U.S. patent number RE48,378 [Application Number 15/583,538] was granted by the patent office on 2021-01-05 for vanadium compensated, si sic single crystals of nu and pi type and the crystal growth process thereof.
This patent grant is currently assigned to II-VI Delaware, Inc.. The grantee listed for this patent is II-VI Delaware, Inc.. Invention is credited to Thomas E. Anderson, Avinash K. Gupta, Varatharajan Rengarajan, Gary E. Ruland, Andrew E. Souzis, Ping Wu, Xueping Xu, Ilya Zwieback.
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United States Patent |
RE48,378 |
Zwieback , et al. |
January 5, 2021 |
Vanadium compensated, SI SiC single crystals of NU and PI type and
the crystal growth process thereof
Abstract
In a crystal growth apparatus and method, polycrystalline source
material and a seed crystal are introduced into a growth ambient
comprised of a growth crucible disposed inside of a furnace
chamber. In the presence of a first sublimation growth pressure, a
single crystal is sublimation grown on the seed crystal via
precipitation of sublimated source material on the seed crystal in
the presence of a flow of a first gas that includes a reactive
component that reacts with and removes donor and/or acceptor
background impurities from the growth ambient during said
sublimation growth. Then, in the presence of a second sublimation
growth pressure, the single crystal is sublimation grown on the
seed crystal via precipitation of sublimated source material on the
seed crystal in the presence of a flow of a second gas that
includes dopant vapors, but which does not include the reactive
component.
Inventors: |
Zwieback; Ilya (Township of
Washington, NJ), Wu; Ping (Warren, NJ), Rengarajan;
Varatharajan (Flanders, NJ), Gupta; Avinash K. (Basking
Ridge, NJ), Anderson; Thomas E. (Morristown, NJ), Ruland;
Gary E. (Morris Plains, NJ), Souzis; Andrew E.
(Hawthorne, NJ), Xu; Xueping (Westport, CT) |
Applicant: |
Name |
City |
State |
Country |
Type |
II-VI Delaware, Inc. |
Wilmington |
DE |
US |
|
|
Assignee: |
II-VI Delaware, Inc.
(Wilmington, DE)
|
Family
ID: |
1000002725401 |
Appl.
No.: |
15/583,538 |
Filed: |
May 1, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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61651143 |
May 24, 2012 |
|
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Reissue of: |
13902016 |
May 24, 2013 |
9090989 |
Jul 28, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01B
3/025 (20130101); C30B 23/002 (20130101); C30B
23/002 (20130101); C30B 29/36 (20130101); C30B
29/36 (20130101); H01B 3/025 (20130101) |
Current International
Class: |
H01B
1/02 (20060101); H01B 3/02 (20060101); C30B
29/36 (20060101); H01B 1/22 (20060101); C30B
23/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO-2012088996 |
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Jul 2012 |
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WO |
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Other References
Bickermann et al., "Preparation of Semi-Insulating Silicon Carbide
by Vanadium Doping During PVT Bulk Crystal Growth", Materials
Science Forum, 2003, pp. 51-54, vols. 433-436. cited by applicant
.
Nunnally et al., "Silicon Carbide Photo-Conductive Switch Results
Using Commercially Available Matrial", IEEE, 2010, pp. 170-173.
cited by applicant .
Sheppard et al., "High-Power Microwave GaN/ AlGaN HEMT's on
Semi-Insulating Silicon Carbide Substrates", IEEE Electron Device
Letters, Apr. 1999, pp. 161-163, vol. 20, No. 4. cited by
applicant.
|
Primary Examiner: Vincent; Sean E
Attorney, Agent or Firm: Blank Rome LLP
Claims
The invention claimed is:
1. A crystal growth method comprising: (a) providing a SiC single
crystal seed and a polycrystalline SiC source material in spaced
relation inside of a growth crucible that is disposed inside of a
furnace chamber, the growth crucible disposed inside of a furnace
chamber defining a growth ambient; and (b) sublimation growing a
SiC single crystal on the SiC seed crystal via precipitation of
sublimated SiC source material on the SiC seed crystal; and (c)
causing a reactive atmosphere to form in the growth ambient that
reacts with background nitrogen and boron present in the growth
ambient forming a solid nitride compound with the background
nitrogen and a gaseous boron halide compound with the background
boron.
2. The method of claim 1, wherein the reactive atmosphere includes
a halide vapor compound and one or more gases.
3. The method of claim 2, wherein: the halide vapor compound is
comprised of (1) fluorine or chlorine, and (2) tantalum or niobium;
and the one or more gases includes argon, hydrogen, or a mixture of
argon+hydrogen.
4. The method of claim 2, further including: (d) following step
(c), changing the atmosphere in the growth ambient to a
non-reactive atmosphere; and (e) following step (d), introducing
into the growth ambient a vanadium dopant that causes the portion
of the SiC single crystal sublimation growing on the SiC seed
crystal after step (d) to be .[.fully compensated and
semi-insulating.]. .Iadd.doped with vanadium.Iaddend..
5. The method of claim 4, wherein step (e) further includes
introducing into the growth ambient a dopant of boron or
nitrogen.
6. The method of claim 4, wherein, in step (e), the vanadium dopant
is introduced into the growth ambient via controlled effusion.
7. The method of claim 4, wherein introducing the vanadium dopant
into the growth ambient in step (e) includes moving the vanadium
dopant from a position outside the growth crucible where the
vanadium dopant is a solid to a position inside the growth crucible
where the vanadium dopant produces vanadium vapors during
sublimation growth of the SiC single crystal.
8. The method of claim 4, wherein a pressure inside of the growth
crucible during sublimation growth of the SiC single crystal is
between 1 and 100 Torr.
9. A crystal growth method comprising: (a) introducing a
polycrystalline source material and a seed crystal into a growth
ambient comprised of a growth crucible disposed inside of a furnace
chamber; (b) in the presence of a first sublimation growth pressure
in the growth ambient, sublimation growing a single crystal on the
seed crystal via precipitation of sublimated source material on the
seed crystal in the presence of a flow of a first gas that includes
a reactive component that reacts with gaseous nitrogen in the
growth ambient forming a solid nitride compound, reacts with boron
in the growth ambient forming a gaseous boron halide compound, or
both; and (c) following step (b) and in the presence of a second
sublimation growth pressure in the growth ambient, sublimation
growing the single crystal on the seed crystal via precipitation of
sublimated source material on the seed crystal in the presence of a
flow of a second gas that includes dopant vapors, but which does
not include the reactive component.
10. The method of claim 9, wherein: each sublimation growth
pressure is between 1 and 100 Torr; and the first and second
sublimation growth pressures can be the same or different.
11. The method of claim 9, further including introducing a source
of the dopant vapors into the growth crucible between steps (b) and
(c).
12. The method of claim 9, wherein steps (b) and (c) are performed
without exposing the growth ambient to room ambient atmosphere
between said steps.
13. The method of claim 9, wherein: the reactive component of the
first gas is a gaseous metal halide; the dopant vapors of the
second gas comprise gaseous vanadium; and the second gas further
comprises hydrogen, nitrogen or hydrogen+nitrogen.
.[.14. A SiC crystal growth method comprising: (a) providing a SiC
single crystal seed and a polycrystalline SiC source material in
spaced relation inside of a growth crucible that is disposed in a
furnace chamber, wherein the crucible disposed in the furnace
chamber defines a growth ambient; (b) initiating sublimation growth
of a SiC single crystal on the SiC single crystal seed in the
growth ambient; (c) following step (b), substantially removing
background impurities of nitrogen and boron from the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient; and (d) following step
(c), introducing vanadium and boron dopants into the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient thereby sublimation
growing a PI-type SiC single crystal on the SiC seed crystal,
wherein the grown PI-type SiC single crystal is semi-insulating,
has a room-temperature resistivity of at least 10.sup.10 Ohm-cm,
and an activation energy of resistivity in the range between
approximately 0.9 and 1.5 eV in the temperature range between room
temperature and 400.degree. C..].
15. .[.The SiC crystal growth method of claim 14.]. .Iadd.A SiC
crystal growth method comprising: (a) providing a SiC single
crystal seed and a polycrystalline SiC source material in spaced
relation inside of a growth crucible that is disposed in a furnace
chamber, wherein the crucible disposed in the furnace chamber
defines a growth ambient; (b) initiating sublimation growth of a
SiC single crystal on the SiC single crystal seed in the growth
ambient; (c) following step (b), substantially removing background
impurities of nitrogen and boron from the growth ambient during
sublimation growth of the SiC single crystal on the SiC single
crystal seed in the growth ambient; and (d) following step (c),
introducing vanadium and boron dopants into the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient thereby sublimation
growing a PI-type SiC single crystal on the SiC seed
crystal.Iaddend., wherein the PI-type SiC single crystal further
comprises: shallow acceptors present in larger concentrations than
shallow donors; and vanadium present in concentrations sufficient
to achieve full compensation.
16. .[.The SiC crystal growth method of claim 14.]. .Iadd.A SiC
crystal growth method comprising: (a) providing a SiC single
crystal seed and a polycrystalline SiC source material in spaced
relation inside of a growth crucible that is disposed in a furnace
chamber, wherein the crucible disposed in the furnace chamber
defines a growth ambient; (b) initiating sublimation growth of a
SiC single crystal on the SiC single crystal seed in the growth
ambient; (c) following step (b), substantially removing background
impurities of nitrogen and boron from the growth ambient during
sublimation growth of the SiC single crystal on the SiC single
crystal seed in the growth ambient; and (d) following step (c),
introducing vanadium and boron dopants into the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient thereby sublimation
growing a PI-type SiC single crystal on the SiC seed
crystal.Iaddend., wherein the PI-type SiC single crystal further
comprises: background nitrogen intentionally reduced in a
concentration between 410.sup.15 and 710.sup.15 atoms-cm.sup.-3;
and intentionally introduced boron and vanadium dopants in
concentrations between 910.sup.15 and 210.sup.16 atoms-cm.sup.-3,
and 910.sup.16 and 210.sup.17 atoms-cm.sup.-3, respectively.
17. The SiC crystal growth method of claim .[.14.].
.Iadd.15.Iaddend., wherein the PI-type SiC single crystal further
comprises a 4H or 6H polytype.
.[.18. A SiC crystal growth method comprising: (a) providing a SiC
single crystal seed and a polycrystalline SiC source material in
spaced relation inside of a growth crucible that is disposed in a
furnace chamber, wherein the crucible disposed in the furnace
chamber defines a growth ambient; (b) initiating sublimation growth
of a SiC single crystal on the SiC single crystal seed in the
growth ambient; (c) following step (b), substantially removing
background impurities of nitrogen and boron from the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient; and (d) following step
(c), introducing vanadium and nitrogen dopants into the growth
ambient during sublimation growth of the SiC single crystal on the
SiC single crystal seed in the growth ambient thereby sublimation
growing a NU-type SiC single crystal on the SiC seed crystal,
wherein the grown NU-type SiC single crystal is semi-insulating,
has a room-temperature resistivity of at least 10.sup.10 Ohm-cm,
and an activation energy of resistivity between approximately 0.78
and 0.82 eV in the temperature range between room temperature and
400.degree. C..].
19. .[.The SiC crystal growth method of claim 18.]. .Iadd.A SiC
crystal growth method comprising: (a) providing a SiC single
crystal seed and a polycrystalline SiC source material in spaced
relation inside of a growth crucible that is disposed in a furnace
chamber, wherein the crucible disposed in the furnace chamber
defines a growth ambient; (b) initiating sublimation growth of a
SiC single crystal on the SiC single crystal seed in the growth
ambient; (c) following step (b), substantially removing background
impurities of nitrogen and boron from the growth ambient during
sublimation growth of the SiC single crystal on the SiC single
crystal seed in the growth ambient; and (d) following step (c),
introducing vanadium and nitrogen dopants into the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient thereby sublimation
growing a NU-type SiC single crystal on the SiC seed
crystal.Iaddend., wherein the NU-type SiC single crystal further
comprises: shallow donors present in larger concentrations than
shallow acceptors, and vanadium present in concentrations
sufficient to achieve full compensation.
20. .[.The SiC crystal growth method of claim 18.]. .Iadd.A SiC
crystal growth method comprising: (a) providing a SiC single
crystal seed and a polycrystalline SiC source material in spaced
relation inside of a growth crucible that is disposed in a furnace
chamber, wherein the crucible disposed in the furnace chamber
defines a growth ambient; (b) initiating sublimation growth of a
SiC single crystal on the SiC single crystal seed in the growth
ambient; (c) following step (b), substantially removing background
impurities of nitrogen and boron from the growth ambient during
sublimation growth of the SiC single crystal on the SiC single
crystal seed in the growth ambient; and (d) following step (c),
introducing vanadium and nitrogen dopants into the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient thereby sublimation
growing a NU-type SiC single crystal on the SiC seed
crystal.Iaddend., wherein the NU-type SiC single crystal further
comprises: background boron intentionally reduced in a
concentration between 210.sup.15 and 810.sup.15 atoms-cm.sup.-3;
and intentionally introduced nitrogen and vanadium dopants in
concentrations between 810.sup.15 and 210.sup.16 atoms-cm.sup.-3,
and 910.sup.16 and 210.sup.17 atoms-cm.sup.-3, respectively.
21. The SiC crystal growth method of claim .[.18.].
.Iadd.19.Iaddend., wherein the NU-type SiC single crystal further
comprises a 4H or 6H polytype.
.Iadd.22. The method of claim 4, wherein the portion of the SiC
single crystal that is doped with vanadium is fully compensated and
semi-insulating..Iaddend.
.Iadd.23. A SiC crystal growth method comprising: (a) providing a
SiC single crystal seed and a polycrystalline SiC source material
in spaced relation inside of a growth crucible that is disposed in
a furnace chamber, wherein the crucible disposed in the furnace
chamber defines a growth ambient; (b) initiating sublimation growth
of a SiC single crystal on the SiC single crystal seed in the
growth ambient; (c) following step (b), substantially removing
background impurities of nitrogen and boron from the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient; and (d) following step
(c), introducing vanadium and boron dopants into the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient thereby sublimation
growing a PI-type SiC single crystal on the SiC seed crystal,
wherein the grown PI-type SiC single crystal is semi-insulating,
has a room-temperature resistivity of at least 10.sup.10 Ohm-cm,
and an activation energy of resistivity in the range between
approximately 0.9 and 1.5 eV in the temperature range between room
temperature and 400.degree. C..Iaddend.
.Iadd.24. A SiC crystal growth method comprising: (a) providing a
SiC single crystal seed and a polycrystalline SiC source material
in spaced relation inside of a growth crucible that is disposed in
a furnace chamber, wherein the crucible disposed in the furnace
chamber defines a growth ambient; (b) initiating sublimation growth
of a SiC single crystal on the SiC single crystal seed in the
growth ambient; (c) following step (b), substantially removing
background impurities of nitrogen and boron from the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient; and (d) following step
(c), introducing vanadium and nitrogen dopants into the growth
ambient during sublimation growth of the SiC single crystal on the
SiC single crystal seed in the growth ambient thereby sublimation
growing a NU-type SiC single crystal on the SiC seed crystal,
wherein the grown NU-type SiC single crystal is semi-insulating,
has a room-temperature resistivity of at least 10.sup.10 Ohm-cm,
and an activation energy of resistivity between approximately 0.78
and 0.82 eV in the temperature range between room temperature and
400.degree. C..Iaddend.
.Iadd.25. The SiC crystal growth method of claim 16, wherein the
PI-type SiC single crystal further comprises a 4H or 6H
polytype..Iaddend.
.Iadd.26. The SiC crystal growth method of claim 23, wherein the
PI-type SiC single crystal further comprises a 4H or 6H
polytype..Iaddend.
.Iadd.27. The SiC crystal growth method of claim 20, wherein the
NU-type SiC single crystal further comprises a 4H or 6H
polytype..Iaddend.
.Iadd.28. The SiC crystal growth method of claim 24, wherein the
NU-type SiC single crystal further comprises a 4H or 6H
polytype..Iaddend.
Description
.Iadd.CROSS REFERENCE TO RELATED APPLICATION .Iaddend.
.Iadd.This application is a reissue of application Ser. No.
13/902,016, which issued as U.S. Pat. No. 9,090,989..Iaddend.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to silicon carbide single crystals
and, in particular, to vanadium-compensated, Semi-Insulating
(hereafter 'SI) SiC single crystals of 4H and 6H polytype intended
for applications in semiconductor, electronic and optoelectronic
devices.
2. Description of Related Art
DEFINITIONS
The following definitions will be used herein.
Donors. Impurities in a semiconductor which are capable of donating
electrons to the Conduction Band (hereafter 'CB) or to other levels
in the bandgap are called donors.
Acceptors. Impurities in a semiconductor which are capable of
capturing electrons from the Valance Band (hereafter 'VB) or from
other levels in the bandgap are called acceptors.
Shallow Donors. Donors which are substantially ionized at room
temperature are called shallow donors. Nitrogen (N) is an element
of the V Group of the Periodic Table having 5 valence electrons. In
the SiC lattice, N substitutes for C and gives four electrons to
form ionic-covalent bonds with four silicon neighbors, thus
assuming a ground state with one extra electron. The binding energy
of this extra electron is about 0.08 eV; accordingly, the energy
level of N in the SiC bandgap is at about 0.08 eV below the CB. Due
to the low binding energy, N easily ionizes by donating one
electron to the CB. As an example, in a 6H SiC crystal including N
at a concentration between 110.sup.16 and 110.sup.17 N
atoms-cm.sup.-3, about 60 to 90% of N donors are ionized at room
temperature producing approximately between 910.sup.15 and
610.sup.16 cm.sup.3 electrons in the CB. A semiconductor having
electric conductivity due to free electrons is called n-type.
Shallow Acceptors. Acceptors which are substantially ionized at
room temperature are called shallow acceptors. Boron (B) is an
element of the III Group of the Periodic Table having 3 valence
electrons. In the SiC lattice, B substitutes for Si and gives these
three electrons to form bonds with the carbon neighbors. It lacks
one electron to finish the tetrahedral covalent configuration and,
therefore, is ready to accept one electron on the orbit, i.e., to
act as an acceptor. Lacking one electron is equivalent of having
one hole on the outer orbit, and accepting one electron from VB is
equivalent to generating one free hole in VB. In the ground state,
the binding energy of the B-bound hole is between 0.2 and 0.3 eV;
accordingly, the energy levels of B in the SiC bandgap are at
0.2-0.3 eV above VB. Note that boron and nitrogen can occupy
several sites in the SiC lattice and produce multiple energy levels
in the SiC bandgap. As an example, a 6H SiC crystal including B at
a concentration between 110.sup.16 and 110.sup.17 B
atoms-cm.sup.-3, will have between 3% and 10% of the B acceptors
ionized at room temperature, thus producing approximately between
110.sup.15 and 310.sup.15 cm.sup.-3 holes in the VB. A
semiconductor having electric conductivity due to free holes is
called p-type.
Deep Donors and Acceptors are donors and acceptors having higher
binding energies for electrons and holes, respectively, and
therefore are not substantially ionized at room temperature.
Compared to shallow donors and acceptors, the energy levels of deep
donors and acceptors are positioned deeper in the bandgap. Vanadium
(V) produces two deep levels in the SiC bandgap--one deep donor at
1.5 eV above VB and one deep acceptor at 0.8 eV below CB. Deep
acceptors can capture electrons, while deep donors can capture
holes.
Compensated Semiconductor. In a semiconductor containing both
donors and acceptors, an electron from the donor can be captured by
the acceptor. This phenomenon is known as compensation. The
consequence of such compensation will be a reduced density of free
charge carriers. Compared to a semiconductor with a dominant donor
or a dominant acceptor, the resistivity of a compensated
semiconductor is higher.
Fully Compensated Semiconductor. A semiconductor is considered
fully compensated when all free charge carriers generated by
thermal emission from shallow donors or shallow acceptors are
removed and the resistivity approaches a theoretical limit
determined by transitions from the deep level to the band edge. The
phenomenon of full compensation in application to vanadium doped
SiC will be discussed hereinafter in more detail.
Compensation with Shallow Levels. A crystal including shallow
acceptors (or donors) can be compensated by the introduction of
shallow donors (or acceptors). Full compensation and maximum
resistivity are achieved when N.sub.D=N.sub.A, where N.sub.D and
N.sub.A are concentrations of shallow donors and acceptors,
respectively. Such compensation requires precise and equal numbers
of donors and acceptors, which is practically impossible to
achieve. In SiC, both nitrogen shallow donors and boron shallow
acceptors are background impurities and their concentrations are
difficult to control.
Compensation with Deep Levels. A more reliable way to achieve
compensation is by the introduction of deep levels. For instance, a
crystal including shallow donors (or acceptors) can be compensated
with a deep acceptor (or donor). This type of compensation does not
require precise matching of concentrations. Instead, deep levels
must be dominant, that is, present in higher concentrations than
the shallow levels.
NU-Type Semiconductor. In the case when a crystal including shallow
donors is compensated with a deep acceptor, full compensation and
maximum resistivity are achieved when the deep acceptor
concentration (N.sub.DA) exceeds that of shallow donors (N.sub.D):
N.sub.DA>N.sub.D. Upon heating of such a fully compensated
semiconductor, the electrons captured by the deep acceptor return
to CB leading to n-type conductivity with the activation energy
equal to the energy level of the deep acceptor counted from the
Conduction Band (CB). This type of fully compensated semiconductor
is called NU-type, NU standing for Greek letter v.
PI-Type Semiconductor. In the case when a crystal including shallow
acceptors is compensated with a deep donor, full compensation and
maximum resistivity are achieved when the deep donor concentration
(N.sub.DD) exceeds that of shallow acceptors (N.sub.A):
N.sub.DD>N.sub.A. Upon heating of this fully compensated
semiconductor, the holes captured by the deep donor return to VB
leading to p-type conductivity with the activation energy equal to
the energy level of the deep donor counted from the Valence Band
(VB). This type of fully compensated semiconductor is called
PI-type, PI standing for Greek letter .pi..
More generally, when a crystal contains shallow donors (N.sub.D)
and shallow acceptors (N.sub.A), its full compensation is achieved
when the density of deep levels exceeds the net shallow impurity
concentration expressed as |N.sub.D-N.sub.A|.
Electronic Properties of SiC Crystals Compensated with Vanadium
Nitrogen (shallow donor) and boron (shallow acceptor) are main
background impurities always present in measurable concentrations
in sublimation-grown SiC crystals. In 4H and 6H SiC, nitrogen
donors have their energy levels at about 0.08 eV below CB, while
boron acceptors have their energy levels at 0.2-0.3 eV above
VB.
Electronic compensation of SiC with vanadium is well known.
Background regarding electronic compensation of SiC with vanadium
can be found in U.S. Pat. No. 5,611,955; U.S. Pat. No. 7,018,597;
U.S. Pat. No. 6,507,046; U.S. Pat. No. 5,856,231; and Bickermann et
al. "Preparation of SI SiC by Vanadium Doping during PVT Bulk
Crystal Growth", J. Mat. Sci. Forum (V. 433-436) pp. 51-54. The
electron configuration of neutral V atom is 3d.sup.34s.sup.2. In
the SiC lattice, vanadium substitutes for Si atom and loses two s
and two d electrons to form ionic-covalent bonds with the four
surrounding C neighbors. This leaves the V.sup.+ ion with one
electron on the 3d-shell. The 3d-shell of vanadium is split by the
SiC crystal field into 3d.sup.1 and 3d.sup.2 orbitals positioned
within the SiC bandgap: the 3d.sup.1 orbital is located .about.1.5
eV above VB, while the 3d.sup.2 orbital is located .about.0.8 eV
below CB. In the absence of shallow impurities, the 3d.sup.1
orbital is filled, while the 3d.sup.2 orbital is empty.
As a result of this electron configuration, vanadium in SiC can
compensate either shallow donors or shallow acceptors, depending on
what element dominates in the shallow impurity background. When a
shallow donor is dominant, i.e. N.sub.D>N.sub.A, vanadium
captures electrons from the shallow donor onto its empty 3d.sup.2
orbital (V.sup.4++e.sup.-.fwdarw.V.sup.3+), thus acting as a deep
acceptor. In the case of full compensation, the Fermi level
coincides with the level of the vanadium deep acceptor at about 0.8
eV below CB. Upon heating, free electrons are released back into
the CB with the activation energy of .about.0.8 eV. This type of
fully compensated SiC is a semiconductor of NU-type. The
theoretical limits for the resistivity of NU-type 6H SiC and 4H SiC
are in the range between 210.sup.11 and 410.sup.11 Ohm-cm at room
temperature.
When a shallow acceptor is dominant, i.e. N.sub.A>N.sub.D,
vanadium captures holes from the shallow acceptor onto its 3d'
orbital VB (V.sup.4++h.sup.+.fwdarw.V.sup.5+), thus acting as a
deep donor. In the case of full compensation, the Fermi level
coincides with the level of the vanadium deep donor at .about.1.5
eV above VB. Upon heating, holes are released back into the VB with
the activation energy of .about.1.5 eV. This type of fully
compensated SiC is a semiconductor of PI-type. The theoretical
limits for the resistivity of PI-type 6H SiC and 4H SiC are as high
as 10.sup.20-10.sup.21 Ohm-cm at room temperature.
In the rare case when an SiC crystal is compensated with vanadium,
while shallow acceptors approximately balance shallow donors, the
Fermi level position and electronic properties of the crystal are
determined by the vacancy-related native point defects, said
defects having their energy levels in the middle portion of the gap
and present in sublimation-grown SiC crystals at concentrations on
the order of 10.sup.15-10.sup.16 cm.sup.-3. In such crystals, the
Fermi level is often found at 0.9 to 1.5 eV from the conduction
band (CB). Upon heating, the compensated crystal can assume either
n- or p-type conductivity, depending on the nature of the
deep-level point defects, with the activation energies ranging from
0.9 to 1.5 eV. Due to the Fermi level position near the mid-gap,
the resistivity of such crystals is higher than in the NU-type
crystals, such as 10.sup.12 Ohm-cm or higher. Conditionally, one
can designate such crystals as PI-type as well.
Normally, vanadium substitutes for silicon in the SiC lattice.
However, vanadium and other impurities can also occupy "abnormal"
sites in the lattice. For instance, vanadium can substitute for
carbon, or can occupy defect-relates sites, such as dislocations
and sub-grain boundaries, or form clusters with vacancies. An
impurity occupying an "abnormal" site in the crystal lattice can
show an electrically "abnormal" behavior or be electrically
inactive.
The technique of Secondary Ion Mass Spectroscopy (SIMS) is commonly
used to determine the concentrations of impurities in SiC. This
technique yields the total impurity concentration, both in
electrically active and electrically inactive states. Therefore,
the impurity concentration determined by SIMS is always higher than
the electrically active impurity concentration.
When the vanadium concentration (N.sub.V) is equal or slightly
higher than |N.sub.D-N.sub.A|, wherein N.sub.V, N.sub.D and N.sub.A
are determined by SIMS, the SiC crystal can still have free charge
carriers due to the fact that not all of the impurities are in the
electrically active state. Therefore, full compensation can be
achieved reliably only when N.sub.V is at least 3-4 times higher
than |N.sub.D-N.sub.A|.
SiC Sublimation Growth--Prior Art
The conventional technique of sublimation, often called Physical
Vapor Transport (PVT), has been widely used for the growth of
commercial-size SiC single crystals. A schematic view of a prior
art PVT growth cell 8 is shown in FIG. 1. The process is carried
out in PVT growth cell 8 which includes a chamber 10, which is
usually water-cooled and made of fused silica, which includes a
growth crucible 11 and thermal insulation 12 which surrounds the
crucible inside of chamber 10. Growth crucible 11 is commonly made
of dense, fine-grain, isostatically molded graphite, while thermal
insulation 12 is made from light-weight, fibrous graphite.
Crucible 11 is sealed with graphite lid 11a and includes SiC
sublimation source 14 and SiC seed crystal 15. Generally, SiC
source 14 is polycrystalline SiC grain disposed at the bottom of
crucible 11. SiC seed 15 is a SiC wafer disposed at the crucible
top. Other graphite components of the growth cell (not shown) can
include heat shields, growth guides, spacers, etc. Inductive and/or
resistive type of heating can be used in SiC crystal growth; as a
non-limiting illustration, FIG. 1 shows RF coil 16 as a heater.
Typical SiC sublimation growth temperatures are between
2000.degree. C. and 2400.degree. C. At these temperatures, SiC
source 14 vaporizes and fills the crucible with SiC vapors 19 that
includes Si.sub.2C, SiC.sub.2 and Si volatile molecules. During
growth, a temperature of SiC source 14 is maintained higher by
10.degree. C.-200.degree. C. than that of SiC seed 15; this forces
SiC vapors 19 to migrate and precipitate on SiC seed 15 causing
growth of a SiC single crystal 17 on SiC seed 15. Vapor transport
is symbolized in FIG. 1 by arrows 19. In order to control the
growth rate of SiC single crystal 17 and ensure high crystal
quality, SiC sublimation growth is carried out under a small
pressure of inert gas, generally, between several and 100 Torr.
All SiC crystals grown by sublimation in accordance with the prior
art include substantial concentrations of nitrogen (N) and boron
(B) as unintentional background impurities. Graphite is the main
source of background nitrogen in SiC crystals. When exposed to air,
the graphite forming PVT growth cell 8 readily adsorbs H.sub.2O,
O.sub.2 and N.sub.2 from the atmosphere. Upon heating, graphite
releases these gases into the interior of growth crucible 11. At
high temperatures of SiC sublimation growth, oxygen and water vapor
react with carbon to form CO, CO.sub.2, and H.sub.2, while nitrogen
causes contamination of SiC single crystal 17.
The graphite forming PVT growth cell 8 is also the main source of
background boron. Inside a graphite lattice, boron forms strong
chemical bonds with the neighboring carbon atoms (hereafter
`carbon-bound boron`). When the Si-bearing vapors 19 attack and
erode the wall of the graphite growth crucible, they react with
boron and transport it to the growing SiC crystal.
SiC sublimation growth in accordance with the prior art employs
conventional measures aimed at the reduction of boron and nitrogen
contamination. It is common practice in SiC growth to use
halogen-purified graphite for parts. However,
commercially-available purified graphite can still contain boron at
levels of 0.1-0.2 weight ppm. This translates to the presence of
background B in the crystals at levels on the order of 10.sup.16
cm.sup.3. Graphite with lower boron levels is not routinely
available from commercial manufacturers.
In order to reduce the presence of nitrogen, pre-growth vacuum
outgassing of PVT growth cell 8 and growing of SiC Crystal 17 under
a continuous flow of a high-purity inert gas are commonly employed
during SiC growth. However, these common measures are only
partially effective and contamination of growing SiC crystals 17
with nitrogen remains a problem.
As a result of insufficient removal of N.sub.2 from the growth
ambient, concentrations of background nitrogen in SiC crystals 17
grown in accordance with the prior art can be as high as 110.sup.17
cm.sup.-3, especially, in the first-to-grow portions of the
crystal.
SUMMARY OF THE INVENTION
Disclosed herein is a SiC sublimation crystal growth process
designed to yield high-quality, vanadium-compensated, SI SiC single
crystals of NU-type and PI-type. The term NU-type refers to a
specific type of fully compensated semiconductor, in which the
shallow impurity background is dominated by donors. The term
PI-type refers to a specific type of fully compensated
semiconductor, in which the shallow impurity background is
dominated by acceptors.
Also disclosed herein is a SiC crystal growth apparatus for the
growth of high-quality, vanadium-compensated, SI 4H--SiC and SI
6H--SiC single crystals of NU-type and PI-type.
Also disclosed herein are high-quality, vanadium-compensated, SI
4H--SiC and SI 6H--SiC single crystals of PI-type.
Also disclosed herein are high-quality, vanadium-compensated, SI
4H--SiC and SI 6H--SiC single crystals of NU-type.
The high-quality, vanadium-compensated, SI SiC single crystals
disclosed herein can be used in ultra-fast Photoconductive
Semiconductor Switches (PCSS), and as lattice-matched, high thermal
conductivity, insulating substrates in epitaxial SiC- and GaN-based
semiconductor devices. Background regarding the use of SI SiC
single crystals in PCSS can be found in Nunnally et al. "SiC
Photo-Conductive Switch Results Using Commercially Available
Material", In Power Modulator and High Voltage Conference (IPMHVC),
2010 IEEE International, 23-27 May 2010, pp. 170-173. Background
regarding the use of SI SiC substrates for GaN-based devices can be
found in Sheppard et al. "High-Power Microwave GaN/AlGaN HEMTs on
Semi-Insulating Silicon Carbide Substrates" in Published in
Electron Device Letters, IEEE Vol. 20, Issue 4, pp. 161-163.
Requirements of the SI SiC crystal depend on the type of the
device. In one example, NU-type SiC is a material of choice for RF
devices where a SI SiC substrate with the Fermi level in the upper
half of the bandgap is required. In another example, PI-type SiC is
a material of choice for devices where a SI SiC substrate having
the Fermi level in the middle portion of the bandgap is preferred.
In yet another example, PI-type SiC is a material of choice for
devices where a SI SiC substrate with extremely high resistivity in
excess of 210.sup.11 Ohm-cm is required. In yet another example, a
preferred material for a PCSS switch triggered by a 1064 nm light
(Nd:YAG laser) is a compensated with vanadium SI SiC crystal of
NU-type.
Disclosed herein is a crystal growth method comprising: (a)
providing a SiC single crystal seed and a polycrystalline SiC
source material in spaced relation inside of a growth crucible that
is disposed inside of a furnace chamber, the growth crucible
disposed inside of a furnace chamber defining a growth ambient; and
(b) sublimation growing a SiC single crystal on the SiC seed
crystal via precipitation of sublimated SiC source material on the
SiC seed crystal.[.in the presence of a reactive atmosphere in the
growth ambient that removes donor and/or acceptor background
impurities from the growth ambient.]..Iadd.; and (c) causing a
reactive atmosphere to form in the growth ambient that reacts with
background nitrogen and boron present in the growth ambient forming
a solid nitride compound with the background nitrogen and a gaseous
boron halide compound with the background boron.Iaddend..
The reactive atmosphere can include a halide vapor compound and one
or more inert gases. The halide vapor compound can be comprised of
(1) fluorine or chlorine, and (2) tantalum or niobium. The one or
more inert gases can include argon, hydrogen, or a mixture of
argon+hydrogen.
The method can further include: .[.(c).]. .Iadd.(d).Iaddend.,
following step .[.(b).]. .Iadd.(c).Iaddend., changing the
atmosphere in the growth ambient to a non-reactive atmosphere; and
.[.(d).]. .Iadd.(e).Iaddend., following step .[.(c).].
.Iadd.(d).Iaddend., introducing into the growth ambient a vanadium
dopant .[.that causes.]..Iadd.. The vanadium dopant can cause
.Iaddend.the portion of the SiC single crystal sublimation growing
on the SiC seed crystal after step .[.(c).]. .Iadd.(d) .Iaddend.to
be fully compensated and semi-insulating.
Step .[.(d).]. .Iadd.(e) .Iaddend.can further include introducing
into the growth ambient a dopant of boron or nitrogen.
In step .[.(d).]. .Iadd.(e).Iaddend., the vanadium dopant is
introduced into the growth ambient via controlled effusion.
Introducing the vanadium dopant into the growth ambient in step
.[.(d).]. .Iadd.(e) .Iaddend.can include moving the vanadium dopant
from a position outside the growth crucible where the vanadium
dopant is a solid to a position inside the growth crucible where
the vanadium dopant produces vanadium vapors during sublimation
growth of the SiC single crystal.
A pressure inside of the growth crucible during sublimation growth
of the SiC single crystal can be between 1 and 200 Torr.
Also disclosed is a SiC single crystal sublimation growth apparatus
comprising: a growth ambient comprised of a growth crucible inside
of a furnace chamber, wherein an interior of the growth crucible is
configured to be charged with a SiC single crystal seed and a SiC
source material in spaced relation; a doping capsule charged with
at least one dopant; means for introducing the doping capsule
charged with at the least one dopant from a position outside the
growth crucible where the at least one dopant is in solid form to a
position inside the growth crucible where the at least one dopant
releases dopant vapors into the growth crucible; and a gas
distribution system operative for: (1) supplying into the growth
ambient during sublimation growth of a SiC single crystal on the
SiC single crystal seed via sublimation of the SiC source material
prior to introducing the doping capsule into the growth crucible, a
first gas which includes a reactive component that chemically binds
to and removes donor and/or acceptor background impurities from the
growth ambient; and (2) supplying into the growth ambient during
sublimation growth of the SiC single crystal on the SiC single
crystal seed via sublimation of the SiC source material following
introducing the doping capsule into the growth crucible, a second
gas comprised of at least one inert gas.
The means for introducing the doping capsule can include a tube in
communication with the growth crucible via a plug that seals an end
of the tube in communication with the growth crucible, and a
pushrod for moving the doping capsule though the tube dislodging
the plug, whereupon the doping capsule can be moved into the growth
crucible via the end of the tube in communication with the growth
crucible.
The doping capsule can include at least one calibrated capillary
for the flow of dopant vapors from an interior of the doping
capsule into the growth crucible.
The at least one dopant can include at least one of the following:
vanadium, or vanadium and boron.
The reactive component of the first gas can be gaseous metal
halide. The second gas can comprise either hydrogen or nitrogen,
but not a reactive component.
The growth crucible, the doping capsule, or both can be made from
graphite.
The SiC source material is disposed in a source crucible which is
spaced from a bottom and a side of the interior of the growth
crucible.
Also disclosed herein is a crystal growth method comprising: (a)
introducing a polycrystalline source material and a seed crystal
into a PVT growth ambient comprised of a growth crucible disposed
inside of a furnace chamber; (b) in the presence of a first
sublimation growth pressure in the growth ambient, sublimation
growing a single crystal on the seed crystal via precipitation of
sublimated source material on the seed crystal in the presence of a
flow of a first gas that includes a reactive component that reacts
with and removes donor and/or acceptor background impurities from
the growth ambient during said sublimation growth; and (c)
following step (b) and in the presence of a second sublimation
growth pressure in the growth ambient, sublimation growing the
single crystal on the seed crystal via precipitation of sublimated
source material on the seed crystal in the presence of a flow of a
second gas that includes dopant vapors, but which does not include
the reactive component.
Each sublimation growth pressure can be between 1 and 200 Torr. The
first and second sublimation growth pressures can be the same or
different.
The method can further include introducing a source of the dopant
vapors into the growth crucible between steps (b) and (c).
Steps (b) and (c) are desirably performed without exposing the
growth ambient to ambient (or room) atmosphere between said
steps.
The reactive component of the first gas can be a gaseous metal
halide. The dopant vapors of the second gas can comprise gaseous
vanadium. The second gas further comprises hydrogen, nitrogen or
hydrogen+nitrogen.
Also disclosed is a method of forming a high-purity SiC single
crystal comprising: (a) providing SiC growth ambient, which
includes a growth crucible and a furnace chamber to hold the growth
crucible, said growth crucible charged with a SiC source and a SiC
seed crystal in spatial relation; (b) providing a reactive
atmosphere in the growth ambient, said atmosphere comprising
gaseous species capable of chemical binding of donor and/or
acceptor background impurities present in the growth ambient and
removing said impurities from said growth ambient by means of
chemical binding; (c) in the presence of the reactive atmosphere,
heating and sublimating the source material, transporting the
sublimated source material to the seed crystal and precipitating
the sublimated source material on said seed crystal causing growth
of high-purity SiC single crystal; and (d) forming a high-purity
SiC single crystal comprising donor and/or acceptor background
impurities, wherein their concentrations are intentionally reduced
by means of chemical binding of said impurities with the gaseous
species of the reactive atmosphere.
The reactive atmosphere can include at least one reactive gaseous
component capable of chemical binding of gaseous nitrogen at
elevated temperatures and removing it from the growth ambient by
forming solid metal nitride. The reactive gaseous component can be
gaseous metal halide. The reactive atmosphere can comprise gaseous
metal halide and hydrogen.
The reactive atmosphere can comprise at least one reactive gaseous
component capable of chemical binding of boron at elevated
temperatures, including carbon-bound boron, and removing it from
the growth ambient by chemical binding of said boron into
boron-bearing volatile molecular associates. The reactive gaseous
component can be gaseous metal halide. The first reactive
atmosphere can comprise gaseous metal halide and hydrogen.
The reactive atmosphere can include gaseous components capable of
reacting between themselves at elevated temperatures to yield
gaseous hydrogen halide. The reactive atmosphere can include a
gaseous metal halide chosen from the group consisting of
TaCl.sub.5, TaF.sub.5, NbCl.sub.5 and NbF.sub.5. The reactive
atmosphere desirably includes gaseous tantalum pentachloride,
TaCl.sub.5.
The high-purity SiC single crystal can include nitrogen as a
background impurity, wherein the concentration of said background
nitrogen is intentionally reduced to concentrations between
410.sup.15 and 710.sup.15 atoms-cm.sup.-3, as measured by SIMS.
Also or alternatively, the high-purity SiC single crystal can
include boron as a background impurity, wherein the concentration
of said background boron is intentionally reduced to concentrations
between 210.sup.15 and 810.sup.15 atoms-cm.sup.-3, as measured by
SIMS.
The high purity SiC single crystal can have a polytype selected
from the group consisting of the 4H and 6H polytypes of silicon
carbide.
Also disclosed is an apparatus for sublimation growth of
high-purity SiC single crystals comprising: a furnace chamber
holding a growth crucible charged with SiC source material and a
SiC seed crystal in spaced relation; a gas distribution system to
supply a flow of gas mixture into the furnace chamber, said gas
mixture forming a reactive atmosphere in the furnace chamber
capable of chemical binding at elevated temperatures of donor
and/or acceptor background impurities in a SiC growth ambient that
includes the furnace chamber and growth crucible, leading to
removal of the donor and/or the acceptor background impurities from
said growth ambient by means of chemical binding; and the crystal
growth crucible containing the SiC source material growing by
sublimation on the SiC seed crystal under reactive atmosphere to
thereby form a high-purity SiC crystal boule on the SiC seed
crystal.
The gas mixture can include at least one reactive gaseous component
capable of chemical binding of gaseous nitrogen at elevated
temperatures and removing it from the growth ambient by
precipitating it in the form of solid nitride.
The reactive gaseous component can be gaseous metal halide. The gas
mixture can be gaseous metal halide and hydrogen.
The gas mixture can include at least one reactive gaseous component
capable of chemical binding of boron, including carbon-bound boron,
and removing it from the growth ambient by chemical binding of said
boron into boron-bearing volatile molecular associates.
The reactive gaseous component can be gaseous metal halide. The gas
mixture can include gaseous metal halide and hydrogen.
The gas mixture can comprise reactive gaseous components capable of
reacting between themselves at elevated temperatures to yield
gaseous hydrogen halide. The gas mixture can comprise gaseous metal
halide chosen from the group consisting of TaCl.sub.5, TaF.sub.5,
NbCl.sub.5 and NbF.sub.5. The gas mixture desirably comprises
gaseous tantalum pentachloride, TaCl.sub.5.
Also disclosed is a sublimation-grown, high purity SiC single
crystal comprising nitrogen as a background impurity, wherein the
concentration of said nitrogen is reduced by removal of the
residual nitrogen from the growth ambient by means of chemical
binding. The concentration of background nitrogen can be reduced to
levels between 410.sup.15 and 710.sup.15 atoms-cm.sup.-3, as
measured by SIMS. The as-grown crystal can have a polytype selected
from the group consisting of the 4H and 6H polytypes of silicon
carbide.
Also disclosed is a sublimation-grown, high purity SiC single
crystal comprising boron as a background impurity, wherein the
concentration of said boron is reduced by removal of the residual
boron from the growth ambient by means of chemical binding. The
concentration of background boron can be intentionally reduced to
concentrations between 2010.sup.15 and 810.sup.15 atoms-cm.sup.-3,
as measured by SIMS. The as-grown crystal can have a polytype
selected from the group consisting of the 4H and 6H polytypes of
silicon carbide.
Also disclosed is a method of forming a fully compensated,
semi-insulating SiC single crystal of PI-type comprising: (a)
providing a SiC growth ambient, which includes a growth crucible
and a furnace chamber holding the growth crucible which is charged
with a SiC source material and a SiC seed crystal in spaced
relation; (b) providing in the growth ambient a reactive atmosphere
comprising gaseous species capable of chemical binding of donor
and/or acceptor background impurities present in the growth ambient
and removing said impurities from said growth ambient by means of
chemical binding; (c) sublimating the SiC source material and
transporting the sublimated SiC source material to the SiC seed
crystal and precipitating the sublimated SiC source material on the
SiC seed crystal causing growth of a SiC single crystal on the SiC
seed crystal, while simultaneously removing donor and/or acceptor
background impurities from the growth ambient by means of the
chemical binding; and (d) following step (c), introducing vanadium
and boron dopants into the growth ambient thereby forming fully
compensated, semi-insulating SiC single crystal of PI-type co-doped
with vanadium and boron.
The semi-insulating SiC single crystal of PI-type co-doped with
vanadium and boron includes one or more of the following:
intentionally reduced levels of background donors and acceptors; a
shallow acceptor intentionally introduced in step (d) in a
concentration exceeding the summary concentration of the residual
donors; vanadium intentionally introduced in step (d) in a
concentration sufficient to achieve full compensation; and/or a
resistivity of at least 10.sup.11 Ohm-cm at room temperature and
activation energy of resistivity of about 0.9-1.5 eV in the
temperature range between room temperature and 400.degree. C.
The vanadium and boron dopants can be introduced into the growth
ambient via a capsule made of an inert material that is introduced
into the growth ambient following step (c). The capsule can be made
of graphite. The capsule can include at least one calibrated
capillary serving as escape pathway for vapors of at least one of
the dopants.
Prior to step (d) the capsule with the dopants can be stored
outside the growth crucible at a relatively low temperature. In
step (d) the capsule with the dopants is moved into the growth
crucible.
The dopants can be elemental vanadium and boron or a boron
compound, such as, without limitation, vanadium di-boride,
VB.sub.2.
Also disclosed is an apparatus for sublimation growth of fully
compensated, semi-insulating SiC single crystals of PI-type
comprising: (a) a furnace chamber holding a growth crucible that
charged with SiC source material and a SiC seed crystal in spaced
relation; (b) a gas distribution system to supply a flow of gas
mixture into the furnace chamber, said gas mixture forming a
reactive atmosphere in the furnace chamber at elevated temperatures
capable of chemical binding to donor and/or acceptor background
impurities in a SiC growth ambient that includes the furnace
chamber and the growth crucible, leading to removal of said
background impurities from said growth ambient by means of chemical
binding; (c) a doping capsule including dopants in the capsule; and
(d) means for moving the capsule with dopants between a position
outside said the growth crucible at relatively low temperatures
during removal of background impurities and to a position inside
the growth crucible during growth of the SiC crystal of
PI-type.
The doping capsule can be made of inert material, such as graphite.
The doping capsule can include at least one calibrated capillary as
an escape pathway for vapors of the dopants.
The dopants can be elemental vanadium and boron, or vanadium
compounds and boron compounds. The dopants can be elemental
vanadium and vanadium di-boride, VB.sub.2.
Also disclosed is a .[.fully compensated with vanadium,
semi-insulating SiC single crystal of PI-type having.]. .Iadd.SiC
crystal growth method comprising: (a) providing a SiC single
crystal seed and a polycrystalline SiC source material in spaced
relation inside of a growth crucible that is disposed in a furnace
chamber, wherein the crucible disposed in the furnace chamber
defines a growth ambient; (b) initiating sublimation growth of a
SiC single crystal on the SiC single crystal seed in the growth
ambient; (c) following step (b), substantially removing background
impurities of nitrogen and boron from the growth ambient during
sublimation growth of the SiC single crystal on the SiC single
crystal seed in the growth ambient; and (d) following step (c),
introducing vanadium and boron dopants into the growth ambient
during sublimation growth of the SiC single crystal on the SiC
single crystal seed in the growth ambient thereby sublimation
growing a PI-type SiC single crystal on the SiC seed crystal. The
grown PI-type SiC single crystal can be semi-insulating, can have
.Iaddend.a room-temperature resistivity of at least 10.sup.11
Ohm-cm.Iadd., .Iaddend.and .Iadd.can have .Iaddend.an activation
energy of resistivity in the range between approximately 0.9 and
1.5 eV in the temperature range between room temperature and
400.degree. C.
The PI-type SiC single crystal can include: shallow acceptors,
shallow donors and vanadium, said shallow acceptors present in
larger concentrations than shallow donors, and said vanadium
present in concentrations sufficient to achieve full compensation
and a room-temperature resistivity of at least 10.sup.11 Ohm-cm and
an activation energy of resistivity of about 0.9-1.5 eV in the
temperature range between room temperature and 400.degree. C.
The PI-type SiC single crystal can include: background nitrogen
impurity in a concentration between 410.sup.15 and 710.sup.15
atoms-cm.sup.-3, and intentionally introduced boron and vanadium
dopants in concentrations between 910.sup.15 to 210.sup.16
atoms-cm.sup.-3 and 910.sup.16 to 210.sup.17 atoms-cm.sup.-3,
respectively.
The PI-type SiC single crystal can include: intentionally
introduced boron and vanadium dopants and having a room-temperature
resistivity of at least 110.sup.10 Ohm-cm and, more desirably,
between 110.sup.11 and 110.sup.21 Ohm-cm, and an activation energy
of the resistivity of approximately 0.9-1.5 eV in the temperature
range between room temperature and 400.degree. C.
The PI-type SiC single crystal can be of 4H or 6H polytype.
Also disclosed is a method of forming a fully compensated,
semi-insulating SiC single crystal of NU-type comprising: (a)
providing a SiC growth ambient that includes a growth crucible and
a furnace chamber to hold the growth crucible, said growth crucible
charged with SiC source and a SiC seed crystal in spaced relation;
(b) providing a reactive atmosphere in the growth ambient, said
reactive atmosphere comprising gaseous species capable of chemical
binding of donor and/or acceptor background impurities present in
the growth ambient and removing said impurities from said growth
ambient by means of chemical binding; (c) in the presence of said
reactive atmosphere, sublimating the source material, whereupon the
sublimated SiC source material transports to and precipitates on
the SiC seed crystal causing growth of a SiC single crystal on the
SiC seed crystal, while simultaneously said reactive atmosphere
removes donor and/or acceptor background impurities from the growth
ambient by means of chemical binding; and (d) following step (c),
introducing vanadium and nitrogen dopants into the growth ambient
and forming fully compensated, semi-insulating SiC single crystal
of NU-type co-doped with vanadium and nitrogen having one or more
of the following: intentionally reduced levels of background donors
and acceptors; a shallow donor intentionally introduced in step (d)
in a concentration exceeding the summary concentration of the
residual acceptors; vanadium intentionally introduced in step (d)
in a concentration sufficient to achieve full compensation; and/or
a resistivity of at least 10.sup.10 Ohm-cm at room temperature and
activation energy of resistivity of about 0.78-0.82 eV in the
temperature range between room temperature and 400.degree. C.
The vanadium dopant can be contained in a capsule made of an inert
material, such as graphite. The capsule can include at least one
calibrated capillary serving as an escape pathway for vapors of the
dopant.
Prior to step (d), the capsule with the vanadium dopant can be
stored outside the growth crucible at a relatively low temperature.
In step (d), the capsule can be brought into the growth
crucible.
The dopants can be elemental vanadium and nitrogen, or vanadium
compounds and nitrogen.
Also disclosed is an apparatus for sublimation growth of fully
compensated, semi-insulating SiC single crystals of NU-type
comprising: (a) a furnace chamber holding a growth crucible charged
with SiC source material and a SiC seed in spaced relation; (b) a
gas distribution system to supply a flow of gas mixture into the
furnace chamber, said gas mixture forming a reactive atmosphere in
the furnace chamber capable of chemical binding at elevated
temperatures to background donor and/or acceptor impurities in an
SiC growth ambient leading to removal of said impurities from said
growth ambient by means of chemical binding; (c) a doping capsule
including vanadium dopant in the capsule; and (d) means for moving
the doping capsule with vanadium dopant between a position outside
the crucible at relatively low temperatures during removal of
background impurities and to a position inside the growth crucible
during growth of the SiC crystal of NU-type.
The doping capsule can be made of inert material, such as graphite.
The doping capsule can include at least one calibrated capillary
serving as an escape pathway for vapors of the vanadium dopant.
The capsule can include elemental vanadium or vanadium
compounds.
Also disclosed is a .[.fully compensated, semi-insulating SiC
single crystal of NU-type having.]. .Iadd.SiC crystal growth method
comprising: (a) providing a SiC single crystal seed and a
polycrystalline SiC source material in spaced relation inside of a
growth crucible that is disposed in a furnace chamber, wherein the
crucible disposed in the furnace chamber defines a growth ambient;
(b) initiating sublimation growth of a SiC single crystal on the
SiC single crystal seed in the growth ambient; (c) following step
(b), substantially removing background impurities of nitrogen and
boron from the growth ambient during sublimation growth of the SiC
single crystal on the SiC single crystal seed in the growth
ambient; and (d) following step (c), introducing vanadium and
nitrogen dopants into the growth ambient during sublimation growth
of the SiC single crystal on the SiC single crystal seed in the
growth ambient thereby sublimation growing a NU-type SiC single
crystal on the SiC seed crystal. The grown NU-type SiC single
crystal can be semi-insulating, can have .Iaddend.a
room-temperature resistivity of at least 10.sup.10 Ohm-cm and
.Iadd.can have .Iaddend.an activation energy of resistivity between
approximately 0.78 and 0.82 eV in the temperature range between
room temperature and 400.degree. C.
The NU-type SiC single crystal can include: shallow acceptors,
shallow donors and vanadium, said shallow donors present in larger
concentrations than shallow acceptors, and said vanadium present in
concentrations sufficient to achieve a room-temperature resistivity
of at least 10.sup.10 Ohm-cm and an activation energy of
resistivity of approximately 0.78-0.82 eV in the temperature range
between room temperature and 400.degree. C.
The NU-type SiC single crystal can include: background boron in
concentrations between 210.sup.15-810.sup.15 atoms-cm.sup.-3 and
intentionally introduced nitrogen and vanadium in concentrations
between 810.sup.15-210.sup.16 atoms-cm.sup.-3 and
910.sup.16-210.sup.17 atoms-cm.sup.-3, respectively.
The NU-type SiC single crystal can include: intentionally
introduced nitrogen and vanadium and having a room-temperature
resistivity of at least 110.sup.1.degree. Ohm-cm and, more
desirably, of at least 110.sup.11 Ohm-cm, and an activation energy
of the resistivity of 0.78-0.82 eV in the temperature range between
room temperature and 400.degree. C.
The NU-type SiC single crystal can be of 4H or 6H polytype.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of a prior art physical vapor
transport (PVT) growth cell;
FIG. 2 is a schematic of a SiC sublimation growth cell that
includes a chamber, that has a gas inlet and a gas outlet, and
which holds a crucible surrounded by thermal insulation, wherein
SiC source material and a SiC crystal which grows on a SiC seed
crystal are shown disposed inside the crucible;
FIG. 3 is a schematic illustration of one embodiment SiC crystal
growth apparatus for growth of high-purity SiC crystals;
FIG. 4 is a schematic illustration of another embodiment SiC
crystal growth apparatus for growing SiC crystals of PI-type;
FIGS. 5A and 5B are isolated schematic views of the growth
crucibles of FIGS. 4 and 7 showing movement of a dopant capsule
from a position outside the growth crucible to a position inside
the growth crucible;
FIGS. 6A and 6B are different embodiments of a doping capsule that
include a single compartment and separate compartments,
respectively, for dopants, wherein each doping capsule can be used
separately with the SiC crystal growth apparatuses shown in FIGS. 4
and 7; and
FIG. 7 is a schematic illustration of another embodiment SiC
crystal growth apparatus for growing SiC crystals of NU-type.
DETAILED DESCRIPTION OF THE INVENTION
The SiC growth process described hereinafter incorporates
conventional elements of the prior art, such as the use of
halogen-purified graphite, pre-growth vacuum outgassing and growth
under continuous purge with high-purity inert gas. In addition, the
SiC growth process described hereinafter comprises the following
novel elements:
1. Growth in a reactive atmosphere leading to removal of residual
background nitrogen and boron from the growth ambient by chemical
binding.
2. A two-stage process for the growth of SI SiC crystals of PI-type
comprising removal of background nitrogen (N) and background boron
(B) from the growth ambient in stage (a) followed by growth using
controlled co-doping of the growing crystal with vanadium (V) and B
in stage (b).
3. A two-stage process for the growth of SI SiC crystals of NU-type
comprising removal of N and B from the growth ambient in stage (a)
followed by growth using controlled co-doping of the growing
crystal with V and N in stage (b).
Growth of High Purity SiC Crystals
The concept of SiC sublimation growth under reactive atmosphere is
disclosed in U.S. Pat. No. 8,361,227 (hereinafter "the '227
patent"), which is incorporated herein by reference. The patent
discloses in-situ purification of the graphite growth cell from
boron by supplying into the SiC growth ambient a gas mixture that
includes halo silane gas.
The SiC growth process described hereinafter improves on the
in-situ purification method disclosed in the '227 patent.
Specifically, the SiC growth method disclosed herein comprises
removal of both boron and nitrogen from the growth ambient via SiC
sublimation growth in the presence of a reactive atmosphere that
includes molecular species capable of binding with gaseous nitrogen
and carbon-bound boron. This reactive atmosphere comprises volatile
reactive species of gaseous metal halide and hydrogen (H.sub.2).
The gaseous metal halide is chosen from the group of TaCl.sub.5,
TaF.sub.5, NbCl.sub.5 and NbF.sub.5. Desirably, the gaseous metal
halide is tantalum pentachloride, TaCl.sub.5.
A flow of inert gas, such as argon (Ar), brings the gaseous metal
halide and H.sub.2 into the SiC growth cell, where they participate
in chemical reactions, including reactions between themselves, as
well as reactions with the gaseous nitrogen impurity and
carbon-bound boron impurity.
FIG. 2 shows a schematic of a SiC sublimation growth cell that
includes a chamber 20, having gas inlet 20a and outlet 20b, holding
graphite crucible 21 surrounded by thermal insulation 22. SiC
source 23 and SiC crystal 24 which grows on a SiC seed crystal 24a
are shown disposed inside crucible 21 in spatial relationship
typical for SiC sublimation growth.
Gas mixture 26, which enters through the inlet 20a, includes an
inert gas, desirably Ar mixed with H.sub.2, and a vapor of volatile
halide compound designated as MeX. Element X is a halogen chosen
from the group of fluorine, F, and chlorine, Cl. Me is a metal
chosen from the group of tantalum, Ta, and niobium, Nb. Desirably,
the volatile metal halide is tantalum pentachloride, TaCl.sub.5.
Upon entering chamber 20, gas mixture 26 creates reactive
atmosphere inside chamber 20.
Thermal insulation 22 is made of light-weight fibrous graphite,
which is fully permeable to gases. After entering chamber 20, gas
mixture 26 permeates the bulk of thermal insulation 22, as shown
schematically in FIG. 2 by arrows 25.
The temperature inside the thermal insulation 22 is spatially
nonuniform. On the outer surface, which is in proximity to the
water-cooled wall of the chamber 20, the temperature can be as low
as 200-300.degree. C. An outer layer 22a of thermal insulation
where the temperature during SiC growth is between 300 and
500.degree. C., approximately, is schematically shown in FIG. 2. On
the interior surface of an inner layer 22c of thermal insulation,
which is in proximity to the crucible 21, the temperature is close
to the SiC sublimation temperature (2000-2400.degree. C.). The
inner layer 22c of thermal insulation, where the temperature is
higher than 900.degree. C., approximately, is shown in FIG. 2. An
intermediate layer 22b of thermal insulation 22, where the
temperature is between 500 and 900.degree. C., approximately, is
shown in FIG. 2.
Performed thermodynamic calculations showed that chemical reactions
between the gaseous species of the reactive atmosphere (MeX,
H.sub.2), nitrogen (N.sub.2) and boron proceed through several
steps. In the first step, as gas mixture 26 permeates the outer
layer 22a of thermal insulation, said outer layer 22a situated at
temperatures approximately between 300 and 500.degree. C., the
gaseous metal halide (MeX) reacts with H.sub.2 according to the
following reaction (1) (reaction (1) is written without
stoichiometric coefficients): MeX+H.sub.2Me.dwnarw.+HX (1)
Reaction (1) is, in essence, CVD deposition of metal Me, and it
yields elemental metal in the form of solid precipitate,
Me.dwnarw.. This reaction is partial and does not consume the
entire amount of gaseous metal halide present in the reactive
atmosphere.
In the second step, which follows the first step and which occurs
as gas mixture 26 carrying the remaining metal halide vapor
permeates the intermediate layer 22b of thermal insulation, said
intermediate layer 22b situated at temperatures approximately
between 300 and 900.degree. C., gaseous metal halide reacts with
hydrogen and nitrogen according to the following reaction (2)
(reaction (2) is written without stoichiometric coefficients):
MeX+H.sub.2+N.sub.2MeN.dwnarw.+HX, (2) where MeN.dwnarw. is a
precipitate of solid metal nitride MeN. This reaction leads to the
removal of residual N.sub.2 from the atmosphere by binding nitrogen
into solid metal nitride, MeN. Reaction (2) is, in essence, CVD
deposition of metal nitride MeN. The residual nitrogen in reaction
(2) comes from N.sub.2 released into the furnace chamber 20 from
graphite parts, such as graphite crucible 21 and thermal insulation
22.
In the third step which follows the second step, the gas mixture 26
carrying the remaining metal halide vapor moves to inner layer 22c
of thermal insulation, said inner layer 22c situated at
temperatures above 900.degree. C., the remaining metal halide
reacts with hydrogen and carbon of thermal insulation to form metal
carbide according to the following reaction (3) (reaction (3) is
written without stoichiometric coefficients):
MeX+H.sub.2+CMeC.dwnarw.+HX, (3) where Med.dwnarw. is a precipitate
of solid metal carbide. Reaction (3) is, in essence, CVD deposition
of metal carbide, MeC.
All three aforementioned reactions produce gaseous hydrogen halide,
HX, as a byproduct. Driven by the flow of gas mixture 26 into
chamber 20 and diffusion, gaseous hydrogen halide permeates the
bulk (the walls, the lid, and the base) of graphite crucible 21
situated at temperatures between 2000 and 2400.degree. C., where
said gaseous hydrogen halide reacts with carbon-bound boron and
converts it into volatile boron halides according to the following
reaction (4) (reaction (4) is written without stoichiometric
coefficients): BC+HXBX.sub.n.uparw.+CH.sub.m.uparw., (4) where BC
symbolizes carbon-bound boron, BX.sub.n.uparw. symbolizes volatile
boron-halogen molecular associates and CH.sub.m.uparw. symbolizes
gaseous hydrocarbons. In the case of hydrogen chloride, HCl, the
dominant products of reaction (4) are BCl, BCl.sub.2 and
C.sub.2H.sub.2.
The volatile products of reactions (1)-(4) are removed from the
crystal growth cell and then from the chamber by the flow of gas
mixture 26 into chamber 20, as symbolized by arrows 25a in FIG.
2.
Due to reactions (1)-(3), the bulk of thermal insulation 22,
becomes coated with thin deposits of metal, metal nitride and metal
carbide. Such coatings reduce to some degree the ability of the
insulation 22 to absorb gases, but they do not affect adversely
thermal properties of said insulation.
At high temperatures of SiC sublimation growth (2000-2400.degree.
C.), gaseous hydrogen halide also reacts with silicon carbide
leading to the appearance of volatile silicon-halogen and
hydrocarbon molecular associates according to the following
reaction (5) (reaction (5) is written without stoichiometric
coefficients): SiC+HXSiX.sub.m.uparw.+CH.sub.n.uparw., (5) where
SiX.sub.m.uparw. symbolizes volatile silicon halides and
CH.sub.n.uparw. symbolizes gaseous hydrocarbons. In the case of
hydrogen chloride, HCl, the dominant products of reaction (5) are
SiCl.sub.2 and C.sub.2H.sub.2. In practical terms, the yield of
reaction (5) is insignificant, and no noticeable silicon or carbon
losses from the crucible occur.
FIG. 3 shows a SiC crystal growth apparatus for the growth of
high-purity SiC crystals. In one desirable, non-limiting
embodiment, the metal halide used for the removal of nitrogen and
boron from the growth ambient is tantalum pentachloride,
TaCl.sub.5.
With reference to FIG. 3, the growth process is carried out in a
growth cell 8 (e.g., the growth cell 8 of FIG. 1) which includes a
chamber 10, which includes growth crucible 11 and thermal
insulation 12. Growth crucible 11 is made of dense, fine-grain,
isostatically-molded graphite, such as "ATJ" available from UCAR
Carbon Company of New York, N.Y. Thermal insulation 12 is made from
light-weight, fibrous graphite, such as Calcarb.RTM. CBCF available
from Mersen USA, St. Mary's, PA. Prior to use in SiC growth, all
graphite parts and components are commercially halogen-purified to
the total ash level of 5 ppm by weight. At present, this is the
purest graphite available commercially.
Growth crucible 11 is charged with SiC sublimation source 14
disposed at the crucible bottom and SiC seed crystal 15 disposed at
the crucible top. RF coil 16 provides heating to growth crucible
11. Upon reaching SiC sublimation growth temperatures between 2000
and 2400.degree. C., source 14 vaporizes and fills the interior of
crucible 11 with SiC vapors 19 that include volatile molecules of
Si.sub.2C, SiC.sub.2 and Si. Driven by temperature gradients, the
SiC vapors 19 migrate towards seed 15, as symbolized by arrows 19,
and precipitate on SiC seed crystal 15 causing growth of SiC single
crystal 17 on SiC seed crystal 15.
The SiC growth apparatus of FIG. 3 includes gas delivery system 30,
which serves to generate the vapor of metal halide, mix the vapor
with the carrier gas (Ar+H.sub.2) and bring the gas mixture 26 into
the furnace chamber 10 through a heated inlet 10a. This gas mixture
has the following composition: H.sub.2 (desirably, between 2 and 5%
by volume), TaCl.sub.5 vapor (desirably, between 100 and 1000 ppm
by volume), Ar (the balance). Argon pre-mixed with hydrogen to a
desired level can be used as a carrier gas.
The pressure and the flows of the gaseous components are controlled
using means known in the art, e.g., U.S. Pat. No. 6,410,433, such
as upstream valves 35 and 36, mass flow controllers 35a, 36a,
valves 35b and 36b, downstream valve 39 and vacuum pump 37. Other
common and conventional parts of the gas delivery system, such as
pressure gauges, solenoid valves, filters, electronic control, etc.
are not shown. During growth of SiC single crystal 17, the total
pressure in chamber 10 is maintained, desirably, between 5 and 50
Torr.
In FIG. 3, the source of gaseous TaCl.sub.5 is solid tantalum
pentachloride 32, which is contained in a sealed vessel 31 having
an interior volume of about 100 cm.sup.3. Vessel 31 is made of
corrosion resistant alloy, such as type 316 stainless steel, and is
heated by a heater 31a to create a spatially uniform temperature
distribution in the vessel. During growth of SiC single crystal 17,
the temperature of vessel 31 is maintained, desirably between 75
and 120.degree. C. At these temperatures, solid TaCl.sub.5
vaporizes and generates a TaCl.sub.5 vapor pressure between 0.1 and
1 Torr.
The Ar+H.sub.2 mixture is supplied into vessel 31 at a flow rate,
desirably, between 20 and 50 sccm. Inside vessel 31, the Ar+H.sub.2
mixture mixes with the TaCl.sub.5 vapor and carries it through the
valve 36b to manifold 38. Valve 36b and manifold 38 are heated by
flexible tape-heaters 38a to a temperature equal or above that of
the vessel 31 and, desirably, to a temperature between 100 and
200.degree. C.
The main flow of the Ar+H.sub.2 mixture is supplied through the
valve 35, mass flow controller 35a and valve 35b to manifold 38 at
a flow rate, desirably, between 50 and 300 sccm. The gaseous
byproducts of the reactions taking place in chamber 10 flow through
an outlet 10b, a valve 39 and vacuum pump 37 to a scrubber (not
shown) for neutralization.
Results of high-purity 6H SiC growth runs carried out in the
apparatus shown in FIG. 3 are shown in the following Table 1. The
nitrogen concentration in grown SiC single crystal 17 was between
410.sup.15 and 710.sup.15 cm.sup.-3, and the boron concentration
was between 210.sup.15 and 810.sup.15 cm.sup.-3. Compared to the
prior art, a 4-10 fold reduction in the levels of background N and
B in SiC single crystals 17 were observed.
TABLE-US-00001 TABLE 1 Activation Energy of Impurity Content,
cm.sup.-3 Rho @ RT, Resistivity Nitrogen Boron Vanadium Ohm-cm
(RT-400.degree. C.) Crystals Type of Growth Background Introduced
Background Introduced Background Introduced - Measured Extrapolated
eV 6H Prior Art 8e15-1e17 8e15-3e16 9e16-2e17 1e5-2e11 Variable 6H
High 4e15-7e15 2e15-8e15 <1e14 1e3-1e7 Variable Purity 6H
Pl-Type 4e15-7e15 8e15-2e16 9e16-2e17 1e12-1e21 0.9-1.5 4H Pi-Type
1e14-1e18 1.1-1.5 6H Nu-Type 8e15-2e16 2e15-8e15 (1-2)e11 0.78-0.80
4H Nu-Type (2-4)e11 0.79-0.82
Growth of SI SiC Single Crystals of PI-Type
The growth process for SI SiC single crystals of PI-type includes
two phases, phase (a) and phase (b). Phase (a) is growth under
reactive atmosphere aimed at removal of background N and B from the
growth ambient, as described above in connection with FIG. 3. The
duration of phase (a) of the growth process is, desirably, between
12 and 24 hours. Phase (b) of the process is growth of the final
product--fully compensated, semi-insulating PI-type SiC single
crystal--said growth carried out using co-doping with V (vanadium)
and B (boron).
FIG. 4 shows a SiC crystal growth apparatus for growth of SiC
crystals of PI-type. The apparatus is similar to the one shown in
FIG. 3, with the exception of the growth crucible 11'. The presence
of vanadium and boron dopants in the heated growth crucible during
phase (a) of the process is undesirable. Therefore, growth crucible
11' was devised to permit vanadium and boron dopants to be stored
at low temperatures during phase (a) and be subsequently brought
into the growth crucible in phase (b). Details regarding growth
crucible 11' and its operation are shown in FIGS. 5A and 5B.
With reference to FIGS. 4, 5A and 5B, growth crucible 11' is made
of dense, fine grain graphite and has a graphite tube 42 attached,
i.e., at the bottom. Desirably, the outside diameter of tube 42 is
between 30 and 40 mm, while the inner diameter is between 15 and 20
mm. A doping capsule 45 containing the dopant(s) is disposed inside
the tube 42 on pushrod 44. Desirably, doping capsule 45 and pushrod
44 are made of an inert material, such as graphite. The prior art
use of a doping capsule is disclosed in U.S. Pat. No. 7,608,524 and
U.S. Pat. No. 8,216,369, both of which are incorporated herein by
reference.
As shown in FIG. 4, tube 42 is supported in the chamber by a
structure 42a that has an opening 42b that facilitates evacuation
and back filling of the inner space of chamber 10 with process
gases. Tube 42, doping capsule 45 and pushrod 44 are included in
chamber 10 and are exposed to the same pressure and flows of
gaseous components as chamber 10.
At its bottom, graphite pushrod 44 is connected to a metal pushrod
44a using means known in the art, such as threading. The threaded
union between graphite pushrod 44 and metal pushrod 44a is shown
schematically as item 44b in FIG. 4. Metal pushrod 44a extends to
the exterior of the chamber 10 and is sealed via a seal 44c, which
forms a vacuum-tight, linear motion feed-through. Seal 44c can be
an O-ring seal, a Ferrofluidic linear motion feed-through (e.g.,
available from FerroTec, Inc. 33 Constitution Drive Bedford, N.H.,
USA 03110), or a bellows-based vacuum feed-through (e.g., available
from Standard Bellows Company, 375 Ella T. Grasso Turnpike, Windsor
Locks, Conn., USA 06096).
During growth of SiC single crystal 17, the total pressure in
chamber 10, including Tube 42, doping capsule 45 and pushrod 44, is
maintained, desirably, between 5 and 50 Torr.
In phase (a) of the process, where growth is carried out in
crucible 11' in the manner described above in connection with FIG.
3, doping capsule 45 is disposed at a distance from crucible 11',
while the opening of tube 42 is sealed with graphite plug 43, as
shown in FIG. 5a, Desirably, the substantially undoped portion of
SiC single crystal 17 grown during phase (a) is a sacrificial
portion. Due to the distance between doping capsule 45 and heated
crucible 11', the temperature of the doping capsule 45 is lower
than that of the crucible 11'. Desirably, the temperature of the
doping capsule 45 during phase (a) of the process does not exceed
1000.degree. C.
SiC source material 14 is disposed in a source crucible 40 at a
distance from the bottom of the crucible 11' via one or more
standoffs 46 that are configured to permit the doping vapors 56
(discussed hereafter) to migrate toward the top of crucible 11',
thus forming a gap or free space 41. Source crucible 40 also forms
an annular gap 41a between the outer diameter of the source
crucible 40 and the inner diameter of the crucible 11'. During
phase (b) of the process, free space 41 and annular gap 41a serve
as conduits for doping vapors 56 to reach the growing SiC single
crystal 17.
Two non-limiting embodiments of doping capsule 45 are shown in
FIGS. 6A and 6B. FIG. 6A is a doping capsule 45a that includes a
single compartment 63 for a single dopant 62, for instance,
vanadium, while FIG. 6B is a doping capsule 45b that includes two
compartments 63a and 63b for two separate dopants 62a and 62b, for
instance, vanadium and boron. Each doping capsule 45a and 45b has
tapered top 60. Doping capsule 45a has at least one calibrated
capillary 61 in communication with compartment 63 serving as a
passageway for doping vapors 56. Doping capsule 45b has at least
two calibrated capillaries 61a and 61b in communication with
compartments 63a and 63b serving as passageways for the doping
vapors 56a and 56b.
The principle of operation of each capsule 45a and 45b 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 (62, 62a, or 62b) inside
of its space (63, 63a, or 63b) forces the vapor (56, 56a, or 56b)
to escape via each capillary (61, 61a, or 61b) in communication
with the corresponding space. If the cross section of each
capillary is sufficiently small, the vapor pressure of the doping
vapors in the capsule does not differ substantially from an
equilibrium value.
The laws of effusion are well known and, for given growth
conditions, temperature, vapor pressure of inert gas, volatility of
the dopant (62, 62a, or 62b), and the diameter and/or length of the
capillary (61, 61a, or 61b), the flux of molecules of doping vapors
56, 56a, or 56b escaping the corresponding capsule via the
corresponding capillary can be readily calculated. Thus, the
dimension of each capillary and the number of capillaries in
communication with each space (63, 63a, and/or 63b) can be tailored
to achieve a steady and well-controlled flux of doping vapors from
the capsule to the growing SiC crystal 17.
Referring back to FIG. 4 and with continuing reference to FIGS.
5A-6, at the completion of phase (a) of the process described above
in connection with FIG. 3, valves 36 and 36b of the gas delivery
system 30 are closed, thus stopping the flow of metal halide vapor
into the furnace chamber 10.
Following termination of the flow of metal halide vapor into the
furnace chamber 10, doping capsule 45 i.e., either doping capsule
45a or doping capsule 45b, is moved upward (FIG. 5B) via upward
movement of pushrod 44. In FIG. 4, the upward movement of pushrod
44 is accomplished via upward movement of the pushrod 44a through
the vacuum seal 44c, said seal operational to preserve the
integrity of the atmosphere in the chamber 10. The outside diameter
of the doping capsule is sized to the inside diameter of tube 42,
so that the doping capsule can be moved via push rod 44 without
undue force. The tapered top 60 of the doping capsule pushes plug
43 out of the end of tube 42, thus bringing the doping capsule into
the crucible interior, as shown in FIG. 5B. The outside diameter of
the doping capsule is sized to the inside diameter of tube 42, so
that the doping capsule can be moved via push rod 44 without undue
force. The tapered top 60 of the doping capsule pushes plug 43 out
of the end of tube 42, thus bringing the doping capsule into the
crucible interior, as shown in FIG. 5B.
During phase (b) of the growth process, co-doping of the growing
SiC single crystal 17 with vanadium and boron takes place. The
dopant(s) are chosen from a group that includes, without
limitation, elemental vanadium, elemental boron, vanadium carbide
(VC.sub.0.9), boron carbide (B.sub.4C), vanadium boride (VB) and/or
vanadium diboride (VB.sub.2).
In one embodiment, for vanadium-boron co-doping, doping capsule 45a
is used. Alternatively, doping capsule 45b can be used with
vanadium and boron in spaces 63a and 63b, respectively, or vice
versa. Doping capsule 45a comprises a single capillary which is 1
mm in diameter and 6 mm long. The single-compartment 63 in doping
capsule 45a contains vanadium metal as a source of vanadium and
vanadium diboride, VB.sub.2, as a source of boron. Vanadium
diboride is taken in the weight ratio to vanadium, desirably,
between 1 and 10%.
Results of growth runs aimed at producing vanadium-compensated,
semi-insulating PI-type 6H SiC crystals are shown in Table 1 above.
Based on SIMS impurity analysis, the grown crystals included
between 410.sup.15 and 710.sup.15CM.sup.3 of unintentional
background nitrogen. The levels of intentionally introduced boron
and vanadium were between 910.sup.15 and 210.sup.16 cm.sup.3 and
between 910.sup.16 and 210.sup.17 cm.sup.3, respectively.
The resistivity of the wafers sliced from the grown SI SiC crystals
was measured at room temperature using COREMA, a non-contact
capacitance-based instrument. The results were, typically, above
the measurement limit of 110.sup.12 Ohm-cm of the instrument. In
order to approximately estimate the room-temperature resistivity,
the wafers were measured at elevated temperatures between 100 and
400.degree. C. using a Variable Temperature version of COREMA
(VT-COREMA). The results were extrapolated to room temperature,
yielding room-temperature resistivity values on the order of
10.sup.12-10.sup.21 Ohm-cm with the activation energies between
about 0.9 and 1.5 eV. This indicated PI-type with full compensation
of boron shallow acceptors by vanadium.
Growth of SI SiC Single Crystals of NU-Type
In similarity to the growth of semi-insulating SI SiC single
crystals of PI-type, the growth process for SI SiC crystals of
NU-type also includes two phases. Phase (a) of the process is
growth of substantially undoped, sacrificial portion of the SiC
single crystal under reactive atmosphere aimed at removal of
background N and B from the growth ambient. Phase (a) of the growth
process is carried out as described above in connection with FIG.
3. The duration of phase (a) is, desirably, between 12 and 24
hours. Phase (b) of the process is growth of NU-type SiC using
co-doping with V (vanadium) and N (nitrogen).
FIG. 7 shows a SiC crystal growth apparatus for the growth of
semi-insulating SiC single crystals of NU-type. The apparatus shown
in FIG. 7 is similar to the one shown in FIG. 4, with the exception
of gas delivery system 30. For simplicity of illustration, pushrod
44a, vacuum seal 44c, threading 44b, and structure 42a including
opening 44b have been omitted from FIG. 7. However, it is to be
appreciated these elements or their equivalents would also present
in the apparatus shown in FIG. 7. In order to achieve precise
co-doping with nitrogen, gas delivery system 30 includes an
additional gas line comprising valves 74, 74b and mass flow
controller 74a which is not required for the gas delivery system 30
of FIG. 4. Other than the addition of the gas line comprising
valves 74, 74b, and mass flow controller 74a, the SiC crystal
growth apparatus shown in FIG. 7 is the same as the SiC crystal
growth apparatus shown in FIG. 4. Accordingly, details regarding
the elements common to the SiC crystal growth apparatuses shown in
FIGS. 4 and 7 will not be described further herein to avoid
unnecessary redundancy.
An Ar+N.sub.2 gas mixture is supplied to valve 74. The
concentration of N.sub.2 in the Ar+N.sub.2 gas mixture is,
desirably, between 50 and 200 ppm by volume.
In one embodiment, metallic vanadium is used as a dopant. During
growth of SiC single crystal 17, vanadium is disposed in doping
capsule 45a shown in FIG. 6A. Doping capsule 45a comprises a single
capillary 61 which is 1 mm in diameter and 6 mm long.
With ongoing reference to FIG. 7, the growth process for
vanadium-compensated SiC single crystals 17 of NU-type is carried
out as follows. At the completion of phase (a) of the process,
described above in connection with FIG. 3, valves 36 and 36b are
closed, thus stopping the flow of metal halide vapor into furnace
chamber 10. Recall that during phase (a) of the process, Ar+H.sub.2
flows into furnace chamber 10 via valves 35 and 35b and mass flow
controller 35a. Desirably, the portion of SiC single crystal 17
grown during phase (a) is a sacrificial portion.
In phase (b) of the process and following termination of the flow
of metal halide vapor into furnace chamber 10, valves 74 and 74b
are opened, and the mass flow controller 74a is activated allowing
the Ar+N.sub.2 mixture to flow into the furnace chamber 10 with the
flow of Ar+H.sub.2. Desirably, the flow of the Ar+N.sub.2 mixture
is between 1 and 10% of the flow of the Ar+H.sub.2 mixture.
Following this, doping capsule 45a is moved upward using pushrod
44. The tapered top of doping capsule 45a pushes plug 43 out of the
tube 42, thus bringing doping capsule 45a into the crucible
interior, as shown for example in FIG. 5B.
Results of the growth runs of vanadium-compensated, SI SiC crystals
of NU-type are shown in Table 1 above. Based on SIMS impurity
analysis, the grown SI SiC single crystals included between
210.sup.15 and 810.sup.15 cm.sup.-3 of unintentional background
boron. The levels of intentionally introduced nitrogen and vanadium
were between 810.sup.15 and 210.sup.16 cm.sup.3 and 910.sup.16 and
210.sup.17 cm.sup.3, respectively.
The resistivity of the wafers sliced from the grown SI SiC crystals
was measured at room temperature using COREMA. The resistivity
values were between 110.sup.11 Ohm-cm and 410.sup.11 Ohm-cm. The
activation energy of resistivity in the temperature range between
25 and 400.degree. C. measured using VT COREMA was between 0.78 and
0.82 eV. This pointed to NU-type with full compensation of nitrogen
shallow donors by vanadium.
The present invention has been described with reference to the
accompanying figures. 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.
* * * * *