Single crystal manufacturing method and graphite crucible

Abstract

To simplify a single crystal manufacturing method.SOLUTION: A single crystal manufacturing method comprises preparing a pedestal fitted with a seed crystal substrate for growing silicon carbide single crystal and a graphite crucible having a raw material housing space. The method includes: a crucible preparation process; a raw material feeding process of feeding carbon raw material and silicon raw material to be heated for synthesis into the raw material housing space; a heating preparation process of closing the graphite crucible with a lid with a graphite sheet arranged to partition off the raw material housing space and the space where the seed crystal substrate is arranged, and making heating preparations for the graphite crucible; and a heating process of heating the graphite crucible to heat and synthesize a silicon carbide precursor in the graphite crucible from the carbon raw material and the silicon raw material, and subliming the heat-synthesized silicon carbide precursor to grow the silicon carbide single crystal, the graphite sheet being thin enough to be opened in the middle of the heating process.SELECTED DRAWING: Figure 2A

Description

[Technical field]

[0001] The present invention relates to a method for producing a silicon carbide single crystal from a carbon raw material and a silicon raw material before thermal synthesis, and to a graphite crucible used in the method. [Background technology]

[0002] Conventionally, a method of producing silicon carbide single crystals by heating a crucible containing raw materials and using a sublimation recrystallization method has been widely known. As a raw material for producing silicon carbide single crystals, polycrystalline silicon carbide is required. Since polycrystalline silicon carbide can also be considered a precursor of silicon carbide single crystals, in this specification, the raw material for producing silicon carbide single crystals may be referred to as a "silicon carbide precursor."

[0003] In order to produce a silicon carbide precursor, it is necessary to prepare a crucible separate from the crucible used to produce the silicon carbide single crystal, and to place the raw material elemental silicon and elemental carbon in the separate crucible and heat and synthesize them.

[0004] Patent Document 1 describes a method of heating a crucible for producing a silicon carbide precursor, which contains elemental silicon and elemental carbon as raw materials, and forming a silicon carbide polycrystalline ingot (silicon carbide precursor) by sublimation recrystallization, and a method of using only the lid of the crucible containing the silicon carbide polycrystalline ingot as part of a crucible for producing a single crystal, and producing a silicon carbide single crystal by sublimation recrystallization. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2012-136391 A Summary of the Invention [Problem to be solved by the invention]

[0006] In the conventional method for producing silicon carbide single crystals, two crucibles are required: one for producing a silicon carbide precursor and the other for producing a silicon carbide single crystal. In addition, a transfer operation is required in which the silicon carbide precursor is taken out of the crucible for producing the silicon carbide precursor and put into the crucible for producing the single crystal. Therefore, this method requires the cost of preparing the two crucibles, and the time and cost required for transferring the raw material and performing two heating operations.

[0007] In the method described in Patent Document 1, it is considered that the work of transferring raw materials can be omitted by sharing some of the parts of the crucible. However, even in the method described in Patent Document 1, it is necessary to return the inside of the heating furnace to room temperature once and rearrange the crucible. Therefore, the method described in Patent Document 1 still requires time and cost for performing two heating steps, a heating step for producing a silicon carbide precursor and a heating step for growing a single crystal. In addition, the method described in Patent Document 1 requires preparation costs for parts of the crucible that are not shared, and time and cost for rearranging the crucible.

[0008] The problem to be solved by the present invention is to simplify a method for producing a single crystal, and to provide a graphite crucible for realizing the simplified method for producing a single crystal. [Means for solving the problem]

[0009] The present invention relates to a method for producing a single crystal, a crucible preparation step of preparing a graphite crucible having a base on which a seed crystal substrate for growing a silicon carbide single crystal is attached and a space for accommodating a raw material; a raw material charging step of charging a carbon raw material and a silicon raw material before thermal synthesis into the raw material storage space; a heating preparation step of closing a lid of the graphite crucible in a state where a graphite sheet is placed so as to separate the raw material accommodation space from a space in which the seed crystal substrate is placed, and preparing the graphite crucible for heating; a heating step of heating the graphite crucible to thermally synthesize a silicon carbide precursor from the carbon raw material and the silicon raw material, and then sublimating the thermally synthesized silicon carbide precursor to grow the silicon carbide single crystal on the seed crystal substrate; The graphite sheet has a thickness that allows the graphite sheet to open up during the heating step.

[0010] In the single crystal manufacturing method, a graphite crucible is used that performs the entire process from the carbon raw material and the silicon raw material before thermal synthesis to the production of a silicon carbide single crystal. In the heating step, both of the following are performed in one graphite crucible: obtaining a silicon carbide precursor by thermal synthesis of the carbon raw material and the silicon raw material; and obtaining the silicon carbide single crystal from the silicon carbide precursor by a sublimation recrystallization method. In the heating step, thermal synthesis is performed until the graphite sheet opens, and after the graphite sheet opens, crystal growth is performed. The thickness of the graphite sheet is set to a thickness that allows the graphite sheet to naturally open by heat during the heating step without applying a mechanical force to the graphite sheet from outside the graphite crucible. Whether or not the thickness is "enough to allow the graphite sheet to open during the heating step" can be confirmed, for example, by heating the graphite crucible in which the graphite sheet is placed at an actual heating temperature for a predetermined time.

[0011] Since only one graphite crucible is used, there is no need to transfer the raw materials, and both the thermal synthesis and the crystal growth can be performed in a single heating step. This reduces the number of steps required to transfer the raw materials, and shortens the time required to produce a single crystal. In addition, only one graphite crucible is required, which also leads to cost reduction.

[0012] The heating step comprises: creating a vacuum in a heating furnace in which the graphite crucible is placed; Prior to the step of creating a vacuum, creating a pressure in the heating furnace higher than the vacuum; The initial stage of the step of creating a pressure higher than a vacuum is a first heating phase in which a silicon carbide precursor is thermally synthesized from the carbon raw material and the silicon raw material. The step of creating a vacuum mainly corresponds to a second heating phase in which the thermally synthesized silicon carbide precursor is sublimated to grow the silicon carbide single crystal. This does not prevent the step of creating a pressure higher than a vacuum from including a second heating phase or the vacuum environment from including a first heating phase.

[0013] The vacuum step may be 10 kPa or less, and the higher pressure step may be 50 kPa or more.

[0014] The graphite sheet may be a flexible graphite sheet.

[0015] The graphite sheet may have a thickness such that 90 wt % or more of the graphite sheet disappears when the heating step is completed.

[0016] The graphite crucible of the present invention is A container having a raw material storage space; a lid having a seed crystal mounting portion for mounting a seed crystal substrate therein; a graphite sheet arranged to separate the raw material accommodation space from a space in which the seed crystal attachment portion is located; Both thermal synthesis for producing a silicon carbide precursor from a carbon raw material and a silicon raw material, and sublimation of the silicon carbide precursor for single crystal growth on the seed crystal substrate are carried out.

[0017] The graphite sheet may be fixed in place by sandwiching the graphite sheet between the container and the lid.

[0018] The container may be closed with the lid by screwing the lid onto the container. Effect of the Invention

[0019] This simplifies the method for producing the single crystal, thereby reducing the time and cost required for producing the silicon carbide single crystal. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a flow diagram of a single crystal manufacturing method. [Figure 2A] FIG. 2 is a cross-sectional view of the graphite crucible after a heating preparation step is completed. [Figure 2B] FIG. 2 is a cross-sectional view of the graphite crucible immediately after the start of a second heating phase of the heating process. [Figure 2C] FIG. 2 is a cross-sectional view of a graphite crucible in which the graphite sheet separating the two spaces has disappeared. [Diagram 3] 1 is a graph showing experimental results regarding the thermal properties of a graphite sheet. [Figure 4] FIG. 4 is a cross-sectional view of a graphite crucible according to a second embodiment. [Diagram 5] FIG. 11 is a cross-sectional view of a graphite crucible according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The following description will be given with reference to the drawings as appropriate. Note that the drawings disclosed in this specification are schematic illustrations, except for graphs and flow diagrams. In other words, the dimensional ratios in the drawings do not necessarily match the actual dimensional ratios, and the dimensional ratios between the drawings do not necessarily match.

[0022] In this specification, the XYZ coordinate system is appropriately referred to for explanation. In this specification, when a direction is expressed and a positive or negative direction is to be distinguished, it is described with a positive or negative sign, such as "+X direction" and "-X direction". In addition, when a direction is expressed without distinguishing between positive and negative directions, it is simply described as "X direction". In other words, in this specification, when it is simply described as "X direction", both "+X direction" and "-X direction" are included. The same applies to the Y direction and the Z direction. In this embodiment, the X direction and the Y direction are horizontal directions, and the -Z direction is the direction of gravity.

[0023] [Outline of single crystal production method] Fig. 1 is a flow diagram of the single crystal manufacturing method. As shown in Fig. 1, the single crystal manufacturing method includes a crucible preparation step S10 for preparing a graphite crucible, a raw material introduction step S20 for introducing raw materials into the prepared graphite crucible, a heating preparation step S30 for preparing the graphite crucible for heating by closing the lid 16 of the graphite crucible with a graphite sheet placed therein after introducing the raw materials, and a heating step S40 for heating the graphite crucible after the heating preparation.

[0024] The raw material input in the raw material input step S20 is a mixture of a carbon raw material and a silicon raw material. The carbon raw material is in the form of small pieces or powder. The carbon raw material may be produced by crushing coke. The average particle size of the carbon raw material is preferably, for example, 10 μm to 200 μm, and more preferably, 50 μm to 150 μm. The silicon raw material may be in the form of small pieces or powder. The silicon raw material may be a crystal of a simple silicon or may be silicon oxide (silica, SiO2). In the case of silicon oxide, it may be quartz sand that has been mined and processed. The average particle size of the silicon raw material is preferably, for example, 0.1 mm to 50 mm, and more preferably, 0.5 mm to 5 mm. The purity of both the carbon raw material and the silicon raw material is preferably high, and a high-purity treatment may be carried out to obtain a carbon raw material or a silicon raw material with a high purity. The purity of carbon in the carbon raw material is preferably 99% or more, and the purity of silicon in the silicon raw material is preferably 99% or more. The higher the purity of the carbon raw material and the silicon raw material, the fewer the impurities that can be contained in the target single crystal.

[0025] It should be noted that the raw materials put into the graphite crucible are in a state before the carbon raw material and the silicon raw material are heated and synthesized. The amounts of the carbon raw material and the silicon raw material put into the graphite crucible should be set so that the amount of C in the carbon raw material and the amount of Si in the silicon raw material are 1 (mol):1 (mol). The carbon raw material and the silicon raw material may be put into the graphite crucible in a mixed state, or the carbon raw material and the silicon raw material may be stirred in the crucible after being put into the graphite crucible. For example, the carbon raw material and the silicon raw material may be put into the crucible in multiple batches, alternately, and then a stirring rod may be inserted into the crucible to stir.

[0026] First Embodiment [Graphite crucible] There are several possible embodiments of the graphite crucible. Here, a graphite crucible according to a first embodiment will be described. FIG. 2A is a cross-sectional view of the graphite crucible 100 after the heating preparation step S30. As shown in FIG. 2A, the graphite crucible 100 has a container 12 and a lid 16 fitted to the container 12. Both the container 12 and the lid 16 are mainly composed of graphite. In this specification, the term "main component" refers to a carbon content of the graphite that is 90 wt% or more of the total mass. The carbon content of the container 12 and the lid 16 may be different.

[0027] The container 12 includes a bottom and a sidewall extending upward from the periphery of the bottom of the container 12. In this embodiment, the bottom surface of the container 12 is circular, and the sidewall is cylindrical. The tip 12a of the sidewall is configured to be thin. Therefore, the container 12 as a whole has a hollow cylindrical shape with an open upper end. The hollow portion is a raw material storage space for storing raw material 1.

[0028] The lid 16 closes the opening at the top end of the released container 12. The lid 16 of this embodiment includes a top plate of the lid 16 and a side wall extending downward from the main extension of the top plate. In this embodiment, the top plate of the lid 16 is circular, and the side wall is cylindrical. The bottom surface of the container 12 and the top plate of the lid 16 do not have to be circular. The bottom surface of the container 12 and the top plate of the lid 16 may be, for example, rectangular or hexagonal. The tip 16a of the side wall of the lid 16 is configured to be fitted with the tip 12a of the side wall of the container 12. The portion where both tips (12a, 16a) come into contact is threaded. The lid 16 of the graphite crucible 100 is closed by fitting the lid 16 to the container 12 as if the lid 16 is screwed into the container 12. Thus, the inside and the outside of the graphite crucible 100 are separated.

[0029] The thickness of the side wall of the container 12 is preferably 2 mm to 50 mm, more preferably 5 mm to 30 mm, and more preferably 10 mm to 20 mm. The thickness of the bottom of the container 12 is preferably 5 mm to 40 mm. The thicker the side wall or bottom of the container 12, the greater the amount of heat transferred per unit time. However, if the side wall or bottom of the container 12 is too thick, the volume inside the graphite crucible 100 will be reduced by the thickness, limiting the amount of raw material 1 to be charged or causing the graphite crucible 100 to become larger. For this reason, the above-mentioned preferable upper limit value is set.

[0030] The inside of the top plate of the lid 16 has a pedestal 17 for mounting a seed crystal substrate 19. The graphite crucible 100 shown in Fig. 2A is shown in a state in which the raw material 1 has already been charged and the seed crystal substrate 19 has already been mounted on the pedestal 17, but the raw material 1 and the seed crystal substrate 19 are not components of the graphite crucible 100 itself.

[0031] 2A, the graphite crucible 100 further includes a graphite sheet 11. Details of the graphite sheet 11 will be described later. In a crucible preparation step S10, a graphite crucible is prepared, in a raw material introduction step S20, raw material 1 is introduced into the prepared graphite crucible 100, and in a heating preparation step S30, the graphite sheet 11 is placed in the graphite crucible 100 into which the raw material 1 has been introduced. Then, in the heating preparation step S30, the lid 16 of the graphite crucible 100 is closed with the graphite sheet 11 placed therein.

[0032] [Details of the heating process] The heating step S40 is performed by placing the graphite crucible 100 after the heating preparation step S30 in a heating furnace and heating the graphite crucible 100. The heating furnace may be a resistance heating furnace, a high-frequency induction heating furnace, or a heating furnace of another heating method. The heating temperature is preferably 1850°C or more and 2500°C or less, and more preferably 2200°C or more and 2400°C or less.

[0033] The heating step S40 includes a first heating phase and a second heating phase that starts after the first heating phase. In the first heating phase, a carbon raw material and a silicon raw material are thermally synthesized to obtain a silicon carbide precursor. In the second heating phase, the sublimated gas of the thermally synthesized silicon carbide precursor is recrystallized on the seed crystal substrate 19 to grow a silicon carbide single crystal.

[0034] During the first heating phase, the graphite crucible 100 is in the state shown in FIG. 2A, that is, the graphite sheet 11 is arranged to separate the raw material storage space in which the raw material 1 is stored from the space in which the seed crystal substrate 19 is arranged. Therefore, the sublimation gas of the hydrocarbon is prevented by the graphite sheet 11 from moving to the space in which the seed crystal substrate 19 is located. Therefore, in the graphite crucible 100 in the first heating phase, the growth of silicon carbide single crystals on the seed crystal substrate 19 is not performed, and only the thermal synthesis of silicon carbide precursors from carbon raw materials and silicon raw materials is performed. When the temperature of the raw material 1 rises, first, silicon, which has a melting point lower than that of carbon, melts around the particulate carbon, and then the silicon is impregnated into the carbon, and the silicon carbide forms a polycrystalline state. The thermally synthesized silicon carbide precursor is an aggregate of polycrystalline particles of silicon carbide. Note that the thermal synthesis is not necessarily performed throughout the entire first heating phase. In some cases, the thermal synthesis may be completed during the first heating phase.

[0035] Fig. 2B is a cross-sectional view of graphite crucible 100 immediately after the start of the second heating phase. Fig. 2B shows graphite sheet 11 with hole H1 in the center. Graphite sheet 11 is set to a thickness that will open when heated for a certain period of time. Graphite sheet 11 may be set to a thickness such that 90 wt% or more of graphite sheet 11 disappears at the end of heating step S40 (the end of the second heating phase).

[0036] The arrow indicated by G1 in Fig. 2B indicates the direction in which the sublimation gas of silicon carbide flows. The sublimation gas flows from the hole H1 in the direction of G1, that is, from the raw material storage space to the space in which the seed crystal substrate 19 is disposed, and a single crystal begins to grow on the seed crystal substrate 19. When the graphite crucible 100 is in the state shown in Fig. 2B, a silicon carbide single crystal grows in the graphite crucible 100. Note that when Fig. 2B begins (when a hole is formed in the graphite sheet 11), the thermal synthesis process of the silicon carbide precursor is completed.

[0037] When the graphite sheet 11 is further heated, the hole H1 expands to the periphery, and finally, as shown in FIG. 2C, the graphite sheet 11 between the raw material storage space and the space in which the seed crystal substrate 19 is disposed disappears. When the graphite sheet 11 disappears, the graphite sheet 11 no longer blocks the flow direction G1 of the sublimation gas of silicon carbide, as shown in FIG. 2C. The flow of the sublimation gas is unstable from when the hole H1 is formed in the graphite sheet 11 until the graphite sheet 11 disappears. In order to stabilize the flow of the sublimation gas, it is preferable that the time from when the hole H1 is formed in the graphite sheet 11 until the graphite sheet 11 disappears is short. In order to shorten such time, for example, a process that locally reduces the strength of the graphite sheet 11, such as a scribing process, may be performed on the surface of the graphite sheet 11.

[0038] As described above, by arranging graphite sheet 11, which prevents the flow of sublimation gas, so as to separate the raw material storage space from the space in which seed crystal substrate 19 is disposed, two manufacturing processes, namely, generation of a silicon carbide precursor by thermal synthesis of raw material 1 and growth of a silicon carbide single crystal, can be consistently performed in one graphite crucible 100. Furthermore, in the first heating phase, impurities (substances containing carbon atoms or atoms other than silicon) contained in raw material 1 and sublimation gas of silicon alone may appear, and since graphite sheet 11 prevents the adhesion of such impurities and silicon to seed crystal substrate 19, arranging graphite sheet 11, which prevents the flow of sublimation gas, is also effective.

[0039] The heating furnace in which the graphite crucible 100 is placed may include a step of evacuating the inside of the heating furnace and, prior to this step, a step of evacuating the inside of the heating furnace to a pressure higher than that in the step of evacuating. The vacuum in the step of evacuating is preferably 30 kPa or less, more preferably 10 kPa or less.

[0040] In the second heating phase for the purpose of growing a single crystal, it is preferable to set the inside of the heating furnace to a vacuum. Since the graphite crucible 100 itself has a porous structure, impurities that do not contribute to the growth of a single crystal can be intentionally allowed to leak out of the graphite crucible 100 from the inside of the graphite crucible 100 by placing the graphite crucible 100 in a vacuum environment.

[0041] In contrast, in the first heating phase aimed at thermal synthesis, the environment does not necessarily have to be a vacuum environment. It may be a pressure environment higher than vacuum. In the "higher pressure environment than vacuum", the pressure of the environment is preferably 50 kPa or more, more preferably 70 kPa or more. In the first heating phase aimed at thermal synthesis, the pressure of the environment is preferably atmospheric pressure (about 100 kPa) or less, more preferably 90 kPa or less. Thus, in the first heating phase aimed at thermal synthesis, the pressure of the environment is preferably 50 kPa or more and 100 kPa or less, more preferably 70 kPa or more and 90 kPa or less. The gas in the heating furnace may be purged with an inert gas. The inert gas is preferably nitrogen or argon.

[0042] The advantages of the single crystal manufacturing method disclosed in this embodiment over the conventional single crystal manufacturing method will be summarized and explained. As described in the "Background Art" section, conventionally, in order to manufacture a single crystal, two crucibles were required: a crucible for manufacturing a silicon carbide precursor and a crucible for manufacturing a silicon carbide single crystal. When using two crucibles, it was necessary to return the heating furnace to room temperature once, remove the crucible from the heating furnace, crush and remove the silicon carbide precursor in the crucible using a tool, put the silicon carbide precursor into a crucible for manufacturing a single crystal, put the crucible for manufacturing a single crystal into a heating furnace, and heat it again. In contrast, in this embodiment, a silicon carbide precursor can be manufactured and a single crystal can be manufactured in one crucible. Therefore, it is not necessary to transfer the raw material of the silicon carbide precursor to the crucible for manufacturing a single crystal. When a tool is used to crush the silicon carbide precursor for transfer, there is a risk that the metal components of the tool may be mixed into the silicon carbide precursor, but since no tool is used in this embodiment, the metal components of the tool do not mix with the silicon carbide precursor. Furthermore, when the crucible of this embodiment is used, the silicon carbide precursor and the silicon carbide single crystal can be produced in one heating step, and it is not necessary to return the crucible to room temperature between the production of the silicon carbide precursor and the production of the single crystal. Furthermore, the graphite sheet 11 has high thermal conductivity and has a heat retention effect. Therefore, when the graphite sheet 11 covers the upper part of the raw material 1, which is particularly at a low temperature, the temperature uniformity of the raw material 1 in the crucible is improved. In this way, various effects can be obtained by using the crucible of this embodiment.

[0043] [Graphite sheet details] Graphite sheet 11 is a flexible sheet manufactured by expanding acid-treated natural graphite and compressing it into a sheet. Graphite sheet 11 is made of carbon. Graphite sheet 11 is highly airtight and does not allow the passage of sublimation gas composed of at least one of carbon and silicon. As described above, graphite sheet 11 disappears when heated and becomes integrated with silicon carbide, but since carbon, which is a component of graphite sheet 11, is also a component of silicon carbide, even if graphite sheet 11 disappears, no impurities are generated and the growth of the single crystal is not affected.

[0044] As described above, graphite crucible 100 is closed so that lid 16 fits into container 12. In this embodiment, graphite sheet 11 is arranged and fixed such that graphite sheet 11 is sandwiched between container 12 and lid 16. In particular, in this embodiment, tip 16a of the side wall of lid 16 is threaded at a portion where it abuts tip 12a of the side wall of container 12, and graphite sheet 11 fits between the thread and the screw groove, so that graphite sheet 11 can be arranged with reduced slack.

[0045] Fig. 3 is a graph showing the experimental results regarding the thermal properties of graphite sheet 11. This graph shows the relationship between the heating time (unit: hour) of graphite crucible 100 in which graphite sheet 11 is placed as in Fig. 2A and the thickness (unit: mm) of graphite sheet 11. This experimental result was obtained under the following experimental conditions. Graphite crucible heating temperature: 2350℃ Graphite sheet 11 used: Nikafilm (2.0 mm thick) manufactured by Nippon Carbon Co., Ltd. Thickness measurement method: After heating at the above-mentioned set heating temperature for 1 hour, 2 hours, 4 hours, or 10 hours, graphite sheet 11 was removed from graphite crucible 100, and the thickness of graphite sheet 11 was measured at multiple points with a micrometer, and the thickness of the thinnest part was recorded as the "thickness" shown on the vertical axis of the graph.

[0046] The graph in FIG. 3 shows that as the heating time (cumulative heating time) increases, graphite sheet 11 gradually becomes thinner, and when the heating time reaches 10 hours, holes are formed in graphite sheet 11. Considering the graph in FIG. 3 and the fact that the heating synthesis time of raw material 1 does not require 10 hours, the thickness of graphite sheet 11 is preferably 2.0 mm or less, and more preferably 1.5 mm or less. In particular, and taking into consideration the mechanical strength and the like of graphite sheet 11, the thickness of graphite sheet 11 is preferably 0.3 mm or more, and more preferably 0.5 mm or more. The thickness of graphite sheet 11 may be set in consideration of the graph showing the relationship between the heating time and the desired thickness of graphite sheet 11 as shown in FIG. 3, and the heating synthesis time of raw material 1 or the time from the start of heating to the start of single crystal growth.

[0047] Second Embodiment FIG. 4 is a diagram showing a graphite crucible of the second embodiment. Except for the matters described below, the embodiment can be carried out in the same manner as the above-mentioned embodiment. In the graphite crucible 200 of this embodiment, the end 21 of the graphite sheet 11 is attached and fixed to the inner wall of the container 12. An adhesive is used to attach the end 21 of the graphite sheet 11 to the inner wall. It is preferable to use an adhesive that does not contain metal elements (except silicon). It is preferable to use an organic adhesive as the adhesive. Before the graphite crucible 200 is heated, the adhesive bonds the end 21 of the graphite sheet 11 to the inner wall of the container 12, but when the graphite crucible 200 is heated, the adhesive component is decomposed or oxidized, and is unlikely to remain in the graphite crucible 200 as an impurity. On the other hand, although the adhesive component is lost during the heating process, the graphite sheet 11 is fixed to the inner wall of the container 12 by heat. Therefore, the end 21 of the graphite sheet 11 maintains a state of being fixed to the inner wall. In this embodiment, since it is not necessary to place the graphite sheet 11 at the boundary between the container 12 and the lid 16, the position of the graphite sheet 11 (the distance from the raw material 1) can be adjusted to a desired value.

[0048] <Third embodiment> 5 is a diagram showing a graphite crucible of the third embodiment. Except for the matters described below, the third embodiment can be carried out in the same manner as the above-mentioned embodiments. In the graphite crucible 300 of this embodiment, another method of fixing the graphite sheet 11 is adopted. The container 12 has a recess 12b on the inner wall of the container 12. The graphite sheet 11 is inserted into the recess 12b, and a plug 22 for closing the recess 12b is inserted. The graphite sheet 11 is sandwiched and fixed between the container 12 and the plug 22.

[0049] Although the first to third embodiments have been described above, the present invention is not limited to the above-mentioned embodiments and their modified examples, and various improvements and modifications are possible within the scope of the present invention. For example, a plurality of graphite sheets 11 may be stacked and arranged. The stacked graphite sheets 11 may be arranged so as to be in contact with each other, or may be arranged at intervals from each other. EXAMPLES

[0050] An example of a method for producing a single crystal is described below.

[0051] [Crucible preparation process S10] A graphite crucible 100 was prepared, which had a container 12 for accommodating a raw material 1 and a lid 16 having a seat 17 for attaching a seed crystal substrate 19 to its inner surface. A seed crystal substrate 19 was attached to the lid 16 of the prepared graphite crucible 100.

[0052] [Raw material input process S20] The carbon raw material and the silicon raw material were alternately charged into the container 12 of the prepared graphite crucible 100 in three separate portions. After that, a stirring rod was inserted into the raw material 1 in the container 12 and stirred to obtain a mixture of the carbon raw material and the silicon raw material. The amounts of the carbon raw material and the silicon raw material charged were set so that the substance amount of C in the carbon raw material and the substance amount of Si in the silicon raw material were 1 (mol):1 (mol). The carbon raw material was carbon powder with an average particle size of 100 μm. The carbon powder used was highly purified to a purity of 99.99% or more. The silicon raw material was silicon chips with an average particle size of 1 mm. The silicon chips used were highly purified to a purity of 99.999% or more.

[0053] [Heating preparation process S30] 2A, graphite crucible 100 in which graphite sheet 11 was placed was prepared by fitting lid 16 to container 12 so as to sandwich graphite sheet 11 between container 12 and lid 16. Nikafilm (thickness: 1.0 mm) manufactured by Nippon Carbon Co., Ltd. was used as graphite sheet 11.

[0054] [Heating process S40] The graphite crucible 100 prepared for heating was placed in a resistance heating furnace, and the graphite crucible was heated. From the graph in FIG. 3, it was found that the thickness of the Nikafilm manufactured by Nippon Carbon Co., Ltd. was reduced by 1.0 mm over two hours, so it was assumed that the graphite sheet 11 of the Nikafilm (1.0 mm thick) manufactured by Nippon Carbon Co., Ltd. would open two hours after the start of heating. Therefore, the first heating phase was set to the period from the start of heating until two hours had elapsed, and the second heating phase was set to the period from the end of the first heating phase (after two hours had elapsed) until 120 hours had elapsed. In the first heating phase, the heating furnace was heated to 2350° C., and the inside of the furnace was set to an 80 kPa atmosphere filled with argon gas. In the second heating phase, the temperature of the heating furnace was kept at 2350° C., and the degree of vacuum in the furnace was improved to an atmosphere of 0.3 kPa. After 120 hours from the start of heating, the atmosphere was returned to 80 kPa filled with argon gas, and heating of the heating furnace was stopped. When graphite crucible 100 was opened after completion of heating step S40, graphite sheet 11 was no longer remaining in the space of graphite crucible 100, and a silicon carbide single crystal was grown on seed crystal substrate 19. [Explanation of symbols]

[0055] 1: Raw materials 10: Crucible 11: Graphite sheet 12: Container 12a: (of a container) tip 12b: (container) depression 16: Lid 16a: (lid) tip 17: Pedestal 19: Seed crystal substrate 21: Edge (of graphite sheet) 22: Plug 100, 200, 300: Graphite crucible H1: Hole (in graphite sheet) S10: Crucible preparation process S20: Raw material input process S30: Heating preparation process S40:Heating process

Claims

1. a crucible preparation step of preparing a graphite crucible having a base on which a seed crystal substrate for growing a silicon carbide single crystal is attached and a space for accommodating a raw material; a raw material introduction step of introducing a carbon raw material and a silicon raw material before thermal synthesis into the raw material storage space; a heating preparation step of closing a lid of the graphite crucible in a state where a graphite sheet is placed so as to separate the raw material accommodation space from a space in which the seed crystal substrate is placed, and preparing the graphite crucible for heating; a heating step of heating the graphite crucible to thermally synthesize a silicon carbide precursor from the carbon raw material and the silicon raw material, and then sublimating the thermally synthesized silicon carbide precursor to grow the silicon carbide single crystal on the seed crystal substrate; The method for producing a single crystal, wherein the thickness of the graphite sheet is such that the graphite sheet opens during the heating step.

2. The heating step comprises: creating a vacuum in a heating furnace in which the graphite crucible is placed; Prior to the step of creating a vacuum, creating a pressure in the heating furnace higher than the vacuum; The method for producing a single crystal according to claim 1, comprising:

3. 3. The method for producing a single crystal according to claim 2, wherein the step of creating a vacuum is 10 kPa or less, and the step of creating a pressure higher than the vacuum is 50 kPa or more.

4. The method for producing a single crystal according to any one of claims 1 to 3, wherein the graphite sheet is a flexible graphite sheet.

5. The method for producing a single crystal according to any one of claims 1 to 3, characterized in that the thickness of the graphite sheet is such that 90 wt % or more of the graphite sheet disappears at the end of the heating step.

6. A container having a raw material storage space; a lid having a seed crystal mounting portion for mounting a seed crystal substrate therein; a graphite sheet arranged to separate the raw material accommodation space from a space in which the seed crystal attachment portion is located; A graphite crucible, characterized in that it is capable of performing both a thermal synthesis for producing a silicon carbide precursor from a carbon raw material and a silicon raw material, and a single crystal growth on the seed crystal substrate by sublimating the silicon carbide precursor.

7. 7. The graphite crucible according to claim 6, wherein the graphite sheet is fixed by sandwiching the graphite sheet between the container and the lid.

8. 8. The graphite crucible according to claim 7, characterized in that the vessel is closed with the lid by screwing the lid onto the vessel.

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