Patent Publication Number: US-2021189591-A1

Title: Large diameter silicon carbide single crystals and apparatus and method of manufacture thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/584,583, filed on May 2, 2017, which claims the benefit of U.S. Provisional Patent Application No. 62/332,731, filed on May 6, 2016, the disclosures of each of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     Field of the Invention 
     Disclosed herein is high-quality, large-diameter silicon carbide (SiC) single crystals, and an apparatus and method of growth thereof. 
     Description of Related Art 
     SiC single crystals find their use in a variety of semiconductor, electronic, and optoelectronic devices where SiC wafers serve as substrates for the growth of epitaxial layers of SiC or GaN. The epilayers are then fabricated into devices, such as power switching devices, RF/microwave devices and LEDs. Compared to traditional Si-based devices, SiC-based and GaN-based devices can operate at much higher temperature, power level, frequency—all combined with improved efficiency. 
     Wide-spread application of SiC-based and GaN-based devices is hampered by the high cost the SiC substrate, which is a major contributor to the overall device cost. Currently, the largest 4H—SiC and 6H—SiC substrates available commercially are of 100 mm and 150 mm in diameter, while development of 200 mm SiC substrates has been announced. Implementation of large-size SiC substrates, such as of 200 mm, 250 mm or 300 mm in diameter in the device technology can substantially reduce the cost of SiC- and GaN-based devices. 
     Crystal defects in the SiC substrate are harmful to the device performance, especially in the SiC-based power switching devices formed on N-type 4H—SiC substrates. It is known that threading dislocations cause charge leakage and device degradation, while basal plane dislocations and stacking faults can cause terminal device failure. Stress and strain in the SiC substrate are negative factors in device processing. 
     Industrial-size SiC single crystals, e.g., 100 mm and 150 mm in diameter, are grown by the technique of Physical Vapor Transport (PVT). A sectional view of a conventional PVT growth apparatus is shown schematically in  FIG. 1 , wherein a graphite growth crucible  1  charged with a SiC source material  2  and a single crystal SiC seed  3  in spaced relation is placed in a growth chamber  20 . A heating means  4 , for heating the interior of crucible  1  to a sublimation growth temperature, e.g., between 2000° C. and 2400° C., can be provided about the exterior of chamber  20 , which can be water-cooled and formed from fused silica. In this example, heating means  4  can be an exterior RF heating coil. However, heating means in the form of a resistance heater inside chamber  20  is envisioned. For the purpose of this description, heating means  4  will be described as being an RF heating coil. However, this is not to be construed in a limiting sense. 
     Crucible  1  is surrounded by thermal insulation  5  inside of chamber  20 . A top window  5   a  having a relatively small diameter is provided in thermal insulation  5 . This window  5   a  is provided for heat dissipation from the backside of SiC seed  3  attached to a top or lid  22  of growth crucible  1 . Window  5   a  can also serve for measuring the temperature of the crucible top using an optical pyrometer via a sealed viewing port  30  in top or lid  22  of chamber  20 . 
     In preparation for growth of a SiC single crystal  6  on SiC seed  3 , chamber  20  and, hence, growth crucible  1  are evacuated via a vacuum pump and are filled to a desired pressure with a suitable process gas or gases, e.g., argon, nitrogen, boron, supplied through an inlet  36 . By controlling the flow of process gas introduced into chamber  20  and, hence, growth crucible  1 , via gas inlet  36 , and controlling the operation of a vacuum pump connected to an outlet  38  of chamber  20 , the gas pressure inside of chamber  20  and, hence, growth crucible  1 , can be controlled to a pressure suitable for growth of SiC single crystal  6  on single crystal SiC seed  3 . In an example, this pressure can be between 5 and 300 Torr. A window  5   b  can be provided in thermal insulation adjacent a bottom of growth crucible  1  for temperature measurement of crucible  1  using an optical pyrometer via a sealed viewing port  30  in a bottom  24  of crucible  20 . 
     In use, crucible  1  is heated by heating means  4  to a temperature between 2000° C. and 2400° C. sufficient to vaporize the SiC source material  2  and fill the crucible material with vapor species  7  of SiC 2 , Si 2 C and Si in the presence of a suitable pressure of the process gas within chamber  20  and, hence, growth crucible  1 . Because growth crucible  1  is formed of porous graphite, process gas introduced into chamber  20  appears almost immediately in the interior of growth crucible  1 . Similarly, a vacuum applied to chamber  20  appears almost immediately in the interior of growth crucible  1 . 
     An axial temperature gradient is introduced in crucible  1  by properly positioning heating means  4  with respect to crucible  1  such that the temperature of SiC source material  2  is higher than that of SiC seed  3 . In an example, this temperature gradient can be between 30° C. and 150° C. However, this is not to be construed in a limiting sense. Driven by this temperature gradient, the vapor species  7  migrate toward SiC seed  3 , as illustrated schematically by arrow  7 , and condense on SiC seed  3  causing growth of a SiC single crystal  6  on SiC seed  3 . 
     The pattern of heat transport in the PVT growth apparatus is shown schematically in  FIG. 1  by arrows  8  and  9 . The heat absorbed by crucible  1  from heating means  4  is transported into the crucible interior by horizontal heat fluxes  8 . Heat escapes crucible  1  via the top of crucible  1  and top window  5   a  in the form of vertical heat flux  9 . As a result of such a heat transport pattern, curved isotherms  10  emerge in the crucible  1  and in the growing SiC single crystal  6 . 
     Two features of the growth process, which are not independent, are critical to the quality of PVT-grown SiC crystals: (i) the shape of growth interface of the SiC single crystal  6  on the SiC seed  3  and (ii) the magnitude of radial temperature gradient(s) in the growing SiC single crystal  6 . It is known that the growth interface follows the isotherm shape. A strongly curved growth interface leads to stacking faults, foreign polytypes, and other defects. The presence of a steep radial temperature gradient in the growing SiC single crystal  6  causes stress, strain, and related defects, such as basal plane dislocations. It is recognized that the most favorable for crystal quality is a flat growth interface, especially in the case of large-diameter SiC crystal growth. 
     U.S. Pat. No. 6,800,136 discloses a PVT growth apparatus and method, wherein harmful radial gradients in SiC sublimation growth can be reduced. The apparatus utilizes two flat heaters disposed below and above the exterior of the crucible. Disadvantages of this arrangement include: (i) in the case of RF heating, poor coupling of the flat RF coils to the crucible; and (ii) in the case of resistive heating, concave radial temperature gradients in the growing crystal lead to a concave growth interface. U.S. Pat. No. 6,800,136 is incorporated herein by reference. 
     U.S. Pat. No. 8,741,413 discloses a SiC sublimation growth apparatus and method, wherein the flux of SiC bearing vapors is restricted to the central area of the growing crystal. SiC boules grown using this arrangement suffer from the growth interface being too convex. U.S. Pat. No. 8,741,413 is incorporated herein by reference. 
     U.S. Pat. No. 9,228,274 discloses another SiC sublimation growth apparatus and method, wherein the heating arrangement includes a growth crucible positioned between two resistive heaters, a top heater and a bottom heater. The heaters are disposed coaxially with the crucible: the top, ring-shaped heater is disposed above the crucible, while the bottom, cup-shaped heater is disposed below and around the crucible. This arrangement has the following disadvantage: during growth, the SiC source vaporizes predominantly from the hotter areas located at the crucible walls, thus supplying the vapor mostly to the periphery of the growing SiC crystal. This leads to a concave growth interface, especially when the boule diameter is large. U.S. Pat. No. 9,228,274 is incorporated herein by reference. 
     In order to grow a large-diameter SiC single crystal, a large-diameter SiC seed wafer is needed. Large-diameter SiC seeds can be produced in a process commonly called “diameter expansion”. US 2012/0285370 and U.S. Pat. No. 8,313,720 disclose SiC sublimation growth combined with expansion of boule diameter using a growth guide. During growth, the boule shape and the rate of diameter expansion are determined by the growth guide geometry. US 2012/0285370 and U.S. Pat. No. 8,313,720 are incorporated herein by reference. 
     Alternatively, a large size seed can be produced by intergrowth of smaller-size, tiled seed crystals, as disclosed in U.S. Pat. No. 6,805,745, which is incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     Disclosed herein is a growth apparatus that produces near-zero radial temperature gradients and a flat growth interface in the growing SiC boule, yielding high-quality SiC single crystals suitable for the fabrication of high-quality SiC substrates or wafers of standard diameters of 200 mm, 250 mm, and 300 mm. 
     Disclosed herein are large-diameter, high crystal quality SiC single crystal boules of 4H and 6H polytype grown by sublimation, and large-diameter, high-quality SiC single crystal substrates, N-type and semi-insulating fabricated from said boules, including substrates of standard diameters of 200 mm, 250 mm, and 300 mms. 
     Also disclosed is a method of growing large-diameter SiC single crystal boules of 4H and 6H polytype by sublimation, said boules suitable for the fabrication of high crystal quality SiC substrates having standard diameters of 200 mm, 250 mm, and 300 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a conventional view (prior art) example PVT growth apparatus; 
         FIG. 2  is a sectional view of an example large-diameter PVT growth apparatus in accordance with the principles described herein; 
         FIGS. 3A-3D  are examples of the heating means (e.g., resistive heaters) that can be used in the example PVT growth apparatus shown in  FIG. 2 , wherein each heating means has a different conductive pattern for connection to delta and star three-phase electrical power ( FIGS. 3A-3B ) and single-phase electrical power ( FIGS. 3C-3D ); 
         FIGS. 4A-4E  are different configuration insulation inserts that can be used with the PVT growth apparatus shown in  FIG. 2 ; 
         FIG. 5  is a flow diagram of a method for producing by diameter expansion a SiC single crystal boule from which a SiC single crystal wafer can be sliced and used as the single crystal SiC seed in the PVT growth apparatus of  FIG. 2 ; 
         FIG. 6  is a photograph of an N-type 4H—SiC boule grown in accordance with Example 1 growth described herein; 
         FIG. 7  is a cross-polarizer image of a wafer extracted from the boule shown in  FIG. 6 ; 
         FIG. 8A  is a graph of lattice curvature versus distance along the wafer diameter of the wafer shown in  FIG. 7 ; 
         FIG. 8B  is a graph of FWHM versus distance along the wafer diameter of the wafer shown in  FIG. 7 ; 
         FIG. 9  is a micropipe density map of the wafer shown in  FIG. 7 ; 
         FIGS. 10A-10C  are maps of overall dislocation density, screw dislocation density, and basal plane dislocation density, respectively, measured on the wafer shown in  FIG. 7 ; and 
         FIG. 11  is a photograph of a vanadium doped, semi-insulating 4H—SiC boule grown in accordance with Example 2 growth run described herein. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Various non-limiting examples will now be described with reference to the accompanying figures where like reference numbers correspond to like or functionally equivalent elements. 
     A sectional view of an example large-diameter PVT growth apparatus for the growth of large-diameter SiC single crystals is shown schematically in  FIG. 2 . The PVT growth apparatus of  FIG. 2  includes a graphite growth crucible  1 , thermal insulation  12  (including cylindrical side insulation  12   a , top insulation  12   b , and bottom insulation  12   c ) and a heating means in the form of a single flat resistive heater  40  positioned between a bottom  26  of growth crucible  1  and bottom insulation  12   c . In an example, thermal insulation  5  can be made of lightweight fiberous graphite, such as graphite felt or foam. In an example, chamber  20  can be optionally water-cooled. In this example, because resistive heater  40  is in the form of a flat-resistive heater positioned in the interior of chamber  20  between bottom  26  of growth crucible  1  and the bottom of insulation  5 , chamber  20  can be formed of any suitable and/or desirable material capable of withstanding the heat produced by resistive heater  40  during growth of growing SiC single crystal  6  on single crystal SiC seed  3 . In an example, chamber  20  can be formed of fused silica or a suitable metal or a metal alloy, such as stainless steel. However, this is not to be construed in a limiting sense since it is envisioned that chamber  20  can be formed of any suitable and/or desirable material. 
     In an example, resistive heater  40  can be made of dense graphite. The graphite can be iso-molded or extruded. Resistive heater  40  can be shaped as a flat disc and can be positioned under growth crucible  1  as shown in  FIG. 2 . 
     Referring to  FIGS. 3A-3D , electric configurations of resistive heater  40  can include three-phase “Delta” ( FIG. 3A ), three phase “Star” ( FIG. 3B ), or single phase ( FIGS. 3C-3D ). The patterns of resistive heaters  40  shown in  FIGS. 3A-3D  are not to be construed in a limiting sense. 
     In preparation for growth, growth crucible  1 , charged with SiC source material  2  and a single crystal SiC seed  3  in spaced relation, can be placed in a growth chamber  20 , wherein the resistive heater  40  can be disposed below the crucible bottom  26 , as shown in  FIG. 2 . Growth crucible  1  can be positioned inside chamber  20  which can be optionally water-cooled and optionally made from fused silica, a metal, or a metal alloy. However, this is not to be construed in a limiting sense. 
     Thermal insulation  12  can surround crucible  1  and can be positioned between crucible  1  and chamber  20 . In contrast to the PVT growth apparatus of  FIG. 1 , the top window  5   a  in the insulation  5  in  FIG. 1  can be replaced in  FIG. 2  with a thermal insulation insert  12   d  positioned in an opening  14  of thermal insulation  12   b  that extends across at least a portion of the exterior top of crucible  1 , whereupon the thermal insulation above the top or lid of growth crucible  1  includes top insulation  12   b  and thermal insulation insert  12   d . The thickness and shape of insulation insert  12   d  are discussed below. 
     Small-diameter windows  16  and  18  can be provided in the bottom insulation piece  12   c  and in heater  40 , respectively. These windows serve for temperature measurement, via an optical pyrometer, via a sealed viewing port  34  in the bottom  26  of crucible  1 . 
     In use, crucible  1  can be heated by resistive heater  40  to SiC sublimation temperature, e.g., between 2000° C. and 2400° C., to vaporize SiC source material  2  and fill the crucible  1  with C and Si-bearing vapor species  7  in the presence of a suitable gas pressure within chamber  20  and, hence, within growth crucible  1 . Driven by an axial temperature gradient formed within growth crucible  1  by resistive heater  40 , these vapor species  7  migrate toward single crystal SiC seed  3  and condense on single crystal SiC seed  3  causing growth of a SiC single crystal  6  on single crystal SiC seed  3 . 
     The pattern of heat transport in PVT growth apparatus shown in  FIG. 2  differs from that of the example PVT growth apparatus shown in  FIG. 1 . Heat transport in the PVT growth apparatus of  FIG. 2  is vertical, as shown schematically by arrows  8  and  9  in  FIG. 2 . Heat from resistive heater  40 , which is disposed below the bottom  26  of growth crucible  1 , is transported into the interior of crucible  1  by the axial heat fluxes shown by arrows  8 . Heat escapes from a top or lid  22  of growth crucible  1  in the form of the axial heat fluxes shown by arrows  9 , which diffuse across thermal insulation insert  12   d . As a result of such purely axial heat fluxes, flat isotherms  10  emerge in the interior of growth crucible  1  and in the growing SiC single crystal  6 . 
     Insulation insert  12   d  can be made of fibrous graphite insulation board. A value for the thickness t of insulation insert  12   d  can be determined from SiC growth experiments. However, an approximate value of t (in mm) can be calculated using the following equation: 
     
       
         
           
             
               
                 
                   t 
                   = 
                   
                     λ 
                      
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           T 
                           · 
                           A 
                         
                       
                       Q 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     In Eq. 1. λ is the thermal conductivity of the insulation insert  12   d  (W/mK), ΔT is the temperature difference across the insulation insert  12   d  (° K.), A is the area (m 2 ) of the insulation insert  12   d , and Q is the heat flux across the insulation insert  12   d  (kW). Assuming λ≈0.5 W/mK, ΔT≈2000° K., Q≈0.5 kW and A=0.02 m 2 , Eq. 1 yields the insert thickness t≈40 mm. 
     Example dimensions of the flat, disc-shaped insulation insert  12   d  can include: thickness desirably between 20 and 50 mm, or more desirably between 25 and 45 mm; and diameter between 90% and 120% of the diameter of the single crystal SiC seed  3 . 
     Isotherms  10  and radial temperature gradients in growth crucible  1  can be further controlled by using non-flat, shaped insulation inserts  12   d   1 - 12   d   4 . Example configurations of insulation inserts  12   d - 12   d   4  are shown in  FIGS. 4A-4E . In an example, insulation insert  12   d  ( FIG. 4A ) can be a flat insert with parallel top and bottom surfaces. Insulation inserts  12   d   1  and  12   d   2  ( FIGS. 4B-4C ) can have planar top surfaces and bottom surfaces (which, in use, faces growth crucible  1 ) with sloped edges. Insulation inserts  12   d   3  and  12   d   4  can have planar top surfaces and bottom surfaces (which, in use, faces growth crucible  1 ) with rounded edges. Inserts  12   d   1  and  12   d   3  ( FIGS. 4B-4D ) can have concave shapes. Inserts  12   d   2  and  12   d   4  ( FIGS. 4C and 4E ) can have convex shapes. Inserts  12   d   1  and  12   d   3  can have a flat central portion and the edge area concave or sloped towards growth crucible  1 . These inserts  12   d   1  and  12   d   3  can be used when the growth interface of SiC single crystal  6  is concave and the crystal attaches to a growth guide  11  of crucible  1 . Insulation inserts  12   d   2  and  12   d   4  can have a flat central portion and the edge area convex sloped away from growth crucible  1 . These inserts  12   d   2  and  12   d   4  can be used when the growth interface of SiC single crystal  6  is convex. 
     In an example, the radius or curvature or the angle of slope of the concave ( 12   d   1  and  12   d   3 ) or convex ( 12   d   2  and  12   d   4 ) edge areas, and/or where the flat central portion of insulation insert(s)  12   d   1 - 12   d   4  ends and where said radius of curvature or slope begins can be selected or tuned as necessary to cause flat (or substantially flat) isotherms  10  to be present at the growth interface of growing SiC single crystal  6 , inside of the growing SiC single crystal  6 , and within growth crucible  1 . 
     Also, the aspect ratio of the growth crucible  1 , defined as a ratio of the outside diameter (OD) of crucible  1  to the height of crucible  1 , can be greater than one, i.e., the crucible OD is larger than its height. It has been observed that increasing the crucible OD helps further reduce undesirable radial temperature gradients in the central areas of crucible  1 . In an example, for 200 mm crystal growth, the aspect ratio of growth crucible  1  can desirably be between 1 and 3, or more desirably between 1.5 and 2.5. In another example, for 300 mm crystal growth, the aspect ratio of growth crucible  1  can desirably be between 1.5 and 4, or more desirably between 2 and 3. 
     To aid in forming flat (or substantially flat) isotherms within growth crucible  1 , at least the interior facing surface of top or lid  22  of growth chamber  20  is colored black, whereupon said black coloring enables top  22  to absorb heat flux coming into contact therewith while avoiding or reducing reflection of said heat flux back into the interior of chamber  20 . The interior surface of top  22  can be colored black in any suitable or desirable manner such as, for example, a high temperature black paint that is compatible with the material forming top or lid  22 . However, this is not to be construed in a limiting sense. 
     By coloring its interior facing surface black, top or lid  22  of chamber  20  more closely approximates a black body (known in classical physics) that absorbs radiation that falls on its surface. Of course, an ideal black body that absorbs all radiation does not exist. However, by coloring at least the interior surface of top or lid  22  of chamber  20  black, or forming top  22  of a material that is black in color, the reflection of heat flux from top  22  back into chamber  20  can be reduced or avoided enabling at least some control of heat flux being reflected by the interior facing surface of top or lid  22  back into chamber  20 . While not ideal, the black colored interior facing surface of top or lid  22  has been observed to be advantageous in the formation of flat (or substantially flat) isotherms  10  within growth crucible  1  and, more particularly, at the growth interface of growing SiC single crystal  6 , and inside of the growing SiC single crystal  6 . 
     Herein, a substantially flat isotherm (as determined by finite element analysis) is considered to be one that corresponds to a radial temperature variation in the growth crucible  1  of less than 10° C. between a central axis  42  of the growth crucible  1  and the outside diameter of the SiC single crystal  6  growing on SiC seed  3 . However, this is not to be construed in a limiting sense. 
     By optimizing the radial gradients using the aforementioned approaches, large-diameter SiC crystal  6  boules having flat growth interfaces and low levels of stress can be grown. 
     Example 1. Run GQ0090: Growth of High Crystal Quality 200 mm Diameter N-Type 4H—SiC Boule 
     For this growth run, a high quality 4H—SiC single crystal seed  3  wafer was prepared having a diameter of 225 mm and oriented 4 degrees off-axis. This single crystal SiC seed  3  wafer was sliced from a large-diameter 4H SiC boule produced by diameter expansion in accordance with the teaching of US 2012/0285370 and/or U.S. Pat. No. 8,313,720. 
     Expansion of the SiC boule diameter was accomplished through a number of sequential crystal growth runs, according to the flowchart shown in  FIG. 5 . Diameter expansion starts with selection of a quality single crystal SiC seed  3  of smaller diameter. This seed can be used in a PVT growth process tuned to produce defect-free or substantially defect-free diameter expansion. The thus grown SiC single crystal  6  boule can be sliced into wafers which are larger in diameter than the initial SiC seed  3 . The wafers can be thoroughly characterized, and only best quality wafers can be used as seeds  3  in the next growth run (expansion round). As the SiC seed  3  diameter increases from one growth run (expansion round) to the next, the growth process in each run can be tuned, for example, to optimize SiC single crystal  6  growth for each round of expansion. 
     The PVT growth apparatus shown in  FIG. 2  was used in the growth run of this Example 1. The graphite growth crucible  1  of this PVT growth apparatus had a 250 mm OD. The growth crucible  1  was loaded with SiC source grain material  2  disposed at the crucible bottom. The aforementioned 4H—SiC single crystal seed  3  wafer of 225 mm in diameter was disposed at the interior top or lid  22  of growth crucible  1 . The three-phase flat serpentine heater  40  shown schematically in  FIG. 3A  was positioned under the bottom  26  of crucible  1 . 
     In this run, a flat insulation insert  12   d  (shown in  FIGS. 2 and 4A ) was used. Insulation insert  12   d  was made of a rigid insulation board. The insulation insert  12   d  dimensions were 220 mm diameter and 25 mm thick. During this run, temperatures and pressures typical for the prior art PVT growth were used. In order to produce N-type material, gaseous nitrogen (versus argon) was added to the growth atmosphere as the process gas. 
     The growth run of this Example 1 yielded the 4H—SiC boule shown in  FIG. 6 . The boule had a flat growth interface; accordingly, a small c-facet was visible at the very edge of the boule. The flat growth interface was the consequence of flat isotherms and a sign of low crystal stress. 
     The as-grown boule of Example 1 was processed into 200 mm diameter wafers. A cross-polarizer image taken of one these wafers is shown in  FIG. 7 . The image shows neither local nor global crystal stress. 
     Wafers sliced from the as-grown boule of Example 1 were polished and characterized using X-ray rocking curves. The wafers were scanned in two orthogonal directions: parallel to the &lt;1-210&gt; crystallographic direction and parallel to the &lt;10-10&gt; direction. The results obtained from one of these wafers are shown in  FIGS. 8A-8B . As can be seen, the lattice curvature ( FIG. 8A ) in this wafer is below 0.1°, while FWHM ( FIG. 8B ) across the entire wafer area does not exceed 25 arc-seconds. This testifies to the excellent crystal quality of this as-grown SiC wafer. 
       FIG. 9  is a micropipe density map measured on one of the 200 mm wafers sliced from the 4H—SiC boule of Example 1. As can be seen, the wafer is practically micropipe free, with only a few micropipes visible near the wafer edge. The wafer-average micropipe density (MPD) in this wafer was measured to be 0.09 cm −2 , thus demonstrating very high crystal quality. 
     Dislocation density maps measured on one of the 200 mm wafers sliced from the 4H—SiC boule of Example 1 are shown in  FIGS. 10A-10C . Dislocation etch pits were revealed by etching the wafer in molten KOH. The wafer had a total dislocation density of 2800 cm −2  ( FIG. 10A ), threading screw dislocation (TSD) density of 620 cm −2  ( FIG. 10B ), and basal plane dislocation (BPD) density of 170 cm −2  ( FIG. 10C ). All these results point to excellent crystal quality. 
     Example 2. Run GP0105: Growth of High Crystal Quality 200 mm Diameter Vanadium-Doped Semi-Insulating 4H—SiC Boule 
     For this growth run, a high quality 4H—SiC single crystal seed  3  wafer of 210 mm in diameter was prepared. This SiC seed  3  wafer was sliced from a larger diameter 4H—SiC boule grown by the process of diameter expansion discussed above. The SiC seed  3  wafer was sliced on-axis, that is, with its faces parallel to the basal (0001) plane. 
     Growth in this Example 2 was carried out in a PVT growth apparatus similar to that shown in  FIG. 2 . In order to produce a semi-insulating crystal, vanadium doping was employed combined with precise control over the presence of background impurities of nitrogen and boron. Vanadium compensation of silicon carbide was in accordance with the teachings of U.S. Pat. Nos. 7,608,524 and 8,216,369. 
     Growth in this Example 2 yielded a 4H—SiC boule of 209 mm in diameter shown in  FIG. 11 . The as-grown boule was fabricated into a plurality of 200 mm wafers, which were characterized. The characterization included evaluation of electrical properties and crystal quality. The results showed that the produced wafers were of Nu-type with their room-temperature resistivity above 1·10 12  Ohm-cm and the activation energy of resistivity of about 1 eV. 
     SIMS analysis performed on one of the wafers sliced from the boule of this Example 2 showed vanadium concentration of 1·10 17  atoms cm −3 , nitrogen at 6·10 15  atoms cm −3 , and boron at 3·10 15  atoms cm −3 . 
     Etching of a wafer sliced from the boule of this Example 2 in molten KOH revealed a wafer-averaged micropipe density of 0.1 cm −2 . Scanning of a polished wafer with X-ray rocking curves showed a lattice curvature of below 0.1° and a full width at half maximum (FWHM) of the X-ray reflection below 25 arc-seconds. 
     Various non-limiting examples of an apparatus and method described herein can include one or more or any combination of the features described in the following paragraphs. 
     As can be seen, disclosed herein is a PVT growth apparatus for PVT growing an SiC single crystal comprising growth chamber  20 . Growth crucible  1  is positioned in the growth chamber  20 . The growth crucible is configured to be charged with a SiC source material  2  at a bottom  26  of the growth crucible  1  and a single crystal SiC seed  3  at a top or lid  28  of the growth crucible  1  with the SiC source material and the single crystal SiC seed in spaced relation. Thermal insulation  12  surrounds the growth crucible  1  inside of the growth chamber  20 . The thermal insulation  12  includes a side insulation piece  12   a  between a side  30  of the growth crucible  1  and a side  25  of the growth chamber  20 , a bottom insulation piece  12   c  between the bottom  26  of the growth crucible  1  and a bottom  24  of the growth chamber  20 , a top insulation piece  12   c  between the top  28  of the growth crucible  1  and a top  22  of the growth chamber  20 . The thermal insulation  12  also includes an insulation insert  12   d  positioned in an opening  32  in the top insulation piece  12   b . The insulation insert  12   d  has a thickness between 20 mm and 50 mm and a largest dimension between 90% and 120% of a largest dimension of the single crystal SiC seed  3 . A geometry of the insulation insert can be tuned to control heat flux in the SiC single crystal that grows on the single crystal SiC seed in use of the PVT growth apparatus. A heater  40  is positioned between the bottom  26  of the growth crucible  1  and the bottom insulation piece  12   c.    
     The side  30  of the growth crucible  1  have a cylindrical shape about a central axis  42 . 
     In an example, at least an interior facing surface of the top  22  of the growth chamber  20  that faces the insulation insert  12   d  can be black in color. The growth chamber  20  is made from a metal or metal alloy, such as stainless steel. 
     In an example, at least the exterior facing surface of the top  22  of the growth chamber  20  that faces away from the insulation insert  12   d  can also be black in color. 
     In an example, a growth guide  11  can depend from the top  28  of the growth crucible  1  toward the bottom  26  of the growth crucible  1  and terminate above a top level of the SiC source material  2 . The growth guide  11  can be spaced from an interior of the side  30  of the growth crucible  1 . 
     In an example, the side, top, and bottom insulation pieces  12   a .  12   b ,  12   c  can each have a thickness greater than or equal to the thickness of the insulation insert  12   d . The side, top, and bottom insulation pieces  12   a ,  12   b , and  12   c  can each have a thickness that is at least two times the thickness of the insulation insert  12   d.    
     In an example, the heater  40  can comprise a flat resistance heater. The heater  40  can have a largest dimension greater than a largest dimension of the bottom  26  of the growth crucible  1 . 
     In an example, desirably no part of the heater  40  resides in a space between the side  30  of the growth crucible  1  and the side  25  of the growth chamber  20  occupied by the side insulation piece  12   a.    
     In an example, the heater  40  can reside exclusively between the bottom  26  of the growth crucible  1  and the bottom insulation piece  12   c.    
     In an example, a ratio of an outside diameter of the growth crucible  1  over a height of the growth crucible  1  can be between 1 and 3 or between 1.5 and 4. 
     Also disclosed is a PVT growth apparatus for PVT growing an SiC single crystal comprising a growth crucible  1  having a side  30 , a top  28 , and a bottom  26 , and an aspect ratio of an outside diameter over a height between 1 and 4. The top  28  of the growth crucible  1  is configured to support a single crystal SiC seed  3  in an interior of the growth crucible  1 . Insulation  12  surrounds an exterior of the growth crucible  1 . The insulation  12  includes side, top, and bottom insulation pieces  12   a .  12   b ,  12   c  positioned adjacent the respective side, top, and bottom  30 ,  28 ,  26  of the growth crucible  1 . The insulation  12  further includes an insulation insert  12   d  positioned in an opening  32  in the top insulation piece  12   b . The insulation insert  12   d  has a thickness less than a thickness of any one or more of the side, top, and bottom insulation pieces  12   a .  12   b .  12   c . A geometry of the insulation insert can be tuned to control heat flux in the SiC single crystal that grows on the single crystal SiC seed in use of the PVT growth apparatus. A heater  40  is positioned exclusively between a bottom  26  of the growth crucible  1  and the bottom insulation piece  12   c.    
     In an example, the insulation insert  12   d  can have a thickness between 20 mm and 50 mm and a diameter between 90% and 120% of a diameter of a the single crystal SiC seed  3 . 
     In an example, the PVT growth apparatus can further include a growth chamber  20  in which the growth crucible  1 , insulation  12 , and heater  40  are positioned. The growth chamber  20  can include a top  22  in spaced relation to the top  28  of the growth crucible  1 . At least an interior facing surface of the top  22  of the growth chamber  20  can be black in color. 
     In an example, the PVT growth apparatus can further include at least one of the following: a window  5   b  in the bottom insulation piece  12   c ; and a window  5   c  in the heater  40 . 
     Also disclosed herein is a method of PVT growing a SiC single crystal boule comprising providing the PVT growth apparatus described above with the single crystal SiC seed  3  and SiC source material  2  positioned in spaced relation in the growth crucible  1 ; and causing the heater  40  to heat the growth crucible  1  such that the SiC source material  2  sublimates and is transported via a temperature gradient that forms in the growth crucible  1  in response to the heater heating the growth crucible  1  to the single crystal SiC seed  3  where the sublimated SiC source material  2  condenses forming a growing SiC single crystal  6 . 
     In an example, step (b) of the method can include purely axial heat fluxes  8 ,  9  passing through the bottom  26  and the top  28  of the growth crucible  1  forming a flat isotherm at least at a growth interface of the growing SiC single crystal  6  on the single crystal SiC seed  3 . 
     In an example, it is envisioned that the growth apparatus and method described herein can be utilized to PVT grow a SiC single crystal boule having a first, outside diameter of at least 305 mm, e.g., between 205 mm and 305 mm; a thickness of at least 10 mm; and, within a second diameter of the SiC single crystal boule that is less than the first diameter, a total thickness variation of less than 20%, e.g., less than 10%. Herein, the total thickness variation is defined as the difference between maximum and minimum values of boule thickness within the second diameter divided by the average boule thickness. 
     In an example, the second diameter can be at least 5 mm less than the first diameter. Stated differently, the first diameter can be at least 5 mm more than the second diameter. 
     In an example, the SiC single crystal boule, and a wafer prepared (sliced) therefrom, can be a 4H or 6H polytype. 
     In an example, the SiC single crystal boule can be a nitrogen-doped N-type SiC single crystal boule. 
     In an example, the SiC single crystal boule can be vanadium-doped and semi-insulating. 
     In an example, a wafer having a diameter of 200 mm can be prepared (sliced) from a 205 mm SiC single crystal boule. In another example, a wafer having a diameter of 250 mm can be prepared (sliced) from a 255 mm SiC single crystal boule. In another example, a wafer having a diameter of 300 mm can be prepared (sliced) from a 305 mm SiC single crystal boule. 
     In an example, a 200 mm, 250 mm, or 300 mm diameter N-type 4H—SiC wafer can be prepared in accordance with the principles described herein and can have a wafer-average micropipe density of less than 0.1 cm −2 , e.g., less than 0.05 cm −2 ; a wafer-average threading screw dislocation density of less than 1000 cm −2 , e.g., less than 300 cm −2 ; and a wafer-average basal plane dislocation density of less than 500 cm 2 ; e.g., less than 200 cm 2 . 
     In an example, a 200 mm, 250 mm, or 300 mm diameter n-type 4H—SiC wafer can be prepared in accordance with the principles described herein and can have a lattice curvature of less than 0.3° e.g., less than 0.1°; and a full width at half maximum (FWHM) of a rocking curve of less than 25 arc-seconds, e.g., less than 15 arc-seconds. 
     In an example, a 200 mm, 250 mm, or 300 mm diameter N-type 4H—SiC wafer can be prepared in accordance with the principles described herein and can have total areas populated with stacking faults of less than 10%. e.g., less than 2% of the total wafer area. 
     In an example, a 200 mm diameter semi-insulating, vanadium doped SiC wafer can be prepared in accordance with the principles described herein and can have a micropipe density of less than 0.1 cm 2 ; a lattice curvature of less than 0.3, e.g., less than 0.1°; and a full width at half maximum (FWHM) of the rocking curve of less than 25 arc-seconds, e.g., less than 15 arc-seconds. 
     In an example, a 200 mm diameter semi-insulating, vanadium doped SiC wafer can be prepared in accordance with the principles described herein and can have a room temperature resistivity greater than 1·10 11  Ohm-cm. 
     In an example, a 200 mm diameter semi-insulating, vanadium doped. Nu-type 6H—SiC wafer can be prepared in accordance with the principles described herein and can have a room temperature resistivity greater than 1·10 11  Ohm-cm, or an activation energy of resistivity of at least 0.8 eV, or both 
     In an example, a 200 mm diameter semi-insulating, vanadium doped, Pi-type 6H—SiC wafer can be prepared in accordance with the principles described herein and can have a room temperature resistivity greater than 1·10 12  Ohm-cm- or an activation energy of resistivity of at least 1.2 eV, or both. 
     In an example, a 200 mm diameter semi-insulating, vanadium doped, Nu-type 4H—SiC wafer can be prepared in accordance with the principles described herein and can have a room temperature resistivity greater than 1·10 12  Ohm-cm; or an activation energy of resistivity of at least 1 eV, or both. 
     In an example, a 200 mm diameter semi-insulating, vanadium doped, Pi-type 4H—SiC wafer can be prepared in accordance with the principles described herein and can have a room temperature resistivity greater than 1·10 12  Ohm-cm; and an activation energy of resistivity of at least 1.5 eV, or both. 
     The foregoing examples have been described with reference to the accompanying figures. Modifications and alterations will occur to others upon reading and understanding the foregoing examples which are provided for the purpose of illustration and are not to be construed in a limiting sense. Accordingly, the foregoing examples are not to be construed as limiting the disclosure.