U.S. patent application number 10/949577 was filed with the patent office on 2005-06-09 for method and apparatus for growing silicon carbide crystals.
Invention is credited to Kordina, Olle Claes Erik, Paisley, Michael James.
Application Number | 20050120943 10/949577 |
Document ID | / |
Family ID | 23645557 |
Filed Date | 2005-06-09 |
United States Patent
Application |
20050120943 |
Kind Code |
A1 |
Kordina, Olle Claes Erik ;
et al. |
June 9, 2005 |
Method and apparatus for growing silicon carbide crystals
Abstract
A method and apparatus for controlled, extended and repeatable
growth of high quality silicon carbide boules of a desired polytype
is disclosed which utilizes graphite crucibles coated with a thin
coating of a metal carbide and in particular carbides selected from
the group consisting of tantalum carbide, hafnium carbide, niobium
carbide, titanium carbide, zirconium carbide, tungsten carbide and
vanadium carbide.
Inventors: |
Kordina, Olle Claes Erik;
(Durham, NC) ; Paisley, Michael James; (Garner,
NC) |
Correspondence
Address: |
SUMMA & ALLAN, P.A.
11610 NORTH COMMUNITY HOUSE ROAD
SUITE 200
CHARLOTTE
NC
28277
US
|
Family ID: |
23645557 |
Appl. No.: |
10/949577 |
Filed: |
September 24, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10949577 |
Sep 24, 2004 |
|
|
|
09415402 |
Oct 8, 1999 |
|
|
|
6824611 |
|
|
|
|
Current U.S.
Class: |
117/11 |
Current CPC
Class: |
Y10S 117/90 20130101;
C30B 25/00 20130101; Y10S 117/902 20130101; C30B 23/00 20130101;
C30B 29/36 20130101 |
Class at
Publication: |
117/011 |
International
Class: |
C30B 001/00; C30B
023/00; C30B 025/00; C30B 028/12; C30B 028/14 |
Claims
1. A seeded SiC crystal growth system for high temperature SiC
crystal growth, comprising: a container for receiving a silicon
carbide seed crystal, said container comprising: a first inlet to
the interior of said container; a second inlet to the interior of
said container; a graphite core; and a coating on said graphite
core, said coating being characterized by a melting point above the
sublimation temperature of SiC, chemical inertness with respect to
silicon and hydrogen at the sublimation temperature of SiC, and a
coefficient of thermal expansion sufficiently similar to said
graphite core to prevent cracking between said graphite core and
said coating during heating and cooling of said container to and
from the sublimation temperature of SiC; a source of silicon
connected to said first inlet of said container; and a source of
carbon connected to said second inlet of said container.
2. The seeded SiC crystal growth system according to claim 1,
wherein said source of silicon is silane.
3. A seeded SiC crystal growth system according to claim 1 wherein
said coating comprises a refractory metal compound selected from
the group consisting of tantalum carbide, hafnium carbide, niobium
carbide, titanium carbide, zirconium carbide, tungsten carbide,
vanadium carbide, tantalum nitride, hafnium nitride, niobium
nitride, titanium nitride, zirconium nitride, tungsten nitride,
vanadium nitride and mixtures thereof.
4. A seeded SiC crystal growth system according to claim 1 wherein
said coating comprises tantalum carbide.
5. A seeded SiC crystal growth system according to claim 1 further
comprising: a heating element for heating said container; a seed
holder attached to the interior of said container; and a seed
crystal on said seed holder; wherein vaporized species containing
silicon and carbon enter said system and propagate sublimation
growth of silicon carbide on said seed crystal.
6. A seeded SiC crystal growth system according to claim 5, wherein
said inlets provide separate pathways for a silicon source gas and
a carbon source gas to enter said core.
7. A seeded SiC crystal growth system, comprising: a graphite
enclosure having an inner surface defining a hollow reaction area
therein; a coating on said inner surface, said coating comprising a
material characterized by a melting point above the sublimation
temperature of silicon carbide; a first pathway for introducing a
silicon source gas to the reaction area; a second pathway for
introducing a carbon source gas to the reaction area; a heating
element for heating the reaction area to a temperature sufficient
to allow the silicon source gas and carbon source gas to form
vaporized species of silicon carbide in the reaction area; and a
silicon carbide seed crystal positioned in sufficient proximity to
said reaction area to allow vaporized species of silicon carbide to
deposit on said silicon carbide seed crystal.
8. A seeded SiC crystal growth system according to claim 7, wherein
said coating comprises a material characterized by chemical
inertness with respect to silicon and hydrogen at the sublimation
temperature of silicon carbide, and a coefficient of thermal
expansion sufficiently similar to said graphite enclosure to
prevent cracking between said graphite enclosure and said coating
during heating and cooling of said enclosure to and from the
sublimation temperature of SiC.
9. A seeded SiC crystal growth system according to claim 7, wherein
said coating comprises a refractory metal compound selected from
the group consisting of tantalum carbide, hafnium carbide, niobium
carbide, titanium carbide, zirconium carbide, tungsten carbide,
vanadium carbide, tantalum nitride, hafnium nitride, niobium
nitride, titanium nitride, zirconium nitride, tungsten nitride,
vanadium nitride and mixtures thereof.
10. A seeded SiC crystal growth system according to claim 7,
wherein said coating has an infrared emissivity ratio between about
0.4 and 0.6.
11. A seeded SiC crystal growth system according to claim 7,
further comprising a seed holder for securing said seed crystal
proximate the reaction area.
12. A seeded SiC crystal growth system according to claim 11,
wherein said seed holder secures said seed crystal sufficiently
close to the reaction area of said graphite enclosure to allow
vaporized species of SiC to deposit on said seed crystal, and
wherein said seed holder secures said seed crystal sufficiently
separated from the inner surface of said graphite enclosure and
said heating element to maintain said seed crystal at a temperature
that is between about 150.degree. C. and 200.degree. C. lower than
the temperature of said inner surface of said enclosure.
13. A seeded silicon carbide growth system according to claim 7,
comprising a silicon source gas selected from the group consisting
of silane, chlorosilane, and methyltrichlorosilane, said silicon
source gas being in fluid communication with the reaction area of
said graphite enclosure via said first pathway.
14. A seeded silicon carbide growth system according to claim 7,
comprising a hydrocarbon source gas in fluid communication with the
reaction area of said graphite enclosure via said second
pathway.
15. A seeded silicon carbide growth system according to claim 7,
wherein said first and second pathways separate the carbon source
gas from the silicon source gas until each source gas enters the
reaction area.
16. A seeded silicon carbide growth system according to claim 7,
wherein said first and second pathways for source gases are inner
and outer concentric pathways.
17. A seeded silicon carbide growth system according to claim 16,
wherein said inner concentric pathway supplies a silicon source gas
to the reaction area and said outer concentric pathway supplies a
carbon source gas to the reaction area.
18. A SiC crystal growth system for seeded growth, said system
comprising: a crucible comprising a bottom, a hollow outer cylinder
extending from said bottom and a hollow inner cylinder positioned
inside said outer cylinder, said inner cylinder extending from said
bottom to a height less than the height of said outer cylinder, a
lid attached to said outer cylinder and enclosing said inner and
outer cylinders, wherein the lid and said outer cylinder define a
reaction area inside said crucible above said inner cylinder;
wherein said hollow inner cylinder defines an inner pathway from
said bottom to said reaction area, and wherein the inner wall of
said outer cylinder and the outer wall of said inner cylinder
define an outer pathway between said inner and outer cylinders,
said outer pathway extending from said bottom to said reaction
area, said inner and outer pathways providing points of entry from
outside said crucible into the reaction area; a seed crystal holder
secured to the underside of said lid in the reaction area of said
crucible; a seed crystal secured within the reaction area by said
seed crystal holder; a silicon gas source in fluid communication
with said inner pathway through said bottom; a carbon gas source in
fluid communication with said outer pathway through said bottom; a
heating element outside said crucible for heating the reaction area
to a temperature sufficient to allow the silicon source gas and
carbon source gas to deposit vaporized species of silicon carbide
onto said seed crystal in the reaction area.
19. A SiC crystal growth system according to claim 18, wherein said
silicon gas source is selected from the group consisting of silane,
chlorosilane, and methyltrichlorosilane.
20. A SiC crystal growth system according to claim 18, wherein said
carbon source gas is a hydrocarbon.
21. A SiC crystal growth system according to claim 18, wherein said
crucible, said lid, and said seed crystal holder comprise
graphite.
22. A SiC crystal growth system according to claim 21, wherein said
crucible, said lid and said seed crystal holder are coated with a
material characterized by a melting point above the sublimation
temperature of silicon carbide.
23. A SiC crystal growth system according to claim 22, wherein said
coating comprises a material characterized by chemical inertness
with respect to silicon and hydrogen at the sublimation
temperature, and a coefficient of thermal expansion sufficiently
similar to said graphite enclosure to prevent cracking between said
graphite enclosure and said coating during heating and cooling of
said enclosure to and from the sublimation temperature of SiC.
24. A SiC crystal growth system according to claim 22, wherein said
coating comprises a refractory metal compound selected from the
group consisting of tantalum carbide, hafnium carbide, niobium
carbide, titanium carbide, zirconium carbide, tungsten carbide and
vanadium carbide; and tantalum nitride, hafnium nitride, niobium
nitride, titanium nitride, zirconium nitride, tungsten nitride and
vanadium nitride and mixtures thereof.
25. A SiC crystal growth system according to claim 22, wherein said
coating comprises tantalum carbide.
26. A SiC crystal growth system according to claim 22, wherein said
coating has an infrared emissivity ratio between about 0.4 and
0.6.
27. A SiC crystal growth system according to claim 18, wherein said
seed holder secures said seed crystal sufficiently close to the
reaction area of said crucible to allow vaporized species of SiC to
deposit on said seed crystal, and wherein said seed holder secures
said seed crystal sufficiently separated from said heating element
to maintain said seed crystal at a temperature that is between
about 150.degree. C. and 200.degree. C. lower than the temperature
of said inner and outer cylinders.
28. A SiC crystal growth system according to claim 27, wherein said
outer cylinder comprises a spacer ring extending from the top of
said outer cylinder to said lid, thereby increasing the height of
said reaction area in said crucible.
29. A SiC crystal growth system according to claim 18, comprising a
gas outlet port extending from said reaction area and through said
lid, said gas outlet port providing a passage for evacuating gas
from said reaction area.
30. A SiC crystal growth system according to claim 18, wherein said
seed holder is secured by a graphite rod extending through said lid
into said reaction area.
31. A SiC crystal growth system for high temperature seeded SiC
crystal growth, said system comprising a container for receiving a
silicon carbide seed crystal, a source of silicon and a source of
carbon, wherein said container comprises: a graphite core; a
coating on said graphite core for penetrating the pores of said
graphite core and sufficiently reinforcing the graphite grain
boundaries to keep said graphite intact even after a portion of
said coating has been removed from said core; said coating being
characterized by a melting point above the sublimation temperature
of SiC, chemical inertness with respect to silicon and hydrogen at
the sublimation temperature, and a coefficient of thermal expansion
sufficiently similar to said graphite core to prevent cracking
between said graphite core and said coating during heating and
cooling of said container to and from the sublimation temperature
of SiC.
32. A SiC crystal growth system according to claim 31 wherein said
coating comprises a refractory metal compound selected from the
group consisting of tantalum carbide, hafnium carbide, niobium
carbide, titanium carbide, zirconium carbide, tungsten carbide and
vanadium carbide; and tantalum nitride, hafnium nitride, niobium
nitride, titanium nitride, zirconium nitride, tungsten nitride and
vanadium nitride and mixtures thereof.
33. A SiC crystal growth system according to claim 32 wherein said
refractory metal compound is tantalum caride.
Description
CROSS-REFERENCE TO PRIOR APPLICATIONS
[0001] This application is a divisional of application Ser. No.
09/415,402 filed Oct. 8, 1999 entitled "Method and Apparatus for
Growing Silicon Carbide Crystals."
FIELD OF THE INVENTION
[0002] The present invention relates to the high temperature growth
of large single crystals, and in particular relates to methods and
apparatus for the growth of high-quality single crystals of silicon
carbide.
BACKGROUND
[0003] Silicon carbide is a perennial candidate for use as a
semiconductor material. Silicon carbide has a wide bandgap, a low
dielectric constant, and is stable at temperatures far higher than
temperatures at which other semiconductor materials, such as
silicon, become unstable. These and other characteristics give
silicon carbide excellent semiconducting properties. Electronic
devices made from silicon carbide can be expected to perform, inter
alia, at higher temperatures, faster speeds and at higher radiation
densities, than devices made from other commonly used semiconductor
materials such as silicon.
[0004] Those familiar with solid-state physics and the behavior of
semiconductors know that a semiconductor material must have certain
characteristics to be useful as a material from which electrical
devices may be manufactured. In many applications, a single crystal
is required, with low levels of defects in the crystal lattice,
along with low levels of unwanted chemical and physical impurities.
If the impurities cannot be controlled, the material is generally
unsatisfactory for use in electrical devices. Even in a pure
material, a defective lattice structure can prevent the material
from being useful.
[0005] Silicon carbide possesses other desirable physical
characteristics in addition to its electrical properties. It is
very hard, possessing a hardness of 8.5-9.25 Mohs depending on the
polytype [i.e., atomic arrangement] and crystallographic direction.
In comparison, diamond possesses a hardness of 10 Mohs. Silicon
carbide is brilliant, possessing a refractive index of 2.5-2.71
depending on the polytype. In comparison, diamond's refractive
index is approximately 2.4. Furthermore, silicon carbide is a tough
and extremely stable material that can be heated to more than
2000.degree. C. in air without suffering damage. These physical
characteristics make silicon carbide an ideal substitute for
naturally occurring gemstones. The use of silicon carbide as
gemstones is described in U.S. Pat. Nos. 5,723,391 and 5,762,896 to
Hunter et al.
[0006] Accordingly, and because the physical characteristics and
potential uses for silicon carbide have been recognized for some
time, a number of researchers have suggested a number of techniques
for forming crystals of 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
reactants and gases are introduced into a system within which they
form silicon carbide crystals upon an appropriate substrate.
[0007] The other main technique for growing silicon carbide
crystals is generally referred to as the sublimation technique. As
the designation "sublimation" implies, sublimation techniques
generally use some kind of solid silicon carbide starting material,
which is heated until the solid silicon carbide sublimes. The
vaporized silicon carbide starting material is then encouraged to
condense on a substrate, such as a seed crystal, with the
condensation intended to produce the desired crystal polytype.
[0008] One of the first sublimation techniques of any practical
usefulness for producing better crystals was developed in the 1950s
by J. A. Lely, and 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 a temperature at which silicon carbide sublimes, and then
allowing it to condense, re-crystallized silicon carbide is
encouraged to deposit along the lining of the vessel.
[0009] The Lely sublimation technique was modified and improved
upon by several researchers. Hergenrother, U.S. Pat. No. 3,228,756
("Hergenrother '756") discusses another sublimation growth
technique, which utilizes a seed crystal of silicon carbide upon
which other silicon carbide condenses to grow a crystal.
Hergenrother '756 suggests that in order to promote proper growth,
the seed crystal must be heated to an appropriate temperature,
generally over 2000.degree. C. 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.
[0010] Ozarow, U.S. Pat. No. 3,236,780 ("Ozarow '780") discusses
another unseeded sublimation technique which utilizes a lining of
silicon carbide within a carbon vessel. Ozarow '780 attempts to
establish a radial temperature gradient between the silicon carbide
lined inner portion of the vessel and the outer portion of the
vessel.
[0011] Knippenberg, U.S. Pat. No. 3,615,930 ("Knippenberg '930")
and U.S. Pat. No. 3,962,406 ("Knippenberg '406") discuss
alternative methods for growing silicon carbide in a desired
fashion. The Knippenberg '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. The dopant material is then
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 thereby forming a p-n junction.
[0012] The Knippenberg '406 patent 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 that will form silicon carbide. The
packed mass of silicon carbide and silicon dioxide is then 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.
[0013] Vodakov, 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 a parallel close
proximity relationship to another.
[0014] Addamiano, U.S. Pat. No. 4,556,436 ("Addamiano '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. C. and 2700.degree. C. to another temperature of less
than 1800.degree. C. Addamiano '436 notes that large single
crystals of cubic (beta) silicon carbide are simply not available
and that growth of silicon carbide or other materials such as
silicon or diamond is rather difficult.
[0015] 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 the sublimation
technique.
[0016] German (Federal Republic) Patent No. 3,230,727 to Siemens
Corporation discusses a silicon carbide sublimation technique in
which the emphasis of the discussion is the minimization of the
thermal gradient between a silicon carbide seed crystal and silicon
carbide source material. This patent suggests limiting the thermal
gradient to no more than 20.degree. C. 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.
[0017] Davis, U.S. Pat. No. Re. 34,861 ("Davis '861") discuss a
method of forming large device quality single crystals of silicon
carbide. This patent presents a sublimation process enhanced by
maintaining a constant polytype composition and size distribution
in the source materials. These patents also discuss specific
preparation of the growth surface and seed crystals and controlling
the thermal gradient between the source materials and the seed
crystal.
[0018] Barrett, U.S. Pat. No. 5,746,827 ("Barrett '827") discusses
a method for producing large diameter silicon carbide crystals
requiring two growth stages. The first growth stage is to
isothermally grow a seed crystal to a larger diameter. The second
growth stage is to grow a large diameter boule from the seed
crystal under thermal gradient conditions.
[0019] Hopkins, U.S. Pat. No. 5,873,937 ("Hopkins '937") discusses
a method for growing 4H silicon carbide crystals. This patent
teaches a physical vapor transport (PVT) system where the surface
temperature of the crystal is maintained at less than about
2160.degree. C. and the pressure inside the PVT system is decreased
to compensate for the lower growth temperature.
[0020] Kitoh, U.S. Pat. No. 5,895,526 ("Kitoh '526") teaches a
sublimation process for growing a single silicon carbide crystal
where the sublimed source material flows parallel with the surface
of a single crystal substrate.
[0021] Although significant progress in the production of SiC
crystals has occurred over the years, commercially significant
goals still remain for SiC crystal production. For example, faster
and more powerful prototype devices are being developed that
require larger SiC crystals that maintain or improve upon current
crystal quality. Boules large enough to produce 50-mm diameter SiC
wafers are currently at the far end of commercially viable SiC
production. 75-mm diameter wafers of good quality have been
demonstrated but are not yet commercially available and there is
already a need for 100-mm wafers. Many SiC crystal production
techniques are simply incapable of economically and consistently
producing crystals of the size and quality needed. The primary
reason for the inability of most crystal production techniques to
keep up with commercial demand lies within the chemistry of
SiC.
[0022] The chemistry of silicon carbide sublimation and
crystallization is such that the known methods of growing silicon
carbide crystals are difficult, even when carried out successfully.
The stoichiometry of the crystal growth process is critical and
complicated. Too much or too little silicon or carbon in the
sublimed vapor may result in a crystal having an undesired polytype
or imperfections such as micropipes.
[0023] Likewise, the high operating temperatures, typically above
2100.degree. C. and the necessity of forming specific temperature
gradients within the crystal growth system pose significant
operational difficulties. The traditional graphite sublimation
containers utilized in most sublimation systems possess infrared
emissivities on the order of 0.85 to 0.95 depending upon the
container's surface characteristics. Seed crystals are heat
sensitive to infrared radiation. Therefore, the infrared radiation
emitted by the graphite containers can overheat the seed crystal
thereby complicating the precise temperature gradients necessary
for successful operation of sublimation systems.
[0024] Recently, the SiC group at Linkoping University presented a
technique for the growth of SiC called High Temperature Chemical
Vapor Deposition ("HTCVD"). O. Kordina, et al., "High Temperature
Chemical Vapor Deposition," paper presented at the International
Conference on SiC and Related Materials, Kyoto, Japan, 1995; See
also O. Kordina, et al., 69 Applied Physics Letters, 1456 (1996).
In this technique, the solid silicon source material is replaced by
gases such as silane. The use of gaseous source materials improves
control of the reaction stoichiometry. The solid carbon source
material may also be replaced by a gas such as propane; however,
most of the carbon utilized in this technique actually comes from
the graphite walls of the crucible. Theoretically, this technique's
utilization of a continuous supply of gas would allow continuous
and extended SiC boule growth. Unfortunately, the HTCVD technique
has not proven commercially useful for boule growth primarily
because the reaction destroys the graphite crucibles used in the
process. Furthermore, the addition of hydrocarbon gases in this
particular process tends to produce Si droplets encrusted with SiC
which decreases efficiency and also ties up Si and C thereby
altering the stoichiometry of the system.
[0025] Perhaps the most difficult aspect of silicon carbide growth
is the reactivity of silicon at high temperatures. Silicon reacts
with the graphite containers utilized in most sublimation processes
and, as noted above, is encouraged to do so in some applications.
This reaction is difficult to control and usually results in too
much silicon or too much carbon being present in the system thus
undesirably altering the stoichiometry of the crystal growth
process. In addition, silicon's attack on the graphite container
pits the walls of the container destroying the container and
forming carbon dust which contaminates the crystal.
[0026] In attempts to resolve these problems, some research has
evaluated that the presence of tantalum in a sublimation system,
e.g., Yu. A. Vodakov et al, "The Use of Tantalum Container Material
for Quality Improvement of SiC Crystals Grown by the Sublimation
Technique," presented at the 6th International Conference on
Silicon Carbide, September 1995, Kyoto, Japan. Some researchers
opine that the presence of tantalum helps maintain the required
stoichiometry for optimal crystal growth. Such an opinion is
supported by reports that sublimation containers comprising
tantalum are less susceptible to attack by reactive silicon.
[0027] In a related application WO97/27350 ("Vodakov '350") Vodakov
presents a sublimation technique similar to that presented in U.S.
Pat. No. 4,147,572 and attempts to address the problem of silicon
attacking the structural components of the sublimation system.
Vodakov '350 describes a geometry oriented sublimation technique in
which solid silicon carbide source materials and seed crystals are
arranged in parallel close proximity relationship to another.
Vodakov '350 utilizes a sublimation container made of solid
tantalum. The inner surface of Vodakov's tantalum container is
described as being an alloy of tantalum, silicon and carbon.
Page11, line 26 through page12, line 10. Vodakov claims that such a
container is resistive to attack by silicon vapor and contributes
to well-formed silicon carbide crystals.
[0028] The cost of tantalum is, however, a drawback to a
sublimation process utilizing the container described in Vodakov. A
sublimation container of solid tantalum is extremely expensive and
like all sublimation containers, will eventually fail, making its
long-term use un-economic. A solid tantalum sublimation container
is also difficult to machine. Physically forming such a container
is not an easy task. Lastly, the sublimation process of Vodakov
'350 suffers the same deficiency shown in other solid source
sublimation techniques in that it is not efficient at forming the
large, high quality boules needed for newly discovered
applications.
[0029] Therefore, a need exists for a process that provides for
controlled, extended and repeatable growth of high quality SiC
crystals. Such a system must necessarily provide a container that
is resistive to attack by silicon. Such a system should also be
economical to implement and use.
OBJECT AND SUMMARY OF THE INVENTION
[0030] Accordingly, an object of the present invention is to
provide a method and apparatus for the controlled, extended and
repeatable growth of high quality silicon carbide crystals of a
desired polytype.
[0031] A further object of the present invention is to provide a
method of growing high quality single crystals of silicon carbide
by controlling the stoichiometry of the crystal growth process.
[0032] A further object of the present invention is to provide a
method of growing high quality single crystals of silicon carbide
by controlling the temperature of the crystal growth process.
[0033] A further object of the present invention is to provide a
method and apparatus for growing high quality single crystals of
silicon carbide by reducing or eliminating impurities resulting
from degradation of the physical components of the system.
[0034] A still further object of the present invention is to
provide for a system for SiC crystal growth that resists reaction
with vaporized silicon.
[0035] The invention meets these objects with a method and
apparatus for growing large single crystals of SiC for use in
producing electrical devices and for use as gemstones. In
particular, the invention encompasses introducing a monocrystalline
seed crystal of SiC of a desired polytype and a source of silicon
and a source of carbon into SiC crystal growth system typically
comprising a crucible and a furnace. The source of silicon and
carbon is then raised to a temperature sufficient for the formation
of vaporized species containing silicon and carbon. The temperature
of the seed crystal is raised to a temperature approaching but
lower than the temperature of the silicon and carbon vapors and
lower than that at which SiC will sublime faster than deposit under
the gas pressure conditions within the crucible, thus creating a
temperature gradient within the crucible.
[0036] A suitable flow of a vaporized species containing silicon
and carbon derived from the source of silicon and the source of
carbon is generated and maintained within the crucible. The flow of
vapor is directed to the growth surface of the seed crystal for a
time sufficient to produce a desired amount of macroscopic growth
of monocrystalline SiC while substantially preventing any silicon
containing species from reacting with material utilized in
constructing the SiC crystal growth system.
[0037] The foregoing and other objects, advantages and features of
the invention, and the manner in which the same is 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:
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a schematic representation of a crystal growth
system in accordance with the invention.
[0039] FIG. 2 is a cross-sectional diagram of a gas fed sublimation
system used in accordance with the method of the claimed
invention.
DETAILED DESCRIPTION
[0040] As will be readily apparent to those skilled in the art, the
following disclosure may be easily adapted and incorporated into
virtually all known methods for producing SiC crystals.
Accordingly, the following detailed description will begin with a
general discussion of the invention. Additional embodiments
demonstrating the versatility of the invention will follow.
[0041] A preferred embodiment of the invention is a gas fed
sublimation (GFS) system in which the source of silicon and the
source of carbon are gaseous. The gaseous silicon and carbon
sources are fed to a reaction chamber where they react at high
temperatures, typically above 2000.degree. C., to form vaporized
species containing silicon and carbon. In addition to carbon (C)
and silicon (Si), such species typically include SiC, Si.sub.2C,
and SiC.sub.2 The vaporized species are then deposited onto a
monocrystalline seed crystal of a desired polytype. Such a system
is schematically disclosed in FIG. 1.
[0042] The GFS system of FIG. 1 comprises a crucible broadly
designated at 10. It is to be understood that the crucible 10 is a
substantially enclosed structure similar to the type normally used
in SiC sublimation techniques. Reference is made to the crucible 12
in Barrett '827; the growth chamber 10 of Hopkins '937; and the
crucibles shown in FIGS. 1, 4, 5 and 6 of Davis '861 as being
exemplary, but not limiting, of the crucibles, vessels, or
containers of the present invention. These references also
demonstrate that the broad parameters of sublimation growth are
relatively well understood in this art. Accordingly, these will not
be addressed in detail herein, other than to describe the features
of the present invention. The crucible 10 is generally cylindrical
in shape and includes a cylindrical wall 11 having an outer surface
12 and an inner surface 13. The cylindrical wall 11 is made of
graphite coated with material characterized by a melting point
above the sublimation temperature of SiC. The coating material is
also characterized by chemical inertness with respect to silicon
and hydrogen at the temperatures in question. Metal carbides and
particularly the carbides of tantalum, hafnium, niobium, titanium,
zirconium, tungsten and vanadium and mixtures thereof exhibit the
desired characteristics of the required coating. Metal nitrides,
and particularly the nitrides of tantalum, hafnium, niobium,
titanium, zirconium, tungsten and vanadium and mixtures thereof
also exhibit the desired characteristics of the required coating.
Furthermore, mixtures of metal carbides and metal nitrides such as
those listed previously may be used as the coating substance. For
ease of discussion and reference, the remainder of the detailed
description will refer to metal carbides although it is understood
that the concepts and principles discussed herein are equally
applicable to metal nitride coatings.
[0043] In all instances described herein, it is to be understood
that graphite components exposed to the source materials are coated
with a metal carbide coating. The metal carbide coating may be
provided by any of several commercially available coating processes
such at that practiced by Ultramet Corporation of Pacoima, Calif.
or Advance Ceramics Corporation of Lakewood, Ohio. Additionally,
the graphite components described herein are made from a graphite
which has approximately the same coefficient of thermal expansion
as the selected metal carbide. Such materials are commercially
available. The relative similarities of thermal coefficients of
expansion are a particular requirement for materials heated to the
extremely high temperatures described herein. In this manner, the
likelihood of the graphite or metal carbide coating cracking during
the crystal growth process is substantially reduced and the
lifetime of the crucible will generally be increased.
[0044] The cylindrical wall 11 radially encloses a reaction area
generally designated at 14. Outer 16 and inner 18 concentric source
gas pathways supply the source gas materials to the reaction area
14. Although the source gases could be mixed prior to entering the
reaction area 14, separation of the source gases until each gas is
heated to approximately the reaction temperature helps prevent any
undesired side reactions between the silicon source gas and the
carbon source gas. The concentric source gas pathways keep the
source gas materials separated from one another until the point
where the source gases enter the reaction area 14. In a preferred
embodiment the outer concentric source gas pathway 16 supplies the
carbon source gas to the reaction area 14 and the inner concentric
source gas pathway 18 supplies the silicon source gas.
[0045] In typical sublimation systems the graphite walls of the
crucible are used as a source of carbon. The metal carbide coating
of the claimed invention diminishes the availability of this source
of carbon although it appears that under certain circumstances the
coated graphite may still act as a source of some carbon for the
system. Accordingly, the majority of the carbon needed is supplied
from an outside source, such as a carbon source gas. Suitable
carbon source gases include any hydrocarbon capable of reacting
with Si to form SiC. C.sub.2 to C.sub.8 hydrocarbons and in
particular ethylene (C.sub.2H.sub.4) work well in the claimed
invention. The carbon source gas stream may also comprise one or
more carrier gases such as He or H.sub.2.
[0046] Suitable silicon source gases include any gas which will
react with available carbon to form SiC. Silane (SiH.sub.4) is
probably the most well-known of the possible silicon source gases
and works well in the claimed invention. Other suitable sources of
silicon include chlorosilane (SiH.sub.4-xCl.sub.x) and
methyltrichlorosilane (CH.sub.3SiCl.sub.3). Chlorosilanes require
H.sub.2 to react, however. The silicon source gas stream may also
comprise a suitably inert carrier gas such as He.
[0047] A seed crystal 22 is secured on a seed holder 20 and lowered
into the reaction area 14. The source gases react within the
reaction area 14 to form SiC vapor which eventually deposits on the
surface of the seed crystal 22 to form a boule 24. It is believed
that at least a portion of the SiC first deposits on the inner wall
13, then sublimes to recondense on the growth surface (seed crystal
22 or boule 24). Under most circumstances, the seed crystal is
preferably SiC of the same polytype as the desired growth.
[0048] The composition of the source gases may be kept constant or
varied during the growth process depending upon the required
stoichiometry, type of crystal desired and the physical
characteristics of the crystal growth system.
[0049] Those familiar with the physical chemistry of solids,
liquids and gases know that crystal growth is in most circumstances
encouraged on a growth surface if the surface is at a somewhat
lower temperature than the fluid (either gas or liquid) which
carries the molecules or atoms to be condensed. The GFS system is
no exception. A thermal gradient is established between the growth
surface and the source material. Although the exact dimensions of
the temperature gradient may vary depending upon the pressure of
the system, desired polytype, source gas composition, etc., the
following general principle is usually applicable to all types of
SiC crystal growth processes, including the GFS system. The
temperature of the silicon source and carbon source should be
raised to a temperature sufficient for the formation of the
vaporized species while the temperature of the crystal growth
surface is elevated to a temperature approaching the temperature of
silicon and carbon sources, but lower than the temperature of the
silicon and carbon sources, and lower than that at which SiC will
sublime faster than deposit under the gas pressure conditions
utilized.
[0050] As stated above, numerous variables determine the
appropriate temperature gradient for a given system. However, a
system such as that described in FIG. 1 has been discovered to
operate well at seed temperatures between about 1900 .degree. C.
and about 2500.degree. C. with the inner walls of the reaction area
being about 150.degree. C. to about 200.degree. C. hotter than the
seed. The maximum growth rate for such a system has yet to be
determined. Higher temperatures are known to generally translate
into faster growth rates. Higher temperatures, however, can result
in sublimation of the seed, which alters the equilibrium of the
system and requires additional source gas and potentially other
adjustments as well.
[0051] The GFS system of FIG. 1 has demonstrated the ability to
produce very large high quality crystals of SiC. More importantly,
the GFS system of FIG. 1 has demonstrated an ability to withstand
attack from the Si compounds that eventually destroy typical
graphite crucibles. A test crucible of graphite coated with an
approximately 30 micron thick coating of TaC emerged from a crystal
growth session unaffected by the harsh environment. Only after
several runs have cracks appeared in test crucibles, usually near a
sharp corner where the metal carbide coating was less than optimum.
However, even when the coating cracks, the crystal growth system is
not subject to the carbon dust typically formed when a graphite
crucible's integrity is compromised.
[0052] The explanation for this surprising property is not fully
understood. Although the inventors do not wish to be bound by any
particular theory, one possible explanation is that when uncoated
graphite is attacked by Si, the Si predominately attacks the weak
parts of the graphite, i.e., at the grain boundaries penetrating
into the pores. The Si forms SiC which sublimes and is removed as a
volatile species. Eventually Si completely erodes the graphite
surrounding the grain, leaving the grain behind as a carbon dust
particle. It is believed that the metal carbide coating penetrates
deep within the graphite pores causing the Si to attack the
graphite in a more uniform manner, thereby avoiding the generation
of carbon dust.
[0053] Suprisingly, a graphite crucible once coated with a metal
carbide resists the formation of carbon dust even after substantial
spalling of the metal carbide coating. Accordingly, an alternative
embodiment of the invention is a GFS system comprising a graphite
crucible which has at one time been coated with a metal carbide
coating but which through use or other circumstances has lost some
or all of its metal carbide coating. Such a system is capable is
producing quality SiC crystals without contamination from carbon
dust.
[0054] Additionally, the GFS system of FIG. 1 has demonstrated the
ability to provide improved control of the temperature gradients
within the crystal growth system. As discussed previously, seed
crystals are sensitive to infrared radiation and graphite possesses
an infrared emissivity of between about 0.85 to about 0.95
depending upon the surface of the graphite. In contrast the
infrared emissivity of the metal compound coatings of the invention
range from approximately 0.4 for ZrC to approximately 0.5 for TaC
to approximately 0.6 for NbC. The lower emissivities of the metal
compound coatings of the claimed invention substantially reduce the
amount of infrared radiation impinging upon the seed crystal during
crystal growth and can result in a 100.degree. C. or more reduction
in seed temperature when compared to uncoated graphite systems.
Reducing the amount of infrared radiation removes a potential
source of excess heat from the system thereby improving control of
the temperature gradients within the system.
[0055] It is readily apparent to one skilled in the art that the
utilization of a metal carbide coated crucible as described above
is readily adaptable to existing SiC crystal growth systems. It
will be additionally apparent to those familiar with this art that
the use of metal carbide-coated crucibles according to the present
invention need not be limited to the sublimation growth of SiC.
Thus, although the invention offers particular advantages with
respect to SiC growth, the coatings and coated crucibles, vessels
or containers described herein offer structural and functional
advantages for the growth of other materials, including other wide
band-gap semiconductor materials such as the Group III nitrides,
and particularly including gallium nitride (GaN). For example, some
researchers have reported a link between the presence of carbon and
a yellow luminescence in GaN and non-uniform electrical behavior in
In-containing nitrides. Pearton et al, GaN: Processing, Defects and
Devices, 86 Applied Physics Reviews, 1 (July 1999). The utilization
of the coated apparatus and method of the invention advantageously
reduces the availability of carbon as a potential residual impurity
in MOCVD nitrides. Additional embodiments evidencing the
versatility of the claimed invention follow.
[0056] FIG. 2 illustrates a cross-sectional view of another GFS
system used in accordance with the method of the present invention.
The crucible is broadly designated at 10. The crucible 10 is
located within a furnace indicated generally at 8. Methods and
apparatus, such as a furnace, for supplying heat to SiC and other
crystal growth systems are well known to those skilled in the art,
and thus will not be otherwise discussed in detail herein.
[0057] The crucible 10 is generally cylindrical in shape and
includes a lid 26 and a bottom 28 that substantially encloses an
intermediate cylindrical portion 30. The intermediate cylindrical
portion 30 comprises an outer cylinder 32 having a top and a bottom
and an inner diameter and an outer diameter. Situated within the
inner diameter of the outer cylinder 32 is an inner cylinder 34
also having a top and a bottom, and an inner diameter and an outer
diameter. The outer cylinder 32 and the inner cylinder 34 form
inner 38 and outer 36 concentric gas pathways.
[0058] In a preferred embodiment the intermediate cylindrical
portion 30 also comprises at least one spacer ring 40 situated
between the outer cylinder 32 and the lid 26. The spacer ring 40 is
defined by an inner diameter and an outer diameter with said inner
diameter being less than the outer diameter of the inner cylinder
34. The spacer ring 40 and the lid 26 generally define a reaction
area 42 above the outer and inner cylinders 32 and 34 respectively.
It is to be understood that the spacer ring 40 is an optional
component. When used, however, the spacer ring 40 preferably
incorporates the refractory metal carbide coating of the present
invention. Alternatively, the outer cylinder 32 can be extended to
replace the spacer ring 40. However, the use of a spacer ring or
rings is recommended because of the flexibility provided in
adjusting the size of the reaction area 42 and thus the thermal
gradient. In a further alternative, the spacer ring 40 can be used
in conjunction with other similarly shaped devices such as a growth
disk (a ring with a venturi-like opening that focuses upward
flowing SiC vapor) or a collection disk (a porous disk that allows
SiC vapor to flow upward while collecting solid particles that fall
from the walls of the crucible). Collecting these particles onto a
hot collection disk permits them to resublime and contribute to the
growth of the crystal.
[0059] Extending into the reaction area 42 from the lid 26 is a
seed crystal 44 supported by a seed holder 46 and a graphite rod
48. The seed crystal 44 acts as a substrate for the growth of a SiC
boule 50.
[0060] Two gas sources 52 and 54 are in fluid communication with
the inner and outer concentric gas pathways and provide the silicon
and carbon source gases utilized in the SiC crystal growth process.
In a preferred embodiment one gas source 52 supplies the carbon
source gas to the outer concentric gas pathway 36 and the other gas
source 54 supplies the silicon source gas to the inner concentric
gas pathway 38. The reaction to form SiC vapor and the desired SiC
boule proceeds as previously described with respect to FIG. 1. A
gas outlet 27 incorporated into the lid 26 and extending through
the underlying seed holder 46 provides a means for evacuation of
gas from the reaction area 42.
[0061] It will be further understood that relevant portions of the
systems referred to earlier (e.g., Davis, Vodakov, etc.) could be
modified and improved to incorporate the coated surfaces, vessels,
and systems described herein, and would thus fall within the
parameters of the present invention.
[0062] The invention has been described in detail, with reference
to certain preferred embodiments, in order to enable the reader to
practice the invention without undue experimentation. However, a
person having ordinary skill in the art will readily recognize that
many of the components and parameters may be varied or modified to
a certain extent without departing from the scope and spirit of the
invention. Furthermore, titles, headings, or the like are provided
to enhance the reader's comprehension of this document, and should
not be read as limiting the scope of the present invention.
Accordingly, the intellectual property rights to the invention are
defined only by the following claims and reasonable extensions and
equivalents thereof.
* * * * *