Abstract:
A sublimation grown SiC single crystal includes vanadium dopant incorporated into the SiC single crystal structure via introduction of a gaseous vanadium compound into a growth environment of the SiC single crystal during growth of the SiC single crystal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention claims priority from U.S. Provisional Patent Application No. 61/767,318, filed Feb. 21, 2013, the disclosure of which is hereby incorporated in its entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to silicon carbide (SiC) single crystals and, more particularly, to a method of fabricating SiC single crystals using a gaseous source of deep level dopant. 
     2. Description of Related Art 
     Single crystals of silicon carbide of 4H and 6H polytypes serve as lattice-matched substrates in SiC- and AlGaN-based semiconductor devices, including ultra-high-frequency AlGaN-based transistors and SiC-based devices for power switching. Other applications include ultra-fast photoconductive switches, sensors for harsh environments, radiation detectors and many others. In the cases of high-frequency devices and photoconductive switches, the SiC substrates must be semi-insulating, that is having very high electric resistivity. 
     In the past, the term ‘semi-insulating’ in application to SiC meant simply that the crystal resistivity was above 1·10 5  Ohm-cm. In more stringent terms, ‘semi-insulating’ is a synonym for ‘fully compensated’. Many of the present day semiconductor devices built on SiC substrates require the substrate to have a resistivity on the order of 10 10 -10 11  Ohm-cm or higher. 
     Compensation of silicon carbide with vanadium is well known and has been used to produce SiC single crystals having high electric resistivity. The Prior Art related to vanadium doping includes U.S. Pat. Nos. 5,611,955; 7,608,524; 8,216,369; US 2008/0190355 and US 2011/0303884, which are all incorporated herein by reference. Vanadium produces two deep levels in the SiC bandgap, one deep acceptor and one deep donor, and, therefore, can electronically compensate either shallow donors (when they dominate over shallow acceptors), or shallow acceptors (when they dominate over shallow donors). 
     Large-size commercial SiC single crystals are commonly grown by the sublimation technique of Physical Vapor Transport (PVT). A simplified schematic diagram of conventional PVT system is shown in  FIG. 1 , wherein a double-wall, water-cooled furnace chamber  1  is desirably made of fused silica. A PVT crystal growth cell is disposed inside furnace chamber  1 . The PVT growth cell includes crystal growth crucible  20  charged with polycrystalline SiC grain  21  (SiC source) and a SiC single crystal seed  22  in spaced relationship. Commonly, SiC source  21  is disposed at the bottom of growth crucible  20 , while SiC seed  22  is disposed at the top of growth crucible  20 . Desirably, growth crucible  20  is made of dense, fine-grain graphite. 
     Conventionally, inductive type of heating is employed in PVT growth of silicon carbide. This type of heating is shown in  FIG. 1  by RF coil  11  which is disposed outside the chamber  1 . Graphite growth crucible  20  serves as an RF susceptor which couples electromagnetically to an RF field produced by excitation of RF coil  11 . Growth crucible  20  is surrounded by thermal insulation  10  which is usually made of light-weight porous graphite, such as graphite felt or fibrous graphite board. These thermally insulating materials do not couple substantially to the RF field of RF coil  11 . Resistive-type heating (in place of or in addition to RF coil  11 ), different types of thermal insulation, furnace chambers made of stainless steel, and RF coils disposed inside the chamber can also or alternatively be successfully employed for SiC sublimation growth. Other common and ordinary parts of the PVT crystal growth apparatus, such as gas and vacuum lines, valves, pumps, electronic controls, etc. are not shown in  FIG. 1 . 
     In preparation for PVT growth, chamber  1  is loaded with growth crucible  20  charged with SiC source  21  and SiC seed  22 , and thermal insulation  10 . Chamber  1  and, hence, growth crucible  20  are then evacuated and filled with a process gas (most commonly argon) to a desired pressure—generally between several and 100 Torr. Following this, growth crucible  20  is heated via energized RF coil  11  to growth temperature, which is generally between 2000° C. and 2400° C. Growth crucible  20  is heated such that a vertical temperature gradient is created between SiC source  21  and SiC seed  22 , with the temperature of SiC source  21  higher than that of SiC seed  22 . 
     At high temperatures, SiC source  21  sublimes releasing into the atmosphere of growth crucible  20  a spectrum of volatile molecular species, such as Si, Si 2 C and SiC 2 . Driven by the vertical temperature gradient, these species migrate to SiC seed  22  (vapor transport in  FIG. 1  is shown by arrow  23 ) and condense on it causing growth of SiC single crystal  24  on SiC seed  22 . Prior art in the area of PVT growth of silicon carbide includes U.S. Pat. Nos. 6,805,745; 5,683,507; 5,667,587 and 5,746,827, which are all incorporated herein by reference. 
     In the past, vanadium-doped SiC crystals (such as SiC crystal  24 ) were obtained by admixing a small amount of solid vanadium dopant directly to the SiC source (such as SiC source  21 ), as disclosed in U.S. Pat. No. 5,611,955 and US 2008/0190355, both of which are incorporated herein by reference. This solid vanadium dopant could be in the form of elemental metallic vanadium or in the form of a solid vanadium compound such as vanadium carbide. A major disadvantage of this type of vanadium doping is the physical contact between the solid vanadium dopant and the SiC source  21 . Specifically, at high temperatures, multi-step chemical reactions take place between the vanadium and the SiC source  21  leading to the formation of multiple intermediary compounds, such as vanadium carbides, silicides, carbo-silicides and various eutectic compositions. This makes the partial vapor pressure of the vanadium comprising the volatile molecular species unstable and varying with time, and leads to spatially nonuniform vanadium doping of the grown SiC crystal (such as SiC crystal  24 ). 
     The aforementioned problem of spatially nonuniform vanadium doping was addressed in U.S. Pat. Nos. 7,608,524; 8,216,369 and US 2011/0303884, which are all incorporated herein by reference, wherein vanadium doping was accomplished by disposing the source of vanadium inside a doping capsule made of an inert material thus eliminating direct contact between the vanadium source and the SiC source  21 . This doping arrangement is shown in  FIG. 2 . 
     With reference to  FIG. 2 , vanadium solid dopant  225  is included in an inert capsule  226  which, generally, is made of graphite. Capsule  226  includes at least one calibrated capillary  227  of predetermined diameter and length. Each capillary  227  allows controlled effusion of vanadium vapor from capsule  226 . Doping capsule  226  can be placed on the surface of SiC source  221 , as illustrated in  FIG. 2 , beneath SiC source  221  (on the bottom of growth crucible  220 ), or buried in the bulk of SiC source  221 .  FIG. 2  also shows vapor transport  223  of volatile molecular species to SiC seed  222  where the species condense on SiC seed  222  causing growth of SiC single crystal  224  on SiC seed  222 . 
     Implementation of doping capsule  226  improved the uniformity of vanadium doping, but only for vanadium concentrations around 1·10 17  atoms-cm −3  and around 1·10 16  atoms-cm −3 . This was due to the fact that the temperature of the solid vanadium source  225  could not be controlled independently. Accordingly, the partial pressure of vanadium inside growth crucible  220  could not be controlled independently as well. Therefore, when elemental vanadium was used in doping capsule  226  as a vanadium doping source  225 , the vanadium concentration in the crystal was about 1·10 17  atoms-cm −3 . However, when vanadium carbide (VC) was used in doping capsule  226  as a vanadium doping source  225 , the vanadium concentration in the crystal was about 1·10 16  atoms-cm −3 . Thus, vanadium concentrations between 1·10 16  atoms-cm −3  and 1·10 17  atoms-cm −3  or vanadium concentrations below 1·10 16  atoms-cm −3  could be achieved reliably. 
     Gas-assisted PVT processes are known generally in the art. Such PVT processes include: APVT, HTCVD, HCVD, CF-PVT and M-PVT. All these modifications of SiC sublimation growth were created with the aim of achieving better crystal purity, longer growth cycle, steady-state growth, control over the vapor phase stoichiometry, and improved doping. 
     Advanced PVT (APVT).  FIG. 3  is a schematic representation of an APVT growth cell, e.g., of the type disclosed in U.S. Pat. No. 5,985,024. In APVT growth, pure silicon  321  is included at the bottom of growth crucible  320  and melted upon heating. A gaseous carbon precursor (propane, C 3 H 8 ) is introduced via a gas conduit  331 . This carbon-bearing gas precursor  331  reacts with silicon vapor emanating from the molten silicon  321 . The products of reaction migrate towards SiC seed  322  and precipitate on it causing growth of SiC single crystal  324  on SiC seed  322 . Gaseous byproducts leave the crucible through open passages  333 .  FIG. 3  also shows chamber  31  (similar to chamber  1 ) and RF coil  311  (similar to RF coil  11 ). 
     High Temperature CVD (HTCVD).  FIG. 4  is a schematic diagram of a HTCVD SiC growth cell. Details regarding HTCVD growth can be found in M. B. J. Wijesundara and R. Azevedo, “Silicon Carbide Microsystems for Harsh Environments”, Chapter 2: SiC Materials and Processing Technology, pp. 40-44. Springer Science and Business Media, LLC 2011, EP 0835336 and EP 0859879. Silicon and carbon gaseous precursor gases, namely, silane and propane, respectively, are input into crucible  420  via coaxial inlets  431  and  432 . Once inside crucible  420 , silane undergoes thermal dissociation leading to the formation of Si clusters. These Si clusters react with the carbon precursor gas and form Si x C y  clusters. Driven by a vertical vapor transport  423 , the Si x C y  clusters enter a higher-temperature zone, where they, in similarity to the conventional PVT, sublimate to form Si and C including vapor species, such as Si, SiC 2  and Si 2 C. These species migrate towards SiC seed  422  and precipitate on SiC seed  422  causing growth of SiC single crystal  424  on SiC seed  422 . Gaseous byproducts leave the crucible through open passages  433 .  FIG. 4  also shows chamber  41  (similar to chamber  1 ) and RF coil  411  (similar to RF coil  11 ). 
     Halide CVD (HCVD). A HCVD growth cell is shown schematically in  FIG. 5 . Details regarding the HCVD growth can be found in A. Polyakov et al. “Halide-CVD Growth of Bulk SiC Crystals”, J. Mat. Sci. Forum (2006) Vol. 527-529, 21-26. Fanton et al. US 2005/0255245 “Method and Apparatus for the Chemical Vapor Deposition of Materials”. The HCVD growth process is similar to the HTCVD growth process, with the exception of different chemical reactions involved due to the presence of halogen (chlorine) in the system. A chlorinated silicon precursor (SiCl 4  diluted by Ar) and a carbon precursor (C 3 H 8  or CH 4  diluted by H 2 /Ar) are supplied upward into crucible  520  via coaxial inlets  531  and  532 , respectively. At high temperatures and while still inside inlets  531  and  532 , these precursors dissociate yielding gaseous molecules of SiCl 2  and C 2 H 2 . In a mixing zone  581 , which is situated near SiC seed  522 , SiCl 4 , SiCl 2 , C 2 H 2  and H 2  react in the gas phase according to the following summary equation (written without stoichiometric coefficients):
 
SiCl 2 ( g )+SiCl 4 ( g )+C 2 H 2 ( g )+H 2 ( g )           SiC( s )+SiCl( g )+HCl( g )
 
The net effect of the above reaction is precipitation of solid SiC on SiC seed  522  and growth of a SiC single crystal  524  on SiC seed  522 . Gaseous byproducts (HCl, SiCl) and carrier gases (Ar, H 2 ) leave crucible  520  through the open bottom passages  533 .  FIG. 5  also shows crucible  51  (similar to crucible  1 ) and RF coil  511  (similar to RF coil  11 ).

     Continuous Feed PVT (CF-PVT). A CF-PVT growth cell is shown in  FIG. 6 . Details regarding CF-PVT growth can be found in D. Chaussende et al. “Continuous Feed Physical Vapor Transport Toward High Purity and Long Boule Growth of SiC”. J. Electrochem. Soc. 2003, Vol. 150, issue 10, G653-G657. The method of CF-PVT growth is a hybrid between the PVT and HTCVD growth processes. The CF-PVT growth cell is divided into two zones: PVT zone  6101  and HTCVD zone  6102 , said zones separated by graphite foam  635  which supports SiC source  621 . Tetramethylsilane (TMS)  634  including both silicon and carbon is used as a single gaseous SiC precursor. TMS  634  is input into crucible  610  via inlet  638  by a flow of argon carrier gas. In order to dilute and remove reaction products from the growth cell, pure argon is supplied through lateral inlets  637 . Thermal dissociation of TMS  634  occurs in HTCVD zone  6102  and leads to the formation of microscopic SiC clusters. These SiC clusters are transported by the argon flow to a higher-temperature sublimation zone where they vaporize. These vapors diffuse through porous graphite foam  635  and feed solid SiC source  621  disposed on foam  635 . The solid SiC source  621  vaporizes leading to the growth of SiC single crystal  624  on SiC seed  622 . Gaseous byproducts from the HTCVD zone  6102  leave crucible  610  through open passages  633 .  FIG. 6  also shows chamber  61  (similar to chamber  1 ) and RF coil  611  (similar to RF coil  11 ). 
     Modified PVT Method (M-PVT). A M-PVT cell is shown in  FIG. 7A . Details regarding M-PVT growth can be found in R. Muller et al., “Growth of SiC Bulk Crystals with a Modified PVT Technique”, Chem. Vap. Deposition (2006), 12, 557-561. In essence, the M-PVT growth method is a PVT process with the added capability of delivering small amounts of Si and/or C gaseous precursors and/or dopants into a growth crucible  720  via a gas conduit  731 . The M-PVT growth method has been used for the growth of aluminum-doped SiC crystals. See T. L. Straubinger et al. “Aluminum p-type doping of SiC crystals using a modified physical vapor transport growth method”. J. Cryst. Growth 240 (2002) 117-123. In one embodiment of M-PVT, trimethylaluminum (TMA) is used as a gaseous Al precursor supplied via a gas conduit  731 . In another embodiment of M-PVT shown in  FIG. 7B , elemental aluminum  790  is included in an external reservoir  791  connected to gas conduit  731  ( FIG. 7A ). The temperature of reservoir  791  is controlled by placing it at a pre-determined distance from growth crucible  720 . The temperature of reservoir  791  is sufficiently high to melt aluminum and generate aluminum (Al) vapors  792  which are delivered into the growth crucible  720  with the flow of argon.  FIG. 7A  also shows chamber  71  (similar to chamber  1 ) and RF coil  711  (similar to RF coil  11 ). 
     The above-cited prior art gas-assisted PVT techniques had potential advantages, such as superior purity and stoichiometry control, but also had limitations and drawbacks. In the cases of APVT, HTCVD, HCVD and CF-PVT growth, the drawback is the open nature of the growth crucible. In all of the aforementioned processes, the presence of open passages leads to severe losses of vapors and gases and to very low crystallization efficiency. In the case of M-PVT ( FIG. 7A ), the drawback is interference by the gas flow or vertical vapor transport  723  coming from gas conduit  731  with the growth of SiC single crystal  724 . These and other drawbacks avoided these techniques from becoming viable commercial competitors to standard PVT sublimation growth. 
     Vanadium doping of SiC using vanadium gaseous precursors has been explored in 4H—SiC CVD epitaxy. Ferrocene-type vanadium metalorganic compounds have been used in CVD SiC epitaxy carried out at 1370-1440° C. See H. Song et al., “Homoepitaxial Growth of Vanadium-Doped Semi-Insulating 4H—SiC Using Bis-Trimethylsilymethane and Bis-Cyclopentadienylvanadium Precursors”. J. Electrochem. Soc. 155 (2008) p. H11-H16. The ferrocene bath (bubbler) was maintained at temperatures between 50° C. and 110° C., and H 2  was used as a carrier gas flowing at a rate of 10 sccm. In the epilayers grown at 1440° C., the maximum achieved resistivity was about 10 7  Ohm-cm. In the epilayers grown at 1370° C., higher resistivity values were observed, but the epilayer quality was poor. 
     Organometallic vanadium precursors were used by B. Landini et al. in CVD growth of semi-insulating SiC epilayers. See Landini et al., “CVD Growth of Semi-Insulating 4H—SiC Epitaxial Layers by Vanadium Doping”. Abstracts of 39 th  Electronic Materials Conference, Jun. 25-27, 1997, Fort Collins, Colo. Landini et al., “Vanadium Precursors for Semi-Insulating SiC Epilayers”, 1998 DoD-MDA SBIR/STTR Phase I Award ID: 41218. Landini et al., U.S. Pat. Nos. 6,329,088 and 6,641,938. The growth temperatures were, between 1200° C. and 1700° C. No details are available on the composition of the precursors, resistivity and quality of the produced SiC epilayers. 
     Generally, vanadium organometallic compounds dissociate at relatively low temperatures, typically, between 200 and 300° C., leading to precipitation of solid vanadium carbide(s). Such precipitation can occur even before the precursor is delivered into the heated SiC growth (reaction) zone. 
     Vanadium tetrachloride (VCl 4 ) as a precursor in CVD chloro-carbon epitaxy was explored in B. Krishnan et al., “Vanadium Doping Using VCl 4  Source during the Chloro-Carbon Epitaxial Growth of 4H—SiC”. J. Cryst. Growth, 321 (2011) pp. 8-14. The goal was to produce strongly compensated 4H—SiC epilayers. CVD growth was performed in a hot-wall CVD reactor at 1450° C. and 1600° C. with H 2  as a carrier gas. CH 3 Cl and SiCl 4  were used as chlorinated carbon and silicon precursors, respectively. Delivery of VCl 4  into the growth zone was achieved by bubbling H 2  through liquid VCl 4  maintained at 20° C. Depending on the H 2  flow rate, the vanadium concentration in the epilayers was between 1·10 16  atoms-cm −3  and (2-3)·10 17  atoms-cm −3 . The highest resistivity observed was about 5·10 5  Ohm-cm. 
     It is believed that sublimation growth of vanadium-doped, bulk SiC single crystals using a gaseous vanadium source (precursor) injected into the growth cell during growth is not known in the art or obvious in view of the prior art. 
     SUMMARY OF THE INVENTION 
     Disclosed herein is a SiC sublimation crystal growth process capable of yielding semi-insulating SiC single crystals of 4H and 6H polytype uniformly doped with vanadium in a broad range of vanadium concentrations, from 10 15  atoms-cm −3  to 10 17  atoms-cm −3 . 
     Also disclosed herein is a process for doping a growing SiC crystal with vanadium using a gaseous vanadium compound as a doping source. 
     Also disclosed herein are SiC single crystals of 4H and 6H polytype including spatially uniform concentrations of vanadium in the range between 10 15  and 10 17  atoms-cm −3 . 
     More specifically, disclosed herein is a method of growing vanadium-doped SiC single crystals comprising: (a) providing a growth crucible having SiC source material and a SiC single crystal seed in spaced relation therein; (b) heating the growth crucible of step (a) such that the SiC source material is heated to sublimation and a temperature gradient forms between the SiC source material and the SiC single crystal seed that causes the sublimated SiC source material to be transported to and precipitate on the SiC single crystal seed thereby growing a SiC crystal on the SiC single crystal seed; and (c) concurrent with step (b), introducing into the growth crucible a doping gas mixture that includes a carrier gas and a gaseous vanadium compound such that the growing SiC crystal is doped during the growth thereof with vanadium from the gaseous vanadium compound. 
     The carrier gas can include an inert gas and hydrogen. The gaseous vanadium compound can include a halogen. 
     The gaseous vanadium compound can be vanadium chloride (VCl n ), where n=2, 3, or 4. The gaseous vanadium compound can be vapors of the VCl n . 
     The doping gas mixture of step (c) can be comprised of the VCl n  vapors mixed with the carrier gas. The VCl n  vapors can be mixed with the carrier gas by passage of the carrier gas through a pool of liquid VCl n . The VCl n  vapors can be mixed with the carrier gas outside the growth crucible. 
     During step (c), the gaseous vanadium compound can undergo dissociation releasing byproducts that exit the growth crucible along with the carrier gas by diffusion across a porous wall of the growth crucible. The dissociated byproducts of the gaseous vanadium compound can react with unwanted impurities in the porous wall of the growth crucible to form therewith volatile molecules that exit the growth crucible with the carrier gas diffusing across the porous wall of the growth crucible. 
     The carrier gas can include a halogen. The halogen can be chlorine. 
     The gaseous vanadium compound of step (c) can be formed by reaction between the halogen in the carrier gas and a solid vanadium source. The reaction between the halogen in the carrier gas and the solid vanadium source can occur outside the growth crucible. 
     Also disclosed herein is an apparatus for growing vanadium-doped SiC single crystals comprising: a growth crucible inside of a sealed chamber, the growth crucible having SiC source material and a SiC single crystal seed in spaced relation therein; a gas conduit connected to the growth crucible via the chamber and operative for delivering a doping gas mixture comprised of a carrier gas and a gaseous vanadium compound to the growth crucible; a heater for heating the growth crucible such that the SiC source material is heated to sublimation and a temperature gradient forms between the SiC source material and the SiC single crystal seed that causes the sublimated SiC source material to be transported to and precipitate on the SiC single crystal seed causing a SiC crystal to grow on the SiC single crystal seed; and a source of the gaseous vanadium compound coupled to the gas conduit. 
     The source of the gaseous vanadium compound is one of the following: (1) a pool of a vanadium bearing liquid through which a carrier gas flows picking-up vapors of the vanadium bearing liquid that become the gaseous vanadium compound; or (2) a compartment housing solid vanadium through which a halogen bearing carrier gas flows, wherein reaction between the halogen in the carrier gas and a solid vanadium source forms the gaseous vanadium compound. 
     Also disclosed herein is a sublimation grown SiC single crystal including vanadium dopant incorporated into the SiC single crystal via introduction of a gaseous vanadium compound into a growth environment of the SiC single crystal during growth of the SiC single crystal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a prior art conventional physical vapor transport (PVT) growth system including a growth crucible inside of a furnace chamber; 
         FIG. 2  is an isolated schematic drawing of a prior art growth crucible (like the growth crucible shown in  FIG. 1 ) including a prior art inert capsule charged with a dopant; 
         FIGS. 3-6  are schematic drawings of prior art APVT, HTCVT, HCVD, and CF-PVT growth cells; 
         FIG. 7A  is a schematic drawing of a prior art M-PVT growth cell; 
         FIG. 7B  is a schematic view of dopant contained in a prior art external reservoir that, in one embodiment of  FIG. 7A , can be connected to a gas conduit of the M-PVT growth cell shown in  FIG. 7A ; 
         FIG. 8  is an isolated schematic view of a growth crucible charged with SiC source material and a SiC crystal in spaced relation in accordance with the principle of the present invention; 
         FIG. 9A  is a schematic drawing of a growth system in accordance with one embodiment of the present invention; 
         FIG. 9B  is an isolated schematic view of the growth crucible of  FIG. 9A  including a source crucible disposed in spaced relation to interior surfaces of the growth crucible and charged with SiC source material in spaced relation to a SiC seed crystal; 
         FIG. 10A  is a schematic view of another embodiment growth system in accordance with the present invention; and 
         FIG. 10B  is an isolated and partially exploded view of the dopant compartment and upper and lower conduits of the second embodiment doping system shown in  FIG. 10A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described with reference to the accompanying figures where like reference numbers correspond to like elements. 
     Vanadium doping from a gaseous vanadium source (precursor) is believed to have advantages over the solid vanadium sources of the prior art. A gas mixture including volatile vanadium-bearing molecules can be delivered into the growth crucible very accurately and at a desired rate. Therefore, the partial pressure of vanadium inside the growth crucible can be controlled, and SiC single crystals precisely and uniformly doped with vanadium in a broad range of vanadium concentrations can be obtained. 
     With reference to  FIG. 8 , gas-assisted PVT, which combines SiC sublimation growth with vanadium doping using a gaseous vanadium source, includes a graphite growth crucible  20  loaded with a SiC source  21  and a SiC single crystal seed  22  in spaced relationship typical for PVT. It is to be understood that growth crucible  20  is surrounded by insulation  10 , a chamber  1 , and a heating means, e.g., RF coil  11 , as shown in  FIG. 1 . Insulation  10 , chamber  1 , and heating means have been omitted from  FIG. 8  for simplicity. 
     At high temperatures of SiC sublimation growth, SiC source  21  vaporizes and generates Si- and C-bearing vapors, in similarity to the conventional PVT process. Driven by a vertical temperature gradient inside growth crucible  20  caused by the heating means, e.g., RF coil  11 , these vapors migrate towards SiC seed  22 . The migration of these vapors towards SiC seed  22  is illustrated in  FIG. 9  by arrow  23  which represents vertical vapor transport inside of growth crucible  20 . Upon approaching SiC seed  22 , the vapors condense on SiC seed  22  causing growth of SiC single crystal  24  on SiC seed  22 . 
     Simultaneously, a flow of a doping gas (arrow  25 ) including a gaseous vanadium compound (precursor) is introduced into growth crucible  20 . At high temperatures of SiC sublimation, this gaseous vanadium precursor  25  undergoes chemical transformations in accordance with the system thermodynamics. These chemical transformations yield gaseous molecular associates that include vanadium. These gaseous molecular associates migrate towards growing SiC single crystal  24  and adsorb on the growth interface causing doping of SiC single crystal  24  with vanadium. 
     In similarity to the M-PVT growth process ( FIG. 7A ), growth crucible  20  does not have open passages for the removal of gaseous byproducts. Rather, the gaseous byproducts escape from growth crucible  20  by filtering across the porous graphite wall of growth crucible  20 , as shown schematically by arrows  27 . 
     Referring back to  FIGS. 3, 4, 5 and 6 , the prior art APVT, HTCVD, HCVD and CF-PVT processes required relatively high flows of gases. For instance, typical flows for the carrier gases were on the order of Liters per minute. This was the main reason why the open passages  33  were essential in the design of APVT, HTCVD, HCVD and CF-PVT growth cells. Otherwise, a dangerous raise in the gas pressure inside the growth crucible would occur. 
     In  FIG. 8 , growth crucible  20  does not have open passages for gas escape. Instead, the carrier gas and gaseous byproducts filter out of growth crucible  20  by diffusing across the porous graphite wall of growth crucible  20 . In order to avoid an unwanted rise in pressure inside growth crucible  20 , the flow  25  of doping gas into growth crucible  20  is kept relatively small—desirably ≦50 sccm. Gas assisted PVT using a gaseous vanadium source will now be described in more detail with reference to two embodiments. 
     First Embodiment 
       FIG. 9A  shows a SiC crystal growth system in accordance with a first embodiment. In similarity to prior art PVT, growth crucible  20  is placed inside furnace chamber  1 , where it is surrounded by thermal insulation  10 . Growth crucible  20  is charged with SiC source grain  21  and SiC single crystal seed  22  in spaced relationship: with SiC source grain  21  disposed in the lower portion of growth crucible  20  and SiC single crystal seed  22  disposed in the upper portion of growth crucible  20 . Heating of growth crucible  20  is via RF coil  11  (or other suitable heating means). Common and ordinary parts of the SiC crystal growth system of  FIG. 10 , such as gas and vacuum lines, valves, vacuum pumps, electronic controls, etc. are not shown for simplicity. 
     In similarity to the M-PVT process of  FIG. 7A , a gas conduit  40  is attached to growth crucible  20 . Conduit  40  can be attached at the bottom of the growth crucible  20 , as shown in  FIG. 9A , or, without limitation, at any other suitable location on the wall of growth crucible  20 . Conduit  40  opens into the interior of growth crucible  20  and serves for the delivery of a gaseous vanadium compound (precursor) into growth crucible  20 . Both growth crucible  20  and conduit  40  are desirably made from high-density, fine-grain graphite, such as grade 2020 available from Mersen USA Bay City-MI Corp., 900 Harrison Street, Bay City, Mich. 48708, grade IG-11 available from Toyo Tanso USA, Inc., 2575 NW Graham Circle, Troutdale, Oreg. 97060, or similar, without limitation. Growth crucible  20  and conduit  40  are connected in a manner known in the art, e.g., by threading. In order to reduce possible gas leaks, carbon-based thread sealants can be applied to the threaded connections. 
     Inside growth chamber  1  and outside growth crucible  20 , gas conduit  40  is connected to a metal gas line  52  in a gas-tight fashion. This graphite-to-metal connection, which is, desirably, maintained at temperatures not exceeding 200° C., is accomplished in a manner known in the art, for instance, by using threaded metal adapters  57 . Outside chamber  1 , gas line  52  is connected to a temperature-controlled bubbler bath  51  that includes a volatile liquid vanadium precursor  54 , such as, without limitation, vanadium tetrachloride (VCl 4 ). 
     In preparation for PVT growth, growth crucible  20  is loaded with SiC source  21  and SiC single crystal seed  22  and placed inside furnace chamber  1 , as shown in  FIG. 9A . Inside chamber  1 , growth crucible  20  is surrounded by thermal insulation  10 . Chamber  1  and, hence, growth crucible  20  are then evacuated via vacuum pump(s)  4 , and filled with a process gas from process gas(es) source  6  to a desired pressure, e.g., between several and 100 Torr. Since the graphite forming growth crucible  20  and thermal insulation  10  are highly permeable to gases, the vacuum produced in chamber  1  by vacuum pump(s)  4  appears almost immediately in the interior of growth crucible  20 . Similarly, the process gas introduced into chamber  1  from process gas(es) source  6  appears almost immediately in the interior of growth crucible  20 . The process gas can be pure inert gas, such as argon (Ar) or helium (He). Alternatively, the process gas can comprise a hydrogen-including gas additive, such as hydrogen (H 2 ). Desirably, the H 2  content in the latter process gas is between 0 and 20%. 
     In a sequence typical for conventional PVT growth, RF coil  11  is energized to heat growth crucible  20  to desired temperatures of SiC sublimation growth, which is generally between 2000° C. and 2400° C. Growth crucible  20  is heated such that a vertical temperature gradient is created making the temperature of SiC source  21  higher than that of SiC seed  22 . At the temperatures of SiC sublimation growth, the silicon carbide of the SiC source  21  sublimes releasing a spectrum of volatile molecular species, such as Si, Si 2 C and SiC 2 . Driven by the vertical temperature gradient inside growth crucible  20 , these species are transported via vapor transport  23  to SiC seed  22  where they condense on SiC seed  22  causing growth of SiC single crystal  24  on SiC seed  22 . 
     Once SiC sublimation growth conditions are established in growth crucible  20 , a flow of carrier gas  53  is introduced from a carrier gas(es) source  5  into bubbler bath  51  which is maintained at a predetermined temperature. Desirably, carrier gas  53  is pure argon (Ar) or helium (He). Alternatively, carrier gas  53  is a gas mixture comprising a hydrogen-including gas additive, such as pure hydrogen (H 2 ). Desirably, the H 2  content in this latter carrier gas  53  is between 0 and 20%. Desirably, the flow of carrier gas  53  does not exceed 50 sccm. In one nonlimiting embodiment, the source of carrier gas  53  can be process gas(es) source  6 . However, this is not to be construed as limiting the invention since it is envisioned that carrier gas  53  can originate from a separate source, such as carrier gas(es) source  5 . 
     Carrier gas  53  bubbles through the liquid vanadium precursor  54  disposed in the temperature-controlled bubbler bath  51 . In the process of bubbling, vapor of the vanadium precursor (VCl 4 ) mixes with carrier gas  53 , thus transforming said carrier gas  53  into a doping gas mixture  25 . Doping gas mixture  25  is then introduced or injected into growth crucible  20  via gas line  52  and gas conduit  40 . 
     A description of a prior art metal precursor delivery system can be found in U.S. Pat. No. 6,410,433, which is incorporated herein by reference. 
     It was observed that best results are obtained when the vanadium precursor  54  in bubbler bath  51  is maintained at temperatures, desirably, between 18° C. and 24° C., and the flow of carrier gas  53  bubbling through vanadium precursor  54  is maintained, desirably, between 1 and 20 sccm. However, this temperature and flow rate is not to be construed as limiting the invention. Under these conditions, the estimated content of vanadium precursor  54  in the doping gas mixture  25  is between 10 ppm and 1000 ppm by volume with carrier gas  53  being the balance. 
     The prior art M-PVT process ( FIG. 7A ) suffered from interference of the gas flow with the SiC growth process, namely gas flow supplied from gas conduit  731  disposed in close proximity to the growing crystal  724 . As a result of this geometry, the grown SiC crystal  724  had unwanted and disadvantageous shapes at the growth interface, such as concave. In the embodiment shown in  FIG. 9A , doping gas flows in a space  26  provided between the wall of growth crucible  20  and SiC source  21 . This path for doping gas  25  flow avoids any deleterious effects that said doping gas  25  flow could have on vapor transport and crystal growth. 
     One exemplary spatial relationship between growth crucible  20  and the source  21  is shown in  FIG. 9B . The spatial relationship shown in  FIG. 9B , however, is not to be construed as limiting the invention since it is envisioned that other spatial relationships are possible. A similar gas path geometry can be found in U.S. Pat. No. 8,361,227, which is incorporated herein by reference. 
     In  FIG. 9B , polycrystalline SiC source material  21  is disposed in a source crucible  21   a , which is disposed inside growth crucible  20  in spaced relation to interior surfaces of growth crucible  20 . An exterior of a base of source crucible  21   a  is disposed in spaced relation to an interior floor of growth crucible  20 , thereby defining a first gap  30  therebetween. First gap  30  is desirably between 2 and 10 mm wide and, more desirably, between 4 and 7 mm wide. In order to produce first gap  30 , spacers  71  can be used, said spacers  71  including holes  71   a  allowing for the doping gas flow. 
     An exterior of a wall of the source crucible  21   a  is disposed in spaced relation to an interior of a wall of growth crucible  20 , thereby defining a second gap  32  therebetween. Desirably, second gap  32  is between 2 and 10 mm wide and, more desirably, between 4 and 7 mm wide. 
     Doping gas mixture  25  enters growth crucible  20  via conduit  40 , flows in first and second gaps  30  and  32 , and delivers gaseous vanadium-bearing molecules to the growing SiC crystal  24 , as shown by arrows  25   a . The flow of doping gas mixture  25  in first and second gaps  30  and  32  avoids interference with the vapor transport  23  from SiC source  21  to the growing SiC crystal  24  and avoids interference with the temperature distribution in the vicinity of the growing SiC crystal  24 . The path for the flow of doping gas mixture  25  in  FIG. 9B  avoids deleterious effects that said flow may have on vapor transport  23  and the growth of SiC single crystal  24 . 
     Due to heat conductance from the heated growth crucible  20 , the temperature in the upper portion of the gas conduit  40  is higher than in its lower portion. After entering the upper portion of gas conduit  40 , the gaseous molecules of the vanadium precursor  54  (VCl 4 ) dissociate, initially into VCl 3 . In the case when doping gas mixture  25  does not include hydrogen, the mechanism of VCl 4  dissociation is purely thermal, yielding monoatomic Cl as a byproduct. However, when the doping gas mixture  25  includes hydrogen, it participates in chemical reduction of VCl 4  to VCl 3  and then further to VCl 2 , yielding HCl as a byproduct. 
     Upon entering growth crucible  20 , doping gas mixture  25  becomes exposed to the temperatures of SiC sublimation growth, which are, generally, between 2000° C. and 2400° C. At these high temperatures, molecules of VCl 3  in doping gas mixture  25  transform into molecules of VCl 2 . Where hydrogen is present in doping gas mixture  25 , its presence facilitates this transformation. The VCl 2  molecules migrate towards growing SiC crystal  24  and adsorb on the growth interface causing doping of SiC crystal  24  with vanadium. The liberated chlorine desorbs from the growth interface and diffuses into the atmosphere of growth crucible  20 . 
     The gaseous chlorine and hydrogen chloride (which are byproducts of vanadium halide dissociation, chemical reduction by hydrogen and doping processes) diffuse across the graphite wall of growth crucible  20  together with carrier gas  53 , as shown by arrows  27  in  FIGS. 9A  and  9 B. During their diffusion, Cl and HCl react with impurities in the graphite forming the wall of growth crucible  20 . In particular, they react with boron and form volatile molecular associates with boron, such as BCl and BCl 2 , which are removed from growth crucible  20  by the gas flow. Thus, vanadium doping of SiC using gaseous vanadium halide precursors has an additional benefit, namely, removal of unwanted impurities from growth crucible  20 . 
     Due to the flowing of gas(es) inside growth crucible  20 , the pressure inside growth crucible  20  can exceed useful limits and make SiC sublimation growth unsustainable. In order to keep the gas pressure inside growth crucible  20  within useful limits for PVT growth, the thickness of the wall of the growth crucible  20  is, desirably, between 4 and 20 mm thick, and, more desirably, between 8 and 16 mm thick. 
     After escaping growth crucible  20  by diffusing across the walls of growth crucible  20 , the various gaseous byproducts and carrier gas  53  leave growth chamber  1  via a chamber port  3  which is coupled to vacuum pump(s)  4 . 
     Second Embodiment 
     A second embodiment growth system shown in  FIG. 10A  is similar in many respects to the first embodiment growth system shown in  FIGS. 9A and 9B  with the following exceptions: bubbler bath  51  is omitted, carrier gas  53  is introduced directly into gas line  52 , and conduit  40  (comprised of upper conduit part  40   b  and lower conduit part  40   a ) includes an in-line graphite compartment  41 . In similarity to the M-PVT cell shown in  FIGS. 7A and 7B , conduit  40  includes graphite compartment  41  situated at a distance from growth crucible  20 . Compartment  41  is charged with a solid source of vanadium  42 , e.g., in the form of elemental metallic vanadium or solid vanadium compound, such as vanadium carbide (VC). 
     Compartment  41  can be disposed in the bulk of thermal insulation  10 , as shown in  FIG. 10A . In this case, heating of the compartment  41  is provided from the heated growth crucible  20  via thermal conductivity along conduit  40 . Also or alternatively, thermal insulation  10  can be carved to form a well around compartment  41 , whereupon at least partial heating of compartment  41  is provided by heat radiated from heated growth crucible  20 . Also or alternatively, heating of the compartment  41  can be via a separate heater  43 , which can be resistive or inductive. Any combination of one or more of the foregoing means of heating compartment  41  is envisioned. 
     After SiC sublimation growth conditions are established in growth crucible  20  by evacuation of chamber  1  via vacuum pump(s)  4  and filling of chamber  1  with process gas (e.g., Ar or He) from process gas(es) source  6  to a desired pressure between several and 100 Torr, a small flow of doping gas mixture  25  is allowed into growth crucible  20  via the upper part  40   b  of gas conduit  40 . According to this embodiment, doping gas mixture  25  is comprised of a carrier gas  53 , such as argon (Ar) or helium (He), and a halogen-including additive from carrier gas(es) source  5 . Desirably, the halogen additive is gaseous halogen selected from the group of chlorine (Cl 2 ) and fluorine (F 2 ) and present in carrier gas  53  in concentrations between 10 ppm and 1000 ppm by volume. Desirably, the halogen additive is chlorine, Cl 2 . The flow of carrier gas  53  is, desirably, less than 50 sccm and, more desirably, between 1 and 20 sccm. 
     Alternatively, carrier gas  53  further comprises a hydrogen-including gaseous additive, such as pure hydrogen (H 2 ), in addition to Ar or He plus the halogen additive. Desirably, the H 2  content in carrier gas  53  is between 0 and 20%. 
     After entering the lower part  40   a  gas conduit  40  in  FIG. 10A , carrier gas  53  reaches compartment  41 , which is maintained at elevated temperature. Inside compartment  41 , the chlorine of carrier gas  53  reacts with vanadium of the solid vanadium source  42 , forms volatile vanadium chlorides, and exits compartment  41  as doping gas mixture  25 . Depending on the temperature of compartment  41 , the dominating products of reaction between solid vanadium source  42  and the gaseous chlorine of carrier gas  53  are VCl 4  (at temperatures below 600° C.), VCl 3  (at temperatures between 600 and 900° C.) or VCl 2  (at temperatures above 900° C.). 
     Desirably, the temperature of compartment  41  is high enough to avoid any possible kinetic limitations that can slow down reaction between the gaseous chlorine of carrier gas  53  and the solid vanadium source  42 . At the same time, the temperature of compartment  41  is low enough to avoid melting of solid vanadium source  42  (vanadium melting point is 1890° C.) and/or any possible chemical reactions between solid vanadium source  42  and the graphite forming compartment  41 . Based on the aforementioned considerations, the temperature of compartment  41  is, desirably, between 1000° C. and 1600° C. 
     One exemplary design of compartment  41  is shown in  FIG. 10B . However, this design of compartment  41  is not to be construed as limiting the invention since it is envisioned that other designs are possible. In the nonlimiting exemplary embodiment shown in  FIG. 10B , compartment  41  is about 30 mm in diameter, about 30 mm high with a wall thickness between 6 and 10 mm. Compartment  41  and graphite conduits  40   a  and  40   b  on either side of compartment  41  have threaded connections  70 . The floor of compartment  41  comprises several through holes  71 , each of about 1 mm in diameter. These holes  71  provide a pathway for the flow of carrier gas  53  into the interior of compartment  41  while preventing pieces of solid vanadium source  42  from falling into lower conduit  40   a . In the case when compartment  41  is buried in the bulk of thermal insulation  10 , as shown in  FIG. 10A , the distance between compartment  41  and growth crucible  20  is desirably between 40 and 70 mm. 
     Thermodynamic analysis of the ternary V—C—Cl system shows that within the preferred temperature range of 1000° C.-1600° C. of compartment  41  there are only two main products of reaction between the solid vanadium source  42  (elemental vanadium or vanadium carbide) and chlorine, namely, a higher vanadium chloride, VCl 3 , and a lower vanadium chloride, VCl 2 . The latter (VCl 2 ) should be present at substantially higher levels than the former (VCl 3 ). Increasing the temperature of compartment  41  from 1000° C. to 1600° C., as well as adding hydrogen to carrier gas  53  generally result in increased molecular ratio of VCl 2 :VCl 3  in the gas phase of doping gas mixture  25 . 
     When compartment  41  is maintained at a temperature within the preferred temperature range of 1000° C.-1600° C., the total amount of vanadium in doping gas mixture  25  (mostly in the form of VCl 2  molecules) depends only on the total amount of available chlorine. That is, the total amount of vanadium in doping gas mixture  25  is directly proportional to the concentration of chlorine in carrier gas  53  and the flow rate of carrier gas  53 . 
     Apart from the reaction between solid vanadium source  42  in compartment  41  and the chlorine of carrier gas  53 , the process of doping growing SiC single crystal  24  with vanadium in accordance with the second embodiment is similar to that of the first embodiment. That is, after entering growth crucible  20 , which is generally maintained at temperatures between 2000° C. and 2400° C., the higher vanadium chloride VCl 3  dissociates into VCl 2  with Cl as a byproduct. In the case when hydrogen is present in carrier gas  52 , this hydrogen chemically reduces VCl 3  to VCl 2  with HCl as a byproduct. The VCl 2  molecules migrate towards the growing SiC crystal  24  (as shown for example by arrows  25   b  in  FIG. 10A ) and adsorb on the growth interface causing doping of the SiC single crystal  24  with vanadium. The liberated chlorine desorbs from the growth interface and diffuses into the atmosphere of growth crucible  20 . The halogenated gaseous byproducts and carrier gas  53  diffuse across the wall of graphite growth crucible  20  causing removal  27  of unwanted impurities from the interior and wall of graphite growth crucible  20 . In this embodiment, the path for the gas flow inside the growth crucible  20  is the same or similar to that shown in  FIG. 9B . 
     Several 6H SiC crystals  24  have been grown in accordance with the two embodiment growth cells described herein. In the growth runs carried out in accordance with the first embodiment growth cell, VCl 4  disposed in a controlled temperature bubbler bath  51  was used as a volatile liquid vanadium precursor  54 . The VCl 4  bath was maintained at temperatures between 18° C. and 30° C., and pure argon was used as the carrier gas  53  flowing at rates between 1 and 20 sccm. 
     In the growth runs carried out in accordance with the second embodiment growth cell described herein, pure metallic vanadium was used as the solid vanadium source  42 . Vanadium pieces were disposed in graphite compartment  41  similar to that shown in  FIG. 10B . Compartment  41  was maintained at a temperature around 1400° C. in all growth experiments. Argon pre-mixed with 500 ppm of Cl, was used as carrier gas  53  and its flow rate was varied between 1 and 20 sccm. 
     6H SiC single crystals  24  grown in accordance with either embodiment growth cell were manufactured into standard on-axis wafers, 100 mm in diameter and 500 microns thick. Representative wafer samples were selected and the vanadium content in them was measured using the method of Secondary Ion Mass Spectroscopy (SIMS). The measured values were within the range between 2·10 15  and 1·10 17  atoms-cm −3 . As a general trend, the vanadium concentration in the grown crystals increased with increase in the vanadium chloride content in the carrier gas  53  and with increase in the flow rate of the carrier gas  53 . 
     The lowest vanadium concentration of 2·10 15  atoms-cm −3  was measured in the sample grown in accordance with the second embodiment growth cell when the flow rate of the Ar+Cl 2  carrier gas  53  was 1 sccm. The highest usable vanadium concentration of 1·10 17  atoms-cm −3  was measured in the sample grown in accordance with the first embodiment growth cell when the VCl 4  bath was maintained at 24° C. and the flow rate of the carrier gas  53  (i.e., argon) was 20 sccm. Increase in the temperature of the bath  51  beyond 24° C. and increase in the flow of carrier gas  53  beyond 20 sccm led to the appearance of vanadium precipitates in the grown SiC crystals  24 . 
     The obtained SIMS results showed excellent spatial uniformity of vanadium doping in both axial and radial directions. In all SiC single crystals  24  grown in accordance with either embodiment growth cell and analyzed by SIMS, variations in the vanadium concentration were within ±10% from the ingot&#39;s mean value. 
     The SIMS results obtained on the SiC single crystals  24  grown in accordance with either embodiment growth cell showed very low concentrations of background boron, equal or below 4·10 15  atoms-cm −3 . 
     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.