.
3. The semiconductor structure of claim 1, wherein the peak of the
sum of the concentrations of Al, Ga, and In in the substrate is
less than about 10.sup.17/cm.
4. The semiconductor structure of claim 1, wherein the peak
concentration of Al, Ga, and/or In in the substrate is less than
about 10.sup.17/cm.sup.3.
5. The semiconductor structure of claim 1, wherein the substrate
comprises a surface region associated with the surface of the
substrate, and the surface region comprises a low-conductivity
parasitic channel or the substrate is free of a parasitic
channel.
6. A method of forming a semiconductor structure, comprising:
forming a diffusion barrier region comprising an elemental diboride
over a substrate comprising silicon, the substrate comprising at
least a layer having a resistivity of greater than 10.sup.2
Ohms-cm; and forming a III-nitride material region over the
diffusion barrier region.
7. The semiconductor structure of claim 1, wherein the III-nitride
material region comprises GaN.
8. The semiconductor structure of claim 1, wherein the
semiconductor structure comprises a transistor located over the
substrate.
9. The semiconductor structure of claim 1, wherein the III-nitride
material region comprises a III-nitride nucleation layer.
10. The semiconductor structure of claim 1, wherein the III-nitride
material region comprises a III-nitride transition layer.
11. The semiconductor structure of claim 16, wherein the
III-nitride transition layer is compositionally graded.
12. The semiconductor structure of claim 1, wherein the III-nitride
material region comprises a III-nitride buffer layer.
13. The semiconductor structure of claim 1, wherein the III-nitride
material region comprises a III-nitride device region.
14. The semiconductor structure of claim 1, wherein the elemental
diboride comprises zirconium diboride, aluminum diboride, hafnium
diboride and/or one or more alloys of these.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 14/847,279, filed Sep. 8, 2015, which is incorporated herein by
reference in its entirety.
TECHNICAL FIELD
[0002] III-nitride materials are generally described, including
gallium nitride materials and associated material structures
including substrates comprising silicon.
BACKGROUND
[0003] III-nitride materials include gallium nitride (GaN),
aluminum nitride (AlN), indium nitride (InN) and their respective
alloys (e.g., AlGaN, InGaN, AlInGaN and AlInN). In particular,
gallium nitride materials include gallium nitride (GaN) and its
alloys such as aluminum gallium nitride (AlGaN), indium gallium
nitride (InGaN), and aluminum indium gallium nitride (AlInGaN).
These materials are semiconductor compounds that have a relatively
wide, direct bandgap which permits highly energetic electronic
transitions to occur. Such electronic transitions can result in
gallium nitride materials having a number of attractive properties
including the ability to efficiently emit blue light, the ability
to transmit signals at high frequency, and others.
[0004] In many applications, III-nitride materials are typically
grown heteroepitaxially on a substrate. However, property
differences between III-nitride materials (e.g., gallium nitride
materials) and many substrate materials can present challenges. For
example, gallium nitride materials (e.g., GaN) have a different
thermal expansion coefficient (i.e., thermal expansion rate) and
lattice constant than many substrate materials and, in particular,
silicon. These differences may lead to formation of cracks and/or
other types of defects in gallium nitride material layers that are
grown heteroepitaxially on silicon. In some methods, a transition
layer is used to mitigate the effects of these differences in order
to grow high quality gallium nitride material on silicon. However,
these differences (and others) have limited the performance and
commercialization of structures and devices that include gallium
nitride material formed on silicon substrates.
[0005] III-nitride materials (e.g., gallium nitride materials) are
being investigated in high frequency (e.g., RF and power
management) device applications. When energy is dissipated in high
frequency devices through undesirable mechanisms (e.g., parasitic
losses and capacitive coupling), the performance of the device may
be impaired. These so-called parasitic losses can reduce output
power, switching speed, power gain, and efficiency. Therefore, it
is generally desirable to limit the parasitic losses in high
frequency (and other types of) RF and power management devices.
SUMMARY
[0006] III-nitride materials are generally described herein,
including material structures comprising III-nitride material
regions and silicon-containing substrates. Certain embodiments are
related to gallium nitride materials and material structures
comprising gallium nitride material regions and silicon-containing
substrates. The subject matter of the present invention involves,
in some cases, interrelated products, alternative solutions to a
particular problem, and/or a plurality of different uses of one or
more systems and/or articles.
[0007] Certain embodiments are related to semiconductor structures.
Some embodiments are related to methods of forming semiconductor
structures.
[0008] According to certain embodiments, the semiconductor
structure comprises a substrate comprising silicon, and a
III-nitride material region located over a surface region of the
substrate, wherein the surface region of the substrate comprises a
low-conductivity parasitic channel or the substrate is free of a
parasitic channel, and at least a region of the substrate comprises
at least one species having a relative atomic mass of less than 5
at a concentration of at least about 10.sup.19/cm.sup.3.
[0009] In some embodiments, the method of forming a semiconductor
structure comprises implanting a species having a relative atomic
mass of less than 5 into a substrate comprising silicon to produce
a surface region comprising no parasitic channel or comprising a
low-conductivity parasitic channel wherein, during the implanting
step, at least a portion of the species is implanted through a
III-nitride material region.
[0010] The method of forming a semiconductor structure comprises,
according to some embodiments, implanting a species having a
relative atomic mass of less than 5 into a structure comprising a
III-nitride material region and a substrate comprising silicon,
wherein at least a portion of the species is implanted through the
substrate without being implanted through the III-nitride material
region, and implanting the species produces a surface region
comprising no parasitic channel or a low-conductivity parasitic
channel.
[0011] In certain embodiments, the semiconductor structure
comprises a substrate comprising silicon, and a III-nitride
material region located over a surface region of the substrate,
wherein the substrate comprises at least one p-type dopant defining
a p-type dopant concentration profile, and the substrate comprises
at least one n-type dopant defining an n-type dopant concentration
profile that is substantially matched to the p-type dopant
concentration profile.
[0012] The method of forming a semiconductor structure comprises,
according to certain embodiments, implanting a counter-dopant into
a semiconductor structure comprising a III-nitride material region
and a substrate comprising silicon such that a concentration
profile of the counter-dopant substantially matches a concentration
profile of a second dopant present within the substrate.
[0013] The semiconductor structure comprises, according to some
embodiments, a substrate comprising silicon and at least one active
species coupled with an external species or capable of reacting
with an external species, and a III-nitride material region located
over a surface region of the substrate, wherein the concentration
of the active species is at least about 10.sup.19/cm.sup.3.
[0014] In certain embodiments, the method of forming a
semiconductor structure comprises forming a III-nitride material
region over a surface region of a substrate comprising silicon such
that a species within the substrate reacts with at least a portion
of an external species that contacts the substrate during the
formation of the III-nitride material region.
[0015] According to some embodiments, the semiconductor structure
comprises a substrate comprising silicon and comprising at least a
layer having a resistivity of greater than 10.sup.2 Ohms-cm; a
low-temperature AlN region located over the substrate; a
high-temperature AlN region located over the substrate; and a
III-nitride material region located over the low-temperature AlN
region and over the high-temperature AlN region.
[0016] The method of forming a semiconductor structure comprises,
in certain embodiments, forming a first AlN region over a substrate
comprising silicon and comprising at least a layer having a
resistivity of greater than 10.sup.2 Ohms-cm, wherein the
temperature of the environment in which the first AlN region is
formed is between about 700.degree. C. and about 950.degree. C.;
forming a second AlN region over the substrate, wherein the
temperature of the environment in which the second AlN region is
formed is from about 950.degree. C. to about 1150.degree. C.; and
forming a III-nitride material region over the first AlN region and
over the second AlN region.
[0017] The semiconductor structure comprises, according to some
embodiments, a substrate comprising silicon and comprising at least
a layer having a resistivity of greater than 10.sup.2 Ohms-cm; a
diffusion barrier region comprising a rare-earth oxide and/or a
rare-earth nitride located over a surface of the substrate; and a
III-nitride material region located over the diffusion barrier
region comprising the rare-earth oxide and/or the rare-earth
nitride.
[0018] The method of forming a semiconductor structure comprises,
according to certain embodiments, forming a diffusion barrier
region comprising a rare-earth oxide and/or a rare-earth nitride
over a substrate comprising silicon, the substrate comprising at
least a layer having a resistivity of greater than 10.sup.2
Ohms-cm; and forming a III-nitride material region over the
diffusion barrier region comprising the rare-earth oxide and/or the
rare-earth nitride.
[0019] In certain embodiments, the semiconductor structure
comprises a substrate comprising silicon and comprising at least a
layer having a resistivity of greater than 10.sup.2 Ohms-cm; a
diffusion barrier region comprising silicon carbide located over a
surface of the substrate; and a III-nitride material region located
over the diffusion barrier region comprising silicon carbide.
[0020] In some embodiments, the method of forming a semiconductor
structure comprises forming a diffusion barrier region comprising
silicon carbide over a substrate comprising silicon, the substrate
comprising at least a layer having a resistivity of greater than
10.sup.2 Ohms-cm; and forming a III-nitride material region over
the diffusion barrier region comprising silicon carbide.
[0021] The semiconductor structure comprises, according to some
embodiments, a substrate comprising silicon and comprising at least
a layer having a resistivity of greater than 10.sup.2 Ohms-cm; a
diffusion barrier region comprising an elemental diboride located
over a surface of the substrate; and a III-nitride material region
located over the diffusion barrier region.
[0022] In certain embodiments, the method of forming a
semiconductor structure comprises forming a diffusion barrier
region comprising an elemental diboride over a substrate comprising
silicon, the substrate comprising at least a layer having a
resistivity of greater than 10.sup.2 Ohms-cm; and forming a
III-nitride material region over the diffusion barrier region.
[0023] According to certain embodiments, the semiconductor
structure comprises a substrate comprising silicon; a III-nitride
material region located over a surface region of the substrate; and
an implanted species arranged within the surface region of the
substrate in a pattern spatially defined across at least one
lateral dimension of the substrate, wherein the implanted species
is present within at least a portion of the surface region of the
substrate at a concentration of at least about
10.sup.19/cm.sup.3.
[0024] The method of forming a semiconductor structure comprises,
according to some embodiments, implanting a species into a surface
region of a substrate comprising silicon such that the implanted
species forms a pattern spatially defined across at least one
lateral dimension of the substrate, wherein, during the implanting
step, at least a portion of the species is implanted through a
III-nitride material region, and after the implanting step, the
implanted species is present within at least a portion of the
surface region of the substrate at a concentration of at least
about 10.sup.19/cm.sup.3.
[0025] According to some embodiments, the semiconductor structure
comprises a substrate comprising silicon; a III-nitride material
region located over a surface region of the substrate; a first
implanted species arranged within the surface region of the
substrate in a first pattern spatially defined across at least one
lateral dimension of the substrate; and a second implanted species
arranged within the III-nitride material region in a second pattern
spatially defined across at least one lateral dimension of the
substrate.
[0026] The method of forming a semiconductor structure comprises,
according to some embodiments, implanting a first species and a
second species into a semiconductor structure comprising a
substrate comprising silicon and a III-nitride material region
located over the substrate, wherein the first species is implanted
into a surface region of the substrate such that the first species
forms a pattern spatially defined across at least one lateral
dimension of the substrate; and the second species is implanted
into the III-nitride material region such that the second species
forms a pattern spatially defined across at least one lateral
dimension of the III-nitride material region.
[0027] Other advantages and novel features of the present invention
will become apparent from the following detailed description of
various non-limiting embodiments of the invention when considered
in conjunction with the accompanying figures. In cases where the
present specification and a document incorporated by reference
include conflicting and/or inconsistent disclosure, the present
specification shall control.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Non-limiting embodiments of the present invention will be
described by way of example with reference to the accompanying
figures, which are schematic and are not intended to be drawn to
scale. In the figures, each identical or nearly identical component
illustrated is typically represented by a single numeral. For
purposes of clarity, not every component is labeled in every
figure, nor is every component of each embodiment of the invention
shown where illustration is not necessary to allow those of
ordinary skill in the art to understand the invention. In the
figures:
[0029] FIG. 1A is a cross-sectional schematic illustration of a
semiconductor structure, according to certain embodiments;
[0030] FIG. 1B is a cross-sectional schematic illustration of a
semiconductor structure comprising an optional diffusion barrier
region, according to certain embodiments;
[0031] FIG. 1C is, according to some embodiments, a cross-sectional
schematic illustration of a multi-layered diffusion barrier
region;
[0032] FIG. 1D is a cross-sectional schematic illustration of a
multi-layered III-nitride material region, according to some
embodiments;
[0033] FIG. 1E is a cross-sectional schematic illustration of a
multi-layered III-nitride material device region, according to
certain embodiments;
[0034] FIG. 2A is, according to some embodiments, a cross-sectional
schematic illustration of a semiconductor structure comprising a
transistor;
[0035] FIG. 2B is a cross-sectional schematic illustration of a
semiconductor structure comprising an optional diffusion barrier
region and a transistor, according to certain embodiments;
[0036] FIG. 2C is, according to certain embodiments, a top-view
schematic illustration of a semiconductor structure comprising a
transistor;
[0037] FIG. 2D is a side-view, cross-sectional schematic
illustration of the semiconductor structure illustrated in FIG.
2C;
[0038] FIG. 2E is a side-view, cross-sectional schematic
illustration of a portion of the semiconductor structure
illustrated in FIG. 2C;
[0039] FIG. 3A is, according to some embodiments, a cross-sectional
schematic illustration showing the transport of species into a
semiconductor structure;
[0040] FIG. 3B is a cross-sectional schematic illustration showing
the transport of species into a semiconductor structure comprising
multiple layers within a III-nitride material region, according to
certain embodiments;
[0041] FIG. 3C is, according to certain embodiments, a top-view
schematic illustration of a semiconductor structure comprising a
transistor, including an implantation mask;
[0042] FIG. 3D is a side-view, cross-sectional schematic
illustration of the semiconductor structure illustrated in FIG.
3C;
[0043] FIG. 3E is a side-view, cross-sectional schematic
illustration of a portion of the semiconductor structure
illustrated in FIG. 3C;
[0044] FIG. 3F is, according to certain embodiments, a top-view
schematic illustration of a semiconductor structure comprising a
transistor, including an implantation mask;
[0045] FIG. 3G is a side-view, cross-sectional schematic
illustration of the semiconductor structure illustrated in FIG.
3F;
[0046] FIG. 3H is a side-view, cross-sectional schematic
illustration of a portion of the semiconductor structure
illustrated in FIG. 3F;
[0047] FIG. 3I is, according to certain embodiments, a top-view
schematic illustration of a semiconductor structure comprising
multiple transistors;
[0048] FIG. 3J is a side-view, cross-sectional illustration of the
semiconductor structure illustrated in FIG. 3I;
[0049] FIG. 3K is a schematic illustration of an implantation mask,
according to certain embodiments;
[0050] FIG. 3L is a schematic illustration of an implantation mask,
according to certain embodiments;
[0051] FIG. 3M is a schematic illustration of an implantation mask,
according to certain embodiments;
[0052] FIG. 4A is, according to some embodiments, a cross-sectional
schematic illustration showing the transport of species into a back
side of a substrate of a semiconductor structure;
[0053] FIG. 4B is a cross-sectional schematic illustration showing
the transport of species into a back side of a substrate of and
into a semiconductor structure comprising multiple layers within a
III-nitride material region, according to certain embodiments;
[0054] FIG. 5 is an exemplary plot of free carrier concentration as
a function of depth in a substrate, according to certain
embodiments;
[0055] FIG. 6 is a plot of free carrier concentration as a function
of depth into a substrate, including examples in which protons
(H.sup.+) were implanted into a substrate and examples in which
protons (H.sup.+) were not implanted into a substrate; and
[0056] FIG. 7 is a plot of free carrier concentration as a function
of depth into a substrate, including examples in which oxygen
(O.sup.+) was implanted into a substrate and examples in which
oxygen (O.sup.+) was not implanted into a substrate.
DETAILED DESCRIPTION
[0057] III-nitride materials are generally described herein,
including material structures comprising III-nitride material
regions and silicon-containing substrates. Certain embodiments are
related to gallium nitride materials, and material structures
comprising gallium nitride material regions and silicon-containing
substrates.
[0058] Some embodiments are related to mitigating the effect of
parasitic channels (also sometimes referred to as parasitic
conducting channels) present within semiconductor structures and/or
inhibiting or preventing the formation of parasitic channels within
semiconductor structures. Parasitic channels can arise when growing
III-nitride materials (e.g., gallium nitride materials) on
substrates comprising silicon, and particularly on substrates
comprising silicon that are highly resistive (e.g., having an
electrical resistivity of greater than or equal to about 10.sup.2
Ohm-cm). It is believed that the parasitic channel is formed by the
diffusion of dopants that are unintentionally introduced into the
substrate prior to, or during, the growth of layers/regions (e.g.,
III-nitride material region) on the substrate. As described further
below, the dopants may include the elements (e.g., Group III
element(s) and/or nitrogen) from the reactive species which
participate in the reaction that forms the III-nitride material
region. For example, the dopants may be gallium and/or aluminum;
though, it should be understood that other dopants may also
contribute to forming the parasitic channel. Once diffused into the
substrate, the dopants can generate free carriers (i.e., electrons
or holes) which, in effect, form a conductive channel at or near
the substrate surface.
[0059] Parasitic channels are typically formed in a surface region
(e.g., at or near a surface of the substrate over which the
III-nitride material is grown also referred to herein as the "top"
surface of the substrate). A "surface region" of a substrate can
include an external surface of a substrate and a portion of the
substrate underneath and close to the external surface. In some
embodiments, the surface region of a substrate extends to a depth
of about 5 microns, to a depth of about 2 microns, to a depth of
about 1 micron, to a depth of about 500 nm, or to a depth of about
200 nm, or less. The "top surface region" is the surface region
associated with the top surface of a substrate. In any instances in
which a surface region of a substrate is recited herein, the
surface region may correspond to the top surface region of the
substrate. According to certain embodiments, the surface region of
the substrate is all or part of a silicon layer.
[0060] According to certain embodiments, semiconductor structures
comprising a parasitic channel (e.g., a high-conductivity parasitic
channel) can be processed such that the adverse impact of an
existing parasitic channel is reduced or eliminated. The adverse
impact of an existing parasitic channel can be reduced or
eliminated, for example, by reducing the free carrier concentration
in the parasitic channel. In some such embodiments, parasitic
losses and/or capacitive coupling associated with undesired
conduction through the parasitic channel are reduced or minimized.
In some embodiments, one or more species can be implanted into a
semiconductor structure such that the electronic conductivity of
the high-conductivity parasitic channel within the semiconductor
structure is reduced (e.g., such that it becomes a low-conductivity
parasitic channel or such that the parasitic channel is
eliminated). Reduction of the conductivity of the high-conductivity
parasitic channel may be achieved, for example, by implanting a
species (e.g., a small-atom species) that disrupts the structure of
the high-conductivity parasitic channel. For example, the implanted
species may disrupt the crystalline lattice structure of the
substrate and/or the high-conductivity parasitic channel. The
disruption of the crystalline lattice structure of the substrate
and/or the high-conductivity parasitic channel may reduce the
ability of the dopants to contribute to conduction (e.g., by
reducing the mobility of the dopants). Reduction of the
conductivity of the parasitic channel can also be achieved, for
example, by implanting one or more counter dopants into the
semiconductor structure such that the concentration profile of the
counter dopant substantially matches the concentration profile of
the dopant within the high-conductivity parasitic channel. In this
way, the contribution of the original dopant to the electronic
conductivity of the high-conductivity parasitic channel can be at
least partially negated by the counter dopant.
[0061] Some embodiments relate to methods (and associated
structures) that can be used to inhibit or prevent the diffusion of
material that increases the conductivity of the substrate, which
can lead to the formation of high-conductivity parasitic channels.
For example, in some embodiments, one or more species capable of
reacting with a species external to the semiconductor structure are
located within a substrate of the semiconductor structure, and a
III-nitride material region is formed over the substrate. During
the formation of the III-nitride material region, the reactive
species within the substrate can react with a diffusing Group III
element(s), nitrogen, or other species originating from outside the
semiconductor structure, which can, according to certain
embodiments, inhibit or prevent the formation of a
high-conductivity parasitic channel. The diffusion of material can
also be inhibited or prevented, for example, via the use of one or
more layers positioned between the III-nitride material region and
the substrate. The one or more layers positioned between the
III-nitride material region and the substrate may act, in some
embodiments, as a diffusion barrier. According to certain
embodiments, careful selection of an appropriate diffusion barrier
material is considered, to allow for heteroepitaxial formation of
high quality gallium nitride material and/or gallium nitride
material microelectronic and/or optoelectronic device layers.
[0062] As described further below, mitigating the effects of
parasitic channels can be desirable because parasitic channels can
be particularly significant in leading to parasitic losses in
semiconductor structures, including semiconductor structures formed
on highly resistive substrates (e.g., highly resistive silicon
substrates). The parasitic channels can provide a mechanism for
undesired energy absorption within the structure. When processing
methods are used to mitigate the effects of parasitic channels
present in semiconductor structures and/or to inhibit or prevent
the formation of such parasitic channels, the parasitic losses in
the resulting semiconductor structures may be significantly reduced
which can result in performance improvements. As one non-limiting
example, certain devices (such as High Electron Mobility
Transistors (HEMTs) and RF devices) formed of certain structures
described herein may have higher output power, power gain, and/or
efficiency, (even at higher operating frequencies), amongst other
advantages. In yet another non-limiting example, certain power
switching devices may exhibit a lower Rds(on) shift and/or
degradation.
[0063] Certain embodiments are related to inventive semiconductor
structures. Certain inventive semiconductor structures can comprise
a substrate (e.g., a substrate comprising silicon) and a
III-nitride material region located over a surface region of the
substrate. FIG. 1A is a cross-sectional schematic illustration of a
semiconductor structure 100A, according to certain embodiments.
Semiconductor structure 100A comprises substrate 110 and a
III-nitride material region 120 located over surface 135 of
substrate 110 (and, thus, over surface region 130 of substrate
110). According to certain embodiments, surface 135 of substrate
110 can be a silicon surface. For example, surface 135 may
correspond to a surface of a bulk silicon wafer, in some
embodiments. In certain embodiments, the top surface (e.g., surface
135 in the figures) may correspond to a silicon surface of a
composite substrate (e.g., comprising a silicon layer and one or
more additional underlying layers). For example, in some
embodiments, surface 135 may correspond to a surface of a silicon
portion of a silicon-on-insulator substrate, surface 135 may
correspond to a surface of a silicon-on-sapphire substrate, or
surface 135 may correspond to a silicon surface of a separation by
implantation of oxygen (SIMOX) substrate.
[0064] According to certain embodiments, the substrates of the
semiconductor structures described herein comprise silicon (i.e., a
substrate containing the element silicon in any form). Examples of
substrates comprising silicon that can be used in various
embodiments include, but are not limited to, silicon carbide
substrates, bulk silicon wafers, and silicon on insulator
substrates. In some embodiments, the substrate comprises a silicon
substrate. As used herein, a silicon substrate refers to any
substrate that includes a silicon surface. Examples of suitable
silicon substrates include substrates that are composed entirely of
silicon (e.g., bulk silicon wafers), silicon-on-insulator (SOI)
substrates, silicon-on-sapphire substrate (SOS), and separation by
implantation of oxygen (SIMOX) substrates, amongst others. Suitable
silicon substrates also include composite substrates that have a
silicon wafer bonded to another material such as diamond or other
crystallographic forms of carbon, aluminum nitride (AlN), silicon
carbide (SiC), or other crystalline or polycrystalline materials.
Silicon substrates having different crystallographic orientations
may be used, though single crystal silicon substrates may be
preferred in certain, but not necessarily all, embodiments. In some
embodiments, silicon (111) substrates are used. In certain
embodiments, silicon (100) or (110) substrates are used.
[0065] As used herein, a silicon carbide substrate refers to any
substrate that includes a silicon carbide surface. Examples of
suitable silicon carbide substrates include substrates that are
composed entirely of silicon carbide (e.g., bulk silicon carbide
wafers), silicon carbide composite wafers (e.g., wafers comprising
a silicon carbide layer and a second layer of a material that is
not silicon carbide), and the like.
[0066] In certain embodiments, the substrate may have various
device layers, homojunctions, heterojunctions, or circuit layers
embedded in the substrate, or formed on the front-side or back-side
of the substrate. Such substrates may be semi-spec standard
thickness, or thicker, or in some implementations thinner than
semi-spec standards. In some cases, for example, the Si substrate
may have a diameter of less than one hundred millimeters (100 mm),
while in other implementations, the substrate may have a diameter
in a range from approximately 100 mm to approximately 150 mm. In
certain embodiments, the substrate diameter may be in a range from
approximately 150 mm to approximately 200 mm, or larger. In still
other embodiments, the substrate may include a textured surface or
may have a non-planar surface. The substrate may also have any of a
variety of suitable thicknesses. For example, in some embodiments,
the substrate has a thickness greater than or equal to about 250
micrometers, greater than or equal to about 500 micrometers,
greater than or equal to about 625 micrometers, greater than or
equal to about 675, greater than or equal to about 1 mm, or thicker
(e.g., having a thickness of up to about 2 mm, up to about 3 mm, up
to about 5 mm, up to about 10 mm, or thicker). In some embodiments,
the substrate has a thickness of less than about 10 mm, less than
about 5 mm, less than about 3 mm, less than about 2 mm, less than
about 1 mm, less than about 500 microns, less than about 200
microns, less than about 150 microns, less than about 100 microns,
less than about 50 microns, or less. According to certain
embodiments, the thickness of the substrate may be selected based
on the final device and heteroepitaxial specifications (e.g., wafer
warp and bow), for example, as needed for successful high yielding
semiconductor fabrication.
[0067] In some embodiments, the substrate comprises at least a
layer having a high resistivity. For example, in certain
embodiments in which silicon substrates are used, the silicon
substrate (or at least the silicon portion of the substrate for
substrates that include a silicon portion formed on another
material) is highly resistive. According to certain embodiments,
the substrate comprises at least a layer having a resistivity of
greater than or equal to about 10.sup.2 Ohms-cm (or greater than or
equal to about 10.sup.4 Ohms-cm, or greater than or equal to about
10.sup.5 Ohms-cm). For example, in certain embodiments in which
silicon substrates are used, the resistivity of the silicon
substrate (or the silicon portion of the substrate) may be greater
than or equal to about 10.sup.2 Ohms-cm (or greater than or equal
to about 10.sup.4 Ohms-cm, or greater than or equal to about
10.sup.5 Ohms-cm). In certain embodiments, the resistivity of the
surface region of the substrate may be greater than or equal to
about 10.sup.2 Ohms-cm (or greater than or equal to about 10.sup.4
Ohms-cm, or greater than or equal to about 10.sup.5 Ohms-cm).
Highly resistive substrates comprising silicon (e.g., silicon
substrates or other substrates comprising silicon) may be
particularly useful in some (but not necessarily all) structures
that are used to form devices that operate at high frequencies
(e.g., RF devices). According to certain embodiments, the high
resistivity can reduce so-called substrate losses which otherwise
may arise and sacrifice performance. These substrate losses may
render substrates comprising silicon with lower resistivities
unsuitable in high frequency devices.
[0068] It has been observed that parasitic channels generally have
a significantly greater effect in structures that include highly
resistive substrates comprising silicon as compared to structures
that include substrates having more conventional resistivities
(e.g., 0.01-0.1 Ohms-cm). Because substrates comprising silicon
having more conventional resistivities typically have bulk free
carrier concentrations on the order of 10.sup.18/cm.sup.3, the
dopant diffusion phenomena described above may not substantially,
or even at all, change the free carrier concentration at the
substrate surface. Thus, a parasitic channel may not be generated
in such substrates. In contrast, the dopant diffusion phenomena can
have a significant effect on the free carrier concentration at the
surface region in substrates having high resistivities which
typically have bulk free carrier concentrations on the order of
10.sup.14/cm.sup.3 or lower. For these reasons, it is generally
more critical to reduce the effects of the parasitic channel in
structures that include highly resistive substrates comprising
silicon.
[0069] As used herein, the term "III-nitride material" refers to
any Group III element-nitride compound. Non-limiting examples of
III-nitride materials include boron nitride (BN), aluminum nitride
(AlN), gallium nitride (GaN), indium nitride (InN), and thallium
nitride (TIN), as well as any alloys including Group III elements
and Group V elements (e.g., Al.sub.xGa.sub.(1-x)N,
Al.sub.xIn.sub.yGa.sub.(1-x-y)N, In.sub.yGa.sub.(1-y)N,
Al.sub.xIn.sub.(1-x)N, GaAs.sub.aP.sub.bN.sub.(1-a-b),
Al.sub.xIn.sub.yGa.sub.(1-x-y)As.sub.aP.sub.bN.sub.(1-a-b), and the
like). Typically, when present, arsenic and/or phosphorus are at
low concentrations (e.g., less than 5 weight percent). III-nitride
materials may be doped n-type or p-type, or may be intrinsic.
III-nitride materials may have any polarity including but not
limited to Ga-polar, N-polar, semi-polar, or non-polar crystal
orientations. A III-nitride material may also include either the
Wurtzite, Zincblende, or mixed polytypes, and may include
monocrystalline, polycrystalline, or amorphous structures.
[0070] In some embodiments, the III-nitride material region
comprises a gallium nitride material, as described further below.
As used herein, the phrase "gallium nitride material" refers to
gallium nitride (GaN) and any of its alloys, such as aluminum
gallium nitride (Al.sub.xGa.sub.(1-x)N), indium gallium nitride
(In.sub.yGa.sub.(1-y)N), aluminum indium gallium nitride
(Al.sub.xIn.sub.yGa.sub.(1-x-y)N), gallium arsenide phosphoride
nitride (GaAs.sub.aP.sub.bN.sub.(1-a-b)), aluminum indium gallium
arsenide phosphoride nitride
(Al.sub.xIn.sub.yGa.sub.(1-x-y)As.sub.aP.sub.bN.sub.(1-a-b)),
amongst others. Typically, when present, arsenic and/or phosphorus
are at low concentrations (i.e., less than 5 weight percent). In
certain embodiments, the gallium nitride material has a high
concentration of gallium and includes little or no amounts of
aluminum and/or indium. In high gallium concentration embodiments,
the sum of (x+y) may be less than 0.4, less than 0.2, less than
0.1, or even less. In some cases, it is preferable for the gallium
nitride material layer to have a composition of GaN (i.e., x+y=0).
Gallium nitride materials may be doped n-type or p-type, or may be
intrinsic.
[0071] When a structure (e.g., layer and/or device) is referred to
as being "on," "over," or "overlying" another structure (e.g.,
layer or substrate), it can be directly on the structure, or an
intervening structure (e.g., a layer, air gap) also may be present.
A structure that is "directly on" or "in direct contact with"
another structure means that no intervening structure is present.
It should also be understood that when a structure is referred to
as being "on" or "over" another structure, it may cover the entire
structure, or a portion of the structure.
[0072] In certain embodiments, the substrate and the III-nitride
material region can be in direct contact, as illustrated in FIG.
1A. In some such embodiments, a silicon portion of the substrate
and the III-nitride material region are in direct contact.
Inventive semiconductor structures are not limited to arrangements
in which the substrate and the III-nitride material region are in
direct contact, and in some embodiments, one or more regions may be
positioned between the substrate and the III-nitride material
region. For example, in some embodiments, a diffusion barrier
region is positioned between the substrate and the III-nitride
material region. FIG. 1B is a cross-sectional schematic
illustration of a semiconductor structure 100B, according to
certain embodiments. In FIG. 1B, semiconductor structure 100B
comprises optional diffusion barrier region 140 positioned between
substrate 110 and III-nitride material region 120.
[0073] In some embodiments in which the diffusion barrier region is
present, the diffusion barrier region is in direct contact with the
substrate (e.g., a silicon surface of the substrate). For example,
in the illustrative embodiment shown in FIG. 1B, diffusion barrier
region 140 is in direct contact with substrate 110. In other
embodiments, one or more layers may be positioned between the
diffusion barrier region and the substrate (e.g., between the
diffusion barrier region and a silicon surface of the
substrate).
[0074] In certain embodiments, the diffusion barrier region is in
direct contact with the III-nitride material region. For example,
in the illustrative embodiment shown in FIG. 1B, diffusion barrier
region 140 is in direct contact with III-nitride material region
120. In other embodiments, one or more layers may be positioned
between the diffusion barrier region and the III-nitride material
region.
[0075] In some embodiments, the diffusion barrier region may be
formed prior to the introduction into the reaction chamber of
reactive species (e.g., Al, Ga species) that react to form the
III-nitride material region. In some such embodiments, the
diffusion barrier region limits, or prevents, dopant accumulation
on the substrate surface, amongst other functions. Because the
dopant concentration accumulated on the substrate surface is
reduced, dopant diffusion into the substrate is also decreased,
which can result in the parasitic conducting channel (e.g., within
the surface region of the substrate) having a lower conductivity.
It should be understood, however, that in some cases, some dopant
diffusion may occur through the diffusion barrier region and into
the substrate.
[0076] The diffusion barrier region may be formed of a number of
materials, as described in more detail below. In some embodiments,
the diffusion barrier region has an amorphous (i.e.,
non-crystalline) crystal structure. In some embodiments, the
diffusion barrier region may have a single crystal or
poly-crystalline structure.
[0077] In some embodiments, the diffusion barrier region may be
very thin. For example, according to certain embodiments, the
diffusion barrier region has a thickness of less than about 500 nm,
less than about 200 nm, less than about 100 nm, less than about 50
nm, less than about 20 nm, or less than about 10 nm (and/or, in
some embodiments, as thin as 5 nm, as thin as 1 nm, or thinner).
Thicker diffusion barrier regions could also be used. For example,
in some embodiments, the diffusion barrier region has a thickness
of less than about 5 microns, less than about 4 microns, less than
about 3 microns, less than about 2 microns, or less than about 1
micron. In some embodiments, the diffusion barrier region has a
thickness of at least about 10 nm or at least about 100 nm.
[0078] In some, but not necessarily all, embodiments, it may be
beneficial to employ a very thin diffusion barrier region when the
diffusion barrier region is amorphous. Not wishing to be bound by
any particular theory, very thin layers formed directly on the
substrate may absorb strain associated with lattice and thermal
expansion differences between the substrate and overlying
layers/regions (e.g., III-nitride material region). This absorption
of strain may reduce generation of misfit dislocations (and other
types of defects) and limit/prevent crack generation in the
overlying layers/regions. In certain although not necessarily all
embodiments, it may be beneficial to use a thin diffusion barrier
if the diffusion barrier is made of a poor thermal conductor. In
some such cases, using a thin diffusion barrier layer can minimize
the adverse effect on thermal conduction of the final III-nitride
material-based device.
[0079] In the embodiment illustrated in FIG. 1B, diffusion barrier
region 140 covers substantially the entire top surface 135 of
substrate 110. This arrangement may be preferable, in certain but
not necessarily all embodiments, for example, to minimize dopant
diffusion and the generation of dislocations in overlying regions.
In other embodiments, the diffusion barrier region does not
completely cover the top surface of the substrate. In some
embodiments, the layer may cover a majority of the top surface of
the substrate (e.g., greater than about 50 percent or greater than
about 75 percent of the top surface area).
[0080] It should be understood that the term "region" may refer to
one layer or may refer to multiple layers. As one non-limiting
example, the diffusion barrier region may be made of a single layer
or it may comprise a plurality of layers. FIG. 1C is a
cross-sectional schematic illustration of a diffusion barrier
region 140 comprising multiple layers 140A and 140B. In one set of
embodiments, layers 140A and 140B correspond to a high-temperature
AlN layer and a low-temperature AlN layer, as described in more
detail below. Multi-layer diffusion barrier region 140 illustrated
in FIG. 1C can be used in association with any of the embodiments
described herein. Alternatively, as noted above, the diffusion
barrier regions described herein may be single-layer
structures.
[0081] Similarly, the III-nitride material region may be made of a
single layer or it may comprise a plurality of layers. In some
cases, the III-nitride material region may also comprise a
non-III-nitride material layer or feature. FIG. 1D is a
cross-sectional schematic illustration of a III-nitride material
region 120 comprising multiple layers. Multi-layer III-nitride
material region 120 illustrated in FIG. 1D can be used in
association with any of the embodiments described herein.
Alternatively, as noted above, the III-nitride material regions
described herein may be single-layer structures.
[0082] In certain embodiments, the III-nitride material region
comprises an optional III-nitride material nucleation layer. For
example, referring to the exemplary embodiment of FIG. 1D,
III-nitride material region 120 comprises III-nitride material
nucleation layer 155. It should be understood that nucleation layer
155 is optional, and in other embodiments, III-nitride material
region 120 does not include nucleation layer 155.
[0083] The nucleation layer can, according to certain embodiments,
prepare a surface of the substrate for growth of III-nitride
material over the substrate. In certain cases, III-nitride material
(e.g., gallium nitride materials and/or other III-nitride
materials) can be difficult to grow heteroepitaxially directly on
the substrate (and/or another region that is over a surface of the
substrate), for example, because the III-nitride material one
wishes to grow may have a lattice structure and/or a lattice
constant which is significantly different than the substrate or
other underlying region. According to certain embodiments, the
nucleation layer forms an appropriate template to transition from
the lattice of the substrate (or other underlying layer) to a
template more suitable for III-nitride growth. In certain
embodiments, the nucleation layer can accommodate the difference in
the lattice constants of an overlying layer in the III-nitride
material region (e.g., the III-nitride material region portion in
direct contact with the nucleation layer) and the region underneath
the nucleation layer (e.g., the substrate and/or another underlying
region, which in some cases, may be in direct contact with the
nucleation layer). In some embodiments, the nucleation layer can
accommodate the difference in the thermal expansion coefficients of
an overlying layer in the III-nitride material region (e.g., the
III-nitride material region portion in direct contact with the
nucleation layer) and the region underneath the nucleation layer
(e.g., the substrate and/or another underlying region, which in
some cases, may be in direct contact with the nucleation layer).
According to certain embodiments, the nucleation layer can
accommodate both the difference in lattice constants and the
difference in thermal expansion coefficients of an overlying layer
in the III-nitride material region and the region underneath the
nucleation layer.
[0084] According to certain embodiments, the nucleation layer
comprises an aluminum nitride material. As used herein, the phrase
"aluminum nitride material" refers to aluminum nitride (AlN) and
any of its alloys, such as aluminum gallium nitride
(Al.sub.(1-x)Ga.sub.(x)N), aluminum indium nitride
(Al.sub.(1-x)In.sub.(x)N), aluminum indium gallium nitride
(Al.sub.(1-x-y)In.sub.(x)Ga.sub.(y)N), aluminum indium gallium
arsenide phosphoride nitride
(Al.sub.(1-x-y)In.sub.xGa.sub.yAs.sub.aP.sub.bN.sub.(1-a-b)),
amongst others. In certain embodiments, the aluminum nitride
material has a high concentration of aluminum and includes little
or no amounts of gallium and/or indium. In high aluminum
concentration embodiments, the sum of (x+y) may be less than 0.4,
less than 0.2, less than 0.1, or even less. In some cases, it is
preferable for the aluminum nitride material to have a composition
of AlN (i.e., x+y=0). Aluminum nitride materials may be doped
n-type or p-type, or may be intrinsic. In certain embodiments, the
use of an aluminum nitride material as the nucleation layer may be
preferred in certain cases in which the III-nitride material is
formed on the substrate without the use of a diffusion barrier
region between the III-nitride material and the substrate. In
certain embodiments, it may be preferred to forego the use of a
nucleation layer if a diffusion barrier region between the
substrate and the III-nitride material region is employed that
comprises a lattice type and/or a lattice constant that is more
suitable or favorable for the growth of the III-nitride material
region. Non-limiting examples of such diffusion barrier regions
include, but are not limited to, aluminum nitride diffusion barrier
regions, rare-earth oxide and/or rare-earth nitride diffusion
barrier regions, silicon carbide diffusion barrier regions, and
elemental diboride diffusion barrier regions, examples of which are
described in more detail below.
[0085] According to certain embodiments, the nucleation layer may
comprise one or more layers. When multiple nucleation layers are
present, the nucleation layers may be made of the same material or
different materials. In addition, in certain embodiments in which
multiple nucleation layers are present, the nucleation layers may
be formed using different semiconductor growth conditions. For
example, in some embodiments, the nucleation layers may comprise
two or more aluminum nitride material layers formed at different
growth temperatures (e.g., one at a relatively low temperature and
another at a relatively high temperature). In some embodiments,
other growth conditions (e.g., pressure, reactant flow rates, etc.)
may be varied from the growth of one nucleation layer to
another.
[0086] Suitable materials from which the III-nitride material
nucleation layer may be formed include, but are not limited to,
aluminum nitride materials (e.g., aluminum nitride, aluminum
nitride alloys) and gallium nitride materials. The III-nitride
material nucleation layer typically has a constant composition.
[0087] In some embodiments, the nucleation layer comprises a first
aluminum nitride-based layer and a second aluminum nitride-based
layer. The aluminum nitride-based nucleation layer can include
aluminum nitride as well as other optional elements, such as
silicon and/or oxygen. For example, in some embodiments, the
aluminum nitride-based nucleation layer can be a thin (e.g., from
about 10 Angstroms to about 20 Angstroms, or thinner) amorphous or
non-crystalline (unordered) material comprising aluminum and
nitrogen, and optionally silicon and/or oxygen. In some
embodiments, the amorphous aluminum nitride-based layer may also
act as a diffusion barrier region as discussed further below. In
other embodiments, one or more separate diffusion barrier layers
can be used in combination with the aluminum-nitride based
layer.
[0088] In certain embodiments, a III-nitride material nucleation
layer has a single crystal structure. It may be advantageous, in
some but not necessarily all embodiments, for a III-nitride
material nucleation layer to have a single crystal structure
because such structures can facilitate formation of one or more
single crystal layers (e.g., gallium nitride material layers) above
the III-nitride material nucleation layer.
[0089] It should also be understood that a III-nitride material
nucleation layer may not have a single crystal structure and may be
amorphous or polycrystalline, though certain of the advantages
associated with the single crystal nucleation layers may not be
achieved in some such embodiments.
[0090] The III-nitride material nucleation layer may have any
suitable thickness. For example, the III-nitride material
nucleation layer may have a thickness of between about 10
nanometers and about 5 microns, though other thicknesses are also
possible. In certain embodiments in which more than one nucleation
layer is employed, the combined thickness of the nucleation layers
may be between about 10 nanometers and about 5 microns, though
other thicknesses are also possible.
[0091] In certain embodiments, the III-nitride material region
comprises an optional III-nitride material transition layer. For
example, referring to the exemplary embodiment of FIG. 1D,
III-nitride material region 120 comprises III-nitride material
transition layer 170. It should be understood that transition layer
170 is optional, and in other embodiments, III-nitride material
region 120 does not include transition layer 170.
[0092] In FIG. 1D, transition layer 170 is formed directly on
nucleation layer 155. In other embodiments, one or more materials
may be positioned between transition layer 170 and nucleation layer
155.
[0093] In some embodiments, the III-nitride material transition
layer comprises a compositionally graded III-nitride material.
Examples of such materials are described, for example, in U.S. Pat.
No. 6,649,287, issued Nov. 18, 2003, and entitled "Gallium Nitride
Materials and Methods," which is incorporated herein by reference
in its entirety for all purposes. Compositionally-graded transition
layers have a composition that is varied across at least a portion
of the layer (e.g., across at least a portion of the thickness of
the layer). For example, according to certain embodiments in which
the transition layer comprises a III-nitride material layer, the
concentration of at least one of the elements (e.g., Ga, Al, In) of
the III-nitride material is varied across at least a portion of the
thickness of the transition layer. Compositionally-graded
transition layers are particularly effective, according to certain
embodiments, in reducing crack formation in gallium nitride
material regions formed on the transition layer, for example, by
lowering thermal stresses that result from differences in thermal
expansion rates between the gallium nitride material and the
substrate (e.g., silicon). Compositionally-graded transition layers
may also contribute to reducing generation of screw dislocations in
the III-nitride material layer(s)/region(s) (e.g., gallium nitride
material layer(s)). In some cases, the compositionally-graded
transition layers may also contribute to reducing mixed and edge
dislocation densities.
[0094] The composition of a compositionally-graded III-nitride
material layer can be graded, for example, discontinuously (e.g.,
step-wise) or continuously. The composition of the
compositionally-graded layer can be graded across the entire
thickness of the layer, or across only a portion of the thickness
of the layer.
[0095] According to one set of embodiments, the transition layer is
compositionally-graded and formed of an alloy of gallium nitride
such as Al.sub.xIn.sub.yGa.sub.(1-x-y)N, Al.sub.xGa.sub.(1-x)N, and
In.sub.yGa.sub.(1-y)N. In some such embodiments, the concentration
of at least one of the elements (e.g., Ga, Al, In) of the alloy is
varied across at least a portion of the thickness of the transition
layer. In certain embodiments in which the transition layer has an
Al.sub.xIn.sub.yGa.sub.(1-y)N composition, x and/or y may be
varied. In certain embodiments in which the transition layer has a
Al.sub.xGa.sub.(1-x)N composition, x may be varied. In certain
embodiments in which the transition layer has a
In.sub.yGa.sub.(1-y)N composition, y may be varied.
[0096] In certain embodiments, it is desirable for the transition
layer to have a low gallium concentration at a back surface which
is graded to a high gallium concentration at a front surface. It
has been found that such transition layers can be particularly
effective in relieving internal stresses within overlying gallium
nitride material layers. For example, the transition layer may have
a composition of Al.sub.xGa.sub.(1-x)N, where x is decreased from
the back surface to the front surface of the transition layer
(e.g., x is decreased from a value of 1 at the back surface of the
transition layer to a value of 0 at the front surface of the
transition layer).
[0097] In some embodiments, the semiconductor structure includes an
aluminum nitride nucleation layer and a compositionally-graded
transition layer. In some embodiments, the compositionally-graded
transition layer has a composition of Al.sub.xGa.sub.(1-x)N, where
x is continuously graded from a value of 1 at the back surface of
the transition layer to a value of 0 at the front surface of the
transition layer. One discontinuous grade may include steps of AlN,
Al.sub.0.6Ga.sub.0.4N, and Al.sub.0.3Ga.sub.0.7N (step grades)
proceeding in a direction toward the gallium nitride material
layer. In another example of a discontinuously graded III-nitride
material transition layer, there may be periodic layers and/or
intervening layers inserted between one or more of the step layers
making up the step grade. The periodic layers and/or intervening
layers, for example, may be layers of aluminum nitride material
(e.g., AlN or AlGaN) formed at the same or different (e.g., lower)
temperatures than are used to form the step grade layers. Another
example of periodic layers or intervening layers include silicon
nitride and/or aluminum silicon nitride layers, which can act as
masking layers to pin the vertical threading and screw dislocations
which may extend from one layer to the next.
[0098] It should be understood that, in other cases, the transition
layer may have a constant composition and may not be
compositionally-graded. In some cases (e.g., in certain cases in
which the substrate is not a silicon substrate and/or in certain
cases in which a diffusion barrier layer is positioned between the
substrate and the III-nitride material region), the transition
layer may have a constant composition. Suitable compositions
include, but are not limited to, aluminum nitride-based materials
(e.g., aluminum nitride, aluminum nitride alloys) and gallium
nitride materials. In these constant composition embodiments, the
transition layer may be similar to the nucleation layer described
above. In certain embodiments which utilize a diffusion barrier,
the constant composition of the III-nitride transition layer may
have a lattice constant that approximates the lattice constant of
the diffusion barrier.
[0099] According to certain embodiments, the transition layer may
be made of, at least in part, one or more superlattices (including
strained layer superlattices (SLS) or multiple quantum wells (MQW))
and/or a compositionally-graded superlattice or compositionally
graded MQW.
[0100] In certain embodiments, the III-nitride material region
comprises an optional III-nitride material buffer layer. For
example, referring to the exemplary embodiment of FIG. 1D,
III-nitride material region 120 comprises III-nitride material
buffer layer 180. It should be understood that buffer layer 180 is
optional, and in other embodiments, III-nitride material region 120
does not include buffer layer 180.
[0101] The buffer layer can, according to certain embodiments,
provide a surface for the growth of epitaxial III-nitride material
above the buffer layer.
[0102] According to certain embodiments, the buffer layer comprises
an aluminum gallium nitride material. In some such embodiments, the
buffer layer comprises Al.sub.xGa.sub.(1-x)N. In certain
embodiments in which the buffer layer comprises
Al.sub.xGa.sub.(1-x)N, x may be less than about 0.2, less than
about 0.1, less than about 0.05, or less than about 0.01. In some
embodiments, the buffer layer comprises GaN.
[0103] The buffer layer may be formed over the transition layer,
for example, using any of a number of known growth techniques. For
example, according to certain embodiments, the buffer layer may be
formed over the transition layer using molecular-beam epitaxy (MBE)
or metalorganic vapor phase epitaxy (MOVPE). In certain embodiments
(including certain embodiments in which the desired epitaxial
material structure will be used in the fabrication of transistors
such as field effect transistors (FETs) and/or High Electron
Mobility Transistors (HEMTs)), it may be desirable to form a
channel layer above the buffer layer. Typically, in some such
embodiments, it would then be desirable for the buffer layer
bandgap to be greater than or equal to the bandgap of the channel
layer. For example, in certain transistor designs using back
barriers, the channel layer may comprise gallium nitride while the
buffer layer may comprise aluminum gallium nitride. In some such
cases, the buffer layer may comprise of a substantially uniform
composition of AlGaN with a bandgap larger than the channel layer
but smaller than the underlying transition layer alloy composition.
In other examples, the buffer layer may itself be a compositionally
graded layer which has a larger aluminum composition formed near
the transition layer, and a smaller aluminum concentration formed
near the channel layer.
[0104] In some embodiments, the composition of the channel layer
and the buffer layer may be substantially the same. Although the
intrinsic material properties of GaN materials can allow for the
formation of high performance devices in theory, conventional
growth environments of GaN nitride materials typically include
impurity sources. For example, carbon impurities resulting from
metalorganic precursors may be, in some cases, introduced into the
GaN materials grown using MOCVD, also known more generically as
organometallic vapor phase epitaxy (OMVPE). The presence of these
impurities in the GaN growth environment can cause unintentional
doping in or near critical device layers, for example the channel
layer. In certain embodiments (e.g., including certain embodiments
in which enhancing the standoff voltage is desired), it may be
desirable to incorporate impurities into the buffer layer. For
example, the addition of impurities such as carbon (C) and iron
(Fe) into the buffer layers of gallium nitride based transistors
may increase the vertical and lateral breakdown voltage capability
and/or reduce the leakage levels of the device. However, addition
of certain impurities within close proximity to the channel layer
may result in dispersive device performance (e.g., exhibited by
high levels of drain and gate lag). As such, in certain
embodiments, it may be beneficial to form the buffer layer such
that there is a substantially low impurity concentration throughout
the thickness of the buffer layer. In certain other embodiments,
there may be a graded (continuously or discontinuously) impurity
level within the buffer layer, with a higher impurity concentration
near the transition layer, and a lower impurity concentration
formed nearer to the channel layer. In certain other embodiments,
the transition layer(s) may also include one or more impurities. In
some such embodiments, the concentration of impurities in the
transition layer(s) may be higher than the concentration(s) of the
impurities within the buffer layer and/or within the channel
layer.
[0105] The III-nitride material region comprises, according to
certain embodiments, an optional III-nitride material device
region. For example, referring to the exemplary embodiment of FIG.
1D, III-nitride material region 120 comprises III-nitride material
device region 190. It should be understood that device region 190
is optional, and in other embodiments, III-nitride material region
120 does not include device region 190.
[0106] In some embodiments, the III-nitride material region
includes at least one gallium nitride material layer. For example,
in certain embodiments, the III-nitride material device region
comprises at least one gallium nitride material layer. Referring to
FIG. 1D, for example, in some embodiments, III-nitride material
device region 190 can include at least one gallium nitride material
layer. As described further below, oftentimes, the structure
includes more than one gallium nitride material layer which form,
in part, the active region of the device.
[0107] As noted above, in some embodiments, the device region
comprises one or more III-nitride material layers. FIG. 1E is a
cross-sectional schematic illustration of a III-nitride material
device region 190, according to certain embodiments. In some
embodiments, the III-nitride material device region comprises an
optional back barrier layer. According to certain embodiments, when
present, the optional back barrier layer is the layer of the
III-nitride material region that is the closest to the underlying
substrate. For example, in FIG. 1E, exemplary III-nitride material
device region 190 comprises optional back barrier layer 190A, which
as shown in FIG. 1E, is the closest of the layers within
III-nitride material region to substrate 110. When used, the
optional back barrier can create a double heterostructure (e.g.,
due to the bandgap off sets with the buffer layer, such as a GaN
buffer layer). This may be desirable, in certain cases, in devices
which operate under higher drain bias as they can prevent injection
of electrons from the channel layer into the buffer layer, thereby
reducing drain leakage and punch through of the device.
Additionally, in some cases, the buffer layer may contain higher
levels of impurities, intentionally (e.g., iron and carbon used to
increase breakdown voltage) or unintentionally (e.g., carbon
impurities incorporated into the buffer layer as byproducts from
the crystal growth methodologies employed). These impurities and/or
other defects may, in certain cases, act as trapping centers and
cause deleterious performance results (e.g., memory effects) for
the device if electrons from the channel layer penetrate into the
underlying buffer layer. Back barrier layers can, in certain cases,
help confine the electrons in the channel layer and prevent spill
over into the more defective buffer layer and/or other underlying
III-nitride layers. In certain embodiments, one or more AlGaN back
barrier layers may be used. In certain embodiments, one or more
InGaN back barrier layers may be used. In some embodiments, one or
more AlInN back barrier layers may be employed. According to
certain embodiments, the thickness of the back-barrier (either in
the form of a single layer or a combination of layers) is in the
range of about 1-300 angstroms.
[0108] In some embodiments, the III-nitride material device region
comprises an optional channel layer. The channel layer may be
positioned, according to certain embodiments, over the back barrier
layer when present. For example, in FIG. 1E, exemplary III-nitride
material device region 190 comprises optional channel layer 190B,
which as shown in FIG. 1E, is positioned over optional back barrier
layer 190A. In other embodiments in which the back barrier layer is
not present, the channel layer can be the layer within the
III-nitride material device region that is closest to the
underlying substrate. According to certain embodiments, the channel
layer composition is selected with a smaller bandgap than either
the spacer and or front-barrier layers (described in more detail
below). Such arrangements can create a heterostructure forming a
two-dimensional electron gas (2DEG) near the interface between the
channel layer and an overlying layer (e.g., the spacer layer and/or
the front barrier layer, described in more detail below). Such
arrangements may be present, for example, in High Electron Mobility
Transistors (HEMTs). Electron flow through HEMTs and HFETs between
the source and the drain of the device can, in some instances, be
controlled by the gate of the device which acts to interrupt
electron current flow between the source and drain. The channel
layer can be formed, in certain cases, such that impurities or
other point defects (which can act as trapping centers) are kept at
a relatively low level, for example, to avoid impeding the mobility
of the electrons and/or to avoid adding memory effects to the
device. Trapping centers can adversely impact linearity in RF
devices and turn-on and turn-off (switching) speeds in power
management devices. The thickness of the channel layer can vary,
for example, depending on the operational voltage desired for the
device. As the drain voltage is increased, the depth of the
depletion area between the gate and drain generally increases. In
certain cases, if the channel is formed too thin, punch through
into the buffer layer can occur, which can result in drain leakage
and breakdown of the device.
[0109] According to certain embodiments, the III-nitride material
device region comprises an optional spacer layer (sometimes also
referred to as an interlayer). The spacer layer may be positioned,
according to certain embodiments, over the channel layer and/or the
back barrier layer when present. For example, in FIG. 1E, exemplary
III-nitride material device region 190 comprises optional spacer
layer 190C, which as shown in FIG. 1E, is positioned over optional
back barrier layer 190A and over optional channel layer 190B. In
some embodiments, the interface between the channel layer and the
spacer layer can form a 2-dimensional electron gas region (i.e., a
"2DEG region"). For example, in FIG. 1E, 2DEG region 191 is located
at the interface of spacer layer 190C and channel layer 190B.
Typically the spacer layer, when used, is formed with a high
aluminum content. In some embodiments, the spacer layer is
configured to have a relatively high bandgap offset with the
underlying channel layer (e.g., by using a relatively high aluminum
content in the spacer layer), which can lead to enhancement of the
2DEG. In certain embodiments, the spacer layer comprises
Al.sub.xGa.sub.(1-x)N. In certain such embodiments, (1-x)=0.5 or
greater. In some embodiments, the spacer layer comprises AlN.
According to certain embodiments, the spacer layer is relatively
thin (for example, less than about 50 Angstroms, less than about 20
Angstroms, or less). The use of a relatively thin spacer layer can
avoid, in some cases, adversely impacting the ohmic contact
resistance of the source and drain to the 2DEG and channel layer of
the device.
[0110] The III-nitride material device region comprises, in some
embodiments, an optional front barrier layer. The front barrier
layer may be positioned, according to certain embodiments, over the
spacer layer, the channel layer, and/or the back barrier layer when
present. For example, in FIG. 1E, exemplary III-nitride material
device region 190 comprises optional front barrier layer 190D,
which as shown in FIG. 1E, is positioned over optional back barrier
layer 190A, over optional channel layer 190B, and over optional
spacer layer 190C. According to certain embodiments (and as
described above), if the device structure is a HEMT and/or if a
2DEG is desired, the optional front-barrier can be formed over the
channel (or spacer, if used) to form a heterojunction The
composition of the front-barrier is selected, according to certain
embodiments, such that the carrier density and/or sheet charge of
the 2DEG is tailored (e.g., optimized) for the device desired. In
certain embodiments, the front-barrier layer comprises AlGaN.
According to certain embodiments in which an AlGaN-containing
front-barrier layer is used, the aluminum concentration of the
AlGaN front-barrier is less than about 30 atomic percent (at %),
less than about 25 at %, or between about 15 at % and about 20 at
%. In certain embodiments, it may be desirable to match or
substantially match the lattice constant between the channel and
the front-barrier, and to maintain a bandgap offset, for example,
to create a 2DEG. In some such cases, an AlInN or InGaN
front-barrier layer can be created to provide the bandgap offset
and match or substantially match the lattice constants.
[0111] According to some embodiments, the III-nitride material
device region comprises an optional cap layer. The cap layer may be
positioned, according to certain embodiments, over the front
barrier layer, the spacer layer, the channel layer, and/or the back
barrier layer when present. For example, in FIG. 1E, exemplary
III-nitride material device region 190 comprises optional cap layer
190E, which as shown in FIG. 1E, is positioned over optional back
barrier layer 190A, over optional channel layer 190B, over optional
spacer layer 190C, and over optional front barrier layer 190D. Cap
layers have been found useful in optimizing the semiconductor
surface of the device structure, according to certain embodiments.
For example, in certain cases in which the cap layer(s) comprises
GaN, the resulting surface morphology may be smoother and/or
include fewer defects, relative to surfaces formed when the cap
layer(s) is not present. In addition, in some cases, a more uniform
source composition may also be provided (i.e., terminated with
gallium atoms rather than a mixture of gallium and aluminum atoms),
which may, in some instances, aid in surface chemistry processing
of the semiconductor surface and/or reduce the number of surface
defects under the gate of the HFET. Such surface defects may, for
example, act as shallow trapping centers and compromise the
performance of the device, for example, due to increased gate
leakage or lateral breakdown of the device, increased dispersion,
increased gate and drain lag of the device, amongst other
reductions in performance. Additionally, in certain cases, by
providing a more consistently terminated semiconductor surface, the
repeatability and consistency of the contact resistance may be
enhanced, which can lead to higher manufacturing yields. In some
embodiments, it may be desired to dope the cap layer(s). In some
instances in which the barrier is doped, it may be desirable to use
a GaN cap layer which is n-type doped (e.g., using silicon). The
use of an n-type doped cap layer may, according to certain
embodiments, reduce Idss degradation of the device. In some
embodiments, the cap layer(s) may comprise a conductive GaN
material layer, for example, used as a conductive field plate.
[0112] In certain embodiments, the cap layer(s) may be doped
p-type, for example, with magnesium. Such doping may be desirable,
for example, in certain cases in which a normally OFF or
enhancement mode HFET is being fabricated. As one non-limiting
example, by forming a localized p-type GaN region under the gate of
the HFET, the 2DEG can be disrupted and the channel depleted such
that under no bias, the device is normally off. In some such cases,
a positive bias to the gate would then be needed to restore the
2DEG locally under the gate and allow current flow from the source
to the drain. It should be noted that one or more layers may be
used as the cap layer(s), and that whether a single cap layer or
multiple cap layers are employed may depend, for example, on the
specifics of the design device structure.
[0113] In some embodiments, the cap layer may include an in-situ
silicon nitride cap layer and/or passivation layer. Such layer(s)
may be used to terminate the III-Nitride structure and/or stabilize
the surface of the GaN material.
[0114] In some embodiments, the III-nitride material device region
comprises at least three device layers. In some embodiments, the
first device layer can be the closest of the three layers to the
substrate, the second device layer can be an intermediate layer
(i.e., between the first device layer and a third device layer),
and the third device layer can be the farthest of the three layers
from the substrate. In some such embodiments, the first layer can
be a channel layer, the second device layer can be a front-barrier
layer, and the third device layer can be a cap layer. Referring to
FIG. 1E, for example, III-nitride material device region 190 can
comprise channel layer 190B, front-barrier layer 190D, and cap
layer 190E. In some such embodiments, back-barrier layer 190A and
spacer layer 190C are each optional, and one or both may be present
or not present. According to some such embodiments, channel layer
190B can be the closest of these three layers (i.e., channel layer
190B, front barrier layer 190D, and cap layer 190E) to the
underlying substrate. In certain embodiments, it may be preferable
for the second device layer to have an aluminum concentration that
is greater than the concentration of aluminum in the first layer
and/or the third layer. For example, referring to FIG. 1E, in some
embodiments, front-barrier layer 190D can have an aluminum
concentration that is greater than the concentration of aluminum in
channel layer 190B and cap layer 190E. In some embodiments, the
first and second layers (e.g., channel layer 190B and front-barrier
layer 190D in FIG. 1E, respectively) can be gallium nitride
material layers, and the value of x (as used elsewhere herein with
reference to gallium nitride material layers, in subscripts to
denote the relative amount of aluminum in a compound (e.g., the "x"
in Al.sub.xGa.sub.(1-x)N)) in the second gallium nitride material
layer may have a value that is between about 0.15 and about 0.3
greater, or between about 0.15 and about 0.75 greater than the
value of x in the first gallium nitride material layer. For
example, the second device layer may be formed of
Al.sub.0.26Ga.sub.0.74N, while the first device layer may be formed
of GaN. This difference in aluminum concentration may lead to
formation of a highly conductive region at the interface of the
second and first device layer (i.e., a 2DEG region). In some
embodiments, the first device layer may be formed of GaN.
[0115] According to certain embodiments, the III-nitride material
device region (e.g., which may comprise at least one gallium
nitride material layer) has a low crack level. As described above,
the transition layer (particularly when compositionally-graded)
and/or the nucleation layer may reduce crack formation. Gallium
nitride materials and other III-nitride materials having low crack
levels have been described, for example, in U.S. Pat. No.
6,649,287, which is incorporated herein by reference in its
entirety for all purposes. In some cases, the III-nitride material
device region (e.g., which may comprise at least one gallium
nitride material layer) has a crack level of less than 0.005
.mu.m/.mu.m.sup.2. In some embodiments, the III-nitride material
device region (e.g., which may comprise at least one gallium
nitride material layer) has a very low crack level of less than
0.001 .mu.m/.mu.m.sup.2. In certain cases, it may be preferable for
the III-nitride material device region (e.g., which may comprise at
least one gallium nitride material layer) to be substantially
crack-free as defined by a crack level of less than 0.0001
.mu.m/.mu.m.sup.2.
[0116] In certain cases, the III-nitride material device region
(e.g., which may comprise at least one gallium nitride material
layer) has a single crystal (i.e., monocrystalline) structure. In
some cases, the III-nitride material device region (e.g., which may
comprise at least one gallium nitride material layer) includes one
or more layers having a Wurtzite (hexagonal) structure.
[0117] The thickness of the III-nitride material device region
(e.g., which may comprise at least one gallium nitride material
layer) and the number of different layers within the III-nitride
material device region are dictated, at least in part, on the
application in which the structure is used. According to certain
embodiments, at a minimum, the total thickness of the III-nitride
material device region (or any individual layer within the
III-nitride material device region) is sufficient to permit
formation of the desired structure or device. The total thickness
of the III-nitride material device region is, according to some
embodiments, greater than about 0.1 micron, though not always. In
some embodiments, the total thickness is greater than about 2.0
microns, or even greater than about 5.0 microns. In some
embodiments, the thickness of at least one layer within the
III-nitride material device region is greater than about 0.1
micron, greater than about 2.0 microns, or greater than about 5.0
microns.
[0118] The optional III-nitride material nucleation layer, the
optional III-nitride material transition layer, and the optional
III-nitride material buffer layer are not typically (though may be)
part of the active region of devices formed from structures of the
embodiments described herein. As described above, these layers may
be formed to facilitate deposition of the layer(s) of the
III-nitride material device region. However, in some cases, the
optional III-nitride material nucleation layer, the optional
III-nitride material transition layer, and/or the optional
III-nitride material buffer layer may have other functions
including functioning as a heat spreading layer that helps remove
heat from active regions of the semiconductor structure during
operation of a device. For example, such transition layers that
function as heat spreading layers have been described in U.S.
Patent Application Publication No. 2002/0117695, published Aug. 29,
2002, entitled "Gallium Nitride Materials Including
Thermally-Conductive Regions," which is incorporated herein by
reference in its entirety for all purposes.
[0119] Active regions of devices formed from certain of the
structures described herein may be formed, in part, in one or more
layers of the III-nitride material device region (e.g., gallium
nitride material layers). Suitable gallium nitride material layer
arrangements have been described, for example, in U.S. Pat. No.
7,071,498, entitled "Gallium Nitride Material Devices Including an
Electrode-Defining Layer and Methods of Forming the Same," issued
on Jul. 4, 2006, which is incorporated herein by reference in its
entirety for all purposes. Other commonly used III-Nitride material
device layers include channel layers, spacer layers, barrier
layers, capping layers, and P-type layers used under the gate
electrodes used for the design of enhancement mode (normally OFF)
transistor designs. These III-Nitride material device layers may
also include, according to certain embodiments, intentionally doped
layers in addition to various III-Nitride layers exhibiting
different alloy compositions.
[0120] The semiconductor structures described herein may, according
to certain embodiments, form the basis of a variety of
semiconductor devices. Suitable devices include, but are not
limited to, transistors (e.g., FETs), Schottky diodes, as well as
light-emitting devices including LEDs and laser diodes. It may be
particularly advantageous, according to certain but not necessarily
all embodiments, to use structures of the invention in devices that
operate at high frequencies. Non-limiting examples of applications
using III-nitride devices operating at higher frequencies include
power management discretes and integrated circuits used to switch,
rectify, monitor, or control electric power from a source to a load
(e.g., buck converters, boost converters, half bridges, H-bridges,
full bridges, three-phase bridges and multi-phase bridges). Other
non-limiting examples of RF applications include discretes and
integrated circuits used for wireless and wireline communications,
RF energy, RF plasma lighting, wireless charging, RF induction and
microwave heating, RF spark-plugs, ISM, medical devices, RADAR, and
electronic warfare and countermeasure devices. In certain
embodiments, there may be integrated circuits and/or multiple dice
on a chip combining both RF devices and switching devices used to
monitor, switch, or control the electric power delivery from a
source to a load.
[0121] According to certain embodiments, the devices have active
regions that are typically, at least in part, formed within the
III-nitride material region (e.g., in one or more layers of the
III-nitride material device region, such as one or more gallium
nitride material layers). According to some embodiments, the
devices include a variety of other functional layers and/or
features (e.g., electrodes, dielectric layers, field plate layers,
etc.).
[0122] According to certain embodiments, the semiconductor
structure comprises a transistor (e.g., a field effect transistor
(FET)). The transistor can comprise, according to certain
embodiments, a source electrode and a drain electrode. The source
electrode and the drain electrode can be electronically isolated
from each other. For example, in some embodiments, the source
electrode and the drain electrode are spatially separated by a
dielectric material. In some embodiments, the transistor further
comprises a gate electrode. The gate electrode may be a Schottky
gate or an insulated gate electrode. According to certain
embodiments, during use, application of a voltage at the gate
electrode can create and/or modify an electric field at least
partially positioned between the source electrode and the drain
electrode, such that electrons are transferred from the source
electrode to the drain electrode. Suitable transistors (e.g., FETs)
that may be used in association with certain of the embodiments
described herein include depletion mode (normally-ON) transistors
and enhancement mode (normally OFF) transistors. A transistor can
be associated with any of the semiconductor structures described
elsewhere herein, including but not limited to those described with
respect to FIGS. 1A-1B.
[0123] FIG. 2A is an exemplary cross-sectional schematic
illustration of a semiconductor structure 200A comprising
transistor 210, according to certain embodiments. In FIG. 2A,
transistor 210 comprises source electrode 220 and drain electrode
230. Transistor 210 also comprises gate electrode 240. Source
electrode 220, drain electrode 230, and gate electrode 240 are
positioned on III-nitride material region 120. The device also
includes dielectric layer(s) 250. The dielectric layer can be a
passivating layer that protects and passivates the surface of the
III-nitride material region. Via 260 is formed within the
dielectric layer 250 in which gate electrode 240 is, in part,
formed. In FIG. 2A, as described above, III-nitride material region
120 is formed directly on substrate 110. It is also noted that the
configuration of gate electrode 240 in direct contact with the
III-nitride material device region 190 forms a Schottky-gated FET.
In other embodiments (not shown) there may be an insulator layer
formed between gate electrode 240 and III-nitride material device
region 190 configured as a Metal Insulated Field Effect Transistor
(MISFET) or more generically an insulated gate transistor. It
should also be noted that, although FIG. 2A shows via 260 formed
within dielectric layer 250, in certain other embodiments, via 260
could extend down and terminate within III-nitride material region
120.
[0124] FIG. 2B is another exemplary cross-sectional schematic
illustration of a semiconductor structure 200B comprising
transistor 210 (e.g., a FET). In FIG. 2B, the electrodes of
transistor 210 are formed over semiconductor structure 100B, as
illustrated in FIG. 1B. Unlike in FIG. 2A, the III-nitride material
region 120 is formed above optional diffusion barrier region 140,
and optional diffusion barrier region 140 is formed above substrate
110.
[0125] The transistor structure illustrated in FIGS. 2A-2B is
exemplary, and other structures could also be used. For example,
FIGS. 2C-2E are schematic illustrations of another exemplary
semiconductor structure 200C, according to certain embodiments.
FIG. 2C is a top view schematic illustration of semiconductor
structure 200C. FIG. 2D is a front view cross-sectional
illustration of semiconductor structure 200C from FIG. 2C, with the
cross-section taken along line 2D shown in FIG. 2C. FIG. 2E is a
side view cross-sectional illustration of semiconductor structure
200C from FIG. 2C, with the cross-section taken along line 2E shown
in FIG. 2C.
[0126] In some embodiments, the transistor comprises one or more
source pads coupled to a plurality of source finger electrode
portions. For example, in FIG. 2C, transistor 210 of semiconductor
structure 200C comprises a plurality of source pads 220A coupled to
source finger electrode portions 220B. In some embodiments, the
transistor comprises one or more drain pads coupled to a plurality
of drain finger electrode portions. For example, in FIG. 2C,
transistor 210 of semiconductor structure 200C comprises a
plurality of drain pads 230A coupled to drain finger electrode
portions 230B. According to certain embodiments, the transistor
comprises one or more gate pads coupled to a plurality of gate
finger electrode portions. For example, in FIG. 2C, transistor 210
of semiconductor structure 200C comprises a plurality of gate pads
240A coupled to gate finger electrode portions 240B via gate
conductors 240C. Transistor 210 can also comprise other components,
such as one or more dielectric layers, but such additional
components are omitted from FIG. 2C for purposes of clarity.
[0127] According to certain embodiments, the plurality of source
finger electrode portions can be elongated (e.g., having an aspect
ratio of at least about 2:1, at least about 3:1, at least about
5:1, at least about 10:1, or longer). In some embodiments, the
plurality of source finger electrode portions are substantially
parallel to each other (i.e., the longitudinal axes of the source
finger electrode portions are parallel to within about 5.degree.,
within about 2.degree., or within about 1.degree. of each other).
For example, in FIG. 2C, transistor 210 of semiconductor structure
200C comprises elongated source finger electrode portions 220B,
which have longitudinal axes (extending horizontally across the
page) that are parallel to each other. In some embodiments, the
plurality of drain finger electrode portions can be elongated
(e.g., having an aspect ratio of at least about 2:1, at least about
3:1, at least about 5:1, at least about 10:1, or longer). In some
embodiments, the plurality of drain finger electrode portions are
substantially parallel to each other (i.e., the longitudinal axes
of the drain finger electrode portions are parallel to within about
5.degree., within about 2.degree., or within about 1.degree. of
each other). For example, in FIG. 2C, transistor 210 of
semiconductor structure 200C comprises elongated drain finger
electrode portions 230B, which have longitudinal axes (extending
horizontally across the page) that are parallel to each other. In
certain embodiments, the plurality of gate finger electrode
portions can be elongated (e.g., having an aspect ratio of at least
about 2:1, at least about 3:1, at least about 5:1, at least about
10:1, or longer). In some embodiments, the plurality of gate finger
electrode portions are substantially parallel to each other (i.e.,
the longitudinal axes of the gate finger electrode portions are
parallel to within about 5.degree., within about 2.degree., or
within about 1.degree. of each other). For example, in FIG. 2C,
transistor 210 of semiconductor structure 200C comprises elongated
gate finger electrode portions 240B, which have longitudinal axes
(extending horizontally across the page) that are parallel to each
other.
[0128] According to certain embodiments, at least some of the
source finger electrode portions are interdigitated with drain
finger electrode portions. In some embodiments, at least some of
the drain finger electrode portions are interdigitated with source
finger electrode portions. Generally, a first finger electrode
portion is said to be "interdigitated" with other finger electrode
portions when the longitudinal axis of the first finger electrode
portion extends into a spatial cavity created by the other finger
electrode portions. In FIG. 2C, for example, drain finger electrode
portion 230B-1 is interdigitated with source finger electrode
portions 220B-1 and 220B-2.
[0129] It should be understood that other structures and devices
may be within the scope of the present invention including
structures and devices that are not specifically described herein.
Other structures may include other layers and/or features, amongst
other differences.
[0130] In some cases, a parasitic channel can be present at (or
proximate to) a surface of the substrate (e.g., a top surface of
the substrate). In some embodiments, the parasitic channel can be
formed in a surface region of the substrate, such as the top
surface region of the substrate. As noted elsewhere, the surface
region of the substrate can include an external surface of a
substrate and/or a portion of the substrate underneath and close to
the external surface. In some embodiments, the surface region of
the substrate extends to a depth of about 5 microns, to a depth of
about 2 microns, to a depth of about 1 micron, to a depth of about
500 nm, to a depth of about 200 nm, or less. For example, referring
back to FIG. 1A, in some cases, a parasitic channel may be formed
in surface region 130 of substrate 110, at or near substrate
surface 135. The parasitic channel, when present, can generally be
formed in a top surface region of the substrate, and can extend for
a depth (d) into the substrate surface. In most cases, the
parasitic channel extends to the top surface, but it is possible
for the top surface region to include a thin portion at the top
surface that is not parasitic and for the parasitic channel to be
beneath that portion and, thus, proximate to the top surface. In
general, depth (d) is the distance from the top surface at which
the free carrier concentration of the parasitic channel is equal to
the free carrier concentration of the bulk substrate. Thus, depth
(d) depends, in part, on the free carrier concentration of the bulk
substrate. In some embodiments, such as when the silicon substrate
is highly resistive, the free carrier concentration of the bulk
substrate may be less than about 10.sup.14/cm.sup.3 (e.g.,
10.sup.12/cm.sup.3). Typical parasitic channel depths may be, in
some cases, at least about 1 micron and may be less than about 5
microns. In some embodiments, the depth of the parasitic channel is
less than or equal to about 5 microns, less than or equal to about
2 microns, less than or equal to about 1 micron, less than or equal
to about 500 nm, less than or equal to about 200 nm, or less.
[0131] In some embodiments, the devices that are formed are free of
parasitic channels or include only low-conductivity parasitic
channels. In certain embodiments, the surface region of a device
(e.g., the top surface region of the device) is free of a parasitic
channel or includes only a low-conductivity parasitic channel.
"Low-conductivity parasitic channels" are parasitic channels having
a peak free carrier concentration that is less than about
10.sup.17/cm.sup.3 and/or having a total integrated surface region
charge of less than about 10.sup.12/cm.sup.2. In some embodiments,
the low-conductivity parasitic channel has a peak free carrier
concentration that is less than about 10.sup.17/cm.sup.3, less than
about 5.times.10.sup.16/cm.sup.3, less than about
10.sup.16/cm.sup.3, or less than about 10.sup.15/cm.sup.3.
Low-conductivity parasitic channels can have, in some embodiments,
peak free carrier concentrations as low as about
10.sup.14/cm.sup.3, or lower. In some embodiments, the
low-conductivity parasitic channel has a total integrated surface
region charge that is less than about 10.sup.11/cm.sup.2, less than
about 10.sup.10/cm.sup.2, less than about 10.sup.9/cm.sup.2, or
less than about 10.sup.8/cm.sup.2. Low-conductivity parasitic
channels can have, in some embodiments, total integrated surface
region charges as low as about 10.sup.8/cm.sup.2, as low as about
10.sup.5/cm.sup.2, or lower. Portions of parasitic channels having
relatively low free carrier concentrations (e.g., less than about
10.sup.14/cm.sup.3) and/or relatively low total integrated surface
region charges may not have a significant, if any, impact on device
performance. "High-conductivity parasitic channels" are parasitic
channels having a peak free carrier concentration that is at least
about 10.sup.17/cm.sup.3 and having a total integrated surface
region charge of at least about 10.sup.12/cm.sup.2. In some cases,
a high-conductivity parasitic channel can have a peak free carrier
concentration that is at least about 5.times.10.sup.17/cm.sup.3, at
least about 10.sup.18/cm.sup.3, or at least about
10.sup.19/cm.sup.3. In some cases, a high-conductivity parasitic
channel can have a total integrated surface region charge that is
at least about 10.sup.13/cm.sup.2, at least about
10.sup.14/cm.sup.2, at least about 10.sup.15/cm.sup.2, or at least
about 10.sup.16/cm.sup.2.
[0132] In some embodiments, various of the processing (e.g.,
implantation) methods described herein can be used to reduce the
peak free carrier concentration from above a first threshold level
to below a second threshold level (which can be the same as or
different from the first threshold level). For example, in some
embodiments, processing (e.g., implantation) can be performed such
that the peak free carrier concentration is reduced from a level
above about 10.sup.17/cm.sup.3 to below about 10.sup.17/cm.sup.3
(or below about 5.times.10.sup.16/cm.sup.3, below about
10.sup.16/cm.sup.3, below about 10.sup.15/cm.sup.3, or below about
10.sup.14/cm.sup.3); from a level above about
5.times.10.sup.16/cm.sup.3 to below about
5.times.10.sup.16/cm.sup.3 (or below about 10.sup.16/cm.sup.3,
below about 10.sup.15/cm.sup.3, or below about 10.sup.14/cm.sup.3);
from a level above about 10.sup.16/cm.sup.3 to a level below about
10.sup.16/cm.sup.3 (or below about 10.sup.15/cm.sup.3, or below
about 10.sup.14/cm.sup.3); from a level above about
10.sup.15/cm.sup.3 to a level below about 10.sup.15/cm.sup.3 (or
below about 10.sup.14/cm.sup.3); or from a level above about
10.sup.14/cm.sup.3 to a level below about 10.sup.14/cm.sup.3. Such
reductions may be achieved within any of the regions described
herein in which the reduction of free carrier concentrations is
described.
[0133] In some embodiments, various of the processing (e.g.,
implantation) methods described herein can be used to reduce the
total integrated surface region charge from above a first threshold
level to below a second threshold level (which can be the same as
or different from the first threshold level). For example, in some
embodiments, processing (e.g., implantation) can be performed such
that the total integrated surface region charge is reduced from a
level above about 10.sup.11 km.sup.2 to below about
10.sup.11/cm.sup.2 (or below about 10.sup.10/cm.sup.2, below about
10.sup.9/cm.sup.2, below about 10.sup.8/cm.sup.2, or below about
10.sup.5/cm.sup.2); from a level above about 10.sup.10/cm.sup.2 to
below about 10.sup.10/cm.sup.2 (or below about 10.sup.9/cm.sup.2,
below about 10.sup.8/cm.sup.2, or below about 10.sup.5 cm.sup.2);
from a level above about 10.sup.9/cm.sup.2 to a level below about
10.sup.9/cm.sup.2 (or below about 10.sup.8/cm.sup.2, or below about
10.sup.5/cm.sup.2); or from a level above about 10.sup.8/cm.sup.2
to a level below about 10.sup.8/cm.sup.2 (or below about
10.sup.5/cm.sup.2). Such reductions may be achieved within any of
the regions described herein in which the reduction of free carrier
concentrations is described.
[0134] The peak free carrier concentration of a parasitic channel
may depend on a number of factors including the particular method
used to reduce free carrier concentration. The peak free carrier
concentration of a parasitic channel refers to the highest (or
"peak") concentration of free carriers within the parasitic
channel, and may be measured using standard techniques known to
those of ordinary skill in the art including spreading resistance
profiling (SRP) and Secondary Ion Mass Spectroscopy (SIMS). The
free carrier concentration in the parasitic channel typically
decreases with distance from the substrate top surface or from a
maximum free carrier concentration that is displaced some depth
below the substrate top surface. As one example, referring to FIG.
1A, the free carrier concentration may, in some embodiments,
decrease from top surface 135 downward through the thickness of
substrate 110. The free carrier concentration in the parasitic
channel can, according to certain embodiments, exhibit a profile
resulting from SRP measurement techniques similar to that shown in
FIG. 5. In some embodiments, the free carrier concentration
exhibits a maximum, which is referred to as the "peak free carrier
concentration." The peak free carrier concentration can, in some
cases, be located at or near a top surface of the substrate. For
example, in FIG. 5, free carrier concentration curve 510 exhibits a
maximum substantially at or near the top surface of the substrate
(e.g., surface 135 in the figures) indicated as point 512 in FIG.
5. In some cases, the free carrier concentration decreases to a
minimum at some depth within the substrate (e.g., at a depth of
about 1 micron or less, at a depth of about 0.5 microns or less, or
at a depth of about 0.2 microns or less). For example, in FIG. 5,
free carrier concentration curve 510 exhibits a minimum at point
514, which corresponds to a depth of about 1.33 microns. In some
cases, the substrate background free carrier concentration
dominates the profile at the minimum.
[0135] As noted above, certain of the devices described herein can
have relatively low total integrated surface region charges. The
"total integrated surface region charge" refers to the average
amount of free carriers per unit volume, determined by integrating
the free carrier concentration profile curve from a surface of the
substrate to the depth at which the parasitic channel free carrier
concentration matches the background free carrier concentration of
the substrate. The free carrier concentration profile curve can be
determined using standard techniques known to those of ordinary
skill in the art including spreading resistance profiling (SRP) and
Secondary Ion Mass Spectroscopy (SIMS). As one example, the total
integrated surface region charge for the device shown in FIG. 5
would be calculated by determining the area under curve 510 from
x=0 (i.e., the surface of the substrate) to x=1.33 (i.e., the
location on the x-axis corresponding to the depth within the
substrate at which the value of curve 510 equals the background
free carrier concentration, which is indicated by dotted line 516),
and dividing the determined area by 1.33. In other words, the total
integrated surface region charge for a given free carrier
concentration profile is determined by integrating the free carrier
concentration profile curve from x=0 (i.e., the surface of the
device) to x=d (i.e., the depth at which the parasitic channel free
carrier concentration matches the background free carrier
concentration of the substrate).
[0136] In some cases, the free carrier within the parasitic channel
region may be of a first type and the native background free
carrier may be of the opposite type. For example, in some cases,
the free carrier within the parasitic layer may be an acceptor,
whereas the native background free carrier concentration of the
substrate may be a donor. As such, a P-N junction can be formed in
the substrate. For example, the P-N junction can comprise a P-type
parasitic layer and an N-type background free carrier region (e.g.,
of a high resistivity silicon substrate). According to certain
embodiments, the P-N junction does not exist within the substrate
prior to forming the III-nitride material region. Rather, the P-N
junction and parasitic layer in the silicon substrate are generally
formed in such embodiments during the semiconductor growth process
of the III-nitride material region (which may include, for example,
the optional nucleation layer, the optional transition layer(s),
the optional buffer layer(s), and/or optional layer(s) of the
III-nitride material device region).
[0137] One feature of certain embodiments of the invention is that
the peak free carrier concentration may be relatively low. For
example, in some embodiments, the peak free carrier concentration
may be less than about 10.sup.17/cm.sup.3; in some cases, less than
about 10.sup.16/cm.sup.3; in some cases, less than about
10.sup.15/cm.sup.3; in some cases, less than about
10.sup.14/cm.sup.3; in some cases, less than about
10.sup.13/cm.sup.3; or lower. In some embodiments, the peak free
carrier concentration is less than about 100 times the free carrier
concentration in the bulk of the substrate, less than about 10
times the free carrier concentration in the bulk of the substrate,
less than about 5 times the free carrier concentration in the bulk
of the substrate, less than about 2 times the free carrier
concentration in the bulk of the substrate, less than about 1.5
times the free carrier concentration in the bulk of the substrate,
or less than about 1.1 times the free carrier concentration in the
bulk of the substrate. According to certain embodiments, both the
peak free carrier concentration and the background bulk substrate
free carrier concentration are less than about 10.sup.13/cm.sup.3.
Depth profiles may be generated using the SRP and SIMS techniques
noted above.
[0138] According to certain embodiments, the substrate includes a
bulk region below the top surface region. For example, referring to
FIGS. 1A-1B, 2A-2B, 2D-2E, 3A-3B, 3D-3E, 3G-3H, 3J, and 4A-4B,
substrate 110 can comprise bulk region 195 below top surface region
130. In some embodiments, the bulk region of the substrate has a
lower peak free carrier concentration than the top surface region.
For example, the top surface region of the substrate may have a
relatively high peak free carrier concentration (e.g., when a
low-conductivity or a high-conductivity parasitic channel is
present), and the bulk region of the substrate may have a lower
peak free carrier concentration. In some embodiments, the bulk
region of the substrate is doped with a first free carrier type and
the top surface region is doped with a second free carrier type
(e.g., a Group III species such as Al, Ga, and/or In). In some
embodiments, the second free carrier type is Al and/or Ga.
According to certain embodiments, the peak free carrier
concentration in the bulk region is less than about
10.sup.13/cm.sup.3, less than about 10.sup.12/cm.sup.3, less than
about 10.sup.11/cm.sup.3, less than about 10.sup.10/cm.sup.3, or
less than about 10.sup.9/cm.sup.3.
[0139] As noted above, in some embodiments, semiconductor
structures can be formed such that the adverse impact of an
existing parasitic channel is reduced or eliminated. The impact of
an existing parasitic channel can be reduced or eliminated, for
example, by implanting one or more species into the semiconductor
structure. Implanting the species into the semiconductor structure
can, according to certain embodiments, disrupt the parasitic
channel and/or reduce the electronic conductivity within the
parasitic channel, or eliminate the parasitic channel entirely.
[0140] In certain embodiments, a method of forming a semiconductor
structure is described comprising implanting a species having a
relative atomic mass of less than 5 into a substrate comprising
silicon. Those of ordinary skill in the art are familiar with
"relative atomic mass" (also sometimes referred to as "atomic
weight"), which refers to the ratio of the average mass of atoms of
an element to 1/12 of the mass of an atom of carbon-12 (known as
the unified atomic mass unit). As one example, atomic hydrogen has
a relative atomic mass of 1.008. Atomic helium has a relative
atomic mass of 4.003. Examples of species having a relative atomic
mass of less than 5 include, but are not limited to, hydrogen and
helium. The species having a relative atomic mass of less than 5
can be atomic (e.g., atomic helium (He)), ionic (e.g., hydrogen
cations (H.sup.+) also referred to herein as protons), or molecular
(e.g., molecular hydrogen (H.sub.2)). In some embodiments, a single
species having a relative atomic mass of less than 5 is implanted
into the substrate, while in other embodiments, multiple species
having a relative atomic mass of less than 5 are implanted into the
substrate.
[0141] In some embodiments, implanting the species having a
relative atomic mass of less than 5 produces a surface region
comprising a low-conductivity parasitic channel, or a surface
region that does not comprise a parasitic channel. For example,
referring to FIG. 1A, in some embodiments, a species having a
relative atomic mass of less than 5 can be implanted into substrate
110. In some such embodiments, implantation of the species into
substrate 110 can result in the formation of a low-conductivity
parasitic channel in surface region 130. In some such embodiments,
implantation of the species into substrate 110 can result in
surface region 130 being free of a parasitic channel.
[0142] According to certain embodiments, during the implanting
step, at least a portion of the species having a relative atomic
mass of less than 5 is implanted through a III-nitride material
region of the semiconductor structure. For example, referring to
FIG. 1A, in some embodiments, a species having a relative atomic
mass of less than 5 is implanted through III-nitride material
region 120, through surface 135, and into substrate 110. In some
such embodiments, the III-nitride material region is formed over
the substrate in a first step (e.g., via any of the methods
described elsewhere herein), after which the species having a
relative atomic mass of less than 5 is implanted through the
III-nitride material region.
[0143] It should be understood that, as described herein, a species
is considered to be implanted through a III-nitride material region
when it is implanted through the finally formed III-nitride
material region as well as when it is implanted through only an
intermediate portion of the III-nitride material region, which may
have only a fraction of the thickness of the fully-formed
III-nitride material region. For example, if a species is implanted
through a partially-formed III-nitride material region (or a
partially-formed III-nitride material layer), and subsequent to the
implantation of the species, additional potion(s) of III-nitride
material (or additional portion(s) of the layer of III-nitride
material) are formed directly on the III-nitride material through
which the species was implanted, the species would still be
considered to have been implanted through a III-nitride material
region.
[0144] In certain embodiments, the species having a relative atomic
mass of less than 5 can be implanted through an optional
III-nitride material nucleation layer. For example, referring to
FIG. 1D, in some embodiments, a species having a relative atomic
mass of less than 5 can be implanted through optional III-nitride
material nucleation layer 155 (and subsequently through surface 135
and into substrate 110). In some such embodiments, the optional
III-nitride material nucleation layer (or a portion thereof) is
formed over the substrate in a first step (e.g., via any of the
methods described elsewhere herein), after which the species having
a relative atomic mass of less than 5 is implanted through the
III-nitride material nucleation layer.
[0145] In some embodiments, the species having a relative atomic
mass of less than 5 can be implanted through an optional
III-nitride material transition layer. For example, referring to
FIG. 1D, in some embodiments, a species having a relative atomic
mass of less than 5 can be implanted through optional III-nitride
material transition layer 170 (and subsequently through surface 135
and into substrate 110). In some such embodiments, the optional
III-nitride material transition layer (or a portion thereof) is
formed over the substrate in a first step (e.g., via any of the
methods described elsewhere herein), after which the species having
a relative atomic mass of less than 5 is implanted through the
III-nitride material transition layer.
[0146] In some embodiments, the species having a relative atomic
mass of less than 5 can be implanted through an optional
III-nitride material buffer layer. For example, referring to FIG.
1D, in some embodiments, a species having a relative atomic mass of
less than 5 can be implanted through optional III-nitride material
buffer layer 180 (and subsequently through surface 135 and into
substrate 110). In some such embodiments, the optional III-nitride
material buffer layer (or a portion thereof) is formed over the
substrate in a first step (e.g., via any of the methods described
elsewhere herein), after which the species having a relative atomic
mass of less than 5 is implanted through the III-nitride material
buffer layer.
[0147] In some embodiments, the species having a relative atomic
mass of less than 5 can be implanted through at least one layer of
a III-nitride material device region (e.g., a GaN device layer).
For example, referring to FIG. 1D, in some embodiments, a species
having a relative atomic mass of less than 5 can be implanted
through at least one layer of III-nitride material device region
190 (and subsequently through surface 135 and into substrate 110).
In some such embodiments, a layer of the III-nitride material
device region (or a portion thereof) is formed over the substrate
in a first step (e.g., via any of the methods described elsewhere
herein), after which the species having a relative atomic mass of
less than 5 is implanted through the layer of the III-nitride
material device region. In some such embodiments, the layer of the
III-nitride material device region through which the species is
implanted can be an epitaxial III-nitride material device material
layer (e.g., an epitaxial GaN layer). In some such embodiments, the
layer of the III-nitride material device region through which the
species is implanted can be a monocrystalline III-nitride material
device layer (e.g., a monocrystalline GaN layer).
[0148] According to certain embodiments, the species having a
relative atomic mass of less than 5 can be implanted through a
transistor (e.g., FET) of the semiconductor structure. For example,
referring to FIG. 2A, in some embodiments a species having a
relative atomic mass of less than 5 can be implanted through
transistor 210 and III-nitride material region 120 (and
subsequently through surface 135 and into substrate 110).
[0149] The species having a relative atomic mass of less than 5 can
be implanted, in some embodiments, between III-nitride material
formation steps. For example, in some embodiments, after the
species has been implanted through a first III-nitride material
region, a second III-nitride material region is formed over the
first III-nitride material region through which the species was
implanted. In some such embodiments, the second III-nitride
material can be formed directly on the first III-nitride material
region. In some such embodiments, the second III-nitride material
can be a monocrystalline III-nitride material (e.g., a
monocrystalline GaN layer). In some such embodiments, the second
III-nitride material can be an epitaxial III-nitride material
(e.g., an epitaxial GaN layer). In certain embodiments, the second
III-nitride material region can include a 2DEG region. According to
certain embodiments, by forming the second III-nitride material
after the implantation step has been performed, one can avoid
damaging the second III-nitride material region via the
implantation step. For example, according to some embodiments, a
species (e.g., having a relative atomic mass of less than 5) can be
implanted through an optional III-nitride material nucleation
layer, an optional III-nitride material transition layer, and/or an
optional III-nitride material buffer layer. In some such
embodiments, after the implantation step, at least a layer of a
III-nitride material device region can be formed over the substrate
(and/or the optional III-nitride material nucleation layer, the
optional III-nitride material transition layer, and/or the optional
III-nitride material buffer layer). In some embodiments, a species
(e.g., having a relative atomic mass of less than 5) can be
implanted through an optional III-nitride material nucleation layer
and/or an optional III-nitride material transition layer. In some
such embodiments, after the implantation step, a III-nitride
material buffer layer and/or at least a layer of a III-nitride
material device region can be formed over the substrate (and/or the
optional III-nitride material nucleation layer and/or the optional
III-nitride material transition layer). In some embodiments, a
species (e.g., having a relative atomic mass of less than 5) can be
implanted through an optional III-nitride material nucleation
layer. In some such embodiments, after the implantation step, a
III-nitride material transition layer, a III-nitride material
buffer layer, and/or at least a layer of a III-nitride material
device region can be formed over the substrate (and/or the optional
III-nitride material nucleation layer). According to certain
embodiments, a species (e.g., having a relative atomic mass of less
than 5) can be implanted through an optional III-nitride material
nucleation layer. In some such embodiments, after the implantation
step, a III-nitride material transition layer, a III-nitride
material buffer layer, and/or at least a layer of a III-nitride
material device region can be formed over the substrate (and/or the
optional III-nitride material nucleation layer).
[0150] According to certain embodiments, after the implantation has
been performed, a transistor (e.g., a FET) may be formed over the
III-nitride material region(s). For example, in some embodiments, a
III-nitride material region is formed over the substrate, a species
having a relative atomic mass of less than 5 is implanted into the
substrate, after which one or more electrodes of the transistor are
formed. In certain embodiments, at least part of the transistor is
formed prior to the implantation of the species having a relative
atomic mass of less than 5. For example, in some embodiments, a
III-nitride material region is formed over the substrate, a
transistor is formed over the III-nitride material region, and
subsequently, a species having a relative atomic mass of less than
5 is implanted into the substrate (e.g., through the transistor and
the III-nitride material region).
[0151] In some embodiments, a species having a relative atomic mass
of less than 5 is implanted into the semiconductor substrate from
the back side of the substrate. In some such embodiments, transport
of the implanted species through the III-nitride material region is
eliminated, which, in some cases, can help preserve the
functionality and/or structure of the III-nitride material
region.
[0152] Accordingly, certain inventive methods comprise implanting a
species having a relative atomic mass of less than 5 into a
structure comprising a III-nitride material region and a substrate
comprising silicon, wherein at least a portion of the species is
implanted through the substrate without being implanted through the
III-nitride material region. This may be achieved, according to
certain embodiments, by implanting the species having a relative
atomic mass of less than 5 into the substrate via the back side of
the substrate (i.e., the side of the substrate opposite the
substrate surface over which the diffusion barrier region, the
III-nitride material region, and/or the transistor is positioned).
One example of such an implantation method is illustrated in FIG.
4A. In FIG. 4A, species 315 is implanted through back side 410 of
substrate 110. Species 315 is not implanted, however, through
III-nitride material region 120. In addition, species 315 is not
implanted through transistor 210 (e.g., when transistor 210 is
present during the implantation step). Another example of such
implantation is illustrated in FIG. 4B. In FIG. 4B, species 315 is
implanted through back side 410 of substrate 110. In the embodiment
shown in FIG. 4B, however, species 315 is not implanted through
III-nitride material region 120. In certain embodiments, the
species having a relative atomic mass of less than 5 is not
implanted through the optional III-nitride material nucleation
layer. For example, in FIG. 4B, species 315 is not implanted
through optional III-nitride material nucleation layer 155. In
certain embodiments, the species having a relative atomic mass of
less than 5 is not implanted through the optional III-nitride
material transition layer. For example, in FIG. 4B, species 315 is
not implanted through optional III-nitride material transition
layer 170. In certain embodiments, the species having a relative
atomic mass of less than 5 is not implanted through the III-nitride
material device region. For example, in FIG. 4B, species 315 is not
implanted through III-nitride material device region 190.
[0153] In certain embodiments, the portion of the species that is
not implanted through a particular region (e.g., the III-nitride
region) may be implanted into the region but not completely through
the region. For example, referring to FIGS. 4A-4B, in some cases,
at least a portion of species 315 is implanted through substrate
110, across surface 135, and into III-nitride material region 120,
but is not implanted through III-nitride material region 120. In
other cases, the portion of the species that is not implanted
through a particular region is not even implanted into the region.
For example, referring to FIGS. 4A-4B, in some embodiments, at
least a portion of species 315 is implanted into substrate 110
without crossing surface 135 (and, thus, without passing into
III-nitride material region 120). Thus, said species is not even
implanted into III-nitride material region 120 (and said species
would not be implanted through III-nitride material region
120).
[0154] According to certain embodiments, at least a portion of the
species having a relative atomic mass of less than 5 is not
implanted through the transistor. For example, in FIG. 4B, species
315 is not implanted through transistor 210 (e.g., when transistor
210 is present during the implantation step). In certain
embodiments, the transistor may not be formed until after the
implantation of the species having a relative atomic mass of less
than 5 is implanted.
[0155] Various of the implantation methods described herein can
produce semiconductor structures in which the substrate comprises a
region having a relatively high concentration of species having a
relative atomic mass of less than 5. In some embodiments, at least
a region of the substrate comprises at least one species having a
relative atomic mass of less than 5 at a concentration of at least
about 10.sup.19/cm.sup.3 (or at least about 10.sup.20/cm.sup.3, at
least about 10.sup.21/cm.sup.3, at least about 10.sup.22/cm.sup.3,
at least about 10.sup.23/cm.sup.3, or more). The concentration of
species having relative atomic masses of less than 5 can be
measured using standard techniques known to those of ordinary skill
in the art including spreading resistance profiling (SRP) and
Secondary Ion Mass Spectroscopy (SIMS). In embodiments in which
hydrogen is used as an implanted species, hydrogen forward
scattering spectrometry (HFS) can be used to determine the
concentration of hydrogen within regions of the semiconductor
structure.
[0156] In some such embodiments, the region having a relatively
high concentration of species having a relative atomic mass of less
than 5 can be part of a surface region and/or a parasitic channel
in the substrate. For example, referring to FIGS. 3A-3B, in some
embodiments, one or more of regions 330 can include species having
a relative atomic mass of less than 5 at a concentration within any
of the ranges described above.
[0157] According to certain embodiments in which species having a
relative atomic mass of less than 5 are implanted into the
substrate, the substrate may also include a relatively high peak
concentration of Group III species (e.g., Al, Ga, In, Tl, and B).
The relatively high peak concentration(s) of Group III species can
be present before the implantation or species having a relative
atomic mass of less than 5, according to some embodiments. In
certain embodiments, the relatively high peak concentration(s) of
Group III species can be present after the implantation of species
having a relative atomic mass of less than 5. The presence of the
species having a relative atomic mass of less than 5 can reduce the
electronic conductivity of the Group III species, inhibiting the
formation of a high conductivity parasitic channel. In some
embodiments in which species having a relative atomic mass of less
than 5 are implanted into the substrate, the peak of the sum of the
concentrations of Group III species in the substrate (e.g., the
peak of the sum of the concentrations of Al, Ga, In, Tl, and B in
the substrate) is at least about 10.sup.17/cm.sup.3, at least about
10.sup.18/cm.sup.3, at least about 10.sup.19/cm.sup.3, at least
about 10.sup.20/cm.sup.3, at least about 10.sup.21/cm.sup.3, at
least about 10.sup.22/cm.sup.3, or at least about
10.sup.23/cm.sup.3. In some embodiments in which species having a
relative atomic mass of less than 5 are implanted into the
substrate, the peak of the sum of the concentrations of Al, Ga, and
In in the substrate is at least about 10.sup.17/cm.sup.3, at least
about 10.sup.18/cm.sup.3, at least about 10.sup.19/cm.sup.3, at
least about 10.sup.20/cm.sup.3, at least about 10.sup.21/cm.sup.3,
at least about 10.sup.22/cm.sup.3, or at least about
10.sup.23/cm.sup.3. In some embodiments in which species having a
relative atomic mass of less than 5 are implanted into the
substrate, the peak concentration of Al, Ga, and/or In in the
substrate is at least about 10.sup.17/cm.sup.3, at least about
10.sup.18/cm.sup.3, at least about 10.sup.19/cm.sup.3, at least
about 10.sup.20/cm.sup.3, at least about 10.sup.21/cm.sup.3, at
least about 10.sup.22/cm.sup.3, or at least about
10.sup.23/cm.sup.3.
[0158] In some embodiments, the substrate comprises a
high-conductivity parasitic channel before the implantation of
species having a relative atomic mass of less than 5 and a
low-conductivity parasitic channel (or no parasitic channel) after
the implantation of species having a relative atomic mass of less
than 5. In some embodiments, the bulk region of the substrate has a
lower peak free carrier concentration than the top surface region
of the substrate before and/or after the implantation of species
having a relative atomic mass of less than 5. In some embodiments,
the bulk region of the substrate is doped with a first free carrier
type and the top surface region is doped with a second free carrier
type (e.g., a Group III species such as Al, Ga, In, and/or Tl)
before and/or after the implantation of species having a relative
atomic mass of less than 5. According to certain embodiments, the
peak free carrier concentration in the bulk region is less than
about 10.sup.13/cm.sup.3, less than about 10.sup.12/cm.sup.3, less
than about 10.sup.11/cm.sup.3, less than about 10.sup.10/cm.sup.3,
or less than about 10.sup.9/cm.sup.3 before and/or after the
implantation of species having a relative atomic mass of less than
5.
[0159] In some embodiments, at least one species is implanted into
the substrate in a spatially defined pattern. As a result, certain
of the inventive semiconductor structures described herein include
species located within the substrate (e.g., in a surface region of
a substrate, such as surface region 130 in the figures) in a
spatially defined pattern. Implanting species in a spatially
defined pattern can allow one to select the regions of the
semiconductor structure through which the species is transported,
potentially limiting or eliminating damage caused by the implanted
species within certain regions. Patterned implantation of the
species can be performed using any of the semiconductor structures
described herein.
[0160] Those of ordinary skill in the art would be capable of
determining the amount of implanted species within various regions
of a given semiconductor device using, for example, Secondary Ion
Mass Spectroscopy (SIMS).
[0161] Certain embodiments comprise implanting a species into the
substrate (e.g., into a surface region of the substrate) such that
the implanted species forms a pattern spatially defined across at
least one lateral dimension of the substrate (and, in some
embodiments, across two lateral dimensions of the substrate). A
"lateral dimension" of a substrate corresponds to a dimension of
the substrate that is perpendicular to the thickness of the
substrate. The lateral dimensions of the substrate can define the
face of the substrate on which the barrier region, the III-nitride
material region, and/or transistors (or other devices) are formed.
The thickness of the substrate can be the smallest of the three
coordinate dimensions of the substrate. For example, in the case of
a silicon wafer, the wafer can have a thickness corresponding to
its thinnest coordinate dimension and two lateral dimensions--which
form the face of the wafer--each perpendicular to each other, and
each perpendicular to the thickness of the wafer. Referring to FIG.
1A as a non-limiting example, substrate 110 comprises thickness 196
and lateral dimension 197. (Substrate 110 also has a second lateral
dimension that extends into and out of the plane across which FIG.
1A is drawn.) In certain embodiments, the implanted species can
form a pattern that is also spatially defined across the second
lateral dimension of the substrate. In some embodiments, the
spatially defined pattern in which the species is implanted can
vary across the depth of (i.e., through the thickness of) the
substrate. Implanting species according to such patterns can
provide, according to certain embodiments, a number of advantages
discussed in more detail below.
[0162] In some embodiments, at least one species can be implanted
into a surface region of the substrate (e.g., into surface region
130 in the figures) such that the implanted species forms a pattern
spatially defined across at least one lateral dimension (and, in
some embodiments, across two lateral dimensions) of the surface
region of the substrate. In some such embodiments, the species that
is implanted into the substrate can be implanted through a
III-nitride material region and subsequently into the substrate. In
some such embodiments, after the implanting step, the implanted
species is present within at least a portion of the surface region
of the substrate at a concentration of at least about
10.sup.19/cm.sup.3 (and/or, in some embodiments, at least about
10.sup.20/cm.sup.3, at least about 10.sup.21/cm.sup.3, at least
about 10.sup.22/cm.sup.3, or at least about 10.sup.23/cm.sup.3).
According to some such embodiments, after the implanting step, at
least a second portion of the surface region of the substrate is
substantially free of the first implanted species (i.e., the
implanted species is present within the second portion in an amount
of less than about 10.sup.15/cm.sup.3, and, in some cases, less
than about 10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3,
less than about 10.sup.12/cm.sup.3, less than about
10.sup.11/cm.sup.3, less than about 10.sup.10/cm.sup.3, or
lower).
[0163] According to certain embodiments, the species that is
implanted into the substrate in a spatially defined pattern can
have a relative atomic mass of less than 5. The species having a
relative atomic mass of less than 5 can be atomic (e.g., atomic
helium (He)), ionic (e.g., hydrogen cations (H.sup.+)), or
molecular (e.g., molecular hydrogen (H.sub.2)). Other species could
also be used including, but not limited to, p-type dopants (e.g.,
boron, aluminum, gallium, and/or indium) and n-type dopants (e.g.,
nitrogen, phosphorus, oxygen, and/or arsenic). In some embodiments,
the spatially defined pattern in which the species is implanted can
be a preselected spatially defined pattern. Such preselected
spatially defined patterning may be achieved, for example, using an
implantation mask. According to certain embodiments, the implanting
is performed through a mask such that a first portion of the
semiconductor structure is not substantially exposed to the
implanted species and a second portion of the semiconductor
structure is exposed to the implanted species. For example, an
implantation mask comprising one or more open regions (which allow
the implanted species to pass through the mask) and one or more
blocking regions (which inhibit or prevent the implanted species
from passing through the mask) can be employed. An exemplary
schematic illustration showing such patterning is shown in FIG. 3A.
In FIG. 3A, mask 300 comprises open regions 310, which allow
implanted species 315A to pass through the mask and blocking region
320 which inhibits (or prevents) implanted species 315B from
passing through mask 300. Another example of such patterning is
shown in FIG. 3B, in which mask 300 comprises open regions 310,
which allow implanted species 315A to pass through the mask and
blocking region 320 which inhibits (or prevents) implanted species
315B from passing through mask 300. Although the open regions in
FIGS. 3A and 3B are substantially rectangular in shape, mask
openings with other shapes (e.g., circular or substantially
circular, hexagonal or substantially hexagonal, parallelograms,
etc.) may also be used.
[0164] According to some embodiments, the patterned implantation is
performed such that at least one region of a parasitic channel
(e.g., within a surface region of the substrate, such as surface
region 130 in the figures) is exposed to the implanted species. In
some such embodiments, at least a portion of the III-nitride
material region is not exposed to the implanted species. In certain
such embodiments, damage of the portions of the III-nitride
material region that are not exposed to the implanted species can
be avoided. For example, referring to FIGS. 3A-3B, in some
embodiments, regions 330 of the parasitic channel within surface
region 130 (and/or regions 331 of the III-nitride material region
120) are exposed to the implanted species while region 335 of
III-nitride material region 120 (which region 335 can include a
2DEG region) and region 340 of a parasitic channel within surface
region 130 are not exposed to the implanted species. According to
some such embodiments, the structure of the material within region
335 (including, in some embodiments, the 2DEG region within region
335) is preserved while the number of free carriers within regions
330 is reduced or eliminated. According to certain embodiments, a
2DEG within region 335 is preserved while the 2DEG within regions
331 is reduced, eliminated, or interrupted, and the number of free
carriers within regions 330 is reduced or eliminated.
[0165] According to certain embodiments, patterned implantation can
reduce the conductivity of the parasitic channel region (or
eliminate the parasitic channel region) while preserving the
structure of one or more functional regions of the III-nitride
material region (e.g., a 2DEG region). In some embodiments, after
the species has been implanted into the semiconductor structure,
the species is not substantially present within the III-nitride
material region. Thus, in some embodiments, the III-nitride
material region of the semiconductor structure (through which the
species may be implanted) can be substantially free of the
implanted species (i.e., the implanted species is not present or is
present in an amount of less than about 10.sup.15/cm.sup.3, and, in
some cases, less than about 10.sup.14/cm.sup.3, less than about
10.sup.13/cm.sup.3, less than about 10.sup.12/cm.sup.3, less than
about 10.sup.11/cm.sup.3, less than about 10.sup.10/cm.sup.3, or
lower). In some embodiments, the III-nitride material region
comprises a 2DEG region, and the 2DEG region (through which the
species may be implanted) is substantially free of the first
implanted species (e.g., after the species is implanted).
[0166] In some embodiments, the implanted species is not implanted
into at least a portion of a region between a source of the
transistor and a drain of the transistor. The region between the
source of the transistor and the drain of the transistor through
which the species is not implanted may correspond to, in some
embodiments, a 2DEG region. Thus, in some embodiments, the
III-nitride material region of the semiconductor comprises a 2DEG
region that is substantially free of the implanted species (i.e.,
the implanted species is not present or is present in an amount of
less than about 10.sup.15/cm.sup.3, and, in some cases, less than
about 10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3, less
than about 10.sup.12/cm.sup.3, less than about 10.sup.11/cm.sup.3,
less than about 10.sup.10/cm.sup.3, or lower). According to certain
embodiments, the species is not implanted into at least a portion
of a region between a source of the transistor and a gate of the
transistor. In some embodiments, the species is not implanted into
at least a portion of a region between a drain of the transistor
and a gate of the transistor.
[0167] In certain embodiments, after the implanting step, at least
a portion of the substrate underneath a channel between a source of
the transistor and a drain of the transistor is substantially free
of the implanted species (i.e., the implanted species is not
present or is present in an amount of less than about
10.sup.15/cm.sup.3, and, in some cases, less than about
10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3, less than
about 10.sup.12/cm.sup.3, less than about 10.sup.11/cm.sup.3, less
than about 10.sup.10/cm.sup.3, or lower). In some embodiments, at
least a portion of the substrate underneath a region between a
source of the transistor and a gate of the transistor is
substantially free of the implanted species. For example, referring
to FIGS. 3A-3B, in some embodiments, region 340 (which is
underneath a region between source 220 and drain 230 of transistor
210 (and which has portions between source 220 and gate 240 of
transistor 210, and between gate 240 and drain 230 of transistor
210) is substantially free of the implanted species.
[0168] In some embodiments, the implanted species is not implanted
into at least a portion of a region between at least a portion of a
source electrode of the transistor and at least a portion of a
drain electrode of the transistor. For example, in some
embodiments, the implanted species is not implanted into at least a
portion of a region between a source finger electrode portion and a
drain finger electrode portion. Such arrangement of the implanted
species can be achieved, for example, using an implantation mask.
FIG. 3C is a top-view schematic illustration showing an embodiment
in which a mask comprising blocking regions 320 (shown as shaded
areas inside dotted rectangles) is used to avoid implanting a
species in multiple regions between source finger electrode
portions and drain finger electrode portions of a transistor (which
can correspond to transistor 210 of semiconductor structure 200C
from FIGS. 2C-2E). FIG. 3D is a front view cross-sectional
illustration of semiconductor structure 200C from FIG. 3C, with the
cross-section taken along line 3D shown in FIG. 3C. FIG. 3E is a
side view cross-sectional illustration of semiconductor structure
200C from FIG. 3C, with the cross-section taken along line 3E shown
in FIG. 3C. As shown in FIGS. 3D-3E, mask 300 comprises open
regions 310, which allow implanted species 315A to pass through the
mask and blocking region 320 which inhibits (or prevents) implanted
species 315B from passing through mask 300. As a result, in some
embodiments, regions 330 of the parasitic channel within surface
region 130 (and/or regions 331 of the III-nitride material region
120) are exposed to the implanted species while region 335 of
III-nitride material region 120 (which region 335 can include a
2DEG region) and region 340 of a parasitic channel within surface
region 130 are not exposed to the implanted species. According to
some such embodiments, the structure of the material within region
335 (including, in some embodiments, the 2DEG region within region
335) is preserved while the number of free carriers within regions
330 is reduced or eliminated. According to certain embodiments, a
2DEG within region 335 is preserved while the 2DEG within regions
331 is reduced, eliminated, or interrupted and the number of free
carriers within regions 330 is reduced or eliminated.
[0169] According to certain embodiments, the implanted species is
implanted into at least a portion of a substrate region (e.g., a
substrate surface region) underneath a source electrode, a gate
electrode, and/or a drain electrode of the transistor. In some
embodiments, implantation of the species can be inhibited across at
least a portion of (and, in some embodiments, a relatively large
percentage of (e.g., at least about 50% of, at least about 75% of,
or at least about 90% of)) the active area of the transistor, while
that species is implanted into one or more regions underneath a
source electrode, a drain electrode, and/or a gate electrode of the
transistor. For example, in some embodiments, implantation of the
species can be inhibited within at least one substrate surface
region between a source electrode and a gate electrode and/or
within at least one substrate surface region between a gate
electrode and a drain electrode while, at the same time, the
species can be implanted into at least one substrate surface region
underneath a source electrode, a gate electrode, and/or a drain
electrode. Such patterned implantation can be achieved, for
example, using an implantation mask. For example, in some
embodiments, the implantation mask can be used to inhibit the
implantation of species within an active area of the transistor
(and, in some cases, across a relatively large percentage of the
active area of the transistor) but can allow for implantation of
species underneath one or more regions underneath the source
electrode (e.g., a source electrode pad), the gate electrode (e.g.,
a gate electrode pad), and/or the drain electrode (e.g., a drain
electrode pad).
[0170] FIG. 3F is a top-view schematic illustration showing an
embodiment in which a mask comprising blocking region 320 (shown as
a shaded area inside a dotted rectangle) is used to avoid
implanting within a large area of the active region of transistor
210 of semiconductor structure 200C (which can correspond to
transistor 210 of semiconductor structure 200C from FIGS. 2C-2E).
FIG. 3G is a front view cross-sectional illustration of
semiconductor structure 200C from FIG. 3F, with the cross-section
taken along line 3G shown in FIG. 3F. FIG. 3H is a side view
cross-sectional illustration of semiconductor structure 200C from
FIG. 3F, with the cross-section taken along line 3H shown in FIG.
3F. In the exemplary embodiment shown in FIGS. 3F-3H, substantially
the entire active area of the transistor is masked to prevent
implantation of the species into the active area. However,
implantation of the implanted species can occur outside of the
active region (e.g., in the field region of the transistor, such as
under the source, gate, and/or drain pads; in the isolation regions
of the transistor, and/or in saw street(s) between the transistor
and an adjacent transistor). According to certain embodiments, by
implanting outside the active area of the transistor, a
high-conductivity parasitic channel area can remain intact in the
active area of the transistor, while only a low-conductivity
parasitic channel (or no parasitic channel) remains outside the
active area, due to interaction of the implanted species into the
surface region of the substrate. For example, as shown in FIGS.
3F-3H, mask 300 comprises open regions 310, which allow implanted
species 315A to pass through the mask and blocking region 320 which
inhibits (or prevents) implanted species 315B from passing through
mask 300. As a result, in some embodiments, regions 330 of the
parasitic channel within surface region 130 (and/or regions 331 of
the III-nitride material region 120) are exposed to the implanted
species while region 335 of III-nitride material region 120 (which
region 335 can include a 2DEG region) and region 340 of a parasitic
channel within surface region 130 are not exposed to the implanted
species. According to some such embodiments, the structure of the
material within region 335 (including, in some embodiments, the
2DEG region within region 335) is preserved while the number of
free carriers within regions 330 is reduced or eliminated.
According to certain embodiments, a 2DEG within region 335 is
preserved while the 2DEG within regions 331 is reduced, eliminated,
or interrupted and the number of free carriers within regions 330
is reduced or eliminated.
[0171] As noted above, in some embodiments, a species can be
implanted in a field region of a semiconductor structure to reduce
the conductivity of or eliminate a parasitic channel. Such
implantation schemes may be useful, for example, in eliminating
high-conductivity parasitic channels underneath contact pads of a
transistor and/or between adjacent transistors of a semiconductor
structure.
[0172] In some embodiments, the semiconductor structure comprises a
first transistor and a second transistor laterally spaced apart
from the first transistor. For example, referring to FIGS. 3I-3J,
semiconductor structure 390 comprises a first transistor 210-1 and
a second transistor 210-2 laterally spaced apart from first
transistor 210-1 (i.e., spaced apart along lateral dimension 197 of
substrate 110). In certain embodiments, more than two transistors
may be present. For example, in FIGS. 3I-3J, semiconductor
structure 390 also comprises an optional third transistor 210-3
laterally spaced apart from first transistor 210-1 and second
transistor 210-2, and an optional fourth transistor 210-4 laterally
spaced apart from first transistor 210-1, second transistor 210-2,
and third transistor 210-3. In FIGS. 3I-3J, the electrodes (e.g.,
the source, drain, and gate electrodes) and the insulating
materials between the electrodes are shown simply as block 362, for
purposes of maintaining clarity in the figures. Accordingly, each
of blocks 362A, 362B, 362C, and 362D can include a source
electrode, a gate electrode, a drain electrode, and/or one or more
dielectric layers, arranged in any suitable configuration
(including, but not limited to, any of the configurations described
herein).
[0173] According to certain embodiments, after an implanting step,
at least a portion of the surface region of the substrate between
the first transistor and the second transistor has a concentration
of the implanted species of at least about 10.sup.19/cm.sup.3 (or,
in some embodiments, at least about 10.sup.20/cm.sup.3, at least
about 10.sup.21/cm.sup.3, at least about 10.sup.22/cm.sup.3, at
least about 10.sup.23/cm.sup.3, or more). For example, referring to
FIG. 3J, in some embodiments, region 330 of surface region 130 of
substrate 110 between first transistor 210-1 and second transistor
210-2 (and/or between second transistor 210-2 and third transistor
210-3, and/or between third transistor 210-3 and fourth transistor
210-4) has a concentration of the implanted species of at least
about 10.sup.19/cm.sup.3. According to certain embodiments, after
an implanting step, the presence of the implanted species within
the surface region of the substrate between the first transistor
and the second transistor (and/or between the second transistor and
the third transistor, and/or between the third transistor and the
fourth transistor) can reduce the conductivity of a parasitic
channel region within surface region 130 of substrate 110 such that
a formerly high-conductivity parasitic channel region becomes a
low-conductivity parasitic channel region or ceases to be a
parasitic channel region. According to certain embodiments, after
an implanting step, at least a portion of the surface region of the
substrate below the first transistor and/or the second transistor
is substantially free of the implanted species (i.e., the implanted
species is not present or is present in an amount of less than
about 10.sup.15/cm.sup.3, and, in some cases, less than about
10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3, less than
about 10.sup.12/cm.sup.3, less than about 10.sup.11/cm.sup.3, less
than about 10.sup.10/cm.sup.3, or lower). For example, referring to
FIG. 3J, in some embodiments, region 340 under first transistor
210-1 can be substantially free of the implanted species. In some
embodiments, region 340 under second transistor 210-2 (and/or third
transistor 210-3, and/or fourth transistor 210-4) can be
substantially free of the implanted species.
[0174] According to certain embodiments, after an implanting step,
the presence of the implanted species within the surface region of
the substrate between the first transistor and the second
transistor (and/or between the second transistor and the third
transistor, and/or between the third transistor and the fourth
transistor) can reduce the conductivity of a parasitic channel
region within the surface region of the substrate such that a
formerly high-conductivity parasitic channel region becomes a
low-conductivity parasitic channel region (or ceases to be a
parasitic channel region), while a 2DEG within the III-nitride
material region is reduced, eliminated, or interrupted.
[0175] FIGS. 3K-3M are top-view schematic illustrations showing
exemplary implantation masks that can be used to implant one or
more species into a semiconductor structure comprising multiple
devices (e.g., transistors, such as FETs), according to certain
embodiments. Transistors can be laterally spaced across the top
surface of substrate, for example, in an array. For example, in
FIGS. 3K-3M, transistors 210 (including transistors 210-1, 210-2,
210-3, and 210-4) are arranged in a two-dimensional array. The mask
can include multiple blocking regions, which can inhibit or prevent
the implantation of species into the underlying semiconductor
structure. For example, in FIGS. 3K-3M, the illustrated masks
include blocking regions 320, which inhibit or prevent implantation
of species into the semiconductor structure underlying regions 320.
FIG. 3K includes a pattern similar to the pattern described with
respect to FIG. 3F, repeated over each of the transistors on the
substrate. FIG. 3L includes a pattern similar to the pattern
described with respect to FIG. 3C, repeated over each of the
transistors on the substrate. In certain embodiments, the mask can
be configured such open regions cover a relatively large percentage
(e.g., at least about 80%, at least about 90%, at least about 95%,
or at least about 99%) of the area of the substrate covered by
electrode pads, and blocking regions cover a relatively large
percentage (e.g., at least about 80%, at least about 90%, at least
about 95%, or at least about 99%) of the remaining substrate. One
particular example is shown in FIG. 3M, in which the mask comprises
open regions 310 positioned over contact pads of transistors 210,
and blocking region 320 covers the remaining part of substrate 110.
Although the open regions in FIGS. 3K-3M are substantially
rectangular in shape, mask openings with other shapes (e.g.,
circular or substantially circular, hexagonal or substantially
hexagonal, parallelograms, etc.) may also be used.
[0176] Certain embodiments comprise implanting a first species and
a second species into the semiconductor structure. According to
certain such embodiments, the first species and the second species
can be implanted into the semiconductor structure at different
depths. This can produce, for example, a semiconductor structure in
which a first species is implanted at a first depth (e.g., with a
surface region of the substrate) and a second species is present at
a different depth (e.g., within the III-nitride material region,
such as within a 2DEG region).
[0177] According to certain embodiments, the first implanted
species and the second implanted species are compositionally
different. In some embodiments, the first species that is implanted
into the semiconductor structure can have a relative atomic mass of
less than 5. The species having a relative atomic mass of less than
5 can be atomic (e.g., atomic helium (He)), ionic (e.g., hydrogen
cations (H.sup.+)), or molecular (e.g., molecular hydrogen
(H.sub.2)). In certain embodiments, more than one first species
having an atomic mass of less than 5 may be implanted. In some
embodiments, the second species that is implanted into the
semiconductor structure can be a p-type dopant (e.g., boron,
aluminum, gallium, and/or indium) or an n-type dopants (e.g.,
nitrogen, oxygen, phosphorus, and/or arsenic).
[0178] In some embodiments, the first species is implanted into the
substrate of the semiconductor structure, such as into a surface
region of the substrate (e.g., surface region 130 in the figures).
In some such embodiments, the first implanted species is arranged
within the substrate (e.g., within the surface region of the
substrate) in a first pattern spatially defined across at least one
lateral dimension of the substrate (and, in some embodiments,
across two lateral dimensions of the substrate). Various such
patterns are described above, for example, with respect to FIGS.
3A-3J. In some embodiments, during the implanting of the first
species, at least a portion of the first species is implanted
through the III-nitride material region. For example, referring to
FIGS. 3A-3B, in some embodiments, a first species is implanted
through III-nitride material region 120 into surface region 130 of
substrate 110 (e.g., into regions 330 of surface region 130).
Similarly, referring to FIGS. 3D-3E and 3G-3H, in some embodiments,
a first species is implanted through III-nitride material region
120 into surface region 130 of substrate 110 (e.g., into regions
330 of surface region 130).
[0179] According to certain embodiments, after the first species is
implanted, the first species is present within at least a portion
of the surface region of the substrate at a concentration of at
least about 10.sup.19/cm.sup.3 (or, in some embodiments, at least
about 10.sup.20/cm.sup.3, at least about 10.sup.21/cm.sup.3, at
least about 10.sup.22/cm.sup.3, at least about 10.sup.23/cm.sup.3,
or more). For example, referring to FIGS. 3A-3B, in some
embodiments, after a first species is implanted through III-nitride
material region 120 into surface region 130 of substrate 110 (e.g.,
into regions 330 of surface region 130), the concentration of the
first species within surface region 130 (e.g., within region 330 of
surface region 130) can be at least about 10.sup.19/cm.sup.3. As
another example, referring to FIGS. 3D-3E and 3G-3H, in some
embodiments, after a first species is implanted through III-nitride
material region 120 into surface region 130 of substrate 110 (e.g.,
into regions 330 of surface region 130), the concentration of the
first species within surface region 130 (e.g., within region 330 of
surface region 130) can be at least about 10.sup.19/cm.sup.3.
[0180] In some embodiments, after the first species is implanted,
at least a second portion of the surface region of the substrate is
substantially free of the first implanted species (i.e., the first
implanted species is not present or is present in an amount of less
than about 10.sup.15/cm.sup.3, and, in some cases, less than about
10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3, less than
about 10.sup.12/cm.sup.3, less than about 10.sup.11/cm.sup.3, less
than about 10.sup.10/cm.sup.3, or lower). For example, referring to
FIGS. 3A-3B, in some embodiments, after a first species is
implanted through III-nitride material region 120 into surface
region 130 of substrate 110 (e.g., into regions 330 of surface
region 130), region 340 of surface region 130 of substrate 110 can
be substantially free of the first implanted species. As another
example, referring to FIGS. 3D-3E and 3G-3H, according to certain
embodiments, after a first species is implanted through III-nitride
material region 120 into surface region 130 of substrate 110 (e.g.,
into regions 330 of surface region 130), region 340 of surface
region 130 of substrate 110 can be substantially free of the first
implanted species.
[0181] According to certain embodiments, after the first species is
implanted, the III-nitride material region is substantially free of
the first species (i.e., the first species is not present or is
present in an amount of less than about 10.sup.15/cm.sup.3, and, in
some cases, less than about 10.sup.14/cm.sup.3, less than about
10.sup.13/cm.sup.3, less than about 10.sup.12/cm.sup.3, less than
about 10.sup.11/cm.sup.3, less than about 10.sup.10/cm.sup.3, or
lower). For example, referring to FIGS. 3A-3B, in some embodiments,
after a first species is implanted through III-nitride material
region 120 into surface region 130 of substrate 110 (e.g., into
regions 330 of surface region 130), III-nitride material region 120
can be substantially free of the first implanted species. As
another example, referring to FIGS. 3D-3E and 3G-3H, according to
certain embodiments, after a first species is implanted through
III-nitride material region 120 into surface region 130 of
substrate 110 (e.g., into regions 330 of surface region 130),
III-nitride material region 120 can be substantially free of the
first implanted species. In some such embodiments, the III-nitride
material region comprises a 2DEG region, and the 2DEG region (e.g.,
after the first species is implanted) is substantially free of the
first implanted species. For example, referring to FIGS. 3A-3B, in
some embodiments, III-nitride material region 120 comprises a 2DEG
region and, after a first species is implanted through III-nitride
material region 120 into surface region 130 of substrate 110 (e.g.,
into regions 330 of surface region 130), the 2DEG region can be
substantially free of the first implanted species. As another
example, referring to FIGS. 3D-3E and 3G-3H, according to certain
embodiments, III-nitride material region 120 comprises a 2DEG
region and, after a first species is implanted through III-nitride
material region 120 into surface region 130 of substrate 110 (e.g.,
into regions 330 of surface region 130), the 2DEG region can be
substantially free of the first implanted species.
[0182] In certain embodiments, the second species is implanted into
a III-nitride material region of the semiconductor structure. The
second implanted species may be arranged, according to certain
embodiments, within the III-nitride material region in a second
pattern spatially defined across at least one lateral dimension
(and, in some embodiments, across two lateral dimensions) of the
substrate. The second implanted species may also be arranged within
the III-nitride material region such that the second species forms
a pattern spatially defined across at least one lateral dimension
(and, in some embodiments, across two lateral dimensions) of the
III-nitride material region.
[0183] According to some embodiments, the first implanted species
can be used to form a spatially defined implantation pattern in a
parasitic channel region within a semiconductor structure, and the
second implanted species can be used to form a spatially defined
implantation pattern in a III-nitride material region (e.g., a 2DEG
region within a III-nitride material region) within the
semiconductor structure. The first implanted species can be used,
in some such embodiments, to reduce or eliminate the conductivity
of certain regions of a parasitic channel within a surface region
of a substrate (e.g., as described above). The second implanted
species can be used, in some such embodiments, to reduce or
eliminate the conductivity of certain portions of a 2DEG region
within a III-nitride material region of the semiconductor
structure. In some embodiments, the second implanted species can be
used as an implant isolation step (e.g., in addition to or instead
of an etched isolation step) in the fabrication process. The second
implant species could be used, in some embodiments, to enhance
ohmic contact from the source and/or drain electrodes of the device
and the 2DEG and/or channel layer. In some such embodiments, the
implant species may be a n-type dopant such as silicon or
germanium.
[0184] An example of such patterned implantation is shown in FIG.
3A. In FIG. 3A, an optional second implantation step can be
performed in which a second species (e.g., high energy boron or
nitrogen to damage the III-nitride crystal lattice and disrupt the
2DEG) has been implanted into regions 331 of semiconductor
structure 200A. In some such embodiments, the semiconductor
structure includes both a first implanted species to reduce or
eliminate the conductivity of a parasitic channel in patterned or
selected regions of the substrate (e.g., in selected regions of a
surface region of the substrate), as well as a second implanted
species to reduce or eliminate the conductivity of patterned or
selected regions of a 2DEG region in the III-nitride material
region. The implantation of multiple species in this way may be
advantageous according to certain, although not necessarily all,
embodiments, such as, for example, in certain cases when implant
isolation is used in the field regions of semiconductor structures
to electrically isolate one device (e.g., transistor) form another
device (e.g., transistor), and when implant species are also used
to eliminate or reduce the impact of an existing parasitic channel.
Another example illustrating the implantation of multiple species
at different depths is shown in FIGS. 3B and 3H. In FIGS. 3B and
3H, an optional second implantation step can be performed in which
a second species has been implanted into regions 331 of
semiconductor structure 200A.
[0185] In certain embodiments, a semiconductor device may comprise
a region in which the conductivity of a 2DEG region has been
reduced or eliminated, for example, by implanting a second species
(e.g., boron or nitrogen). In some such embodiments, the
semiconductor device may also include a region--positioned under
the region in which the conductivity of the 2DEG region has been
reduced or eliminated--in which the conductivity of a parasitic
channel has been reduced or eliminated. For example, referring to
FIGS. 3A-3B, in certain embodiments, the semiconductor structure
200 may comprise a region 331 in which the conductivity of a 2DEG
region within III-nitride material region 120 has been reduced,
disrupted, or eliminated (for example, by implanting a second
species) and a region 330, positioned under region 331, in which
the conductivity of a parasitic channel has been reduced (or even
eliminated). In some such embodiments, region 331 has a
concentration of both the parasitic species (e.g., Ga, Al, etc.)
and a concentration of the first implanted species (e.g., a species
having a relative atomic mass of less than 5). In some embodiments,
the semiconductor device may also comprise a region having a
high-conductivity 2DEG and a region--positioned under the
high-conductivity 2DEG region--having a high-conductivity parasitic
channel. For example, referring to FIGS. 3A-3B, in some
embodiments, the semiconductor device may have a region 335 which
includes a high-conductivity 2DEG, as well as a region 340 formed
under region 335 which includes a high-conductivity parasitic
channel (for example, due to a relatively high concentration of one
or more parasitic species, such as Ga, Al, and the like).
[0186] According to certain embodiments, implantation of the second
species can reduce the electrical conductivity of the 2DEG by at
least about 5%, at least about 10%, at least about 25%, at least
about 50%, at least about 75%, at least about 90%, at least about
95%, at least about 99%, or more, relative to the electrical
conductivity of the 2DEG prior to the implantation of the second
species.
[0187] The patterned implantation of the second species can be
achieved, for example, using an implantation mask. As shown in
FIGS. 3A, 3B, and 3G, the second species is implanted into regions
331 using the same implantation mask 300 that is used to pattern
the implantation of the first species. In other embodiments, the
implantation of the second species can be achieved using a second
implantation mask that is different from the first implantation
mask. For example, in certain embodiments in which the implantation
mask shown in FIG. 3C is used to implant the first species, a
different implantation mask may be used to implant the second
species (e.g., to avoid implanting the second species within active
regions of the transistor). As one example, in some embodiments,
the mask illustrated in FIG. 3C can be used to implant the first
species and the mask illustrated in FIG. 3F can be used to implant
the second species.
[0188] The III-nitride material region into which the second
species is implanted may comprise, in some embodiments, a 2DEG
region. In some such embodiments, the second implanted species can
be implanted within the 2DEG region of the III-nitride material
region. In some such embodiments, after implantation of the second
species into the 2DEG region, the second implanted species forms a
pattern spatially defined across at least one lateral dimension of
the 2DEG region (and, in some embodiments, across two lateral
dimensions of the 2DEG region).
[0189] In some embodiments, the concentration of the second
implanted species within the III-nitride material region is
relatively high. For example, in some cases, after the second
species is implanted into the semiconductor structure, the second
species is present within at least a portion of the III-nitride
material region (e.g., within at least a portion of a 2DEG region
of the III-nitride material) at a concentration of at least about
10.sup.16/cm.sup.3 (and/or, in some embodiments, at least about
10.sup.17/cm.sup.3, at least about 10.sup.18/cm.sup.3, at least
about 10.sup.19/cm.sup.3, at least about 10.sup.20/cm.sup.3, or
more). Referring to FIGS. 3A-3B, for example, in some embodiments,
after a second species is implanted into III-nitride material
region 120, the concentration of the second species within
III-nitride material region 120 (e.g., within region 331 of
III-nitride material region 120) can be at least about
10.sup.16/cm.sup.3. As another example, referring to FIG. 3G, in
some embodiments, after a second species is implanted into
III-nitride material region 120, the concentration of the second
species within surface region 130 (e.g., within region 331 of
III-nitride material region 120) can be at least about
10.sup.16/cm.sup.3.
[0190] In some embodiments, after the second species is implanted,
at least a second portion of the III-nitride material region is
substantially free of the second implanted species (i.e., the
second implanted species is not present or is present in an amount
of less than about 10.sup.15/cm.sup.3, and, in some cases, less
than about 10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3,
less than about 10.sup.12/cm.sup.3, less than about
10.sup.11/cm.sup.3, less than about 10.sup.10/cm.sup.3, or lower).
Referring to FIGS. 3A-3B, for example, in some embodiments, after a
second species is implanted into III-nitride material region 120,
region 335 of III-nitride material region 120 can be substantially
free of the second implanted species. As another example, referring
to FIG. 3G, in some embodiments, after a second species is
implanted into III-nitride material region 120, region 335 of
III-nitride material region 120 can be substantially free of the
second implanted species. The first and second species can be
implanted, according to certain embodiments, into a semiconductor
structure comprising a transistor (e.g., a FET) located over the
substrate. According to some such embodiments, after the first
species is implanted into the semiconductor structure, at least a
portion of the substrate underneath a channel between a source of
the transistor and a drain of the transistor is substantially free
of the first implanted species (i.e., the first implanted species
is not present or is present in an amount of less than about
10.sup.15/cm.sup.3, and, in some cases, less than about
10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3, less than
about 10.sup.12/cm.sup.3, less than about 10.sup.11/cm.sup.3, less
than about 10.sup.10/cm.sup.3, or lower). For example, referring to
FIGS. 3A-3B, in some embodiments, after the first species is
implanted (e.g., using mask 300 shown in FIGS. 3A-3B), regions 335
and 340 can be substantially free of the first implanted species.
As another example, referring to FIGS. 3E and 3G, in some
embodiments, after the first species is implanted, regions 335 and
340 can be substantially free of the first implanted species. In
some such embodiments, after the second species is implanted, at
least a portion of the III-nitride material region underneath a
channel between a source of the transistor and a drain of the
transistor is substantially free of the second implanted species
(i.e., the second species is not present or is present in an amount
of less than about 10.sup.15/cm.sup.3, and, in some cases, less
than about 10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3,
less than about 10.sup.12/cm.sup.3, less than about
10.sup.11/cm.sup.3, less than about 10.sup.10/cm.sup.3, or lower).
For example, referring to FIGS. 3A-3B, in some embodiments, after
the second species is implanted, regions 335 and 340 can be
substantially free of the second implanted species. As another
example, referring to FIGS. 3E and 3G, in some embodiments, after
the second species is implanted, regions 335 and 340 can be
substantially free of the second implanted species.
[0191] In some embodiments, the first and/or second species can be
implanted between first and second transistors in a semiconductor
structure comprising multiple transistors. For example, as noted
above, in some embodiments, the semiconductor structure comprises
first transistor located over the substrate and a second transistor
located over the substrate and laterally spaced apart from the
first transistor. In some such embodiments, after the first species
is implanted, a portion of the surface region of the substrate
between the first transistor and the second transistor has a
concentration of the first implanted species of at least about
10.sup.19/cm.sup.3 (or, in some embodiments, at least about
10.sup.20/cm.sup.3, at least about 10.sup.21/cm.sup.3, at least
about 10.sup.22/cm.sup.3, at least about 10.sup.23/cm.sup.3, or
more). For example, referring to FIG. 3J, in some embodiments,
after the first species is implanted, region 330 of surface region
130 of substrate 110 between first transistor 210-1 and second
transistor 210-2 has a concentration of the first implanted species
of at least about 10.sup.19/cm.sup.3. In some embodiments, region
330 between second transistor 210-2 and third transistor 210-3
and/or region 330 between third transistor 210-3 and fourth
transistor 210-4 has a concentration of the first implanted species
of at least about 10.sup.19/cm.sup.3.
[0192] In some such embodiments, after the second species is
implanted, a portion of the III-nitride material region between the
first transistor and the second transistor has a concentration of
the second implanted species of at least about 10.sup.16/cm.sup.3
(or, in some embodiments, at least about 10.sup.17/cm.sup.3, at
least about 10.sup.18/cm.sup.3, at least about 10.sup.19/cm.sup.3,
at least about 10.sup.20/cm.sup.3, or more). The second species can
be implanted, according to some embodiments, such that a portion of
a 2DEG region between the first transistor and the second
transistor has a concentration of the second implanted species of
at least about 10.sup.16/cm.sup.3 (or, in some embodiments, at
least about 10.sup.17/cm.sup.3, at least about 10.sup.18/cm.sup.3,
at least about 10.sup.19/cm.sup.3, at least about
10.sup.20/cm.sup.3, or more). For example, referring to FIG. 3J, in
some embodiments, after the second species is implanted, region 331
of III-nitride material region 120 between first transistor 210-1
and second transistor 210-2 (which can include a portion of a 2DEG
region 191 between first transistor 210-1 and second transistor
210-2) has a concentration of the second implanted species of at
least about 10.sup.16/cm.sup.3. In some embodiments, region 331
between second transistor 210-2 and third transistor 210-3 and/or
region 331 between third transistor 210-3 and fourth transistor
210-4 (which can include portions of a 2DEG region 191 between
second transistor 210-2 and third transistor 210-3 and/or between
third transistor 210-3 and fourth transistor 210-4, respectively)
has a concentration of the second implanted species of at least
about 10.sup.16/cm.sup.3.
[0193] According to certain embodiments, the second species can be
implanted such that it surrounds at least one transistor of the
semiconductor structure. As one example, in some embodiments, an
implantation mask similar to the mask shown in FIG. 3K can be used
to pattern the implantation of the second species. In some such
cases, when the second species is implanted across open region 310,
the second implanted species can surround each of transistors 210
positioned over substrate 110.
[0194] Those of ordinary skill in the art, given the guidance
provided by the instant specification, would be capable of
selecting appropriate implantation conditions to achieve a desired
implantation pattern. When implanting multiple species at different
depths within semiconductor structures, each implantation step may,
in some cases, be performed using different implant conditions
(e.g., dose, energy, and/or implant species) may be employed. For
example, in some cases, the conditions (e.g., dose, energy, etc.)
used to implant the first species (e.g., having an atomic mass of
less than 5) to reduce or eliminate the conductivity of an existing
parasitic channel may be different than the conditions (e.g., dose,
energy, etc.) used to implant the second species (e.g., a p-type
dopant or an n-type dopant) or introduce nitrogen vacancies to
reduce or eliminate the conductivity of a 2DEG region of a
III-Nitride material device region). This difference can, in
certain cases, be due to (at least in part) the different depths at
which the parasitic channel and the 2DEG region are positioned. For
example, the parasitic channel may be located at a relatively deep
location (e.g., 2-5 microns below surface 135 of the substrate, or
more) while the 2DEG region may be relatively shallow.
[0195] According to certain embodiments, patterned implantation of
the first and/or second species may result in the formation of at
least one p-n junction defined laterally across a portion of the
semiconductor structure.
[0196] For example, patterned implantation of the first species
into a surface region of the substrate may form, according to
certain embodiments, a p-n junction defined laterally across the
surface region of the substrate. Referring to FIG. 3A, surface
region 130 of substrate 110 may have an original doping profile
(e.g., n-type or p-type). In some cases, formation of III-nitride
material region 120 may result in the diffusion of one or more
dopants (e.g., Ga, In, As, N, and the like) into substrate 110,
causing the doping type of surface region 130 to change (e.g., from
an n-type region to a p-type region, or from a p-type region to an
n-type region). In some embodiments, patterned implantation of the
first species into regions 330 of surface region 130 can cause the
doping type of regions 330 to change again (e.g., from n-type to
p-type, or from p-type to n-type). In some such embodiments, region
340, because it is not exposed to the first implanted species,
retains the doping type present after diffusion of the parasitic
species into surface region 130. In some such embodiments, regions
330 form p-n junctions with region 340.
[0197] As another example, patterned implantation of the second
species into a III-nitride material region (e.g., into a 2DEG
region of a III-nitride material region) may form, according to
certain embodiments, a p-n junction defined laterally across the
III-nitride material region (e.g., defined laterally across a 2DEG
region within a III-nitride material region) of the semiconductor
structure. Referring to FIG. 3A, for example, III-nitride material
region 120 may have an original doping profile (e.g., n-type or
p-type). In some embodiments, patterned implantation of the second
species into regions 331 of III-nitride material region 120 can
cause the doping types of regions 331 to change (e.g., from p-type
to n-type, or from n-type to p-type). In some such embodiments,
region 335, because it is not exposed to the second implanted
species, retains the doping type present after formation of the
III-nitride material region. In some such embodiments, regions 331
form p-n junctions with region 335.
[0198] According to certain embodiments, one or more
counter-dopants can be used to counteract the electronic effect of
a dopant within the parasitic region of the semiconductor
structure. The counter-doping may be performed, according to some
such embodiments, to reduce the free carrier concentration within
the parasitic region of the semiconductor structure.
[0199] Accordingly, certain embodiments comprise implanting a
counter-dopant into the semiconductor structure such that a
concentration profile of the counter-dopant substantially matches a
concentration profile of a second dopant present within the
substrate. Generally, the concentration profile of a first dopant
type matches the concentration profile of a second dopant type
within a region (e.g., within a substrate) when both the first and
second dopant types are present in the substrate, and the maximum
free carrier concentration through the thickness of the region
(e.g., through the thickness of the substrate) is less than about
10.sup.17/cm.sup.3. In some embodiments, a concentration profile of
a first dopant type matches the concentration profile of a second
dopant type within a region (e.g., within a substrate) such that
the maximum free carrier concentration through the thickness of the
region (e.g., through the thickness of the substrate) is less than
about 10.sup.16/cm.sup.3, less than about 10.sup.15/cm.sup.3, less
than about 10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3,
less than about 10.sup.12/cm.sup.3, less than about
10.sup.11/cm.sup.3, or less than about 10.sup.10/cm.sup.3. One of
ordinary skill in the art would understand that, when determining
the n-type dopant concentration profile, the contributions of all
n-type dopants are considered as a whole. Thus, when multiple
n-type dopants are present, the n-type dopant concentration profile
corresponds to the profile generated when summing the
concentrations of all n-type dopants through the thickness of the
substrate. Similarly, when determining the p-type dopant
concentration profile, the contributions of all p-type dopants are
considered as a whole. Thus, when multiple p-type dopants are
present, the p-type dopant concentration profile corresponds to the
profile generated when summing the concentrations of all p-type
dopants through the thickness of the substrate.
[0200] The counter-dopant and the second dopant originally present
in the parasitic channel of the substrate can be of the opposite
type. For example, in some embodiments, the parasitic channel
comprises a p-type dopant (e.g., Al and/or Ga), and the
counter-dopant comprises an n-type dopant (e.g., As and/or P). In
certain embodiments, the parasitic channel comprises an n-type
dopant (e.g., As and/or P), and the counter-dopant comprises a
p-type dopant (e.g., Al and/or Ga).
[0201] Those of ordinary skill in the art are familiar with a
variety of systems and methods that can be used to perform
counter-doping. For example, counter-doping may be performed by
growing a counter-doped layer (e.g., a counter-doped epitaxial
layer) over the substrate (e.g., ex-situ). Counter-doping may also
be performed, for example, by implanting (or diffusing) a
counter-dopant into a surface of the substrate (e.g., surface 135
of substrate 110 in the figures). As yet another example,
counter-doping may be performed by implanting a counter-dopant
using high energies (e.g., such that the surface region of the
substrate becomes non-conductive).
[0202] Certain embodiments are related to semiconductor structures
comprising substrates with multiple dopants whose concentration
profiles are substantially matched. Such structures may be formed,
for example, using the counter-doping methods described above.
According to certain embodiments, the substrate of the
semiconductor structure (e.g., any of the semiconductor structures
described elsewhere herein) comprises a p-type dopant having a
first concentration profile, and an n-type dopant having a second
concentration profile that is substantially matched to the first
concentration profile. The n-type dopant may be, for example, As
and/or P. The p-type dopant may be, for example, Al and/or Ga.
[0203] According to certain embodiments in which counter-dopant(s)
are implanted into the substrate, the substrate may also include a
relatively high peak concentration of Group III species (e.g., Al,
Ga, In, Tl, and B). The relatively high peak concentration(s) of
Group III species can be present before the implantation step,
according to some embodiments. In certain embodiments, the
relatively high peak concentration(s) of Group III species can be
present after the implantation step. In some embodiments, the
presence of the counter-dopant(s) can reduce the electronic
conductivity of the Group III species, inhibiting the formation of
a high conductivity parasitic channel. In some embodiments in which
counter-dopant(s) are implanted into the substrate, the peak of the
sum of the concentrations of Group III species in the substrate
(e.g., the peak of the sum of the concentrations of Al, Ga, In, Tl,
and B in the substrate) is at least about 10.sup.17/cm.sup.3, at
least about 10.sup.18/cm.sup.3, at least about 10.sup.19/cm.sup.3,
at least about 10.sup.20/cm.sup.3, at least about
10.sup.21/cm.sup.3, at least about 10.sup.22/cm.sup.3, or at least
about 10.sup.23/cm.sup.3. In some embodiments in which
counter-dopant(s) are implanted into the substrate, the peak of the
sum of the concentrations of Al, Ga, and In in the substrate is at
least about 10.sup.17/cm.sup.3, at least about 10.sup.18/cm.sup.3,
at least about 10.sup.19/cm.sup.3, at least about
10.sup.20/cm.sup.3, at least about 10.sup.21/cm.sup.3, at least
about 10.sup.22/cm.sup.3, or at least about 10.sup.23/cm.sup.3. In
some embodiments in which counter-dopant(s) are implanted into the
substrate, the peak concentration of Al, Ga, and/or In in the
substrate is at least about 10.sup.17/cm.sup.3, at least about
10.sup.18/cm.sup.3, at least about 10.sup.19/cm.sup.3, at least
about 10.sup.20/cm.sup.3, at least about 10.sup.21/cm.sup.3, at
least about 10.sup.22/cm.sup.3, or at least about
10.sup.23/cm.sup.3.
[0204] In some embodiments, the substrate comprises a
high-conductivity parasitic channel before the implantation of
counter-dopant(s) and a low-conductivity parasitic channel (or no
parasitic channel) after the implantation of counter-dopant(s). In
some embodiments, the bulk region of the substrate has a lower peak
free carrier concentration than the top surface region of the
substrate before and/or after the implantation of counter
dopant(s). In some embodiments, the bulk region of the substrate is
doped with a first free carrier type and the top surface region is
doped with a second free carrier type (e.g., a Group III species
such as Al, Ga, In, and/or Tl) before and/or after the implantation
of counter-dopant(s). According to certain embodiments, the peak
free carrier concentration in the bulk region is less than about
10.sup.13/cm.sup.3, less than about 10.sup.12/cm.sup.3, less than
about 10.sup.11/cm.sup.3, less than about 10.sup.10/cm.sup.3, or
less than about 10.sup.9/cm.sup.3 before and/or after the
implantation of counter-dopant(s).
[0205] According to certain embodiments, one or more active species
capable of reacting with a species originating in a location
external to the substrate (also referred to herein as an "external
species") is incorporated into the substrate (e.g., into a surface
region of a substrate, which can be all or part of a silicon
layer). The active species within the substrate can inhibit (or
prevent) the formation of parasitic channels within the substrate
(e.g., within the surface region of the substrate) by, for example,
reacting with one or more external species that diffuses into or is
otherwise transported into the substrate (e.g., during growth of
the III-nitride material region). In some such embodiments, the
presence of the active species ensures that the peak free carrier
concentration within the substrate (e.g., within the surface region
of the substrate) remains lower than it otherwise would be in the
absence of the active species but under otherwise identical
conditions.
[0206] Accordingly, certain inventive methods comprise forming a
III-nitride material region over a surface of a substrate such that
an active species within the substrate (e.g., within a surface
region of the substrate) reacts with at least a portion of an
external species that contacts the substrate during the formation
of the III-nitride material region. For example, referring to FIGS.
1A-1B, in some embodiments, during at least a portion of the time
during which at least a portion of III-nitride material region 120
is formed, an active species within substrate 110 (e.g., within
surface region 130 of substrate 110) may react with a species
external to substrate 110. In some such embodiments, the free
carrier concentration within the parasitic channel within surface
region 130 may remain relatively low. In certain embodiments, the
parasitic channel within surface region 130 may even be completely
eliminated from substrate 110.
[0207] A variety of active species capable of reacting with the
species originating from a location external to the substrate can
be used. Generally, such species are non-silicon species. The
active species may be molecular or atomic, and may be charged or
uncharged. In some embodiments, the species capable of reacting
with the external species comprises oxygen (e.g., O.sub.2,
O.sup.+), nitrogen (e.g., N.sub.2), carbon, copper (e.g., in
metallic form), and/or iron (e.g., in metallic form). According to
certain embodiments, the species capable of reacting with the
external species comprises O.sup.+. In some embodiments, the active
species capable of reacting with the external species comprises
oxygen, and the oxygen oxidizes an external species that diffuses
into the substrate during at least a portion of the time during
which the III-nitride material region is formed.
[0208] The active species capable of reacting with a species
external to the substrate can be, in some embodiments, formed in
the substrate prior to the III-nitride material region growth step.
Suitable processes for forming such regions in the substrate are
known to those of ordinary skill in the art. For example, in some
embodiments, the active species can be formed in the substrate by
growing a layer comprising the active species over the substrate
(e.g., ex-situ). In certain embodiments, the active species can be
formed in the substrate by implanting (or diffusing) the active
species into a surface of the substrate (e.g., surface 135 of
substrate 110 in the figures). As yet another example, the active
species can be formed in the substrate by implanting the active
species using high energies.
[0209] The species external to the substrate (with which the active
species reacts) may arise from a variety of sources. In some cases,
the species external to the substrate may accumulate on the
substrate surface (or an overlying layer/region), for example,
after being introduced into a reaction chamber in which the
substrate is placed, but prior to formation of the III-nitride
material region on the substrate. For example, the species external
to the substrate can be a contaminant originating from a reactant
fed to the reactor. The species external to the substrate can also
be a contaminant originating from a non-reactant source outside the
reactor. For example, in some embodiments, the species external to
the substrate may accumulate on the substrate (or an overlying
layer) before the substrate is introduced into the reactor and may
accompany the substrate as the substrate is introduced into the
reactor. In certain cases, the species external to the substrate
may be present within the reactor before the substrate is inserted
into the reactor. For example, in some cases, the species external
to the substrate may be a species that forms a part of the reactor
(e.g., a metal such as Fe, Ni, and/or Cr) in which the III-nitride
material region is grown. As another example, in some cases, the
species external to the substrate may be a contaminant within the
reactor that was present prior to insertion of the substrate into
the reactor. In still further cases, the species external to the
substrate can be a residual reactant left over from a previous
reaction process. In certain embodiments, the external species is
all or part of a precursor of the III-nitride material. In some
embodiments, the species external to the substrate is an organic
species (e.g., an organic component of a reaction precursor of the
III-nitride material). The species external to the substrate could
also be, in some embodiments, a group III element (e.g., Ga, In,
and/or Al). Combinations of any or all of the above species
external to the substrate are also possible.
[0210] Certain embodiments are related to semiconductor structures
comprising substrates containing at least one active species. The
active species, when present in the substrate (e.g., a surface
region of the substrate), can be coupled with an external species
(i.e., coupled with a species that originated from outside the
substrate) or can be capable of reacting with a species external to
the substrate (e.g., any of the external species mentioned above).
In some embodiments, the concentration, within the substrate (e.g.,
within the surface region of the substrate), of the active species
coupled to the external species or capable of reacting with the
external species is at least about at least about
10.sup.19/cm.sup.3 (or at least about 10.sup.20/cm.sup.3, at least
about 10.sup.21/cm.sup.3, at least about 10.sup.22/cm.sup.3, at
least about 10.sup.23/cm.sup.3, or more). The concentration of
active species (either coupled to an external species or capable of
reacting with an external species) can be measured using standard
techniques known to those of ordinary skill in the art including
Secondary Ion Mass Spectroscopy (SIMS).
[0211] According to certain embodiments in which active species are
included in the substrate, the substrate may also include a
relatively low peak concentration of Group III species (e.g., Al,
Ga, In, Tl, and B), for example, after growth of the III-nitride
region and/or after formation of the transistor. In some such
embodiments, the one or more active species can react with Group
III species, inhibiting the buildup of Group III species in the
substrate. In some embodiments in which active species are included
in the substrate, the peak of the sum of the concentrations of
Group III species in the substrate (e.g., the peak of the sum of
the concentrations of Al, Ga, In, Tl, and B in the substrate) is
less than about 10.sup.17/cm.sup.3, less than about
10.sup.16/cm.sup.3, less than about 10.sup.15/cm.sup.3, less than
about 10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3, less
than about 10.sup.12/cm.sup.3, less than about 10.sup.11/cm.sup.3,
or less than about 10.sup.10/cm.sup.3 (e.g., after the formation of
the III-nitride region and/or after formation of the transistor).
In some embodiments in which active species are included in the
substrate, the peak of the sum of the concentrations of Al, Ga, and
In in the substrate is less than about 10.sup.17/cm.sup.3, less
than about 10.sup.16/cm.sup.3, less than about 10.sup.15/cm.sup.3,
less than about 10.sup.14/cm.sup.3, less than about
10.sup.13/cm.sup.3, less than about 10.sup.12/cm.sup.3, less than
about 10.sup.11/cm.sup.3, or less than about 10.sup.10/cm.sup.3
(e.g., after formation of the III-nitride region and/or after
formation of the transistor). In some embodiments in which active
species are included in the substrate, the peak concentration of
Al, Ga, and/or In in the substrate is less than about
10.sup.17/cm.sup.3, less than about 10.sup.16/cm.sup.3, less than
about 10.sup.15/cm.sup.3, less than about 10.sup.14/cm.sup.3, less
than about 10.sup.13/cm.sup.3, less than about 10.sup.12/cm.sup.3,
less than about 10.sup.11/cm.sup.3, or less than about
10.sup.10/cm.sup.3 (e.g., after formation of the III-nitride region
and/or after formation of the transistor).
[0212] As noted above, certain embodiments relate to methods (and
associated structures) in which diffusion barrier regions are used
to inhibit or prevent the diffusion of material that increases the
conductivity of the substrate (which increased conductivity can
lead to the formation of high-conductivity parasitic channels).
Dopants and/or other species can generally diffuse into the
substrate after accumulating on the substrate surface, or on the
surface of a layer overlying the substrate through which the
dopants also diffuse. Thus, as described further below, certain
methods can include forming one or more layers that inhibits or
prevents the diffusion of dopants or other species into the
substrate.
[0213] According to certain embodiments in which diffusion barrier
regions are included in the semiconductor structure, the substrate
may also include a relatively low peak concentration of Group III
species (e.g., Al, Ga, In, Tl, and B), for example, after growth of
the III-nitride region and/or after formation of the transistor. In
some such embodiments, the diffusion barrier region can inhibit or
prevent the diffusion of Group III species into the substrate,
inhibiting the buildup of Group III species in the substrate.
[0214] In some embodiments in which in which diffusion barrier
regions are included in the semiconductor structure, the peak of
the sum of the concentrations of Group III species in the substrate
(e.g., the peak of the sum of the concentrations of Al, Ga, In, Tl,
and B in the substrate) is less than about 10.sup.17/cm.sup.3, less
than about 10.sup.16/cm.sup.3, less than about 10.sup.15/cm.sup.3,
less than about 10.sup.14/cm.sup.3, less than about
10.sup.13/cm.sup.3, less than about 10.sup.12/cm.sup.3, less than
about 10.sup.11/cm.sup.3, or less than about 10.sup.10/cm.sup.3
(e.g., after formation of the III-nitride region and/or after
formation of the transistor). In some embodiments in which
diffusion barrier regions are included in the semiconductor
structure, the peak of the sum of the concentrations of Al, Ga, and
In in the substrate is less than about 10.sup.17/cm.sup.3, less
than about 10.sup.16/cm.sup.3, less than about 10.sup.15/cm.sup.3,
less than about 10.sup.14/cm.sup.3, less than about
10.sup.13/cm.sup.3, less than about 10.sup.12/cm.sup.3, less than
about 10.sup.11/cm.sup.3, or less than about 10.sup.10/cm.sup.3
(e.g., after formation of the III-nitride region and/or after
formation of the transistor). In some embodiments in which
diffusion barrier regions are included in the semiconductor
structure, the peak concentration of Al, Ga, and/or In in the
substrate is less than about 10.sup.17/cm.sup.3, less than about
10.sup.16/cm.sup.3, less than about 10.sup.15/cm.sup.3, less than
about 10.sup.14/cm.sup.3, less than about 10.sup.13/cm.sup.3, less
than about 10.sup.12/cm.sup.3, less than about 10.sup.11/cm.sup.3,
or less than about 10.sup.10/cm.sup.3 (e.g., after formation of the
III-nitride region and/or after formation of the transistor).
[0215] According to certain embodiments, the diffusion barrier can
inhibit or prevent the formation of a parasitic channel in the
substrate during a subsequent step of forming a III-nitride
material region over the substrate. For example, some embodiments
comprise forming a III-nitride material region over a substrate
over which a diffusion barrier region has been positioned (e.g.,
formed). According to some such embodiments, after forming the
III-nitride material region over the diffusion barrier region, the
surface region of the substrate over which the III-nitride material
is formed comprises a low-conductivity parasitic channel or the
surface region (and/or the remainder of the substrate) is free of a
parasitic channel. This can lead to the formation of semiconductor
devices comprising III-nitride material regions positioned over
substrates in which the substrate has no parasitic channel or
includes only a low-conductivity parasitic channel (e.g., in the
surface region over which the III-nitride material region is
formed). For example, referring to FIG. 1B, in some embodiments,
after forming III-nitride material region 120 over diffusion
barrier region 140, surface region 130 of substrate 110 over which
III-nitride material region 120 is formed comprises a
low-conductivity parasitic channel or surface region 130 (and/or
the remainder of substrate 110) is free of a parasitic channel.
[0216] The diffusion barrier region can comprise, in some
embodiments, a single-crystal diffusion barrier layer. In some
embodiments, the diffusion barrier region comprises an epitaxial
layer.
[0217] According to certain embodiments, the diffusion barrier
region comprises an AlN region. In some such embodiments, the
diffusion barrier comprises a low-temperature AlN region located
over the substrate and a high-temperature AlN region located over
the substrate. For example, in the embodiment illustrated in FIG.
1C, diffusion barrier region 140 comprises low-temperature AlN
region 140A (which can be positioned over substrate 110). In
addition, diffusion barrier region 140 comprises high-temperature
AlN region 140B (which can be positioned over substrate 110 and
over low-temperature AlN region 140A). In addition, as shown in
FIG. 1B, semiconductor structure 100B comprises III-nitride
material region 120, which can be located over low-temperature AlN
region 140A and over high-temperature AlN region 140B when
diffusion barrier region 140 includes such layers. While a
multi-layer AlN diffusion barrier region is illustrated and
described in association with FIG. 1C, it should be understood that
such diffusion barrier regions could be used in any of diffusion
barrier regions 140 shown in the figures.
[0218] Those of ordinary skill in the art would be capable of
determining whether a particular AlN region is formed at low
temperatures (e.g., temperatures below about 950.degree. C.) or
high temperatures (e.g., temperatures of about 950.degree. C. or
higher). Low-temperature AlN regions typically exhibit a relatively
rough morphology, often including a three-dimensional surface
structure. Such low-temperature AlN regions can include protrusions
that are triangular and/or columnar in cross-section.
High-temperature AlN regions, on the other hand, generally have a
smoother morphology and develop a two-dimensional step-wise
surface. According to certain embodiments, the low-temperature AlN
region can be formed first, after which the high-temperature AlN
region can be formed. In some such embodiments, the low-temperature
AlN region is unsuitable for gallium nitride material growth, and
the high-temperature AlN region can be used to provide a suitable
substrate for subsequent gallium nitride material growth. Another
artifact of using a low temperature and high temperature AlN
nucleation bilayer is a change in impurity levels found within the
two layers, for example the carbon impurity concentration.
Typically, higher growth temperature AlN layers contain lower
levels of carbon impurities, and lower growth temperature AlN
contain higher levels of carbon impurities. Therefore, in certain
cases, if two different growth temperatures were used for the AlN
layers, there would be two distinct levels of carbon impurities
present. Accordingly, in some embodiments, the semiconductor
structure comprises a first AlN layer with a first level of
impurities (e.g., carbon impurities) and a second AlN layer with a
second level of impurities (e.g., carbon impurities) different from
the first level of impurities (e.g., at least about 2 at %
different, at least about 5 at % different, or more, relative to
the level of impurities in the first AlN layer).
[0219] Certain embodiments relate to inventive methods for forming
semiconductor structures comprising multiple AlN regions. According
to certain embodiments, a first AlN region is formed over a
substrate, wherein the temperature of the environment in which the
first AlN region is formed is below about 950.degree. C. (e.g.,
between about 700.degree. C. and about 950.degree. C.). Some
embodiments comprise forming a second AlN region over the
substrate, wherein the temperature of the environment in which the
second AlN region is formed is at least about 950.degree. C. (e.g.,
from about 950.degree. C. to about 1150.degree. C.). Certain such
embodiments comprise forming a III-nitride material region over the
first AlN region and over the second AlN material region.
[0220] In some embodiments, the low-temperature AlN region is
formed first and the high-temperature AlN region is formed after
the low-temperature AlN region is formed. For example, referring to
FIG. 1C, in some embodiments, low-temperature AlN region 140A is
formed over substrate 110, and subsequently, high-temperature AlN
region 140B is formed over substrate 110 and over low-temperature
AlN region 140A. In some such embodiments, the III-nitride material
region is then formed. For example, in FIG. 1C, III-nitride
material region 120 can be formed over substrate 110 after
low-temperature AlN region 140A and high-temperature AlN region
140B have been formed over substrate 110.
[0221] In certain embodiments, the high-temperature AlN region is
formed first and the low-temperature AlN region is formed after the
high-temperature AlN region is formed. In such embodiments, the
high-temperature AlN region can be positioned over the substrate
and the low-temperature AlN region can be positioned over both the
substrate and the high-temperature AlN region.
[0222] In certain embodiments, the device comprises an amorphous or
non-single crystalline silicon nitride-based or aluminum-nitride
based layer between the substrate and the III-nitride material
nucleation layer. In some such embodiments, the amorphous or
non-single crystalline layer may be formed of Si.sub.xN.sub.y
and/or Al.sub.xSi.sub.(1-x)N, and may optionally comprise one or
more additional elements (e.g., oxygen). The amorphous or
non-single crystalline layer may be relatively thin, according to
some embodiments. For example, in some embodiments, the amorphous
or non-single crystalline layer may have a thickness of less than
about 30 Angstroms, less than about 20 Angstroms, or less than
about 10 Angstroms (and/or, in some embodiments, down to 5
Angstroms thick, or less). The use of a relatively thin layer can
allow one to avoid destroying the epitaxial template used to
perform heteroepitaxial III-nitride material growth.
[0223] In some embodiments, the amorphous or non-single crystalline
silicon nitride-based or aluminum-nitride based layer can be
amorphous. In certain embodiments, the amorphous or non-single
crystalline silicon nitride-based or aluminum-nitride based layer
can be formed directly on the substrate (e.g., a silicon portion of
the substrate). In some such embodiments, the amorphous or
non-single crystalline silicon nitride-based or aluminum-nitride
based layer may be formed by nitridating a top surface of the
silicon substrate. In a nitridation process, nitrogen reacts with a
top surface region of the silicon substrate to form a silicon
nitride-based layer. The top surface may be nitridated by exposing
the silicon substrate to a gaseous source of nitrogen at elevated
temperatures. For example, ammonia may be introduced into a process
chamber in which a silicon substrate is positioned. The temperature
in the process chamber may be between about 1000.degree. C. and
about 1100.degree. C. and the pressure may be between about 20 torr
and about 40 torr (in some cases, about 30 torr). The reaction
between nitrogen and the silicon substrate can be allowed to
proceed for a reaction time selected to produce a layer having a
desired thickness. It should be understood that other processes may
be used to form silicon nitride-based layers including processes
(e.g., CVD processes) that use separate nitrogen and silicon
sources.
[0224] According to certain embodiments, the diffusion barrier
region comprises a layer comprising a rare-earth oxide and/or a
rare-earth nitride. The category "rare-earth oxide and/or
rare-earth nitride" includes rare-earth oxides, rare-earth
nitrides, and combination rare-earth oxides and rare-earth nitrides
(e.g., rare-earth oxynitrides). In some embodiments, referring to
FIG. 1B, diffusion barrier region 140 can comprise, a layer
comprising a rare-earth oxide and/or a rare-earth nitride.
[0225] Some embodiments are related to methods of forming
semiconductor structures comprising layers comprising a rare-earth
oxide and/or a rare-earth nitride. For example, in some
embodiments, a layer comprising a rare-earth oxide and/or a
rare-earth nitride is formed over a substrate. In some such
embodiments, a III-nitride material region is formed over the layer
comprising the a rare-earth oxide and/or a rare-earth nitride.
Referring to FIG. 1B, for example, in some embodiments, a layer
comprising a rare-earth oxide and/or a rare-earth nitride is formed
as part (or all) of diffusion barrier region 140. In some such
embodiments, III-nitride material region 120 is subsequently formed
over diffusion barrier region 140.
[0226] While a diffusion barrier region containing a rare-earth
oxide and/or a rare-earth nitride is illustrated and described in
association with FIG. 1B, it should be understood that such
diffusion barrier regions could be used in any of diffusion barrier
regions 140 shown in the figures.
[0227] A variety of types of rare-earth oxides and/or rare-earth
nitrides can be used in the diffusion barrier region. As used
herein, a "rare-earth" element refers to an element selected from
the group consisting of cerium (Ce), dysprosium (Dy), erbium (Er),
europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La),
lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm),
samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium
(Yb), and yttrium (Y).
[0228] A "rare-earth oxide" is any oxide species containing at
least one rare-earth element. In some embodiments, the rare-earth
oxide contains a single rare-earth element. In other embodiments,
the rare-earth oxide contains two or more rare-earth elements. In
some embodiments, the diffusion barrier region comprises an erbium
oxide (e.g., Er.sub.2O.sub.3), a gadolinium oxide (e.g.,
Gd.sub.2O.sub.3), a cerium oxide (e.g., CeO.sub.2), and/or a
yttrium oxide (e.g., Y.sub.2O.sub.3). In some embodiments, the
rare-earth oxide is free of nitrogen. In certain embodiments, the
rare-earth oxide is not a rare-earth oxynitride.
[0229] A "rare-earth nitride" is any nitride species containing at
least one rare-earth element. In some embodiments, the rare-earth
nitride contains a single rare-earth element. In other embodiments,
the rare-earth nitride contains two or more rare-earth elements. In
some embodiments, the diffusion barrier region comprises an erbium
nitride (e.g., Er.sub.2N), a gadolinium nitride (e.g., GdN), a
cerium nitride (e.g., CeN), and/or a yttrium nitride (e.g., YN). In
some embodiments, the rare-earth nitride is free of oxygen. In
certain embodiments, the rare-earth nitride is not a rare-earth
oxynitride.
[0230] A "rare-earth oxynitride" is a compound in which both oxygen
and nitrogen are present, along with at least one rare-earth
element. A rare-earth oxynitride is considered to be both a
rare-earth oxide and a rare-earth nitride. In some embodiments, the
rare-earth oxynitride contains a single rare-earth element. In
other embodiments, the rare-earth oxynitride contains two or more
rare-earth elements. In some embodiments, the diffusion barrier
region comprises an erbium oxynitride, a gadolinium oxynitride, a
cerium oxynitride, and/or a yttrium oxynitride.
[0231] In certain embodiments, the amount(s) of the rare-earth
element(s) of the rare-earth oxide and/or rare-earth nitride may be
graded across the thickness of the rare-earth oxide and/or rare
earth nitride in various grading schemes (e.g., compositionally
graded, step-wise graded, continuously or discontinuously graded).
According to some embodiments, the amount(s) of oxygen and/or
nitrogen may be graded across the thickness of the rare-earth oxide
and/or rare-earth nitride in various grading schemes (e.g.,
compositionally graded, step-wise graded, continuously or
discontinuously graded). In some embodiments, both the amount(s) of
rare-earth element(s) as well as the amount(s) of oxygen and/or
nitrogen may be graded in various grading schemes (e.g.,
compositionally graded, step-wise graded, continuously or
discontinuously graded).
[0232] According to certain embodiments, the diffusion barrier
region containing the rare-earth oxide and/or rare-earth nitride
can be made up of a relatively high percentage of rare-earth oxide
and/or rare-earth nitride material. For example, in some
embodiments, at least about 80 weight percent (wt %), at least
about 90 wt %, at least about 95 wt %, at least about 98 wt %, at
least about 99 wt %, or at least about 99.9 wt % of the diffusion
barrier region is made up of rare-earth oxide and/or rare-earth
nitride material. In some embodiments, at least about 80 wt %, at
least about 90 wt %, at least about 95 wt %, at least about 98 wt
%, at least about 99 wt %, or at least about 99.9 wt % of the
diffusion barrier region is made up of rare-earth oxide material.
In some embodiments, at least about 80 wt %, at least about 90 wt
%, at least about 95 wt %, at least about 98 wt %, at least about
99 wt %, or at least about 99.9 wt % of the diffusion barrier
region is made up of rare-earth nitride material. In some
embodiments, at least about 80 wt %, at least about 90 wt %, at
least about 95 wt %, at least about 98 wt %, at least about 99 wt
%, or at least about 99.9 wt % of the diffusion barrier region is
made up of rare-earth oxynitride material.
[0233] In some embodiments, diffusion barrier region 140 may
comprise multiple layers of rare-earth oxide and/or rare-earth
nitride material. For example, diffusion barrier region 140 may
comprise at least one rare-earth oxide containing layer and at
least one rare-earth nitride containing layer. In some embodiments,
diffusion barrier region 140 may comprise at least two rare-earth
oxide containing layers and/or at least two rare-earth nitride
containing layers.
[0234] In some embodiments in which the diffusion barrier region
comprises a rare-earth oxide and/or rare-earth nitride material,
the diffusion barrier region comprising the rare-earth oxide and/or
rare-earth nitride can be relatively thin. For example, in some
embodiments, the diffusion barrier region comprising the rare-earth
oxide and/or rare-earth nitride (either in the form of a single
layer or in the form of multiple layers) can have a thickness of
less than about 500 nm, less than about 200 nm, less than about 100
nm, less than about 50 nm, less than about 20 nm, or less than
about 10 nm (and/or, in some embodiments, as thin as 5 nm, as thin
as 1 nm, or thinner).
[0235] According to certain embodiments, the diffusion barrier
region comprises a silicon carbide region. For example, referring
to FIG. 1B, diffusion barrier region 140 can comprise, according to
some embodiments, a layer comprising silicon carbide.
[0236] Some embodiments are related to methods of forming
semiconductor structures comprising layers comprising silicon
carbide. For example, in some embodiments, a silicon carbide region
is formed over a substrate. In some such embodiments, a III-nitride
material region is formed over the silicon carbide region and the
substrate.
[0237] While diffusion barrier regions comprising silicon carbide
regions are illustrated and described in association with FIG. 1B,
it should be understood that such diffusion barrier regions could
be used in any of diffusion barrier regions 140 shown in the
figures.
[0238] In some embodiments, the silicon carbide region comprises an
epitaxial layer of 3C--SiC (e.g., formed on the surface of the
substrate). In certain embodiments, a silicon carbide region is
formed via reaction and/or consumption of a silicon surface of a
silicon substrate with a carbon species. For example, in some
embodiments, a silicon carbide region is formed by contacting
carbon with a near surface layer of a silicon substrate, and
carbonizing or carburizing the near surface layer of a silicon
substrate. In some embodiments, a silicon carbide region may be
formed of two or more layers of 3C--SiC formed using different
growth techniques. For example, in one embodiments, the silicon
carbide region comprises a thin carbonized layer formed on the
silicon surface, followed by a thicker epitaxially grown 3C--SiC
layer above the carbonized layer. In some embodiments, the two or
more layers of silicon carbide may include other types of silicon
carbide, not limited to 3C--SiC polytypes. For example, the silicon
carbide region can comprise, in some embodiments, a layer of
4H--SiC, 6H--SiC, or another silicon carbide polytype.
[0239] In certain embodiments, the silicon carbide containing
diffusion barrier region can be relatively thick. For example, in
some embodiments, the silicon carbide region can be greater than
about 0.1 microns thick, greater than about 1 micron thick, or
greater and about 2 microns thick (and/or, in some embodiments, up
to 3 microns thick, or thicker). Such large thicknesses can be
used, according to certain embodiments, due to the relatively high
thermal conductivity of silicon carbide.
[0240] In certain embodiments, the diffusion barrier region may
comprise at least one doped 3C--SiC layer. For example, epitaxial
3C--SiC as deposited may have a relatively low resistivity (for
example, about 100 ohm-cm or less). In certain embodiments, it may
be desired to use a highly resistivity 3C--SiC diffusion barrier.
Certain embodiments comprise doping the 3C--SiC (e.g., with
vanadium dopants). Doping the 3C--SiC may increase the resistivity
of the 3C--SiC layer, for example, to greater than about 100
ohm-cm, greater than about 1000 ohm-cm, or greater than about
10,000 ohm-cm. In certain embodiments, it may be desired to us a
conductive substrate. In some such cases, N- or P-type dopants may
be added to the 3C--SiC layer(s) to increase electronic
conductivity and/or to create a P-N junction within the 3C--SiC
diffusion barrier.
[0241] According to certain embodiments, the diffusion barrier
region comprises an elemental diboride diffusion barrier layer. For
example, referring to FIG. 1B, diffusion barrier region 140 can
comprise, according to some embodiments, a layer comprising an
elemental diboride. "Elemental diboride," as used herein, refers to
compounds having the formula (X).sub.1B.sub.2, wherein B is boron
and X is any other element or combination of elements. The formula
(X).sub.1B.sub.2 is used to indicate that the stoichiometric ratio
of the total number of atoms of X elements in the compound (whether
a single X element is present, or more than one X element is
present) to the number of boron atoms in the compound is 1:2. In
some embodiments, X can be a metal or combination of metals. In
some embodiments, X can comprise a transition metal (or a
combination of transition metals). In some embodiments, X can be
selected from the group consisting of aluminum (Al), zirconium
(Zr), Hafnium (Hf), and combinations and/or alloys of these.
Examples of such compounds include ZrB.sub.2, HfB.sub.2, AlB.sub.2,
Hf.sub.(x)Zr.sub.(1-x)B.sub.2, Hf.sub.(x)Al.sub.(1-x)B.sub.2,
Al.sub.(x)Zr.sub.(1-x)B.sub.2, and
Al.sub.(x)Hf.sub.(y)Zr.sub.(1-x-y)B.sub.2. In some embodiments, the
elemental diboride comprises zirconium diboride (ZrB.sub.2).
[0242] The elemental diboride layer can be, in some embodiments, a
single-crystal elemental diboride layer. In some embodiments, the
elemental diboride layer can be an epitaxial layer. In some
embodiments, impurity atoms may be present within the elemental
diboride layer (e.g., in interstitial spaces within the elemental
diboride lattice).
[0243] Some embodiments are related to methods of forming
semiconductor structures comprising diffusion barrier regions
comprising an elemental diboride material layer. For example, in
some embodiments, a diffusion barrier region comprising an
elemental diboride material layer (e.g., a zirconium diboride
(ZrB.sub.2) material layer or any of the other elemental diboride
materials described herein) is formed over a substrate. In some
such embodiments, a III-nitride material region is formed over the
diffusion barrier region comprising the elemental diboride.
[0244] While barrier regions comprising an elemental diboride are
illustrated and described in association with FIG. 1B, it should be
understood that such barrier regions could be used in any of
barrier regions 140 shown in the figures.
[0245] In some embodiments in which the diffusion barrier region
comprises an elemental diboride material, the diffusion barrier
region comprising the elemental diboride can be relatively thin.
For example, in some embodiments, the diffusion barrier region
comprising the elemental diboride can have a thickness of less than
about 500 nm, less than about 200 nm, less than about 100 nm, less
than about 50 nm, or less than about 25 nm (and/or, in some
embodiments, as thin as 15 nm, as thin as 10 nm, or thinner).
[0246] In certain embodiments in which the diffusion barrier region
140 comprises an elemental diboride, it may be (although is not
necessarily) advantageous to use a miscut silicon substrate. Using
the miscut silicon substrate can, according to certain embodiments,
aid in the quality of the epitaxial elemental diboride layer
according to some such embodiments.
[0247] According to certain embodiments, two or more of the
parasitic channel mitigation/prevention strategies described herein
can be used in combination. For example, in some embodiments,
multiple diffusion barrier layers (e.g., two or more of AlN,
rare-earth oxide and/or rare-earth nitride, silicon carbide, and
elemental diboride) can be employed in combination, with one or
more types of diffusion barrier materials within each diffusion
barrier layer. In some embodiments, one or more diffusion barriers
may be employed in combination with the implantation of a species
capable of reacting with an external species, implantation of a
species having an atomic mass of less than 5, and/or counter-doping
of one or more species. In addition, the implantation of a species
capable of reacting with an external species can be used in
combination with the implantation of a species having an atomic
mass of less than 5, in combination with counter-doping of one or
more species, and/or in combination with the use of one or more
diffusion barriers. According to certain embodiments, the
implantation of a species having an atomic mass of less than 5 can
be used in combination with the implantation of a species capable
of reacting with an external species, the counter-doping of one or
more species, and/or in combination with the use of one or more
diffusion barriers. In some embodiments, counter-doping of one or
more species can be used in combination with the implantation of a
species having an atomic mass of less than 5, the implantation of a
species capable of reacting with an external species, and/or in
combination with the use of one or more diffusion barriers.
Patterned implantation, at one or multiple depths, may also be used
in combination with any of these strategies.
[0248] Various of the devices described herein may be made using
conventional semiconductor processing techniques. Such processing
techniques can involve, for example, growing layers on the
substrate in a process chamber under vacuum conditions.
[0249] Some methods may include cleaning the substrate surface
prior to growing overlying layers and, typically, before
introduction into the process chamber. The substrate surface may be
cleaned to remove residual dopant species that may diffuse into the
substrate during processing. For example, the substrate may be
cleaned by wet chemical cleaning agents such as buffered oxide etch
(BOE), hydro-fluoric acid (HF), RCA clean (which is a commercial,
proprietary silicon surface cleaning agent), etc. Substrates may
also be cleaned by a combination of such agents. The surface of the
substrate may be cleaned with organic solvents such as acetone,
methanol, trichloroethylene, isopropyl alcohol, etc., for example,
to rid a surface of organic contamination.
[0250] In some embodiments, methods may include controlling the
residual (e.g., residual reaction by-products) amounts of dopant in
the process chamber. For example, the amount of residual dopant may
be reduced by purging the chamber with a gas (e.g., NH.sub.3) while
heating to an elevated temperature, prior to introducing the
substrate into the chamber. Purging has been found to minimize
accumulation of reaction-by-products on reaction chamber walls and
components.
[0251] In certain embodiments in which a diffusion barrier layer is
present, the diffusion barrier layer may be formed in-situ with
overlying layers (e.g., the III-nitride material region) of the
structure. That is, the diffusion barrier layer may be formed
during the same deposition step as the III-nitride material region
(e.g., including the optional III-nitride material nucleation
layer, the optional III-nitride material transition layer, the
optional III-nitride material buffer layer, and/or the III-nitride
material device region).
[0252] The III-nitride material region may be formed using known
growth techniques.
[0253] In some embodiments, the optional III-nitride nucleation
layer, the optional III-nitride transition layer, the optional
III-nitride buffer layer, and/or the III-nitride device region are
grown using a metalorganic chemical vapor deposition (MOCVD)
process. It should be understood that other suitable techniques
known in the art may also be utilized to deposit these layers
including molecular beam epitaxy (MBE), hydride vapor phase epitaxy
(HVPE), and the like. In certain embodiments, more than one growth
technique may be used to grow different III-nitride material
layers. For example, in one set of embodiments, MBE could be used
to grow the nucleation layer, and the remaining III-nitride
material layers may be formed using MOCVD. Other combinations are
also possible.
[0254] Generally, the MOCVD process involves introducing different
reactive source gases (e.g., Al source gases, Ga source gases, N
source gases) into the process chamber and providing conditions
which promote a reaction between the gases to form a layer. The
reaction proceeds until a layer of desired thickness is achieved.
The composition of the layer may be controlled, as described
further below, by several factors including gas composition, gas
concentration, and the reaction conditions (e.g., temperature and
pressure).
[0255] Examples of suitable source gases for MOCVD growth of the
optional III-nitride material nucleation layer, the optional
III-nitride material transition layer, the optional III-nitride
material buffer layer, and/or the III-nitride material device
region include trimethylaluminum (TMA) or triethylaluminum (TEA) as
sources of aluminum; trimethylindium (TMI) or triethylindium (TEI)
as sources of indium; trimethylgallium (TMG) or trimethylgallium
(TEG) as sources of gallium; and ammonia (NH.sub.3) as a source of
nitrogen. The particular source gas used depends upon the desired
composition of the layers. For example, an aluminum source (e.g.,
TMA or TEA), a gallium source (TMG or TEG), and a nitrogen source
are used to deposit films having an Al.sub.xGa.sub.1-xN
composition.
[0256] The flow rates of the source gases, the ratios of the source
gases, and the absolute concentrations of the source gases may be
controlled to provide layers (e.g., transition layers and gallium
nitride material regions) having a desired composition. For the
growth of Al.sub.xGa.sub.1-xN layers, typical TMA flow rates are
between about 5 .mu.mol/min and about 50 .mu.mol/min with a flow
rate of about 20 .mu.mol/min being preferred in some cases; typical
TMG flow rates are between about 5 .mu.mol/min and 250 .mu.mol/min,
with a flow rate of 115 .mu.mol/min being preferred in some cases;
and the flow rate of ammonia is typically between about 3 slpm to
about 10 slpm. According to certain embodiments, relatively high
flow rates (and also higher gas velocities) can be used, which have
been found to be particularly effective in minimizing accumulation
of dopants.
[0257] According to certain embodiments, the reaction temperatures
are generally between about 900.degree. C. and about 1200.degree.
C. In some embodiments, the process pressures are between about 1
Torr and about 760 Torr. It is to be understood that the process
conditions, and in particular the flow rate, are highly dependent
on the process system configuration. Typically, smaller throughput
systems require less flow than larger throughput systems.
[0258] When forming a compositionally-graded layer (e.g., a
compositionally graded transition layer, which might be formed, for
example, within transition layer 170), process parameters may be
suitably adjusted to control the compositional grading. The
composition may be graded by changing the process conditions to
favor the growth of particular compositions. For example, to
increase incorporation of gallium in the transition layer thereby
increasing the gallium concentration, the flow rate and/or the
concentration of the gallium source (e.g., TMG or TEG) may be
increased. Similarly, to increase incorporation of aluminum into
the transition layer thereby increasing the aluminum concentration,
the flow rate and/or the concentration of the aluminum source
(e.g., TMA or TEA) may be increased. The manner in which the flow
rate and/or the concentration of the source is increased (or
decreased) can control the manner in which the composition is
graded. In other embodiments, the temperature and/or pressure is
adjusted to favor the growth of a particular compound. Growth
temperatures and pressures favoring the incorporation of gallium
into the transition layer differ from the growth temperatures and
pressures favoring the incorporation of aluminum into the
transition layer. Thus, the composition may be graded by suitably
adjusting temperature and pressure.
[0259] When depositing a layer having a constant composition (e.g.,
a transition layer, a gallium nitride material layer, etc.),
however, the process parameters can be maintained constant so as to
provide a layer having a constant composition. When III-nitride
material regions (e.g., gallium nitride material regions) include
more than one material layer (e.g., more than one gallium nitride
material layer) having different respective compositions, the
process parameters may be changed at the appropriate time to change
the composition of the layer being formed.
[0260] It should be understood that all of the layers/regions on
the substrate (e.g., the optional III-nitride material nucleation
layer, the optional III-nitride material transition layer, the
optional III-nitride material buffer layer, and/or the III-nitride
material device region) may be grown in the same process, or
respective layers/regions may be grown separately.
[0261] The processes described herein have been described as
involving growing the layers/regions (e.g., the optional
III-nitride material nucleation layer, the optional III-nitride
material transition layer, the optional III-nitride material buffer
layer, and/or the III-nitride material device region) in vertical
growth processes. That is, these layers/regions have been described
as being grown in a vertical direction with respect to underlying
layers/regions (including the substrate). However, in other
embodiments of the invention (not shown), it is possible to grow at
least a portion of the layer(s) of the III-nitride material region
(e.g., gallium nitride material layer(s)) using a lateral epitaxial
overgrowth (LEO) technique, for example, as described in U.S. Pat.
No. 6,051,849; or a pendeoepitaxial technique that involves growing
sidewalls of gallium nitride material posts into trenches until
growth from adjacent sidewalls coalesces to form a gallium nitride
material region, for example, as described in U.S. Pat. No.
6,265,289. U.S. Pat. No. 7,071,498 entitled "Gallium Nitride
Material Devices Including an Electrode-Defining Layer and Methods
of Forming the Same," filed Dec. 17, 2003, and issued Jul. 4, 2006,
which is incorporated herein by reference above, further describes
techniques used to grow other layers and features shown in the
various embodiments described herein.
[0262] It should also be understood that other processes may be
used to form structures and devices of the present invention as
known to those of ordinary skill in the art.
[0263] Certain of the layers and/or regions are referred to as
being "formed on," "formed over," "formed directly on," "formed
directly over," and/or "covering" another layer or region (e.g.,
the substrate). It should be understood that such phrases include
situations in which a top surface of an underlying region or layer
(e.g., substrate) is converted to the layer or region that is being
formed. Such phrases also refer to situations in which new layers
are formed by depositing the new, separate layer on the top surface
of the underlying layer and/or region (e.g., a substrate).
[0264] As noted above, the term "region" may refer to one layer or
may refer to multiple layers. It should also be understood that,
wherever a single layer is described, the single layer may be
replaced, according to certain embodiments, with multiple layers.
For example, in certain instances, single layers described herein
can be replaced with multiple layers that perform a similar
function.
[0265] The following examples are intended to illustrate certain
embodiments of the present invention, but do not exemplify the full
scope of the invention.
Example 1
[0266] This example describes ion implantation of a species having
a relative atomic mass of less than 5 into a GaN-on-Si HEMT
epitaxial structure to reduce the impact of an existing parasitic
channel. In this example, protons (H.sup.+) were used as the
implanted species. FIG. 6 is a plot of carrier concentration (in
cm.sup.-3) as a function of depth into the substrate (in microns),
as determined by SRP.
[0267] Two similar GaN-on-Si HEMT epitaxial wafers were used in
this demonstration. The parasitic channel level of the unimplanted
wafer is shown as the "control" in FIG. 6. For the "control," the
free carrier concentration was measured in the middle of the wafer,
to characterize parasitic channel.
[0268] The second wafer (shown as the "implanted wafer" in FIG. 6)
received a proton implant with an energy of 400 keV and a proton
implant dose of 10.sup.14 cm.sup.2. SRP was performed at three
locations across this wafer (at the crown, at the center, and at
the flat regions) to assess parasitic channel level after proton
implantation. The effect of the proton implant on the parasitic
channel level was profound. Compared to the control wafer, the
proton implant reduced the peak free carrier concentration by three
orders of magnitude. The integrated background carrier charge was
decreased by more than two orders of magnitude. The proton implant
also completely changed the carrier type from p-type to n-type.
[0269] Parasitic channel reduction by proton implant was confirmed
on more than one pair of control and implanted wafers.
Example 2
[0270] This example describes a set of experiments in which oxygen
was implanted into a silicon substrate to reduce the free carrier
concentration in the substrate after GaN formation. FIG. 7 is a
plot of carrier concentration (in cm.sup.-3) as a function of depth
into the substrate (in microns), as determined by SRP.
[0271] Two silicon wafers comprising similar GaN-on-Si HEMT
epitaxial structures were used. One silicon wafer was implanted
with oxygen (O.sup.+) prior to growth of an epitaxial GaN layer
over the substrate. The oxygen implantation was performed using a
40 keV accelerating voltage and an oxygen implant dose of
5.times.10.sup.15 cm.sup.-2. The control substrate received no
oxygen implant prior to GaN growth. Epitaxial GaN films were
deposited on each substrate using the same epitaxial growth
conditions.
[0272] As shown in FIG. 7, the wafer into which oxygen was
implanted prior to GaN growth (shown as "O+ implanted" in the
figure) had a parasitic channel substantially reduced compared to
the wafer on which GaN was grown without oxygen implantation (shown
as "control" in the figure). In particular, for the oxygen doped
substrate, the peak carrier concentration was almost two orders of
magnitude smaller and the total integrated parasitic channel charge
was almost three orders of magnitude smaller, relative to the
non-doped substrate.
[0273] While several embodiments of the present invention have been
described and illustrated herein, those of ordinary skill in the
art will readily envision a variety of other means and/or
structures for performing the functions and/or obtaining the
results and/or one or more of the advantages described herein, and
each of such variations and/or modifications is deemed to be within
the scope of the present invention. More generally, those skilled
in the art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the teachings of the present invention
is/are used. Those skilled in the art will recognize, or be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. It is, therefore, to be understood that the foregoing
embodiments are presented by way of example only and that, within
the scope of the appended claims and equivalents thereto, the
invention may be practiced otherwise than as specifically described
and claimed. The present invention is directed to each individual
feature, system, article, material, and/or method described herein.
In addition, any combination of two or more such features, systems,
articles, materials, and/or methods, if such features, systems,
articles, materials, and/or methods are not mutually inconsistent,
is included within the scope of the present invention.
[0274] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0275] The phrase "and/or," as used herein in the specification and
in the claims, should be understood to mean "either or both" of the
elements so conjoined, i.e., elements that are conjunctively
present in some cases and disjunctively present in other cases.
Other elements may optionally be present other than the elements
specifically identified by the "and/or" clause, whether related or
unrelated to those elements specifically identified unless clearly
indicated to the contrary. Thus, as a non-limiting example, a
reference to "A and/or B," when used in conjunction with open-ended
language such as "comprising" can refer, in one embodiment, to A
without B (optionally including elements other than B); in another
embodiment, to B without A (optionally including elements other
than A); in yet another embodiment, to both A and B (optionally
including other elements); etc.
[0276] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list, "or"
or "and/or" shall be interpreted as being inclusive, i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of," will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e. "one or the other but not both") when preceded
by terms of exclusivity, such as "either," "one of," "only one of,"
or "exactly one of." "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0277] As used herein in the specification and in the claims, the
phrase "at least one," in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example, "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A, and at least one, optionally including more than one, B
(and optionally including other elements); etc.
[0278] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," and the like are to
be understood to be open-ended, i.e., to mean including but not
limited to. Only the transitional phrases "consisting of" and
"consisting essentially of" shall be closed or semi-closed
transitional phrases, respectively, as set forth in the United
States Patent Office Manual of Patent Examining Procedures, Section
2111.03.
* * * * *