U.S. patent number RE34,861 [Application Number 07/594,856] was granted by the patent office on 1995-02-14 for sublimation of silicon carbide to produce large, device quality single crystals of silicon carbide.
This patent grant is currently assigned to North Carolina State University. Invention is credited to Calvin H. Carter, Jr., Robert F. Davis, Charles E. Hunter.
United States Patent |
RE34,861 |
Davis , et al. |
February 14, 1995 |
**Please see images for:
( Certificate of Correction ) ** |
Sublimation of silicon carbide to produce large, device quality
single crystals of silicon carbide
Abstract
The present invention is a method of forming large device
quality single crystals of silicon carbide. The sublimation process
is enhanced by maintaining a constant polytype composition in the
source materials, selected size distribution in the source
materials, by specific preparation of the growth surface and seed
crystals, and by controlling the thermal gradient between the
source materials and the seed crystal.
Inventors: |
Davis; Robert F. (Raleigh,
NC), Carter, Jr.; Calvin H. (Raleigh, NC), Hunter;
Charles E. (Durham, NC) |
Assignee: |
North Carolina State University
(Raleigh, NC)
|
Family
ID: |
22350170 |
Appl.
No.: |
07/594,856 |
Filed: |
October 9, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
113565 |
Oct 26, 1987 |
0486600 |
Sep 12, 1989 |
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Current U.S.
Class: |
117/86; 117/105;
117/107 |
Current CPC
Class: |
C30B
23/00 (20130101); H01L 33/0054 (20130101); C30B
29/36 (20130101); Y10S 148/021 (20130101); Y10S
148/148 (20130101) |
Current International
Class: |
C30B
23/00 (20060101); H01L 021/365 () |
Field of
Search: |
;148/DIG.148
;156/610,612,614,DIG.64,DIG.68 ;437/100 ;117/105 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1467085 |
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Jul 1964 |
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DE |
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3230727 |
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Feb 1984 |
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DE |
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56-96883 |
|
Aug 1981 |
|
JP |
|
59-35099 |
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Aug 1982 |
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JP |
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62-66000 |
|
Mar 1987 |
|
JP |
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63-283014 |
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Nov 1988 |
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JP |
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Other References
Tairov et al.; Progress in Controlling the Growth of Polytypic
Crystals; Electrical Eng. Institute, Leningrad, p-22, 197022 USSR;
Aug. 24, 1982; pp. 111-161. .
Tairov et al.; General Principles of Growing Large-Size Single
Crystals of Various Silicon Carbide Polytypes; Jrnl of Crystal
Growth 52 (1981), pp. 146-150. .
Scace et al.; Solubility of Carbon in Silicon and Germanium; Jrnl
of Chemical Physics, vol. 60, No. 6, Jun., 1959, pp. 1551-1555.
.
Ziegler et al.; Single Crystal Growth of SiC Substrate Material for
Blue Light Emitting Diodes; Trans. on Electron Devices, vol. ED-30,
No. 4, Apr. 1983, pp. 277-281. .
Thermal Oxidation of 3C Silicon Carbide Single-Crystal Layers on
Silicon; Fung et al.; Appl. Phys. Lett. 45(7), Oct. 1, 1984; pp.
757-759. .
Metal-Oxide-Semiconductor Characteristics of Chemical Vapor
Deposited Cubic-SiC; Shibahara et al.; Japanese Jrnl. of Appl.
Physics; vol. 23, No. 11, pp. L862-L864, Nov. 1984. .
C-V Characteristics of SiC Metal-Oxide-Semiconductor Diode with a
Thermally Grown SiO.sub.2 Layer; Suzuki et al.; Appl. Phys. Lett.
vol. 39, No. 1; Jul. 1, 1981; pp. 89-90. .
Thermal Oxidation of SiC and Electrical Properties of Al-SiO.sub.2
-SiC MOS Structure; Suzuki et al.; Jap. Jrnl. of Appl. Physics;
vol. 21, No. 4, 4-82; pp. 579-585. .
Behavior of Inversion Layers in 3C Silicon Carbide; Avila et al.;
Appl. Phys. Lett. 49(6); Aug. 11, 1986; pp. 334-336..
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Primary Examiner: Fourson; George
Attorney, Agent or Firm: Bell, Seltzer, Park &
Gibson
Claims
That which is claimed is:
1. A method of reproducibly controlling the growth of large single
crystals of the use of impurities as a primary mechanism for
controlling polytype growth, and which crystals are suitable for
use in producing electrical devices, the method comprising:
introducing a monocrystalline seed crystal of silicon carbide of
desired polytype and a silicon carbide source powder into a
sublimation system;
raising the temperature of the silicon carbide source powder to a
temperature sufficient for the source powder to sublime; while
elevating the temperature of the growth surface of the seed crystal
to a temperature approaching the temperature of the source powder,
but lower than the temperature of the source powder and lower than
that at which silicon carbide will sublime under the gas pressure
conditions of the sublimation system; and
generating and maintaining a substantially constant flow of
vaporized Si, Si.sub.2 C, and SiC.sub.2 per unit area per unit time
from the source powder to the growth surface of the seed crystal
for a time sufficient to produce a desired amount of macroscopic
growth of monocrystalline silicon carbide of desired polytype upon
the seed crystal.
2. A method according to claim 1 further comprising the step of
preparing a polished seed crystal of silicon carbide prior to the
step of introducing the seed crystal of silicon carbide into the
closed system.
3. A method according to claim 1 wherein the step of introducing a
seed single crystal of silicon carbide into a closed system
containing silicon carbide source powder further comprises
initially segregating the source powder and the seed crystal from
one another.
4. A method according to claim 1 wherein the step of raising the
temperature of the silicon carbide source powder comprises raising
the temperature of the silicon carbide source powder to between
about 2250.degree. and 2350.degree. centigrade.
5. A method according to claim 1 wherein the step of raising the
temperature of the silicon carbide source powder comprises raising
the temperature of the silicon carbide source powder to about
2300.degree. centigrade.
6. A method according to claim 2 wherein the step of elevating the
temperature of the seed crystal comprises elevating the temperature
of the seed crystal to between about 2150.degree. and 2250.degree.
centigrade.
7. A method according to claim 2 wherein the step of elevating the
temperature of the seed crystal comprises elevating the temperature
of the seed crystal to about 2200.degree. centigrade.
8. A method according to claim 1 wherein the step of introducing a
single seed crystal of silicon carbide comprises introducing a seed
crystal for which a face corresponding to a low integer Miller
index face has been cut to expose a face which is nonperpendicular
to an axis normal to the low integer Miller index face which was
cut.
9. A method according to claim 1 wherein the step of generating and
maintaining a substantially constant flow of vaporized Si, Si.sub.2
C, and SiC.sub.2 per unit time comprises introducing a source
powder having a selected composition of polytypes and maintaining
the selected composition of polytypes in the source powder
substantially constant throughout the growth process.
10. A method according to claim 9 wherein the step of maintaining
the originally selected composition of polytypes in the source
powder comprises replenishing the source powder during the
sublimation process using source powder replenishment having a
composition of polytypes which will maintain the originally
selected composition of polytypes in the source powder
substantially constant in the sublimation system.
11. A method according to claim 1 wherein the step of generating
and maintaining a substantially constant flow of vaporized Si,
Si.sub.2 C, and SiC.sub.2 per unit area per unit time comprises
introducing a source powder having a selected predetermined
distribution of surface areas and maintaining the selected
distribution of surface areas in the source powder substantially
constant throughout the growth process.
12. A method according to claim 11 wherein the step of maintaining
the originally selected predetermined distribution of surface areas
comprises replenishing the source powder during the sublimation
process using source powder replenishment having a distribution of
surface areas which will maintain the originally selected
distribution of surface areas substantially constant in the source
powder in the sublimation system.
13. A method according to claim 1 wherein the step of generating
and maintaining a substantially constant flow of vaporized Si,
Si.sub.2 C, and SiC.sub.2 per unit area per unit time comprises
introducing a source powder having a selected predetermined
distribution of particle sizes and maintaining the selected
distribution of particle sizes in the source powder substantially
constant throughout the growth process.
14. A method according to claim 11 wherein the step of maintaining
the originally selected predetermined distribution of particle
sizes comprises replenishing the source powder during the
sublimation process using source powder replenishment having a
distribution of particle sizes which will maintain the originally
selected distribution of particle sizes substantially constant in
the source powder in the sublimation system.
15. A method according to claim 10, claim 12 or claim 14 wherein
the step of replenishing the source powder during the sublimation
process comprises feeding silicon carbide to the sublimation system
using a screw conveying mechanism.
16. A method according to claim 10, claim 12 or claim 14 wherein
the step of replenishing the source powder during the sublimation
process comprises feeding silicon carbide to the sublimation system
using ultrasonic energy to move silicon carbide powder into the
system.
17. A method according to claim 15 wherein the step of increasing
the temperature gradient between the seed crystal and the source
powder comprises increasing the temperature of the source powder
while maintaining the temperature of the growth surface of the seed
crystal at the initial lower temperature than the source
powder.
18. A method according to claim 15 wherein the step of introducing
the thermal gradient commprises introducing a thermal gradient of
20.degree. centrigrade per centimeter.
19. A method according to claim 15 wherein the step of increasing
the thermal gradient comprises increasing the thermal gradient from
about 20.degree. centigrade per centimeter to about 50.degree.
centigrade per centimeter.
20. A method according to claim 15 wherein the steps of raising the
temperature of the source powder, introducing a thermal gradient
and increasing the thermal gradient comprise using a resistance
heating device to raise the temperature, introduce the thermal
gradient and increase the thermal gradient.
21. A method according to claim 16 wherein the step of maintaining
a fixed thermal gradient between the growth surface of the seed
crystal and the source powder comprises providing relative movement
between the growth surface of the seed crystal and the source
powder as the seed crystal grows while maintaining the source
powder at the temperature sufficient for silicon carbide to sublime
and the seed crystal at the temperature approaching the temperature
of the source powder but lower than the temperature of the source
powder and lower than that at which silicon carbide will
sublime.
22. A method according to claim 16 wherein the step of maintaining
a fixed thermal gradient between the growth surface of the seed
crystal and the source powder comprises maintaining a fixed
distance between the growth surface of the seed crystal and the
source powder as the crystal grows.
23. A method according to claim 16 wherein the step of maintaining
a constant thermal gradient between the growth surface of the seed
crystal and the source powder comprises independently controlling
the source powder and seed crystal temperatures by separately
monitoring the temperature of the source powder and the temperature
of the seed crystal and separately adjusting the temperature of the
source powder and the temperature of the seed crystal.
24. A method according to claim 14 wherein the step of replenishing
the source powder during the sublimation process using source
powder having a selected distribution of particle sizes comprises
introducing silicon carbide powder having the following size
distribution as determined by the weight percentage of a sample
which will pass through a designated Tyler mesh screen:
25. A method according to claim 1 wherein the step of generating
and maintaining a substantially constant flow of vaporized Si,
Si.sub.2 C, and SiC.sub.2 per unit area per unit time from the
source powder to the growth surface of the seed crystal comprises
increasing the thermal gradient between the seed crystal and the
source powder as the crystal grows and the source powder is used up
to thereby maintain an absolute temperature difference between the
source powder and seed crystal which continues to be most favorable
for crystal growth and to continuously encourage further crystal
growth beyond that which would be obtained by maintaining a
constant temperature gradient.
26. A method according to claim 1 wherein the step of generating
and maintaining a substantially constant flow of vaporized Si,
Si.sub.2 C, and SiC.sub.2 per unit area per unit time from the
source powder to the growth surface of the seed crystal comprises
maintaining a constant thermal gradient as measured between the
growth surface of the seed crystal and the source powder as the
crystal grows and as the source powder is used up while maintaining
the growth surface of the seed crystal and the source powder at
their respective different temperatures to thereby maintain a
constant growth rate of the single seed crystal and a consistent
growth of a single polytype upon the single growth surface of the
seed crystal.
27. A method according to claim 1 including the step of rotating
the seed crystal as the seed crystal grows and as the source powder
is used up to thereby maintain a constant temperature profile
across the growth surface of the seed crystal, to dampen the effect
of flux variations, and to prevent the growing crystal from
becoming attached to undesired mechanical portions of the closed
system.
28. A method of reproducibly controlling the growth of large single
crystals of a single polytype of silicon carbide independent of the
use of impurities as a primary mechanism for controlling polytype
growth, and which crystals are suitable for use in producing
electical devices, the method comprising:
introducing a monocrystalline seed crystal of silicon carbide of
desired polytype and a silicon carbide source powder into a
sublimation system, with the source powder having a selected
composition of polytypes, a selected predetermined distribution of
surface areas, and a selected predetermined distribution of
particle sizes;
raising the temperature of the silicon carbide source powder to a
temperature sufficient for the source powder to sublime; while
elevating the temperature of the growth surface of the seed crystal
to a temperature approaching the temperature of the source powder,
but lower than the temperature of the source powder and lower than
that at which silicon carbide will sublime under the gas pressure
conditions of the sublimation system; and
maintaining the selected composition of polytypes in the source
powder substantially constant throughout the growth process;
while
maintaining the selected distribution of surface areas in the
source powder substantially constant throughout the growth process;
and while
maintaining the selected distribution of particle sizes in the
source powder substantially constant throughout the growth process,
to thereby generate and maintain a substantially constant flow of
vaporized Si, Si.sub.2 C, and SiC.sub.2 per unit area per unit time
from the source powder to the growth surface of the seed crystal,
and all for a time sufficient to produce a desired amount of
macroscopic growth of monocrystalline silicon carbide of desired
polytype upon the seed crystal. .Iadd.
29. A method according to claim 1 wherein the step of generating
and maintaining a substantially constant flow of vaporized Si,
Si.sub.2 C, and SiC.sub.2 per unit area per unit time from the
source powder to the growth surface of the seed crystal further
comprises introducing a thermal gradient between the source powder
and the seed crystal and then increasing the thermal gradient
between the seed crystal and the source powder as the crystal grows
and the source powder is used up to thereby maintain an absolute
temperature difference between the source powder and seed crystal
which continues to be most favorable for crystal growth and to
continuously encourage further crystal growth beyond that which
would be obtained by maintaining a constant temperature gradient.
.Iaddend.
Description
FIELD OF THE INVENTION
The present invention is a method for controlling the sublimation
growth of silicon carbide to produce high quality single
crystals.
BACKGROUND OF THE INVENTION
Silicon carbide is a perennial candidate for use as a semiconductor
material. Silicon carbide has a wide bandgap (2.2 electron volts in
the beta polytype, 2.8 in the 6H alpha), a high thermal
coefficient, a low dielectric constant, and is stable at
temperatures far higher than those at which other semiconductor
materials such as silicon remain stable. These characteristics give
silicon carbide excellent semiconducting properties, and electronic
devices made from silicon carbide can be expected to perform at
higher temperatures, and at higher radiation densities, than
devices made from the presently most commonly used semiconductor
materials such as silicon. Silicon carbide also has a high
saturated electron drift velocity which raises the potential for
devices which will perform at high speeds, at high power levels,
and its high thermal conductivity permits high density device
integration.
As is known to those familiar with solid state physics and the
behavior of semiconductors, in order to be useful as a material
from which useful electrical devices can be manufactured, the basic
semiconductor material must have certain characteristics. In many
applications, a single crystal is required, with very low levels of
defects in the crystal lattice, along with very low levels of
unwanted impurities. Even in a pure material, a defective lattice
structure can prevent the material from being useful for electrical
devices, and the impurities in any such crystal are preferably
carefully controlled to give certain electrical characteristics. If
the impurities cannot be controlled, the material is generally
unsatisfactory for use in electrical devices.
Accordingly, the availability of an appropriate crystal sample of
silicon carbide is a fundamental requirement for the successful
manufacture of devices from silicon carbide which would have the
desirable properties described above. Such a sample should be of a
single desired crystal polytype (silicon carbide can form in at
least 150 types of crystal lattices), must be of a sufficiently
regular crystal structure of the desired polytype, and must be
either substantially free of impurities, or must contain only those
impurities selectively added to give the silicon carbide any
desired n or p character.
Accordingly, and because the physical characteristics and potential
uses for such silicon carbide have been recognized for some time, a
number of researchers have suggested a number of techniques for
forming crystalline silicon carbide.
These techniques generally fall into two broad categories, although
it will be understood that some techniques are not necessarily so
easily classified. The first technique is known as chemical vapor
deposition ("CVD") in which reactant gases are introduced into some
sort of system within which they form silicon carbide crystals upon
an appropriate substrate. Novel and commercially significant
improvements in such CVD techniques are discussed in currently
co-pending applications which are assigned to the assignee of the
present invention, "Growth of Beta-SiC Thin Films and Semiconductor
Devices Fabricated Thereon." Ser. No. 113,921, filed Oct. 26, 1988;
and "Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor
Devices Fabricated Thereon." Ser. No. 113,573, filed Oct. 26,
1988.
The other main technique for growing silicon carbide crystals is
generally referred to as the sublimation technique. As the
designation sublimation implies and described. sublimation
techniques generally use some type of solid silicon carbide
material other than a desired single crystal of a particular
polytype, as a starting material, and then heat the starting
material until solid silicon carbide sublimes. The vaporized
material is then encouraged to condense, with the condensation
intended to produce the desired crystals.
As is known to those familiar with the physical chemistry of
solids, liquids and gases, crystal growth is encouraged when the
seed or surface upon which a crystal is being formed is at a
somewhat lower temperature than the fluid, either gas or liquid,
which carries the molecules or atoms to be condensed.
One technique for producing solid silicon carbide when crystal-type
impurity is of little consideration is the Acheson furnace process,
which is typically used to produce silicon carbide for abrasive
purposes. One of the first sublimation techniques of any practical
usefulness for producing better crystals, however, was developed in
the 1950's by J. A. Lely, one technique of whom is described in
U.S. Pat. No. 2,854,364. From a general standpoint, Lely's
technique lines the interior of a carbon vessel with a silicon
carbide source material. By heating the vessel to temperatures at
which silicon carbide sublimes, and then allowing it to condense,
recrystallized silicon carbide is encouraged to redeposit itself
along the lining of the vessel. Although the Lely process can
generally improve upon the quality of the source material, it has
to date failed to produce on a consistant or repeatable basis,
single crystals of silicon carbide suitable for electrical
devices.
Hergenrother, U.S. Pat. No. 3,228,756, discusses another
sublimation growth technique which utilizes a seed crystal of
silicon carbide upon which other silicon carbide can condense to
form the crystal growth. Hergenrother suggests that in order to
promote proper growth, the seed crystal must be heated to an
appropriate temperature, generally over 2000.degree. centigrade, in
such a manner that the time period during which the seed crystal is
at temperatures between 1800.degree. C. and 2000.degree. C. is
minimized.
Ozarow, U.S. Pat. No. 3,236,780, discusses another unseeded
sublimation technique which utilizes a lining of silicon carbide
within a carbon vessel, and which attempts to establish a radial
temperature gradient between the silicon carbide-lined inner
portion of the vessel and the outer portion of the vessel.
Knippenberg, U.S. Pat. No. 3,615,930 and 3,962,406, discuss
alternative attempts at growing silicon carbide in a desired
fashion. The '930 patent discusses a method of growing p-n
junctions in silicon carbide as a crystal grows by sublimation.
According to the discussion in this patent, silicon carbide is
heated in an enclosed space in the presence of an inert gas
containing a donor-type dopant atom, following which the dopant
material is evacuated from the vessel and the vessel is reheated in
the presence of an acceptor dopant. This technique is intended to
result in adjacent crystal portions having opposite conductivity
types and forming a p-n junction.
In the '406 patent, Knippenberg discusses a three-step process for
forming silicon carbide in which a silicon dioxide core is packed
entirely within a surrounding mass of either granular silicon
carbide or materials which will form silicon carbide when heated.
The system is heated to a temperature at which a silicon carbide
shell forms around the silicon dioxide core, and then further
heated to vaporize the silicon dioxide from within the silicon
carbide shell. Finally, the system is heated even further to
encourage additional silicon carbide to continue to grow within the
silicon carbide shell.
Vodadkof, U.S. Pat. No. 4,147,572, discusses a geometry-oriented
sublimation technique in which solid silicon carbide source
material and seed crystals are arranged in parallel close proximity
relationship to one another.
Addamiano, U.S. Pat. No. 4,556,436, discusses a Lely-type furnace
system for forming thin films of beta silicon carbide on alpha
silicon carbide which is characterized by a rapid cooling from
sublimation temperatures of between 2300.degree. centigrade and
2700.degree. centigrade to another temperature of less than
1800.degree. centigrade. Addamiano notes that large single crystals
of cubic (beta) silicon carbide are simply not available and that
growth of silicon carbide on other materials such as silicon or
diamond is rather difficult.
Hsu, U.S. Pat. No. 4,664,944, discusses a fluidized bed technique
for forming silicon carbide crystals which resembles a chemical
vapor deposition technique in its use of non-silicon carbide
reactants, but which includes silicon carbide particles in the
fluidized bed, thus somewhat resembling a sublimation
technique.
Some of the more important work in the silicon carbide sublimation
techniques, however, is described in materials other than United
States patents. For example, German (Federal Republic) Patent No.
3,230,727 to Siemens Corporation discusses a silicon carbide
sublimation technique in which the emphasis of the dicussion is the
minimization of the thermal gradient between silicon carbide seed
crystal and silicon carbide source material. This patent suggests
limiting the thermal gradient to no more than 20.degree. centigrade
per centimeter of distance between source and seed in the reaction
vessel. This patent also suggests that the overall vapor pressure
in the sublimation system be kept in the range of between 1 and 5
millibar and preferably around 1.5 to 2.5 millibar.
This German technique, however, can be considered to be a
refinement of techniques thoroughly studied in the Soviet Union,
particularly by Y. M. Tairov; see e.g. General Principles of
Growing Large-Size Single Crystals of Various Silicon Carbide
Polytypes, J. Crystal Growth, 52 (1981)46-150, and Progress in
Controlling the Growth of Polytypic Crystals, from Crystal Growth
and Characterization of Polytype Structures, edited by P. Krishna,
Pergammon Press, London, 1983, p. 111. Tairov points out the
disadvantages of the Lely method, particularly the high
temperatures required for crystal growth (2600.degree.-2700.degree.
C.) and the lack of control over the resulting crystal polytype. As
discussed with reference to some of the other investigators in
patent literature, Tairov suggests use of a seed as a method of
improving the Lely process. In particular, Tairov suggests
controlling the polytype growth of the silicon carbide crystal by
selecting seed crystals of the desired polytype or by growing the
recondensed crystals on silicon carbide faces worked at an angle to
the 0001 face of the hexagonal lattice. Tairov suggests axial
temperature gradients for growth of between approximately
30.degree. and 40.degree. centigrade per centimeter.
In other studies, Tairov investigated the effects of adjusting
various parameters on the resulting growth of silicon carbide,
while noting that particular conclusions are difficult to draw.
Tairov studied the process temperatures and concluded that growth
process temperature was of relatively smaller importance than had
been considered by investigators such as Knippenberg. Tairov
likewise was unable to draw a conclusion as to the effect of growth
rate on the formation of particular polytypic crystals, concluding
only that an increase in crystal growth rate statistically
corresponds to an increase in the percentage of disordered
structured crystals. Tairov was similarly unable to draw any
conclusions between vapor phase stoichiometry and crystal growth,
but pointed out that certain impurities will favor the growth of
particular silicon carbide polytype crystals. For example, high
nitrogen concentrations favor cubic polytype silicon carbide
crystals, aluminum and some other materials favor the growth of
hexagonal 4H polytype, and oxygen contributes to the 2H polytype.
Tairov concluded that no understanding of the mechanisms leading to
these effects had yet been demonstrated.
In Tairov's experiments, he also attempted using silicon carbide
single crystals of particular polytypes as the vapor source
material and suggested that using such single crystals of
particular polytypes as vapor sources could result in particular
polytypes of crystal growth. Of course, it will be understood that
although the use of single crystals as source materials is
theoretically interesting, a more practical goal, particularly from
a commercial standpoint, is the production of single crystals from
more common sources of silicon carbide other than single
crystals.
Finally, Tairov concluded that the treatment of the substrate
surface upon which sublimation growth was directed could affect the
growth of the resulting crystals. Nevertheless, the wide variety of
resulting data led Tairov to conclude that additional unidentified
factors were affecting the growth he observed in silicon carbide
crystals, and these unknown factors prevented him from reaching a
fundamental understanding of the mechanisms of crystal growth.
Therefore, in spite of the long recognized characteristics of
silicon carbide, and the recognition that silicon carbide could
provide an outstanding, if not revolutionary, semiconductor
material and resulting devices, and in spite of the thorough
investigations carried out by a number of researchers including
those mentioned herein, prior to the present invention there
existed no suitable technique for repeatedly and consistently
growing large single crystals of desired selected polytypes of
silicon carbide.
Accordingly, it is an object of the present invention to provide a
method for the controlled, repeatable growth of large single
crystals of silicon carbide of desired polytypes.
It is a further object of the present invention to provide a method
of growing large single crystals of silicon carbide by controlling
the polytype of the source material.
It is another object of this invention to provide a method of
growing such silicon carbide single crystals using source materials
other than single crystals of silicon carbide.
It is a further object of this invention to provide a method of
growing such silicon carbide crystals by selecting source materials
having a particular surface area.
It is another object of this invention to provide a method of
growing large silicon carbide single crystals by selecting source
materials with predetermined particle size distributions.
It is a further object of this invention to provide a method of
growing such silicon carbide single crystals using sublimation
techniques and in which the thermal gradient between the source
materials and the seed is continuously adjusted to maintain the
most favorable conditions possible for continued growth of silicon
carbide crystals over longer time periods and into larger crystals
than have previously ever been accomplished.
The foregoing and other objects, advantages and features of the
invention, and the manner in which the same are accomplished will
become more readily apparent upon consideration of the following
detailed description of the invention taken in conjunction with the
accompanying drawings, which illustrate preferred and exemplary
embodiments and wherein:
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional diagram of a sublimation crucible used
in accordance with the method of the present invention;
FIG. 2 is an enlarged view of the seed crystal holder of the
crucible of FIG. 1;
FIG. 3 is a cross-sectional diagram of a sublimation furnace used
in accordance with the method of the present invention;
FIG. 4 is a diagram of a sublimation system illustrating a screw
type mechanism for continuously introducing silicon carbide source
powder into a system;
FIG. 5 is a diagram of a sublimation system showing a gas feed
mechanism for introducing silicon carbide precursor materials into
the sublimation system; and
FIG. 6 is a diagram of a sublimation system illustrating
independent heating elements used in accordance with the method of
the present invention.
DETAILED DESCRIPTION
FIG. 1 illustrates a cross-sectional view of a sublimation crucible
used in accordance with the method of the present invention. The
crucible is broadly designated at 10 and is typically formed of
graphite. Crucible 10 is generally cylindrical in shape and
includes a porous graphite liner 11, a lid 12, and a seed holder
13, an enlarged view of which is illustrated in FIG. 2. The
remainder of the crucible is defined by the walls 14 and the floor
15. As further illustrated in FIG. 1, the porous graphite liner 11
is formed in such a manner as to provide an annular chamber 16
between lower portions of the porous graphite liner 11, the
crucible walls 14 and the crucible lid 12. A central sublimation
chamber is illustrated at 20.
In all of the apparatus described herein, the crucibles described
are preferably formed of graphite and most preferably of a graphite
which has approximately the same coefficient of thermal expansion
as silicon carbide. Such materials are commercially available. The
relative similarities of thermal coefficients of expansion are a
particular requirement for materials which are being heated to the
extremely high temperatures described herein and at which these
processes take place. In this manner, the crucible can be prevented
from cracking during the sublimation process and the lifetime of
the crucible will generally be increased.
Furthermore, as is recognized by those familiar with attempts at
growing silicon carbide crystals, the presence of graphite in the
system encourages the growth of silicon carbide by providing an
equilibrium source of carbon atoms as the sublimation process takes
place and by dampening variations in the flux.
Furthermore, graphite is one of the few economically viable
materials which can both withstand the high temperatures of these
processes and avoid introducing undesired impurities into the vapor
flux.
the seed holder 13 is illustrated in more detail in FIG. 2. A seed
crystal 17 rests on upper portions of the seed holder 13 which
extend into the chamber 20. A graphite washer 21 is positioned
between the lower portions of the seed holder 13 and the floor of
the crucible 15. FIG. 2 also shows an optical opening 22, which in
preferred embodiments of the invention provides optical access to
the seed so that the temperature of the seed can be monitored with
an optical pyrometer.
A sublimation crucible such as illustrated in FIG. 1 is typically
used in conjunction with a sublimation furnace broadly designated
at 23 in FIG. 3, in which the crucible is again designated 10.
Furnace 23 is generally cylindrical in shape and includes a
cylindrical heating element 24, opposite portions of which are
shown in the drawing. Furnace 23 is also surrounded by carbon fiber
insulation 25 and includes optical ports 26, 27, and 28 through
which optical pyrometers can measure the temperature of portions of
the interior of the furnace. A power feed-through is generally
designated at 30 and the outer housing of the furnace at 31.
In a first embodiment of the invention, a single seed crystal of
silicon carbide having a desired polytype and silicon carbide
source power are introduced into a system such as the sublimation
crucible and furnace illustrated in FIGS. 1-3. Where the crucible
is of the type illustrated in FIG. 1, the silicon carbide source
powder is positioned in the annular chamber 16. In this first
embodiment of the invention, it has been discovered that by
utilizing silicon carbide source powder substantially all of which
has a constant polytype composition, the production of a desired
crystal growth upon the seed crystal can be greatly improved.
Although applicant does not wish to be bound by any particular
theory, it is known that different polytypes of silicon carbide
have different evaporation activation energies. Specifically, for
cubic (3C) silicon carbide the evaporation activation energy is 108
kilocalories (kcal) per mole; for hexagonal 4H silicon carbide, 144
kcal/mole; and for hexagonal 6H silicon carbide, 199 kcal/mole.
These differences are important, because when silicon carbide
sublimes, it forms three basic vaporized materials: Si, Si.sub.2 C,
and SiC.sub.2. Depending upon the polytype of the source powder,
the amount or "flux" of each of the species which is generated will
differ. In a corresponding manner, the amount of each of the
species in the overall vapor flux will tend to influence the type
of polytypes which will grow when the species recondense.
As used herein, the term "flux" refers to the amount of matter or
energy passing through a designated plane of a given area during a
given period of time. Accordingly, when used to describe the flow
of vaporized species, flux can be measured and designated in units
of matter, area and time such as grams per square centimeter per
second (g/cm.sup.2 /sec).
As used herein, the term "constant polytype composition" refers to
a source powder or powders which are made up of a constant
proportion of certain polytypes, including single polytypes. For
example, a source powder which was formed substantially entirely of
6H alpha silicon carbide would exhibit a constant polytype
composition, as would source powder that was 50 percent alpha
polytype and 50 percent beta polytype. In other words, the
composition--whether homogeneous or heterogeneous with respect to
polytypes--must be controlled so as to remain the same throughout
the sublimation process.
Stated more directly, if the source powder is selected and
controlled so that substantially it has a constant polytype
composition, the relative amounts or ratios of Si, Si.sub.2 C, and
SiC.sub.2 which are generated will remain constant and the other
parameters of the process can be appropriately controlled to result
in the desired single crystal growth upon the seed crystal.
Alternatively, if the source powder is a variable mixture of
various proportions of polytypes of silicon carbide, the relative
amounts (ratios) of Si, Si.sub.2 C, and SiC.sub.2 which are
generated will continually vary and correspondingly continually
encourage alternative polytypes to simultaneously grow upon the
seed crystal. This results in growth upon the seed crystal of a
number of crystals of different polytypes, an undesirable
result.
Once the silicon carbide source powder and the seed crystal are
introduced, the temperature of the silicon carbide source powcer is
raised to a temperature sufficient for silicon carbide to sublime
from the source powder, typically a temperature on the order of
2300.degree. C. While the temperature of the source powder is being
raised, the temperature of the growth surface of the seed crystal
is likewise raised to a temperature approaching the temperature of
the source powder, but lower than the temperature of the source
powder and lower than that at which silicon carbide will sublime.
Typically, the growth surface of the seed crystal is heated to
about 2200.degree. C. By maintaining the silicon carbide source
powder and the growth surface of the silicon carbide seed crystal
at their respective temperatures for a sufficient time, macroscopic
growth of monocrystalline silicon carbide of a desired polytype
will form upon the seed crystal.
It will be understood by those familiar with phase changes that
sublimation and condensation are equilibrium processes, and are
affected by the vapor pressure of a system as well as absolute and
relative temperatures. Accordingly, it will be further understood
that in the processes and systems described herein, the vapor
pressures are suitably controlled in a manner which permits these
processes to proceed and be controlled and adjusted based upon the
temperature and thermal gradient considerations described
herein.
Further to the present invention, it has been discovered that in
addition to maintaining a constant polytype composition, in order
to form appropriate single crystals by the sublimation method,
selecting silicon carbide source powder of a consistent particle
size distribution similarly enhances the technique.
In a manner similar to that set forth earlier, the control of
particle size in a consistent manner results in a consistent flux
profile of the species which evolve from the silicon carbide source
powder, with a corresponding consistency in the sublimation growth
of silicon carbide upon the seed crystal. In one embodiment, a
powder having the following particle size distribution enhanced the
process, the distribution being defined by the weight percentage of
a sample which will pass through a designated Tyler mesh
screen:
______________________________________ Tyler Mesh Screen Weight
Percent Passed ______________________________________ 20-40 43%
40-60 19% 60-100 17% Over 100 21%
______________________________________
Additionally, for a given powder morphology, the exposed surface
area of the source powder is proportional to the particle size. A
consistency in exposed surface area in turn enhances the overall
consistency of the vapor flux, so that controlling the size
distribution in this manner enhances the consistency of the flux
profile.
As in the other embodiments discussed, the silicon carbide source
powder and the growth face of the seed crystal are both heated to
respective different temperatures, with the growth face of the seed
crystal being somewhat cooler than the source powder so as to
encourage condensation of the sublimed species from the source
powder onto the seed crystal.
In another embodiment of the invention, it has been discovered that
controlling the thermal gradient between the growth surface of the
seed crystal and the source powder results in appropriate control
and growth of large single crystals having a desired polytype. In
this respect, the thermal gradient can be controlled in a number of
ways. For example, under certain circumstances the thermal gradient
is controlled so as to remain constant between the growth surface
of the seed crystal while under other circumstances, controllably
changing the thermal gradient between the source powder and the
growth surface of the seed crystal is preferred.
As is known to those familiar with various sublimation techniques,
a thermal gradient is often introduced by physically separating the
source powder from the seed crystal while they are being maintained
at their respective different temperatures. The resulting thermal
gradient is thus a function of geometric separation between the
source powder and the growth surface of the seed crystal; e.g.
20.degree. C. per centimeter and the like. Thus, if the source
powder is initially maintained at a temperature of, for example,
2300.degree. C., and the growth surface of the seed crystal is
maintained at a temperature of, for example, 2200.degree. C. and a
distance of 10 centimeters is initially maintained between the
source powder and the seed crystal, a thermal gradient of
100.degree. C. divided by 10 centimeters, i.e. 10.degree. C. per
centimeter, will be established.
In one embodiment of thermal gradient control, the invention
comprises introducing the seed single crystal of silicon carbide of
a desired polytype and a silicon carbide source powder into a
sublimation system. The temperature of the silicon carbide source
powder is raised to a temperature sufficient for the silicon
carbide to sublime and a thermal gradient is introduced between the
growth surface of the seed crystal and the source powder by
elevating the temperature of the seed crystal to a temperature
approaching the temperature of the source powder, but lower than
the temperature of the source powder and lower than that at which
silicon carbide will sublime, under the vapor pressure conditions
of the system. As the crystal grows and the source powder generally
nearest the top of the crucible is used up, the thermal gradient
between the growth surface of the seed crystal and the source
powder is increased to thereby continuously encourage further
crystal growth beyond that which would be obtained by maintaining a
constant thermal gradient.
During the sublimation growth process, gas species which contain
silicon carbide evolve near the hotter top of the crucible and are
transported via the thermal gradient to the seed at its respective
lower temperature in the cooler lower portion of the crucible. The
source material, however, is also in the thermal gradient and
sublimation of the source material tends to occur at a much faster
rate in the upper portion of the source material than in the lower
portion. As a result if the temperature gradient remains constant,
a rapid decrease in flux with time occurs as the upper source
material is depleted. In a similar manner, as the crystal grows,
its growth surface increases in temperature as a result of its
change in position with respect to the thermal gradient. This
causes a decrease in the sticking coefficient as a function of time
and likewise reduces the growth rate.
According to the present invention, however, it has been discovered
that if the thermal gradient is continually increased as the source
powder is depleted and as the seed crystal grows, the absolute
temperature difference between the source and seed can be
maintained at an amount which continues to be most favorable for
crystal growth.
In one embodiment of the invention, control of the thermal gradient
comprises the step of increasing the thermal gradient between the
growth surface of the seed crystal and the source powder, and the
same is accomplished by increasing the temperature of the source
powder while maintaining the temperature of the growth surface of
the seed crystal at the initial lower temperature than the source
powder.
In another embodiment, the invention comprises maintaining a
constant thermal gradient as measured between the growth surface of
the seed crystal and the source powder as the crystal grows and as
the source powder is used up. It will be understood that the
temperature of the growth surface is the most critical temperature
with respect to the crystal as the growth surface is the surface at
which thermodynamic conditions will either favor or disfavor
continued desired growtth of the crystal.
Accordingly, in another embodiment of the invention, the step of
maintaining a fixed thermal gradient between the growth surface of
the seed crystal and the source powder comprises providing relative
movement between the growth surface of the seed crystal and the
source powder as the seed crystal grows while maintaining the
source powder and the growth face of the seed crystal at their
respective different, but constant, temperatures.
In another embodiment, the step of maintaining a fixed thermal
gradient between the growth surface of the seed crystal and the
source powder comprises maintaining a fixed geometric distance
between the growth surface of the seed crystal and the source
powder as the crystal grows.
In yet another embodiment, the method of maintaining a constant
thermal gradient between the growth surface of the seed crystal and
the source powder can comprise independently controlling the source
powder and seed crystal temperatures by separately monitoring the
temperature of the source powder and the temperature of the seed
crystal and separately adjusting the temperature of the source
powder and the temperature of the seed crystal maintain the desired
thermal gradient.
In another embodiment of the invention, it has been discovered that
growth of the single crystal of silicon carbide can be enhanced
using the methods of the present invention by providing a silicon
carbide seed crystal which presents a sublimation surface which is
slightly off-axis with respect to one of the Miller index faces. In
effect, off-axis silicon carbide crystals tend to transfer three
dimensional crystalographic information to the condensing atoms
during sublimation. Accordingly, such an off axis growth surface
can be used to encourage the repeatable growth of a desired
specific silicon carbide polytype. This technique is particularly
important when a silicon carbide crystal is being doped with an
impurity during sublimation growth. As is known to those familiar
with the properties of silicon carbide, particular impurities tend
to encourage the growth of specific polytypes of silicon carbide.
For example, doping with aluminum is known to favor growth of 4H
silicon carbide, but 6H crystals of silicon carbide can be grown
with aluminum doping according to the present invention if an
off-axis seed is used.
It has further been discovered according to the present invention
that the thermal gradient control and indeed the entire process of
controlling and maintaining temperatures can be enhanced by using
resistance heating, rather than radio frequency (RF) induction
heating in the method of the present invention.
Resistance heating offers a number of advantages in the overall
sublimation process. First, resistance heating allows the process
to be scaled up to larger crystal diameters than can be handled
using induction heating. Induction heating techniques have several
limitations which prevent any silicon carbide sublimation processes
developed using induction techniques from being similarly scaled up
to useful commercial scales. For example, in induction heating, the
induction coil must be positioned outside of the vacuum vessel in
which the sublimation takes place in order to prevent ionization of
the gas (e.g. argon) present in the vessel. Secondly, if the
diameter of the sublimation crucibles are increased, the coils used
in the induction heating tend to heat only the outside layer of the
crucible resulting in an undesirable and unacceptable radial
thermal gradient. Finally, induction heating requires the use of a
glass vacuum vessel to transmit the RF power. As a result, in order
to prevent the glass vessel from overheating, either the thermal
insulation present must be increased in thickness or the glass must
be cooled, typically with water. Increasing the amount of thermal
insulation reduces the practical size of the crystal that can be
grown, and cooling the vessel with water dramatically reduces the
energy efficiency of the entire system.
Alternatively, resistance heating is significantly more energy
efficient than induction heating, resistance heating elements can
be present within the vacuum vessel, skin heating or radial thermal
gradient effects are almost entirely eliminated, and resistance
heating permits improved temperature stability and repeatability of
processes and control over the entire thermal gradient.
FIGS. 4, 5 and 6 illustrate some of the apparatus which can be used
to accomplish the methods of the present invention. FIG. 4 shows a
silicon carbide seed crystal 32 upon which a growing crystal 33 has
epitaxially attached. The respective crystals 32 and 33 are
maintained upon a graphite seed holder 34 which in turn is
positioned upon a shaft 35. The remainder of the crucible is
defined by graphite walls 36 and a porous graphite barrier 37. The
silicon carbide source powder 40 is maintained in a bed 41. In
order to ensure a constant supply of silicon carbide powder to a
desired position, a rotating shaft 42 which carries a screw lifting
mechanism 43 is positioned with a high density graphite cylinder
44. As illustrated in FIG. 4, as shaft 42 rotates, the screw
mechanism 43 will lift silicon carbide source powder 40 to the top
of the screw mechanism to a position adjacent the porous graphite
barrier 37. As described earlier, in particular embodiments, the
silicon carbide source powder at the top of the high density
graphite cylinder 44 is maintained at a temperature of about
2300.degree. C., while the temperature of the growth surface of the
growing crystal 33 is maintained at a somewhat lower temperature,
typically 2200.degree. C.
Moving a continuous supply of silicon carbide source powder to the
sublimation region offers several advantages. In particular, and as
set forth with respect to the other techniques disclosed herein,
the continuous supply further ensures that the subliming source
powder generates a consistent flux density. In effect, new source
powder is continuously moved into the sublimation area, providing a
constant flux as sublimation proceeds.
An optical sight hole 45 is also illustrated, and can be used to
either monitor the temperature of the growing crystal 33 using an
optical pyrometer or to determine the exact position of the crystal
with respect to the silicon carbide source powder 40 at the top of
the high density graphite cylinder 44.
In certain embodiments of the invention, the shaft 35 can be pulled
in a manner which moves the growth face of the growing crystal 33
away from, or if desired towards, the silicon carbide source powder
40.
In yet another embodiment of the invention, the shaft can be
rotated to ensure that the temperature profile across the growth
face is constant. In such a manner, the crystal can be encouraged
to grow symmetrically as the effect of flux variations are dampened
out and the growing crystal can be prevented from attaching itself
to the graphite enclosure.
FIG. 6 illustrates a number of the same features as FIG. 4, but
with the separate and independent heating elements illustrated. In
FIG. 6, the separate and independently controlled resistance
heating elements are shown at 46 and 47. As described earlier
herein, the upper element 46 can be used to control the temperature
of the seed crystal 32 and the growing crystal 33, while the lower
heating element 47 can be used to control the temperature of the
silicon carbide source powder 40 at the top of the high density
graphite cylinder 44.
In order to monitor the respective temperatures generated by
heating elements 46 and 47, optical sight holes 50 and 51 are
provided to permit optical pyrometers to monitor the temperatures
generated.
FIG. 5 illustrates an apparatus used to carry out yet another
embodiment of the invention. In this embodiment, the silicon
carbide which sublimes and then recondenses as the growing crystal,
is not supplied as a powder, but instead is introduced into the
system by providing respective gas feeds of silane (SiH.sub.4) and
ethylene (C.sub.2 H.sub.4) into the system at a temperature at
which they will immediately react to form silicon carbide vapors
which will then migrate in the manner in which vapors generated
from source powders will migrate through the porous graphite
barrier and onto the growing crystal.
As in the earlier described embodiments, the system includes seed
crystal 32, growing crystal 33, graphite seed holder 34, shaft 35,
graphite walls 36, porous graphite barrier 37, and the optical
sight hole 45. Instead of a bed of silicon carbide source powder,
however, the system includes a silane gas feed 52 and an ethylene
gas feed 53. In order to keep these molecules from dissociating
under the high temperatures of the system, they are insulated in a
water cooled molybdenum jacket until they reach a point in the
sublimation system where the temperature is maintained at
approximately 2400.degree. C., and at which the materials are
released and immediately react to form silicon carbide.
Once the silane and ethylene have left the jacket 54 and have
reacted to form silicon carbide containing species, they behave in
the same manner as would silicon carbide containing species which
had sublimed from a source powder. They pass through the porous
graphite barrier 37 and lodge upon the growth face of the growing
crystal 33.
The use of such a gas feed system for sublimation purposes offers
several advantages, the primary one being the delivery of a
constant flux of SiC vapor to the growing crystal surface. Another
advantage is the high purity in which silane and ethylene can be
obtained in commercial quantities so that a resultingly pure
crystal results from this technique.
EXAMPLE 1
A seed was prepared from a 6H alpha polytype silicon carbide. The
seed crystal was lapped to insure flatness and then polished with
progressively smaller sized diamond paste, finishing with a 0.1
micrometer paste. The seed was cleaned in hot sulfuric acid
(H.sub.2 SO.sub.4) for a period of five minutes, in a one-to-one
mixture of ammonium hydroxide (NH.sub.4 OH) and hydrogen peroxide
(H.sub.2 O.sub.2) for five minutes, in hydrofluroic acid (HF) for
one minute, and then finally rinsed in deionized water. The seed
was oxidized in dry oxygen at 1200.degree. C. for 90 minutes to
remove residual polishing damage. The oxide was removed by etching
with HF.
The seed and source powder were then loaded into the crucible. The
source powder consisting of 6H silicon carbide grains having the
following size distribution:
______________________________________ Percentage Passing Through
Tyler Mesh Size (By Weight) ______________________________________
20-40 43 percent 40-60 19 percent 60-100 17 percent Over 100 21
percent ______________________________________
The loaded crucible was then placed in the sublimation furnace
while a slight overpressure of argon was maintained in the furnace
to inhibit water contamination, and thus reducing the furnace pump
down time. The furnace was evacuated to a base pressure below
5.times.10.sup.-6 Torr. The furnace was heated in a vacuum
(5.times.10.sup.-4 Torr) to 1200.degree. C. for about ten minutes.
It will be understood by those familiar with low pressure systems
that an absolute vacuum can never be achieved. Therefore, the term
"vacuum" as used herein refers to various systems which are at
pressures less than atmospheric pressure, and where appropriate,
specific pressures will be employed to best describe the particular
conditions. The furnace was then backfilled with argon to a
pressure of 400 Torr.
The temperature of the system was then increased until the top of
the crucible is approximately 2260.degree. C. and the temperature
of the seed is approximately 2160.degree. C., which in the
particular system used corresponded to a thermal gradient of
31.degree. C. per centimeter (cm). The system was then evacuated
slowly over a period of 85 minutes from the pressure of 400 Torr to
a pressure of about 10 Torr. The system was maintained under these
conditions for six hours, after which the system was backfilled
with argon to 760 Torr and the temperature reduced to 200.degree.
C. over a period of 90 minutes.
When the furnace was unloaded, the process had resulted in a
transparent 6H alpha silicon carbide crystal 12 millimeters (mm) in
diameter and 6 mm thick.
EXAMPLE 2
A 6H Alpha-SiC seed was prepared by cutting the (0001) plane
3.degree. towards the [1120] direction. The seed was then lapped to
assure flatness, polished with progressively smaller diamond paste,
cleaned, oxidized and etched, all as described in Example 1.
The source material was doped with aluminum in a quantity of 0.2
weight percent. The seed and source powder having the same powder
size distribution as set forth in Example 1. The crucible was
loaded, the vessel evacuated, initially heated, and backfilled with
argon, all as set forth in Example 1.
The temperature was then increased until the top of the crucible
was 2240.degree. C. and the seed was 2135.degree. C., corresponding
to a thermal gradient of 32.degree. C./cm.
The furnace was evacuated from 400 Torr to 10 Torr as described in
Example 1 and the sublimation conditions were maintained for a
period of four hours. The furnace was then backfilled with argon to
atmospheric pressure (760 Torr) and the temperature reduced to
200.degree. C. over a period of 90 minutes.
When the furnace was unloaded, the process had resulted in a dark
blue 6H Alpha-SiC crystal 12 mm in diameter and 6 mm thick. The
resulting crystal was P type and had a carrier concentration of
approximately 10.sup.18 carrier atoms per cubic centimeter.
In the description, there have been set forth preferred and
exemplary embodiments of the invention which are set forth by way
of example and not by way of limitation, the scope of the invention
being set forth in the following claims.
* * * * *