PRODUCTION METHOD FOR AN SIC VOLUME MONOCRYSTAL OF INHOMOGENEOUS SCREW DISLOCATION DISTRIBUTION AND SIC SUBSTRATE

Abstract

An SiC volume monocrystal is processed by sublimation growth. An SiC seed crystal is placed in a crystal growth region of a growing crucible and SiC source material is introduced into an SiC storage region. During growth, at a growth temperature of up to 2,400 C. and a growth pressure between 0.1 mbar and 100 mbar, an SiC growth gas phase is generated by sublimation of the SiC source material and by transport of the sublimated gaseous components into the crystal growth region, where an SiC volume monocrystal grows by deposition from the SiC growth gas phase on the SiC seed crystal. A mechanical stress is introduced into the SiC seed crystal at room temperature prior to the start of the growth to cause seed screw dislocations present in the SiC seed crystal to undergo a dislocation movement so that seed screw dislocations recombine.

Claims

1. A method for producing at least one SiC volume monocrystal by sublimation growth, the method comprising: a) prior to a start of the growth: a1) arranging an SiC seed crystal having a growth surface in a crystal growth region of a growing crucible; and a2) introducing SiC source material into an SiC storage region of the growing crucible; and b) during the growth at a growth temperature of up to 2400 C. and a growth pressure between 0.1 mbar and 100 mbar by means of a sublimation of the SiC source material and by way of a transport of sublimated gaseous components into the crystal growth region, producing an SiC growth gas phase in the crystal growth region, in which an SiC volume monocrystal grows on the SiC seed crystal by deposition from the SiC growth gas phase; and c) prior to the start of the growth, introducing a mechanical stress into the SiC seed crystal at room temperature in order to cause seed screw dislocations present in the SiC seed crystal to undergo a dislocation movement under an influence of the mechanical stress, to cause seed screw dislocations which approach each other in connection with respective dislocation movements thereof to recombine with each other and cancel each other out.

2. The method according to claim 1, which comprises thermally activating the dislocation movements of the seed screw dislocations by heating the SiC seed crystal.

3. The method according to claim 1, which comprises introducing the mechanical stress rotationally symmetrically into the SiC seed crystal.

4. The method according to claim 1, which comprises introducing the mechanical stress by bending the SiC seed crystal.

5. The method according to claim 1, which comprises bending the SiC seed crystal to introduce the mechanical stress with a maximum bending distance between 0.1 mm and 5 mm.

6. The method according to claim 1, which comprises bending the SiC seed crystal by way of at least one punch to introduce the mechanical stress.

7. The method according to claim 6, which comprises placing the at least one punch centrally to act on a center of the SiC seed crystal.

8. The method according to claim 6, wherein the at least one punch is one of several punches acting on the SiC seed crystal.

9. The method according to claim 8, which comprises placing at least a portion of the punches along a notional circular line around a center of the SiC seed crystal.

10. The method according to claim 9, which comprises placing the punches equidistantly along the notional circular line around the center of the SiC seed crystal.

11. The method according to claim 1, wherein the step of introducing the mechanical stress into the SiC seed crystal comprises firmly connecting the SiC seed crystal to an uneven contact surface of a shaped seed holder.

12. A monocrystalline SiC substrate, comprising: a total main surface, said total main surface having an accumulation sub-area formed by at most 20% of said total main surface, and said accumulation sub-area comprising at least 80% of all substrate screw dislocations present on said total main surface.

13. The SiC substrate according to claim 12, wherein said accumulation sub-area has at least 85% of all substrate screw dislocations that are present on said total main surface.

14. The SiC substrate according to claim 12, wherein said accumulation sub-area has at least 90% of all substrate screw dislocations that are present on said total main surface.

15. The SiC substrate according to claim 12, wherein said accumulation sub-area has a size of at most 15% of said total main surface.

16. The SiC substrate according to claim 12, wherein the SiC substrate has a total screw dislocation density of at most 1000 per cm.sup.2.

17. The SiC substrate according to claim 12, wherein the SiC substrate has a total screw dislocation density of at most 500 per cm.sup.2.

18. The SiC substrate according to claim 12, wherein said total main surface has a substrate diameter of at least 150 mm.

19. The SiC substrate according to claim 12, wherein said total main surface has a substrate diameter of at least 200 mm.

20. The SiC substrate according to claim 12, wherein the SiC substrate has an SiC crystal structure with only one single SiC polytype.

21. The SiC substrate according to claim 12, wherein the SiC substrate has an SiC crystal structure with one SiC polytype selected from the group consisting of 4H, 6H, 15R and 3C.

22. The SiC substrate according to claim 12, wherein the SiC substrate has an electrical resistivity of 8 mcm to 26 mcm.

23. The SiC substrate according to claim 22, wherein the electrical resistivity of the SiC substrate is 10 mcm to 24 mcm.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0043] FIG. 1 shows an exemplary embodiment of a growth arrangement for sublimation growth of an SiC volume monocrystal;

[0044] FIG. 2 shows an exemplary embodiment of an SiC volume monocrystal grown on an SiC seed crystal by means of the growth arrangement according to FIG. 1 and having inhomogeneously distributed screw dislocations in a longitudinal sectional representation in the growth direction;

[0045] FIG. 3 shows an exemplary embodiment of a seed holder used in the growth arrangement according to FIG. 1 which can be bent by means of a punch;

[0046] FIGS. 4 to 6 show exemplary embodiments of the punch used in the bendable seed holder according to FIG. 3;

[0047] FIG. 7 shows the seed holder according to FIG. 3, which can be bent by means of a single punch, in a schematic plan view;

[0048] FIG. 8 in a schematic plan view, shows a further example of a seed holder used in the growth arrangement according to FIG. 1 which can be bent by means of several punches;

[0049] FIGS. 9 and 10 show exemplary embodiments of shaped seed holders used in the growth arrangement according to FIG. 1, each with an uneven contact surface for the abutting SiC seed crystal;

[0050] FIG. 11 shows an exemplary embodiment of an SiC substrate, in a plan view, obtained from an SiC volume monocrystal similar to the one shown in FIG. 2; and

[0051] FIG. 12 shows an exemplary embodiment of an SiC substrate, in a plan view, obtained from an SiC volume monocrystal with inhomogeneously distributed screw dislocations concentrated in a central region.

DETAILED DESCRIPTION OF THE INVENTION

[0052] Parts that correspond to each other are marked with the same reference signs in FIGS. 1 to 12. Details of the exemplary embodiments described in more detail below may also constitute an invention in their own right or form part of a subject-matter of an invention.

[0053] FIG. 1 shows an exemplary embodiment of a growth arrangement 1 for the production of an SiC volume monocrystal 2 by means of sublimation growth. The growth arrangement 1 contains a growing crucible 3, which comprises an SiC storage region 4 and a crystal growth region 5. The SiC storage region 4 contains, for example, powdery SiC source material 6, which is filled into the SiC storage region 4 of the growing crucible 3 as a prefabricated starting material before the start of the growth process.

[0054] In the region of a crucible end wall 7 of the growing crucible 3 opposite the SiC storage region 4, an SiC seed crystal 8 extending axially into the crystal growth region 5 is attached. The manner in which the latter is attached will be described in more detail below. The SiC seed crystal 8 is in particular monocrystalline. In the exemplary embodiment shown, the crucible end wall 7 is formed as the crucible lid of the growing crucible 3. However, this is not mandatory. The SiC volume monocrystal 2 to be grown grows on the SiC seed crystal 8 by means of deposition from an SiC growth gas phase 9 forming in the crystal growth region 5. The growing SiC volume monocrystal 2 and the SiC seed crystal 8 have approximately the same diameter. If at all, there is a deviation of at most 10% by which a seed diameter of the SiC seed crystal 8 is smaller than a monocrystal diameter of the SiC volume monocrystal 2. However, a gap not shown in FIG. 1 may be present between the inner side of a crucible side wall 13 on the one hand and the growing SiC volume monocrystal 2 and the SiC seed crystal 8 on the other hand.

[0055] In the exemplary embodiment according to FIG. 1, the growing crucible 3 including the crucible lid 7 consists of an electrically and thermally conductive graphite crucible material with a density of, for example, at least 1.75 g/cm.sup.3. A thermal insulation layer 10 is arranged around it. The latter consists, for example, of a foam-like graphite insulation material whose porosity is in particular significantly higher than that of the graphite crucible material.

[0056] The thermally insulated growing crucible 3 is placed inside a tubular container 11, which in the exemplary embodiment is designed as a quartz glass tube and forms an autoclave or reactor. For heating the growing crucible 3, an inductive heating device in the form of a heating coil 12 is arranged around the container 11. The growing crucible 3 is heated to the temperatures required for growth by means of the heating coil 12. In the exemplary embodiment shown, these growth temperatures are at least 2,250 C. The heating coil 12 inductively couples an electric current into the electrically conductive crucible side wall 13 of the growing crucible 3. This electric current flows substantially as a circular current in the circumferential direction within the circular and hollow cylindrical crucible side wall 13, thereby heating the growing crucible 3. If needed, the relative position between the heating coil 12 and the growing crucible 3 can be changed axially, i.e., in the direction of a central longitudinal axis 14 of the growing SiC volume monocrystal 2, in particular in order to adjust and, if necessary, also change the temperature or the temperature profile within the growing crucible 3. The axially variable position of the heating coil 12 during the growth process is indicated in FIG. 1 by the double arrow 15. In particular, the heating coil 12 is displaced in accordance with the growth progress of the growing SiC volume monocrystal 2. The displacement preferably takes place downwards, i.e., in the direction of the SiC source material 6, and preferably by the same length by which the SiC volume monocrystal 2 grows, e.g., by a total of approximately 20 mm. For this purpose, the growth arrangement 1 comprises correspondingly designed monitoring-, control and adjustment means not shown in more detail.

[0057] The SiC growth gas phase 9 in the crystal growth region 5 is fed by the SiC source material 6. The SiC growth gas phase 9 contains at least gas components in the form of Si, Si.sub.2C and SiC.sub.2 (SiC gas species). The material transport from the SiC source material 6 to a growth interface 16 at the growing SiC volume monocrystal 2 takes place on the one hand along an axial temperature gradient. In the sublimation method (=PVT method) used for SiC crystal growth, the growth conditions including the material transport are adjusted and controlled via the temperatures prevailing in the growing crucible 3. At the growth interface 16 there is a relatively high growth temperature of at least 2,250 C., in particular even of at least 2,350 C. or 2,400 C.. Furthermore, an axial temperature gradient of at least 5 K/cm, preferably of at least 15 K/cm, measured in the direction of the central longitudinal axis 14, is set at the growth interface 16 in particular. The temperature within the growing crucible 3 decreases towards the growing SiC volume monocrystal 2. The highest temperature of approximately 2,450 C. to 2,550 C. prevails in the region of the SiC storage region 4. This temperature profile with a temperature difference of in particular 100 C. to 150 C. between the SiC storage region 4 and the growth interface 16 can be achieved by various measures. For example, axially varying heating can be provided by dividing the heating coil 12 into two or more axial sections, which is not shown in more detail. Furthermore, a stronger heating effect can be set in the lower section of the growing crucible 3 than in the upper section of the growing crucible 3, e.g., by a corresponding axial positioning of the heating coil 12. Moreover, the thermal insulation can be designed differently at the two axial crucible end walls. As schematically indicated in FIG. 1, the thermal insulation layer 10 can have a greater thickness at the lower crucible end wall than at the upper crucible end wall. Furthermore, it is possible that the thermal insulation layer 10 has a central cooling opening 17 arranged around the central longitudinal axis 14 adjacent to the upper crucible end wall 7, through which cooling opening 17 heat is dissipated. This central cooling opening 17 is indicated by the dashed lines in FIG. 1.

[0058] In addition, a growth pressure of in particular 0.1 hPa (=mbar) to 10 hPa (=mbar) prevails in the growing crucible 3 during the actual crystal growth.

[0059] The SiC volume monocrystal 2 grows on a growth surface 18 of the SiC seed crystal 8. Said growth takes place in a growth direction 19, which in the exemplary embodiment shown in FIG. 1 is oriented from top to bottom, i.e., from the crucible lid 7 towards the SiC storage region 4. The growth direction 19 runs parallel to the centrally arranged central longitudinal axis 14. Since the growing SiC volume monocrystal 2 is arranged concentrically within the growth arrangement 1 in the exemplary embodiment shown, the centrally arranged central longitudinal axis 14 can also be assigned to the growth arrangement 1 as a whole.

[0060] The growing SiC volume monocrystal 2 has an SiC crystal structure of the 4H polytype. In principle, however, another polytype (=another crystal modification), such as 6H-SiC, 3C-SiC or 15R-SiC, is also possible. Advantageously, the SiC volume monocrystal 2 has only one SiC polytype, which in the exemplary embodiment is said 4H-SiC. The SiC volume monocrystal 2 grows with a high modification stability and in this respect has essentially only one single polytype. The latter is favorable with regard to a very low defect high crystal quality.

[0061] The growth method carried out by means of the growth arrangement 1 to produce the SiC volume monocrystal 2 is also characterized in other respects by a high crystal quality that is achieved. For instance, within any cross-sectional area perpendicular to the growth direction 19, the growing SiC volume monocrystal 2 has an inhomogeneous distribution of volume monocrystal screw dislocations 20 (see FIG. 2). This inhomogeneous distribution preferably includes a division into a (single-part or multi-part) accumulation sub-region 21, in which the largest part of the volume monocrystal screw dislocations 20 is concentrated, and into a (single-part or multi-part) utilization sub-region 22, which, compared to the accumulation sub-region 21, contains significantly fewer volume monocrystal screw dislocations 20 but at the same time has a larger area. In the example shown in FIG. 2, the accumulation sub-region 21 is single-part and formed by the radial edge region of the SiC volume monocrystal 2. Accordingly, the utilization sub-region 22 is also single-part and formed by the central region of the SiC volume monocrystal 2 arranged around the centrally arranged central longitudinal axis 14. The boundary between the accumulation sub-region 21 and the utilization sub-region 22 is indicated in FIG. 2 by a (notional) boundary line 23. The larger utilization sub-region 22 contains only very few volume monocrystal screw dislocations 20 and is therefore of very high quality.

[0062] This high quality of the SiC volume monocrystal 2 is also due to the underlying SiC seed crystal 8, which has a similar favorable inhomogeneous distribution of its seed screw dislocations 24 as the SiC volume monocrystal 2 growing thereon during the subsequent growth. In particular, the seed screw dislocations 24 present in the SiC seed crystal 8 continue in the growth direction 19 in the growing SiC volume monocrystal 2, which is shown in the representation according to FIG. 2. The seed screw dislocations 23 lead to the formation of the volume monocrystal screw dislocations 20 on the growth surface 18 of the SiC seed crystal 8 in the growing SiC volume monocrystal 2.

[0063] In this respect, it is favorable if the SiC seed crystal 8 has, in particular at the latest at the time when the actual growth of the SiC volume monocrystal 2 starts, a distribution of its seed screw dislocations 24 and/or a total seed screw dislocation density related to its growth surface 18, which should and can be inherited at least approximately in this form in the SiC volume monocrystal 2.

[0064] In order to achieve this, the SiC seed crystal 8 is subjected to a treatment prior to its use for growing the SiC volume monocrystal 2, which leads to an at least partial recombination and mutual cancellation of the seed screw dislocations 24 that may still be quite numerous in the SiC seed crystal 8 at this initial stage. For this purpose, the SiC seed crystal 8 is preferably subjected to a mechanical stress while still at room temperature and then heated in particular. This leads to a dislocation movement of initially existing seed screw dislocations 24. In the course of this dislocation movement, seed screw dislocations 24 with opposing Burgers vectors can approach each other to such an extent that recombination and mutual cancellation of the respective pair of seed screw dislocations 24 occurs. This advantageous selective reduction of the seed screw dislocations 24 is thus mechanically induced (by the mechanical stress introduced into the SiC seed crystal 8) and thermally activated (by the thermal treatment carried out thereafter). During the thermal treatment that initiates the favorable dislocation movement, the mechanically strained SiC seed crystal 8 is brought to a temperature of up to 200 C. below the growth temperature that will later prevail on the SiC seed crystal 8 during the actual growth. For example, the treatment temperature is about 2,100 C. This thermal treatment preferably lasts about 200 minutes. After completion of this upstream treatment, the SiC seed crystal 8 has the favorable inhomogeneous distribution of the seed screw dislocations 24 indicated by the example in the representation according to FIG. 2, with only very few seed screw dislocations 24 in the central region.

[0065] Examples for introducing the advantageous mechanical stress into the seed crystal 8 are described below.

[0066] FIG. 3 shows an exemplary embodiment of a seed holder 25 used in the growth arrangement 1, which has a holding plate 27 resting on a wall projection 26 of the crucible side wall 13 and a punch 28 that is adjustable in the direction of the central longitudinal axis 14. The SiC seed crystal 8 is attached, in particular glued, to the front side of the holding plate 27 facing the crystal growth region 5. The punch 28 is in contact with the holding plate 27 on the rear side facing away therefrom. By means of a feed movement of the punch 28 in the direction of the crystal growth region 5, the unit consisting of the holding plate 27 and the bonded SiC seed crystal 8 is bent. The bow is greatest in the center. There, the SiC seed crystal 8 is bent by a bending distance 29 of approximately 1 mm. The bow of the SiC seed crystal 8 is correlated with a mechanical stress a introduced into it. In FIG. 3, the profile of this mechanical stress a introduced into the SiC seed crystal 8 is schematically illustrated in a diagram shown. The maximum of this stress is also located in the center of the SiC seed crystal 8, where the mobility of the seed screw dislocations 24 is highest, since this mobility depends on the strength of the mechanical stress at the respective location. During the heating that takes place during the annealing treatment, the seed screw dislocations 24 present in the central region of the SiC seed crystal 8 in particular will therefore start to move and recombine with each other with a particularly high probability.

[0067] The adjustable punch 28 has a cylindrical shape. Its punch tip, which presses against the back of the holding plate 27, can have different shapes. Three exemplary embodiments are shown in FIGS. 4 to 6. The punch 28a shown in FIG. 4 has a flat punch tip 30a, the punch 28b shown in FIG. 5 has a tapered conical punch tip 30b and the punch 28c shown in FIG. 6 has a semi-spherically rounded punch tip 30c.

[0068] In FIG. 7, the seed holder 25 according to FIG. 3, which can be bent by means of the single punch 28, is shown in a schematic plan view of the rear side of the holding plate 27 in the growth direction 19 of the SiC volume monocrystal 2 to be grown.

[0069] In FIG. 8, a further exemplary embodiment of a seed holder 31 used in the growth arrangement 1 is shown in the same representation as the seed holder 25 in FIG. 7. The seed holder 31 has essentially the same construction as the seed holder 25, and is also bendable, thus allowing mechanical stress to be applied to the SiC seed crystal 8. In contrast to the seed holder 25, however, the seed holder 31 has, in addition to the centrally arranged punch 28, further punches 32 which are placed concentrically around the punch 28 equidistantly along a circular line 33. The punches 32 are also cylindrical and adjustable in the direction of the central longitudinal axis 14. Their punch diameter is somewhat smaller than that of the central punch 28. The seed holder 31 thus has several punches 28, 32 for its bending, whereby the bow of the SiC seed crystal 8 and thus also the mechanical stress to be introduced into the SiC seed crystal 8 can be adjusted very specifically.

[0070] By means of the seed holders 25 and 31, the bow of the SiC seed crystal 8 can only be adjusted during the pre-treatment to reduce the seed screw dislocations 24 present in the SiC seed crystal. After completion of the pre-treatment, the punches 28, 32 can be retracted and the seed holder 25 or 31 including the SiC seed crystal 8 attached thereto can be returned to the unbent state, in which the actual growth of the SiC volume monocrystal 2 is then subsequently carried out.

[0071] In contrast, there are further exemplary embodiments shown in FIGS. 9 and 10 of shaped seed holders 34 and 35, respectively, used in the growth arrangement 1, each of which has an uneven contact surface 36 and 37, respectively, for abutting with the SiC seed crystal 8 and also rests on the wall projection 26 of the crucible side wall 13. In contrast to the seed holders 25 and 31, the seed holders 34 and 35 are not adjustable or bendable. In their case, the SiC seed crystal 8 bonded to the respective contact surface 36 or 37 undergoes permanent bending, wherein in these exemplary embodiments, too, the mechanical stress that is decisive for reducing the seed screw dislocations 24 is introduced into the SiC seed crystal 8 in accordance with the uneven shape of the respective contact surface 36 or 37. In the case of the seed holder 34, the contact surface 36 is concavely curved. Accordingly, the SiC seed crystal 8 glued thereto is bent backwards, i.e., away from the crystal growth region 5. In the case of the seed holder 35, the contact surface 37 is convexly curved, so that the SiC seed crystal 8 glued thereto is bent forward in the direction of the crystal growth region 5, similarly to the case of the seed holders 34 and 35. In principle, any other shapes for uneven contact surfaces can also be realized, the exact shape of which can be adapted to the mechanical stress profile to be introduced into the SiC seed crystal 8 in each individual case.

[0072] If necessary, the SiC seed crystal 8 can first be examined for the presence of seed screw dislocations 24. This examination is carried out e.g., by X-ray topography. A sub-region that has been found to be particularly loaded with seed screw dislocations 24 during this examination can then be selectively subjected to a mechanical stress in order to reduce the unusually (or above-average) high number of seed screw dislocations 24 especially there.

[0073] Overall, the growth arrangement 1 in conjunction with the described different variants for reducing the seed screw dislocations 24 present in the SiC seed crystal 8 enables the growth of a high-grade SiC volume monocrystal 2 which has only a few volume monocrystal screw dislocations 20, wherein the latter are inhomogeneously distributed and preferably concentrated within the accumulation sub-region 21, which can advantageously be left out in the further use of the SiC volume monocrystal 2 for the production of high-grade electronic components.

[0074] From these high-grade SiC volume monocrystals 2, equally high-grade SiC substrates 38, 39 (see schematic plan view representations according to FIGS. 11 and 12) can then be produced. These disc-shaped SiC substrates 38, 39 are obtained from the SiC volume monocrystal 2 in question by cutting or sawing them off axially successively as discs perpendicular to the growth direction 19 or to the central longitudinal axis 14. The position of such a disc 40 forming an SiC substrate within the SiC volume monocrystal 2 is exemplarily indicated by a dashed rectangle in the representation according to FIG. 2. Such an SiC substrate 38 or 39 is large and thin. In one possible embodiment, its total main surface 41 or 42 has a substrate diameter of at least 150 mm, for example 200 mm, whereas a substrate thickness is approximately 500 m. The SiC substrate 38 or 39, like the SiC volume monocrystal 2 from which it is made, has a low total screw dislocation density of preferably at most 1,000 cm.sup.2 and an inhomogeneous distribution of the remaining volume monocrystal screw dislocations 20, which in the context of the substrates 38, 39 can also be understood and referred to as substrate screw dislocations 43. Both improve the suitability of the SiC substrate 38 or 39 for the use in production of components. The total screw dislocation density refers to a complete cross-sectional area of the SiC volume monocrystal 2 perpendicular to the central longitudinal axis 14 or to the growth direction 19 in the case of the SiC volume monocrystal 2 and to the complete total main surface 41 or 42 in the case of the SiC substrate 38 or 39. The inhomogeneous screw dislocation distribution can be seen in the illustrations according to FIGS. 11 and 12, in which the Si side of the respective SiC substrate 38 or 39 is shown in each case.

[0075] In the SiC substrate 38 according to FIG. 11, the substrate screw dislocations 43 are concentrated in an accumulation sub-region 44 formed by the axial edge region, whereas there are significantly fewer substrate screw dislocations 43 in a utilization sub-region 45 formed by the central region. Accordingly, the latter is particularly suitable for use in the production of high-grade components. In this respect, a similar screw dislocation distribution is present as in the case of the volume monocrystal 2 shown schematically in FIG. 2 and as can be achieved by using an SiC seed crystal 8 which has been subjected to a pre-treatment with a bow according to the representation in FIG. 3, 9 or 10 as well as with a thermal pre-treatment before the actual growth.

[0076] In the case of the SiC substrate 39 according to FIG. 12, the conditions are reversed. In this case, the substrate screw dislocations 43 are concentrated in an accumulation sub-region 46 formed by the central region, whereas there are significantly fewer substrate screw dislocations 43 in a utilization sub-region 47 formed by the axial edge region. Accordingly, the latter is again particularly well suited for use in the production of high-grade components.

[0077] In the case of the SiC substrates 38, 39, the respective utilization sub-region 45 or 47 is significantly larger than the respective accumulation sub-region 44 or 46. The respective accumulation sub-region 44 or 46 has an associated accumulation sub-region 48 or 49, which is part of the respective total main surface 41 or 42 and accounts for at most 20% thereof. Nevertheless, at least 80% of all substrate screw dislocations 43 present on the associated total main surface 41 or 42 are located within this accumulation sub-region 48 or 49. The boundary between the smaller accumulation sub-region 44 or 46 and the larger usage sub-region 45 or 47 intended for further use is indicated in FIGS. 11 and 12 by a dashed (notional) boundary line 50 in each case.