Abstract:
A physical vapor transport growth system includes a growth chamber charged with SiC source material and a SiC seed crystal in spaced relation and an envelope that is at least partially gas-permeable disposed in the growth chamber. The envelope separates the growth chamber into a source compartment that includes the SiC source material and a crystallization compartment that includes the SiC seed crystal. The envelope is formed of a material that is reactive to vapor generated during sublimation growth of a SiC single crystal on the SiC seed crystal in the crystallization compartment to produce C-bearing vapor that acts as an additional source of C during the growth of the SiC single crystal on the SiC seed crystal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 61/163,668, filed Mar. 26, 2009, entitled “SiC Single Crystal Sublimation Growth Method and Apparatus”, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to SiC sublimation crystal growth. 
         [0004]    2. Description of Related Art 
         [0005]    Wafers of silicon carbide of the 4H and 6H polytype serve as lattice-matched substrates to grow epitaxial layers of SiC and GaN, which are used for fabrication of SiC- and GaN-based semiconductor devices for power and RF applications. 
         [0006]    With reference to  FIG. 1 , large SiC single crystals are commonly grown by the technique of Physical Vapor Transport (PVT).  FIG. 1  shows a schematic view of a typical PVT growth cell, wherein PVT growth of a SiC single crystal  15  is carried out in a graphite crucible  11  sealed with a graphite lid  12  and loaded with a sublimation source  13  disposed at a bottom of crucible  11  and a single crystal SiC seed  14  disposed at the crucible top. Sublimation source  13  is desirably polycrystalline SiC grain synthesized in a separate process. Loaded crucible  11  is placed inside of a growth chamber  17  where it is surrounded by thermal insulation  18 . Inductive or resistive heating is used to bring crucible  11  to a suitable temperature, generally, between 2000° C. and 2400° C., for the PVT growth of a SiC single crystal  15  on SiC single crystal seed  14 . 
         [0007]      FIG. 1  shows a typical inductive heating arrangement with a RF coil  19  placed outside growth chamber  17 , which is desirably made of fused silica. RF coil  19  is positioned with respect to crucible  11  such that during growth of single crystal  15 , a temperature of sublimation source  13  is maintained higher than a temperature of the seed crystal  14 , typically, by 10° C. to 200° C. 
         [0008]    Upon reaching suitable high temperatures, sublimation source  13  vaporizes and fills crucible  11  with vapor  16  of Si, Si 2 C and SiC 2  molecules. The temperature difference between sublimation source  13  and seed crystal  14  forces vapor  16  to migrate and condense on seed crystal  14  thereby forming single crystal  15 . In order to control the growth rate, PVT growth is carried out in the presence of a small pressure of inert gas, typically, between several and 100 Torr. 
         [0009]    Generally, SiC crystals grown using this basic PVT arrangement suffer from numerous defects, stress, and cracking. To this end, it is difficult to grow long boules of SiC single crystal  15  using conventional PVT due to carbonization of sublimation source  13  and subsequent massive incorporation of carbon inclusions in single crystal  15 . Cracking becomes a major yield loss when the conventional PVT technique is utilized to grow large-diameter SiC single crystals. 
         [0010]    Inclusions in PVT-grown crystals, e.g., single crystal  15 , include carbon inclusions (particles), silicon droplets, and foreign polytypes. Carbon particles in single crystal  15  can be traced to SiC sublimation source  13  and the graphite forming crucible  11 . Specifically, silicon carbide sublimes incongruently producing a silicon-rich vapor and carbon residue in the form of very fine carbon particles. During growth of single crystal  15 , these fine particles become airborne and, transferred by the flow of vapor  16 , incorporate into growing single crystal  15 . Massive carbon incorporation into single crystal  15  happens at the end of the growth of single crystal  15  when a large amount of carbon residue is present in crucible  11 . 
         [0011]    Vapor erosion of the graphite forming crucible  11  can also produce carbon inclusions. During growth, the inner walls of crucible  11  are in contact with Si-rich vapor  16  which attack the graphite forming crucible  11  and erode it. Structurally, the graphite forming crucible  11  includes graphitic grains embedded into the matrix of graphitized pitch. The graphitized pitch is attacked by vapor  16  first. This leads to liberation of graphite grains which are transferred to the growth interface of single crystal  15 . 
         [0012]    Silicon inclusions (droplets) usually form at the beginning of the growth of single crystal  15 , when the SiC sublimation source  13  source is fresh. Vapor  16  over SiC sublimation source  13  can contain a too high fraction of silicon, which can cause the formation of Si liquid on the growth interface of single crystal  15  and incorporation of Si droplets into single crystal  15 . 
         [0013]    A large number of polytypic modifications of silicon carbide exist, and inclusion of foreign polytypes in sublimation-grown 4H and 6H single crystal  15  is common (15R inclusions are most frequent). The origin of polytypic inclusions is often tied to the appearance of macrosteps on the growth interface of single crystal  15 . The facets formed on the macrosteps are not stable against stacking faults. These stacking faults latter evolve during growth of single crystal  15  into foreign polytypes in single crystal  15 . 
         [0014]    Two technological factors affect the stability of the 6H and 4H polytypes during growth of single crystal  15 . One is the curvature of the growth interface of single crystal  15 . A flat or slightly convex growth interface of single crystal  15  is believed to be more stable against polytypic perturbations than a more curved interface, convex or concave. Another factor is the stoichiometry of vapor  16 . It is believed that stable growth of the SiC crystals  15  of hexagonal 4H and 6H polytypes requires a vapor phase enriched with carbon, while a too high atomic fraction of Si in the vapor can lead to the appearance of foreign polytypes. 
         [0015]    Three types of dislocations can generally exist in SiC single crystal  15  grown by PVT: threading screw dislocations, threading edge dislocations, and basal plane dislocations. The lines of the threading dislocations tend to position along the crystallographic c-direction, which is often used as a growth direction of SiC single crystals  15 . Basal plane dislocations are dislocations with their lines parallel to the basal c-plane. 
         [0016]    A micropipe is a threading screw dislocation with a large Burgers vector. When the Burgers vector exceeds (2-3)·c, the crystal relieves the stress caused by the dislocation by forming a hollow core, from a fraction of a micron to 100 microns in diameter. 
         [0017]    Upon nucleation, growing SiC single crystal  15  inherits some of the dislocations from seed crystal  14 . During growth of SiC single crystal  15 , micropipes and dislocations participate in reactions with other micropipes and dislocations. This leads to a progressive reduction in the micropipe/dislocation densities during growth. In the case of growth disturbance, such as incorporation of a carbon particle or foreign polytype, new micropipes and dislocations are generated. 
         [0018]    It has been observed that the magnitude of growth-related stress increases with the increase in the length and diameter of a SiC single crystal boule formed by the growth of SiC single crystal  15 . More specifically, SiC single crystal  15  grown by conventional PVT exhibits nonuniform thermo-elastic stress and its shear component often exceeds the critical value of 1.0 MPa leading to plastic deformation. Plastic deformation occurs via generation, multiplication and movement of dislocations. Unresolved stress accumulated during growth of a boule of SiC single crystal  15  can lead to cracking of the boule formed by the growth of SiC single crystal  15  during cooling of said boule to room temperature or during subsequent wafer fabrication. 
         [0019]    With reference to  FIG. 2 , since the inception of the PVT growth technique, a number of process modifications have been developed. In one such modification, a cylindrical, gas-permeable divider  25 , made of either thin-walled dense graphite or porous graphite, is utilized to divide a crucible  20  into two concentric compartments: a source storage compartment  24  containing a solid SiC sublimation source material  21  and a crystal growth compartment  26  with a SiC single crystal seed  22  at the bottom. For the purpose of simplicity, an RF coil and a growth chamber have been omitted from  FIG. 2 . 
         [0020]    At high temperatures, SiC sublimation source  21  vaporizes and vapor  27  fills compartment  24 . The volatile Si- and C-bearing molecules in vapor  27  diffuse across divider  25  and enter crystal growth compartment  26 , as shown by the arrows in  FIG. 2 . Then, driven by the axial temperature gradient, vapor  27  migrate downward to SiC single crystal seed  22  and condense on it causing growth of a SiC single crystal  23 . 
         [0021]    The PVT process shown and described in connection with  FIG. 2  has drawbacks, including, without limitation, the nucleation of polycrystalline SiC on the graphite walls of crucible  20  and/or divider  25 , the nucleation of polycrystalline SiC on the edges of SiC single crystal seed  22 , and a high degree of stress in the grown SiC single crystal  23 . This PVT modification is considered inapplicable to the growth of industrial size SiC boules. 
         [0022]    With reference to  FIG. 3 , in another modification of the basic PVT growth technique, PVT is used in combination with High Temperature Chemical Vapor Deposition (HTCVD) to achieve continuous growth of SiC single crystals of unlimited thickness. In the schematic diagram of a Continuous Feed PVT process (CF-PVT) shown in  FIG. 3 , a crystal growth crucible  30  is divided into two chambers: a lower chamber  33  for the HTCVD process, and an upper chamber  34 , which includes a SiC single crystal seed  36 , for PVT. Chambers  33  and  34  were separated by one or more members  35  made of gas-permeable graphite foam. Solid SiC source material  39  is placed atop the upper surface of foam member  35  that faces SiC single crystal seed  36 . Heating of SiC source material  39  is provided by an RF coil  31  coupled to a graphite susceptor  32  in a manner known in the art. 
         [0023]    Gaseous trimethylsilane (TMS)  37  is supplied to lower chamber  33  assisted by a peripheral flow of argon  38 . At high temperatures, the TMS molecules undergo various chemical transformations. The gaseous products of these transformations diffuse through foam member  35  and form solid SiC, either in the bulk of foam member  35  or on the upper surface of foam member  35 . In upper chamber  34 , a conventional PVT growth process takes place. Namely, solid SiC source material  39  sublimates, its vapor migrates to SiC single crystal seed  36  and condenses thereon causing growth of SiC single crystal  36 ′. 
         [0024]    It was believed that gas-feeding through foam member  35  would prolong the life of the SiC source material  39  and prevent its carbonization. However, thick and/or long boules of SiC single crystal  36 ′ where unable to be grown due to the erosion of foam member  35 , source carbonization, formation of graphite inclusions and other defects in the growing SiC single crystal  36 ′. For the purpose of simplicity, the growth chamber has been omitted from  FIG. 3 . 
         [0025]    With reference to  FIG. 4 , another modification of the basic PVT growth technique includes a susceptor  46 , a crucible  43  containing semiconductor purity silicon  42 , a SiC seed  40  attached to a seed-holder  41 , and a high-purity, gas-permeable membrane  47  disposed between seed  40  and silicon  42 . Membrane  47  can be in the form of porous graphite disc or in the form of dense graphite disc with multiple holes. 
         [0026]    Upon heating, silicon  42  melts and vaporizes. The Si vapor emanating from the molten silicon  42  diffuses through porous membrane  47 , where it reacts with carbon of membrane  42  producing volatile Si 2 C and SiC 2  molecular associates. Vapor  43  including the volatile Si 2 C and SiC 2  molecular associates escape from membrane  47 , migrate to seed  40 , and condense on it causing growth of single crystal  45 . Thus, membrane  47  serves as a source of carbon. For the purpose of simplicity, an RF coil and a growth chamber have been omitted from  FIG. 4 . 
         [0027]    One of the shortcomings of prior art SiC sublimation growth techniques is the phenomenon of vapor erosion of graphite. With reference to  FIG. 5 , in conventional PVT growth a crystal growth crucible  50  includes solid a SiC source  51  at the bottom, a SiC seed  52  attached to the crucible top, and a SiC single crystal  54  growing on seed  52 . Usually, the edge of the boule of SiC single crystal  54  is in close proximity to (sometimes touching) a graphite sleeve  55  disposed in the vicinity of the growing SiC single crystal  54 . This sleeve  55  can be a heat shield, growth guide, or the crucible wall, all generally made of graphite. The distance between the SiC single crystal  54  and SiC source  51  is usually much more significant. 
         [0028]    During growth of SiC single crystal  54 , SiC source  51  sublimes and generates Si-rich vapor  53 , with an Si:C atomic ratio generally between 1.1 and 1.6, and carbon residue  51   a.  Vapor  53  in the space  57  adjacent to the SiC source  51  is in equilibrium with the SiC+C mixture. Driven by the temperature gradient, vapor  53  moves axially toward SiC seed  52 . This movement of vapor  53  is in the form of Stefan gas flow with the linear rate of about 1-10 cm/s. 
         [0029]    Upon reaching the growth interface, vapor  53  condenses causing growth of the SiC single crystal  54 . Precipitation of stoichiometric SiC from the Si-rich vapor  53  makes the vapor even more Si-rich in the space  58  adjacent SiC crystal  54 . Therefore, the vapor phase composition in this space does not correspond anymore to the SiC+C equilibrium. Instead, vapor  53  is now in equilibrium with either SiC of a certain stoichiometry or, in the extreme case, with the two-phase SiC+Si mixture. A too high content of Si in vapor  53  can lead to the formation of the liquid Si phase on the growth interface and incorporation of Si droplets into the growing crystal. 
         [0030]    The atomic fraction of Si in vapor  53  in space  58  is the highest inside crucible  50 , and this forces excessive Si to diffuse out of space  58 . Due to the significant distance between SiC single crystal  54  and SiC source  51  and the presence of the axial Stefan flow in crucible  50 , the excessive Si does not reach SiC source  51 . Rather, it diffuses from SiC single crystal  54  toward and reaches the nearest graphite part—sleeve  55 . This diffusion is shown by arrows  56 . This Si-rich vapor (which is not in equilibrium with carbon) attacks graphite sleeve  55  and erodes it producing SiC 2  and Si 2 C gaseous molecules. 
         [0031]    In a typical PVT geometry, the temperature of sleeve  55  is higher than that of the SiC single crystal  54 . Driven by this radial temperature gradient, the gaseous products of graphite erosion (SiC 2  and Si 2 C) diffuse back toward SiC single crystal  54 , as shown by arrows  56   a,  and enrich space  58   a  in the peripheral area  54   b  of SiC single crystal  54  in front of the growth interface with carbon. In other words, a zone of vapor circulation emerges at the edges of SiC single crystal  54  with silicon acting as a transport agent and transporting carbon from sleeve  55  to the lateral regions of growing SiC single crystal  54 . In SiC single crystals  54  grown by the PVT technique, carbon from sleeve  55  can comprise up to 20% of the total carbon content of the crystal. 
         [0032]    The net result of this vapor circulation is the formation of two distinct regions in the vapor in the vicinity of the growing crystal. The vapor in central region  58  has a higher atomic fraction of silicon than the vapor in the lateral region  58   a.  Accordingly, central area  54   a  of SiC single crystal  54  grows from Si-rich vapor, while the peripheral area  54   b  of SiC single crystal  54  grows from the vapor containing a higher fraction of carbon. 
         [0033]    Such compositional nonuniformity of the vapor phase has negative consequences for the crystal quality, including:
       Spatial nonuniformity of the crystal composition (stoichiometry) resulting in a high degree of crystal stress, cracking and spatially nonuniform incorporation of impurities and dopants;   Formation of foreign polytypes and related defects;   Inclusion of carbon particles transported from the source;   Inclusion of carbon particles transported from the eroded sleeve; and   Inclusion of Si droplets in central areas of the crystal.       
 
         [0039]    For the purpose of simplicity, an RF coil and a growth chamber have been omitted from  FIG. 5 . 
       SUMMARY OF THE INVENTION 
       [0040]    The invention is a physical vapor transport growth system. The system includes a growth chamber charged with SiC source material and a SiC seed crystal in spaced relation and an envelope that is at least partially gas-permeable disposed in the growth chamber. The envelope separates the growth chamber into a source compartment that includes the SiC source material and a crystallization compartment that includes the SiC seed crystal. The envelope is formed of a material that is reactive to vapor generated during sublimation growth of a SiC single crystal on the SiC seed crystal in the crystallization compartment to produce a C-bearing vapor that acts as an additional source of C during the growth of the SiC single crystal on the SiC seed crystal. 
         [0041]    The envelope can be comprised of a sleeve that surrounds sides of the SiC seed crystal and the growing SiC single crystal and a gas-permeable membrane disposed between the SiC source material and a surface of the SiC seed crystal that faces the SiC source material. 
         [0042]    The sleeve can be disposed between 0.5 mm and 5 mm from the sides of the SiC seed crystal and the growing SiC single crystal. 
         [0043]    The gas-permeable membrane can be disposed between 15 mm and 35 mm from the surface of the SiC seed crystal that faces the SiC source material. 
         [0044]    The gas-permeable membrane can be made of porous graphite having a density between 0.6 and 1.4 g/cm 3  and a porosity between 30% and 70%. 
         [0045]    The graphite forming the gas-permeable membrane can be comprised of graphite grains, each of which has a maximum dimension between 100 and 500 microns. 
         [0046]    The gas-permeable membrane can have a thickness between 3 mm and 12 mm. 
         [0047]    The sleeve can have a wall thickness between 4 mm and 15 mm. 
         [0048]    The sleeve can be cylindrical and the membrane can be disposed at one end of the sleeve. 
         [0049]    The invention is also a physical vapor transport growth method that comprises: (a) providing a growth chamber that is separated by an envelope that is at least partially gas-permeable into a source compartment that is charged with a SiC source material and a crystallization compartment that includes a SiC seed crystal; and (b) heating the interior of the growth crucible such that a temperature gradient forms between the SiC source material and the SiC seed crystal, the SiC source material is heated to a sublimation temperature, and the temperature gradient is sufficient to cause sublimated SiC source material to diffuse from the source compartment through the gas-permeable part of the envelope into the crystallization compartment where the sublimated SiC source material condenses on the SiC seed crystal and forms a SiC single crystal, wherein said envelope is comprised of a material that is reactive to vapor generated during sublimation growth of the SiC single crystal on the SiC seed crystal in the crystallization compartment to produce a C-bearing vapor that acts as an additional source of C during the growth of the SiC single crystal on the SiC seed crystal. 
         [0050]    Step (b) can occur in the presence of between 1 and 100 Torr of inert gas. 
         [0051]    A capsule can be disposed in the source compartment. The capsule can have an interior that is charged with a dopant. The capsule can have one or more capillaries of pre-determined diameter and length that extend between the interior and an exterior of said capsule. The diameter and the length of each capillary can be selected whereupon the dopant is disposed spatially uniformly in the grown SiC single crystal. 
         [0052]    The capsule can be made of graphite. The dopant can be either elemental vanadium or a vanadium compound in quantity sufficient for full electronic compensation of the grown SiC single crystal. 
         [0053]    The method can further include: charging the growth chamber with elemental Si and C; and prior to heating the SiC source material to the sublimation temperature, heating the elemental Si and C to a temperature below the sublimation temperature for synthesis of the elemental Si and C into a solid SiC that comprises the SiC source material. 
         [0054]    The mean, room temperature electrical resistivity of the grown SiC single crystal is above 10 9  Ohm-cm with a standard deviation below 10% of the mean value. The grown SiC single crystal is of the 4H or 6H polytype. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0055]      FIGS. 1-5  are cross-sectional schematic views of different embodiment prior art physical vapor transport (PVT) growth cells; 
           [0056]      FIGS. 6-8  are cross-sectional schematic views of different embodiment PVT growth cells in accordance with the present invention; 
           [0057]      FIG. 9   a  is a photograph of an as-grown, vanadium-compensated 6H SiC single crystal boule that was grown in a PVT growth cell like the one shown in  FIG. 7 ; 
           [0058]      FIG. 9   b  is the axial resistivity distribution in the crystal boule shown in  FIG. 9   a  determined from standard wafers fabricated from the boule; 
           [0059]      FIG. 9   c  is a resistivity map for one of the wafers fabricated from the boule shown in  FIG. 9   a ; and 
           [0060]      FIG. 10  is a micropipe density map obtained from a wafer fabricated from a vanadium-compensated 6H SiC single crystal boule that was grown in a PVT growth cell like the one shown in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0061]    The invention describes an improved SiC sublimation crystal growth process and apparatus for the growth of high quality SiC single crystals suitable for the fabrication of industrial size substrates, including those of 3″ and 100 mm diameter. The crystal growth crucible of the invention is divided into two compartments by a gas-permeable porous graphite membrane, which is positioned in close proximity to the seed. During growth, the membrane interacts with the Si-rich vapor and supplies additional carbon to the growing crystal. The membrane enriches the vapor phase with carbon and makes the vapor composition in front of the growing crystal more uniform. It also prevents particles originated from the source from contaminating the growth interface. It also makes the isotherms more flat. 
         [0062]    The invention leads to SiC boules with reduced densities of inclusions, such as foreign polytypes, silicon droplets and carbon particles, and it reduces stress and cracking. The growth cell design of the invention permits incorporation of in-situ synthesis of SiC into the SiC sublimation growth process. 
         [0063]    The process and apparatus can be used for the growth of SiC single crystals of 6H and 4H polytypes, both undoped and doped, including those doped with vanadium. 
         [0064]    With reference to  FIG. 6 , PVT growth in accordance with the present invention is carried out in a graphite crucible  60  that includes SiC source  61  at the bottom of crucible  60  and a SiC single crystal seed  63  at the top of crucible  60 . During growth of a SiC single crystal  64  on SiC single crystal seed  63 , crucible  60  is disposed inside of a growth chamber  60   a  where crucible  60  is heated, either resistively or by an inductive heating means  59 , to a suitable temperature for the growth of a SiC single crystal  64  on SiC single crystal seed  63 . 
         [0065]    An envelope  66 , that is at least in-part porous and gas-permeable, at least partially surrounds SiC seed crystal  63  and SiC single crystal  64 . SiC seed crystal  63  can be attached directly to a lid of crucible  60  or, as shown in  FIG. 6 , to a suitable standoff disposed between the lid of crucible  60  and SiC seed crystal  63 . Envelope  66  forms a quasi-closed vapor circulation space  67  around the surfaces, sides, edges, and faces of SiC single crystal seed  63  and growing SiC single crystal  64  that face SiC source  61 . Envelope  66  is made of porous, gas-permeable graphite and is positioned a short distance from growing SiC single crystal  64 . 
         [0066]    Upon reaching the desired growth temperature, SiC source  61  sublimes and fills the interior of crucible  60  with Si-rich vapor  62 . During evaporation, carbon residue  61   a  is formed in SiC source  61 . Vapor  62  in the space  68  adjacent to SiC source  61  is in equilibrium with the SiC+C mixture. 
         [0067]    Driven by a temperature gradient in the interior of crucible  60 , vapor  62  migrates axially toward SiC single crystal seed  63  and enters space  67  by diffusing through the front wall (membrane)  69  of envelope  66 . In the process of diffusion, small-size particles emanating from SiC source  61  are filtered from the vapor  62  by envelope  66 . Thus, porous envelope  66  helps to avoid contamination of the growth interface with particulates. 
         [0068]    After passing through membrane  69 , vapor  62  reaches the growth interface and condenses on it causing growth of SiC single crystal  64 . As a result of precipitation of stoichiometric SiC from the Si-rich vapor  62 , vapor  62  becomes even more enriched with Si and forms vapor  65 . This Si-rich vapor  65  diffuses in space  67  in the direction from the growth interface toward the inner surface of envelope  66 . The distance between growing SiC single crystal  64  and the interior wall of membrane  69  is selected so that diffusing Si-bearing molecules in vapor  65  reach the interior wall of envelope  66  in spite of the Stefan gas flow in the opposite direction. 
         [0069]    Upon contact with the interior wall of envelope  66 , the excess Si in vapor  65  (which is not in equilibrium with carbon) attacks and erodes it generating volatile molecular associates Si 2 C and SiC 2 , whereupon the initially Si-rich vapor  65  will now include these C-bearing species. 
         [0070]    The temperature of envelope  66  is controlled to be higher than that of SiC single crystal  63 . This forces vapor  65  now including these C-bearing species to diffuse toward SiC single crystal  63  and participate in SiC crystallization, thereby forming SiC single crystal  64 . As can be seen, Si acts as a transport agent for carbon and envelope  66  serves as a sacrificial carbon body supplying additional carbon to the growing SiC single crystal  64 . 
         [0071]    Porous, gas-permeable envelope  66  has a wall thickness and is positioned a relatively small distance from SiC seed crystal  63 . The thickness of the front wall  69  of envelope  66  is chosen by taking into account the following factors:
       A polycrystalline SiC deposit can form on the front wall  69  of envelope  66 . Therefore, envelope  66  is desirably mechanically strong enough to support the weight of this deposit.   Envelope  66  should be sufficiently thick to make the vapor migration across the membrane the limiting stage of mass transport in the crucible. If the envelope  66  is too thin, solid SiC will form on the top surface of front wall  69  of envelope  66  and lead to deterioration in the quality of growing SiC single crystal  64 .   A too thick envelope  66  will impede vapor transport in the crucible and reduce the growth rate of SiC single crystal  64 .       The distance between the seed and the membrane is chosen on the basis of the following:
       If envelope  66  is positioned too far from SiC single crystal  63 , the Si-rich vapor generated as a result of crystallization will not reach envelope  66 .   If envelope  66  is positioned too close to SiC single crystal  63 , the crystal thickness will be limited.   
       
 
         [0078]    Exemplary dimensions of porous, gas-permeable envelope  66  are described in the embodiments described hereinafter. The geometry of the PVT growth cell shown in  FIG. 6  has several advantages in the growth of SiC single crystal  64 :
       The presence of the sacrificial carbon envelope in close proximity to the growing SiC single crystal  64  increases the carbon content in the vapor phase in the space adjacent to the growth interface. A more carbon-rich vapor phase leads to better stability of the hexagonal polytypes (6H and 4H) and suppression of non-hexagonal polytypes, such as 15R.   Envelope  66  reduces or eliminates spatial nonuniformity of the vapor phase composition in front of the growth interface, thus reducing or eliminating the compositional nonuniformity of the growing SiC single crystal  64 . This leads to a reduced stress and cracking in SiC single crystal  64 .   The more spatially uniform vapor phase makes incorporation of impurities and dopants into the growing SiC single crystal  64  more spatially uniform.   The higher carbon content in vapor  65  surrounding the growing SiC single crystal  64  avoids or eliminates the formation of liquid silicon on the growth interface and inclusion of Si droplets.   Envelope  66  prevents particles generated in SiC source  61  from reaching and incorporating into the growing SiC single crystal  64 .   The graphite forming envelope  66  positively affects the geometry of the thermal field in the vicinity of the growing SiC single crystal  64 . Specifically, the flat front wall  69  of envelope (membrane)  66  makes the isotherms adjacent the growth interface more flat. Flatter isotherms, in-turn, make the growth interface more flat, which is beneficial to the polytype stability and stress reduction.       
 
       EMBODIMENT 1: GROWTH OF SEMI-INSULATING SiC CRYSTALS 
       [0085]    A schematic diagram of a PVT growth cell for the growth of semi-insulating SiC crystals fully compensated by dopant, such as vanadium, is shown in  FIG. 7 . SiC crystal growth is carried out in a cylindrical crucible  70  made of graphite, desirably, dense, low-porosity isostatically molded graphite, such as ATJ or similar. Crucible  70  contains a solid SiC source  71  disposed at the bottom of crucible  70  and a SiC seed crystal  72  at the crucible top of crucible  70 , for instance, attached to the crucible lid  74 , as shown in the  FIG. 7 . SiC source  71  is desirably in the form of pure polycrystalline SiC grain synthesized separately. 
         [0086]    In accordance with the doping procedure disclosed in U.S. Patent Publication No. 2006/0243984, which is incorporated herein by reference, crucible  70  includes a time-release capsule  80  charged with a dopant  82 . Capsule  80  includes a stable form of dopant  82 , desirably, elemental vanadium, vanadium carbide or vanadium oxide. Capsule  80  is desirably made of an inert material, desirably, dense, low-porosity graphite, such as ATJ, and it includes one or more capillaries  81  of predetermined diameter and length. A more detailed description of the doping capsule is given in U.S. Patent Publication No. 2006/0243984. Capsule  80  loaded or charged with vanadium is buried in the bulk of SiC source  71 , as shown in  FIG. 7 . 
         [0087]    SiC seed crystal  72  is a wafer of 4H or 6H SiC polytype sliced from a previously grown SiC crystal. The growth face of SiC seed crystal  72  is polished to remove scratches and sub-surface damage. The preferred orientation of SiC seed crystal  72  is “on-axis”, that is, parallel to the crystallographic c-plane. However, other orientations of SiC seed crystal  72  can also be used, such as, without limitation, off-cut from the c-plane by several degrees. In the case of 6H, the Si-face of SiC seed crystal  72  is the growth face. In the case of 4H, the C-face of SiC seed crystal  72  is the growth face. 
         [0088]    SiC seed crystal  72  (and later the growing SiC single crystal  73 ) is surrounded by a porous, gas-permeable envelope comprised of a horizontal membrane  75  and a cylindrical sleeve  76 . SiC seed crystal  72 , crucible lid  74 , membrane  75  and sleeve  76  define the boundaries of a vapor circulation space  79 . 
         [0089]    Membrane  75  and sleeve  76  are made of porous graphite with a density, desirably, between 0.6 and 1.4 g/cm 3  and a porosity, desirably, between 30% and 70%. In order to avoid contamination of growing SiC single crystal  73  with micron-size graphitic particles generated as a result of graphite erosion of membrane  75  and sleeve  76 , the material forming membrane  75  and sleeve  76  is porous graphite with large grain sizes, desirably, from 100 to 500 microns. When grains of this size are liberated by graphite erosion, they are too heavy to be transported by the Stefan gas flow. 
         [0090]    Membrane  75  has a thickness, desirably, between 3 and 12 mm and is disposed at a distance from the SiC seed crystal  72 , desirably, between 15 and 35 mm. In the example shown in  FIG. 7 , sleeve  76  is cylindrical, but it can also have other useful shapes deemed desirable by those skilled in the art, such as, without limitation, a truncated cone or a hexagonal pyramid. The wall thickness of sleeve  76  is, desirably, between 4 and 15 mm and the distance between the interior surface of sleeve  76  and the edge of SiC seed crystal  72  is, desirably, between 0.5 and 5 mm. 
         [0091]    Loaded crucible  70  is placed inside a gas-tight chamber  78 , which is evacuated and filled with an inert gas, such as argon or helium, to a pressure between 1 to 100 Torr. Crucible  70  is then heated to a temperature between 2000 and 2400° C. using inductive or resistive heating means  83 . During growth, the temperature of SiC source  71  is controlled to be higher than the temperature of membrane  75 , typically, by 10° C. to 150° C. At the same time, the temperature of membrane  75  is controlled to be 20° C. to 50° C. higher that the temperature of SiC seed crystal  72 . 
         [0092]    Upon reaching SiC sublimation temperatures, SiC source  71  vaporizes and fills the interior of crucible  70  with Si-rich vapor  84  comprised of Si, Si 2 C and SiC 2  volatile molecules. During initial stages of the growth of SiC single crystal  73  on SiC seed crystal  72 , vapor  84  migrates to and precipitates on porous membrane  75  forming a polycrystalline SiC deposit  77 . Then, the SiC deposit  77  sublimes and vapor  85  emanating from SiC deposit  77  diffuses across membrane  75  and reaches SiC seed crystal  72 . The thickness of membrane  75  is selected such that the migration of vapor  85  across membrane  75  is the limiting stage in the overall mass transport. 
         [0093]    After passing through membrane  75 , vapor  85  reaches the growth interface and condenses causing the growth of SiC single crystal  73  on SiC seed crystal  72 . As a result of SiC crystallization, silicon enrichment of vapor  85  adjacent the growth interface takes place and forms vapor  85   a.  Vapor  85   a  including excessive silicon diffuses in space  79  toward the membrane  75  and sleeve  76  and attacks them forming Si 2 C and SiC 2  volatile molecules. Driven by temperature gradients, vapor  85   a  including these Si 2 C and SiC 2  molecules is transported to the growth interface. 
         [0094]    During growth, capsule  80  releases vanadium-containing vapor into the interior of crucible  70  through the one or more capillaries  81 . The dimensions of each of the one or more capillaries  81  are selected to cause the vanadium concentration in the grown SiC single crystal  73  to be sufficient for complete compensation without generation of crystal defects. The presence of porous graphite membrane  75  does not prevent the transport of vanadium to the growth interface. At the same time, membrane  75  improves the spatial uniformity of vanadium doping, thus making the resistivity of the grown SiC single crystal  73  spatially uniform. 
         [0095]    Growth of semi-insulating SiC single crystal  73  requires strict adherence to the purity of SiC source  71  and materials of growth crucible  70 . Halogen purification of growth crucible  70  and other graphite parts used in the growth of SiC single crystal  73  is commonplace. However, porous membrane  75  and sleeve  76  are sacrificial carbon bodies supplying carbon to the growing crystal. Therefore, their purity, especially with respect to boron, is critical. Accordingly, the boron content in membrane  75  and sleeve  76  is, desirably, controlled to be below 50 ppb by weight and the contents of other metals in membrane  75  and sleeve  76  are desirably below their GDMS detection limits. 
         [0096]    Another desired treatment of membrane  75  and sleeve  76  prior to PVT growth is the removal of small graphite particles from their surfaces and bulk. Such particles are generated during machining and handling of these parts. The preferred treatment includes ultrasonic cleaning in deionized water for 15 minutes followed by drying in a circulation oven. 
       EMBODIMENT 2: PVT GROWTH OF SIC CRYSTAL COMBINED WITH IN-SITU SYNTHESIS OF SiC SOURCE 
       [0097]      FIG. 8  is an illustration of a growth cell similar to the growth cell shown and described in connection with  FIG. 7 , except that the growth cell of  FIG. 8  includes an interior graphite crucible  90  loaded with a mixture of Si and C raw materials  91  for in-situ synthesis of SiC from elemental Si and C. The elemental Si and C raw materials  91  desirably have atomic ratio of 1:1 and can be in the form of finely divided powders or, desirably, in the form of small lumps or pellets of 0.5 to 3 mm in size. 
         [0098]    The initial heating of crucible  70  is carried out in vacuum, that is, under continuous evacuation of the growth chamber. A diffusion or turbomolecular pump of a suitable capacity can be used for such pumping. During heating, the pressure in chamber  78  and, hence, crucible  70  is, desirably, not higher than 5·10 6  Torr. 
         [0099]    Heating of crucible  70  continues until the temperature of crucible  70  reaches about 1600° C., which is above the melting point of pure Si (1460° C.). Crucible  70  is soaked at this temperature for 1 hour to complete the reaction between elemental Si and C. 
         [0100]    The enthalpy of direct reaction between Si and C is high, about 100 kJ/mol. Therefore, synthesis of SiC from elemental Si and C can lead to a rise in the temperature of the SiC charge. Here, in this embodiment, membrane  75  plays another role: it acts as a heat shield that avoids SiC seed crystal  72  from overheating and carbonization, which otherwise could be caused by the release of the heat of reaction between Si and C. Membrane  75  also prevents contamination of the surface of SiC Seed  72  by particles generated during the reaction between Si and C. 
         [0101]    After the reaction between elemental Si and C is completed and solid SiC is formed in crucible  90 , chamber  78  and, hence, crucible  70  is filled with inert gas, such as argon or helium, to a pressure of about 500 Torr and the temperature of crucible  70  is raised to a desired growth temperature between 2000° and 2400° C. Following this, PVT growth of SiC single crystal  73  on SiC seed crystal  72  is carried out as described in the previous embodiment. 
         [0102]    For the growth of vanadium-compensated semi-insulating SiC crystals, a doping capsule, similar to doping capsule  80  in the embodiment of  FIG. 7 , is used. Such doping capsule is buried in the bulk of the elemental Si and C mixture  91 . It has been observed that the reaction between elemental Si and C does not affect the vanadium source inside the capsule. 
         [0103]    It has been observed that the use of the above-described gas-permeable porous envelope comprised of porous membrane  75  and porous sleeve  76  in the sublimation growth of 6H and 4H SiC single crystals yields SiC boules with reduced densities of inclusions, such as foreign polytypes, silicon droplets and carbon particles. It has also been observed to reduce the degree of growth-related stress, which is the cause for subsequent boule/wafer cracking. 
         [0104]    The above-described gas-permeable porous envelope comprised of porous membrane  75  and porous sleeve  76  also permits incorporation of in-situ synthesis of the SiC source into the sublimation growth process. This leads to a reduction of the process cycle time. 
         [0105]    Two examples of 6H SiC growth runs will now be described. 
       EXAMPLE 1 
     Growth of Semi-Insulating 6H SiC Crystal 
       [0106]    This growth run was carried out in accordance with the embodiment 1 growth of semi-insulating SiC crystals described above. Specifically, a crystal growth crucible  70  made of dense, isostatically molded graphite (grade ATJ) was prepared. Pure SiC grain 0.5 to 2 mm in size was synthesized prior to growth using a separate synthesis process. A charge of about 600 g of the pure SiC grain was disposed at the bottom of crucible  70  and served as SiC source  71  for the growth run. 
         [0107]    A doping capsule  80  made of dense ATJ graphite was prepared having a single capillary of 1 mm in diameter and 2 mm long. This capsule  80  was loaded with 1 gram of metallic vanadium of 99.995% purity. The loaded capsule  80  was buried in the source  71  on the bottom of crucible  70 , as shown in  FIG. 7 . 
         [0108]    A 3.25″ diameter SiC wafer of the 6H polytype was prepared and used as SiC seed crystal  72 . The wafer was oriented on-axis, that is, with its faces parallel to the basal c-plane. The growth surface of the wafer (Si face) was polished using a chemical-mechanical polishing technique (CMP) to remove scratches and sub-surface damage. SiC seed crystal  72  was attached to crucible lid  74  using a high-temperature carbon adhesive. 
         [0109]    Gas-permeable membrane  75 , in the form of a disc, and cylindrical sleeve  76  were prepared. Membrane  75  and sleeve  76  were machined of porous graphite with the density of 1 g/cm 3 , porosity of 47% and average grain size of 200 microns. The thickness of membrane  75  was 4 mm, while the wall thickness of sleeve  76  was 10 mm. Prior to use in growth, membrane  75  and sleeve  76  were purified in halogen-containing atmosphere to remove boron and other impurities and to reduce the level of residual boron to below 50 ppb by weight. 
         [0110]    Porous membrane  75  and sleeve  76  were positioned in crucible  70 , as shown in  FIG. 7 . Membrane  75  was located a distance of 25 mm below the downward facing face of SiC seed crystal  72 . The distance between sleeve  76  and the periphery (or edge or sides) of SiC seed crystal  72  was 3 mm. 
         [0111]    Crucible  70  was loaded into a water-cooled chamber  78 , made of fused silica, of an RF furnace where crucible  70  served as an RF susceptor. Thermal insulation made of fibrous light-weight graphite foam was placed in the space between crucible  70  and chamber  78 . The interior of chamber  78  and, hence, the interior of crucible  70  were evacuated to a pressure of 1·10 −6  Torr and flushed several times with 99.9995% pure helium to remove any absorbed gases and moisture. Then, the interior of chamber  78  and, hence, the interior of crucible  70  was backfilled with He to 500 Torr and the temperature of crucible  70  was raised to about 2100° C. over a period of eight hours. Following this, the position of RF coil  83  and the furnace power were adjusted to achieve a temperature of SiC source material  71  of 2120° C. and a temperature of SiC crystal seed  72  of 2090° C. The He pressure was then reduced to 10 Torr to start sublimation growth. Upon completion of the run, the interior of chamber  78  and the interior of crucible  70  were cooled to room temperature over a period of 12 hours. 
         [0112]      FIG. 9   a  shows a photograph of the as-grown, vanadium-compensated 6H SiC single crystal boule. The boule weighed 250 grams and included 30 grams of carbon transported from the porous membrane and sleeve. Neither carbon particles, nor Si droplets, nor inclusions of the 15R polytype were found in this high quality crystal boule. The micropipe density in this crystal boule was below 25 cm −2 . 
         [0113]    The boule was fabricated into standard 3″ diameter wafers, and their resistivity was measured and mapped using a contactless resistivity tool. The axial resistivity distribution in this crystal boule and a resistivity map for one of the sliced wafers are shown in  FIGS. 9   b  and  9   c , respectively. The resistivity of the grown crystal boule was above 5·10 10  Ohm-cm, with a majority of the sliced wafers having a resistivity above 1·10 11  Ohm-cm, and a standard deviation below 10%. 
       EXAMPLE 2 
     Growth of Semi Insulating 6H SiC Crystal 
       [0114]    With reference to  FIG. 8 , growth of a vanadium-compensated 6H SiC single crystal was carried out in accordance the embodiment 2 growth of semi-insulating SiC crystals described above. The growth crucible used for this growth was similar to that used in Example 1 above. A thin-walled interior graphite crucible ( 90  in  FIG. 8 ) was machined from dense ATJ graphite. The interior of crucible  90  was loaded with 600 g of a raw material mixture of elemental Si and C in a 1:1 atomic ratio. The Si and C forming this mixture was in the form of small lumps or pellets, 0.5 mm to 1 mm in dimension. 
         [0115]    A doping capsule  80  containing 1 gram of vanadium was placed at the bottom of crucible  90 , under the Si+C mixture. The geometry of this capsule  80  was similar to that described in the previous example. 
         [0116]    Gas-permeable membrane  75  and sleeve  76  having the same dimensions as in Example 1 were machined from porous graphite of the same grade as in Example 1. Membrane  75  and sleeve  76  were halogen-purified to reduce the level of boron to below 50 ppb by weight. Porous membrane  75  and sleeve  76  were positioned in crucible  70 , as shown in  FIG. 8 . 
         [0117]    The crucible  70  including the doping capsule  80 , the raw material Si+C mixture  91 , SiC crystal seed  72 , porous membrane  75 , and sleeve  76  surrounding SiC crystal seed  72  was placed into crystal growth chamber  78 . Chamber  78  was then evacuated, flushed with pure helium, as described in the previous example, and then again evacuated to a pressure of 1·10 −6  Torr. 
         [0118]    Crucible  70  was then heated to 1600° C. under continuous evacuation of chamber  78  and crucible  70  using a turbomolecular pump. During heating, the pressure in chamber  78  and crucible  70  remained below 5·10 −6  Torr. Upon approaching the temperature of 1600° C., an increase in pressure and temperature was noticed. This served as an indication that the reaction between the elemental Si and C raw material mixture  91  leading to the formation of solid SiC had started. Crucible  70  was soaked at 1600° C. for 1 hour to complete the reaction of the elemental Si and C raw material mixture  91  to a solid SiC. 
         [0119]    After completing the synthesis of the solid SiC, the chamber  78  and, hence, crucible  70  were filled with pure helium to 500 Torr and the temperature of crucible  70  was raised to about 2100° C. Following this, PVT growth of SiC single crystal  73  was carried as in the previous example 1. During growth of SiC single crystal  73  in this example 2, the temperatures of SiC source  91  and SiC seed crystal  72  were controlled to reach 2170° C. and 2110° C., respectively, and the He pressure inside chamber  78  and crucible  70  was reduced to 20 Torr. 
         [0120]    Investigation of the SiC single crystal  73  boule grown in accordance with this example 2 and the wafers sliced therefrom showed that the grown SiC single crystal  73  boule included no visible carbon particles, Si droplets, or inclusions of the 15R polytype. The average micropipe density in this SiC single crystal  73  boule was below 1 cm −2 , as shown in  FIG. 10 . 
         [0121]    The SiC single crystal  73  boule grown in accordance with this example 2 was fabricated into wafers yielding 25 standard 3″ substrates. These wafers were evaluated for their electrical resistivity. All 25 wafers were semi-insulating with a resistivity above 1·10 10  Ohm-cm and standard deviation below 10%. 
         [0122]    The invention has been described with reference to preferred embodiments. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.