WO2014202284A1 - Crystallisation system and crystallisation method for crystallisation from electrically conductive melts, and ingots that can be obtained by means of the method - Google Patents

Abstract

The invention relates to a crystallisation system for producing crystals by means of directional solidification from electrically conductive melts, comprising: • i. at least one crucible (10) for receiving the melt; • ii. a heater-magnet module (20), which has one or more coils (21) and associated electrical supply lines (200), wherein a heater-magnet module is associated with each crucible; and • iii. a control and power-supply unit for the heater-magnet module (20). The heater-magnet module (20) associated with the respective crucible is arranged and electrically controlled in such a way that a Lorentz force density field (40) can be produced, which is directed in such a way that the resulting force effect acts outward against the growth direction in relation to a geometric centre axis (50) of the crucible (10). The invention further relates to a method in which the crystallisation system according to the invention is used and to an ingot that can be produced by means of the method and/or the crystallisation system.

Description

 Crystallization plant and crystallization process for the crystallization of electrically conductive melts and ingots available via the process

The invention relates to a crystallization plant for the production of ingots by directional solidification of electrically conductive melts, a process for growing crystals by means of the aforementioned crystallization plant, as well as ingots produced or preparable by the process with homogeneous properties.

Technological background

The current development in crystal growth is aimed above all at increasing the productivity of the crystallization from the melt. Here, the approach of simultaneous crystallization in several crucibles is pursued (DE 10 2007 026 298 A1). In addition to the increase in productivity, another focus of the process development of crystalline materials lies in the improvement of the material properties, with the properties focusing on the homogeneity and perfection of the materials.

 In DE 10 2009 045 680 A1, a heater-magnet module arrangement (HMM) is disclosed specifically for the crystallization of PV silicon in rectangular containers, which incorporates a rectangular, concentric ceiling heater for the generation of Lorentz force densities. A combination of lateral heater magnet module and ceiling heater is shown. The expected Lorentz force density in this arrangement is directed in an analogous manner to a laterally arranged heater-magnet module inwardly to the imaginary center. The electrical leads of the ceiling heater, which are routed directly horizontally across the coil windings, would completely extinguish a magnetic traveling field generated in this area, so that the ceiling heater is unsuitable for targeted generation of Lorentz force density distributions in electrically conductive melts.

 By means of the arrangement described in DE 10 2007 026 298 A1 for the simultaneous crystal growth in several crucibles, it is not possible to produce a symmetrical Lorentz force field in electrically conductive melts.

DE 10 2010 041 061 A1 discloses a method and an apparatus for producing crystals by directional solidification from electrically conductive melts. The device has a crucible for holding the melt, a ceiling heater and a Power supply unit. By means of the device, a magnetic field can be set, which generates a crystallization front with a convex contour

 DE 10 2008 027 359 B4 describes a possibility of changing the magnetic field direction and strength when using a laterally arranged heater-magnet module. There it is disclosed that it is possible by means of such an arrangement to achieve better mixing of the melt by increasing velocities under the influence of a time-varying TMF.

 In DE 10 2007 028 548 B4 a combination of side heater magnet module with a bottom heater heater heater magnet module is presented. The illustrated embodiment shows an arrangement according to Czochralski.

 All of the mentioned methods have the general structure in FIG. 1 in common. This results in the marked Lorentz force densities. With these methods, crystals of good quality can be produced. However, there is an unfavorable effect of the magnetic field on the solid / liquid phase boundary during crystal growth in the edge region. The concave profile of the phase boundary in the edge region indicated in FIG. 1 can lead to polycrystalline structures in the edge region of single crystals or to ingots with a particularly small grain structure.

 Ingots can be monocrystalline or polycrystalline. Monocrystalline ingots can be produced by different crystal growing methods. As a rule, the growth takes place from the melt, wherein the Czochralski method is usually used in silicon and other semiconductor materials. Polycrystalline ingots (also referred to as multicrystalline ingots) are mainly used as Si ingots in photovoltaics for the production of solar cells and in micromechanics. If in the course of the application case by case of crystal or single crystal is mentioned, so hereby a monocrystalline ingot is meant.

 The object of the invention is to provide a method and suitable arrangements in order to further increase the crystal quality, in particular the homogeneity.

Summary of the invention

One or more problems of the prior art are directed by means of the crystallization plant according to the invention for the production of crystals Solidification dissolved from electrically conductive melts or at least reduced. The crystallization plant includes:

i. at least one crucible for receiving the melt;

ii. a heater-magnet module, with one or more coils and associated electrical leads, each crucible is associated with a heater-magnet module; and

iii. a control and power supply unit for the heater-magnet module.

 The heater-magnet module (HMM) assigned to the respective crucible is arranged and electrically driven in such a way that a Lorentz force density field can be generated, which is directed in such a way that the resulting force acts outward in the direction of growth relative to a geometric center axis of the crucible ,

 The crystallization plant according to the invention accordingly has one or more crucibles. Each crucible is assigned a heater-magnet module. The heater-magnet module is arranged above in such a way that it is aligned centrally or substantially centrally to the geometric center axis of the crucible (hereinafter also referred to as ceiling heater-magnet module (DHHM)). The heater-magnet modules are driven in a conventional manner by specifying amplitude, phase angle, etc. for generating a traveling magnetic field in the melt. In this case, the magnetic field in the melt is influenced so that a directed from the inside to the outside and thereby in the direction of the crystallization front flow is generated in the melt. The resulting force action of the Lorentz field thus extends at an angle to the geometric center axis of the crucible. With the crystallization plant according to the invention, in particular, the curvature of a solid / liquid phase boundary can be reduced and in this way the homogeneity of the solidification process can be improved over the entire crucible volume.

The invention thus provides a novel apparatus for directional crystallization of preferably semiconductor crystals of electrically conductive melts in crucible arrangements according to the vertical gradient method (VGF) as the preferred method. The invention enables the generation of Lorentz forces directed downwardly and radially outwardly from any imaginary point of the melt, and thereby simultaneously mixing the melt and producing a convexly curved solid-liquid phase boundary without inflection points. The Lorentz forces are by means of one above the crucible arranged heater magnet module (ceiling heater magnet module (DHMM)) produced analogously to the so-called KristMAG method. The coils of the ceiling heater magnet module are preferably fed with alternating current at a phase angle, wherein the phase angle between the coils may increase from the center, but is variable. An alignment of the Lorentz forces in the direction shown to the crucible edge has not been described and allows completely new ways to influence the shape of the solid / liquid phase boundary during crystallization. When using laterally arranged heater-magnet modules, as known from the prior art [z. Ch. Frank-Rotsch, U. Juda, B. Ubbenjans, P. Rudolph: "VGF growth of 4 in. Ga-doped germanium crystals under magnetic and ultrasonic fields" Journal of Crystal Growth 352 (2012) 16-20] , In principle, only Lorentz force densities are inducible in the melt, which have a slope from the crucible edge to the imaginary center line of the crucible. Convex phase boundaries are also adjustable using this arrangement, but there is always an undesirable concave portion of the phase boundary shape near the crucible edge. This concave portion can trigger polycrystalline structures in single crystal growth. The new arrangement can also be applied to Bridgman or DS crystallization plants, in which case the outer edge of the ceiling heater module preferably follows the outer crucible edge shape. Also in the crystallization of multicrystalline material, such. As PV silicon, the avoidance of W-shaped solid / liquid phase boundary is positive for the yield of higher quality material, since the beginning of the crucible edge ingrowth of very small grains can then be reduced.

In order to ensure that the resulting Lorentz force field can be calculated and thus influenced, according to a preferred embodiment of the device according to the invention, the power supply lines to the heater magnet module form an angle which ensures that the magnetic field induced by the power supply lines suppresses the magnetic field of the heater magnet. Module not affected. In other words, a crystallization system with inventive heater-magnet module is preferred, the electrical leads are arranged parallel to the central axis of the crucible. In the described novel crystallization system so the power supply lines are arranged so that they are close to the Heizerwindung (coils) perpendicular and are performed radially only at the maximum distance. As a result, the generated magnetic field in the (ceiling) heater-magnet module is not extinguished and the homogeneity of the Lorentz force density distribution in the melt remains. A preferred embodiment of the invention provides that the outer shape of the heater-magnet module according to the invention is concentric. As a result, it is preferable, but not essential, for the inner crucible shape to follow a circle as well. The resulting advantage is that the resulting Lorentz force field and, as a result, the flow field in the melt can be set highly symmetrical with a less expensive and thus less error-prone control and has a minimum density of perturbations. In other words, one or more coils of a heater-magnet module preferably form a circle or a spiral. Embodiments are possible in which the heater magnet module, in particular the ceiling heater magnet module, consists of any number of coils. Here, various types of arrangement are possible.

 According to one embodiment, the coils of the heater-magnet module thus form a spiral whose spiral winding is divided into two or more coils electrically isolated from each other (preferably 2 to 4 coils). In particular, the spiral may take the form of an Archimedes spiral (equivalent pitch), logarithmic spiral or spring-form spiral.

 Circular embodiments of the heater-magnet module can be obtained, for example, by two or more coils arranged annularly around the center point of the heater-magnet module. The annular coils may in each case comprise a plurality of turns running around the middle point, wherein a transition between the individual turns of a coil preferably takes place stepwise in order to obtain largely concentrically constructed coil turns.

 Another possibility for designing the heater-magnet modules is to use coils of different winding height or with a different winding cross-section. Furthermore, the coils can be arranged at different distances from the crystallization front; the coils are then not arranged in a common plane of the heater-magnet module. Alternatively or additionally, a variation of the winding height, the winding cross-section or distance to the crystallization front is also conceivable within a single coil. The variation of the Lorentz forces and the heat distribution with a heater-magnet module, which includes only a single coil, is particularly preferred because the power supply is much easier.

The crystallization plant preferably has two or more crucibles (More crucible assembly). Each crucible is associated with a heater-magnet module of the embodiment described above. The number of crucibles can be chosen arbitrarily. The heater-magnet modules induce an equal or largely equal Lorentz force field in each crucible. Thus, it is possible to grow crystals of very high similarity and quality simultaneously.

 A further aspect of the invention is the provision of a method for crystal growth from electrically conductive melts, which comprises the following (successive) steps:

i. Providing a crystallisation plant of the previously described embodiment; ii. Feeding the at least one crucible with the material to be processed and melting the material, preferably using the associated heater magnet module;

iii. Initiating crystallization from the bottom of the crucible; and

iv. Generating a time-dependent magnetic field in such a way that, contrary to the growth direction of a crystal to be grown outwards to an edge of the

Tiegel directed Lorentz field results.

 The method according to the invention enables the preparation of crystals with high homogeneity with respect to crystal quality, in particular a homogeneous, low dislocation density in single crystals.

In order to obtain crystals of high homogeneity, a Lorentz force field is preferably induced, which is centrosymetric metric, in which therefore the resulting Lorentz forces act zentrosym metric. This leads to a uniform, convex curvature of the solid / liquid phase boundary and a homogeneous distribution of the temperature in the melt. Both effects influence the crystal quality, in particular the homogeneity of the grown crystals positively.

 To generate the Lorentz force field according to the invention, various control variables are to be considered, which are described in more detail below:

The ceiling heater magnet modules of the crystallization system can be operated by means of predetermined current amplitudes. It is also conceivable to modulate the applied alternating current for the generation of the Lorentz force densities in the melt, for example in the form of a sinusoidal modulation. The modulation can be a variation of the amplitudes of the AC, the amplitudes of the modulation itself, as well as the periods of modulation include. The temporal modulation of the current can be done with different parameters. Due to the modulation particularly symmetrical temperature and magnetic field distributions can be generated in the crucible used, because slightly existing asymmetries in the generated magnetic field are averaged.

 Furthermore, the geometry and relative position of the individual coils in the heater-magnet module is observed. As a rule, the coils are driven sequentially in such a way that they generate a time-dependent magnetic traveling field in the melt. The frequency significantly affects the penetration depth and strength of the Lorentz forces thereby affecting the direction of the Lorentz forces. When using high frequencies, the penetration depth decreases and the intensity of the Lorentz forces and the inclination angle of the Lorentz force to the winding surface increase. The choice of frequency still depends on the electrical conductivity of the melt and thus also on the desired penetration depth. The phase shift affects the Lorentz force intensity only relatively weak (weak at low frequencies, but the influence of the phase shift increases at higher frequencies). However, the influence of the phase shift on the direction of the Lorentz forces is significant. As the phase shift increases, the intensity decreases, with the optimum depending on the frequency. In general, the phase shift should be set such that the resulting Lorentz force is at a slope away from the central vertical axis of the melt. By appropriate specifications of the phase shift or alignment of the Lorentz force, the direction of the magnetic traveling fields generated can be influenced. The current amplitude directly determines the Lorentz force intensity; with increasing amplitude, the Lorentz force increases sharply. The Lorentz force should be higher than the buoyancy force in the melt.

 Particularly in photovoltaics or in the general semiconductor industry crystals of very high perfection are needed. The method according to the invention can be used both for semiconductor materials (arsenides, silicon, germanium, etc.) and other electrically conductive materials such as molten metal or oxides.

Using the crystallization plant according to the invention, the process described leads to monocrystalline ingots which differ in their crystal properties from conventionally prepared ingots. These ingots provide one another aspect of the invention.

 Monocrystalline ingots which can be prepared or prepared according to the invention can be distinguished from known monocrystalline ingots as described below. This is defined

i. a core portion constructed of a body having the same central axis as the outer shape of the ingot and whose base G _K corresponds to seven tenths of the root area Gi of the ingot; and

ii. an edge region constructed from the according to i. remaining hollow body whose base G _R corresponds to three tenths of the base G _{K of} the ingot. The edge area is thus defined so that it is perpendicular to each cut surface perpendicular to

Central axis of the ingot corresponds to one third of the total area of the cut surface and always has the greatest possible distance from the central axis.

 The ingots according to the invention can now be characterized, inter alia, by a mean etch pit density EPD (R) in the edge region exceeding a mean etch pit density EPD (K) in the core region by a maximum of 75%. It therefore applies:

EPD R) <1 ^ EPD (K).

The orientation according to the invention of the Lorentz forces in the direction of the crucible edge avoids the occurrence of a point of inflection in the crystallization front near the edge of the crystal. As a result, ingots produced by the process have significantly altered crystal properties, which can be characterized, for example, by a deviating radial profile of the dislocation density p. The dislocation density p is the total length of all dislocation lines per unit volume in a crystalline solid. One known way of visualizing dislocations and determining their density is to etch the crystals in question on a surface. This results in so-called etching pits whose density can be counted in a light microscope (eg according to DIN specifications). The resulting etch pit density, engl. Etching pit density, EPD for short, is a measure of the quality of semiconductor wafers, especially in the semiconductor industry.

In the present case, the EPD is determined radially, ie along a line from the center of the crystal center to the edge of the crystal. This line runs along a half-shell on a cutting plane transverse to the growth direction of the crystal. Unlike conventional VGF Process grown crystals, the radial EPD curve in ingots according to the invention is not W-shaped, that is, the course does not decrease from the middle to the edge, then again increase sharply in the outer edge region. Rather, a nearly constant radial EPD results. For example, for a conventionally grown GaAs crystal in the crystallographic <100> direction, the EPD is 120% higher in the edge region than at the lowest point in the core region and increased by 85% over the mean in the core region. The increase in the dislocation density in the edge region of the crystals is particularly pronounced in the <100> direction (investigations on a VGF GaAs crystal with a diameter of 100 mm). In the case of ingots according to the invention, by contrast, a smaller increase in the dislocation density is found in the edge region of the semiconductor wafers: the average EPD is at most 75% above the mean value of the EPD of the core region, in particular at most 50% above the mean value of the core region, ie EPD (R) <li EPD (K), more preferably at most 25% above the EPD in the core region.

Another characteristic quantity which is used to determine the crystal quality is the residual stress. St. Eichler, for example, shows in "Crystal Growth Technology", Chapter 9 "Recent Progress in GaAs Growth Technologies at Freiberger" Page 231 -266, edited by HJ. Schell and P. Capper, WILEY-VCH Verlag GmbH & Co. KGaA, 2008 show a typical distribution of residual stress in GaAs wafers. There, values of 0.1-0.4 MPa are given, whereby the shear stresses in the edge area are significantly increased. The crystals according to the invention are characterized in that they also have lower thermal stresses during growth due to a solid / liquid phase boundary with a smaller concave deflection. These thermal stresses in the cultivation have radially seen from the crystal edge to the crystal center lower increases, so that even in the crystal after the cultivation measurable residual stress is less or distributed more homogeneously in the radial direction. Analogous to the property "dislocation density", therefore, the radial homogeneity of the residual stress of crystallographic directions can be used to determine the crystal quality and differentiation of the inventive ingots from conventionally produced ingots Ingots according to the invention now have a stress distribution (SV) at which the values for the stress distribution SV (R) in the edge region are not more than 100%, in particular not more than 75%, particularly preferably not more than 50% are increased compared to Stress distribution SV (K) in the core area. The edge region and core region are defined in the same way as explained above for the determination of the Ätzgrubendichte.

Another measure that reflects the crystal quality of GaAs crystals is the intrinsic defect density. The defect concentration is measured in certain areas. Defective concentration refers to the ratio of the number of defects in the lattice structure of a solid to the total number of lattice sites. Lattice defects in a solid occur because they bring an entropy gain through disorder. To delimit the ingots according to the invention from conventionally produced ingots, the homogeneity of the intrinsic defect density over the crystal can now be used. Specifically, the intrinsic defect density (EL2) is determined in the edge region and core region, the edge region and core region being specified again, as previously in the determination of the etching pit density. The self-defect density is usually considerably lower in the edge region than in the core region. According to the invention, the self-defect density EL2 (R) in the edge region is at most <50%, preferably <25%, in particular <10% lower than the self-defect density EL2 (K) in the core region.

 Furthermore, the electrical resistance distribution (EU) can also be determined radially and used to delineate the ingots according to the invention from conventionally produced ingots. By way of example, the electrical resistance distribution EU (R) in the edge region is increased by at most <10%, preferably <5%, in comparison with a resistance distribution EU (K) in the core region. Edge and core area are as defined above.

By using the invention, an improved mixing of the melt is achieved, which, for example in the case of GaAs crystals, has an effect on the distribution of As precipitates in the crystal and thus on the particle number on the polished surface of a cut in the radial direction of the ingot. Particularly large As precipitates appear on the polished surface as particles (diameter> 0.3 μηι). By reducing the As precipitates and distribution of the As excess in GaAs in uniformly distributed precipitates in sizes <0.3 μηι therefore smaller numbers of particles could be achieved. In fact, the ingots according to the invention, in particular monocrystalline ingots, can also be distinguished from conventionally produced ingots in that the particle number PZ (R) in the edge region shows a smaller deviation from the particle number PZ (K) in the core region. Specifically, the particle number PZ (R) in the edge region is at most <10%, preferably <5% higher than the particle density PZ (K) in the core region. Is inevitable also the cumulative particle number PZ is lowered over the edge and core region in the ingot produced according to the invention. In the special case of GaAs ingots, particle numbers PZ <40 per wafer (100 mm diameter) and for a crystal with 150 mm diameter <75 cm ^{"2 result} .

In summary, ingots which can be produced or produced according to the invention, in particular monocrystalline ingots, can thus be distinguished by one, several or all of the following properties from known ingots:

a) a ratio of etch pit density EPD (K) in the core region to etch pit density EPD (R) in the edge region satisfies the formula: EPD (R) <EPD (K); b) a stress distribution SV (R) in the edge region is not increased by more than 100% in comparison to a stress distribution SV (K) in the core region;

c) an electrical resistance distribution EU (R) in the edge area is increased by at most <10% compared to a resistance distribution EU (K) in the core area; and d) a particle number PZ (R) in the edge region is increased by at most <10% in comparison to a particle density PZ (K) in the core region,

 The core region is constructed from a body having the same central axis and corresponding to the outer shape of the ingot, the base area of which corresponds to seven tenths of the base area of the ingot. The edge area is constructed from the remaining hollow body, the base area of which corresponds to three tenths of the base area of the ingot. The base of the ingot corresponds to the area which results in a section perpendicular to the central axis of the ingot.

The crystallization process according to the invention can be used in particular for the production of ingots from materials having electrical conductivities in the range from 10 to 10 ⁸ S / m. The method according to the invention is preferably suitable for the growth of single crystals, such as GaAs with larger diameters (100-200 mm). The GaAs single crystal has in particular one or more of the properties mentioned under a) to d). Brief description of the figures

 The invention will be explained in more detail with reference to embodiments and associated drawings. Show it:

 1 is a schematic sectional view through a crystallization system with laterally arranged a crucible heater magnet modules according to the prior

Technology and a resulting Lorentz force field,

 2A-D are schematic sectional views through a crystallization system with above a crucible arranged heater magnet modules in four embodiments,

Fig. 3A-C are schematic representations of arranged in heater-magnet modules

 Coils in three embodiments of a device according to the invention,

4A-C are schematic representations of winding shapes in coils of a device according to the invention,

 5 shows a ceiling heater magnet module in the form of an Archimedes spiral with two

 Coils, as well as their power supply as a single-crucible design,

6A / B show a simulated Lorentz force density distribution in a GaAs melt generated by a ceiling heater magnet module consisting of 3 coils, generated with differing AC frequency,

 FIGS. 7A-E show time-sinusoidal modulations of the Lorentz force in the melt, FIGS. 8A / B simulated three-dimensional temperature distributions of a crystal after one

 Embodiment of the method according to the invention in the case of a) sinusoidally modulated current intensity and b) constant alternating current amplitude in the control of a heater-magnet module according to the invention,

 9 shows a radially determined course of an etching pit density (EPD) on a half-shell of a doped GaAs single crystal produced by the VGF method

(solid line with measuring points) and expected course using the invention (dashed)

 Fig. 10 is a schematic representation of a constructible core and

Edge region in a crystal produced by erfindungsmäßig process, and Fig. 1 1 is a schematic representation of a ceiling heater magnet module according to a further embodiment of the invention, each with a coil as a multi-pot arrangement of four crucibles.

Detailed description of the invention

 Figure 1 shows in a highly schematic manner a sectional view through a crystallization unit 1 according to the prior art. The crystallization unit 1 comprises a crucible 10 for receiving an electrically conductive melt 30. Above and below the crucible 10, standard heaters 60 and 62, respectively, are arranged in the lid or base of the crystallization unit 10. Laterally arranged on the crystallization unit 1 are heater-magnet modules 20, which comprise three coils 21 in the illustrated embodiment. The heater-magnet module 20 can be controlled by means not shown power supply lines such that a directional Lorentz force field 40 results. The resulting Lorentz force field 40 points to a geometric center axis 50 of a growing single crystal 32, wherein the geometric center axis 50 of the crystal coincides with the geometric center axis 50 of the crucible. In this case, the force field of the displacement direction of a solid / liquid phase boundary 31 is directed opposite. A directed Lorentz density force field 40 acts as a function of its parameters (for example, strength, direction, distribution, etc.) on the particle movement of an electrically conductive melt 30, so that by controlling the parameters of the Lorentz force field 40, a thorough mixing of the melt 30, and the shape of the solid / liquid-phase boundary 31 can be induced and controlled. When using laterally arranged heater-magnet modules 20, only Lorentz force densities are in principle inducible in the melt 30, which have a slope from the edge of the crucible 10 to the geometric center axis 50 of the crucible 10. Convex phase boundaries 33 can also be set using this application, but there is always an undesirable concave portion of the phase boundary 34 in the vicinity of the crucible edge. This concave portion of the phase boundary 34 can trigger polycrystalline structures.

FIGS. 2A-C show crystallization plants 1 in four embodiments. The crystallization unit 1 allows the directed crystallization of single crystals of electric melts in crucible arrangements according to the vertical gradient method (VGF), but is also applicable to crystallization systems according to the Bridgman or DS method, wherein the outer edge of the lid preferably follows the outer crucible shape. In each case arranged laterally are standard heaters 65, which do not induce magnetic fields. In the present case, a heater-magnet module 20 is arranged above the crucible. In the illustrated embodiments of FIGS. 2A-D, the heater magnet module 20 comprises two coils each. As already described in FIG. 1, the solid / liquid phase boundary 31 can be influenced by generating a Lorentz force field. In the exemplary embodiments illustrated in FIGS. 2A-C, alternating current is fed with a phase angle into the coils of the heater-magnet module 20, wherein the phase angle between the coils increases from the center to the outside. Alignment of the Lorentz forces in the direction shown to the edge of the crucible has not previously been described and enables novel possibilities of influencing the shape of the solid / liquid phase boundary 31 (crystallization front) during crystallization. The resulting Lorentz force field 40 is directed outwardly as shown in FIGS. 2A-C and thus induces a convexly curved solid-liquid phase boundary 31. Unlike the arrangement in the prior art in FIG. 1, this also applies to the edge region. Degrees of freedom in the alignment of the resulting Lorentz field are adjustable by the arrangement of the turns of the individual coils 21 and the coils to each other. In the embodiment of the crystallization plant according to the invention shown in FIG. 2A, the two windings, for example, of an Archimedes spiral are arranged at the same height. The Lorentz force field 40 is controlled by the parameters of the supplied alternating current, such as current and amplitude. Another embodiment of the crystallization plant according to the invention are spirals 21 with variable winding height (see FIGS. 2C / D); Here, by varying the winding height, the distribution of the Lorentz force density 40 and the heat distribution in the melt 32 can be adjusted in a targeted manner. The winding height allows the variation of the Lorentz forces 40 and the heat distribution even in arrangements with only one coil 21. In the described arrangements, embodiments are also possible in which the turns of the coil 21 are not in one plane. The freely selectable distance of the individual windings to the crystallization front opens up further freedom in the adjustment of the temperature and magnetic fields in the melt (see FIG. 2A).

Figures 3A-C show a selection of possible embodiments of the coils 21 in concentrically shaped heater-magnet modules. 3A, a so-called star-shaped coil 21 is shown (the heater-magnet module here comprises only a single coil), while FIG. 3C shows a typical Archimedes spiral composed of three coils 21.1. The spiral arrangement of two coils shown in FIG. 3B 21.1 and 21.2 has stepped windings.

 Beyond the geometric shape, the coils arranged in the heater-magnet module 20 according to the invention can also vary in the arrangement of the turns. For example, in addition to the classic Archimedes spiral (FIG. 4B), logarithmic forms (FIG. 4A) and spring-forming spirals (FIG. 4C) are also possible.

 FIG. 5 shows an exemplary embodiment of a heater-magnet module 20 according to the invention, which is arranged as a ceiling heater above the crucible 10. The heater-magnet module 20 is composed of an inner coil 21.1 and an outer coil 21.2, which are designed concentrically in the form of an Archimedes spiral. Also shown are the power supply lines 221 and 222 of the inner coil 21.1, as well as the power supply lines 231 and 232 for the outer coil 21.2, designed as a connection for AC voltage. In order to ensure the generation of a Lorentz force field in accordance with the invention, it is important to perform the power supply lines so that they are perpendicular to the windings of the heater magnet module and parallel only at the greatest possible distance. Otherwise, the Lorentz force field generated by the heater-magnet module 20 could be completely or partially extinguished, which in turn would lead to inhomogeneities in the melt and ultimately to polycrystalline inclusions.

 A crystallization plant 1 with a multi-seal arrangement is shown in FIG. 11. The crystallization unit 1 comprises a total of four crystallization units of the same structure. A unit comprises in each case a crucible 10 and a heater magnet module 20 with a coil 21 arranged above the crucible opening. Each coil 21 has two power supply lines 211, 212 with a connection for alternating current at the inner starting point of the coil 21 and a further connection on the coil outer end point of the coil 21st The power supply lines 21 1, 212 of the individual coils run first perpendicular to the turns of the coils 21 upwards and then in a star shape to a central terminal 200, which in turn is arranged concentrically above the crucible arrangement. Above the central terminal 200, the terminal 201 for the coil starts and the terminal 202 for the coil ends again apart to be connected to a power source, not shown.

FIGS. 6A / B show simulations of a Lorentz force density distribution in a GaAs melt produced by a heater magnet module according to the invention consisting of 3 coils. The distribution shown in FIG. 6A is a model with one AC frequency of f = 10 Hz and a phase angle Δφ = 100 ° between the coils. The calculations for FIG. 6B were based on f = 60 Hz AC frequency and a phase angle of Δφ = 100 ° between the coils. The distribution of the Lorentz force field is controllable by parameters of the applied alternating current, such as the frequency, as well as the phase angle between the coils.

 A device according to the invention can be operated both by the supply of predetermined current amplitudes, as well as driven by an additional novel method. In this case, the alternating current which is used for generating the Lorentz force densities in the melt is preferably time-modulated sinusoidally. It is possible to change the amplitudes of the alternating current, the amplitudes of the modulation, as well as the period of the modulation. The temporal modulation of the current can be done with different parameters, as shown in Figures 7A-E. In one embodiment, modulation with a period of 20 seconds (Figure 7D) proved to be particularly suitable. The mentioned variants of the method can produce particularly symmetrical temperature and magnetic field distributions in the crucibles used and contain slightly existing asymmetries in the generated magnetic field.

 FIGS. 8A / B three-dimensionally show the temperature distributions in a crucible. The distance between the lines is a measure of the temperature profile, the temperature increases with increasing line spacing. The temperature shows in a range of 1550 K to 1600 K particularly closely spaced lines. The partial representations illustrate the influence of sinusoidally modulated current intensity (FIG. 8A) and the influence of a constant alternating current amplitude (FIG. 8B) on the temperature field.

An ingot produced according to the invention is characterized in that it is homogeneously crystalline over its entire radial extent, ie from its core region to the edge region, in comparison to ingots which are cultivated according to the prior art. A measure of this radial homogeneity is, for example, the dislocation density. The dislocation density over etch pit density (EPD) is experimentally accessible. This is determined by treating the crystal surface to be considered with a selectively etching reagent. This creates trenches at the dislocations, which are visible under a microscope as lines whose number per unit area corresponds to the Ätzgrubendichte. FIG. 9 shows the course, determined experimentally for the surface, of the etched pit density of a half-shell of a GaAs produced by the known KristMAG process, which surface is determined experimentally. Crystal (solid line), the expected after a model course of a crystal prepared by the method according to the invention (dashed line). The abscissa shows the distance a from the central axis of the crystal. The central axis falls in the coordinate origin. The ordinate carries the experimental EPD of the two crystals.

 Clearly seen is the typical W-shape (seen in the figure only as a "half W" since only a half-shell is shown), which is the dislocation density of a prior art crystal. The higher the etch pit density, the higher As can be seen, in the crystal nucleus, a high monocrystalline fraction predominates, which decreases towards the outside and, before reaching a maximum value, passes through a local minimum.The maximum value of the etched pit density of the investigated crystal is almost double in the edge region For crystals produced by the process according to the invention, it is to be expected that they do not show this typical W-curve of the etch pit density. As FIG. 9 shows, the etch pit density increases moderately from the core region to the edge region. but does not go through a local minimum.

 The definition for edge region 56 and core region 55 of a crystal 32 (ingot) is shown schematically in FIG. The core region 55 is to be understood as a body whose outer shape corresponds to the crystal form. The base 51 of the body in the core region 55 is reduced by a third at the same height. Cutting the core region 55 out of the crystal 32 results in a hollow body forming the edge region 56. The base area 51 of the hollow body and thus of the edge area 56 corresponds to one third of the base area of the crystal 32. The base area of the crystal 32 thus results from the sum of the base areas 51 of the core area 55 and the base area 52 of the edge area 56. The central axes 50 of all three bodies , So the core portion 55, edge portion 56 and crystal 32 coincide.

Claims

claims 1. crystallization plant (1) for the production of crystals by directional solidification of electrically conductive melts, comprising:  i. at least one crucible (10) for receiving the melt;  ii. a heater-magnet module (20) having one or more coils (21) and associated electrical leads (200), each heater being associated with a heater-magnet module; and  iii. a control and power supply unit for the heater-magnet module (20), wherein the heater associated with the respective crucible magnet module (20) is arranged and electrically controlled so that a Lorentzkraftdichtefeld (40) can be generated, which is so directed in that the resulting force action, relative to a geometric central axis (50) of the crucible (10), acts outwards in the direction of growth. 2. crystallization system according to claim 1, wherein the electrical leads (200) are arranged parallel to the central axis (50) of the crucible (10). 3. crystallization plant according to one of the preceding claims, wherein the one or more folded coils (21) of a heater-magnet module (10) form a circle or a spiral. 4. crystallization plant according to one of the preceding claims, wherein the crystallization plant comprises two or more crucibles (10). 5. A method for growing crystals from electrically conductive melts comprising the following steps: i. Providing a crystallisation plant (1) according to one of the preceding claims; ii. Feeding the at least one crucible (10) with the material to be processed and melting (30) the material;  iii. Initiating crystallization from the bottom of the crucible (10); and iv. Producing a time-dependent magnetic field in such a way that a Lorentz field directed against the growth direction of a crystal to be grown outwards towards an edge of the crucible (10) to the edge of the crucible results. 6. The method of claim 5, wherein the Lorentz forces act centrosymmetric. 7. The method according to any one of claims 5 and 6, wherein resulting Lorentz forces over a temporal, in particular sinusoidal modulation of at least one of the parameters of an applied alternating current are set. 8. The method according to any one of claims 5 to 7, wherein the electrically conductive melts consist of or comprise semiconductor compounds. 9. Ingot, in particular monocrystalline ingot, producible or prepared by a process according to one of claims 5 to 8. 10. Ingot, in particular monocrystalline ingot, having one, several or all of the following properties:  a) a ratio of etch pit density EPD (K) in the core region to etch pit density EPD (R) in the edge region satisfies the formula: EPD (R) <EPD (K); b) a stress distribution SV (R) in the edge region is not increased by more than 100% in comparison to a stress distribution SV (K) in the core region; c) an electrical resistance distribution EU (R) in the edge area is at most increased by <10% compared to a resistance distribution EU (K) in the core area; and  d) a particle number PZ (R) in the edge region is increased by at most <10% compared to a particle density PZ (K) in the core region,  wherein the core portion is constructed as a body corresponding to one of the outer shape of the ingot and having the same central axis, the base area of which corresponds to seven tenths of the base area of the ingot, and the edge portion is constructed of the remaining hollow body whose base area corresponds to three tenths of the base area of the ingot. 1 1. The ingot according to claim 10, wherein the ingot is a monocrystalline GaAs ingot. 12. Ingot according to claim 1 1, wherein a self-defect density EL2 (R) of the GaAs ingot in the edge region is reduced by at most <50% compared to a self-defect density EL2 (K) in the core region.