Abstract:
In SiC sublimation crystal growth, a crucible is charged with SiC source material and SiC seed crystal in spaced relation and a baffle is disposed in the growth crucible around the seed crystal. A first side of the baffle in the growth crucible defines a growth zone where a SiC single crystal grows on the SiC seed crystal. A second side of the baffle in the growth crucible defines a vapor-capture trap around the SiC seed crystal. The growth crucible is heated to a SiC growth temperature whereupon the SiC source material sublimates and forms a vapor which is transported to the growth zone where the SiC crystal grows by precipitation of the vapor on the SiC seed crystal. A fraction of this vapor enters the vapor-capture trap where it is removed from the growth zone during growth of the SiC crystal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to Physical Vapor Transport growth of SiC single crystals. 
         [0003]    2. Description of Related Art 
         [0004]    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. 
         [0005]    Large, industrial-size SiC single crystals are grown by a sublimation technique commonly known as Physical Vapor Transport (PVT). PVT growth is usually carried out in a graphite crucible that includes solid SiC sublimation source material disposed, typically, at the crucible bottom and a SiC single crystal seed disposed, typically, at the crucible top. The sublimation source material is, usually, polycrystalline SiC grain synthesized separately. The loaded crucible is placed in a furnace and heated to the growth temperature, which is, generally, between 2000° C. and 2400° C. During growth, the source material temperature is maintained higher than that of the seed crystal, typically, by 10° to 200°. 
         [0006]    Upon reaching a suitable high temperature, the sublimation source vaporizes and fills the interior of the crucible with vapor species, such as Si, Si 2 C and/or SiC 2 . The temperature difference between the sublimation source and the seed crystal forces the vapor species to migrate and condense on the seed crystal causing a SiC single crystal to grow on the seed crystal. In order to control the growth rate and thus facilitate good crystal quality, PVT growth is carried out under a small pressure of inert gas, typically, between 1 and 100 Torr. 
         [0007]    Generally, SiC crystals grown using this basic PVT arrangement suffer from structural defects, such as inclusions, micropipes and dislocations. It is commonly believed that inclusions of carbon, silicon and foreign polytypes are caused by deviations in the vapor phase stoichiometry, which is conventionally expressed as the Si:C atomic ratio. It is well-known that SiC sublimes incongruently with the Si:C atomic ratio in the vapor larger than 1 Depending on the SiC source conditions (such as the grain structure and size, polytype composition, stoichiometry, temperature, etc.) the Si:C ratio in the vapor over the sublimation source material can be as high as 1.5 or even higher. When the Si:C ratio in the vapor is too high, silicon inclusions form in the growing SiC crystal. Conversely, when the Si:C atomic ratio in the vapor is too low, carbon inclusions form in the growing SiC crystal. 
         [0008]    It is also believed that stable growth of SiC single crystals of hexagonal 4H and 6H polytypes requires a carbon-rich vapor phase, whereas inclusions of foreign polytypes such as 15R are caused by deviations in the vapor stoichiometry. 
         [0009]    Inclusions of metal carbides can appear in grown SiC single crystals when the SiC sublimation source material contains metallic contaminants. 
         [0010]    Inclusions in a PVT grown SiC single crystal leads to local stress, which is relieved via generation, multiplication and movement of dislocations and micropipes. When SiC single crystal wafers are used as substrates in GaN or SiC epitaxy, the presence of inclusions, micropipes and dislocations in the substrate is harmful to the quality of the epilayers and the performance of semiconductor devices formed on said epilayers. 
         [0011]    Since the inception of the PVT growth technique, a number of process modifications have been developed with the aim to improve the grown crystal quality and reduce defect densities. 
         [0012]    For example, U.S. Pat. No. 5,858,086 to Hunter (hereinafter “the &#39;086 patent”) discloses a system for the growth of AlN (aluminum nitride) crystals by sublimation. A schematic diagram of the system disclosed in the &#39;086 Hunter patent is shown in  FIG. 1 , wherein vapor  2  from AlN source material  4  enters a space  6  in front of an AlN seed crystal  8  and precipitates on said seed crystal  8  causing an AlN crystal  10  to grow. As the growth of AlN crystal  10  progresses, vapor  2  surrounding the growing crystal  10  becomes stagnant, contaminated and generally unsuitable for the growth of a high-quality AlN crystal  10 . In order to avoid this deficiency, a perforated baffle  12  is placed around AlN seed crystal  8  and the space where AlN crystal  10  is to grow. As shown in  FIG. 1 , baffle  12  extends toward AlN source  4 . The portion of growth crucible  14  surrounding baffle  12  is configured to define therewith a gap  16  that enables a portion of vapor  2 , shown by arrows  18 , that passes through perforated baffle  12  to escape from inside growth crucible  14  to a space outside growth crucible  14  via one or more holes of vents  19 . 
         [0013]    U.S. Pat. No. 5,985,024 to Balakrishna et al. discloses a system for the growth of high-purity SiC single crystals. A schematic diagram of the system disclosed in the Balakrishna et al. patent is shown in  FIG. 2 , wherein silicon vapor  20  from Si sublimation source material  22  rises toward a SiC seed crystal  24  where it mixes with carbon-containing gas  26  supplied from an external source. The SiC vapor  28  produced as a result of reaction between Si- and C-containing vapors reaches SiC seed crystal  24 , precipitates on it and causes a SiC crystal  30  to grow on SiC seed crystal  24 . Spent SiC vapors  28 , gases and gaseous contaminants escape from inside growth crucible  32  to a space outside growth crucible  32  via a gap  34  between SiC crystal  30  and a protective liner  36 , desirably made from high purity silicon carbide or tantalum carbide, and one or more holes or vents  38  at the top of growth crucible  32 . A porous graphite wall  40  desirably supports protective liner  36  at an appropriate position within growth crucible  32 . 
         [0014]    U.S. Pat. No. 6,045,613 to Hunter (hereinafter “the &#39;613 patent”) discloses the SiC crystal growth system shown in  FIG. 3 , wherein Si vapors  48  from Si sublimation source material  50  along with C or N gas  52  rise toward a SiC or SiN single crystal seed  54  where they form a growing SiC or SiN crystal  56 , respectively. (The growth system shown in  FIG. 3  can also be utilized to grow MN crystals.) Similar to the &#39;086 patent ( FIG. 1 ), spent or contaminated gases and vapors  48 ,  52  escape growth crucible  59  through one or more vents or holes  58  provided at the top of growth crucible  59 . Once outside growth crucible  59 , the escaped vapors  48 ,  52  are disposed of in a special gettering furnace external to the growth crucible (not shown). 
         [0015]    U.S. Pat. No. 6,086,672 to Hunter discloses a system for the growth of AlN—SiC alloy crystals that is similar to the growth system disclosed in the &#39;086 Hunter patent ( FIG. 1 ) described above. 
         [0016]    U.S. Pat. No. 7,323,052 to Tsvetkov et al. discloses sublimation growth of SiC single crystals containing reduced densities of point defects. The cause of such defects is believed to be vapor species that contain too much silicon. A schematic diagram of the apparatus disclosed in this patent is shown in  FIG. 4 , wherein a graphite growth crucible  60  defines a sublimation chamber  62  with SiC sublimation source material  64  at the bottom of chamber  62  and a SiC crystal seed  66  disposed on a holder  68  at the top of chamber  62 . In order to optimize vapor stoichiometry during the growth of a SiC crystal  70  on seed crystal  66 , a fraction of the SiC vapor  74  is vented from inside growth crucible  60  to a chamber or space  76  outside of growth crucible  60  via one or more outlets  72  at the top of growth crucible  60 . Chamber  76  is defined between the exterior of growth crucible  60  and an interior of an outer wall  78  of the furnace chamber. A suitable insulation  80  typically resides in chamber  76 . 
         [0017]    Generally, crucibles made of high-density, small-grain graphite are utilized in SiC sublimation crystal growth. Herein, high-density or dense graphite is graphite having a density between 1.70 and 1.85 g/cm 3 , grain sizes between several and tens of microns, and porosity on the order of 10%. Those skilled in the art recognize that such graphite is highly permeable to common gases, such as N 2 , Ar, He, CO, CO 2 , HCl, etc. However, dense graphite shows very low permeability to the vapors formed as a result of SiC sublimation: Si, Si 2 C and SiC 2 . Vapor losses from an enclosed crucible made of dense graphite incurred during SiC sublimation growth, typically, do not exceed several grams, and this is not enough to provide for sufficient or desirable removal of the vapor from the crucible. This low permeability of dense graphite to the Si-bearing vapors is the main reason why special holes or vents are made in the growth crucibles of the prior art discussed above for the purpose of venting. 
         [0018]    It is also known that low-density, porous graphite can show higher permeability to the Si-containing vapor species formed as a result of SiC sublimation. Herein, low-density graphite is graphite having a density between 0.8 and 1.6 g/cm 3 ; a porosity between 30% and 60%; and pore sizes between 1 and 100 microns. These properties of low-density graphite are utilized in U.S. Pat. No. 7,323,052 to Tsvetkov et al., where, alternatively to outlets  72  shown in  FIG. 4 , one or more sections of growth crucible  60  can be made of lower-density graphite permeable to atomic silicon vapor in particular. Atomic Si escapes from inside growth crucible  60  by diffusing through said lower-density graphite into chamber  76 , thus reducing the Si content of vapor  74  in the zone of chamber  62  where SiC crystal  70  grows. 
         [0019]    In summary, the aforementioned prior art teaches partial removal of vapor from the space surrounding the growing crystal by way of venting said vapor from inside the growth crucible to a space outside the growth crucible, e.g., into a chamber or space formed between the growth crucible and the outer wall of the furnace chamber where thermal insulation typically resides. 
         [0020]    Venting the vapor into this chamber, however, has its problems. Specifically, the chamber or space surrounding the growth crucible is usually filled with thermal insulation made of purified, light-weight, fibrous graphite. The Si-containing vapor is very reactive toward graphite, especially when graphite is in such a light-weight form. Degradation of the thermal insulation caused by vapor erosion leads to uncontrollable changes in the temperatures of the crucible and, hence, the source and crystal. This has a negative effect on the growth process and crystal quality. 
         [0021]    Another consequence of the escape of vapor into the chamber from the crucible is a reduced service time of the expensive thermal insulation. Utilization of a special gettering furnace for the disposal of the escaping vapor, as taught in the &#39;613 patent, adds to the complexity and cost of the growth system. 
       SUMMARY OF THE INVENTION 
       [0022]    The present invention is 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 substrates of 2″, 3″, 100 mm, 125 mm and 150 mm in diameter. The crystal growth crucible includes grains of SiC source material and a SiC seed crystal disposed inside a sealed graphite crucible in spaced relationship. During growth, the SiC source material vaporizes producing volatile vapor species, such as Si, Si 2 C and SiC 2 . Driven by a temperature gradient inside of the crucible, these vapor species migrate toward the seed crystal and precipitate on it causing the growth of a SiC single crystal on the seed crystal. 
         [0023]    The SiC crystal growth crucible includes a baffle disposed around the seed crystal in the growth crucible, said baffle defining on a first side thereof in said growth crucible a growth zone where the SiC single crystal grows on the seed crystal, said baffle defining on a second side thereof in said growth crucible a vapor-capture trap around the seed crystal. The vapor-capture trap can be located at a position in the growth crucible where the temperature is lower than that of the seed crystal during the growth of the SiC single crystal on the seed crystal. The temperature within the vapor-capture trap can be lower than the temperature of the seed crystal by 3° C. to 20° C. The crucible design includes a pathway that enables the vapor to migrate toward the vapor-capture trap and enter it. 
         [0024]    Upon reaching the vapor-capture trap, the Si-bearing vapor becomes supercooled and precipitates forming solid deposits of polycrystalline SiC within the vapor-capture trap. As a result of this process, part of the vapor is removed from the vicinity of the growing SiC single crystal, i.e., vapor is removed from the vicinity of the SiC single crystal growth interface. Simultaneously, unwanted vapor constituents harmful to the crystal quality are also removed. These harmful components include excessive silicon- or carbon-containing vapors as well as volatile contaminants. 
         [0025]    The SiC crystal growth crucible can further include a porous vapor-absorbing member disposed in the vapor-capture trap and operative for absorbing vapor produced during sublimation growth of the SiC single crystal on the seed crystal. 
         [0026]    The porous vapor-absorbing member can be disposed in the vapor-capture trap at a position where the vapor-absorbing member is at a temperature lower than that of the seed crystal during the growth of the SiC single crystal on the seed crystal. The temperature of the vapor-absorbing member during the growth of the SiC single crystal on the seed crystal can be lower than the temperature of the seed crystal by 3° C. to 20° C. The crucible design desirably includes a pathway that enables the vapor to migrate toward the porous vapor-absorbing member, permeate it, and react with it. 
         [0027]    Upon reaching the vapor-absorbing member, the vapor permeates the pores of the vapor-absorbing member where it chemically reacts with the material of the member to form solid products. As a result of this process, part of the vapor is removed from the vicinity of the growing SiC single crystal. Simultaneously, unwanted vapor constituents harmful to the crystal quality are also removed. These harmful components include excessive silicon- or carbon-containing vapors as well as volatile contaminants. 
         [0028]    In one embodiment, the vapor-absorbing member is made of purified porous graphite having a density, desirably, between 0.8 and 1.6 g/cm 3 ; a porosity, desirably, between 30% and 60%; and pore sizes, desirably, between 1 and 100 microns. 
         [0029]    The use of the vapor-absorbing member inside of the growth crucible facilitates growth of SiC single crystal boules with reduced densities of defects such as inclusions, micropipes and dislocations. 
         [0030]    More specifically, the invention is an apparatus for sublimation growth of a SiC single crystal that includes a growth crucible operative for receiving a source material and a seed crystal in spaced relation and for substantially preventing the escape of vapor produced during sublimation growth of a SiC single crystal on the seed crystal from inside said growth crucible; and a baffle disposed around the seed crystal in the growth crucible, said baffle defining on a first side thereof in said growth crucible a growth zone where the SiC single crystal grows on the seed crystal, said baffle defining on a second side thereof in said growth crucible a vapor capture space, hereinafter a “vapor-capture trap”, around the seed crystal. 
         [0031]    For substantially preventing the escape of vapor produced during sublimation growth of a SiC single crystal on the seed crystal, said growth crucible: can be made from a material that is substantially impermeable to the passage of the vapor produced during sublimation growth of a SiC single crystal on the seed crystal; and can include no intentional pathways or holes for escape of the vapor produced during sublimation growth of a SiC single crystal on the seed crystal from inside the growth crucible to outside the growth crucible. 
         [0032]    The vapor-capture trap can be located at a position in the growth crucible where the temperature is lower than that of the seed crystal during the growth of the SiC single crystal on the seed crystal. 
         [0033]    The apparatus can further include a vapor-absorbing member disposed in the vapor-capture trap and operative for absorbing vapor produced during sublimation growth of the SiC single crystal on the seed crystal. 
         [0034]    The vapor-absorbing member can be disposed in the vapor-capture trap at a position where the vapor-absorbing member is at a temperature lower than that of the seed crystal during the growth of the SiC single crystal on the seed crystal. 
         [0035]    The temperature of the vapor-absorbing member during the growth of the SiC single crystal on the seed crystal can be lower than the temperature of the seed crystal by 3° C. to 20° C. 
         [0036]    The vapor-absorbing member can be made from porous graphite having a density between 0.8 and 1.6 g/cm 3 ; a porosity between 30% and 60%; and pore sizes between 1 and 100 microns. 
         [0037]    The baffle can define a pathway inside said growth crucible for the vapor to flow into the vapor-capture trap. 
         [0038]    The growth crucible can include therein a pedestal for supporting the seed crystal intermediate a top of the growth crucible and the source material. The pedestal can have a height between 5 mm and 25 mm. The pathway can comprise a gap between an inner diameter of the baffle and an outer diameter of the pedestal. The gap can be between 1 mm and 8 mm wide. The pathway can comprise one or more holes in the baffle. 
         [0039]    The invention is also a method of SiC sublimation crystal growth comprising: (a) providing a growth crucible charged with a source material and a seed crystal in spaced relation and a baffle disposed in the growth crucible around the seed crystal, said baffle defining on a first side thereof a growth zone where a single crystal grows on the seed crystal, said baffle defining on a second side thereof a vapor-capture trap around the seed crystal; and (b) heating the growth crucible of step (a) to a growth temperature whereupon a temperature gradient forms in the growth chamber that causes the source material to sublimate and form a vapor which is transported by the temperature gradient to the growth zone of the growth crucible where the single crystal grows by precipitation of the vapor on the seed crystal, wherein a fraction of the vapor enters the vapor-capture trap. 
         [0040]    The vapor entering the vapor-capture trap can be removed during growth of the crystal from the growth zone by forming a deposit therein. One or more of the source material, the seed crystal, and the single crystal can be SiC. 
         [0041]    The vapor-capture trap can be located at a position in the growth crucible where the temperature is lower than that of the seed crystal during the growth of the single crystal on the seed crystal. 
         [0042]    A vapor-absorbing member can be disposed inside the vapor-capture trap. The vapor entering the vapor-capture trap can be removed during growth of the crystal from the growth zone by chemically reacting with the vapor-absorbing member, e.g., without limitation, to form a deposit. 
         [0043]    The vapor-absorbing member can be at a lower temperature than the seed crystal during growth of the single crystal. 
         [0044]    The vapor-absorbing member can be made from porous graphite with a density between 0.8 and 1.6 g/cm 3 ; a porosity between 30% and 60%; and pore sizes between 1 and 100 microns. 
         [0045]    The weight of the deposit formed in the vapor-capture trap can be between 5% and 20% of the weight of the grown crystal. Stated differently, the weight of the vapor absorbed by the vapor-absorbing member can be between 5% and 20% of the weight of the grown crystal. 
         [0046]    The baffle can define a pathway for the vapor to flow to the vapor-capture trap. The growth crucible of step (a) can further include a pedestal for supporting the seed crystal intermediate a top of the growth crucible and the source material. The pathway can comprise a gap formed between an inner diameter of the baffle and an outer diameter of the pedestal. 
         [0047]    The pathway can comprise at least one perforation in a wall of the baffle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0048]      FIGS. 1-4  are prior art systems for the growth of crystals by sublimation; and 
           [0049]      FIGS. 5-7  are systems in accordance with the present invention for the growth of crystals, especially SiC crystals, by sublimation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]    The present invention will now be described with reference to  FIGS. 5-7  where like reference numbers correspond to like elements. 
         [0051]    With reference to  FIG. 5 , PVT growth of a SiC crystal, desirably a SiC single crystal, is carried out in a graphite crucible  102  that includes grains of SiC source material  104  and a SiC seed crystal  106  in spaced relationship. Desirably, source material  104  is disposed at the bottom of crucible  102  and seed crystal  106  at the top of crucible  102 , e.g., seed crystal  106  is attached to a lid  108  of crucible  102 . Upon reaching a desired sublimation growth temperature, SiC source material  104  sublimes and fills the interior of crucible  102  with Si-rich vapor species  110 , such as Si, Si 2 C and SiC 2  volatile molecular species. 
         [0052]    Driven by a vertical temperature gradient inside of crucible  102 , vapor  110  migrates in the axial direction toward seed crystal  106  and condenses on seed crystal  106  causing growth of a SiC single crystal  112  thereon. The growing SiC crystal  112  is surrounded by a baffle  114  which delimits a space  116  adjacent growing SiC crystal  112 . Space  116  is also known as the “growth zone”. During growth, growth zone  116  fills with volatile byproducts emerging as a result of vapor condensation, crystal growth and graphite erosion. These volatile byproducts can contain impurities as well as excessive silicon or carbon. Such uncontrollable changes in the vapor phase composition in growth zone  116  affect negatively the quality of growing SiC crystal  112 . 
         [0053]    Desirably, crucible  102  is formed from high density graphite that “substantially prevents” the escape of vapor  110  from the inside crucible  102 . To “substantially prevent” the escape of vapor  110  from the interior of crucible  102 , the high density graphite forming crucible  102  is “substantially impermeable” to vapors  110  and crucible  102  includes no intentional holes or vents for the escape of vapor  110  from the interior of crucible  102 . Herein, crucible  102  “substantially preventing” the escape of vapor  110  from the interior thereof and crucible  102  being made from high density graphite that is “substantially impermeable” to vapors  110  means that the loss of vapor  110  from the interior of crucible  102  during the growth of SiC single crystal  112  on seed crystal  106  occurs via diffusion of vapor  110  across the wall of crucible  102  and lid  108 , and the total of such loss of vapor  110  from the interior of crucible  102  during the growth of SiC single crystal  112  on seed crystal  106  is between 1% and 5% of the initial weight of SiC source material  104 . 
         [0054]    A vapor-capture trap  117  is provided in the interior of the crucible  102  in order to reduce the aforementioned uncontrollable changes in the vapor phase composition in the growth zone. The thermal field in the crucible is tuned such that vapor-capture trap  117  has the lowest temperature in the crucible interior. In particular, the temperature in vapor-capture trap  117  is desirably lower than the temperature of the seed  106 . A common approach to tuning the temperature field inside the SiC growth crucible is by using finite-element thermal modeling. Driven by the temperature and pressure gradients, vapor  110  migrates toward the crucible top, reaches vapor-capture trap  117 , and precipitates in vapor-capture trap  117  forming a solid polycrystalline SiC deposit  126  in vapor-capture trap  117 , e.g., without limitation, on the interior surface of the wall of crucible  102  adjacent lid  108  and, optionally, on the interior surface of lid  108  adjacent the wall of crucible  102 . As a result of the formation of solid polycrystalline SiC deposit  126 , a fraction of vapor  110  is removed from growth zone  116 . The shape of vapor-capture trap  117  in  FIG. 5  is shown for the purpose of illustration only, and it is not to be construed as limiting the invention, as this space can have any suitable and/or desirable shape. 
         [0055]    A vapor-capture member  117   a  (shown in phantom in  FIG. 5 ) made of vapor-absorbing, porous material can optionally be placed inside crucible  102 , desirably in vapor-capture trap  117 , in order to assist in the reduction of uncontrollable changes in the vapor phase composition in growth zone  116 . The vapor  110  upon reaching member  117   a  permeates its pores and chemically reacts with the material of member  117   a  leading to the formation of solid polycrystalline SiC deposit  128  on or in member  117   a.    
         [0056]    Two possible vapor flows from growth zone  116  toward vapor-capture trap  117  and, if provided, member  117   a  are shown in  FIG. 5  by arrows  118  and  120 . Arrow  118  shows the flow of vapor across baffle  114 , for instance, through one or more perforations in baffle  114 . Arrow  120  shows the flow of vapor around baffle  114 , for instance, through a gap  122  defined between baffle  114  and seed crystal  106 , growing crystal  112 , and/or a seed pedestal  124  upon which seed crystal  106  is mounted. Baffle  114  is shown in  FIG. 5  as having a cone shape with the narrow opening of the cone defining with seed crystal  106  and growing crystal  112 , the gap  122 , and with the wide opening of the cone facing source material  104 . However, the illustration of baffle  114  as having a cone shape is not to be construed as limiting the invention as it is envisioned that baffle  114  can have any suitable and/or desirable shape. 
         [0057]    Desirably, vapor-absorbing member  117   a  is made of purified porous graphite having a density, desirably, between 0.8 and 1.6 g/cm 3 ; a porosity, desirably, between 30% and 60%; and pore sizes, desirably, between 1 and 100 microns, i.e., a low-density graphite. Chemical reaction between vapor  110  and the carbon of the member  117   a  leads to the formation of solid polycrystalline SiC deposit  128  on or inside the pores of the member  117   a . As a result of this reaction and the formation of the SiC deposits  128 , a fraction of vapor  110  is removed from growth zone  116 . Simultaneously, excessive silicon- or carbon-containing vapors, as well as volatile contaminants, are also removed from growth zone  116 . 
         [0058]    With continuing reference to  FIG. 5 , vapor-capture trap  117  can comprise all or part of the space generally bounded by the side of baffle  114  that faces away from growth zone  116 , the portion of lid  108  above baffle  144 , and the portion of the interior wall of crucible  102  between lid  108  and the lower end of baffle  144 . If provided, member  117   a  can be positioned at any suitable and/or desirable location within this space. Desirably, however, vapor-capture trap  117  is comprised of a space  136  (shown in phantom in  FIG. 5 ) adjacent the upper outside portion of the interior of crucible  102 . In  FIG. 5 , lid  108  and the interior wall of crucible  102  adjacent lid  108  define two boundaries of space  136 . However, this is not to be construed as limiting the invention. If provided, member  117   a  is desirably positioned at any suitable and/or desirable location within space  136 , as shown in phantom in  FIG. 5 . 
         [0059]    Two extremes are desirably avoided in order for vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117  to be beneficial to the growth of SiC crystal  112  and the quality of the grown SiC crystal  112 . In one extreme, too much of vapor  110  is removed from growth zone  116 , leading to a dramatic reduction in the growth rate of SiC crystal  112 . Another extreme is when too little vapor  110  is removed from growth zone  116 , whereupon the presence of the vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in crucible  102  has no beneficial effect on the quality of the grown SiC crystal  112 . 
         [0060]    Experimental results show that in order to realize the beneficial effects of vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117 , the weight of SiC deposit  126  or  128  formed in vapor-capture trap  117  or, if provided, vapor-absorbing member  117   a , respectively, is desirably between 5% and 20% of the weight of the grown SiC single crystal  112 . For example, where only vapor-capture trap  117  is present (i.e., without vapor-absorbing member  117   a  in vapor-capture trap  117 ), the weight of SiC deposit  126  formed in vapor-capture trap  117  is desirably between 5% and 20% of the weight of the grown SiC single crystal  112 . On the other hand, where vapor-absorbing member  117   a  is included in vapor-capture trap  117 , the weight of SiC deposit  128  formed in vapor-absorbing member  117   a  is desirably between 5% and 20% of the weight of the grown SiC single crystal  112 . 
         [0061]    It is envisioned, that when vapor-absorbing member  117   a  is included in vapor-capture trap  117 , that some SiC deposit  126  may also form on the wall of crucible  102 , the interior of lid  108 , or both, adjacent space  136 . However, it is envisioned that the total of SiC deposits  126  and  128  will desirably be between 5% and 20% of the weight of the grown SiC single crystal  112 . 
         [0062]    Desirably, control over the amount of vapor  110  absorbed in vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117  is achieved by controlling the temperature of vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117 , and by providing a pathway  118  and/or  120  of desired cross-section, length and geometry for vapor  110  to flow from growth zone  116  to vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117 . 
         [0063]    In order for the SiC deposit to form reliably inside vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117 , the temperature of vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117  is desirably the lowest inside of crucible  102  during the growth of SiC crystal  112 . More specifically, the temperature of vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117  is desirably lower than that of seed crystal  106 . In one embodiment, the temperature of vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117  is lower than that of seed crystal  106 , desirably, by 3° C. to 20° C. 
         [0064]    This difference between the temperatures of seed crystal  106  and vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117  can be realized in a number of ways. In one embodiment, the desired temperature difference between seed  106  and vapor-absorbing member  117   a  in vapor-capture trap  117  is achieved by the following combination: (i) vapor-absorbing member  117   a  (included in vapor-capture trap  117 ) is shaped as a short cylinder, as shown in  FIG. 6 , as a truncated cone, or as a combination thereof, as shown in  FIG. 7 ; (ii) vapor-absorbing member  117   a  is disposed in the upper extreme, e.g., at or adjacent the upper end or top of crucible  102 ; (iii) seed crystal  106  is disposed on pedestal  124 , as shown in  FIG. 5 , whereupon seed crystal  106  is disposed inside crucible  102  away from the top or lid  108  of crucible  102 ; and (iv) the height H of the pedestal  124  is, desirably, between 5 and 25 mm. 
         [0065]    The geometry of the vapor pathway(s) that vapor  110  traverses to reach vapor-capture trap  117  and, if provided, vapor-absorbing member  117   a  in vapor-capture trap  117 , specifically the length and cross-section of such vapor pathway(s), is another factor that can be used to control the amount of removed vapor  110 . Two exemplary vapor pathways are shown schematically in  FIGS. 6 and 7 . These two vapor pathways do not produce deleterious effects on the quality of the growing SiC crystal  112  and can be easily implemented. 
         [0066]    In  FIG. 6 , crucible  102  includes a baffle  114 ′ made of dense graphite that surrounds at least the lower part of pedestal  124 , seed crystal  106 , and the space where SiC crystal  112  grows. An annular gap  130  exists between baffle  114 ′ and pedestal  124 . Gap  130  forms a pathway for vapor  110  to flow to a vapor-capture trap  117 ′ and, if provided, a vapor-absorbing member  117   a ′ in vapor-capture trap  117 ′. In  FIG. 7 , baffle  114 ″ surrounding seed crystal  106  and the space where SiC crystal  112  grows is perforated. That is, baffle  114 ″ includes a plurality of openings  132  that form pathway(s) for vapor  110  to flow to vapor-capture trap  117 ″ and, if provided, vapor-absorbing member  117   a ″ in vapor-capture trap  117 ″. 
         [0067]    Upon reaching vapor-absorbing member  117   a ′ disposed in vapor-capture trap  117 ′, vapor  110  permeates it, diffuses through its bulk and reacts with the carbon forming said member  117   a ′. As a result of such reaction, polycrystalline SiC deposit  134 ′ forms on the member  117   a ′ and/or inside said member  117   a ′ in its coldest spot. It is envisioned that a portion of SiC deposit  134 ′ may also form on the wall of vapor-capture trap  117 ′. 
         [0068]    Smartly, upon reaching vapor-absorbing member  117   a ″ in vapor-capture trap  117 ″, vapor  110  permeates it, diffuses through its bulk and reacts with the carbon forming said member  117   a ″. As a result of such reaction, polycrystalline SiC deposit  134 ″ forms on the member  117   a ″ and/or inside said member  117   a ″ in its coldest spot. It is envisioned that a portion of SiC deposit  134 ″ may also form on the wall of vapor-capture trap  117 ″. 
         [0069]    When vapor-capture trap  117 ′ in  FIG. 6  does not include vapor-absorbing member  117   a ′, SiC deposit  134 ′ will form on the wall(s) of vapor-capture trap  117 ′ in its coldest spot. Similarly, when vapor-capture trap  117 ″ in  FIG. 7  does not include vapor-absorbing member  117   a ″, SiC deposit  134 ″ will form on the wall(s) of vapor-capture trap  117 ″ in its coldest spot. 
       Example 1 
     Growth of 3″ Diameter Semi-Insulating 6H SiC Crystal 
       [0070]    This growth run was carried out in a growth furnace having the crucible, baffle, and vapor-absorbing member  117   a ′ arrangement like the one shown in  FIG. 6 . In this growth run, a crystal growth crucible  102  made of dense, isostatically molded graphite was prepared and purified by high-temperature treatment in a halogen-containing atmosphere. High-purity SiC sublimation source material  104 , i.e., SiC grains 0.5 to 2 mm in size, was synthesized prior to growth of SiC crystal  112  in a separate synthesis process. A charge of 600 g of the SiC source material  104  was disposed at the bottom of crucible  102  and served during growth of SiC crystal  112  as the solid sublimation source. In order to produce semi-insulating SiC crystal  112 , the source material  104  included vanadium as a compensating dopant. The amount of vanadium and other details of vanadium doping were in accordance with the prior art. 
         [0071]    A 3.25″ diameter SiC wafer of the 6H polytype was used as the seed crystal  106 . This wafer was oriented on-axis, with its faces parallel to the basal c-plane. The surface of the wafer where the growth of SiC crystal  112  was to occur was polished prior to the growth of SiC crystal  112  using a chemico-mechanical polishing (CMP) technique to remove scratches and sub-surface damage. This seed crystal  106  was attached to pedestal  124  of crucible lid  108  using a high-temperature carbon adhesive. Pedestal  124  had a height H of 12.5 mm. 
         [0072]    Baffle  114 ′ was machined from dense, isostatically molded and halogen-purified graphite and had a 3 mm thick wall. The inner diameter of baffle  114 ′ was larger than the outer diameter of pedestal  124  to form a 2 mm wide annular gap  130  between pedestal  124  and baffle  114 ′. 
         [0073]    Vapor-absorbing member  117   a ′, shaped as a cylinder in  FIG. 6 , was machined from halogen-purified porous graphite with a density of 1.0 g/cm 3 ; a porosity of 50%; and pore sizes in the range of 20-80 microns. Vapor-absorbing member  117   a ′ was disposed in vapor-capture trap  117 ′ as shown in  FIG. 640 . [068] Crucible  102  was loaded into a water-cooled growth chamber of the growth furnace having an outer wall made of fused silica and an external RF coil that was utilized to inductively heat crucible  102 , which acts as an RF susceptor, in a manner known in the art. Thermal insulation made of fibrous light-weight graphite foam was placed in the growth chamber around crucible  102 . The interior of the growth furnace and, hence, the interior of crucible  102  were evacuated to a pressure of 1·10 −6  Torr and flushed several times with 99.9999% pure argon to remove absorbed gases and moisture. Then, the interior of the growth furnace and, hence, the interior of crucible  102  was backfilled with Ar to 500 Torr and RF power was applied to the RF coil which inductively caused the temperature of crucible  102  to increase to about 2100° C. over a period of six hours. Because of the porosity of crucible  102  to gases, the gas pressure inside crucible  102  very quickly becomes the same as the gas pressure inside of the growth chamber. 
         [0074]    Following this, the RF coil position and the RF power were adjusted to achieve a temperature of source material  104  of 2120° C. and a temperature of seed crystal  106  of 2090° C. The Ar pressure was then reduced to 10 Torr to start sublimation growth of SiC crystal  112  boule. Upon completion of the run, the growth furnace was cooled to room temperature over a period of 12 hours. 
         [0075]    The grown 6H boule of SiC crystal  112  weighed 300 grams. The weight of the polycrystalline SiC deposit  134  formed inside vapor-absorbing member  117 ′ was about 20 grams. The grown boule of SiC crystal  112  contained neither carbon particles, nor Si droplets, nor foreign polytype inclusions. The micropipe density in this boule of SiC crystal  112  was about 0.9 cm −2  and the dislocation density was close to 1·10 4  cm −2 . 
         [0076]    The boule of SiC crystal  112  was fabricated into 25 standard 3″ diameter, 400 micron thick wafers, and their resistivity was measured and mapped using Corema, a contactless resistivity tool. The resistivity of all wafers was close to 1·10 11  Ohm-cm, with a standard deviation below 10%. 
       Example 2 
     Growth of 100 mm Diameter Semi-Insulating 6H SiC Crystal 
       [0077]    This growth run gas was carried out in a growth furnace having the crucible, baffle, and vapor-absorbing member  117   a ″ arrangement like the one shown in  FIG. 7 . The crystal growth crucible  102  was made of dense, isostatically molded and halogen-purified graphite. High-purity SiC grain source material  104 , 0.5 to 2 mm in size, was synthesized prior to growth in a separate synthesis process. A charge of 900 g of the SiC grain source material  104  was disposed at the bottom of crucible  102  and served during growth of SiC crystal  112  as a solid sublimation source. 
         [0078]    A 110 mm diameter SiC wafer of the 6H polytype oriented on-axis was used as the seed crystal  106 . The surface of the wafer where SiC crystal  112  was to grow was CMP polished prior to growth. The seed crystal  106  was attached to pedestal  124  of crucible lid  108  using a high-temperature adhesive. Pedestal  124  had a height of 10 mm. 
         [0079]    Baffle  114 ″ used in this run had the configuration shown in  FIG. 7 . The wall thickness of baffle  114 ″ was 3 mm. Baffle  114 ″ was perforated by drilling twenty-four 2 mm diameter holes in the wall of baffle  114 ″ in three rows of eight holes spaced uniformly around the circumference of baffle  114 ″. The number of perforations, however, can be between 4 and 40 and the diameter of each perforation can be between 1 and 3 mm. 
         [0080]    Vapor-absorbing member  117 ″ had the configuration shown in  FIG. 7 , namely, a combination of cylinder (top) and truncated cone (bottom). Vapor-absorbing member  117 ″ was machined from halogen-purified porous graphite with a density of 1.0 g/cm 3 ; a porosity of 50%; and pore sizes between 20 and 80 microns. 
         [0081]    The growth conditions were as follows: the temperature of source material  104  was 2150° C.; the temperature of seed crystal  106  was 2100° C.; and the pressure of inert gas (Ar) was 20 Torr. 
         [0082]    The grown 6H boule of SiC crystal  112  weighed 380 grams. The weight of the polycrystalline SiC deposit  134  formed inside vapor-absorbing member  117 ″ was about 35 grams. Upon inspection, no inclusions were detected in the boule bulk. The micropipe density in this boule was below 0.3 cm −2  and the dislocation density was about 9·10 3  cm −2 . 
         [0083]    The boule of SiC crystal  112  was fabricated into 23 standard 100 mm diameter, 400 microns thick wafers. The resistivity of all wafers was close to 1·10 11  Ohm-cm, with the standard deviation below 10%. 
         [0084]    As can be seen, sublimation growth of SiC single crystals in accordance with the present invention yields SiC boules with reduced densities of inclusions, such as foreign polytypes, silicon droplets and carbon particles. The invention also leads to reduced densities of micropipes and dislocations. 
         [0085]    The invention has been described with reference to exemplary 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.