U.S. patent application number 13/217668 was filed with the patent office on 2011-12-15 for sic crystals having spatially uniform doping impurities.
This patent application is currently assigned to II-VI Incorporated. Invention is credited to Donovan L. Barrett, Avinash K. Gupta, Edward Semenas, Andrew N. Souzis, Ilya Zwieback.
Application Number | 20110303884 13/217668 |
Document ID | / |
Family ID | 36808698 |
Filed Date | 2011-12-15 |
United States Patent
Application |
20110303884 |
Kind Code |
A1 |
Gupta; Avinash K. ; et
al. |
December 15, 2011 |
SiC Crystals Having Spatially Uniform Doping Impurities
Abstract
A sublimation-grown silicon carbide (SiC) single crystal boule
includes a deep level dopant introduced into the SiC single crystal
boule during sublimation-growth thereof such that in a continuous
section of the boule that is not less than 50% of a continuous
length of said boule, the deep level dopant concentration at the
boule center varies by not more than 25% from the average
concentration of the deep level dopant in the continuous section of
the boule.
Inventors: |
Gupta; Avinash K.; (Basking
Ridge, NJ) ; Semenas; Edward; (Allentown, PA)
; Zwieback; Ilya; (Washington Township, NJ) ;
Barrett; Donovan L.; (Port Orange, FL) ; Souzis;
Andrew N.; (Hawthorne, NJ) |
Assignee: |
II-VI Incorporated
Saxonburg
PA
|
Family ID: |
36808698 |
Appl. No.: |
13/217668 |
Filed: |
August 25, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12573288 |
Oct 5, 2009 |
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13217668 |
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11405368 |
Apr 17, 2006 |
7608524 |
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12573288 |
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60672945 |
Apr 19, 2005 |
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Current U.S.
Class: |
252/512 ;
252/516 |
Current CPC
Class: |
Y10T 117/10 20150115;
Y10T 117/108 20150115; Y10T 117/1016 20150115; C30B 23/00 20130101;
C30B 29/36 20130101; Y10T 117/1008 20150115 |
Class at
Publication: |
252/512 ;
252/516 |
International
Class: |
H01B 1/04 20060101
H01B001/04 |
Claims
1. A sublimation-grown silicon carbide (SiC) single crystal boule
including a deep level dopant introduced into the SiC single
crystal boule during sublimation-growth thereof such that in a
continuous section of the boule that is not less than 50% of a
continuous length of said boule, the deep level dopant
concentration at the boule center varies by not more than 25% from
the average concentration of the deep level dopant in said
continuous section of the boule.
2. The SiC boule of claim 1, wherein the deep level dopant is
vanadium.
3. The SiC boule of claim 2, wherein the concentration of vanadium
is between 2.times.10.sup.6 cm.sup.-3 and 2.times.10.sup.7
cm.sup.-3.
4. The SiC boule of claim 1, wherein a resistivity of each of a
plurality of wafers obtained from the boule is greater than about
5.times.10.sup.9 ohm-cm.
5. The SiC boule of claim 1, wherein a resistivity of each of a
plurality of wafers obtained from the boule is greater than about
1.times.10.sup.10 ohm-cm.
6. The SiC boule of claim 1, wherein a resistivity of each of a
plurality of wafers obtained from the boule is greater than about
5.times.10.sup.10 ohm-cm.
7. The SiC boule of claim 1, wherein a resistivity of each of a
plurality of wafers obtained from the boule is greater than about
1.times.10.sup.11 ohm-cm.
8. The SiC boule of claim 1, wherein at least 10 wafers are
obtained from the boule.
9. The SiC boule of claim 1, wherein at least 13 wafers are
obtained from the boule.
10. The SiC boule of claim 1, wherein at least 17 wafers are
obtained from the boule.
11. A sublimation-grown silicon carbide (SiC) single crystal boule
including a deep level dopant introduced into the SiC single
crystal boule during sublimation-growth thereof such that in a
sublimation-growth direction of the SiC single crystal boule a
concentration of the deep level dopant in a plurality of at least
10 consecutive wafers obtained from the SiC single crystal boule
varies by not more than 25% from the average concentration of the
deep level dopant in any one of the plurality of wafers.
12. The SiC boule of claim 11, wherein the deep level dopant is
vanadium.
13. The SiC boule of claim 12, wherein the concentration of
vanadium is between 2.times.10.sup.6 cm.sup.-3 and 2.times.10.sup.7
cm.sup.-3.
14. The SiC boule of claim 11, wherein the resistivity of each of
the plurality of wafers is greater than about 5.times.10.sup.9
ohm-cm.
15. The SiC boule of claim 11, wherein the resistivity of each of
the plurality of wafers is greater than about 1.times.10.sup.10
ohm-cm.
16. The SiC boule of claim 11, wherein the resistivity of each of
the plurality of wafers is greater than about 5.times.10.sup.10
ohm-cm.
17. The SiC boule of claim 11, wherein the resistivity of each of
the plurality of wafers is greater than about 1.times.10.sup.11
ohm-cm.
18. The SiC boule of claim 11, wherein the plurality of wafers is
at least 13 wafers.
19. The SiC boule of claim 11, wherein the plurality of wafers is
desirably at least 17 wafers.
20. A pair of silicon carbide (SiC) wafers obtained from first and
second parts of a sublimation-grown SiC single crystal boule that
are spaced from each other a distance no less than a thickness of
at least eight other SiC wafers, wherein one of the pair of wafers
has a concentration of the deep level dopant that is different from
the concentration of the deep level dopant in the other of the pair
of wafers by not more than 25%.
21. The SiC wafers of claim 20, wherein the deep level dopant is
vanadium.
22. The SiC wafers of claim 21, wherein the concentration of
vanadium is between 2.times.10.sup.6 cm.sup.-3 and 2.times.10.sup.7
cm.sup.-3.
23. The SiC boule of claim 20, wherein the resistivity of each of
the plurality of wafers is greater than about 5.times.10.sup.9
ohm-cm.
24. The SiC boule of claim 20, wherein the resistivity of each of
the plurality of wafers is greater than about 1.times.10.sup.10
ohm-cm.
25. The SiC boule of claim 20, wherein the resistivity of each of
the plurality of wafers is greater than about 5.times.10.sup.10
ohm-cm.
26. The SiC boule of claim 20, wherein the resistivity of each of
the plurality of wafers is greater than about 1.times.10.sup.11
ohm-cm.
27. The SiC boule of claim 20, wherein the plurality of wafers is
at least 13 wafers.
28. The SiC boule of claim 20, wherein the plurality of wafers is
desirably at least 17 wafers.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of co-pending U.S. patent
application Ser. No. 12/573,288, filed Oct. 5, 2009, which is a
divisional of U.S. patent application Ser. No. 11/405,368, filed
Apr. 17, 2006, now U.S. Pat. No. 7,608,524, which claims priority
from U.S. Provisional Patent Application No. 60/672,945, filed Apr.
19, 2005, all of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] Unique electronic properties of silicon carbide (SiC) make
it a very desirable material for state-of-the-art semiconductor
devices that can operate at high frequencies, high voltages and
current densities, and in harsh conditions. In many such devices,
silicon carbide is utilized as a substrate on which the
semiconductor device structure is formed using epitaxy,
photolithography and metallization. Depending on the device design,
the substrate must possess specified electronic parameters, such as
conductivity type and resistivity. While devices operating at high
and microwave frequencies (RF devices) require semi-insulating (SI)
substrates with very high resistivity, for other devices, such as
high power switching devices, low-resistivity n-type and p-type
substrates are needed.
[0003] Presently, SiC single crystals are grown on the industrial
scale by a sublimation technique called Physical Vapor Transport
(PVT). A schematic diagram of a typical prior art PVT arrangement
is shown in FIG. 1. In PVT, polycrystalline grains of silicon
carbide (SiC source) 1 are loaded on the bottom of a growth
container 2 and a SiC seed crystal 4 is attached to a top of growth
container 2. Desirably, growth container 2 is made of a material,
such as graphite, that is not reactive with SiC or any dopant
(discussed hereinafter) added thereto. The loaded growth container
2 is evacuated, filled with inert gas to a certain, desired
pressure and heated via at least one heating element 3 (e.g., an RF
coil) to a growth temperature, e.g., between 1900.degree. C. and
2400.degree. C. Growth container 2 is heated in such a fashion that
a vertical temperature gradient is created making the SiC source 1
temperature higher than that of the SiC seed 4. At high
temperatures, silicon carbide of the SiC source 1 sublimes
releasing a spectrum of volatile molecular species to the vapor
phase. The most abundant of these gaseous species are Si, Si.sub.2C
and SiC.sub.2. Driven by the temperature gradient, they are
transported to the SiC seed 4 and condense on it causing growth of
a SiC single crystal 5 on the SiC seed 4. Prior art patents in this
area include, for example, U.S. Pat. Nos. 6,805,745; 5,683,507;
5,611,955; 5,667,587; 5,746,827; and Re. 34,861, which are all
incorporated herein by reference.
[0004] Those skilled in the art of semiconductor materials know
that production of SiC substrates with desirable electronic
properties is impossible without purposeful introduction of certain
impurities in a process known as doping. In silicon carbide, the
chemical bonds are so exceptionally strong and solid-state
diffusion of impurities is so slow that doping in the bulk can be
accomplished only at the stage of crystal growth, when the doping
element (dopant) incorporates directly into the lattice of the
growing SiC crystal 5.
[0005] As a particular example of SiC doping during growth, n-type
SiC crystals are produced by adding small amounts of gaseous
nitrogen (N.sub.2) to growth container 2 atmosphere. Nitrogen-doped
SiC single crystals with very uniform electrical properties can be
readily grown by maintaining appropriate partial pressure of
N.sub.2 during growth.
[0006] With the exception of the nitrogen-doped crystals, attaining
uniform electrical properties in other types of SiC crystals,
including semi-insulating, p-type and phosphorus doped n-type
crystals, is much more difficult because the doping compounds are
not gaseous but solid. Vanadium is one particularly important
dopant, which is used to produce a high-resistivity semi-insulating
SiC crystal. Aluminum is another important dopant used for the
growth of conductive crystals of p-type. Other solid dopants
include boron, phosphorus, heavy metals and rare earth
elements.
[0007] Prior art doping of SiC crystals using a solid dopant is
carried out by admixing small amounts of impurity directly to the
SiC source 1. For instance, vanadium can be introduced in the form
of elemental vanadium, vanadium carbide or vanadium silicide.
Aluminum can be introduced in the elemental form, aluminum carbide
or aluminum silicide. Other suitable solid dopants, such as boron
or phosphorus, can be similarly introduced as elements, carbides or
silicides. The doping compound can be in the physical form of
powder, pieces or chips.
[0008] During SiC crystal 5 sublimation growth, multi-step chemical
reactions take place between the SiC source 1 and the dopant
admixed directly in the SiC source. These reactions proceed through
several stages and lead to the formation of multiple intermediary
compounds. In the case of vanadium doping, thermodynamic analysis
shows that the product of reaction between SiC and vanadium dopant
(whether elemental, carbide or silicide) depends on the
stoichiometry of SiC. That is, when the SiC source 1 is Si-rich and
its composition corresponds to the two-phase equilibrium between
SiC and Si, formation of vanadium silicide (VSi.sub.2) is likely.
When the SiC source is C-rich and its composition corresponds to
the two-phase equilibrium between SiC and C, formation of vanadium
carbide (VC.sub.x) is likely.
[0009] It is known that freshly synthesized SiC source 1 is,
typically, Si-rich. Due to the incongruent character of SiC
sublimation, the initially silicon-rich SiC source 1 gradually
becomes carbon-rich. This change in the stoichiometry of the SiC
source 1 during sublimation growth causes the following sequence of
reactions:
[0010] During initial stages of growth, when the SiC source 1 is
Si-rich, reaction between vanadium dopant and SiC yields vanadium
silicide VSi.sub.2.
[0011] As the growth progresses and the SiC source 1 becomes more
carbon-rich, vanadium silicide converts to intermediate
carbo-silicide VC.sub.xSi.sub.y.
[0012] During final stages of growth, when the SiC source 1 is
carbon-rich, vanadium carbo-silicide converts into vanadium carbide
VC.sub.x.
[0013] Accordingly, the partial pressure of vanadium-bearing
species in the vapor phase decreases from high at the beginning of
growth to low at the end. The change in the vanadium partial
pressure results in the characteristic concentration profile with
too much vanadium in the first-to-grow portions of the SiC crystal
5 boule and too little in the last-to-grow portions. For this
reason, electrical properties of SiC crystals grown using the
doping technique of prior art are spatially nonuniform and the
yield of high electronic quality substrates is low.
[0014] The above case of vanadium doping was given for the purpose
of example only. Similar problems exist in the cases when other
solid dopants are added to the SiC source 1 directly, including,
but not limited to aluminum, boron and phosphorus.
SUMMARY OF THE INVENTION
[0015] The present invention is a system for producing spatially
uniform and controlled concentration of a dopant throughout a SiC
crystal boule that avoids or eliminates the too high dopant
concentration in the first-to-grow boule portions and the too low
dopant concentration in the last-to-grow portions. The dopant
concentration can be sufficiently high to achieve the desired
electronic properties of the SiC material while, at the same time,
the dopant concentration can be low enough to avoid generation of
crystal defects. Moreover, the dopant concentration does not change
appreciably as the crystal grows. Therefore, longer boules with
spatially uniform electrical properties can be grown resulting in
higher quality of the SiC substrates, higher yields and
productivity.
[0016] More specifically, the invention is a sublimation-grown
silicon carbide (SiC) single crystal boule including a deep level
dopant introduced into the SiC single crystal boule during
sublimation-growth thereof such that in a continuous section of the
boule that is not less than 50% of a continuous length of said
boule, the deep level dopant concentration at the boule center
varies by not more than 25% from the average concentration of the
deep level dopant in said continuous section of the boule.
[0017] The invention is also a sublimation-grown silicon carbide
(SiC) single crystal boule including a deep level dopant introduced
into the SiC single crystal boule during sublimation-growth thereof
such that in a sublimation-growth direction of the SiC single
crystal boule a concentration of the deep level dopant in a
plurality of at least 10 consecutive wafers obtained from the SiC
single crystal boule varies by not more than 25% from the average
concentration of the deep level dopant in any one of the plurality
of wafers.
[0018] Last, the invention is a pair of silicon carbide (SiC)
wafers obtained from first and second parts of a sublimation-grown
SiC single crystal boule that are spaced from each other a distance
no less than a thickness of at least eight other SiC wafers,
wherein one of the pair of wafers has a concentration of the deep
level dopant that is different from the concentration of the deep
level dopant in the other of the pair of wafers by not more than
25%.
[0019] The deep level dopant is desirably vanadium. The
concentration of vanadium can be between 2.times.10.sup.6 cm.sup.-3
and 2.times.10.sup.7 cm.sup.-3.
[0020] The resistivity of each of the plurality of wafers is
desirably greater than about 5.times.10.sup.9 ohm-cm, is more
desirably greater than about 1.times.10.sup.10 ohm-cm, is even more
desirably greater than about 5.times.10.sup.10 ohm-cm, and is most
desirably greater than about 1.times.10.sup.11 ohm-cm.
[0021] The plurality of wafers is desirably at least 13 wafers and
is most desirably at least 17 wafers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a cross section of a prior art physical vapor
transport arrangement;
[0023] FIG. 2 is a cross section of a time-release capsule in
accordance with the present invention for use in the physical vapor
transport arrangement shown in FIG. 1;
[0024] FIGS. 3a-3c are cross-sectional views of one or more
capsules of FIG. 2 positioned in different locations within the
physical vapor transport arrangement shown in FIG. 1;
[0025] FIG. 4 is a graph of resistivity versus wafer number for
wafers sliced from a boule grown in accordance with the prior
art;
[0026] FIG. 5 is a low magnification optical view of the first
wafer utilized to form the plot shown in FIG. 4;
[0027] FIG. 6 is a plot of the resistivity of each wafer sliced
from a boule grown in accordance with the present invention;
and
[0028] FIG. 7 is another plot of the resistivity of each wafer
sliced from another boule grown in accordance with the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0029] With reference to FIG. 2, the advantages of spatially
uniform and controlled doping are realized using a time-release
capsule 14, which is loaded with a stable form of solid dopant, and
placed inside growth container 2. Capsule 14 is desirably made of
an inert material, which is reactive with neither SiC nor the
dopant. For a majority of applications, dense and low-porosity
graphite is a preferred material for capsule 14. Other possible
materials include refractory metals, their carbides and nitrides.
However, this is not to be construed as limiting the invention.
[0030] Capsule 14 includes a tight lid 15 having one or more
calibrated through-holes or capillaries 16 of predetermined
diameter and length. There are no limitations on the dimensions of
capsule 14 except that it should fit inside growth container 2 and
not restrict the flow of vapor to the SiC seed 4.
[0031] At a suitable time, capsule 14 is loaded with the proper
amount of solid dopant 17. Dopant 17 must be either in a stable
chemical form that is not reactive with the material of capsule 14
or in a form that upon reaction with the material forming capsule
14 produces a stable compound. For the majority of practical
applications, the preferred forms of solid dopant are: (i)
elemental form, (ii) carbide and (iii) silicide. However, this is
not to be construed as limiting the invention.
[0032] During sublimation growth of the SiC crystal 5, capsule 14
is situated inside growth container 2. In one embodiment, shown in
FIG. 3a, a single capsule 14 is positioned on the top surface of
the SiC source 1 near the axis of growth container 2. In another
embodiment, shown in FIG. 3b, several capsules 14 are positioned on
the top surface of the SiC source 1 near the wall of growth
container 2. In yet another embodiment, shown FIG. 3c, capsule 14
is buried within the material forming the SiC source 1.
[0033] The principle of operation of capsule 14 is based on the
well-known phenomenon of effusion, i.e., the slow escape of vapor
from a sealed vessel through a small orifice. At high temperatures,
the vapor pressure of dopant 17 inside capsule 14 forces it to
escape through each capillary 16. If the cross section of each
capillary 16 is sufficiently small, the vapor pressure of dopant 17
in capsule 14 does not differ substantially from the equilibrium
value.
[0034] The laws of effusion are well-known and, for given growth
conditions (temperature, vapor pressure of the inert gas,
volatility of the substance contained in capsule 14, capillary 16
diameter and capillary 16 length), the flux of the molecules of
dopant 17 escaping capsule 14 via each capillary 16 can be readily
calculated. Thus, the dimension of each capillary 16 and number of
capillaries 16 can be tailored to achieve a steady and
well-controlled flux of the impurity dopant 17 atoms from capsule
14 to the growing SiC crystal 5.
[0035] For relatively small doping levels, a capsule 14 having a
single capillary 16 can be used (see embodiment in FIG. 3a). For
higher doping levels or doping with multiple dopants 17, multiple
capsules 14 can be used (see embodiment in FIG. 3b), as well as a
capsule 14 with multiple capillaries. For special purposes, such as
programmable or delayed doping, one or more time-release capsules
14 buried in the depth of the SiC source 1 can be utilized (see
embodiment in FIG. 3c).
[0036] According to prior art SiC doping, a small amount of dopant
is admixed directly to the SiC source 1 material, leading to
chemical reactions between the dopant and SiC source 1. These
reactions, combined with changes in the stoichiometry of the SiC
source 1 material, lead to progressive changes in the partial
pressure of the dopant. As a result, prior art doping produces
initially high concentrations of dopant in the crystal followed by
a decrease in the dopant concentration over the SiC crystal 5
length. Crystals grown according to the prior art have too high a
degree of dopant in the first-to-grow sections and insufficient
dopant in the last-to-grow sections. The dopant level in the
first-to-grow boule sections can be so high that second-phase
precipitates form in the crystal bulk leading to the generation of
crystal defects.
[0037] The present invention eliminates the problems of the prior
art by using one or more time-release capsules 14 for the doping of
SiC crystals 5 during crystal growth. The invention has two
distinct advantages:
[0038] First, the present invention eliminates direct contact
between the dopant 17 and the SiC source 1. This is accomplished by
placing the dopant 17 inside of a capsule 14 made of an inert
material.
[0039] Second, the present invention offers a means for precise
control of the dopant 17 concentration. This is achieved by
choosing the number of capsules 14, the number and dimensions of
the capillaries 16, and the position of each capsule 14 within
growth container 2.
[0040] The present invention offers the following technical
advantages over the prior art. First, it eliminates direct contact
between the dopant 17 and the SiC source 1, so the transient
processes associated with the chemical reactions between the dopant
17 and SiC source 1 are avoided or eliminated. Secondly, the
present invention provides a means to precisely control the flux of
the dopant 17 to the SiC seed 4. These technical advantages lead to
the production of precisely and uniformly doped SiC crystals 5.
[0041] The direct consequence of precise and spatially uniform
doping is SiC single crystals 5 with spatially uniform and
controllable electrical properties. In addition to the superior
electrical properties, the invention avoids or eliminates the
formation of impurity precipitates and associated defects and,
thus, leads to the improvement in the SiC crystal 5 quality and
wafer yield.
[0042] Specifically, for a vanadium doped SiC crystal 5, the
application of the present invention increases the yield of usable
prime quality SiC wafers by as much as 50%. This in-turn leads to
reduced costs and improved profitability.
[0043] The present invention has been applied to the growth of
semi-insulating 6H--SiC single crystals doped during growth with
vanadium. However, this is not to be construed as limiting the
invention since it is envisioned that the invention can also be
applied to the growth of 4H--SiC, 3C--SiC or 15R--SiC single
crystals doped during growth with a suitable dopant. In Examples 2
and 3 below, a single time-release capsule 14 made of pure dense
graphite was used. All other parameters of the SiC growth process,
such as temperature, pressure, temperature gradient, etc., were in
accordance with existing growth techniques used for the production
of SiC crystals 5.
Example 1
[0044] In accordance with the prior art SiC crystal growth method,
an appropriate amount of elemental vanadium was admixed to the SiC
source 1. The SiC source/vanadium mixture and a SiC seed 4 were
loaded into growth container 2 which was then evacuated and filled
with an inert gas to a desired pressure. Following this, the
temperature of growth container 2 was raised to a temperature
sufficient to cause the growth of the SiC crystal 5.
[0045] Thereafter, the grown SiC crystal 5 boule was sliced into
wafers and the impurity content for vanadium and other elements was
measured using Secondary Ion Mass Spectroscopy (SIMS) in wafer #2
and wafer #17 (the last wafer in the boule). The results showed
that wafer #2 contained vanadium at about 1.4.times.10.sup.17
cm.sup.-3 while wafer #17 contained vanadium at about
2.times.10.sup.14 cm.sup.-3.
[0046] With reference to FIG. 4, the resistivity of each wafer
obtained from the grown boule was measured and plotted. In the
plot, each point represents an average resistivity for the
particular wafer. As can be seen, the resistivity of the
first-to-grow wafers is very high (on the order of
2.times.10.sup.11 .OMEGA.cm) while the resistivity of the
last-to-grow wafers is low, below 10.sup.5 .OMEGA.cm. One skilled
in the art would immediately recognize that only those wafers that
have the resistivity above 10.sup.5 .OMEGA.cm are semi-insulating
and can be used in the manufacturing of RF devices, while wafers
with the resistivity below 10.sup.5 .OMEGA.cm would be
rejected.
[0047] Investigation under a low-magnification optical microscope
of the first-to-grow wafers sliced from this boule showed that at
least three of them contained precipitates of V-rich second phase
(see FIG. 5). The precipitates caused generation of defects such as
dislocations and micropipes, which spread from the area populated
by precipitates into other parts of the boule.
[0048] Thus, prior art SiC doping causes nonuniform distribution of
dopant, spatially nonuniform electrical properties, and formation
of crystal defects.
Example 2
[0049] In accordance with a SiC crystal growth method of the
present invention, a capsule 14 having a capillary 16 of 1.5 mm in
diameter and 6 mm long was loaded with 1 g of pure vanadium carbide
(VC.sub.0.88, 99.999+%). Capsule 14 was positioned atop the SiC
source 1 in growth container 2. All other parameters of this growth
run were in accordance with existing standard technological
procedures.
[0050] After finishing this growth run and cooling to room
temperature, capsule 14 was recovered and its content investigated.
A pellet of sintered vanadium carbide was found inside capsule 14.
Chemical analysis of the pellet showed that it consisted of
vanadium and carbon in the stoichiometric ratio of VC.sub.x
(x.apprxeq.0.8) with traces of silicon accounting for less than 3
weight %. Thus, there was no major chemical transformation in
capsule 14 during growth, and vanadium was preserved in its stable
form of vanadium carbide. The traces of silicon could be a result
of silicon diffusion through the capsule wall or silicon vapor back
streaming through the capillary 16. Both these marginal processes
could not change significantly the composition of the dopant 17 in
capsule 14.
[0051] The grown boule was sliced into wafers, two of which, wafer
#03 (near the SiC seed 4) and wafer #15 (near the boule dome), were
analyzed for impurity content using SIMS. The results showed that
wafer #03 contained vanadium at a level of 2.90.times.10.sup.16
cm.sup.-3 while wafer #15 contained vanadium at a level of
2.34.times.10.sup.16 cm.sup.-3. Investigation under a microscope
found no precipitates of secondary phases. Moreover, the density of
micropipes and other defects in this boule was observed to be
low.
[0052] With reference to FIG. 6, the resistivity of each wafer
obtained from the boule grown in accordance with this Example 2 was
measured and plotted. In the plot, each point represents an average
resistivity for the particular wafer. As can be seen, the
resistivity of all 15 wafers sliced from this boule was close to
1.7.times.10.sup.11 .OMEGA.cm, with no visible decrease in the
last-to-grow wafers.
Example 3
[0053] In accordance with a SiC crystal growth method of the
present invention, a capsule 14 having a capillary 16 of 1.5 mm in
diameter and 6 mm long was loaded with 1 g of elemental vanadium of
99.995% purity. Capsule 14 was positioned atop the SiC source 1 in
growth container 2. All other parameters of this growth run were in
accordance with existing standard technological procedures.
[0054] After finishing this growth run and cooling to room
temperature, the capsule content was investigated. It was found
that during heating to the growth temperature, vanadium melted and
reacted with carbon of the capsule wall to form stable vanadium
carbide, VC.sub.x with x.apprxeq.0.9. No further chemical
transformations occurred during growth cycle.
[0055] The grown boule was sliced into wafers, two of which, wafer
#03 and wafer #17 (the last wafer of the boule), were analyzed for
impurity content using SIMS. The results showed that wafer #03
contained vanadium at a level of 3.4.times.10.sup.16 cm.sup.-3
while wafer #17 contained vanadium at a level of about
2.7.times.10.sup.16 cm.sup.-3.
[0056] The resistivity in the wafers sliced from this boule was so
high that it exceeded the upper sensitivity limit of the measuring
instrument. Accordingly, the resistivity data is plotted in FIG. 7
as empty circles at 10.sup.12 .OMEGA.cm indicating that the actual
resistivity is higher. These values of resistivity exceeded by
several orders of magnitude the current requirements for semi-
insulating SiC substrates.
[0057] The level of vanadium in this boule was high enough to cause
full electrical compensation, but much lower than the solubility
limit, so no precipitates of secondary phases were formed. The
grown boule was of good crystal quality with low densities of
micropipes and other defects.
[0058] The invention has been described with reference to the
preferred embodiments. Obvious modifications and alterations will
occur to others upon reading and understanding the preceding
detailed description. It is intended that the invention be
construed as including all such modifications and alterations
insofar as they come within the scope of the appended claims or the
equivalents thereof.
* * * * *