Abstract
The invention relates to a crystal growing unit comprising a crucible for producing and/or enlarging a single crystal. The crystal growing unit has a first thermal insulation with a first thermal conductivity and a second thermal insulation with a second thermal conductivity. The crucible has a crucible base, a crucible side wall and a crucible cover. The crucible side wall is indirectly or directly surrounded by the first thermal insulation. The second thermal insulation is arranged indirectly or directly above the crucible cover. The second thermal conductivity is greater than the first thermal conductivity.
Claims
1. A crystal growing unit comprising a crucible for producing and/or enlarging a single crystal, wherein the crystal growing unit has a first thermal insulation with a first thermal conductivity and a second thermal insulation with a second thermal conductivity, wherein the crucible has a crucible base, a crucible side wall and a crucible cover, wherein the crucible side wall is indirectly or directly surrounded by the first thermal insulation, wherein the second thermal insulation is arranged indirectly or directly above the crucible cover, wherein the second thermal conductivity is greater than the first thermal conductivity, and wherein the second thermal conductivity lies in a range of from 2 to 50 W/(m*K).
2. The crystal growing unit according to claim 1, wherein a source material provided in the crucible can be heated, evaporated and deposited, and wherein the source material comprises SiC.
3. The crystal growing unit according to claim 1, wherein the first thermal insulation is arranged indirectly or directly below the crucible base.
4. The crystal growing unit according to claim 1, wherein the first thermal conductivity lies in a range of from 0.05 to 5 W/(m*K), or the second thermal conductivity lies in a range of from 5 to 20 W/(m*K).
5. The crystal growing unit according to claim 1, wherein the crystal growing unit comprises a cavity arranged between the crucible cover and the second thermal insulation.
6. The crystal growing unit according to claim 5, wherein at least one of (1) the surface of the first thermal insulation adjoining the cavity has a predetermined first emissivity (ε), (2) the surface of the second thermal insulation adjoining the cavity has a predetermined second emissivity (ε), or (3) the surface of the crucible cover adjoining the cavity has a predetermined third emissivity (ε); and wherein at least one of the first, second or third emissivity (ε) is set in a range of between 0.05 and 0.5.
7. The crystal growing unit according to claim 5, wherein at least one of the following has a predetermined relief: (1) the surface of the crucible cover adjoining the cavity, (2) the surface of the first thermal insulation adjoining the cavity, or (3) the surface of the second thermal insulation adjoining the cavity.
8. The crystal growing unit according to claim 1, wherein the single crystal is arranged with the aid of a nucleus suspension device, and wherein the crystal growing unit comprises a nucleus cavity arranged within the crucible between the single crystal and the crucible cover.
9. The crystal growing unit according to claim 8, wherein at least one of (1) the surface of the nucleus suspension device adjoining the nucleus cavity has a predetermined fourth emissivity (ε), (2) the surface of the crucible cover adjoining the nucleus cavity has a predetermined fifth emissivity (ε), or (3) the surface of the single crystal adjoining the nucleus cavity has a predetermined sixth emissivity (ε).
10. The crystal growing unit according to claim 8, wherein at least one of the following has a predetermined relief: (1) the surface of the crucible cover adjoining the nucleus cavity, (2) the surface of the nucleus suspension device adjoining the nucleus cavity, or (3) the surface of the single crystal adjoining the nucleus cavity.
11. The crystal growing unit according to claim 8, wherein the nucleus cavity is filled with a solid material, wherein the solid material is at least one of (1) SiC powder, (2) a polycrystalline or monocrystalline SiC crystal, or (3) porous or solid graphite.
12. The crystal growing unit according to claim 1, wherein the crystal growing unit comprises a heating device for heating the crucible, wherein the heating device comprises at least one of an induction-heating unit or a resistance-heating unit.
13. The crystal growing unit according to claim 12, wherein the heating device is arranged between the crucible base and the first thermal insulation or between the crucible side wall and the first thermal insulation.
14. The crystal growing unit according to claim 1, wherein the crystal growing unit comprises at least one of (1) a first pyrometer access, or (2) a second pyrometer access, and wherein (1) the first pyrometer access penetrates the second thermal insulation up to the crucible cover, (2) the second pyrometer access (15) penetrates the first thermal insulation, or (3) the second pyrometer access penetrates the heating device up to the crucible base.
15. The crystal growing unit according to claim 1, wherein the first thermal insulation is arranged indirectly or directly above a radially outer annular surface of the crucible cover.
16. The crystal growing unit according to claim 1, wherein at least one of the crucible base, the crucible side wall, or the crucible cover of the crucible is formed of at least one of (1) graphite, (2) TaC, or (3) coated graphite.
17. The crystal growing unit according to claim 1, wherein the source material in the crucible can, depending on temperature gradients, be at least one of (1) evaporated, (2) transported, or (3) deposited, and wherein the temperature gradients can be set in a targeted manner in the crucible.
18. The crystal growing unit according to claim 1, wherein the crystal growing unit is formed for the targeted setting of temperature gradients in the crucible, wherein the temperature gradients can be set through the design of at least one of the first thermal insulation or the second thermal insulation in such a way that the isotherms have a convex progression.
19. The crystal growing unit according to claim 1, wherein the temperature gradients in the crucible can be set by the heating device.
20. The crystal growing unit according to claim 1, wherein at least one of (1) the first thermal insulation comprises graphite felt or graphite foam or (2) the second thermal insulation comprises graphite foam or porous graphite.
21. The crystal growing unit according to claim 1, wherein the second thermal insulation is formed of a series of several sheets spaced apart from each other in each case.
22. The crystal growing unit according to claim 21, wherein the second thermal insulation is formed of from two to ten sheets.
23. The crystal growing unit according to claim 21, wherein the sheets are formed of at least one of graphite, coated graphite, or metal carbide.
24. The crystal growing unit according to claim 21, wherein the sheets in each case have a thickness of between 0.1 and 10 mm.
25. The crystal growing unit according to claim 21, wherein successive sheets in each case have a spacing in the range of from 1 to 50 mm.
26. The crystal growing unit according to claim 21, wherein the sheets have, at their surfaces, an emissivity of at most 0.4 or an emissivity of at least 0.6.
27. The crystal growing unit according to claim 26, wherein the emissivities of successive sheets differ.
28. The crystal growing unit according to claim 21, wherein the sheets in each case have several elongate incisions.
29. A method for producing and/or enlarging a single crystal by heating, evaporating and depositing a source material in a crucible of a crystal growing unit, wherein the source material is, depending on temperature gradients, at least one of (1) evaporated, (2) transported, or (3) deposited, wherein the temperature gradients are set in a targeted manner, wherein the crucible has a crucible base, a crucible side wall and a crucible cover, wherein the crucible side wall is indirectly or directly surrounded by a first thermal insulation with a first thermal conductivity, wherein a second thermal insulation with a second thermal conductivity is arranged indirectly or directly above the crucible cover, wherein the second thermal conductivity is greater than the first thermal conductivity, wherein the second thermal conductivity lies in a range of from 2 to 50 W/(m*K), and wherein the temperature gradients are set through the design of at least one of the first thermal insulation or the second thermal insulation in such a way that the isotherms have a convex progression.
Description
[0121] The invention will now be explained in more detail with reference to embodiment examples. There are shown in:
[0122] FIG. 1A a schematic representation of a crucible according to the state of the art, a schematic representation of a crystal growing unit with crucible according to the state of the art,
[0123] FIG. 2 a schematic representation of a first crystal growing unit according to the invention with crucible,
[0124] FIG. 3A a schematic representation of a second crystal growing unit according to the invention with crucible,
[0125] FIG. 3B a schematic representation of a third crystal growing unit according to the invention with crucible,
[0126] FIG. 3C a schematic representation of a fourth crystal growing unit according to the invention with crucible,
[0127] FIG. 3D a schematic representation of a fifth crystal growing unit according to the invention with crucible,
[0128] FIG. 3E a schematic representation of a sixth crystal growing unit according to the invention with crucible,
[0129] FIG. 3F a schematic representation of a seventh crystal growing unit according to the invention with crucible,
[0130] FIG. 4 a schematic representation of an eighth crystal growing unit according to the invention with crucible,
[0131] FIG. 5A a schematic representation of a ninth crystal growing unit according to the invention with crucible,
[0132] FIG. 5B a schematic representation of a tenth crystal growing unit according to the invention with crucible,
[0133] FIG. 6 a schematic representation of an eleventh crystal growing unit according to the invention with crucible,
[0134] FIGS. 7A-7C enlarged detail views with various embodiments of the eleventh crystal growing unit according to the invention,
[0135] FIG. 8 a schematic three-dimensional view of a second thermal insulation as a series of five sheets,
[0136] FIG. 9 a central two-dimensional section through the series, shown in FIG. 8, of five sheets in a first embodiment,
[0137] FIG. 10A a central two-dimensional section through the series, shown in FIG. 8, of five sheets in a second embodiment,
[0138] FIG. 10B a central two-dimensional section through the series, shown in FIG. 8, of five sheets in a third embodiment, and
[0139] FIG. 11 schematic representations of sheets provided with elongate incisions.
[0140] FIG. 1A shows a schematic representation of a crucible according to the state of the art. The cylindrical crucible has a crucible wall 1. The crucible wall 1 is divided into a crucible base, a crucible side wall and a crucible cover. A source material 2, a gas space 3 and a single crystal 4 are accommodated in the interior of the crucible. The source material 2 and the single crystal 4 are arranged at the two axially opposite ends of the interior of the crucible and separated from each other by the gas space 3. The source material 2 has a substantially cylindrical shape. The single crystal 4 has the shape of a cylinder convexly rounded on one side. The source material 2 is for example a silicon carbide powder (written as a formula SiC). A single crystal made of SiC is correspondingly produced as single crystal 4.
[0141] To enlarge the single crystal 4, the crucible is heated such that source material 2 passes into the gas phase by sublimation, is transported through the gas space as gaseous precursor material and crystallizes out on the single crystal 4.
[0142] The progression of the temperature T in the z-direction, i.e. in the axial direction of the crucible, is schematically set against the crucible. The temperature at the boundary surface of the source material 2 adjoining the gas space 3 is T.sub.1. The boundary surface of the source material 2 is preferably flat or substantially flat. The temperature along the boundary surface of the source material 2 is preferably constant. The boundary surface of the source material 2 thus preferably lies on an isotherm with the temperature T.sub.1. The temperature T.sub.1 is set sufficiently high that a sublimation of source material 2 occurs. The temperature at the boundary surface of the single crystal 4 adjoining the gas space 3 is T.sub.2. This boundary surface of the single crystal 4 has a convex shape and can also be referred to as growth boundary surface. The temperature along the convex growth boundary surface is preferably constant. The growth boundary surface is thus preferably formed along an isotherm with the temperature T.sub.2. The temperature T.sub.2 is lower than the temperature T.sub.1. An axial temperature gradient is thus formed between the source material 2 and the single crystal 4. The temperature T.sub.2 is set such that a supersaturation of the gaseous precursor material, in particular a supersaturation of the growth species, and consequently a crystallization on the single crystal 4 occurs. The source material 2 is continuously removed by the sublimation. The single crystal 4 is continuously enlarged by the crystallization. The growth boundary surface preferably constantly forms along an isotherm in the process.
[0143] FIG. 1B shows a schematic representation of a crystal growing unit with crucible according to the state of the art. The crystal growing unit has a thermal insulation 5 and an induction-heating unit 6. The thermal insulation 5 surrounds the crucible up to an opening provided in the area of the crucible cover. This opening has the function of a radiating channel via which heat is transported away from the crucible upwards. The heat is preferably transported away upwards via thermal radiation. Thermal radiation is the heat transport mechanism which plays a role at temperatures from 500° C. and dominates the heat transport in (partially) transparent media at temperatures >1000° C. By transporting the heat away through the radiating channel, an axial temperature gradient is provided in the crucible.
[0144] The radiating channel brings about a concentration of the heat flow 8. A significant radial component of the heat flow 8 is thereby generated within the crucible. The temperature gradients in the crucible correspondingly have a strong radial component. The isotherms 7 therefore have a pronounced convex shape, viewed from the source material 2. As the growth boundary surface of the growing single crystal 4 forms along an isotherm 7, as described above, a highly convex single crystal is thus formed.
[0145] FIGS. 2 to 6 in each case show schematic representations of a first to eleventh crystal growing unit according to the invention comprising a crucible. In all embodiments of the invention, the crystal growing unit and the crucible in each case have all described elements and properties of the crystal growing unit and the crucible from FIG. 1. In addition, the crystal growing unit in each case has both a first thermal insulation 5 with a first thermal conductivity and a second thermal insulation 12 with a second thermal conductivity. The first thermal conductivity is in each case less than the second thermal conductivity. The first thermal insulation 5 consists of a first high-temperature insulation material, for example a graphite felt and/or graphite foam. The first thermal insulation 5 is a high thermal insulation. The second thermal insulation 12 consists of a second high-temperature insulation material, for example a graphite foam and/or porous graphite. The second thermal insulation 12 is a medium-high thermal insulation. The first thermal conductivity is correspondingly low and is for example 0.5 W/(m*K). The second thermal conductivity is medium and is for example 10 W/(m*K).
[0146] In the embodiment examples, the crucible side wall is indirectly or directly completely surrounded by the first thermal insulation. The first thermal insulation is in each case formed as a hollow cylinder or substantially as a hollow cylinder. The second thermal insulation is arranged indirectly or directly above the crucible cover. The second thermal insulation is in each case formed as a solid cylinder and is in each case radially completely surrounded by the first thermal insulation.
[0147] FIG. 2 shows a schematic representation of a first crystal growing unit according to the invention with crucible. The basic principle of the invention is explained with reference to FIG. 2. The beaker-shaped first thermal insulation 5 encloses a lower heat source 11 arranged over the whole surface below the crucible base of the crucible, the crucible as well as the second thermal insulation 12 arranged over the whole surface above the crucible cover.
[0148] By arranging the lower heat source 11 over the whole surface directly below the crucible base of the crucible, by providing a negligibly low thermal conductivity of the first thermal insulation 5 and by arranging the second thermal insulation 12 with medium-high thermal insulation over the whole surface directly above the crucible cover, the heat flow 8 is guided in an axial direction through the crucible and through the growing single crystal 4. In an ideal case, by disregarding the low thermal conductivity of the first thermal insulation 5 an exclusively axial heat flow 8 from bottom to top is realized. The isotherms 7 are horizontal in this ideal case. As the growth boundary surface of the growing single crystal 4 forms along an isotherm 7, as described above, a flat single crystal is thus formed.
[0149] If a first thermal insulation 5 with an actual, high (highest possible) thermal insulation is provided, the transporting of the heat away from the crucible gains a small radial component laterally outwards. As a result, slightly concave isotherms 7 are formed at the crystal growth front, viewed from the source material 2. The growth boundary surface, forming along an isotherm 7, of the growing single crystal 4 thus also adopts a slightly concave shape. However, this results in the massive incorporation of crystal defects. By at least partially heating the crucible side wall, the above-mentioned transporting of the heat away from the crucible being effected laterally outwards can be overcompensated. As a result, the slightly concave formation of the growth boundary surface and the negative effect of the incorporation of crystal defects for crystal growth can be prevented. Therefore, in the case of the second to eleventh crystal growing unit according to the invention in FIGS. 3 to 6, a lateral heat source 6, 9, 13 is provided in each case. As a result, in the area of the growing single crystal 4, a defined axial temperature gradient at the same time as the smallest possible radial temperature gradient is advantageously provided.
[0150] FIG. 3A shows a schematic representation of a second crystal growing unit according to the invention. The beaker-shaped first thermal insulation 5 encloses the crucible as well as the second thermal insulation 12 arranged over the whole surface above the crucible cover. The second crystal growing unit according to the invention has a lateral heat source in the form of an induction-heating unit 6. The induction power generated by means of the induction-heating unit 6 is absorbed in the crucible side wall. For this, the crucible is formed of a conductive material, e.g. graphite. The induction-heating unit 6 thus brings about a heating of the crucible side wall by means of induction. The heating of the crucible side wall generates a heat flow 8 with a radial component from the crucible side wall into the crucible interior. By providing the second thermal insulation 12, the heat flow 8 is guided, in the crucible interior, in an axial direction through the growing single crystal 4. As a result, the isotherms 7 at the crystal growth front are slightly convex, viewed from the source material 2. The growth boundary surface of the growing single crystal 4 forming along an isotherm 7 thus also adopts a slightly convex shape. As a result, favorable conditions for the growth of the single crystal 4 prevail.
[0151] FIG. 3B shows a schematic representation of a third crystal growing unit according to the invention. In addition to the second crystal growing unit according to the invention, a susceptor 31 with a material-free area 32 is arranged between the crucible side wall and the first thermal insulation 5. The susceptor 31 serves for the primary absorption of the induction power generated by means of the induction-heating unit 6. The susceptor 31 is formed, for example, of graphite. The material-free area 32 contains a vacuum or a gas, for example. By providing the susceptor 31, the absorption of the induction power is improved.
[0152] FIG. 3C shows a schematic representation of a fourth crystal growing unit according to the invention. The fourth crystal growing unit according to the invention has a lateral heat source in the form of a resistance-heating unit 9. The resistance-heating unit 9 has a material-free area 32 and surrounds the crucible side wall. In the example shown, the resistance-heating unit 9 completely surrounds the crucible side wall. The beaker-shaped first thermal insulation 5 encloses the crucible together with the resistance-heating unit 9 as well as the second thermal insulation 12 arranged over the whole surface above the crucible cover. The resistance-heating unit 9 brings about a heating of the crucible side wall. The heating of the crucible side wall generates a heat flow 8 with a radial component from the crucible side wall into the crucible interior. As already explained above, by providing the second thermal insulation 12, the heat flow 8 is guided, in the crucible interior, in an axial direction through the growing single crystal 4. As a result, the isotherms 7 at the crystal growth front are slightly convex, viewed from the source material 2. The growth boundary surface of the growing single crystal 4 forming along an isotherm 7 thus also adopts a slightly convex shape. As a result, favorable conditions for the growth of the single crystal 4 prevail.
[0153] FIG. 3D shows a schematic representation of a fifth crystal growing unit according to the invention. The fifth crystal growing unit according to the invention has any desired lateral heat source 13. The fifth crystal growing unit according to the invention can therefore be identical to the second, third or fourth crystal growing unit according to the invention. The lateral heat source 13 is therefore identified merely schematically by an altered representation of the crucible side wall. In particular, the lateral heat source 13 can be a combination of the induction-heating unit 6 discussed in the context of the second crystal growing unit according to the invention and the resistance-heating unit 9 discussed in the context of the fourth crystal growing unit according to the invention. Furthermore, the lateral heat source 13 can be a combination of the induction-heating unit 6 with susceptor 31 discussed in the context of the third crystal growing unit according to the invention and the resistance-heating unit 9 discussed in the context of the fourth crystal growing unit according to the invention. In each case, the advantages correspondingly mentioned above result.
[0154] FIG. 3E shows a schematic representation of a sixth crystal growing unit according to the invention. The sixth crystal growing unit according to the invention has any desired lateral heat source 13 and, in this aspect, corresponds to the second to fifth crystal growing units according to the invention. With respect to the features present and advantages achieved therewith, reference is therefore made to the comments regarding the second to fifth crystal growing units according to the invention. In addition, the sixth crystal growing unit according to the invention has a lower heat source 11 arranged below the crucible base of the crucible. In the example shown, the lower heat source 11 is arranged over the whole surface below the crucible base. The beaker-shaped first thermal insulation 5 thus encloses the lower heat source 11, the crucible optionally surrounded by a resistance-heating unit and/or a susceptor 31 as well as the second thermal insulation 12 arranged over the whole surface above the crucible cover. The lower heat source 11 can in particular be realized as resistance heater. By providing the lower heat source 11, the axial component of the heat flow 8 through the crucible and through the growing single crystal 4 is strengthened. The lower heat source 11 thus cooperates, together with the second thermal insulation 12, in the creation of a heat flow 8 in an axial direction. The additional lateral heat source 13 and the heating of the crucible side wall brought about therewith generate a slight radial component of the heat flow 8 from the crucible side wall into the crucible interior. As a result, the isotherms 7 at the crystal growth front are slightly convex, viewed from the source material 2. The growth boundary surface of the growing single crystal 4 forming along an isotherm 7 thus also adopts a slightly convex shape. As a result, particularly favorable conditions for the growth of the single crystal 4 prevail.
[0155] FIG. 3F shows a schematic representation of a seventh crystal growing unit according to the invention. The seventh crystal growing unit according to the invention corresponds to the sixth crystal growing unit according to the invention and additionally has a first 14 and a second optical pyrometer access 15. The first optical pyrometer access 14 passes through the second thermal insulation 12 and thereby makes a pyrometric measurement of the temperature of the crucible cover from above possible. The second optical pyrometer access 15 passes through the first thermal insulation 5 as well as the lower heat source 11 and thereby makes a pyrometric measurement of the temperature of the crucible base from below possible. The first 14 and/or the second optical pyrometer access 15 preferably run along the axis of symmetry of the crucible.
[0156] FIG. 4 shows a schematic representation of an eighth crystal growing unit according to the invention. The eighth crystal growing unit according to the invention corresponds to the seventh crystal growing unit according to the invention. Unlike the latter, however, the second thermal insulation 12 is not arranged over the whole surface above the crucible cover. In the example shown, the cylindrical second thermal insulation 12 has a diameter of 100% of the diameter 16 of the useful area of the single crystal 4, i.e. of 100% of the diameter of the single crystal planned as product. In another example, the second thermal insulation 12 can have a diameter of 80% of the external diameter 17 of the crucible. In the case of the eighth crystal growing unit according to the invention, the first thermal insulation 5 encroaches on the area lying above the annular surface of the crucible cover lying radially on the outside and not covered by the second thermal insulation 12, with the result that the second thermal insulation 12 is directly surrounded by the first thermal insulation 5. By adapting the diameter of the second thermal insulation 12, the strength of the heat flow 8 or the axial temperature gradient from the crucible can advantageously be set. The eighth crystal growing unit according to the invention, like the sixth or the seventh crystal growing unit according to the invention, can additionally have a lower heat source 11. The reference number 10 is inserted—as well as in the following figures—as representative of any desired heat sources. The reference number 10 indicates that lower heat sources 11 and/or lateral heat sources 13 can be provided. Furthermore, the reference number 10 comprises induction-heating units and/or resistance-heating units.
[0157] FIG. 5A shows a schematic representation of a ninth crystal growing unit according to the invention. The ninth crystal growing unit according to the invention additionally has a cavity 18 arranged between the crucible cover and the second thermal insulation 12. The cavity 18 is therefore delimited at the top by the lower surface 19 of the second thermal insulation 12, at the bottom by the upper surface 20 of the crucible cover and at the side by the inner surface 21 of the first thermal insulation 5. The surfaces 19, 20, 21 adjoining the cavity 18 are designed such that they have an adapted emissivity ε or different adapted emissivities ε. In the example shown, the surfaces 19, 20, 21 have the same adapted emissivity ε. For this, the surfaces 19, 20, 21 are provided with a coating of low emissivity ε, e.g. with TaC with an emissivity ε of approximately 0.3. The coating in particular of the surface 19 of the second thermal insulation 12 and of the surface 20 of the crucible cover has a particularly strong effect on the axial temperature gradient in the cavity 18. The application of coatings with low emissivity ε results in an increased axial temperature gradient in the cavity 18. An increased axial temperature gradient in the cavity 18 is physically accompanied by a reduced heat flow 8 from the crucible. In total, this results in the fact that the same thermal conditions can be achieved in the crucible interior as it is possible to do if the coating of the surfaces 19, 20, 21 adjoining the cavity 18 is dispensed with, but with a heat output which is 10% to 20% lower. In this way, the coating can be utilized to save electrical energy.
[0158] In a modification not shown here, the cavity 18 can be delimited at the side by a hollow graphite cylinder and/or at the top by a graphite disk. These graphite components provide the cavity 18 with a greater mechanical stability. The thickness of the graphite components can be 10 mm. The graphite components can in turn be provided with a coating of low emissivity ε, e.g. with TaC with an emissivity ε of approximately 0.3. Thus, the above-mentioned advantages also result.
[0159] Moreover, the ninth crystal growing unit according to the invention can be embodied like the first to eighth crystal growing units according to the invention. In particular, the ninth crystal growing unit according to the invention, like the sixth or the seventh crystal growing unit according to the invention, can have a lower heat source 11.
[0160] FIG. 5B shows a schematic representation of a tenth crystal growing unit according to the invention. The tenth crystal growing unit according to the invention corresponds to the ninth crystal growing unit according to the invention and additionally has a relief 22 on the surface 20 of the crucible cover facing the cavity 18. As explained above, this surface can also be adapted in terms of its emissivity ε, for example by a coating. In particular, it can be provided with a coating with low emissivity ε, e.g. with TaC with an emissivity ε of approximately 0.3.
[0161] Beyond the setting of the axial temperature gradient through variation of the emissivity ε of the surfaces 19, 20, 21, the embossing of a relief makes it possible to influence the direction of the thermal radiation and thus the radial temperature gradient within the cavity. For this, the direction of the heat flow in the cavity 18 is indicated with the reference number 23 in FIG. 5B. The direction of the heat flow 23 in the cavity 18 is tilted slightly radially inwards from the axial direction through the influence of the relief 22. Thus, the temperature field in the crucible, in particular in the area of the growing single crystal 4, is also influenced to a small degree. This is important for the fine-tuning of the temperature field in the crucible. A small radial temperature gradient with associated slightly convex isotherms can thereby be set in a defined manner at the crystal growth front. The temperature field starting from a radial temperature gradient close to 0 K/cm can thereby be prevented from inadvertently changing into slightly concave isotherms at the crystal growth front through unintended variations in the material properties or geometries of the graphite parts used, which would result in the massive incorporation of crystal defects. Advantageously, by providing slightly convex isotherms at the crystal growth front, the crystal growth can thus be stabilized at a low crystal defect density.
[0162] FIG. 6 shows a schematic representation of an eleventh crystal growing unit according to the invention. The eleventh crystal growing unit according to the invention corresponds to the ninth or tenth crystal growing unit according to the invention and additionally has a nucleus cavity. For this, the single crystal 4 is arranged with the aid of a nucleus suspension device 24 in such a way that the nucleus cavity is formed within the crucible between the single crystal 4 and the crucible cover. The nucleus suspension device 24 can be formed of graphite. The crystal nucleus suspended in the nucleus suspension device 24, from which the single crystal 4 develops through adsorption, is denoted with the reference number 25. By providing the nucleus suspension device 24, the growing single crystal 4 is mechanically separated from the crucible material. As a result, thermally induced mechanical stresses in the single crystal 4 due to the different thermal expansion coefficients of the single crystal 4 and the crucible formed of graphite or the conventional nucleus carrier generally formed of dense graphite can be prevented.
[0163] FIGS. 7A to 7C show enlarged detail views with various embodiments of the eleventh crystal growing unit according to the invention. The nucleus cavity is delimited at the bottom by the upper surface 26 of the crystal nucleus 25 or the single crystal 4, at the top by the lower surface 27 of the crucible cover and at the side by the inner surface 28 of the nucleus suspension device 24. The surfaces 26, 27, 28 adjoining the nucleus cavity are designed such that they have an adapted emissivity ε or different adapted emissivities ε. In the example shown, the upper surface 26 of the crystal nucleus 25 or the single crystal 4 adjoining the nucleus cavity and the lower surface 27 of the crucible cover adjoining the nucleus cavity have the same adapted emissivity ε. For this, the surfaces 26, 27 are provided with a coating of low emissivity ε, e.g. with TaC with an emissivity ε of approximately 0.3. In addition, the inner surface 28 of the nucleus suspension device 24 adjoining the nucleus cavity can also have the same adapted emissivity ε and in particular be provided with a coating of low emissivity ε, e.g. with TaC with an emissivity ε of approximately 0.3. The coating in particular of the upper surface 26 of the crystal nucleus 25 or the single crystal 4 and of the lower surface 27 of the crucible cover has an effect on the axial temperature gradient in the nucleus cavity. The application of coatings with low emissivity ε results in an increased axial temperature gradient in the nucleus cavity. An increased axial temperature gradient in the nucleus cavity results in a higher supersaturation of the growth species at the growth front of the growing single crystal 4. Such a supersaturation is advantageous in the production of SiC with a cubic polytype, i.e. in the production of 3C—SiC.
[0164] In an alternative embodiment, the upper surface 26 of the crystal nucleus 25 or the single crystal 4 adjoining the nucleus cavity and the lower surface 27 of the crucible cover adjoining the nucleus cavity are provided with a coating with high emissivity ε, e.g. with C with an emissivity ε of approximately 0.9. The application of coatings with high emissivity ε results in a reduced axial temperature gradient in the nucleus cavity. A reduced axial temperature gradient in the nucleus cavity results in a low supersaturation of the growth species at the growth front of the growing single crystal 4. That is advantageous in the production of SiC with a hexagonal polytype, for example in the production of 6H—SiC and in particular in the production of 4H—SiC.
[0165] Depending on the coating of the surfaces 26, 27 adjoining the nucleus cavity, the heat can thus be dissipated to different extents. The heat flow directed out of the crucible can thereby be set precisely.
[0166] In the embodiment represented in FIG. 7B, a further relief 29 is additionally provided on the lower surface 27 of the crucible cover adjoining the nucleus cavity. This influences the radial temperature gradient. A small radial temperature gradient with associated slightly convex isotherms can thereby be set in a defined manner at the crystal growth front of the growing single crystal 4. The temperature field starting from a radial temperature gradient close to 0 K/cm can thereby be prevented from inadvertently changing into slightly concave isotherms at the crystal growth front through unintended variations in the material properties or geometries of the graphite parts used, which would result in the massive incorporation of crystal defects. Advantageously, by providing slightly convex isotherms at the crystal growth front, the crystal growth can thus be stabilized at a low crystal defect density.
[0167] In the embodiment represented in FIG. 7C, a nucleus cavity filling 30 is additionally provided in the nucleus cavity. In the example shown, the nucleus cavity is completely filled with the nucleus cavity filling 30. The nucleus cavity filling 30 is a temperature-stable solid material which is chemically inert with respect to SiC, e.g. a SiC powder. The nucleus cavity filling 30 is provided in such a way that it does not hinder the transporting of heat away from the single crystal 4 to the crucible cover. By providing the nucleus cavity filling 30, a further possibility is advantageously provided to guarantee a defined transporting away of the heat of crystallization.
[0168] FIG. 8 shows a schematic three-dimensional view of a second thermal insulation as a series of five sheets 33. The crucible located below the series of sheets 33 is not represented here. The sheets 33 in each case have a thickness of 2 mm, for example. Adjacent sheets 33 have a spacing of 10 mm, for example. The sheets 33 are formed of a material with high temperature stability, e.g. graphite.
[0169] Each individual sheet 33 preferably reflects the highest possible proportion of the thermal radiation 34 incident on it and preferably transmits the lowest possible proportion of the thermal radiation 34 incident on it. The sheets 33 therefore act as radiation shields. The transmitted thermal radiation 34 decreases from sheet 33 to sheet 33. Correspondingly, a much lower temperature prevails above the series of sheets 33 than below the series of sheets 33.
[0170] The strength of the thermal insulation is substantially determined by the number of sheets 33 and the respective emissivity of the surfaces of the sheets 33.
[0171] The crucible is preferably operated in a temperature range of from T=1500° C. to 2500° C. In this temperature range, heat transfer by thermal radiation dominates. In order to obtain the same high-temperature insulation of a graphite foam or graphite felt in this temperature range with the aid of the arrangement of several sheets 33, the following are for example suitable:
[0172] Three to five sheets with an emissivity of 0.3 on both sides. For this, the sheets 33 are e.g. made of graphite with a TaC coating or of TaC.
[0173] Five to eight sheets with an emissivity of 0.7 on both sides. For this, the sheets 33 have e.g. a shiny graphite surface.
[0174] For this, various variations are possible. The emissivity of successive sheets 33 can vary. The upper side and the underside of one sheet 33 or several sheets 33 can differ in terms of the emissivity.
[0175] FIG. 9 shows a central two-dimensional section through the series, shown in FIG. 8, of five sheets 33 in a first embodiment. Here, the sheets 33 are spaced apart from each other by spacers arranged in a radially outer area of the sheets 33. The spacers are designed as thin pins 35.
[0176] FIG. 10A shows a central two-dimensional section through the series, shown in FIG. 8, of five sheets in a second embodiment. Here too, the sheets 33 are spaced apart from each other by spacers arranged in a radially outer area of the sheets 33. The spacers are implemented as rings 36 in this embodiment. For this, in each case one ring 36 and one sheet 33 are arranged alternately one on top of the other. FIG. 10B shows a central two-dimensional section through the series, shown in FIG. 8, of five sheets in a third embodiment. Here, the sheets 33 engage, with a radially outer area, in receiving grooves 37 of an annular receiving body 38. The rings 36 and the receiving body 38 are preferably made from thermally insulating material, for example graphite foam or felt.
[0177] FIG. 11 shows schematic representations of sheets in each case provided with elongate incisions 39. The incisions 39 in each case run in a radial direction starting from an outer circumference of the sheets 33. The incisions 39 do not penetrate into a radially inner area of the sheets and therefore do not meet. The sheets 33 in each case have twelve incisions 39 in each case with an angular spacing of 30° with respect to each other. The sheets 33 shown as (a) and (b) are turned by 15° with respect to each other. As a result, in the case of the arrangement of the sheets 33, shown in (a) and (b), carried out in (c) so that they lie one on top of the other, the incisions 39 are offset with respect to each other.
[0178] By providing the incisions 39, the inductive coupling of induction power into the sheets 33 can advantageously be prevented or at least greatly reduced. A possibly interfering vertical radiation of heat through the incisions 39 can be suppressed through the turned vertical arrangement.
LIST OF REFERENCE NUMBERS
[0179] 1 crucible wall [0180] 2 source material [0181] 3 gas space [0182] 4 single crystal [0183] 5 first thermal insulation [0184] 6 induction-heating unit [0185] 7 isotherm [0186] 8 heat flow [0187] 9 resistance-heating unit [0188] 10 any desired heat source [0189] 11 lower heat source [0190] 12 second thermal insulation [0191] 13 lateral heat source [0192] 14 first optical pyrometer access [0193] 15 second optical pyrometer access [0194] 16 external diameter of the crucible [0195] 17 diameter of the useful area of the single crystal [0196] 18 cavity [0197] 19 lower surface of the second thermal insulation [0198] 20 upper surface of the crucible cover [0199] 21 inner surface of the first thermal insulation [0200] 22 relief [0201] 23 direction of the heat flow in the cavity [0202] 24 nucleus suspension device [0203] 25 crystal nucleus [0204] 26 upper surface of the crystal nucleus or the single crystal [0205] 27 lower surface of the crucible cover [0206] 28 inner surface of the nucleus suspension device [0207] 29 further relief [0208] 30 nucleus cavity filling [0209] 31 susceptor [0210] 32 material-free area [0211] 33 sheet [0212] 34 thermal radiation [0213] 35 pin [0214] 36 ring [0215] 37 receiving groove [0216] 38 receiving body [0217] 39 incision
