Growth chamber for single crystal silicon carbide production

Abstract

The present disclosure relates to a physical vapor transport (PVT) growth chamber comprising: a source of silicon carbide for the sublimation of vapor species derived therefrom, including silicon vapor; a silicon carbide seed crystal; a membrane having a D50 pore size in a range of 10 µm to 200 µm disposed between the source of silicon carbide and the silicon carbide seed crystal, such that the sublimation vapor species migrate through the membrane towards the silicon carbide seed crystal, a crucible for housing the source of silicon carbide, the silicon carbide seed crystal; and the membrane, said crucible comprising an inner surface layer in communication with silicon vapor. The membrane or the inner surface layer of the crucible comprises a composite composed of two or more regions; a first region comprising a carbonaceous material, excluding glassy carbon, composed of an average crystallite size of no more than 27 nm; and / or carbon black or activated carbon.

Description

[0001] 25PC01SC PATENT

[0002] Growth Chamber for Single Crystal Silicon Carbide Production

[0003] Field

[0004] The present disclosure relates to a growth chamber for single crystal silicon carbide production and the process of producing single crystal silicon carbide using said growth chamber and the membrane and composite used therein.

[0005] Background

[0006] Crystalline silicon carbide is a semi-conductor valued for its fast, high-temperature and / or high voltage performance. A significant problem for SIC commercialization has been the elimination of defects: edge dislocations, screw dislocations (both hollow and closed core), triangular defects and basal plane dislocations.

[0007] The formation of single crystal silicon carbide is obtained from a source of silicon carbide, which is heated to elevated temperatures such that the source of silicon carbide undergoes sublimation to form silicon and carbon vapor. Through controlling the homogeneity of the ratio between the silicon vapor to carbon vapor, and the purity thereof, the growth of a single crystal silicon carbide from a seed crystal can be controlled, and therefore, the defects are minimized.

[0008] Despite improvements in growth chamber designs and operating conditions, there is still a need for improved PVT growth chamber designs to enhance the quality and volume of single crystal silicon carbide produced.

[0009] Summary

[0010] In a first aspect of the present disclosure there is provided a physical vapor transport (PVT) growth chamber comprising: a. a source of silicon carbide for the sublimation of vapor species derived therefrom, including silicon vapor; b. a silicon carbide seed crystal; c. a membrane having a D50 pore size in a range of 10 pm to 200 pm disposed between the source of silicon carbide and the silicon carbide seed crystal, such that the sublimation vapor species migrate through the membrane towards the silicon carbide seed crystal; and 25PC01SC PATENT d. a crucible for housing the source of silicon carbide, the silicon carbide seed crystal; and the membrane, said crucible comprising an inner surface layer in communication with silicon vapor; wherein the membrane or the inner surface layer of the crucible comprises a composite composed of two or more regions; a first region comprising a carbonaceous material, excluding glassy carbon, composed of:

[0011] (i) an average crystallite size of no more than 27 nm; and / or

[0012] (ii) carbon black or activated carbon.

[0013] A high reactivity membrane or a high reactivity inner surface layer of the crucible may be advantageously used to control the level of sublimation species through selectively reacting silicon vapor with a high reactivity carbonaceous material. The carbonaceous material functions as a sacrificial material which effectively extends the life span of the crucible or further upstream membranes if present.

[0014] The high reactivity carbonaceous material may be periodically replenished through the recoating of the crucible wall or the replacement of the membrane.

[0015] The carbonaceous material, excluding glassy carbon, may have an average crystallite size of no more than 27 nm or no more than 26 nm or no more than 25 nm or no more than 24 nm or no more than 23 nm or no more than 22 nm or no more than 21 nm or no more than 20 nm or no more than 19 nm or no more than 18 nm or no more than 17 nm or no more than 16 nm.

[0016] Typically, the smaller the crystallite size the more reactive the carbonaceous material is with silicon vapor. Typically, the average crystallite size is at least 1 nm or at least 3 nm or at least 5 nm.

[0017] In some embodiments, the composite further comprises a second region, said second region is composed of:

[0018] (iii) a carbonaceous material with average crystallite size of greater than 27 nm; and / or

[0019] (iv) graphite, glassy carbon, metal carbide or metal nitride.

[0020] The second region preferably provides the main structural support for the membrane and enables the filtering and purification functions of the membrane to be maintained whilst the first region of the membrane is being consumed by reacting with the silicon vapor. 25PC01SC PATENT

[0021] In other embodiments, the composite forming part of the membrane and / or inner surface layer of the crucible may not comprise any significant amounts of other carbonaceous materials (e.g., less than 10 wt% or less than 5 wt%), with the other carbonaceous materials derived from binders used to form the membrane or the inner surface layer of the crucible.

[0022] The membrane may comprise a first surface and an opposing second surface. The distance between the first and second surfaces being the thickness of the membrane. The SiC sublimation vapors flow in through the first surface of the membrane and flow out of the second surface of the membrane. Each of the regions of the membrane may extend from the first surface to the second surface of the membrane. Therefore, different portions of the SiC sublimation vapors will flow through different regions of the membrane thereby interacting with the different carbonaceous properties of each region. As a result, the SiC sublimation vapor flowing out of the membrane may have a different thermal field and / or vapor flow field relative to the SiC sublimation vapor flowing out a membrane with uniform properties.

[0023] In some embodiments, the PVT growth chamber further comprises a second membrane, said second membrane is disposed between the source of silicon carbide and the first membrane. The second membrane preferably has a D50 pore size in a range of 10 pm to 200 pm or in the range of 30 pm to 70 pm. The second membrane may comprise a carbonaceous material with a higher reactivity measured as a rate of consumption in a thermogravimetric analyzer to silicon vapor compared to the first membrane. Alternatively, relative SiC reactivity may be calculated / estimated by TGA analysis performed in accordance with ASTM standard E2550 with the samples heated from room temperature to 900°C in air or by Temperature Programmed Oxidation (TPO) to 850 °C at a rate of 5 °C / min in air. The applicants have found that a higher reactivity of the membrane in air correlates with a higher reactivity of the membrane in silicon vapor. As a result, a higher reactivity of the membrane in air will be deemed to amount to a higher reactivity of the membrane in silicon vapor.

[0024] The difference between reactivity between the second membrane and the first membrane may be in the range of 5% to 500% (or 10% to 400%) higher in terms of consumed silicon vapor.

[0025] The higher reactivity of the carbonaceous material enables the second membrane to promote the formation of SiC vapor species from silicon vapor thereby reducing the quantity of silicon vapor that reaches the first membrane. The presence of the second membrane may increase the yield of single crystal SiC as well as protecting the silicon vapor from reacting with and altering the properties of the first membrane. The reactivity of the first membrane to SiC sublimation vapors is preferably low so that the properties of the first membrane are 25PC01SC PATENT maintained over an extended period of time. The second membrane may be periodically replaced and function as a consumable component of the PVT growth reactor.

[0026] In some embodiments, two regions of carbonaceous material within the membrane comprise two or more regions with different reactivity to silicon vapor. The difference between the lowest and highest regions in terms of silicon vapor reactivity may be in the range of 5% to 500% of consumed silicon vapor (or air) and preferably at least 10% or at least 30% or at least 50% or at least 80%.

[0027] The first region

[0028] The first region may be composed of one or more carbonaceous materials with a higher reactivity to silicon vapor than the second region. The carbonaceous materials may include activated carbon and carbon black. Whilst the first region may comprise activated carbon and carbon black, these materials may be progressively graphitized in operation if they have not already been graphitized prior thereto.

[0029] The reactivity of the carbonaceous material may be correlated to the crystallite size of the carbonaceous material, as determined by Raman analysis (i.e., from analysis of Raman spectroscopy). The first region may have an average crystallite size of no more than 27 nm or no more than 26 nm or no more than 25 nm or no more than 24 nm or no more than 23 nm or no more than 22 nm or no more than 21 nm or no more than 20 nm or no more than 19 nm or no more than 18 nm or no more than 17 nm or no more than 16 nm or no more than 15 nm.

[0030] Typically, the average crystallite size is at least 1 nm or at least 3 nm or at least 5 nm.

[0031] The reactivity of the carbonaceous material may also be defined by reference to Temperature Program Oxidation (TPO) and Thermogravimetric Analysis (TGA). In some embodiments, the first region comprises a TPO with a maximum CO2 count peak of less than 680°C or less than 678°C or less than 675°C or less than 672°C or less than 669°C, in accordance to the methodology later described.

[0032] In some embodiments, TGA analysis (in air or silicon vapor) results in the first region consuming at least 1% or at least 3% or at least 5% or at least 10% more air or silicon vapor than the second region.

[0033] The second region 25PC01SC PATENT

[0034] The second region may be composed of one or more carbonaceous materials with a lower reactivity to silicon vapor than the first region. The materials may include natural graphite, synthetic graphite, graphitized coke, graphitized pitch, graphitized phenolic resin, glassy carbon and metal carbides (e.g., tantalum carbide, zinc carbide, hafnium carbide). Whilst the second region may comprise ungraphitized carbonaceous materials, these materials may be progressively graphitized in operation if they have not already been graphitized prior thereto.

[0035] The second region, if not metal carbides or glassy carbon, may have an average crystallite size of greater than 27 nm or greater than 28 nm or greater than 29 nm or greater than 30 nm or greater than 31 nm or greater than 32 nm or greater than 33 nm or greater than 34 nm or greater than 35 nm or greater than 36 nm or greater than 37 nm or greater than 38 nm or greater than 39 nm or greater than 40 nm. Typically, the average crystallite size is no more than 100 nm or no more than 80 nm or no more than 60 nm.

[0036] The difference in the average crystallite size between the first region and the second region is typically at least 2 nm or at least 3 nm or at least 4 nm or at least 5 nm or at least 6 nm or at least 7 nm or at least 8 nm or at least 9 nm or at least 10 nm or at least 11 nm or at least 12 nm or at least 13 nm or at least 14 nm or at least 15 nm. Typically, the larger the crystallite size difference between the regions results in a higher proportion of the first region being consumed by silicon vapor than the second region. Therefore, the second region can function better as a membrane in filtering carbonaceous particles and distributing the vapor flow through the reactor.

[0037] In some embodiments, both the first and second region comprise an average crystallite size greater than 27 nm. In some embodiments, both the first and second region comprises an average crystallite size no more than 27 nm.

[0038] In some embodiments, the second region may comprise metal carbides, metal nitrides or glassy carbon. In some embodiments, a glassy carbon membrane may be coated with a carbonaceous material with lower reactivity to silicon vapor. For example, this may be carbonaceous material with an average crystallite size of less than 27 nm. The preparation of a glassy carbon membrane may be conducted in accordance with the disclosures in GB2510357.3 which is incorporated herein by reference. 25PC01SC PATENT

[0039] The com

[0040] In a second aspect of the present disclosure there is provided a composite for use in the PVT growth chamber according to the first aspect of the disclosure, comprising a first region of carbonaceous material (excluding glassy carbon) having an average crystallite size of no more than 27 nm and a second region selected from a carbonaceous material with an average crystallite size of greater than 27 nm, a metal carbide, a glassy carbon or a combination thereof.

[0041] The composite may comprise in the range of 0.5 wt% and 99.5 wt% of the first region and in the range of 0.5 to 99.5 wt% of the second region. In another embodiment, the composite may comprise in the range of 5.0 wt% and 95 wt% of the first region and in the range of 5 to 95 wt% of the second region. In a further embodiment, the composite may comprise in the range of 10 wt% and 90 wt% of the first region and in the range of 10 to 90 wt% of the second region. In yet a further embodiment, the composite may comprise in the range of 20 wt% and 80 wt% of the first region and in the range of 20 to 80 wt% of the second region. The lower limit of the second region may be limited to the amount of less reactive carbonaceous material required to maintain a functioning membrane (e.g., comprising a pore size distribution with a D50 between 30 pm and 70 pm. In some embodiments, the proportion of the second region is at least 40 wt% or at least 50 wt% or at least 60 wt% or at least 70 wt% relative to the total weight of the membrane.

[0042] The composite may be composed of two or more regions defined by their crystallite size. Each of the regions may also comprise sub-regions defined by their crystalline size and dispersed through the composite (i.e., a region having an average crystallite size of less than 20 nm may be composed of a series of sub-regions having an average crystallite size less than 20 nm).

[0043] In some embodiments, the first region comprises one or more sub-regions, each sub-region comprising a minimum cross-sectional surface area of at least 20 pm2or at least 30 pm2or at least 60 pm2or at least 90 pm2or at least 120 pm2or at least 150 pm2or at least 180 pm2or at least 210 pm2or at least 240 pm2or at least 270 pm2or at least 300 pm2.

[0044] In some embodiments, the second region comprises one or more sub-regions, each subregion comprising a minimum cross-sectional surface area of at least 20 pm2or at least 30 pm2or at least 60 pm2or at least 90 pm2or at least 120 pm2or at least 150 pm2or at least 180 pm2or at least 210 pm2or at least 240 pm2or at least 270 pm2or at least 300 pm2. 25PC01SC PATENT

[0045] In some embodiments, the composite comprises a first region comprising an average crystallite size of no more than 22 nm and a second region comprising an average crystallite size of greater than 27 nm.

[0046] Membrane

[0047] In a third aspect of the present disclosure, there is provided a membrane for use in a PVT growth chamber comprising the composite of the second aspect of the present disclosure. The membrane preferably comprises a Dso pore size in a range of 10 pm to 200 pm. In some embodiments, the membrane comprises a D50 pore size in a range of 30 pm to 70 pm. The membrane preferably does not comprise any pores greater than 500 pm or greater than 800 pm.

[0048] % surface areas of regions

[0049] In some embodiments, the first region comprises at least 5% or at least 10% or at least 15% or at least 20% of a total cross-sectional surface area of a composite material making up the membrane or inner surface layer of the crucible. In some embodiments, the first region makes up no more than 90% or no more than 80% or no more than 70% or no more than 60% or no more than 50% or no more than 40% or no more than 35% or no more than 30% of a total cross-sectional surface area of a composite material making up the membrane or inner surface layer of the crucible. The % surface area of the first region is preferably determined via a combination of Raman analysis and the use of software (e.g., ImageJ™ software) to process optical images. In some embodiments, the first surface (i.e., facing the source of SiC) comprises a higher % surface area of the first region than the second surface. In some embodiments, the first surface comprises at a surface area of the first region of at least 20% or at least 30% or at least 40% or at least 50% or at least 60% of the total surface area of the first surface.

[0050] Pore size

[0051] Pore sizes typically range from 0.1 to 500 pm. In some embodiments, the maximum pore sizes are limited. For example, less than 5% or less than 4% or less than 3% or less than 2% or less than 1% or less than 0.5% of pores being greater than 400 pm or greater than 300 pm or greater than 200 pm or greater than 100 pm or greater than 80 pm. The D50 pore size is typically at least 10 pm or at least 20 pm or at least 30 pm. In one embodiment, the D50 pore size is between 25 and 100 pm or between 30 and 80 pm or between 45 and 70 pm. 25PC01SC PATENT

[0052] In one embodiment, the pore size distribution comprises at least 60% or, at least 65%, or at least 70%, or at least 75%, or at least 80%, or at least 85% of the pores being in the range of 20 and 80 pm, or between 40 and 70 pm.

[0053] In one embodiment, the pore size distribution of one or both of the regions is the following:

[0054] 9-15 % of pores smaller than 1 pm;

[0055] 2-3 % of pores between 1 and 10 pm;

[0056] 75-87 % of pores between 10 and 100 pm; and

[0057] 2-7 % of pores larger than 100 pm.

[0058] The membrane preferably does not comprise any apertures which penetrate through the membrane, as this serves to by-pass the beneficial interaction between the SiC vapor species and the membrane as well as causing thermal “hot spots” due to the spikes of radiant heat that originate from the apertures.

[0059] These pore size limitations effectively enable the membrane to filter out unwanted contaminants, whilst adjusting the vapor flow field to a target a desired seed crystal growth rate and / or desired seed crystal shape.

[0060] The pore size distribution of the membrane may comprise a Dgo value of no more than 120 pm or no more than 110 pm or no more than 100 pm or no more than 90 pm or no more than 80 pm or no more than 70 pm or no more than 60 pm or no more than 50 pm or no more than 45 pm.

[0061] The pore size distribution of the membrane may comprise a difference between the Dgo and D50 value of no more than 30 pm or no more than 25 pm or no more than 20 pm or no more than 18 pm or no more than 15 pm or no more than 12 pm or no more than 10 pm or no more than 8 pm or no more than 6 pm or no more than 5 pm. The narrower the pore size difference between D50 and Dgo and the narrower the pore size distribution, the more consistent the performance of the membrane is in terms of impurity removal and permeability.

[0062] In some embodiments D50 - D10 > Dgo - Dso- In some embodiments D10 is in the range of 0.1 to 25 pm and preferably in the range of 0.5 and 3.0. 25PC01SC PATENT

[0063] In one embodiment, the PVT growth chamber comprises a first membrane, said first membrane disposed between the source of silicon carbide and a second membrane. The first membrane or material of the first region and the second membrane may comprise material from the second region. The difference in reactivity between the first membrane and the second membrane may be in the range of 5 to 500% (or 10 to 400%) higher in terms of consumed silicon vapor (or consumed air, if reactivity is measured using air).

[0064] The higher reactivity of the carbonaceous material enables the first membrane to promote the formation of SiC vapor species from silicon vapor, thereby reducing the quantity of silicon vapor that reaches the second membrane. The presence of the first membrane may increase the yield of single crystal SiC as well as protect the silicon vapor from reacting with and altering the properties of the second membrane. The reactivity of the second membrane to SiC sublimation vapors is preferably low so that the properties of the first membrane are maintained over an extended period of time.

[0065] In some embodiments, a single membrane comprises two or more regions with different reactivity to silicon vapor. The difference between the lowest and highest regions in terms of silicon vapor reactivity may be in the range of 5 to 500% of consumed silicon vapor and preferably at least 10% or at least 30% or at least 50% or at least 80%.

[0066] In some embodiments comprising a single membrane, the first and second regions are configured to preferentially expose the silicon vapors with a surface of the first region to reduce the quantity of silicon vapor which reacts with the second region. Thus, the first region is configured to be a sacrificial region which is consumed, whilst the second region is configured to provide structural support to the membrane and enable the function of the membrane to purify and filter the sublimation vapors and fine carbonaceous particles to be maintained.

[0067] Method of manufacture

[0068] The present disclosure provides a method for manufacturing a membrane with tunable silicon vapor reactivity for use as a membrane in the PVT growth of single crystal SiC.

[0069] The manufacturing method includes the following steps:

[0070] • Mixing of at least two types of materials, including metal carbides, glassy carbon, or at least one graphite or graphite precursor material with a binder, each type of material corresponding to a heat-treated product with a different silicon vapor reactivity. The graphite precursor materials can be made of carbon-rich materials derived from 25PC01SC PATENT biomass, coal, or petroleum, preferably petroleum coke. The binder system should be chosen to give ample carbon yield upon carbonization. Binders can be polymeric (e.g., phenolic resin), petroleum pitch, preferably coal-derived pitch or tar. Optional fillers and pore forming agents (e.g., ammonium bicarbonate) may also be included in the mixture. Pore forming agents may include organic fibers, such as cotton fibres, carbon or polymeric fibres.

[0071] • Loading the mixture into a mold;

[0072] • Compacting the mixture;

[0073] • Heat treating the mixture in a furnace to roast the mixture to produce a porous composite and then further heat treating the porous composite to induce graphitization and preferably remove contaminants to purify the graphitized porous composite. Roasting may involve several heating steps at temperatures in the range of about 600 to 1700 K, performed over several days. The graphitization temperature may occur under vacuum (e.g., 50 to 150 Pa at temperatures up to about 2600 K for about 24 hours. Freon and / or chlorinated gases may be added during the heat treatment for purification purposes. The heat treated composite preferably comprises a portion of material with an average crystallite size of no more than 27 pm or not more than 25 pm; and either

[0074] • Machining the porous composite into the desired shape; or

[0075] • comminuting to form a solid material with a D50 of less than 2000 pm, which is blended with a carbon-based binder to form a coating paste.

[0076] • The coating paste may alternatively be made by mixing at least two types of materials before comminuting to form a solid material with a D50 of less than 2000 pm, which is blended with a carbon-based binder to form a coating paste. In this embodiment, the coated material is at least partially graphitized in-situ within the PVT growth chamber.

[0077] In some embodiments, the membrane may be produced with a non-reactive graphite grade (e.g. average crystallite size > 27 nm or > 30 nm or a metal carbide (e.g. carbides of Ta, Hf, Nb, Zr, W and V), metal nitride (e.g. nitrides of Ta, Hf, Nb, Zr, W and V) or glassy carbon, with a more reactive carbonaceous grade being impregnated into the membrane to coat the outer surface of the membrane. In this embodiment, the first region (most reactive region) forms an outer layer of the membrane. Within this configuration, the second region functions as a skeletal support structure for the first region. The viscosity and / or wettability of the impregnate solution may be optimized, as known by those skilled in the art, for the impregnate to form a layer on the internal pores without excessively lowering the permeability of the membrane. In some 25PC01SC PATENT embodiments, a lower viscosity and lower concentration impregnate may be used to impregnate the membrane with one or more impregnating steps taken to obtain a targeted layer thickness of the more reactive carbonaceous grade.

[0078] In one embodiment, the membrane comprises an inner layer comprising the second region and an outer layer comprising the first region, said outer layer is in communication with the PVT growth chamber.

[0079] In a further aspect of the present disclosure, there is provided a process for producing the composite for use in a PVT growth chamber, comprising:

[0080] (a) Mixing a first component comprising a carbonaceous material; and a second component selected from materials consisting of natural graphite, synthetic graphite, glassy carbon, metal carbides or precursors thereof; and an optional carbon based binder;

[0081] (b) Heat treating the mixture at sufficient temperature and sufficient time to at least partially graphitize any of the first or second components, which are not already graphitized, wherein the heat treated first component comprises an average crystallite size of less than 27 nm (or what is otherwise defined as the first region). The heat treated second component may comprise an average crystallite size of more than 27 nm (or what is otherwise defined as the second region). The first component may comprise carbon black or activated carbon.

[0082] In some embodiments, the mixture prior to heat treatment, is loaded into a mould and compacted to form a membrane after the heat treatment, said mixture comprises a carbonbased binder (e.g., phenolic resin). The relative particle size of the different components may be configured to promote the reaction of silicon vapor with the first region whilst at least partially shielding the second region from reaction with the silicon vapor or other sublimation vapors. In some embodiments, the particles forming the first region comprise particle size distribution with a D50 lower than the particle size distribution of the particles forming the second region.

[0083] In other embodiments, the first and second components may be packed in different regions of the mould. For example, the first component may form a first layer in the mould and the second component may form a second layer in the mould. In this configuration, the resulting membrane may be configured so that the more reactive first layer faces the source of silicon carbon and the less reactive second layer faces the silicon carbide seed crystal. Alternatively, 25PC01SC PATENT the first component may have a greater concentration towards one surface of the membrane than the other, such that the surface with the greater concentration of the first component may face the source of the silicon carbide.

[0084] In another embodiment, the composite may be formed by first producing a skeletal membrane (i.e., second region comprising the second component). This is produced as mixing the second component, compacting the second component, and optional binder into a mould and heat treating the mixture to form a porous graphite membrane. The first region comprising the first component may then be applied to the second region may liquid impregnation.

[0085] The uncoated membrane or inner layer (second region) is placed inside an impregnation vessel and typically evacuated with a vacuum pump. Liquid impregnant media containing the first component as a slurry or a precursor to the first component is drawn into the second region via pressure. Particles of the first component can range from 1 to 200 microns, preferably 10-100 microns, and most preferably 20-80 microns. Impregnation pressures can range from 10 psi to 20,000 psi, preferably between 50-5,000 psi, and most preferably between 100 and 3000 psi. After impregnation, the chamber is depressurized, and the materials are removed and ready for a standard baking and / or graphitization heat treatment process. The impregnation process provides an outer layer to the skeletal inner layer.

[0086] The slurry solution of the first component may comprise in the range of 5 to 90 wt% solids or 10 to 70 wt% solids or 20 to 50 wt% solids. Carry liquids for the slurry may be resins such as phenolic, alcohols such as ethanol, water, or any other fluid carrying media.

[0087] Alternatively, the uncoated membrane (inner layer) may be dipped coated by dipping the uncoated membrane into the slurry comprising the first component. The coated membrane comprising the first component (outer layer) coated on the second region (inner layer) is then dried and further heat treated as required. This configuration of the components enables the first region to be preferentially exposed to the silicon vapor, thereby enabling the first region to function as a sacrificial component and protect the structural integrity of the second region. In some embodiments, the outer layer comprises in the range of 1.0 to 50 wt% or 2.0 and 40 wt% or 3.0 and 30 wt% or 4.0 and 25 wt% of the total weight of the membrane. The pore size distribution of the uncoated membrane (second region) may be selected such that the coated membrane provides a pore size distribution with a targeted D50 pore size (e.g., between 10 to 200 pm), with the uncoated membrane preferably still possessing a D50 within a targeted D50 pore size. 25PC01SC PATENT

[0088] When forming a paste for applying to the crucible walls, the mixture after heat treatment undergoes comminution to form a solid material with a Dso of less than 2000 pm or less than 1000 pm (and typically greater than 100 pm), which is blended with a carbon-based liquid binder to form a coating paste for application onto the walls of the PVT growth chamber.

[0089] CN116120079, which is disclosed herein by reference, discloses a representative example of how to produce a porous graphite grade with a specified permeability. By changing manufacturing process parameters, as would be known to the skilled artisan, porous graphite properties can be tuned to obtain the desired performance in the PVT SiC growth.

[0090] CN118026684 and CN1186197043, which are disclosed herein by reference, also disclose suitable methods of producing porous graphite for the PVT SiC growth.

[0091] Reference to Dw, D50 and Dgo of the pore size distribution is on a % volume basis. For example, the D50 value is the value at which 50% of the pores by volume are smaller and 50% of the pores by volume are larger.

[0092] In a fifth aspect of the present disclosure, there is provided a physical vapor transport (PVT) growth chamber comprising: a. a source of silicon carbide for the sublimation of vapor species derived therefrom, including silicon vapor; b. a silicon carbide seed crystal; c. a membrane disposed between the source of silicon carbide and the silicon carbide seed crystal, such that the sublimation vapor species migrate through the membrane towards the silicon carbide seed crystal; and d. a crucible for housing the source of silicon carbide, the silicon carbide seed crystal; and the membrane, said crucible comprising an inner surface layer in communication with silicon vapor; wherein the membrane or the inner surface layer of the crucible comprises two or more regions comprising different reactivity to silicon vapor; said two or more regions comprise a first region with higher reactivity to silicon vapor relative to a second region.

[0093] The different reactivity of the first and second regions to silicon vapor may be determined by TGA analysis performed using air in accordance with ASTM standard E2550. Alternatively, Temperature Programmed Oxidation (TPO) in air may also be used. 25PC01SC PATENT

[0094] For the purposes of the present disclosure, reference to a reactive carbon species is inclusive of the graphitized version of that reactive carbon species. For example, carbon black is inclusive of graphitized carbon black and activated carbon is inclusive of graphitized activated carbon.

[0095] A graphitized species is defined as a carbon species which has been exposed to a graphitization heat treatment. For example, heat treatment above 2000°C for sufficient time from the graphitization process to commence.

[0096] For the purposes of the present disclosure, a membrane is a membrane which comprises a continuous layer of carbonaceous material which provides a designated permeability to the sublimation vapors. The membrane is preferably monolithic. The pores in the membrane provide a tortuous pathway through which vapor must travel from a first surface side to a second surface side of the membrane to reach the SiC seed crystal. The membrane is exclusive of a layer of carbonaceous granules or particles. For clarity, the membrane does not include through-holes which bypass the tortuous pathway of the membrane.

[0097] As used herein, the term "tortuous pathway" means a flow pathway having multiple branches, curves, angles, turns, etc., which prevent the straight path of flow. In some embodiments, the tortuous path increases the residence time of the vapor within the membrane to increase concentration uniformity.

[0098] For clarity, glassy carbon is deemed to be a carbonaceous material of the second region.

[0099] Sublimated SiC includes Si, Si2C and SiC2.

[0100] For the purpose of the disclosure “a first surface facing the source of silicon carbide and a second surface facing the silicon carbide seed crystal” means that the first surface is exposed to a pathway of entering sublimation vapor from the source of silicon carbide and the second surface is exposed to a pathway of sublimation vapor exiting the membrane towards the silicon carbide seed crystal.

[0101] For the purposes of the disclosure “the first and second regions are configured such that first region is exposed to preferentially react with the silicon vapor compared to the second region” means that, on a proportional basis, a greater portion of the first region is exposed to the silicon vapor entering the membrane, (e.g. if the membrane was composed of compacted particles comprising 10% of the surface area from particles of the first region and 90 % of the surface area of particles of the second region, then the first region may be configured to 25PC01SC PATENT preferentially react with the silicon vapor if the compacted surface area of the resultant membrane exposed to the silicon vapor was 20% from the first region relative to the total surface area exposed to the silicon vapor. Alternatively, a high proportion of the first region’s carbonaceous material may be disposed towards the first surface of the membrane facing the source of silicon carbide and therefore able to preferentially react with the incoming silicon vapor).

[0102] For the purposes of the disclosure “the first and second regions are configured such that the second region provides the main structural support to the membrane” means that the second region provides the structural integrity of the membrane such that the substantial removal / consumption of the first region of the membrane (e.g. greater than 40 wt% or greater than 50 wt%) does not prevent the membrane from performing to filter out carbonaceous particles (i.e. maintains a D50 pore size distribution of between 10 pm and 200 pm and preferably 20 pm to 70 pm).

[0103] Brief description of the Figure

[0104] Figure 1 is a schematic diagram of a cross-sectional view of an apparatus for providing single crystal SiC comprising a PVT growth chamber wherein the membrane transverses across the walls of the chamber.

[0105] Figure 2 is a schematic diagram of a cross-sectional view of an apparatus for providing single crystal SiC comprising a PVT growth chamber wherein the membrane extends from the base of the top of the chamber.

[0106] Figure 3a is an optical image of a cross section of a composite material comprising graphite and carbon black in accordance with an embodiment.

[0107] Figure 3b is a Raman spectroscopy output array for a 12 x 11 4 pm2grid of the composite in Figure 3a.

[0108] Figure 4a is an optical image of a cross section of a composite material comprising graphite and activated carbon in accordance with an embodiment.

[0109] Figure 4b is a Raman spectroscopy output array for a 12 x 11 4 pm2grid of the composite in Figure 4a.

[0110] Figure 5 is a Temperature Program Oxidation plot comparing PG with and without 10 wt% activated carbon in the pre-baked composition. 25PC01SC PATENT

[0111] Figure 6 is a graph derived from Thermogravimetric Analysis (TGA) comparing PG with and without 10 wt% activated carbon in the pre-baked composition.

[0112] Figure 7 is a Temperature Program Oxidation plot comparing activated carbon and petroleum coke.

[0113] Figure 8 is a graph derived from thermogravimetric analysis (TGA) comparing glassy carbon with graphitized coke.

[0114] Detailed description of a preferred embodiment

[0115] In this disclosure, when information is listed numerically or alphabetically in a list, such a list is not indicative that items in the list must occur in the listed sequence.

[0116] With reference to Figure 1 , there is provided an apparatus 10 for producing single crystal SiC comprising an insulation layer 20, typically made from graphite felt, encompassing a PVT growth chamber 30. The growth chamber 30 comprises a graphite crucible 40. The graphite crucible may be permeable to gases such as nitrogen and argon, although the graphite crucible preferably does not have sufficient permeability to enable SiC sublimation vapors to pass through to any significant extent. Within the growth chamber there is provided a raw material zone 50 which comprises SiC or precursor thereof. At an opposing end of the growth chamber 30 there is provided a silicon carbide seed crystal zone 60 including a seed crystal holder from which the single crystal silicon carbide is to grow. Located between the raw material zone and the silicon carbide seed crystal zone is positioned a membrane 70 and / or a coating 95 on an internal surface of the crucible 40, beneath the membrane, which forms a transverse plane through which the sublimations vapors pass through. The membrane 70 comprises a first surface 72 facing the raw material 50 and a second surface 74 facing the seed crystal 60.

[0117] The membrane 70 and / or the coating 95 comprises two regions of carbonaceous material, with one of the regions being more reactive to silicon vapor than the other. The more silicon vapor reactive region of the membrane or crucible coating preferentially reacts with the silicon vapor to form SiC species.

[0118] The sacrificial nature of the reactive region of the membrane and coating enables the less reactive region of the membrane and coating to retain its mechanical integrity and performance (e.g., function as a membrane). 25PC01SC PATENT

[0119] The membrane may directly or indirectly sealingly connect to the walls of the crucible. When the membrane indirectly sealingly connects to the walls of the crucibles, the membranes connect to another structure which is connected to the crucible walls. The other structure is preferably non-porous such that sublimation vapors are directed through the membrane.

[0120] The membrane may transverse the graphite crucible 40. In operation, induction heating coils 80 heats the contents positioned within depending upon their conductivity, including the crucible 40, raw materials 50 and the membrane 70. The temperature of the raw materials 50 may typically reach in the range of about 2600 to 2800 K, resulting in the sublimation of the raw materials into Si and SiC species. The temperature at the silicon carbide seed crystal zone 60 is typically near approximately 2400 K to enable a sufficient temperature gradient for the sublimed raw materials vapors to flow towards the silicon carbide seed crystal zone 60.

[0121] The PVT growth chambers may have a number of different configurations. For example, Figure 2 illustrates an apparatus 210 comprising a membrane 270 (comprising a region of silicon vapor reactive carbon with an average crystallite size of less than 27 pm or less than 25 pm) which is cylindrical in shape and extends along an axial axis from a base 232 to a top 234 of the PVT growth chamber 230 with the silicon carbide seed crystal 260 on the interior side of the membrane cylinder 270 and the source of raw materials 250 which comprises silicon carbide or precursors thereof, on the exterior of the membrane cylinder. The PVT growth chamber may also comprise a crucible coating 275 comprising a silicon vapor reactive coating (e.g., comprising carbon crystallites with an average size of less than 27 nm or less than 25 nm). The membrane 270 comprises a first surface 272 facing the raw material 250 and a second surface 274 facing the seed crystal 260.

[0122] The apparatus also comprises the insulation layer 220 and heating coils 280. The membrane may directly or indirectly sealingly connect to the base and top of the crucible. When the membrane indirectly sealingly connects to the base 232 and / or top 234 of the crucible 240, the PG or glassy carbon (GC) membrane 270 connects to another structure which is connected to the crucible top and / or base. The other structure is preferably non-porous such that sublimation vapors are directed through the PG or GC membrane 270.

[0123] Figure 3a is an optical image of a composite material which the membrane 70, 270 or coating 95, 275 is composed of. Raman spectroscopy is used to calculate the average crystallite size from a results map of (12 x 11) four pm2areas superimposed over the optical image 320. The composite is composed of a first region 300 derived from carbon black comprising crystallite 25PC01SC PATENT sizes ranging from about 12 to 19 nm and a second region 310 of graphite comprising crystallite sizes ranging from about 25 to 40 nm.

[0124] A minimum of thirty 2 x 2 pm squares are used to calculate the average crystallite sizes from each of the regions. As indicated in the Raman results matrix in Figure 3b, there is a degree of variation in crystallite size within the region and even some impurities. For example, the second region comprises a 4 pm2area with a crystallite size of only 7 nm. Figure 3b also illustrates the transition region between the two white dotted lines covering the interface between the first and second regions. The transition region is not included in the determination of the crystallite size given that the 4 pm2analysis area partially covers the first and second regions.

[0125] Figures 4a and 4b highlight the optical image and Raman results grid for the activated carbon of the first region 400 and the graphite of the second region 410. As indicated in the results map, the variation in crystallite size in region 2 is relatively greater than the region 2 in Figures 3a and 3b, except the region of impurities in Figures 3a and 3b. This is also apparent in the optical image 4a, in which the second region 410 has a heterogeneous appearance.

[0126] Experimental

[0127] Sample preparation

[0128] The raw materials used to make the porous graphite are petroleum coke, coal tar pitch, sulfur, and a reactive carbon additive. Additives can include any carbon species that does not go through significant mesophase during heating up to 2900 °C, such as activated carbon and carbon black. Composite pieces were fabricated by first hot mixing 57-62 wt% petroleum coke, 35 wt% coal tar pitch, 3 wt% sulfur, and 5-10 wt% of a reactive carbon additive at 150 °C. The composite material was subsequently milled, screened to be <300 pm, and packed in a mould with approximately 1 .4 MPa of pressure applied to the top of the load. The composite material was then graphitized up to 2900 °C.

[0129] Example 1 was produced according to the above procedure with the composite comprising 10 wt% carbon black.

[0130] Example 2 was produced according to the above procedure with the composite comprising 10 wt% activated carbon. 25PC01SC PATENT

[0131] Crystallite size was determined via Raman measurements performed on a Horiba LabRam HR Soleil system with 532 nm excitation lasers coupled through a 50x (NA 0.5) objective lens with an incident laser power of 3.6 mW. The spectrometer was coupled with a 600 gr / mm grating and Si-array back illuminated deep-depleted detector (Horiba - Synapse 1024x512 pixel). The integration time for the Raman measurements was 0.9 seconds at each point with 1 accumulation. A total of 132 points (4 pm2each) were measured on each sample in a fast map.

[0132] Peak fitting (Gauss-Lorentzian) was performed in LabSpec6 and the FWHM and peak intensities from the fit were used to calculate the crystallite size (La). Average crystalline sizes were segregated by La color mapping, where, in general, the high crystalline region has a crystalline size >27 nm and low crystalline regions are <27 nm. These distinct regions are evident in the heat mapping in Figures 3b and 4b.

[0133] Determination of average crystalline size of a region should preferably utilize no less than 30 pm2or no less than 60 pm2of the total area taken in the scan, using points that are adjacent to each other. Impurities should be included in the determination of the average crystallite size. Impurities are deemed to be areas of less than 10 pm2or less than 5 pm2or less than a 10% or less than a 5% area of the region. Impurities are a non-carbonaceous material or carbonaceous material with a crystallite size of greater than 15 nm difference to the adjacent points within the region.

[0134] In determination of the average crystallite size of different regions, a transition region may be observed wherein the Raman measure point partially covers two different regions. These measurement points should be discarded and not included in the measurement of the average crystallite size. The use of optical imaging in addition to the Raman measurement result may be used to determine the transition region measurement points to be discarded.

[0135] In Example 1 , Region 1 consisted of 62 x 4 pm2measurement points = 248 pm2with an average crystallite size determined to be 14.9 nm. Region 2 consisted of 64 x 4 pm2measurement points = 256 pm2with an average crystallite size determined to be 29.7 nm.

[0136] Temperature Programmed Oxidation (TPO) may be used to gauge the reactivity of the porous graphite. Temperature was swept from room temperature to 850 °C at a rate of 5 °C / min. Clean dry air was flown through the system at 100 seem. CO2 emissions were recorded from the downstream flow. Sample size was (5 ± 1 ) mg of <45 pm particles. By comparing the 25PC01SC PATENT temperature maximum peak position and early onset temperature the relative reactivity of materials can be compared. A lower temperature onset transition and / or maximum peak is an indication that the material is more reactive than the comparison.

[0137] For the plot shown in Figure 5, the scale is normalized to remove intensity inaccuracy. Maximum peak positions are the temperature values when the curves reach 1.0 CO2 counts and early onset temperature is when the curves first reach 0.2 CO2 counts. In Figure 5, a PG without any reactive additives and PG with 10 wt% activated carbon are compared. The unaltered PG is perceived to be less reactive with an onset and maximum peak positions of 629 °C and 687 °C, respectively. This is compared to the more reactive PG with activated carbon that has an onset and maximum peak positions of 612 °C and 673 °C, respectively.

[0138] Figure 6, derived from TGA analysis, further highlights the greater reactivity of a membrane comprising 10 wt% active carbon and 90 wt% porous graphite compared to the reference porous graphite membrane at temperatures up to 900°C. The TGA analysis was performed in accordance with ASTM standard E2550, with the samples heated from room temperature to 900°C in air with a sample size of 7.9 mg for each sample and particle sizes between 63 to 75 pm, with both powders graphitized. Alternatively, the analysis can be performed using a silicon precursor (e.g., silane or SiC) using TGA or a sublimation furnace. When a sublimation furnace is used, the weight change attributable to the reaction of the materials is evaluated to determine the relative reactivity between materials.

[0139] Furthermore, comparing the individual components of petroleum coke and activated carbon that have undergone graphitization temperatures in Figure 6 indicates the higher reactivity of the additive of activated carbon. In this plot, the onset temperature for both species is nearly equivalent at 693 °C. However, the maximum peak temperatures differ greatly at 753 °C and 785°C for activated carbon and petroleum coke, respectively. Thus, there is evidence that activated carbon begins its maximum burning sooner and is more reactive than a petroleum coke.

[0140] Figure 8 shows Thermogravimetric Analysis (TGA) of glassy carbon prepared using a furfuryl alcohol precursor. Analysis was performed according to ASTM standard E2550 with first an inert atmosphere to 525°C, then an air atmosphere between 525°C and 570°C. Heating in the air atmosphere was set to 0.1C / min. Analysis demonstrates that glassy carbon did not show either mass loss or gain, while the graphitized coke experienced an increase in weight up to almost 7%, presumably due to the uptake of oxygen in the graphitized material. The results confirm that a glassy carbon membrane would be less reactive than a graphitized coke in a 25PC01SC PATENT membrane. Therefore, in a membrane or crucible wall lining comprising glassy carbon and more reactive carbonaceous materials like carbon black or activated carbon, the carbon black or activated carbon would be preferentially reacted (i.e., consumed).

[0141] % surface areas of regions

[0142] Optical imaging, with the assistance of polarized light, if required, may be used to correlate crystallite size and the optical shading of a region, such that the proportion of high and low reactivity regions may be calculated. I mageJ™ software may be able to identify different regions dependent upon the degree of light or dark shading. Once the ImageJ™ software has determined the proportion of each region by shading, Raman measurements, as previously described, may be used to validate the average crystallite size per region.

[0143] The % surface areas in Examples 1 and 2 were about the same as the wt% contents of the two components, with region 1 in both Examples being determined to be about 10% of the total surface area of the cross-sectional area, once the proportion of void area (i.e., pores) was taken into account.

[0144] Pore Size Distribution (PSD) was measured using mercury intrusion porosimetry.

[0145] The pore size distribution in both examples was such that the D50 pore size was in the range of 45 and 70 pm.

[0146] Table 1

[0147] Table 2 25PC01SC PATENT

[0148] Many variants, adaptations, product forms, uses, and applications of the present disclosure will be apparent to the person skilled in the art and are intended to be encompassed by this disclosure.

Claims

25PC01SC PATENTClaims1. A physical vapor transport (PVT) growth chamber comprising: a. a source of silicon carbide for the sublimation of vapor species derived therefrom, including silicon vapor; b. a silicon carbide seed crystal; c. a membrane, having a D50 pore size in a range of 10 pm to 200 pm, disposed between the source of silicon carbide and the silicon carbide seed crystal, such that the sublimation of vapor species migrate through the membrane towards the silicon carbide seed crystal; and d. a crucible for housing the source of silicon carbide, the silicon carbide seed crystal; and the membrane, said crucible comprising an inner surface layer in communication with silicon vapor; wherein the membrane or the inner surface layer of the crucible comprises a composite composed of two or more regions; a first region comprising a carbonaceous material, excluding glassy carbon, composed of:(i) an average crystallite size of no more than 27 nm; and / or(ii) carbon black or activated carbon.

2. The PVT growth chamber of claim 1 , wherein the composite is further comprising a second region, said second region is composed of:(iii) a carbonaceous material with an average crystallite size of greater than 27 nm; and / or(iv) graphite, glassy carbon, metal carbide or metal nitride.

3. The PVT growth chamber of claim 2, wherein the first and second regions are configured such that the first region is exposed to preferentially react with the silicon vapor compared to the second region.

4. The PVT growth chamber of claim 2, wherein the first and second regions are configured such that the second region provides the main structural support to the membrane.25PC01SC PATENT5. The PVT growth chamber according to any one of claims 2 to 4, wherein the membrane comprises an inner layer comprising the second region and an outer layer comprising the first region.

6. The PVT growth chamber of claim 5, wherein the inner layer forms a skeletal membrane coated with the outer layer.

7. The PVT growth chamber of claims 5 or 6, wherein the membrane is composed of in the range of 1.0 to 50 wt% of the inner layer.

8. The PVT growth chamber of any one of claims 2 to 7, wherein the membrane comprises a first surface facing the source of silicon carbide and a second surface facing the silicon carbide seed crystal, wherein the first surface has a higher proportion of the first region compared to the second surface.

9. The PVT growth chamber of any one of the preceding claims, wherein the first region is composed of a carbonaceous material with an average crystallite size of no more than 24 nm.

10. The PVT growth chamber of any one of the preceding claims, wherein the first region is composed of a carbonaceous material with an average crystallite size of at least 1 nm.

11. The PVT growth chamber of any one of claims 2 to 10, wherein the second region is composed of glass carbon or a carbonaceous material with an average crystallite size at least 5 nm greater than an average crystallite size of the material of the first region.

12. The PVT growth chamber of claim 11 , wherein the second region is composed of glassy carbon or a carbonaceous material with an average crystallite size at least 10 nm greater than an average crystallite size of the material of the first region.

13. The PVT growth chamber according to any one of the preceding claims, wherein one or both of the first region and the second regions comprises one or more sub-regions, each sub-region comprising a minimum cross-sectional surface area of at least 60 pm2.25PC01SC PATENT14. The PVT growth chamber according to any one of claims 2 to 13, wherein the second region is composed of one or more materials selected from natural graphite, synthetic graphite, graphitized coke, graphitized pitch; graphitized carbon-based resin, glassy carbon, tantalum carbide, zinc carbide or hafnium carbide.

15. The PVT growth chamber of any one of the preceding claims, wherein the membrane comprises a first surface facing the source of silicon carbide and a second surface facing the silicon carbide seed crystal; wherein the composite extends from the first surface to the second surface.

16. The PVT growth chamber according to any one of claims 2 to 15, wherein the composite comprises a first region which is homogeneously inter-dispersed throughout the second region.

17. The PVT growth chamber of any one of the preceding claims, wherein at least a portion of the inner surface of the crucible comprises the composite.

18. The PVT growth chamber of claim 16, wherein the inner surface of the crucible comprises the first region of the composite and a core of the crucible comprising the second region of the composite.

19. The PVT growth chamber of any one of claims 2 to 18, wherein the in membrane comprises a first membrane and second membrane, said second membrane disposed between the source of silicon carbide and the first membrane.

20. The PVT growth chamber of claim 19, wherein the second membrane comprises or consists of the first region of carbonaceous material and the first membrane comprises or consists of the second region of carbonaceous material.

21. The PVT growth chamber of claims 19 or 20, wherein the average crystallite size of the carbonaceous material in the first membrane is greater than 27 nm; and the average crystallite size of the carbonaceous material in the second membrane is not more than 27 nm.

22. The PVT growth chamber of claim 21 , wherein the difference between the crystallite sizes of the first membrane is at least 3 nm compared to the second membrane.25PC01SC PATENT23. The PVT growth chamber of any one of the preceding claims, wherein the composite is composed of in a range of 0.5 wt% and 99.5 wt% of the first region and in a range of 0.5 to 99.5 wt% of the second region.

24. The PVT growth chamber of claim 23, wherein the composite is composed of in a range of 5.0 wt% and 95 wt% of the first region and in a range of 5 to 95 wt% of the second region.

25. A membrane for use in a PVT growth chamber comprising the composite as defined in any one of the preceding claims, comprising: a first surface; a second surface opposing the first surface; and a thickness between the first and second surfaces of between 2 mm to 15 mm.

26. The membrane of claim 25, wherein the D50 pore size is in a range of 30 pm to 70 pm.

27. A process for producing a composite for use in a PVT growth chamber, comprising:(a) Mixing a first component comprising a carbonaceous material; and a second component selected from materials consisting of natural graphite, synthetic graphite, glassy carbon, metal carbides or precursors thereof; and an optional carbon based binder;(b) Heat treating the mixture at sufficient temperature for sufficient time to at least partially graphitize any of the first or second components, which are not already graphitized wherein the first component comprises carbon black or activated carbon or another carbonaceous material, excluding glassy carbon, with a crystallite size of no more than 27 nm.

28. The process according to claim 27, wherein the mixture prior to heat treatment is loaded into a mould and compacted to form a membrane after the heat treatment, said mixture comprises a carbon based binder.

29. The process according to claim 27 or 28, wherein the mixture after heat treatment undergoes comminution to form a solid material with a D50 of less than 2000 pm, which25PC01SC PATENT is blended with a carbon-based liquid binder to form a coating paste for application onto walls of the PVT growth chamber.

30. A physical vapor transport (PVT) growth chamber comprising: a. a source of silicon carbide for the sublimation of vapor species derived therefrom, including silicon vapor; b. a silicon carbide seed crystal; c. a membrane disposed between the source of silicon carbide and the silicon carbide seed crystal, such that the sublimation of vapor species migrate through the membrane towards the silicon carbide seed crystal; and d. a crucible for housing the source of silicon carbide, the silicon carbide seed crystal; and the membrane, said crucible comprising an inner surface layer in communication with silicon vapor; wherein the membrane or the inner surface layer of the crucible comprises a composite composed of two or more regions comprising different reactivity to silicon vapor, said two or more regions comprising a first region with higher reactivity to silicon vapor relative to a second region.

31. The PVT growth chamber of claim 30, wherein the different reactivity to silicon vapor is determined by TGA analysis in air performed in accordance with ASTM standard E2550.

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