U.S. patent application number 15/258624 was filed with the patent office on 2018-03-08 for cvd reactor and method for nanometric delta doping of diamond.
The applicant listed for this patent is Euclid TechLabs, LLC. Invention is credited to James E Butler.
Application Number | 20180068850 15/258624 |
Document ID | / |
Family ID | 61281475 |
Filed Date | 2018-03-08 |
United States Patent
Application |
20180068850 |
Kind Code |
A1 |
Butler; James E |
March 8, 2018 |
CVD REACTOR AND METHOD FOR NANOMETRIC DELTA DOPING OF DIAMOND
Abstract
An apparatus and method for creating nanometric delta doped
layers in epitaxial diamond includes providing a dummy gas load
with gas impedance equivalent to the reactor, and switching gas
supplied between the reactor and the gas dummy load without
stopping either flow, thereby enabling rapid flow and rapid gas
switching without turbulence. An atomically smooth, undamaged
substrate can be prepared, preferably in the (100) plane, by
etching the surface after polishing to remove subsurface damage. A
gas phase chemical getter reactant such as hydrogen disulfide can
be used to suppress incorporation of residual boron into the
intrinsic layers. Embodiments can produce interfaces between doped
and mobile layers that provide at least 100 cm.sup.2/Vsec carrier
mobility and 10.sup.13 cm.sup.-2 sheet carrier concentration.
Inventors: |
Butler; James E;
(Huntingtown, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Euclid TechLabs, LLC |
Solon |
OH |
US |
|
|
Family ID: |
61281475 |
Appl. No.: |
15/258624 |
Filed: |
September 7, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/02019 20130101;
H01J 37/32449 20130101; H01J 37/32467 20130101; H01L 21/02376
20130101; H01L 21/02634 20130101; H01J 37/32192 20130101; H01L
21/02658 20130101; H01L 21/0262 20130101; H01L 22/12 20130101; H01L
21/02527 20130101; H01L 22/26 20130101; H01J 37/32724 20130101;
H01L 21/02584 20130101; H01L 21/02579 20130101; H01L 21/02024
20130101; H01L 21/02656 20130101; H01J 2237/3321 20130101; H01J
37/32201 20130101 |
International
Class: |
H01L 21/02 20060101
H01L021/02; H01L 21/66 20060101 H01L021/66; H01J 37/32 20060101
H01J037/32 |
Claims
1. An epitaxial CVD reactor for growing delta-doped layers on
diamond substrates, the reactor comprising: a reaction chamber
configured to enable a flow of gas from an inlet thereof through an
interior thereof; a substrate support located within the reaction
chamber and configured for supporting a diamond substrate on a
surface thereof; a plasma generator configured to excite the gas so
as to surround the substrate with a gas plasma; a first gas source;
a second gas source; a dummy gas load configured to allow gas from
one of the gas sources to flow therethrough, the dummy gas load
being configured to present a dummy gas flow impedance to a gas
source that is equivalent to a reactor gas flow impedance of the
reaction chamber; a gas manifold configured to direct a flow from
one of the gas sources to the reaction chamber while directing a
flow from the other of the gas sources to the dummy gas load; a gas
switch configured to switch the gas flows between the reaction
chamber and the dummy gas load, while maintaining both gas flows;
and a switch controller configured to control switching by the gas
switch of the gas flows.
2. The reactor of claim 1, wherein the reaction chamber is a fused
silica tube.
3. The reactor of claim 1, wherein the plasma generator is
configured to excite the gas using electromagnetic radiation.
4. The reactor of claim 3, wherein the plasma generator is a
magnetron that generates electromagnetic radiation at 2.45 GHz.
5. The reactor of claim 1, further comprising a heater embedded in
the substrate support and configured for maintaining the substrate
surface at a desired temperature.
6. The reactor of claim 1, further comprising a heater and control
system configured to maintain a temperature of the diamond
substrate at a value that is within the range of 700.degree. C. to
1100.degree. C. with a precision of +/-5.degree. C.
7. The reactor of claim 1, wherein the dummy load includes a dummy
chamber and dummy support that are substantially identical in their
physical configurations to the reaction chamber and substrate
support.
8. The reactor of claim 1, further comprising a second dummy load
and a third gas source, the gas manifold and gas switch being
configured to direct and switch a gas flow from any of the gas
sources to the reaction chamber while directing flows from the
other two gas sources to dummy gas loads while maintaining all
three gas flows.
9. A method for preparing a diamond substrate for epitaxial
deposition thereupon of a delta doping layer, the method
comprising: polishing a face of the substrate; and etching the
polished face of the substrate, thereby removing subsurface damage
caused by the polishing step.
10. The method of claim 9, wherein the polished face is
approximately in the (100) plane.
11. The method of claim 10, wherein polishing the face includes
applying a rough polish in a polish direction that is within five
degrees of the (010) direction, followed by a fine polish within
five degrees of the (011) direction, the fine polish being
continued until polishing tracks resulting from the rough polish
are no longer detectable by optical interferometry.
12. The method of claim 9, wherein the polished face has a
roughness Sa of less than 0.3 nm over at least an 80.times.80
micron sampling area.
13. The method of claim 9, wherein the etching is applied using an
inductively coupled reactive ion etcher.
14. The method of claim 9, wherein the diamond substrate is a plate
of single crystal type IIa diamond or type 1b HBHT diamond.
15. The method of claim 9, wherein the diamond substrate has a
dislocation density that is less than 10.sup.4/cm.sup.2.
16. A method of depositing epitaxial delta-doped layers on a
diamond substrate, the method comprising: a) providing a CVD
reactor according to claim 1; b) filling the first gas source with
a first gas mixture that includes carbon atoms but not dopant
atoms; c) filling the second gas source with a second gas mixture
that includes both carbon atoms and dopant atoms; d) placing a
diamond substrate having a polished surface on the substrate
holder; e) introducing a flow of the first gas mixture from the
first gas source into the reaction chamber, while a flow of the
second gas mixture from the second gas source flows into the dummy
load; f) causing the plasma generator to surround the polished
surface of the substrate with a plasma of the first gas mixture; g)
waiting until a first epitaxial layer has formed on the polished
surface; h) causing the gas switch to switch the gas sources
between the reaction chamber and the dummy load, so that gas from
the second gas source is caused to flow through the reaction
chamber and the polished surface is surrounded with a plasma of the
second gas mixture; and i) allowing a second epitaxial layer to
form on the first epitaxial layer.
17. The method of claim 16, wherein interfaces between the first
and second epitaxial layers and the diamond substrate provide a
charge carrier concentration of at least 10.sup.13 cm-2 having a
carrier mobility of at least 100 cm.sup.2/Vsec
18. The method of claim 16, wherein the first gas mixture further
comprises a chemical getter reactant that inhibits incorporation of
residual dopant atoms into the first epitaxial layer.
19. The method of claim 18, wherein the dopant atoms are boron
atoms, and the chemical getter reactant is hydrogen disulfide.
20. The method of claim 16, wherein the flows of the first and
second gas mixtures are at a rate of approximately 950 sccm.
21. The method of claim 16, wherein the switching of the gas
sources between the reaction chamber and the dummy load is
completed within a switching time that is less than 20 seconds.
22. The method of claim 21, wherein epitaxial growth during the
switching time is between 1 and 2 Angstroms.
23. The method of claim 16, further comprising repeating steps e)
through i) so as to form third and fourth epitaxial layers.
Description
FIELD OF THE INVENTION
[0001] The invention relates to reactors and methods for epitaxial
growth of diamond, and more particularly to chemical vapor
deposition ("CVD") reactors and methods for creating nanometric
delta doped layers in epitaxially grown single crystals of
diamond.
BACKGROUND OF THE INVENTION
[0002] Diamond is of strong interest as a potential semiconductor
material for high voltage, high frequency, and/or high power active
and passive electronic devices because of its superlative materials
properties, including high electronic carrier mobilities, high
breakdown field strength, high thermal diffusivity, favorable
matrix for quantum devices, as well as many other desirable
optical, chemical, and materials properties. However, a major
barrier to exploiting diamond for active electronic applications is
that there are no dopants known that have a sufficiently low
thermal activation energy barrier to create a concentration of
electronic carriers and a carrier mobility in diamond at room
temperature that is adequate for most devices of interest.
[0003] While there are many known defect and impurity states in the
wide bandgap (5.45 eV) of diamond, several of which can act as
donors or acceptors of electronic charge, only boron (creating an
acceptor state) and phosphorous (creating a donor state) have been
demonstrated to be reliable dopants. Boron, the most commonly used
diamond dopant, has the smallest activation energy of 0.37 eV at
low doping concentrations (<10.sup.17 cm.sup.-3). However, this
activation energy is still high enough to ensure that only a
fraction of the boron present is activated at room temperature,
leading to relatively low concentrations of free carriers.
[0004] Increasing the boron concentration in diamond reduces the
activation energy, such that at a concentration of approximately
3.times.10.sup.20 cm.sup.-3 the metal-to-insulator transition point
occurs and a fully-activated impurity band is formed via the
quantum tunneling of holes between neighboring boron acceptor
states. Unfortunately, as the activation energy of the holes
decreases, so does carrier mobility, not only because of the
increased impurity scattering but also due to the onset of a
low-mobility, hopping-like conduction. The resulting material is
one that has sub-unity carrier mobility and typical sheet carrier
densities in excess of those that are readily controlled, for
example, by a typical field effect transistor (FET).
[0005] One approach to creating both high mobility and high carrier
concentrations for electronic materials in two dimensions is the
formation of "nanometric delta doped" layers, which are heavily
doped layers, typically less than five nanometers in thickness,
that are located adjacent to high mobility intrinsic material, so
that a fraction of the carriers created by the heavily ionized
dopant layer reside in the adjacent high mobility layer.
[0006] The success of "delta doping" requires the epitaxial growth
of a very thin, heavily doped "delta layer" that is typically
between 1 and 2 nm thick and is preferably doped to a concentration
that is above the metal insulator transition, which for boron in
diamond means a concentration of at least approximately
4.times.10.sup.20 cm.sup.-3. Successful delta doping further
requires that the interface between the doped "delta" layer and the
high mobility intrinsic layer (containing less than 10.sup.17
cm.sup.-3 boron atoms) must be abrupt, and must also be atomically
smooth, so as to minimize carrier scattering.
[0007] Recent attempts at delta doping of diamond with boron have
failed to demonstrate the theoretically expected performance, and
have shown low carrier mobilities, low sheet carrier
concentrations, and/or low channel mobility. Some of these studies
have attributed this disappointing performance to poor lateral
homogeneity and interrupted morphology of the delta layers. Carrier
mobilities measured in these studies did not exceed the range of 1
to 4.4 cm.sup.2/Vsec which is typical of bulk diamond doped with
boron above the metal insulator transition level of 4 to
5.times.10.sup.20 cm.sup.-3. These results are well short of the
values of approximately 100 cm.sup.2/Vsec mobility and 10.sup.13
cm.sup.-2 sheet carrier concentrations that are required for the
implementation of doped layers of diamond in practical electronic
devices.
[0008] What is needed, therefore, is an apparatus and method for
creating nanometric delta doped layers in epitaxial diamond with
interfaces between the doped and high mobility layers that are
sufficiently abrupt and smooth to provide at least 100
cm.sup.2/Vsec carrier mobility and 10.sup.13 cm.sup.-2 sheet
carrier concentrations.
SUMMARY OF THE INVENTION
[0009] An apparatus and method is disclosed for creating nanometric
delta doped layers in epitaxial diamond with interfaces between the
doped and high mobility layers that are sufficiently abrupt and
smooth to provide at least 100 cm.sup.2/Vsec carrier mobility and
10.sup.13 cm.sup.-2 sheet carrier concentrations. The disclosed
apparatus includes a novel switched-chamber gas supply that enables
rapid switching between gas sources by maintaining constant loads
or "impedances" on the outlets of the gas sources, thereby
minimizing any turbulence and gas mixing caused by the switching.
The method further includes preparation of an atomically smooth,
undamaged single crystal diamond substrate surface by finely
polishing the surface, preferably in or near the (100) plane, and
then etching the surface after polishing to remove any subsurface
polishing damage. Embodiments of the method further include
maintaining rapid gas flows while adopting extraordinarily slow
growth rates to create the sharpest possible interfaces.
[0010] Embodiments further include using Type IIa single crystal
diamond as the substrate. And in various embodiments a gas phase
chemical getter reactant is used to suppress any residual boron
incorporation in the intrinsic layers. Embodiments further include
heating and maintaining the single crystal diamond substrate
surface at a temperature between 700 and 1100.degree. C. during the
epitaxial growth.
[0011] A first general aspect of the present invention is an
epitaxial CVD reactor for growing delta-doped layers on diamond
substrates. The reactor includes a reaction chamber configured to
enable a flow of gas from an inlet thereof through an interior
thereof, a substrate support located within the reaction chamber
and configured for supporting a diamond substrate on a surface
thereof, a plasma generator configured to excite the gas so as to
surround the substrate with a gas plasma, a first gas source, a
second gas source, a dummy gas load configured to allow gas from
one of the gas sources to flow therethrough, the dummy gas load
being configured to present a dummy gas flow impedance to a gas
source that is equivalent to a reactor gas flow impedance of the
reaction chamber, a gas manifold configured to direct a flow from
one of the gas sources to the reaction chamber while directing a
flow from the other of the gas sources to the dummy gas load, a gas
switch configured to switch the gas flows between the reaction
chamber and the dummy gas load, while maintaining both gas flows,
and a switch controller configured to control switching by the gas
switch of the gas flows.
[0012] In embodiments, the reaction chamber is a fused silica
tube.
[0013] In some embodiments, the plasma generator is configured to
excite the gas using electromagnetic radiation. And in some of
these embodiments, the plasma generator is a magnetron that
generates electromagnetic radiation at 2.45 GHz.
[0014] Various embodiments further include a heater embedded in the
substrate support and configured for maintaining the substrate
surface at a desired temperature. Some embodiments further include
a heater and control system configured to maintain a temperature of
the diamond substrate at a value that is within the range of
700.degree. C. to 1100.degree. C. with a precision of +/-5.degree.
C.
[0015] In certain embodiments, the dummy load includes a dummy
chamber and dummy support that are substantially identical in their
physical configurations to the reaction chamber and substrate
support.
[0016] Other embodiments further include a second dummy load and a
third gas source, the gas manifold and gas switch being configured
to direct and switch a gas flow from any of the gas sources to the
reaction chamber while directing flows from the other two gas
sources to dummy gas loads while maintaining all three gas
flows.
[0017] A second general aspect of the present invention is a method
for preparing a diamond substrate for epitaxial deposition
thereupon of a delta doping layer. The method includes polishing a
face of the substrate, and etching the polished face of the
substrate, thereby removing subsurface damage caused by the
polishing step.
[0018] In embodiments, the polished face is approximately in the
(100) plane. And in some of these embodiments polishing the face
includes applying a rough polish in a polish direction that is
within five degrees of the (010) direction, followed by a fine
polish within five degrees of the (011) direction, the fine polish
being continued until polishing tracks resulting from the rough
polish are no longer detectable by optical interferometry.
[0019] In various embodiments the polished face has a roughness Sa
of less than 0.3 nm over at least an 80.times.80 micron sampling
area. In some embodiments the etching is applied using an
inductively coupled reactive ion etcher. And in other embodiments
the diamond substrate is a plate of single crystal type IIa diamond
or type 1b HBHT diamond.
[0020] And in certain embodiments the diamond substrate has a
dislocation density that is less than 104/cm.sup.2.
[0021] A third general aspect of the present invention is a method
of depositing epitaxial delta-doped layers on a diamond substrate.
The method includes the following steps:
[0022] a) providing a CVD reactor according to the first general
aspect;
[0023] b) filling the first gas source with a first gas mixture
that includes carbon atoms but not dopant atoms;
[0024] c) filling the second gas source with a second gas mixture
that includes both carbon atoms and dopant atoms;
[0025] d) placing a diamond substrate having a polished surface on
the substrate holder;
[0026] e) introducing a flow of the first gas mixture from the
first gas source into the reaction chamber, while a flow of the
second gas mixture from the second gas source flows into the dummy
load;
[0027] f) causing the plasma generator to surround the polished
surface of the substrate with a plasma of the first gas
mixture;
[0028] g) waiting until a first epitaxial layer has formed on the
polished surface;
[0029] h) causing the gas switch to switch the gas sources between
the reaction chamber and the dummy load, so that gas from the
second gas source is caused to flow through the reaction chamber
and the polished surface is surrounded with a plasma of the second
gas mixture; and
[0030] i) allowing a second epitaxial layer to form on the first
epitaxial layer.
[0031] In embodiments, interfaces between the first and second
epitaxial layers and the diamond substrate provide a charge carrier
concentration of at least 10.sup.13 cm.sup.-2 having a carrier
mobility of at least 100 cm.sup.2/Vsec.
[0032] In some embodiments, the first gas mixture further comprises
a chemical getter reactant that inhibits incorporation of residual
dopant atoms into the first epitaxial layer. And in some of these
embodiments the dopant atoms are boron atoms, and the chemical
getter reactant is hydrogen disulfide.
[0033] In various embodiments, the flows of the first and second
gas mixtures are at a rate of approximately 950 sccm.
[0034] In certain embodiments, the switching of the gas sources
between the reaction chamber and the dummy load is completed within
a switching time that is less than 20 seconds. And in some of these
embodiments epitaxial growth during the switching time is between 1
and 2 Angstroms.
[0035] And some embodiments further include repeating steps e)
through i) so as to form third and fourth epitaxial layers.
[0036] The features and advantages described herein are not
all-inclusive and, in particular, many additional features and
advantages will be apparent to one of ordinary skill in the art in
view of the drawings, specification, and claims. Moreover, it
should be noted that the language used in the specification has
been principally selected for readability and instructional
purposes, and not to limit the scope of the inventive subject
matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a cross sectional diagram illustrating an
embodiment of the apparatus of the present invention;
[0038] FIG. 2A is a plot of boron concentration profiles resulting
from an embodiment of the present invention;
[0039] FIG. 2B is a plot of the SIMS-SP profile of a single delta
layer of FIG. 2A;
[0040] FIG. 2C is a plot of a time-resolved plasma optical emission
(line Ar 750.4 nm) of an embodiment of the present invention;
[0041] FIG. 3A is a SIMS-SP plot of the boron concentration in a
sample created using the present invention wherein 2 separate delta
layers were grown over an intrinsic buffer layer on a heavily boron
doped layer; and
[0042] FIG. 3B presents the results of capacitance versus voltage
measurements on mesa structures of the sample of FIG. 3A showing
the apparent profile of the hole concentration in one delta
layer.
DETAILED DESCRIPTION
[0043] The present invention is an apparatus and method for
creating nanometric delta doped layers in epitaxial diamond with
interfaces between the doped and high mobility layers that are
sufficiently abrupt and smooth to provide at least 100
cm.sup.2/Vsec carrier mobility and 10.sup.13 cm.sup.-2 carrier
concentration.
[0044] The apparatus of the present invention is a CVD reactor that
is able to create nanometric delta-doped layers with ultra-sharp
interfaces between doped/undoped material. FIG. 1 is a schematic
illustration of an embodiment of the disclosed reactor, in which a
microwave plasma excitation zone 100 surrounds a substrate 112
supported by a substrate holder 104 in a reduced pressure region of
a vertical quartz tube growth chamber 102. In various embodiments
the growth chamber is a fused silica tube 102. In embodiments, the
substrate holder 104 can heat the substrate 112 to a temperature of
between 700 and 1100.degree. C. as desired.
[0045] The plasma in this embodiment is produced by electromagnetic
radiation generated by a 2.45 GHz magnetron 106 which is
transmitted through a waveguide 108 to a cavity 110. In similar
embodiments, the plasmas is created by another method known in the
art, such as dc, rf, microwave, or terahertz excitation, combustion
flames, or hot wires, filaments, or surfaces. The use of a
cylindrical cavity in the embodiment of FIG. 1 enables the
generation of a symmetrical plasma ball 100 above the substrate 112
and well separated from the walls of the quartz tube 102 by
applying only a low microwave power density in the plasma 100.
[0046] In the embodiment of FIG. 1, during intrinsic (undoped)
diamond growth, a mixture of purified H.sub.2+CH.sub.4 is fed from
a first gas supply 114 through a first manifold 116 to the quartz
tube 102, where it flows past the substrate 112 and out through an
outlet 124 to a gas pumping system (not shown). In embodiments, a
chemical "getter" reactant is also included in this gas mixture, as
discussed in more detail below. Due to the smooth and unobstructed
configurations of the manifold 116 and quartz tube 102, the flow of
gas quickly becomes laminar in the region of the substrate 112 once
the reaction has been initiated.
[0047] So as to provide delta doped layers in the epitaxial
diamond, it is necessary to temporarily substitute a gas containing
a dopant in place of the H.sub.2+CH.sub.4 provided by the first gas
supply 114. In some embodiments where the doping is with boron, the
gas mixture is switched briefly to H.sub.2+CH.sub.4+B.sub.2H.sub.6.
According to the present invention, the dopant gas mixture is
contained in a second gas supply 118, and is delivered to the
quartz tube 102 through a second gas manifold 120 when the gas is
switched between the two gas supplies 114, 118 by a gas supply
switch 122.
[0048] If the gas supplies 114, 118 were simply switched, such that
flow from the first supply 114 was halted and flow from the second
supply 118 was initiated, the result would likely be to temporarily
cause a turbulence and eddy currents in the flow of gas near the
substrate 112, leading to "tails" or mixing of the gases in the
transition region between the gas mixtures, and a correspondingly
broad transition between epitaxial layers in the diamond. This
effect is largely due to the sudden change in load or "impedance"
that is encountered at the outlets of the gas supplies when a gas
flow is interrupted or commenced, and in that regard it is
conceptually similar to the reflections of electromagnetic energy
such as radio frequency or laser beams that occurs when a sudden
change or mismatch of impedance is encountered. Conventional
approaches to minimizing this problem would be to switch the gases
on and off slowly and gradually, and or to use a very low gas flow
rate. However, both of these approaches would tend to broaden the
transition between the epitaxial layers.
[0049] The present invention implements a novel approach to avoid
this problem of turbulence, whereby the gas flows are not stopped,
but are merely switched between the reactor and a second "dummy"
gas load, such as a second dummy quartz tube 124 and manifold 120,
which in embodiments is identical in design to the reactor quartz
tube 102 and manifold 116. Making the dummy load physically similar
or identical to the reactor ensures that the "impedances" are
matched, however dummy loads that do not necessarily duplicate the
physical configuration of the reaction chamber but nevertheless are
configured to provide an impedance that matches the reaction
chamber are included within the scope of the present invention.
[0050] The gas supply switch 122 is configured so as to provide a
continuous, uninterrupted flow of gas simultaneously from both gas
supplies 114, 118 to the two reaction chamber 102 and the dummy
load 124, so that switching between the gasses consists merely in a
change of direction of each gas from one destination to the other.
Because neither gas flow is interrupted, and because the impedances
are matched, i.e. in the embodiment of FIG. 1 the manifolds 116,
120 and quartz tubes 102, 124 are identical, the flow from each
source is continuous, and the flow "impedance" seen by each source
is constant, so that any generation of turbulence due to the
switching process is minimized or eliminated.
[0051] This duel-load gas switching architecture and method allows
embodiments to maintain a high gas flow rate and to switch the gas
quickly and abruptly without triggering undue turbulence. For
example, embodiments employ a gas flow rate of 950 sccm together
with a gas switching time of less than 10 seconds. Keeping the gas
flow rate high has the added advantage in various embodiments of
suppressing gas buoyancy effects from the hot plasma "ball" 100
above the substrate 112, and thereby reducing the effects of any
turbulence resulting from the heating of the plasma.
[0052] Embodiments further sharpen the layer transition by
maintaining an extraordinarily slow epitaxial growth rate of
between 30 and 90 nm per hour, as determined by secondary ion mass
spectroscopy sputter profiling ("SIMS-SP") of previously grown
epitaxial layers. In embodiments, the total gas pressure was 30 to
50 Torr and the microwave power was 1.5 kW. In various embodiments
the gas compositional switching time immediately above the
substrate is approximately equivalent to 1-2 angstrom of growth so
that, for example, a 20 sec gas switching time implies a desired
growth rate of 18-36 nm/hr. These growth rates were only slightly
over the rate of etching of a diamond substrate in a similar carbon
free plasma.
[0053] Further embodiments of the present invention include more
than one "dummy" load and more than two gas sources and manifolds,
so that a plurality of epitaxial layers can be created on the
substrate having differing dopent levels and types. For example,
embodiments with two dummy cylinders are able to produce
alternating boron-doped and phosphorous-doped epitaxial layers,
interspersed with undoped layers.
[0054] Obviously, a sharp transition between adjacent layers in an
epitaxially grown diamond crystal requires that the diamond
substrate surface must be highly polished so as to be as smooth as
possible. However, the present inventors have realized that the
necessary polishing of the substrate surface inevitably results in
some subsurface damage to the substrate. This is problematic,
because success of the delta-doping strategy in terms of high
carrier mobility requires that the undoped regions near the delta
doped layer, i.e. the region of the substrate near the polished
surface, must be free of damage and defects, which would otherwise
hinder the mobility of the carriers. Accordingly, the method of the
present invention includes the additional step of etching the
polished substrate surface so as to remove the subsurface damage
caused by polishing while retaining the substrate smoothness.
[0055] In embodiments, the method includes orienting the polished
surface of the substrate in or near the (100) plane. In various
embodiments, a "rough" polish is applied, for example using a
traditional cast iron skive plate charged with diamond grit of size
larger than 2 microns and revolving at between 1500 to 3000 rpm, so
as to polish the surface to be within 1 degree of the (100) plane,
with the crystal oriented such that skive plate motion is within 5
degrees of the easy "soft" polishing (010) direction. This first
polishing step provides a macroscopically flat surface on the
crystal that is oriented within 1 degree of the (100)
crystallographic plane, but generally leaves polishing tracks or
grooves of a few nm depth.
[0056] In a second polishing step, in embodiments, another
traditional cast iron skive plate, charged with diamond grit of
size less than 0.5 microns and revolving at between 1500 to 3000
rpm, is used to polish the previously polished surface, without
changing the near (100) plane orientation, with the crystal
reoriented in the difficult `hard` direction relative to the skive
plate motion (within 5 degrees of the (011) direction. This step,
which has a much slower polishing rate, is continued until the
polishing tracks or grooves from the previous polishing step are no
longer detectable by optical interferometry, producing a surface
with a roughness, Sa, of less than 0.3 nm over at least an
80.times.80 micron sampling area.
[0057] In various embodiments, the polished surface of the
substrate crystal is then homogeneously etched, for example in an
inductively coupled reactive ion etcher, such as an Oxford
Instruments Plasmalab 80 using argon and chlorine, to remove
between 0.1 and 10 microns of the diamond surface. This step
eliminates the subsurface polishing damage while retaining the
smoothness of the polished surface.
[0058] In embodiments, the substrate is a plate of single crystal
type IIa diamond (no nitrogen detectable by IR absorption
spectroscopy) or type 1b HBHT diamond cut from a single (100)
growth sector of a low dislocation density (less than
10.sup.4/cm.sup.2, preferably less the 10.sup.2) HPHT synthetic
diamond. In various embodiments, the size of the substrate crystal
can be, for example, 3.times.3.times.0.5 mm,
3.5.times.3.5.times.0.5 mm, or larger, and can display little or no
birefringence when viewed between crossed polarizers. In
embodiments, the substrate crystal has a dislocation density below
10.sup.6 cm-2, and more preferably, below 10.sup.-2.
[0059] In embodiments, the residual or inadvertent boron levels in
nominally undoped diamond layers is maintained below 10.sup.17
cm-3, and preferably below 10.sup.16 by employing a gas phase
chemical getter to suppress the residual boron incorporation in the
intrinsic layers. For example, in some of these embodiments
hydrogen disulfide is added to the gas phase to form volatile boron
sulfur complexes which, at elevated substrate surface temperatures
of approximately 850 C or higher, reduces the probability of boron
incorporating in the growing diamond layers. As an example, typical
growth conditions in embodiments employ a flow of 900 sccm hydrogen
(Pd diffusion cell purified), 1.4 sccm methane (ultra pure,
99.999%), 6 to 17 sccm of 0.1% B2H6 diluted in hydrogen, and 6 to
14 sccm of 0.1% H2S diluted in hydrogen.
[0060] Embodiments allow programmable doping for growing one or
more layers of a pre-determined thickness. For example, the
disclosed apparatus and method can be used to form a "two-humped"
delta layer for which, according to calculations, the sheet carrier
concentration should be increased several times compared to the
structure with a single delta-layer.
Measured Results for Representative Embodiments
[0061] Measurements have been made using the apparatus of FIG. 1
with a total gas flow of 950 sccm (standard cubic centimeters per
minute). FIG. 2A is a plot of the resulting boron concentration
profiles, as determined by SIMS-SP (solid line) and of the boron
concentration profiles recovered using an analytical depth
resolution function (dashed line), both as a function of depth in
an epitaxially grown diamond. For this measurement, the methane
flow was 1.4 sccm, and the B/C ratio was 18570 ppm (1), 12860 ppm
(2), 8570 ppm (3) respectively. FIG. 2B is a plot of the SIMS-SP
profile of a single delta layer (open circles) of the Boron
concentration profiles recovered using an analytical depth
resolution function (dashed line), and the resulting fit profile
(solid line).
[0062] To measure the switching time from one gas manifold to the
other, argon was used instead of boron as the "dopant" so that a
spectrometer could be used to measure the time-resolved intensity
of the Ar emission line during gas mixture switching. The plasma
was maintained in the H.sub.2+CH.sub.4 gas mixture at a total flow
of 950 sccm, and was switched to H.sub.2+CH.sub.4 Ar at time t=0.
Then, the gas mixture was switched back to H.sub.2+CH.sub.4 at time
t=147 s. FIG. 2C shows the time-resolved plasma optical emission
(line Ar 750.4 nm), collected using a SOLAR TII spectrometer.
Because of the low reactivity of the inert gas argon, the measured
gas flow switching times (up to 10 s) in FIG. 1 represent the
"gas-dynamic" times.
[0063] FIG. 3A presents a SIMS-SP plot of the boron concentration
in a sample wherein 2 separate delta layers were grown over an
intrinsic buffer layer on a heavily boron doped layer. FIG. 3B
presents the results of capacitance versus voltage measurements on
mesa structures prepared on the sample shown in FIG. 3A, showing
the apparent profile of the hole concentration in one delta layer.
Note that the measured hole concentration outside the delta layer
is more than an order of magnitude above the residual boron
concentration measured by SIMS-SP of less than 4.times.10.sup.17
cm-3.
[0064] There are many parallel electrical conduction paths in a
sample with a capped single delta layer. These consist of
conduction on the surface, in the bulk intrinsic capping layer, in
the heavily doped delta layer, in the intrinsic layers on either
side and adjacent to the delta layer, in the bulk intrinsic buffer
layer, and in the diamond substrate. Surface conduction is
prevented by oxidation of the diamond surface, but no other paths
can be ignored. Conduction in the heavily doped delta layer is
characterized by low mobilities and high carrier concentrations,
typically 3 to 5 cm.sup.2/Vsec and 10.sup.20 cm-3. Conversely,
conduction in the CVD grown bulk intrinsic layers have high
mobilities, greater than 1000 cm.sup.2/Vsec, but low carrier
concentrations. Conduction in the diamond substrate may vary
depending on the unintentional doping of the HPHT substrate with B
and N, but generally, these substrates are insulating.
[0065] For Hall effect measurements with sheet carrier
concentrations below 10.sup.12 cm-2, the dominant conduction path
is likely the high mobility bulk intrinsic buffer and capping
layers, while for carrier concentrations above 10.sup.14 cm-2, the
dominant contribution is from conduction in the low mobility,
heavily doped delta layer.
[0066] The foregoing description of the embodiments of the
invention has been presented for the purposes of illustration and
description. Each and every page of this submission, and all
contents thereon, however characterized, identified, or numbered,
is considered a substantive part of this application for all
purposes, irrespective of form or placement within the
application.
[0067] The invention illustratively disclosed herein suitably may
be practiced in the absence of any element which is not
specifically disclosed herein and is not inherently necessary.
However, this specification is not intended to be exhaustive.
Although the present application is shown in a limited number of
forms, the scope of the invention is not limited to just these
forms, but is amenable to various changes and modifications without
departing from the spirit thereof. One of ordinary skill in the art
should appreciate after learning the teachings related to the
claimed subject matter contained in the foregoing description that
many modifications and variations are possible in light of this
disclosure. Accordingly, the claimed subject matter includes any
combination of the above-described elements in all possible
variations thereof, unless otherwise indicated herein or otherwise
clearly contradicted by context. In particular, the limitations
presented in dependent claims below can be combined with their
corresponding independent claims in any number and in any order
without departing from the scope of this disclosure, unless the
dependent claims are logically incompatible with each other.
* * * * *