U.S. patent application number 13/308574 was filed with the patent office on 2012-03-29 for method and apparatus for fabricating crack-free group iii nitride semiconductor materials.
This patent application is currently assigned to FREIBERGER COMPOUND MATERIALS GMBH. Invention is credited to Vladimir A. Dmitriev, Yuri V. Melnik.
Application Number | 20120076968 13/308574 |
Document ID | / |
Family ID | 34705377 |
Filed Date | 2012-03-29 |
United States Patent
Application |
20120076968 |
Kind Code |
A1 |
Dmitriev; Vladimir A. ; et
al. |
March 29, 2012 |
METHOD AND APPARATUS FOR FABRICATING CRACK-FREE GROUP III NITRIDE
SEMICONDUCTOR MATERIALS
Abstract
Method for producing a III-N (AlN, GaN, Al.sub.xGa.sub.(1-x)N)
crystal by Vapor Phase Epitaxy (VPE), the method comprising:
providing a reactor having: a growth zone for growing a III-N
crystal; a substrate holder located in the growth zone that
supports at least one substrate on which to grow the III-N crystal;
a gas supply system that delivers growth material for growing the
III-N crystal to the growth zone from an outlet of the gas supply
system; and a heating element that controls temperature in the
reactor; determining three growth sub-zones in the growth zone for
which a crystal grown in the growth sub-zones has respectively a
concave, flat or convex curvature; growing the III-N crystal on a
substrate in a growth region for which the crystal has a by desired
curvature.
Inventors: |
Dmitriev; Vladimir A.;
(Gaithersburg, MD) ; Melnik; Yuri V.; (Rockville,
MD) |
Assignee: |
FREIBERGER COMPOUND MATERIALS
GMBH
Freiberg/ Sachsen
DE
|
Family ID: |
34705377 |
Appl. No.: |
13/308574 |
Filed: |
December 1, 2011 |
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Current U.S.
Class: |
428/64.1 |
Current CPC
Class: |
Y10S 117/915 20130101;
C30B 25/02 20130101; C30B 25/00 20130101; C30B 29/403 20130101;
Y10T 428/21 20150115 |
Class at
Publication: |
428/64.1 |
International
Class: |
C01B 21/072 20060101
C01B021/072; C01B 21/06 20060101 C01B021/06; B32B 3/02 20060101
B32B003/02 |
Claims
1. A group III-N (AlN, GaN, Al.sub.xGa.sub.(1-x)N) crystal having:
thickness at least 100 .mu.m; diameter equal to at least 2 inches;
and stress that is less than 0.1 gigapascal (GPa).
2. A crystal according to claim 1 wherein the crystal is
crack-free.
3. A crystal according to claim 1 wherein the desired curvature is
flat.
4. A crystal according to claim 1 having a bow below 50
microns.
5. A crystal according to claim 1 and having a diameter greater
than or equal to about 4 inches.
6. A crystal according to claim 1 and having a diameter greater
than or equal to about 6 inches.
7. A crystal according to claim 1 having thickness equal to or
greater than about 1 mm.
8. A crystal according to claim 1 having thickness equal to or
greater than about 10 mm.
9. A crystal according to claim 1 having thickness equal to or
greater than about 50 mm.
10. A crystal according to claim 1 that appears crack-free under
transmission and reflection optical microscopy at magnifications up
to .times.1000. (12/90)
11. An AlN crystal according to claim 1 having a thermal
conductivity greater than 3.2 W/K-cm.
12. An AlN crystal according to claim 1 having an optical
absorption less than 5% at wavelengths between 200 nm and 6
.mu.m.
13. An AlN crystal according to claim 1 having an the x-ray rocking
curve measured in .omega.-scanning geometry characterized by a full
width half maximum (FWHM) that is less than 300 arc seconds.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of allowed U.S. application
Ser. No. 13/011,879 filed Jan. 22, 2011, which is a continuation of
12/235,370, filed Sep. 22, 2008, which is a continuation of U.S.
application Ser. No. 11/483,455, filed Jul. 10, 2006, now
abandoned, which is a divisional of and claims the benefit of U.S.
application Ser. No. 10/778,633, filed Feb. 13, 2004, now U.S. Pat.
No. 7,501,023;
[0002] U.S. application Ser. No. 10/778,633 claiming the benefit of
the filing date of U.S. Provisional Application No. 60/449,085,
filed Feb. 21, 2003;
[0003] U.S. application Ser. No. 10/778,633 claiming the benefit of
and being a continuation-in-part application of U.S. application
Ser. No. 10/355,426, filed Jan. 31, 2003, now U.S. Pat. No.
6,936,357, which is a continuation-in-part of U.S. application Ser.
No. 09/900,833, filed Jul. 6, 2001, now U.S. Pat. No.
6,613,143;
[0004] U.S. application Ser. No. 10/778,633 claiming the benefit of
and being a continuation-in-part of U.S. application Ser. No.
09/903,047, filed Jul. 11, 2001, which is a continuation of U.S.
application Ser. No. 09/900,833, filed Jul. 6, 2001, now U.S. Pat.
No. 6,613,143;
[0005] U.S. application Ser. No. 10/778,633 claiming the benefit of
and being a continuation-in-part of U.S. application Ser. No.
10/632,736, filed Aug. 1, 2003, now U.S. Pat. No. 7,279,047, which
is a continuation of U.S. application Ser. No. 09/903,299, filed
Jul. 11, 2001, now U.S. Pat. No. 6,656,285, which is a continuation
of U.S. application Ser. No. 09/900,833, filed Jul. 6, 2001, now
U.S. Pat. No. 6,613,143;
[0006] the priority of all of which are claimed under 35 U.S.C.
.sctn..sctn.119 and 120, and the disclosures of all of which are
incorporated herein by reference for any and all purposes.
FIELD OF THE INVENTION
[0007] The present invention relates generally to semiconductor
materials and, more particularly, to a method and apparatus for
growing Group III nitride semiconductor materials with improved
characteristics.
BACKGROUND
[0008] Group III nitride materials (e.g., GaN, AlN, InN, BN, and
their alloys) are perspective semiconductor materials for the next
generation of high power, 5 high frequency, high temperature
electronic devices, including short wavelength opto-electronic
devices. Unfortunately, these materials suffer from a variety of
problems that limit their performance as well as their commercial
viability.
[0009] One of the principal problems associated with Group III
nitride materials is their tendency to crack, a problem that has
been described in numerous scientific papers. During the growth of
the Group III nitride, as soon as its thickness reaches a certain
value, typically on the order of a few microns or less, cracks are
formed in the growing layer. Occasionally cracks even form in the
substrate on which the layer is being grown. As a result, devices
that would otherwise benefit from the use of thick Group III
nitride layers are prohibited.
[0010] Accordingly, a means of fabricating thick Group III nitride
layers and wafers is desired. The present invention provides such a
means.
SUMMARY
[0011] The present invention provides a method and apparatus for
growing low defect, optically transparent, colorless, crack-free
single crystal Group III nitride epitaxial layers with a thickness
exceeding 10 microns. These layers can be grown on large area
substrates. Suitable substrate materials include silicon (Si),
silicon carbide (SiC), sapphire, gallium nitride (GaN), aluminum
nitride (AlN), gallium arsenide (GaAs), aluminum gallium nitride
(AlGaN) and others.
[0012] In one aspect of the invention, monocrystalline, crack-free
Group III nitride layers are grown using gas transport techniques
based on the hydride vapor phase epitaxial (HVPE) approach. During
growth, the shape and the stress of the nitride epitaxial layers
can be controlled, thus allowing concave, convex 30 and flat layers
to be controllably grown. The crack-free Group III nitride layer
can be grown to a thickness of at least 1 micron and, depending
upon the desired application, to a thickness of greater than 5
microns, 10 microns, 15 microns, 20 microns, 30 microns, 50
microns, 1 mm or more. The Group III nitride layer can be grown on
any of a variety of substrates, including substrates of Si, SiC,
sapphire, quartz, GaN, GaAs, AlN and AlGaN, with substrate sizes
ranging from 2 inches to 6 inches or more. Assuming that the grown
Group III nitride layer is formed of AlN, the material is
electrically insulating with an electrical resistivity at 300 K of
at least 10.sup.6 Ohm-cm. Defect density in the as-grown layer is
less than 10.sup.8 cm.sup.-2, and can be held to levels of less
than 10.sup.6 cm.sup.-2 or even less than 10.sup.4 cm.sup.-2.
Thickness uniformity of the as-grown layer is better than 10
percent, typically on the order of between 1 and 5 percent. Thermal
conductivity of the as-grown AlN layer is 3 W/K-cm or greater. The
surface of the grown layer can be polished to a surface roughness
rms of less than 0.5 nm, and if desired to a surface roughness rms
of less than 0.3 nm or less than 0.1 nm.
[0013] In another aspect of the invention, a method and apparatus
for producing free-standing, monocrystalline, crack-free, low
defect Group III nitride wafers is provided. Preferably the Group
III nitride wafers are comprised of AlN and are grown on SiC
substrates. After the growth of the AlN is completed, the substrate
is removed. The thickness of the AlN wafer can exceed 5 mm with
diameters larger than 2, 3, 4 or even 6 inches being achievable. As
such, the volume of the AlN wafer can exceed 10 cm.sup.3, more
preferably 100 cm.sup.3, and still more preferably 200 cm.sup.3.
The defect density of the electrically insulating wafers is less
than 10.sup.8 cm.sup.-2, and preferably less than 10.sup.6
cm.sup.-2. Once initial fabrication of the wafer is complete, the
wafer can be sliced into thinner AlN wafers. The resultant AlN
wafers can be polished and prepared to provide epi ready surfaces
of varying orientation, including (0001) Al face and (000-1)N
face.
[0014] In another aspect of the invention, a semiconductor device
comprising at least one thick, monocrystalline, crack-free AlN
layer is provided. The thickness of the AlN layer is typically in
the range of between 1 micron and 50 microns, although thicker
layers can be used. The semiconductor device can be an electronic
device or an opto-electronic device. The semiconductor device can
contain one or more heterojunctions or homojunctions, for example 3
comprised of AlGaN/AlGaN. The device can also include doped and/or
undoped nitride epitaxial layers. Preferably the substrate is of
SiC or AlN, although other substrates can also be used.
[0015] A further understanding of the nature and advantages of the
present invention can be realized by reference to the remaining
portions of the specification and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic illustration of a horizontal furnace
suitable for use with the invention;
[0017] FIG. 2 illustrates the three growth sub-zones located in the
reactor shown in FIG. 1;
[0018] FIG. 3 illustrates a tilted substrate pedestal located in
the reactor shown in FIG. 1;
[0019] FIG. 4 illustrates a HEMT device fabricated in accordance
with the invention;
[0020] FIG. 5 illustrates a first embodiment of a light emitting
diode fabricated in accordance with the invention;
[0021] FIG. 6 illustrates a second embodiment of a light emitting
diode fabricated in accordance with the invention; and
[0022] FIG. 7 illustrates a third embodiment of a light emitting
diode fabricated in accordance with the invention.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
Methodology
[0023] Gas Phase Growth
[0024] In order to grow crack-free Group III nitride materials from
the gas phase, preferably a modified hydride vapor phase epitaxial
(HVPE) approach is used with a horizontal reactor tube as
illustrated in FIG. 1. Although a horizontal reactor 101 is
preferred as it easily accommodates the required sources, it is
understood that the invention is not limited to a particular
furnace configuration 4 as other configurations (e.g., vertical
furnaces) that offer the required control over the temperature,
temperature zone or zones, gas flow, source and substrate
locations, source configurations, etc., can also be used.
[0025] The furnace is comprised of multiple temperature zones,
preferably obtained through the use of multiple heaters, each of
which at least partially surrounds the reactor tube and each of
which preferably has its own temperature controller. In the
preferred embodiment, a six zone configuration with resistive
heaters 103-108 is used. Although reactor tube 101 preferably has a
cylindrical cross-section, other configurations can be used such as
a `tube` with a rectangular cross-section. Within the reactor tube
are one or more source tubes 111. As noted with respect to the
reactor tube, source tube 111 preferably has a cylindrical
cross-section although the invention is not limited to cylindrical
source tubes. Furthermore, it will be appreciated that as used
herein, the terms source tube and source channel are
interchangeable and considered to be equivalent.
[0026] In order to grow undoped thick crack-free AlN, at least one
single Al source tube is required (e.g., source tube 111). It will
be appreciated that in order to grow other Group III nitride
materials, sources other than, or in combination with, Al must be
used (e.g., Ga). Within source tube is a source boat 113. As used
herein, the term "boat" simply refers to a means of holding the
source material. For example, boat 113 can be comprised of a
portion of a tube with a pair of end portions. Alternately, the
source material can be held within the source tube without the use
of a separate boat. Alternate boat configurations are clearly
envisioned by the inventors.
[0027] In at least one embodiment of the invention, the desired
growth temperature depends upon the stage of crystal growth (e.g.,
crystal nucleation versus high growth rate). Accordingly the
temperature of a source is preferably controllable, for example by
varying the heat applied by specific zone heaters.
[0028] In at least one preferred embodiment of the invention, the
location of a particular source within reactor tube 101 can be
controllably varied, typically by altering the position of the
source. For example, in source tube 111 a control rod 115 is
coupled to boat 113, control rod 115 allowing the position of boat
113 and thus the source within the boat to be varied within the
reactor. Control rod 115 can be manually manipulated, as provided
for in the illustrated configuration, or coupled to a robotic
positioning system (not shown).
[0029] Coupled to each source tube are one or more sources of gas
(e.g., gas sources 117 and 119). The rate of gas flow through a
particular source tube is controlled via valves (e.g., valves 121
and 123), either manually or by an automatic processing system.
[0030] At least one substrate 125 is located on a pedestal 127
within the growth zone of reactor. Although typically multiple
substrates are manually loaded into the reactor for co-processing,
a single substrate can be processed with the invention.
Additionally, substrates can be automatically positioned within the
furnace for automated production runs. In order to vary the
temperature of the growth zone, and thus the temperature of the
substrate or substrates, either the position of the substrates
within the reactor is changed or the amount of heat applied by
heaters proximate to the growth zone is varied.
[0031] Although reactor 100 is preferably a hot-wall, horizontal
reactor and the process is carried out in an inert gas flow at
atmospheric pressure, other reactor configurations can be used to
perform the modified HVPE process of the invention. Preferably
source tube 111 and source boat 113 are comprised of quartz. Other
materials can be used for boat 113, however, such as sapphire or
silicon carbide. Within boat 113, or simply within tube 111 if no
separate boat is used, is source 129. Assuming that the invention
is to be used to grow AlN, source 129 is comprised of aluminum
metal.
[0032] In order to achieve extended growth and thus the growth of
very thick layers, the inventors have found that multiple sources
are preferably used, the sources being maintained at more than one
temperature in order to limit the amount of source participating in
the layer forming reaction. For example, assuming that the intended
layer is to be comprised of AlN, reactor 100 includes at least two
Al sources (e.g., sources 129 and 131). During layer formation, the
temperature of the source designated to participate in the reaction
is held at a relatively high temperature, typically between
750.degree. C. and 850.degree. C. and preferably at a temperature
of approximately 800.degree. C., while the second (or additional)
sources are maintained at a lower temperature. By using multiple
sources it is possible to replace one source (e.g., a depleted
source) while continuing the growth process with a different
source.
[0033] In order to grow thick crack-free AlN according to the
preferred embodiment of the invention using a modified HVPE
approach, a source 117 of halide gas, preferably HCl, is coupled to
the source tube(s) along with a source 119 of inert gas, preferably
Ar, which is used as a carrier gas to transfer materials from the
source tubes to the growth zone. A source 133 of nitrogen
containing gas, preferably ammonia gas, is also coupled to reactor.
Substrate crystal pedestal 127 is preferably fabricated from
quartz, although other materials such as silicon carbide or
graphite can also be used.
[0034] In order to grow thick AlN, preferably substrate(s) 125 is
comprised of SiC or AlN, thus providing a lattice and coefficient
of thermal expansion match between the seed and the material to be
grown. As a result of using AlN substrates, improved quality in the
as-grown material is achieved. Alternately, substrates can be
comprised of sapphire, GaAs, GaN, or other material as previously
noted. Assuming the use of AlN substrates, the substrates can have
less than 10.sup.18 cm .sup.-3 oxygen atomic concentration, less
than 10.sup.19 cm.sup.-3 oxygen atomic concentration, or less than
10.sup.20 cm.sup.-3 oxygen atomic concentration. The FWHM of the
.omega.-scan x-ray (0002) rocking curve for the seed substrate can
range from 60 arc seconds to 10 arc degrees. Although the diameter
of the substrate depends on the size of the reactor, the inventors
have found that the invention is not limited to any specific
substrate size (i.e., diameters of 1 inch, 2 inches, 3 inches, 4
inches, 5 inches, 6 inches and greater can be used). Similarly the
inventors have found that the invention can use substrates of
thickness 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm or greater.
[0035] Prior to layer growth, the substrate can be polished and/or
etched by reactive ion etching (RIE) or wet etching. After
introduction into the growth zone, HCl, aluminum chloride, or a gas
mixture containing HCl and aluminum chloride can be used to etch
the substrate. The surface of the substrate can have a (000-1)N or
a (0001) Al polarity. The surface can be mis-oriented from the
(0001) crystallographic plane at an angle ranging from 0 to 90
degrees. Additionally the seed substrate can contain cracks with a
density from 0 to 10,000 per micron while still resulting in
crack-free layer growth. The substrate(s) can be mounted face up or
face down within the reactor. Alternately, substrates can be
simultaneously fixed to the substrate holder in both the face up
and face down configurations, such configuration increasing the
number of wafers that can be grown in a single run.
[0036] Initially reactor 100 is flushed and filled with an inert
gas, preferably Ar, from gas source 119. The inert gas can enter
the reactor through the source tube(s), thereby flushing the source
tube(s), through a separate entry line (not shown), or both. The
flow of inert gas is controlled by a metering valve and is
typically in the range of 1 to 25 liters per minute. Substrate(s)
125 is then heated to the desired growth temperature. In one
embodiment of the invention the growth zone, and thus the
substrates within the growth zone, are heated to a temperature
within the range of 600.degree. C. to 1500.degree. C., more
preferably within the range of 850.degree. C. to 1050.degree. C.,
and still more preferably within the range of 900.degree. C. to
950.degree. C., and yet still more preferably within the range of
900.degree. C. to 920.degree. C. Although temperatures within the
most preferable range yield relatively slow growth rates, these
temperatures assure a higher quality material in the as-grown
crystal.
[0037] In a preferred embodiment of the invention and as
illustrated in FIG. 2, the growth zone is comprised of three growth
sub-zones defined, in part, by the temperature within the zones.
The growth sub-zone located closest to the source zone and having
the lowest growth temperature yields layers with a generally convex
shape (e.g., sample 201). The growth sub-zone located farthest from
the source zone and having the highest temperature yields layers
with a generally concave shape (e.g., sample 203). The growth
sub-zone located between these first two sub-zones yields
substantially flat layer growth (e.g., sample 205).
[0038] In at least one embodiment of the invention, the gas flows
introduced into the growth zone are directed at an angle to the
substrate holder surface, such geometry improving material
uniformity and reducing defect density. As illustrated in FIG. 3,
the gas flows can be horizontal with the pedestal (i.e., pedestal
301) tilted at an angle to the gas flow. Preferably the angle is
between 1 and 10 degrees. If desired pedestal 301 can be rotated
about axis 303.
[0039] Initially the substrate(s) within a particular growth
sub-zone is heated to a high temperature within the range of
900.degree. C. and 950.degree. C., thus 10 initiating high quality
crystal growth and controlled sample shape. Once crystal growth has
been initiated, the source temperature is lowered and maintained at
a temperature within the range of 850.degree. C. and 1,000.degree.
C., thus allowing rapid crystal growth to be achieved. Preferably
the period of high quality crystal growth is at least 10 minutes
and the period of rapid crystal growth is at least 12 hours. More
preferably the period of high quality crystal growth is at least 30
minutes and the period of rapid crystal growth is at least 24
hours.
[0040] During the growth process, after the source material is
heated a halide reactive gas, preferably HCl, is introduced into
the source tube at a flow rate of 0.1 to 10 liters/minute. Assuming
an Al source, AlCl.sub.3 and other gas components are formed due to
the reaction between the reactive gas and the source. The
AlCl.sub.3 is transported to the reactor's growth zone by the flow
of the inert gas (e.g., Ar), the inert gas having a flow rate of
0.1 to 40 liters/minute. Simultaneously, ammonia gas (NH.sub.3)
from source 133 is delivered to the growth zone at a flow rate of
0.1 to 10 liters/minute. The ammonia gas and the aluminum chloride
gas react to form AlN on the surface of substrate(s) 125. The
growth rate is in the range of 1 to 100 microns per hour, and
preferably in the range of 20 to 40 microns per hour. After the
desired AlN layer thickness has been achieved, the flow of HCl and
NH.sub.3 gas is stopped and the substrate(s) is cooled in the
flowing inert gas.
[0041] During crystal growth the growing layer is not allowed to
come into contact with any portion of the reactor, thus insuring
high quality crystal growth. If required, for example during a long
growth run, the growth can be interrupted in order to allow
parasitic deposits to be etched off of the reactor's internal
components. The quality of the as-grown material can be further
improved by introducing buffer or interrupting layers during
crystal growth. These layers can be single layer or multi-layer
structures and can be comprised, for example, of GaN, InGaN,
InGaAlN or other materials. The thickness of these buffer or
interrupting layers can be in the range of 50 angstroms to 100
microns. Preferably these layers are grown using the same process
used for the boule growth, for example the HVPE process.
[0042] The AlN layer can be grown in the direction parallel to the
(0001), (11-20), (10-10) and other crystallographic directions. AlN
wafers sliced from the as-grown thick AlN layer can have their
surface parallel to the (0001), (11-20), (10-10) or other
crystallographic planes. The surface can be on-oriented or
mis-oriented by an angle from 0 to 90 degrees toward a specific
crystallographic 15 direction, for example a (0001) plane
mis-oriented by 8 degrees to the (11-20) direction.
[0043] During crystal growth, AlN layer (boule) can be doped with
any of a variety of impurities including, but not limited to,
magnesium (Mg), zinc (Zn), silicon (Si), oxygen (O), tin (Sn), iron
(Fe), chromium (Cr), manganese (Mn), erbium (Er) and indium (In).
Doping allows the conductivity of the growing material to be
controlled, thereby resulting in n-type, p-type or i-type
conductivity. The atomic concentration of these impurities can be
varied in the grown material from 10.sup.15 cm.sup.-3 up to
10.sup.20 cm.sup.-3. The impurities can be introduced into the
growth zone using Ar as a carrier gas with a gas flow rate between
0.1 and 50 liters/minute. Metal source temperatures range from
200.degree. C. to 1200.degree. C. Impurity sources (for example Mg
metal) can be etched by HCl before the growth inside the HVPE
reactor. Si doping can be done by supplying gaseous silane (for
example 50 ppm silane in Ar). Doping uniformity in the (0001) plane
is better than 10 percent, preferably better than 5 percent, and
still more preferably better than 1 percent.
[0044] During crystal growth, the substrate can be moved (e.g.,
rotated) in order to maintain the desired gas composition and to
avoid the negative influence of parasitic deposition on reactor
parts.
[0045] Wafer Preparation
[0046] After a thick crystal layer is grown, for example in
accordance with the preferred embodiment previously described,
wafers can be sliced from the grown boule. Preferably the slicing
operation is performed with a diamond wire saw with a cut width of
approximately 200 microns. Depending upon the thickness of the
grown boule, 10, 20, 30 or more wafers can be manufactured from a
single boule. After slicing, the wafers are ground, polished and
etched to remove the damaged surface layer.
[0047] The wafers fabricated by the invention can then be used
directly, for example as a substrate for a device structure.
Alternately, the Group III nitride wafers sliced from the
underlying seed substrate can be polished, prepared and used for a
seed substrate for the growth of additional wafers. For example,
AlN can be initially grown as outlined above using any of a variety
of possible substrates (e.g., SiC). After completion of the growth
of the AlN thick, crack-free layer, it can be sliced from the
underlying substrate and prepared as noted above. Once preparation
is complete, the AlN freestanding wafer can be used to grow
additional AlN material using the process of the invention. In this
example the new AlN material can be grown on either the (000-1)N
face or the (0001) Al face of the AlN substrate. Once growth is
complete, multiple thin wafers can be cut from the boule of
crack-free, AlN material.
[0048] Scrubbing System
[0049] In a preferred embodiment of the invention, the growth
apparatus is equipped with an air scrubbing system to effectively
remove all hazardous components and solid particles from the HVPE
process exhaust. Such a waste utilization system allows the present
HVPE apparatus to operate for the extended periods required to
achieve the desired layer thicknesses.
[0050] The air scrubbing system consists of a wet scrubber
sequentially connected to a wet electrostatic precipitator (ESP)
where the scrubber and ESP 11 are either separate units or placed
within a single unit with the ESP above the scrubber. The air flow
capacity of the scrubbing system is within the range of 50 ACFM to
5000 ACFM. The efficiency to remove HCL and ammonia gases is not
less than 99 percent and the efficiency to remove solid particles
is not less than 99.9 percent. Typically the gas inlet
concentration before the scrubber is up to 15800 PPM for ammonia,
up to 6600 PPM for HCl, and up to 2.8 GR/ACFM for solid particles.
Up to 100 percent of the solid particles may be comprised of
ammonia chloride (NH.sub.4Cl) with a particle size in the range of
0.1 to 3.0 microns.
[0051] The wet ESP's parts having contact with the gas flow to be
scrubbed as well as the wet scrubber and sump tank are preferably
constructed of FRP or Hastelloy C-276. The scrubbing liquid is
water which is circulated in both the scrubber and the ESP. Prior
to discharge, the pH of the scrubbing liquid must be adjusted to be
within an allowed level.
[0052] Process Applicability
[0053] For AlN growth by HVPE processes, layers can be grown on
both the (0001) Al face and the (000-1)N face of an AlN
substrate.
[0054] Applicable to large area substrates (i.e., 2 inch, 3 inch, 4
inch, 6 inch and larger)
[0055] Applicable to a variety of substrates (e.g., SiC, AlN, GaAs,
sapphire, GaN, etc.)
[0056] Applicable to flat, concave, convex or patterned
substrates
[0057] Applicable to oriented or mis-oriented surfaces (preferably
with the mis-orientation angle less than or equal to 0.8
degrees.
[0058] Achievable Material Characteristics
[0059] Crack-free Group III nitride layers (e.g., AlN, AlGaN, GaN,
InN, InGaAlBN, etc.) when the epitaxial growth takes place in the
flat growth subzone, the layers grown either directly on the seed
substrate or on a buffer layer or an intermediate layer
[0060] Crack-free Group III nitride (e.g., AlN, AlGaN, GaN, etc.)
large area wafers by forming thick, crack-free layers of the
desired composition and then separating the grown layer from the
initial substrate
[0061] Layer thickness of 10 microns to 1 cm or more
[0062] Defect density in as-grown thick layers of less than
10.sup.9 cm.sup.-2 preferably less than 10.sup.8 cm.sup.-2, and
still more preferably less than 10.sup.6 cm.sup.-2. These defect
densities were achieved without applying lateral overgrowth
techniques. Defect densities were measured by calculating etch pit
density after etching the samples in hot acid. Low defect densities
were verified by measuring the x-ray diffraction rocking curves
with an x-ray diffractometer (e.g., full width at a half maximum of
the x-ray rocking curve using .omega.-scan geometry was less than
300 arc sec).
[0063] Thermal conductivity in as-grown AlN layers of up to 3.3 W/K
cm
[0064] Electrical resistivity in as-grown layers ranging from
10.sup.7 to 10.sup.15 Ohm cm (at 300 K)
[0065] Colorless
[0066] Optically transparent AlN layers in a wavelength range from
200 nm to 6 microns with an optical absorption of less than 5
percent for AlN wafers polished on both sides
[0067] Shape, stress and lattice constant of the as-grown materials
can be controlled by using the multiple growth sub-zones (i.e.,
concave, convex and flat growth zones) and transferring the
substrates from one growth sub-zone to another during the growth
process.
[0068] Fabrication of semiconductor devices on large area
crack-free single crystal Group III nitride wafers (e.g., AlN
wafers)
[0069] Fabrication of large area substrates (2 inch, 3 inch, 4
inch, 6 inch or larger) of high quality, semi-insulating and of
high thermal conductivity substrates for use in ultra high power
nitride based high frequency devices, the substrates of the
invention allowing the lattice constants and thermal expansion
coefficients to be matched to the desired device structures (e.g.,
AlGaN/GaN-based devices)
[0070] No peripheral polycrystalline regions
Embodiments
Embodiment 1
AlN Material Growth and Wafer Preparation
[0071] The growth of AlN material by the inventive process was
performed in an inert gas flow at atmospheric pressure in a
hot-wall, horizontal reactor chamber. SiC substrates were placed on
a quartz pedestal and loaded into the growth zone of the quartz
reactor. The growth was performed on the (0001) Si on axis 6H-SiC
substrate, the substrates having a surface rms roughness of
approximately 0.3 nm or better.
[0072] Approximately 1 pound of Al metal (5N) was placed in a
sapphire source boat for use in growing the AlN thick layer. For
extended runs, typically those requiring a growth cycle of more
than 48 hours, multiple Al sources/boats were used, either in
parallel or sequentially. The source boat was placed in a quartz
source tube (i.e., source channel) within the source zone of the
reactor. This source tube (or tubes when multiple Al sources were
used) supplied AlCl.sub.3 to the growth zone of the reactor.
Additional quartz tubes (i.e., channels) were used for ammonia
(NH.sub.3) delivery and HCl gas delivery to the growth zone, the
separate HCl tube being use to etch the SiC substrates.
[0073] The reactor was filled with Ar gas, the Ar gas flowing
through the reactor at a rate of between 1 and 25 liters per
minute. The substrates were then heated in the Ar flow to
temperatures in the range of 900.degree. C. to 1150.degree. C. and
the Al was heated to temperatures in the range of 700.degree. C. to
900.degree. C. HCl gas was introduced into the growth zone through
the HCl channel. As a result of the HCl gas flow, the (0001) Si
faces of the SiC substrates were etched prior to film growth. After
substrate etching, the HCl gas was introduced into the source zone,
i.e., the Al channel(s). As a result of the reaction between HCl
and Al, aluminum chloride (AlCl.sub.3) was formed and delivered to
the growth zone by the Ar flow. At the same time, ammonia gas
(NH.sub.3) was introduced into the growth zone. As a result of the
reaction between the AlCl.sub.3 and the NH.sub.3, a single crystal
epitaxial AlN layer was grown on the substrates. The substrate
temperature during the growth was held constant at a temperature
within the range 800.degree. C. to 1200.degree. C., different
temperatures being used for different epitaxial runs.
[0074] Shape controlled epitaxial growth was observed at growth
temperatures within the range of 900.degree. C. to 950.degree. C.
Depending on HCl flow rate, the growth rate of the AlN material
ranged from 0.1 to 1.2 microns per minute. Different epitaxial runs
utilized different growth cycle durations, these durations ranging
from 10 hours to 100 hours. After a particular growth cycle was
completed, all gaseous flows were stopped except for the flow of
Ar. The samples were cooled down in the Ar flow and then unloaded
from the reactor. The as-grown surface had a (0001) Al
orientation.
[0075] The SiC substrates were removed from the grown AlN layers by
grinding on a grinding wheel and/or reactive ion etching (RIE). For
the mechanical grinding process, the sample was glued to a wafer
holder by wax and ground with a liquid abrasive. After ungluing the
wafer, the traces of wax were removed in hot acetone for 20
minutes. Any residual SiC was removed by RIE and/or wet etching in
molten KOH.
[0076] The freestanding AlN wafers were then cleaned using a
conventional cleaning process and placed in the HVPE reactor. AlN
homoepitaxial growth was then performed on the as-grown AlN surface
of the AlN wafers. Once again, multiple epitaxial runs were
performed in which the growth temperature of a particular run was
held constant. The growth temperatures for the various runs were
within the range of 900.degree. C. to 1150.degree. C. The growth
durations for the various runs were between 10 hours and 100 hours
resulting in AlN plates up to 1 cm in thickness. After the sample
cool down procedure was complete, wafers ranging from 0.1 to 1 mm
in thickness were cut from the AlN plates using 0.005'' wire saw.
Both sides of the AlN wafers were ground and polished.
Embodiment 2
AlN Material Growth and Wafer Preparation
[0077] Using a modified HVPE process, a 400 micron thick AlN boule
was grown on a 2 inch SiC substrate at a growth temperature of
900.degree. C. and at a growth rate of 30 microns per hour. The AlN
boule was grown in the growth subzone yielding substantially flat
layer growth. After completion of the growth cycle, the SiC
substrate was removed by a combination of chemical etching, RIE and
mechanical polishing. The resultant AlN wafer was polished, etched
and cleaned and then re-introduced into the flat growth sub-zone of
the HVPE reactor. A 1 centimeter thick AlN boule was grown on the
(0001)N face of the prepared AlN seed wafer, the resultant boule
being crack-free.
[0078] The AlN boule was sliced into 8, 2-inch AlN wafers with
thicknesses ranging from 200 to 500 microns. X-ray diffraction
studies showed that the AlN wafers had a single crystal structure
(e.g., the FWHM of the x-ray RC was less than 300 arc sec).
[0079] The AlN wafers were subjected to chemical-mechanical
polishing, the resultant wafers exhibiting a surface roughness of
less than 0.3 nm The damaged surface sub-layer was removable by wet
and/or dry etching. A RHEED study showed that the surfaces of the
wafers were damage free. The final wafers were crack-free,
colorless and transparent and had less than 20 microns of
bowing.
Embodiment 3
AlN Device Fabrication
[0080] The growth of AlN material by the inventive process was
performed in an inert gas flow at atmospheric pressure in a
hot-wall, horizontal reactor chamber. Two inch SiC substrates were
placed on a quartz pedestal and loaded into the growth zone of the
quartz reactor, positioned for AlN deposition on the (0001)Si
on-axis surface.
[0081] Approximately 1 kilogram of Al metal was placed in the
source boat. After purging the reactor with Ar gas, the growth zone
and the Al source zone were heated to 920.degree. C. and
750.degree. C., respectively. To prepare the substrates for AlN
deposition, HCl gas was introduced into the growth zone to etch the
SiC substrates. The HCl gas was then introduced into the Al source
zone, thereby forming aluminum chloride that was transported into
the growth zone by the Ar carrier gas. Simultaneously, NH.sub.3 gas
was introduced into the growth zone, the NH.sub.3 gas providing a
source of nitrogen. As a result of the reaction between the
aluminum chloride and the NH.sub.3 gases, an AlN layer was grown on
the SiC surface. The NH.sub.3 and aluminum chloride gases were
expelled from the reactor by the flow of the Ar gas. After allowing
the growth process to continue for a period of 2 hours, the flow of
HCl and NH.sub.3 gases was stopped and the furnace was slowly
cooled down to room temperature with Ar gas flowing through all of
the gas channels. The reactor was then opened to the air and the
sample holder was removed. As a result of this growth process, a
crack-free AlN layer 51 microns thick was grown on the SiC
substrates.
[0082] To prepare AlN substrates for further processing, the SiC
substrates were removed from the grown AlN material by chemically
etching the material in molten KOH. The etching was carried out in
a nickel crucible at a temperature within the range of 450.degree.
C. to 650.degree. C. Prior to beginning the etching process, the
molten KOH was maintained at the etching temperature for several
hours to remove the moisture from the melt and the crucible. Once
the substrates were placed within the molten KOH, only a few hours
were required to etch away most of the SiC substrates from the
grown AlN. This process for substrate removal is favored over
either mechanical or laser induced substrate removal. The remaining
SiC substrate was removed by RIE in a Si.sub.3F/Ar gas mixture. For
some of the samples, polycrystalline material was noted in the
peripheral regions, this material being subsequently removed by
grinding. Additionally, in some instances the surface of the
as-grown material required mechanical polishing to smooth the
surface. In these instances, after the polishing was completed, RIE
or chemical etching was used to remove the thin surface layer
damaged during polishing. As a result of this procedure, the
desired AlN seeds were obtained. The high quality of the resultant
material was verified by the x-ray rocking .omega.-scan curves
(e.g., 300 arc sec for the full width at half maximum (FWHM) for
the (0002) AlN reflection). X-ray diffraction measurements showed
that the as-grown material was 2H--AlN.
[0083] The inventors have found that SiC substrates are preferable
over sapphire substrates during the initial growth process as the
resultant material has a defined polarity. Specifically, the
resultant material has a mixture of aluminum (Al) polarity and
nitrogen (N) polarity. The side of the as-grown material adjacent
to the SiC substrates has an N polarity while the opposite,
outermost layer of the material has an Al polarity.
[0084] Prior to growing the next thick AlN layer, those samples
that had had the most material removed during the substrate removal
and surface preparation steps underwent further preparation.
Specifically a thin AlN layer, typically in the range of 10 to 100
microns thick, was grown on one or both sides of the AlN wafers in
question. The additional material improved the mechanical strength
of these substrates and, in general, prepared the AlN surface for
bulk growth. Prior to bulk growth, the AlN seed substrates were
approximately 1 millimeter thick and approximately 6 centimeters in
diameter.
[0085] The growth of the AlN thick layer (boule) used the same
reactor as that used to grow the AlN layers described above. The
substrates were positioned within the reactor such that the new
material would be grown on the (0001) Al on-axis face. It should be
noted that the (0001) surface can be tilted to a specific
crystallographic direction (e.g., [11-20]) and that the tilt angle
can be varied between 0.5 and 90 degrees. In the present
embodiment, the tilt angle was zero.
[0086] In addition to loading the seed substrates into the growth
zone of the reactor, two kilograms of Al was loaded into the source
boats of multiple Al source tubes. After purging the reactor with
Ar gas, the growth zone and the Al source zone were heated to
930.degree. C. and 750.degree. C., respectively. Prior to
initiating AlN growth, a mixture of NH.sub.3 and HCl gas was
introduced in the growth zone to refresh the surfaces of the
substrates. As in the previous growth, HCl was introduced into the
Al source zone to form aluminum chloride that was then transported
to the growth zone by the Ar carrier gas. At the same time,
NH.sub.3 gas used as a source of nitrogen was introduced into the
growth zone. The AlN was formed by the reaction between the gallium
chloride and the NH.sub.3 gases.
[0087] This growth process was allowed to continue for
approximately 40 hours. After that, both HCl flow and NH.sub.3 flow
were stopped. The furnace was slowly cooled down to room
temperature with Ar flowing through all gas channels. The reactor
was then opened to the air and the sample holder was removed from
the reactor. The resultant boule had a diameter of approximately 6
centimeters and a thickness of approximately 1 centimeter. The
crystal had a single crystal 2H polytype structure as shown by
x-ray diffraction measurements.
[0088] After growth, the boule was machined to a perfect
cylindrical shape with a 5.08 centimeter diameter (i.e., 2 inch
diameter), thereby removing defective peripheral areas. One side of
the boule was ground to indicate the (11-20) face. Then the boule
was sliced into 12 wafers using a horizontal diamond wire saw with
an approximately 200 micron diamond wire. Before slicing, the boule
was oriented using an x-ray technique in order to slice the wafers
with the (0001) oriented surface. The slicing rate was about 1
millimeter per minute. The wire was rocked around the boule during
the slicing. Thickness of the wafers was varied from 150 microns to
400 microns. Wafer thickness uniformity was better than 5
percent.
[0089] After slicing, the wafers were polished using diamond
abrasive suspensions. Some wafers were polished only on the Al
face, some wafers were polished only on the N face, and some wafers
were polished on both sides. The final surface treatment was
performed using an RIE and/or a chemical etching technique to
remove the surface layer damaged by the mechanical treatment. The
surface of the wafers had a single crystal structure as shown by
high-energy electron diffraction techniques. The surface of the
finished AlN wafers had a mean square roughness, rms, of 2
nanometers or less as determined by atomic force microscopy
utilizing a viewing area of 5 by 5 microns. The defect density was
measured using wet chemical etching in hot acid. For different
wafers, etch pit density ranged from 10 to 1000 per square
centimeter. Some AlN wafers were subjected to heat treatment in an
argon atmosphere in a temperature range from 450.degree. C. to
1020.degree. C. in order to reduce residual stress. Raman
scattering measurements showed that such heat treatment reduced
stress from 20 to 50 percent.
[0090] In order to compare the performance of devices fabricated
using the AlN substrates fabricated above to those fabricated on
SiC and sapphire, AlN homoepitaxial layers and pn diode multi-layer
structures were grown. Device structures included AlGaN/GaN
structures. Prior to device fabrication, surface 30 contamination
of the growth surface of the AlN wafers was removed in a side
growth reactor with a NH.sub.3--HCl gas mixture. The thickness of
individual layers 19 varied from 0.002 micron to 200 microns,
depending upon device structure. For example, high frequency device
structures (e.g., heterojunction field effect transistors) had
layers ranging from 0.002 to 5 microns. For high power rectifying
diodes, layers ranged from 1 to 200 microns. In order to obtain
p-type layers, a 5 Mg impurity was used while n-type doping was
obtained using a Si impurity. The fabricated device structures were
fabricated employing contact metallization, photolithography and
mesa insulation.
[0091] The structures fabricated on the AlN wafers were studied
using optical and electron microscopy, secondary ion mass
spectrometry, capacitance-voltage and current-voltage methods. The
devices showed superior characteristics compared with devices
fabricated on SiC and sapphire substrates. Additionally, it was
noted that the wafer surface cleaning procedure in the reactor
reduced defect density, including dislocation and crack density, in
the grown epitaxial layers.
Embodiment 4
AlN Device Fabrication
[0092] In this embodiment, AlN material was grown in an inert gas
flow at atmospheric pressure utilizing the hot-wall, horizontal
reactor described in Embodiment 3. Two inch diameter SiC substrates
of a 6H polytype were placed on a quartz pedestal and loaded into
the flat growth sub-zone of the quartz reactor. The substrates were
located such that the (0001) Si on-axis surfaces were positioned
for AlN deposition. Approximately 0.5 kilograms of Al (7N) was
located within a quartz boat in the Al source zone of the reactor.
This channel was used for delivery of aluminum chloride to the
growth zone of the reactor. A second quartz tube was used for
ammonia (NH.sub.3) delivery to the growth zone. A third separate
quartz tube was used for HCl gas delivery to the growth zone.
[0093] The reactor was filled with Ar gas, the Ar gas flow through
the reactor being in the range of 1 to 25 liters per minute. The
substrates were then heated in Ar flow to a temperature of
920.degree. C. and the hot portion of the metal Al source was
heated to a temperature in the range of 750.degree. C. to
800.degree. C. HCl gas was introduced into the growth zone through
the HCl channel. As a result, the SiC seed substrates were etched
at Ar--HCl ambient before initiating the growth procedure.
Additionally the seed was etched with aluminum chloride gas.
[0094] To begin the growth process, HCl gas was introduced into the
Al source zone, creating aluminum chloride that was delivered to
the growth zone by Ar gas flow. Simultaneously, NH.sub.3 was
introduced into the growth zone. As a result of the reaction
between the aluminum chloride gas and the ammonia gas, a single
crystal epitaxial AlN layer was grown on the substrates. The
substrate temperature during the growth process was held constant
at 920.degree. C. After a growth period of 20 hours, the flow of
HCl and NH.sub.3 were stopped and the samples were cooled in
flowing Ar.
[0095] As a result of the growth process, six AlN/SiC samples were
obtained in which the AlN thickness was in the range of 1 to 3
millimeters. To remove the SiC substrates, the samples were first
glued to metal holders using mounting wax (e.g., QuickStick.TM..
135) at a temperature of 130.degree. C. with the AlN layer facing
the holder. The holders were placed on a polishing machine (e.g.,
SBT Model 920) and a thick portion of the SiC substrates were
ground away using a 30 micron diamond suspension at 100 rpm with a
pressure of 0.1 to 3 kilograms per square centimeter. This process
was continued for a period of between 8 and 24 hours. After removal
of between 200 and 250 microns of SiC, the samples were unglued
from the holders and cleaned in hot acetone for approximately 20
minutes.
[0096] The residual SiC material was removed from each sample using
a reactive ion etching (RIE) technique. Each sample was placed
inside a quartz etching chamber on a stainless steel holder. The
RIE was performed using Si.sub.3F/Ar for a period of between 5 and
12 hours, depending upon the thickness of the residual SiC. The
etching rate of SiC in this process is about 10 microns per hour.
After the RIE process was completed, the samples were cleaned to
remove possible surface contamination. As a result of the above
processes, freestanding AlN plates completely free of any trace of
SiC were obtained.
[0097] After completion of a conventional cleaning procedure, the
AlN plates were placed in the HVPE reactor. An AlN homoepitaxial
growth was started on the as-grown (0001) Al surface of the AlN
plates. The growth temperature was approximately 910.degree. C.
After a period of growth of 10 minutes, the samples were cooled and
unloaded from the reactor. The AlN layer grown on the AlN plates
was intended to cover defects existing in the AlN plates.
Accordingly, the samples at the completion of this step were
comprised of 2 inch diameter AlN plates with approximately 10
microns of newly grown AlN. Note that for some samples an AlN layer
was grown not only on the (0001) Al face of the AlN plates, but
also on the (000-1)N face of the plates. Peripheral highly
defective regions of the AlN plates were removed by grinding.
[0098] Three of the AlN plates from the previous process were
loaded into the reactor in order to grow thick AlN layers. Aluminum
chloride (this term includes all possible Al--Cl compounds, for
example AlCl.sub.3) and ammonia gas served as source materials for
growth as previously disclosed. In addition, during the growth
cycle the AlN boules were doped with silicon supplied to the growth
zone by S.sub.2H.sub.4 gas. Growth temperatures ranged from
910.degree. C. to 920.degree. C. and the growth run lasted for 48
hours. Three layers with thicknesses of 5 millimeters, 7
millimeters, and 9 millimeters, respectively, were grown in the
flat growth zone.
[0099] The layers were sliced into AlN wafers. Prior to wafer
preparation, some of the boules were ground into a cylindrical
shape and peripheral polycrystalline AlN regions, usually between 1
and 2 millimeters thick, were removed. Depending upon wafer
thickness, which ranged from 150 to 500 microns, between 7 and 30
wafers were obtained per boule. The wafers were then polished on
either one side or both sides using an SBT Model 920 polishing
machine with a 15 micron diamond suspension at 100 rpm with a
pressure of between 0.5 and 2 kilograms per square centimeter for 9
minutes per side. After cleaning all parts and the holder for 5 to
10 minutes in water with soap, the polishing process was repeated
with a 5 micron diamond suspension for 10 minutes at the same
pressure. After subjecting the parts and the holder to another
cleaning, the wafers were polished using a new polishing cloth and
a 0.1 30 micron diamond suspension for an hour at 100 rpm with a
pressure of between 0.5 and 2 kilograms per square centimeter.
[0100] After cleaning, the AlN wafers were characterized in terms
of crystal structure, electrical and optical properties. X-ray
diffraction showed that the wafers were single crystal AlN with a
2H polytype structure. The FWHM of the x-ray rocking curve measured
in .omega.-scanning geometry ranged from 60 to 760 arc seconds for
different samples. After chemical etching, the etch pit density
measured between 100 and 10,000 per square centimeter, depending
upon the sample. Wafers had n-type conductivity with a
concentration N.sub.d-N.sub.a of between 5.times.10.sup.18 and
9.times.10.sup.18 per cubic centimeter. The wafers were used as
substrates for device fabrication, particularly for AlN/AlGaN
multi-layer device 10 structures grown by the MOCVD process.
Embodiment 5
Fabrication of Thick AlN Wafers
[0101] After growing AlN on SiC substrates and separating the SiC
substrate as disclosed above, a crack-free 5 mm thick AlN layer was
grown at 910.degree. C. by the previously described HVPE process on
the (0001) Al face of the 3 inch diameter freestanding AlN
substrate. The (0001) Al face was prepared for thick AlN epitaxial
growth by RIE. The AlN growth rate was 50 microns per minute, the
duration of the growth cycle was 100 hours, and the growth process
was performed in the flat growth sub-zone.
[0102] The 5 mm thick AlN layer was sliced by diamond wire into
eight AlN wafers. These wafers were polished by a
chemical-mechanical process to reduce the surface roughness rms
down to 0.1 nm as measured by AFM. For some wafers the (000-1) N
face was polished and for other wafers the (0001) Al face was
polished. A sub-surface layer of about 0.1 microns that was damaged
by the mechanical treatment was removed by dry etching. The
resultant 3 inch AlN wafers had more than 90 percent usable area
for device formation. Some wafers were on-axis and some wafers were
mis-oriented from the (0001) surface in the range of 0 to 10
degrees. The wafers had a bow of less than 30 microns. The wafers
contained no polytype inclusions or mis-oriented crystal blocks.
The AlN wafers had a 2H crystal structure. Cathodoluminescence
measurements revealed near band edge luminescence in the wavelength
range from 5.9 to 6.1 eV. The wafers were crack-free, colorless,
and optically transparent. Etch pit 23 density measured by hot wet
etching was less than 10.sup.7 cm.sup.-2. The defect density at the
top of the thick AlN layer was less than in the initial AlN wafer.
The wafers had between 1 and 5 macrodefects with a size larger than
0.1 mm For different samples, the FWHM of x-ray rocking curves
ranged from 60 to 1200 arc sec. The 5 atomic concentrations of Si
and carbon contamination was less than 10.sup.18 cm.sup.-3. The
oxygen concentration in the wafers ranged from 10.sup.18 to
10.sup.21 cm.sup.-3.
Embodiment 6
Device Fabrication
[0103] A number of devices were fabricated to further test the
benefits of the presently developed, crack-free AlN layers. In all
of the devices described within this section, the AlN or other
Group III nitride substrates were fabricated in accordance with the
techniques described above. Additionally, where thick homoepitaxial
layers were grown on the substrate prior to the device fabrication,
these thick layers were grown in accordance with the invention.
[0104] A high electron mobility transistor (HEMT) was fabricated as
shown in FIG. 4. The device was comprised of an AlN substrate 401
and an AlN homoepitaxial layer 403 grown at 1000.degree. C. on
substrate 401 having a (0001) Al surface orientation. Layer 403's
thickness was 12 microns in one device fabrication run and 30
microns in another device fabrication run. Although not required,
the thick AlN homoepitaxial layer reduces defect density in the
final device structure and improves device performance. The AlN
layers were crackfree as verified by transmission and reflection
optical microscopy with magnifications up to 1000.times.. In the
same HVPE growth process, a GaN layer 405 and an AlGaN layer 407
were grown to form the HEMT structure. The thickness of GaN layer
405 was about 0.2 microns and the thickness of AlGaN layer 407 was
about 30 nm. Depending upon the sample, the AlN content in the
AlGaN layer ranged from 10 to 50 mol. %. X-ray diffraction study
verified that all device layers were grown. Source, drain and gate
contacts were also added to the AlGaN active structure (not shown).
It will be appreciated that the GaN/AlGaN structure could have been
fabricated by MOCVD and/or MBE techniques. The HEMT structures
displayed 2DEG mobility up to 2000 cm.sup.2 V sec (300 K),
operating frequency from 1 to 100 GHz, and an operating power for a
single transistor of 10 W, 20 W, 50 W and 100 W or more depending
upon the size of the device.
[0105] Light emitting diodes (LEOs) capable of emitting light in a
color selected from the group consisting of red, green, blue,
violet and ultraviolet were fabricated (shown in FIGS. 5-7). The
tested LEOs had a peak emission wavelength from about 200 to 400 nm
and an output power from 0.001 to 100 mW (20 mA). In at least one
embodiment the Group III nitride substrate 501 was comprised of
n-type AlGaN or AlN while in at least one other embodiment
substrate 601 was comprised of AlN (e.g., substrates 601 and 701).
The embodiment illustrated in FIG. 5 is further comprised of an
n-type layer 503 of AlGaN, an InGaN quantum well layer 505, a
p-type AlGaN layer 507, a p-type GaN layer 509, a first ohmic
contact 511 deposited on substrate 501, and a second ohmic contact
513 deposited on GaN layer 509. The embodiment illustrated in FIG.
6 is further comprised of a buffer layer 603, an n-type layer 605
of AlGaN, an InGaN quantum well layer 607, a p-type AlGaN layer
609, a p-type GaN layer 611, a first ohmic contact 613 deposited on
said n-type layer 605 of AlGaN, and a second ohmic contact 615
deposited on GaN layer 611. The embodiment illustrated in FIG. 7 is
further comprised of an AlN layer 703 at least 10 microns thick, an
n-type layer 705 of AlGaN, an InGaN quantum well layer 20 707, a
p-type AlGaN layer 709, a p-type GaN layer 711, a first ohmic
contact 713 deposited on said n-type layer 705 of AlGaN, and a
second ohmic contact 715 deposited on GaN layer 711.
[0106] As will be understood by those familiar with the art, the
present invention can be embodied in other specific forms without
departing from the spirit or essential characteristics thereof.
Accordingly, the disclosures and descriptions herein are intended
to be illustrative, but not limiting, of the scope of the invention
which is set forth in the following claims.
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