Crucible for the production of multicrystalline silicon by directed solidification, process for its production and its use

Abstract

Crucible for the production of multicrystalline silicon by directed solidification, wherein the inner surface of the crucible base has a coating containing Si3N4 into which azotized silicon particles are introduced, wherein the azotized silicon particles protrude at least partially from the Si3N4-containing coating, wherein the azotized silicon particles each consist of a core, a transition layer which at least partially covers or encases the core, and a cover layer which at least partially covers or encases the transition layer, wherein the cores have a nitrogen content of at most 10 wt.%, the transition layers have a nitrogen content of more than 10 wt.% and up to 25 wt.%, and the cover layers have a nitrogen content of more than 25 wt.%.-%, wherein the azotized silicon particles have a particle size of 50 µm to 3000 µm, and wherein the particle density of the azotized silicon particles at the crucible bottom is 1 to 4000 particles / cm. 2 amounts.

Description

[0001] The present invention relates to a crucible for producing multicrystalline silicon by directional solidification. The inner surface of the bottom of the crucible has a Si3N4-containing coating into which azotized silicon particles are incorporated, wherein the azotized silicon particles protrude at least partially from the Si3N4-containing coating. Furthermore, the present invention also relates to a method for producing and using the crucible. Furthermore, the present invention relates to the use of azotized silicon particles for producing multicrystalline silicon by directional solidification in a crucible.

[0002] Currently, two main types of silicon material are used in the photovoltaics industry to manufacture solar cells. In addition to monocrystalline silicon, which is produced using the Czochralski pulling process, the more cost-effective multicrystalline silicon, produced using the directional solidification process, is preferred. In this manufacturing process, the silicon raw material is melted in a crucible, e.g., a quartz ceramic crucible, and solidified in a direction from the crucible base to the cap area. The inner surfaces of the crucibles used are coated with a silicon nitride-based material to prevent contact between the silicon melt or the silicon crystal and the crucible (so-called "sticking"), thus enabling the solidified silicon block to be removed.

[0003] In recent years, an optimized growth process has been developed that reduces the number of growth-related crystal defects and thus improves material quality. In this so-called HPM process ("high-performance multicrystalline-silicon process"), the silicon raw material is not melted directly above the coated crucible bottom (full melting process), but rather nucleated on an unmelted, fine-grained raw material layer at the crucible bottom (DE 10 2011 087 759 A1, DE 10 2012 023 120 A1). As a consequence, Si blocks result with a small-grained, randomly oriented grain structure with a high proportion of “random” grain boundaries, i.e. grain boundaries which are not symmetrical (see Grimmer, H.; Bollmann, W.; Warrington, HW: Coincidence-site lattices and complete pattern-shift lattices in cubic crystals. In: Acta Cryst. A30 (1974), pp. 197-207).At the same time, the proportion of Σ3 grain boundaries, i.e., symmetric grain boundaries, which are crucial for the formation and propagation of dislocation clusters, is reduced. The resulting solar cells exhibit an increased efficiency of up to 0.5% compared to cells made of conventional multicrystalline silicon (mc-Si) material.

[0004] However, since the HPM process generally increases the process time and the conversion from the conventional multi-process on an industrial scale involves considerable time and associated costs, there are currently approaches to achieve the same material quality using the original full melting process and a corresponding nucleation layer on the crucible bottom. In addition to a rough Si3N4 inner crucible coating (I. Brynjulfsen, L. Arnberg, "Nucleation of silicon on Si3N4 coated SiO2", Journal of Crystal Growth, 2001, 331; DE 10 2015 216 734 A1), this temperature range (T m[Si] ≈ 1400°C), non-melting foreign particles (including SiO2, SiC, and C) are introduced into the system as nucleating agents. Instead of having to nucleate specifically on a raw material layer, the silicon raw material introduced into the crucible can be completely melted. In the subsequent solidification phase, the roughness of the coating or the predetermined nucleation template acts as a nucleation initiator and leads to a grain structure similar to HPM.

[0005] In DE 10 2015 216 734 A1, the positive influence of rough Si3N4 coatings on the material quality of grown mc-Si blocks has already been demonstrated. Several crystallization experiments were conducted on a laboratory scale with varying roughness of the Si3N4 crucible inner coating. Starting with a root mean square roughness of over 20 µm, the proportion of random KG in the grown mc-Si blocks in the crystal base region could be increased by 20%, thus reaching up to 50%. The corresponding value for the HPM standard is approximately 70%. In addition, the average grain size was increased to 1.5 mm. 2 reduced. The corresponding reference had three times the value.

[0006] The tests conducted show that once the limit of 20 µm is reached, the positive effect of roughness no longer increases. The reason for this can be assumed to be abrasion or indentation of the porous, soft Si3N4 coating during the introduction of the Si raw material into the crucible system (approximately 1 kg on a laboratory scale and up to 850 kg on an industrial G5 scale). The actual roughness value after weighing the raw material could therefore be in the range of the limit of 20 µm.

[0007] DE 10 2013 109 024 A1 relates to a reusable crucible and its use in the production of a mono- or multicrystalline semi-metal body from a melt, in particular by directional solidification of a melt held in the crucible or by pulling a single crystal from a melt held in the crucible, preferably from a Si melt.

[0008] Wong et al. (YT Wong et al., "The effect of silica nucleation layers on grain control of multicrystalline silicon in directional solidification", Journal of Crystal Growth, 2014, 404) used porous Si\SiO2 mixed layers in different ratios as heterogeneous nucleation media and investigated their influence on the initial grain structure. All layers were placed in a crucible already coated with Si3N4. It was found that with increasing SiO2 content, the average grain size decreased. However, the proportion of random grain boundary particles was between 20% and 30% in all experiments and therefore significantly below the HPM standard. With increasing Si content in the mixed layer, nucleation also occurred on the underlying Si3N4 layer and resulted in the lowest material quality in terms of the random grain boundary fraction and average grain size.

[0009] Ding et al. (J. Ding et al., "Effect of the fused quartz particle density on nucleation and grain control of high-performance multicrystalline silicon ingots," Journal of Crystal Growth, 2016, 454) investigated the influence of the particle density of amorphous SiO2 nuclei on the initial grain structure. The particles used had a grain size between 600 and 800 µm and were introduced into the uncoated crucible base in the form of a suspension. Subsequently, a Si3N4 layer was sprayed onto the entire interior of the crucible.

[0010] With increasing particle density, the mean grain size in the soil area could be reduced and reached a minimum at a particle density of 220 particles / cm 2The explanation for this is the increasing nucleation area. A further increase in particle density led to an increase in grain size, as the SiO2 particles were too close together, thus causing them to sinter. This resulted in a decrease in the effective nucleation area.

[0011] Zhang et al. (H. Zhang et al.; “Growth of multicrystalline silicon ingot with both enhanced quality and yield through quartz seeded method”, Jour. of Crystal Growth, 2016, 435) used fine-grained SiO2 particles on a µm scale (i.e 50 = 74 µm) as a nucleating agent. First, a crucible was coated with an internal Si3N4 coating. A mesh with a grid thickness of 1 mm and free, square niches with sides of 4 mm was then placed in the crucible bottom.

[0012] The SiO2 particles were then sprayed onto the crucible bottom in the form of a suspension, and the grid was subsequently removed. Ultimately, the crucible bottom was not covered with a homogeneous SiO2 seed layer, but rather with numerous, regularly arranged SiO2 seed mounds. Finally, these mounds were coated with a 30-50 µm thick Si3N4 layer. The resulting Si block was then compared with a corresponding mc-Si and HPM reference.

[0013] The Si block produced with the SiO2 nucleation template is characterized by a significantly finer microstructure than the mc-Si reference and shows comparable results to the HPM block. Another characteristic used to evaluate block quality is the so-called bottom red zone. The bottom red zone is the area of ​​the Si block that is in direct contact with the crucible bottom or the crucible coating. In this area, the electrical parameter, the charge carrier lifetime, is substantially reduced compared to the center of the Si block due to the diffusion of metallic impurities from the crucible and crucible coating into the Si block. The bottom red zone of the nucleation test was less pronounced than in the HPM reference due to the full melting process used, resulting in a higher wafer yield. The cells of the Si block from the nucleation test showed the same high efficiency as those of the HPM reference.

[0014] Ultimately, this germination experiment produced the same material quality as the HPM process. However, the increased oxygen input reveals a disadvantage of the germination process used. This can lead to an increased LiD effect ("light-induced degradation"), which has a lasting negative impact on cell efficiency over an extended period of use (B. Sopori, IEEE PV Specialists Conference 2012).

[0015] In summary, it can be stated that the nucleation templates used previously in the full melt process could not achieve exactly the same material quality as the HPM process. Firstly, the robustness of the nucleation layer, such as the rough Si3N4 coatings, is insufficient, and secondly, the maximum possible nucleation area could not be utilized (SiO2 nucleation templates) because the nuclei sinter. Furthermore, the additional oxygen introduced leads to a sustained decrease in the achieved cell efficiency via the LiD effect.

[0016] Based on this, it was therefore the object of the present invention to provide a crucible or a process with which multicrystalline silicon with a fine-grained structure can be produced in a simple and cost-effective manner by means of directional solidification, which has a better or the same quality or at least a similar quality to multicrystalline silicon produced by the HPM process.

[0017] This object is achieved with respect to a crucible for producing multicrystalline silicon by directional solidification, having the features of patent claim 1, and with respect to a method for producing such a crucible, having the features of patent claim 8. Patent claim 13 relates to the use of the crucible according to the invention for producing multicrystalline silicon by directional solidification. The respective dependent patent claims represent advantageous developments.

[0018] According to the invention, a crucible for producing multicrystalline silicon by means of directional solidification is thus specified, wherein the inner surface of the crucible base has a coating containing Si3N4, into which azotized silicon particles are introduced, wherein the azotized silicon particles protrude at least partially from the coating containing Si3N4, wherein the azotized silicon particles each consist of a core, a transition layer which at least partially covers or encases the core, and a cover layer which at least partially covers or encases the transition layer, wherein the cores have a nitrogen content of at most 10 wt. % (based on the mass of the cores), the transition layers have a nitrogen content of more than 10 wt. % and up to 25 wt. % (based on the mass of the transition layers) and the cover layers have a nitrogen content of more than 25 wt.-% (based on the mass of the cover layers), wherein the azotized silicon particles have a particle size of 50 µm to 3000 µm, and wherein the particle density of the azotized silicon particles at the bottom of the crucible is 1 to 4000 particles / cm. 2 amounts.

[0019] Azotized silicon particles according to the present invention are silicon particles that have partially reacted with nitrogen, with the nitrogen being present in (more or less) tight bonds associated with silicon. Thus, these silicon particles comprise silicon and nitrogen, with the amount of nitrogen being lower than the amount of nitrogen in silicon nitride.

[0020] Particularly pure silicon nitride has a silicon content of up to 60.06 wt.% and a nitrogen content of up to 39.94 wt.%, whereby the proportion of impurities from other metals in these pure products is almost negligible and can be estimated at less than 0.002 wt.%. Technical silicon nitride can have a silicon content of approximately 59.5 to 59.8 wt.% and a nitrogen content of approximately 38.5 to 39.5 wt.%. Other elements include oxygen in concentrations of up to 0.5 wt.% and other metals in concentrations of less than 0.1 wt.%. In any case, technical silicon nitride has a minimum nitrogen content of 38.0 wt.%. Silicon-nitrogen compounds with a nitrogen content of less than 38 wt.% are therefore not to be referred to as silicon nitride.

[0021] Azotized silicon particles according to the present invention are thus silicon particles that preferably have a total nitrogen content of less than 38 wt.% (based on the mass of the particles). Particularly preferred azotized silicon particles according to the present invention have a total nitrogen content of at least 2 wt.% and at most 25 wt.% (based on the mass of the particles).

[0022] The azotized silicon particles incorporated into the Si3N4-containing coating serve as a nucleation template or represent a nucleation template. Because the azotized silicon particles at least partially protrude from the Si3N4-containing coating, the inner surface of the crucible base, on which the Si3N4-containing coating is located, has elevations or macroscopic irregularities (e.g., > 0.1 mm). In this way, the inner surface of the crucible base has a certain roughness, which makes it possible to produce multicrystalline silicon with a fine-grained microstructure using the crucible according to the invention, which has a better or the same quality, or at least a similar quality, to multicrystalline silicon produced using the HPM process.However, a special control of the melting isotherm, as in the HPM process, is advantageously not necessary, which is why the production of multicrystalline silicon by means of directional solidification is possible in a comparatively simple and cost-effective manner using the crucible according to the invention.

[0023] The Si3N4-containing seed layer in the crucible according to the invention, with the azotized silicon particles incorporated therein and partially protruding therefrom, exhibits a certain robustness, which ensures that the specified roughness is maintained even after the silicon melt has been introduced into the crucible and thus also at the beginning of nucleation. In contrast, the Si3N4 coatings used in DE 10 2015 216 734 A1 exhibit significantly lower robustness, which leads to the specified roughness either being overshadowed by the raw material or being lost due to interaction with the aggressive silicon melt.

[0024] The Si3N4-containing coating in the crucible according to the invention, with the azotized silicon particles incorporated therein and partially protruding therefrom, promotes nucleation during the production of multicrystalline silicon in the crucible according to the invention on silicon nitride and azotized silicon, which exhibit wetting properties towards the silicon melt. This also promotes the formation of a fine-grained microstructure.

[0025] Furthermore, the nucleation template used according to the invention (i.e., the azotized silicon particles used according to the invention) offers the advantage over previously used silicon dioxide-based particles that it does not lead to increased oxygen contamination of the grown silicon blocks. This prevents an increased LiD effect ("light-induced degradation"), which would have a lasting negative impact on cell efficiency over a longer period of use. In this respect, too, the multicrystalline silicon produced using the crucible according to the invention is of high quality.

[0026] The azotized silicon particles preferably have a greater or lesser proportion of nitrogen in each layer. The azotized silicon particles are integral particles of azotized silicon which is present in the different layers, namely the core, the transition layer, and the cap layer, with different nitrogen contents; i.e., the nitrogen is present in (more or less) strong bonds with silicon. Thus, both the cores, the transition layers, and the cap layers each comprise silicon and nitrogen. However, neither the core, the transition layer, nor the cap layer consists exclusively of silicon nitride, nor exclusively of silicon, nor of a mixture of silicon and silicon nitride. The cores preferably have a nitrogen content of at least 0.01% by weight, preferably of at least 0.1% by weight (based on the mass of the cores).

[0027] Preferably, neither the cover layer nor the transition layer form a diffusion-resistant layer, i.e., these layers are barrier-free and thus permeable to gases. These permeable layers are permeable to gases, especially nitrogen, regardless of their design as a partially covering or enclosing layer or as a completely covering or enclosing layer.

[0028] Due to the relatively low nitrogen content of the cores, the azotized silicon particles are melted during the melting phase due to the prevailing temperature conditions in the core. However, they still retain their original shape due to the outer layer, which has a relatively high nitrogen content and is therefore not melted. This allows them to serve as a nucleation layer during the formation of multicrystalline silicon through directional solidification. The different nitrogen contents in the various regions of the azotized silicon particles thus advantageously ensure that the roughness on the crucible floor created by the aforementioned elevations and thus the roughness required for nucleation is still present at the crucial time of nucleation. In this way, the production of multicrystalline silicon with even higher quality can be achieved.

[0029] A further preferred embodiment of the crucible according to the invention is characterized in that within the transition layer of the silicon particles, the nitrogen content gradually decreases from the cover layer to the core and / or within the azotized silicon particles, the nitrogen content gradually decreases from the outside to the inside. A gradual progression of the nitrogen content avoids abrupt transitions between regions with high nitrogen content and regions with low nitrogen content, thereby increasing the stability and robustness of the particles.

[0030] In a further preferred embodiment, it is preferred that the cores of the silicon particles - not made of silicon, and / or - have a nitrogen content of 0.5 wt% to 8 wt%, preferably 1 wt% to 5 wt%, particularly preferably 2 wt% to 3 wt% (based on the mass of the cores).

[0031] A further preferred embodiment is characterized in that the cover layers of the silicon particles - not made of Si3N4, and / or - have a nitrogen content of 27 wt.% to 37 wt.%, preferably of 32 wt.% to 37 wt.%, particularly preferably of 35 wt.% to 37 wt.% (based on the mass of the cover layers).

[0032] The nitrogen content can be determined using an N analyzer using the carrier gas hot extraction method.

[0033] The aforementioned preferred nitrogen content in the cores, transition layers, and / or cap layers of the azotized silicon particles influences the durability and robustness of the azotized silicon particles and thus also the roughness of the seed layer. The aforementioned advantageous nitrogen contents can therefore increase the durability and robustness of the azotized silicon particles, thereby maintaining the desired roughness of the seed layer for longer. This increases the quality of the produced multicrystalline silicon.

[0034] Furthermore, it is preferred that in the azotized silicon particles, the transition layer completely covers or encloses the core and / or the cover layer completely covers or encloses the transition layer.

[0035] The azotized silicon particles used in the crucible according to the invention can be produced, for example, by heating polycrystalline silicon to a certain temperature under an argon atmosphere and adding N2 under pressure. In this way, the nitrogen content of the azotized silicon particles can be achieved by decreasing or gradually decreasing from the top layer to the core.

[0036] The total nitrogen content of the azotized silicon particles is preferably 2 wt.% to 25 wt.%, particularly preferably 4 wt.% to 20 wt.%, very particularly preferably 6 wt.% to 15 wt.%. The nitrogen content can be determined using an N analyzer using the carrier gas hot extraction method. The total silicon content of the azotized silicon particles is preferably 73 wt.% to 98 wt.%, particularly preferably 78 wt.% to 96 wt.%, very particularly preferably 83 wt.% to 94 wt.%. The silicon content can be determined by X-ray fluorescence analysis.

[0037] The nitrogen and silicon content of the azotized silicon particles influence the durability and robustness of the azotized silicon particles and thus also the roughness of the seed layer. The aforementioned advantageous nitrogen and / or silicon contents can thus increase the durability and robustness of the azotized silicon particles, thereby maintaining the desired roughness of the seed layer for longer. This increases the quality of the produced multicrystalline silicon.

[0038] A further preferred embodiment is characterized in that the mass fraction of the cores is 10 wt.% to 90 wt.%, preferably 40 wt.% to 75 wt.%, and / or the mass fraction of the transition layers is 9.5 wt.% to 89.5 wt.%, preferably 24.5 wt.% to 59.5 wt.%, and / or the mass fraction of the cover layers is 0.5 wt.% to 40 wt.%, preferably 0.5 wt.% to 5 wt.% (in each case based on the mass of the particles).

[0039] The shape of the azotized silicon particles may be largely spherical or largely cylindrical or largely pyramidal or largely irregular, or mixtures thereof may be present.

[0040] Preferably, no further particles are introduced into the Si3N4-containing coating in addition to the azotized silicon particles.

[0041] Preferably, the azotized silicon particles do not contain silicon dioxide, silicon oxynitride, silicon carbide or mixtures thereof.

[0042] Furthermore, it is preferred that the oxygen content of the azotized silicon particles is less than 2 wt.%, preferably less than 1 wt.%, particularly preferably less than 0.5 wt.% (based on the mass of the particles).

[0043] Furthermore, it is preferred that - the azotized silicon particles have a particle size of 100 µm to 2000 µm, preferably 200 µm to 1000 µm, particularly preferably 200 µm to 800 µm, and / or - the particle density of the azotized silicon particles at the bottom of the crucible is 300 to 3500 particles / cm 2 amounts.

[0044] Particle size can be determined by sieving using a sieve stack with sieves of appropriate mesh sizes. For example, particle size can be determined according to DIN standard DIN 66165. Particle density can be determined by optically counting the number of particles per unit area using a digital microscope.

[0045] Due to the above-mentioned preferred particle sizes and this preferred particle density, a particularly suitable and uniform roughness of the seed layer can be achieved, which in turn leads to a higher quality of the produced multicrystalline silicon.

[0046] A further preferred embodiment is characterized in that the azotized silicon particles are azotized polycrystalline silicon particles or azotized monocrystalline silicon particles. In the case of azotized polycrystalline particles, the particle size is preferably between 0.2 mm and 2 mm, particularly preferably between 0.2 mm and 0.8 mm. In the case of azotized monocrystalline particles, the particle size is preferably between 1 mm and 3 mm.

[0047] According to a further preferred embodiment of the crucible according to the invention, the crucible contains or consists of a material selected from the group consisting of SiC, C, BN, pBN, Si3N4, SiO2, and mixtures and combinations thereof. The material of the crucible mentioned here refers to the base material of the crucible (or the material of the base body of the crucible), from which the walls and the bottom of the crucible are formed. However, the material of the crucible mentioned does not refer to the material of the Si3N4-containing layer or the azotized silicon particles.

[0048] A further preferred embodiment is characterized in that the Si3N4-containing coating - contains up to 100 wt.%, preferably 60 wt.% to 100 wt.%, particularly preferably 80 wt.% to 100 wt.% Si3N4, and / or - has a thickness of 10 µm to 2500 µm, preferably 100 µm to 1000 µm, particularly preferably 300 µm to 600 µm, and / or, - a root mean square roughness value R q from 1 µm to 100 µm, preferably from 5 µm to 20 µm, and / or - has an adhesion strength to the crucible bottom of 0.5 MPa to 5 MPa, preferably 1 MPa to 3 MPa, and / or - has a porosity of 0.5% to 60%, preferably 1% to 40%.

[0049] The root mean square roughness R q can be determined according to Rq=1ln∫0lnz2(x)dx

[0050] With I n = profile line length and z = values ​​in z-direction of the roughness profile, determined according to DIN EN ISO 4287:2010-07.

[0051] The adhesion strength can be determined according to DIN EN ISO 4624:2014-06, Tear-off test for the assessment of adhesion strength, or DIN EN ISO 2409:2013-06, Cross-cut test. Porosity can be determined using mercury porosimetry or BET measurement according to DIN ISO 9277.

[0052] A Si3N4-containing coating with a root mean square roughness within the specified preferred range is characterized by a relatively smooth surface. Such a surface quality can be achieved by using a suspension containing special additives (e.g., binders) in the production of the Si3N4 coating.

[0053] The present invention also relates to a method for producing the crucible according to the invention, in which a Si3N4-containing coating is produced on the inner surface of the crucible bottom of a crucible and azotized silicon particles are introduced into the Si3N4-containing coating in such a way that the azotized silicon particles protrude at least partially from the Si3N4-containing coating and the particle density of the azotized silicon particles on the crucible bottom is 1 to 4000 particles / cm 2wherein the azotized silicon particles each consist of a core, a transition layer which at least partially covers or encases the core, and a cover layer which at least partially covers or encases the transition layer, wherein the cores have a nitrogen content of at most 10 wt.% (based on the mass of the cores), the transition layers have a nitrogen content of more than 10 wt.% and up to 25 wt.% (based on the mass of the transition layers) and the cover layers have a nitrogen content of more than 25 wt.% (based on the mass of the cover layers), and wherein the azotized silicon particles have a particle size of 50 µm to 3000 µm.

[0054] A preferred variant of the method according to the invention for producing the crucible is characterized in that the Si3N4-containing coating is produced by applying a Si3N4-containing suspension to the inner surface of the crucible base, and then drying the moist Si3N4-containing coating thus produced. In other words, a Si3N4-containing suspension is first applied to the inner surface of the crucible base, thereby forming a moist Si3N4-containing layer. This moist layer is then dried, thereby producing the desired Si3N4-containing coating.

[0055] It is preferred that the Si3N4-containing suspension has a composition with the following components: 10 wt% to 60 wt%, preferably 20 wt% to 40 wt% Si3N4, 30 wt% to 80 wt% organic solvent or water, 0 wt% to 30 wt% silicon, 0.5 wt% to 10 wt%, preferably 4 wt% to 8 wt% dispersant, 0.01 wt.% to 0.2 wt.%, preferably 0.02 to 0.07 wt.% defoamer, and 0.05 wt.% to 2 wt.%, preferably 0.1 to 1 wt.% organic binder, the proportions of the components adding up to 100 wt.%.

[0056] Furthermore, it is preferred that - the application of the suspension containing Si3N4 is carried out by means of a spraying process, a brushing process, a spreading process and / or a dipping process, and / or - when applying the suspension containing Si3N4, the crucible has a temperature of 10 °C to 200 °C, preferably a temperature of 20 °C to 100 °C, and / or - the suspension containing Si3N4 is additionally applied to at least one further inner surface of the crucible, preferably to all inner surfaces of the crucible.

[0057] Furthermore, it is preferred that the azotized silicon particles be introduced into the moist Si3N4-containing coating before it is dried. In other words, the Si3N4-containing suspension is first applied to the inner surface of the crucible bottom, thereby forming a moist Si3N4-containing layer or coating. The azotized silicon particles are then introduced into this moist Si3N4-containing layer or coating.

[0058] The coating is then applied. The wet layer or coating is then dried, creating the desired Si3N4-containing coating, into which the azotized silicon particles are then incorporated. This ensures that the particles remain on the surface of the still-wet layer and are firmly fixed in this position during the subsequent drying phase. The retention of the particles on the surface of the still-wet layer is facilitated by the smaller size of the particles.Because the azotized silicon particles are firmly fixed in the Si3N4-containing coating, the introduced particles are not dislodged from their position at the crucible bottom by convective forces during the melting phase of multicrystalline silicon production and introduced into the melt above, preventing them from acting as nucleation initiators at the crucible bottom during the subsequent solidification phase. This ensures that the desired roughness of the nucleation layer at the crucible bottom, which is provided by the azotized silicon particles protruding from the Si3N4-containing coating, is still present at the beginning of nucleation.

[0059] By using special additives, such as an organic binder, in the suspension, the drying time of the applied layer can be increased, thereby achieving a high surface quality of the resulting coating. For example, this can prevent the suspension applied via a spray process from drying out excessively in the spray channel, which would result in a rough coating morphology. The addition of the additive thus leads to a relatively smooth surface of the Si3N4-containing coating. Furthermore, the addition of the additive ensures a homogeneous particle distribution in the suspension or coating, which can significantly reduce the risk of drying cracking.

[0060] The Si3N4 powder content in the coating or suspension is preferably up to 60 wt.%, particularly preferably from 10 wt.% to 40 wt.%, very particularly preferably from 20 wt.% to 30 wt.%. The proportion of the dispersant, which counteracts the agglomeration tendency of the introduced powder, is preferably up to 10 wt.%, particularly preferably from 4 wt.% to 8 wt.%. Since aqueous, ceramic suspensions generally tend to form bubbles, a defoamer is also added to the suspension. The amount added is preferably up to 0.1 wt.%, particularly preferably from 0.02 wt.% to 0.07 wt.%. To regulate the drying time and achieve high abrasion resistance, a binder is finally added to the suspension, preferably in an amount of up to 2 wt.%, particularly preferably from 0.01 to 1 wt.%, very particularly preferably from 0.05 to 0.5 wt.%.The remaining amount of the suspension preferably consists of distilled water and ranges from 30 to 80 wt.%, depending on the suspension. Between the addition of each component, the suspension can be stirred for up to 15 minutes at 4000 rpm to ensure homogeneous distribution of the powder or inorganic additives in the suspension.

[0061] Furthermore, the present invention also relates to the use of the crucible according to the invention for producing multicrystalline silicon by means of directional solidification, in particular for producing fine-grained microstructures from multicrystalline silicon by means of directional solidification.

[0062] Also disclosed is the use of azotized silicon particles for producing multicrystalline silicon by means of directional solidification in a crucible, wherein the inner surface of the crucible bottom has a Si3N4-containing coating into which the azotized silicon particles are introduced, wherein the azotized silicon particles protrude at least partially from the Si3N4-containing coating.

[0063] It is preferred that the azotized silicon particles each consist of a core, a transition layer which at least partially covers or encases the core, and a cover layer which at least partially covers or encases the transition layer, wherein the cores have a nitrogen content of at most 10 wt.% (based on the mass of the cores), the transition layers have a nitrogen content of more than 10 wt.% and up to 25 wt.% (based on the mass of the transition layers) and the cover layers have a nitrogen content of more than 25 wt.% (based on the mass of the cover layers).

[0064] Furthermore, a process for producing multicrystalline silicon by directional solidification is disclosed, in which a) a crucible according to the invention is provided, b) the crucible is filled with silicon raw material, c) the silicon raw material is melted in the crucible to form a silicon melt, and d) the silicon melt is subjected to directional solidification, producing multicrystalline silicon.

[0065] In this case, it is particularly preferred that the crucible according to the invention is provided in step a) by being produced by the method according to the invention for producing a crucible. In other words, step a) preferably comprises a method for producing the crucible according to the invention, in which a coating containing Si3N4 is produced on the inner surface of the crucible base of a crucible and azotized silicon particles are introduced into the coating containing Si3N4 in such a way that the azotized silicon particles at least partially protrude from the coating containing Si3N4, wherein the azotized silicon particles preferably each consist of a core, a transition layer which at least partially covers or encases the core, and a cover layer which at least partially covers or encases the transition layer, wherein the cores have a nitrogen content of at most 10 wt.-% (based on the mass of the cores), the transition layers have a nitrogen content of more than 10 wt.% and up to 25 wt.% (based on the mass of the transition layers) and the cover layers have a nitrogen content of more than 25 wt.% (based on the mass of the cover layers).

[0066] Furthermore, it is preferred that the contact angle of the silicon melt on the Si3N4-containing coating be less than 160°, preferably less than 120°, particularly preferably less than 90°. The contact angle can be determined using the sessile drop method, which is also known in the literature as the sessile drop method.

[0067] The present invention will be explained in more detail with reference to the following figures and examples, without limiting it to the specific embodiments and parameters shown here. Example

[0068] The azotized silicon particles used in the present invention can be produced, for example, as follows: Polycrystalline silicon (metal impurities < 10 ppm each, particle size 0.2 to 0.8 mm; 11.2 kg) was heated to 1250°C at 1.08 bar Ar in a reciprocating hearth furnace under an argon atmosphere. Once the temperature was reached, N2 was added until a pressure of 1.2 bar was reached. As soon as the pressure in the system dropped to 1.1 bar, the system automatically increased the pressure to 1.2 bar with N2. After 72 h, the reciprocating hearth furnace was shut down, and the product was removed after cooling. The product was roughly crushed in the first step (crusher gap width: 5 mm) and crushed in the second step with a gap width of 1 mm. From the resulting product, a fraction 1 with a particle size of less than 0.2 mm, a fraction 2 with a particle size of 0.2 mm to 0.8 mm, and a fraction 3 with a particle size of greater than 0.8 mm were sieved. 72% of the resulting product after the crusher with a gap width of 1 mm is in fraction 2 after sieving.Fraction 2 was used for further experiments. Fraction 2 had a total nitrogen content of 9.6% and a total silicon content of 89.8%. The oxygen content was 0.4%.

[0069] The particle size fractions were determined using a sieve tower with an automatic vibrating plate and a sieving time of 20 minutes. The top sieve was loaded with material to be screened with a bed height of between 3 and 5 mm.

[0070] To determine the nitrogen content in the individual layers of the azotized silicon particles from fraction 2, these were separated into sub-fractions as follows: 200 g of material from the 0.2 to 0.8 mm sieve fraction (corresponding to fraction 2) were placed in a Heubach apparatus and treated at 60 rpm for 15 minutes in the first run, 60 minutes in the second run, and 120 minutes in the third run. The material separated in the collection container in each case was referred to as the first coating, second coating, and third coating. The residue from the Heubach apparatus was treated three times in a mortar for 5 minutes each, and in each case the fraction < 0.16 mm was sieved out, weighed, and the nitrogen content determined. The residue was returned to the mortar. The sieved fractions were designated Transition 1, Transition 2, and Transition 3. The residue was treated in an automatic mechanical mortar under high pre-pressure and treated for a further 10 minutes and then sieved through a sieve. The sieved fraction was referred to as the outer core and the residue as the inner core.The activities result in losses of approximately 1.5% relative to the quantity used.

[0071] The following Table 1 shows the nitrogen contents of the individual fractions as well as the weight proportion of the fractions relative to the total weight of the particles. Table 1: Nitrogen and silicon contents according to particle structure Weight fraction [%] Nitrogen content, based on the weight fraction [wt.%] Silicon content, based on the weight fraction [wt.%] First wrapping 1,5 30,7 69,0 Second wrap 1,2 29,2 70,3 Third wrapping 0,8 27,6 72,0 Transition 1 4,3 18,7 81,1 Transition 2 8,2 12,8 86,8 Transition 3 10,3 11,4 88,2 Core outside 16,4 9,2 90,5 Core inside 55,8 6,6 93,2

[0072] From Table 1, the values ​​given in Table 2 below are calculated for the outer layers (first, second and third cladding), the transition layers (transition 1, 2 and 3) and the cores (outer and inner core). Table 2: Nitrogen and silicon contents of the cover layers, the transition layers and the cores according to the structure of the particles Weight fraction [%] Nitrogen content, based on the weight fraction [wt.%] Silicon content, based on the weight fraction [wt.%] Top layer 3,5 29,4 70,0 Transition layer 22,8 13,3 86,3 core 72,2 7,2 92,6

[0073] In the subsequent growth process, one exemplary embodiment used four different variants of the crucible according to the invention, each differing from one another in their nucleus (i.e., in the azotized silicon particles used). Thus, four different nuclei were used: three polycrystalline and one monocrystalline nuclei. The three polycrystalline nuclei are azotized, polycrystalline silicon particles (Azo-poly) in the size range of 0.5 to 1 mm (Azo-poly 1), 0.2 to 0.8 mm (Azo-poly 2), and 0.2 to 0.3 mm (Azo-poly 3). The Azo-poly 2 nucleus is the material characterized in Table 2. The azotized, monocrystalline silicon particles (Azo-mono) were in the size range of 1 to 3 mm. The following Table 3 shows the test overview as well as the particle densities at the bottom of the crucible of the variants with the respective nucleation templates. Table 3: Overview of the four nucleation templates used in the crucible according to the invention according to the example Germ template Total nitrogen content [%] Particle size [cm] Particle density at the bottom of the crucible [particles / cm 2 ] Azo-poly 1 7,9 0,05 - 0,1 120 Azo-poly 2* 9,6 0,02 - 0,08 400 Azo-poly 3 16,2 0,02 - 0,03 3200 Azo-mono 2,0 0,1 - 0,3 6,3 * corresponds to the material characterized in Table 2

[0074] The amount of introduced germs was approximately 0.06 g / cm in relation to the available area on the bottom of the crucible in all experiments. 2 Thus, in the case of an experiment with a single nuclei sample at the bottom of the crucible, approximately 30 g were introduced.

[0075] For the azotized polycrystalline nuclei, an increase in particle density with decreasing particle size can be observed due to the identical initial weight. The lowest particle density is achieved with the coarsest particles of the azotized monocrystalline nuclei.

[0076] Using four different variants of the inventive crucible, each containing one of the four mentioned nucleation templates, multicrystalline Si blocks were produced by directional solidification. The microstructure of the Si blocks grown in this way was then compared with the microstructure of Si blocks from a standard multi- and HPM process. Furthermore, for comparison, a crystal was produced in a standard multi-process, according to patent DE 10 2015 216 734 A1, using a rough crucible inner coating.

[0077] The results of the tests are in the Fig. 1 to 3 and summarized in Table 4.

[0078] Fig.Figure 1 shows scans of the horizontal sections of all reference and seed experiments at different crystal heights (20 mm and 80 mm) with corresponding coefficients for the grain orientation distribution. CV values ​​less than 1 indicate an inhomogeneous orientation distribution, while larger values ​​represent a homogeneous distribution.

[0079] Fig. Figure 2 shows an overview of the mean grain sizes of all reference and nucleation tests at different crystal heights (5 mm, 20 mm, and 80 mm). In HPM processes, smaller grains are generally formed across the entire crystal height than in conventional full-melt tests.

[0080] Fig. Figure 3 shows the contribution of different grain boundary types, including Σ3 and random grain boundaries, to different crystal heights for all reference experiments and growths with nucleated templates.

[0081] In Fig.Figure 1 compares the orientation distributions (CV values), measured on horizontal sections at different crystal heights, of the corresponding references and experiments with nucleation templates. If the resulting CV value is less than one, the grain orientation distribution is considered inhomogeneous. Otherwise, the distribution is homogeneous.

[0082] Comparing the three references, it can be seen that the Standard Multi test, as expected, exhibits a homogeneous orientation distribution at both crystal heights considered, whereas both the test with the rough crucible inner coating and the HPM test exhibit an inhomogeneous distribution. The coarsest azotized polycrystalline nuclei show comparable results to the Standard Multi reference, with the determined values ​​indicating an even more homogeneous orientation of the grains. The azotized monocrystalline nuclei also show a homogeneous orientation across the crystal height considered. The two fine-grained azotized polycrystalline nuclei, in contrast, lead to comparable or, in some cases, more favorable CV values ​​than the RPM reference.

[0083] As already mentioned in Fig.As can be seen in Figure 1, the seed templates used also influence the grain size of the grown crystals. To verify this and to compare the experiments, the average grain sizes were measured for all experiments at three different crystal heights (5 mm, 20 mm, and 80 mm). The results are shown in Fig. 2 and in Table 4 below. Table 4: Average grain sizes of all tests at a crystal height of 5 mm Germ template Particle size [cm] Average grain size at a crystal height of 5 mm [mm 2 ] HPM Reference - 1,0 ± 0,2 Standard Multi-Reference - 4,5 ± 0,9 Rough Si3N4 coating - 2,3 ± 0,5 Azo-poly 1 0,05 - 0,1 2,6 ± 0,5 Azo-poly 2 * 0,02 - 0,08 1,1 ± 0,2 Azo-poly 3 0,02 - 0,03 1,3 ± 0,3 Azo-mono 0,1 -0,3 1,6 ± 0,3 Poly-Si 0,02 - 0,08 2,0 ± 0,4 * corresponds to the material characterized in Table 2

[0084] As expected, the HPM reference shows smaller grain sizes across the entire crystal height than the standard multi-reference. The test with the rough Si3N4 crucible inner coating follows, as already shown in the results for the orientation distribution in Fig.1, between the other two references. For the azotized polycrystalline nuclei, the mean grain sizes for the coarser particle fraction are in the range of the standard multi-reference with a rough inner crucible coating, so that HPM-equivalent values ​​could not be achieved in this case. For both fine-grained variants, however, the achieved mean grain sizes are within the size range of the HPM reference at all crystal heights considered. Here, the particles in the range of 0.2 to 0.8 mm stand out.

[0085] In Fig.Figure 3 lists the percentage of relevant grain boundary types, including random and Σ3-KG, for all tests at two different crystal heights (20 mm and 80 mm). The results show that, compared to the Si block from the standard multi-process, the proportion of random KG is higher in all other tests. Even the rough Si3N4 coating produces a finer microstructure, and the proportion of random KG can be increased by approximately 20% for both crystal heights considered, but is still noticeably lower compared to the HPM microstructure. The coarse-grained azotized polycrystalline nuclei lead to comparable values ​​to the standard multi-reference. Presumably, the particle density is insufficient in this case (see upper table).

[0086] The fine-grained variants, on the other hand, lead to significantly higher random-KG fractions. The random-KG fraction is only a few percent lower than the value for the HPM material. Despite the difference in particle density between the two azotized variants, no different random-KG fraction could be generated considering the measurement accuracy. This could be an indication that starting at a particle density of approximately 400 particles / cm 2 (see table above) the limit has been reached at which the random-KG proportion no longer increases.

[0087] The azotized monocrystalline nuclei, despite the lowest particle density, result in a high proportion of random KG. This is higher than in the standard multi-reference and is virtually in the range of the fine-grained polycrystalline nuclei. Since the degree of azotization in this case is just 2% and significantly lower than in the other azotized nuclei (see table above), it could be assumed that the upper region of the nuclei is eroded by the Si melt and the actual nucleation occurred on other particle morphologies or densities. Complete dissolution of the nuclei did not occur, as the particles were still clearly visible at the bottom of the crystal.

Claims

[1] Crucible for producing multicrystalline silicon by means of directional solidification, wherein the inner surface of the crucible base has a coating containing Si3N4, into which azotized silicon particles are introduced, wherein the azotized silicon particles protrude at least partially from the coating containing Si3N4, wherein the azotized silicon particles each consist of a core, a transition layer which at least partially covers or encases the core, and a cover layer which at least partially covers or encases the transition layer, wherein the cores have a nitrogen content of at most 10 wt.%, the transition layers have a nitrogen content of more than 10 wt.% and up to 25 wt.% and the cover layers have a nitrogen content of more than 25 wt.-%, wherein the azotized silicon particles have a particle size of 50 µm to 3000 µm, and wherein the particle density of the azotized silicon particles at the bottom of the crucible is 1 to 4000 particles / cm. 2 amounts. [2] Crucible according to claim 1, characterized by that within the transition layer of the azotized silicon particles, the nitrogen content gradually decreases from the top layer to the core. [3] Crucible according to one of claims 1 or 2, characterized by that the cores of the azotized silicon particles - not made of silicon, and / or - have a nitrogen content of 0.5 wt% to 8 wt%, preferably 1 wt% to 5 wt%, particularly preferably 2 wt% to 3 wt%. [4] Crucible according to one of claims 1 to 3, characterized by that the outer layers of the azotized silicon particles - not made of Si3N4, and / or - have a nitrogen content of 27 wt% to 37 wt%, preferably 32 wt% to 37 wt%, particularly preferably 35 wt% to 37 wt%. [5] Crucible according to one of the preceding claims, characterized by , that - the azotized silicon particles have a particle size of 100 µm to 2000 µm, preferably 200 µm to 1000 µm, and / or - the particle density of the azotized silicon particles at the bottom of the crucible is 300 to 3500 particles / cm 2 amounts. [6] Crucible according to one of the preceding claims, characterized by that the crucible contains or consists of a material selected from the group consisting of SiC, C, BN, pBN, Si3N4, SiO2, and mixtures and combinations thereof. [7] Crucible according to one of the preceding claims, characterized by that the Si3N4-containing coating - contains up to 100 wt.%, preferably 60 wt.% to 100 wt.%, particularly preferably 80 wt.% to 100 wt.% Si3N4, and / or - has a thickness of 10 µm to 2500 µm, preferably 100 µm to 1000 µm, particularly preferably 300 µm to 600 µm, and / or, - a root mean square roughness value R q from 1 µm to 100 µm, preferably from 5 µm to 20 µm, and / or - has an adhesion strength to the crucible bottom of 0.5 MPa to 5 MPa, preferably 1 MPa to 3 MPa, and / or - has a porosity of 0.5% to 60%, preferably 1% to 40%. [8] A method for producing a crucible according to any one of the preceding claims, in which a Si3N4-containing coating is produced on the inner surface of the crucible bottom of a crucible and azotized silicon particles are introduced into the Si3N4-containing coating in such a way that they protrude at least partially from the Si3N4-containing coating and the particle density of the azotized silicon particles on the crucible bottom is 1 to 4000 particles / cm 2wherein the azotized silicon particles each consist of a core, a transition layer which at least partially covers or encases the core, and a cover layer which at least partially covers or encases the transition layer, wherein the cores have a nitrogen content of at most 10 wt.%, the transition layers have a nitrogen content of more than 10 wt.% and up to 25 wt.% and the cover layers have a nitrogen content of more than 25 wt.%, and wherein the azotized silicon particles have a particle size of 50 µm to 3000 µm. [9] Method according to claim 8, characterized by that the Si3N4-containing coating is produced by applying a Si3N4-containing suspension to the inner surface of the crucible bottom and drying the moist Si3N4-containing coating produced in this way. [10] Method according to claim 9, characterized bythat the suspension containing Si3N4 has a composition with the following components: 10 wt% to 60 wt% Si3N4, 30 wt% to 80 wt% organic solvent or water, 0 wt% to 30 wt% silicon, 0.5 wt% to 10 wt% dispersant, 0.01 wt% to 0.2 wt% defoamer, and 0.05 wt% to 2 wt% organic binder, whereby the proportions of the components add up to 100 wt%. [11] Method according to one of claims 9 or 10, characterized by , that - the application of the suspension containing Si3N4 is carried out by means of a spraying process, a brushing process, a spreading process and / or a dipping process, and / or - when applying the suspension containing Si3N4, the crucible has a temperature of 10 °C to 200 °C, preferably a temperature of 20 °C to 100 °C, and / or - the suspension containing Si3N4 is additionally applied to at least one further inner surface of the crucible, preferably to all inner surfaces of the crucible. [12] Method according to one of claims 9 to 11, characterized by that the azotized silicon particles are introduced into the still moist Si3N4-containing coating before it is dried. [13] Use of a crucible according to any one of claims 1 to 7 for producing multicrystalline silicon by directional solidification.

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