104 of the growing crucible 102, for example, is located in the SiC supply region 104. The source material 108 may also be densified or provided partly or completely as a solid material in order to enhance the density of the source material 108.

A seed crystal 110 is provided on an inner wall opposing the SiC supply region 104 of the growing crucible 102, for instance on its crucible lid 112, in the crystal growth region 106. The bulk SiC single crystal to be grown grows on the seed crystal 110 by deposition from an SiC growth gas phase forming in the crystal growth region 106. The growing bulk SiC single crystal and the seed crystal 110 may have approximately the same diameter. The growing bulk SiC single crystal may have a larger diameter than the seed crystal 110, if the diameter of the crystal channel is larger than the seed crystal's diameter. However, the usable low-defect diameter of the growing bulk SiC single crystal has normally a size corresponding to the seed crystal's diameter.

The growing crucible 102 including the crucible lid 112 may be fabricated from an electrically and thermally conductive graphite crucible material. Around it is arranged a thermal insulation unit 117, which may comprise, for example, a foam-like or felt-like graphite insulation material, the porosity of which is, in particular, higher than that of the graphite crucible material. A transparent window 119 is provided at one or both of the axial end regions of the thermal insulation unit 117. These transparent windows 119 can be used to measure the temperature of the crucible 102.

In case of an inductive heating, the thermally insulated growing crucible 102 is placed inside a tubular container (not shown in FIG. 1), which may be configured as a quartz glass tube and forms an autoclave or reactor. An inductive heating device in the form of a heating coil 116 is arranged around the container to heat the growing crucible 102. The heating coil 116 generates the required temperature field by inductively coupling an electric current into an electrically conductive crucible wall (the susceptor) of the growing crucible 102. This electric current flows substantially as a circulating current in the peripheral direction within the circular and hollow cylindrical crucible wall and in the process heats the growing crucible 102. The susceptor can be made from graphite, TaC, WC, Ta, W, or other refractory metals, and may either be an integral part of the crucible 102 or may be a separate part close to the crucible wall. The primary purpose of the susceptor is to provide a heat source for inside of the crucible 102. When the susceptor is heated with induction, the surface of the susceptor reaches a high temperature, and the temperature is then transferred to the inside of the crucible 102 through conduction and/or radiation.

As mentioned above, the coil 116 is mounted outside the glass tube in case of an inductive heating and is usually surrounded by a Faraday cage (not visible in FIG. 1) to shield the electromagnetic radiation. In case of a resistive heating, the coil 116 is mounted within the reactor and also within the thermal insulation, thus being in close contact to the crucible 102. The principles of the present disclosure are applicable to both heating techniques. Thus, the coil 116 will more generally be referred to as a heating means and encompasses inductive as well as resistive heating (or a combination thereof) in the following. Furthermore, it should be noted that in the present disclosure the coil windings are depicted as having a rounded, specifically a circular or elliptic cross-section. However, the coil windings may have any other suitable cross-section, such as quadratic or rectangular, as well.

According to the example shown in FIG. 1, all the windings of the coil 116 are shifted along the central axis 120 to have an increased slope. Thus, two opposing regions of each winding are axially offset for instance by a distance d_1. The result is a rotationally asymmetric temperature field that would be generated by the heating means in operation.

However, according to the principles of the present disclosure, the thermal insulation unit 117 has a shape and structure which introduce an asymmetry counteracting the asymmetric temperature field by the asymmetry of the heating means 116.

FIGS. 2 to 6 illustrate various examples of how the thermal insulation unit 117 may be structured in order to provide the asymmetry of the insulation that compensates the impact of the asymmetry of the heating unit 116 and in the end leads to a symmetric temperature field in the crystal growth region 106 during the growth of the single crystal boule.

For instance, the thermal insulating unit 117 may have a body 300 formed from a sheet of carbon felt, which is wrapped around the crucible 102 (see FIG. 1). At a joint 302 of the two ends of the sheet, a stitching 304 is provided to stabilize the body 300. The joint 302 with the stitching 304 constitutes a discontinuity which extends in a longitudinal direction along the central axis 120 (see FIG. 1). This discontinuity has to be positioned at a radial position that causes the discontinuity of the thermal insulation unit 117 to compensate the asymmetry of the heating means 116. Of course, also more than one joint 302 with a respective stitching 304 may be provided in order to achieve a highly homogenous temperature field acting on the growing single crystal.

The stitching 304 is formed with threads made from carbon fibers. Other suitable electrically non-conductive, highly heat resistive materials may also be used.

Instead of a stitching 102 (or in addition thereto), the joint 302 may also be stabilized by one or more cramps 306. This example is shown in FIG. 3. Again, the joint 302 with the cramps 306 constitutes a discontinuity which extends in a longitudinal direction along the central axis 120 (see FIG. 1). This discontinuity has to be positioned at a radial position that causes the discontinuity of the thermal insulation unit 117 to compensate the asymmetry of the heating means 116. Of course, also more than one joint 302 with respective one or more cramps 306 may be provided in order to achieve a highly homogenous temperature field acting on the growing single crystal.

The cramps 306 are for instance manufactures from impregnated and stiffened threads made from carbon fibers. Other suitable electrically non-conductive, highly heat resistive materials may also be used.

Another example of providing a discontinuity in the thermal insulation unit 117 is shown in FIG. 4. According to this advantageous example, the body 300 of the thermal insulation unit 117 is provided with one or more slots 308. The slots 308 may be formed as either extending radially through the complete wall thickness of the body 300, thus forming a gap. Alternatively, the one or more slots 308 may also extend radially only through a part of the wall thickness of the body 300, thus forming trenches in the material of the body 300. The slots 308 constitute a discontinuity which extends in a longitudinal direction along the central axis 120. This discontinuity has to be positioned at a radial position that causes the discontinuity of the thermal insulation unit 117 to compensate the asymmetry of the heating means 116.

It should be noted, that the different examples of introducing a discontinuity into the body 300 of the thermal insulation unit 117 may of course be combined with each other as desired and necessary for achieving a highly homogenous temperature field acting on the growing single crystal.

A further example of a thermal insulation unit 117 is shown in a top view in FIG. 5.

According to this example, the body 300 of the thermal insulation unit 117 is formed by a sheet of thermally insulating material, such as for example a needled carbon felt, which is wound around the central axis 120 in a plurality of turns. In order to present a smooth inner surface 310 and a smooth outer surface 312, the end regions of the sheet have bevels 314, 316 extending along the central axis 120. In case the body 300 is formed by only a single layer of thermally insulating material (not shown in the Figure), the bevels 314 and 316 touch directly and close the thermal insulation unit 117 around the central axis 120. IN the shown example, the body has a triple layer configuration. The sheet forming the body 300 is spirally wound around the central axis 120 so as to form a circular straight cylinder. In the joint region 318, the inner layers have kinked regions that provide slanted supporting surfaces 320, 322 for the bevels 314, 316 of the end regions.

The joint region 318 of the body 300 provides a discontinuity which extends in a longitudinal direction along the central axis 120. This discontinuity has to be positioned at a radial position that causes the discontinuity of the thermal insulation unit 117 to compensate the asymmetry of the heating means 116.

FIG. 6 illustrates schematically a further example of how the thermal insulation unit 117 can be provided with a discontinuity that in operation of the heating means counteracts an asymmetry of the temperature field generated by the heating means. As shown in FIG. 6, the body 300 of the thermal insulation unit 117 has an essentially cylindrical shape with a wall thickness that varies in a radial direction around the circumference of the body 300. For instance, in a region covering more than 10° and less than 180° of the circumference, preferably more than 10° and less than 90° of the circumference, the wall 324 of the body 300 has reduced thickness d2 compared to the rest of the circumference, where the wall 324 has a thickness d1. Of course, the discontinuity may also be created by providing a region with a thickness d2 that is larger than the thickness d1 at the rest of the circumference of the body 300. The discontinuity can be abrupt or evolve gradually.

Thus, a discontinuity is generated which extends in a longitudinal direction along the central axis 120. This discontinuity has to be positioned at a radial position that causes the discontinuity of the thermal insulation unit 117 to compensate the asymmetry of the heating means 116.

Returning now to the various possibilities of providing a heating means 116 which as such and without further counter measures produces an asymmetric temperature field acting on the growing single crystal, FIG. 7 shows a sublimation system 100 according to a further example of the present disclosure. It should be noted that the principles are essentially the same as explained with reference to FIG. 1.

In particular, the sublimation system 100 comprises a growing crucible 102, which contains a source material compartment, in particular an SiC supply region 104 and a crystal growth region 106. The SiC source material 108, which is poured as a pre-finished starting material before the beginning of the growth process into the SiC supply region 104 of the growing crucible 102, for example, is located in the SiC supply region 104. A seed crystal 110 is provided on an inner wall opposing the SiC supply region 104 of the growing crucible 102, for instance on its crucible lid 112, in the crystal growth region 106. The bulk SiC single crystal to be grown grows on the seed crystal 110 by deposition from an SiC growth gas phase forming in the crystal growth region 106.

The growing crucible 102 including the crucible lid 112 may be fabricated from an electrically and thermally conductive graphite crucible material. Around it is arranged the thermal insulation unit 117, which may for instance be fabricated in line with the principles explained above referring to any of the FIGS. 2 to 6.

In case of an inductive heating, the thermally insulated growing crucible 102 is placed inside a tubular container (not shown in FIG. 1), which may be configured as a quartz glass tube and forms an autoclave or reactor. An inductive heating device in the form of a heating coil 116 is arranged around the container to heat the growing crucible 102. The heating coil 116 generates the required temperature field by inductively coupling an electric current into an electrically conductive crucible wall (the susceptor) of the growing crucible 102. This electric current flows substantially as a circulating current in the peripheral direction within the circular and hollow cylindrical crucible wall and in the process heat the growing crucible 102. The susceptor can be made from graphite, TaC, WC, Ta, W, or other refractory metals, and may either be an integral part of the crucible 102 or may be a separate part close to the crucible wall. The primary purpose of the susceptor is to provide a heat source for inside of the crucible 102. When the susceptor is heated with induction, the surface of the susceptor reaches a high temperature, and the temperature is then transferred to the inside of the crucible 102 through conduction and/or radiation.

As mentioned above, the coil 116 is mounted outside the glass tube in case of an inductive heating and is usually surrounded by a Faraday cage (not visible in FIG. 1) to shield the electromagnetic radiation. In case of a resistive heating, the coil 116 is mounted within the reactor and also within the thermal insulation, thus being in close contact to the crucible 102. The principles of the present disclosure are applicable to both heating techniques.

As shown in FIG. 7, the coil 116 has at least one deformed region 115, in which the cross-section of the coil 116 deviates from the circular cross-section of the remaining windings. In FIG. 7, this deviation is depicted as a compression leading to an elliptical cross-section in the deformed region 115. Of course, also other cross-sectional irregularities may be present at the coil 116. In the deformed region 115, the distance between the deformed winding to the adjacent windings is enlarged to a distance d_2 compared to the regular distance d_1 between the undeformed remaining windings. The position and extent of these irregularities may be found upon inspection of the heating system, e. g. when performed crystal growth cycles produced unsatisfactory single crystal boules. The asymmetric thermal insulation according to the present disclosure may then be used to counteract the heating system irregularities.

In operation, the constructional asymmetry due to this deformed region 115, which may cover e. g. 10° to 90° of the circumference, preferably 10° to 45°, causes an irregularity in the temperature field. This irregularity extends in a radial direction (around the circumference of the crucible) as well as in an axial direction (along the center axis 120).

The result is a rotationally asymmetric temperature field that would be generated by the heating means in operation.

However, according to the principles of the present disclosure, the thermal insulation unit 117 has a shape and structure which introduce an asymmetry counteracting the asymmetric temperature field by the asymmetry of the heating means 116. Thus, the temperature field to which the growing single crystal is subjected is homogenous and symmetric, and it is possible to grow high quality crystals that are as free as possible from edge defects.

Furthermore, a constructional asymmetry may also be introduced into the sublimation system 100 when the electrical contacts 124 of the heating coil 116 have to be arranged in an area surrounding the crucible 102. This example is shown in FIG. 4. At least one of the current feeding electrical contacts 124 is located not at a peripheral end of the heating coil 116, but is arranged in an axial region where they constitute a structural asymmetry close to the crucible 102 and in particular, close to the crystal growth region 106. Thus, in operation the heating coil 116 generates a non-uniform temperature field, unless it is counteracted by the asymmetry of the thermal insulation unit 117. As explained above, according to the principles of the present disclosure, the thermal insulation unit 117 has a shape and structure which introduce an asymmetry counteracting the asymmetric temperature field by the asymmetry of the heating means 116. Thus, the temperature field to which the growing single crystal is subjected is homogenous and symmetric, and it is possible to grow high quality crystals that are as free as possible from edge defects.

FIG. 9 shows in a top view a sublimation system 100 according to a further advantageous example. As shown in this Figure, the asymmetry of the temperature field may also be caused by an asymmetric shielding 126 and a metallic holder 132 of the shielding 126. The holder 132 may e. g. extend into the gap between the coil 116 and the shielding 126. The reactor 114 comprises the crucible and the thermal insulation unit explained above.

Thus, in operation the heating coil 116 generates a non-uniform temperature field, which is compensated by the discontinuity (or discontinuities) provided at the thermal insulation unit according to the present disclosure. Thus, the growing single crystal experiences a uniform temperature field which leads to a high quality crystal boule with only few edge defects.

Now turning to FIG. 7, the principles of the present disclosure may further be applied to a sublimation system 200, which is operable to grow two single crystal boules simultaneously. To this end, the crucible 202 comprises a first seed crystal 210A and a second seed crystal 210B. FIG. 7 shows the example analogous to the arrangement shown in FIG. 4. However, it is clear that any of the other possibilities of how an asymmetric temperature field could occur, may also be present in a sublimation system 200 for simultaneously growing more the one single crystal boule as for instance described in the European patent EP 2 664 695 B1.

In particular, FIG. 7 shows a schematic cross sectional view of a physical vapor transport (PVT) growth system 200 for simultaneously growing two SiC bulk crystals. The system 200 comprises a crucible 202 which contains a central source material compartment 234 containing SiC powder 208 as the SiC source material. The source material 208 may also be densified or provided partly or completely as a solid material in order to enhance the density of the source material 208.

The two seed crystals 210A and 210B are arranged in growth regions 206A and 206B. Each of the growth regions 206A and 206B is separated from the powderous SiC source material 208 by means of a gas permeable porous barrier 236A, 236B. Thus, it is ensured that only gaseous Si and C containing components enter the growth regions 206A and 206B.

A heating coil 216 provides the necessary temperature field. The asymmetry of the temperature field is for instance caused by electrical contacts 224 in the vicinity of the crystal growth regions 206 A, 206B.

In order to compensate the asymmetry of the temperature field that would otherwise be generated by the heating means 116, the thermal insulation unit 117 provides a counteracting discontinuity (or more than one discontinuities) in line with the examples explained above. Thus, the growing single crystal experiences a uniform temperature field which leads to a high quality crystal boule with only few edge defects.

In summary, the present disclosure is based on the finding that despite the use of a growth system that produces an inhomogeneous temperature field, it is possible to grow high quality crystals that are as free as possible from edge defects if a special isolation design is used.

Since an ideally homogeneous isolation as provided according to the state of the art in combination with a growth system with an inhomogeneous temperature field is obviously not sufficient to obtain the desired crystal quality, the approach presented in the present disclosure is to balance the overall inhomogeneity by using a locally inhomogeneous isolation for growing SiC single crystals in an inhomogeneous growth system and to generate a homogeneous temperature field in the region of the growing crystal. For this purpose, several conditions have to be fulfilled.

The solution to the technical problem of growing crystals free of edge defects lies, on the one hand, in the use of a growth system that causes at least one inhomogeneous and radially asymmetric heat input into the growth crucible.

Inhomogeneous, asymmetric heat coupling can for instance be caused by the following features of the growth system:

- Non-equidistant spacing of coil turns of the heating system due to an offset of the windings
- Non-equidistant spacing of the coil turns due to non-uniform construction of the turns