Structure Control of Multicrystalline Silicon

Abstract

This work targets multiple approaches for controlling the initial growth conditions of directional solidification, and their effect on the final material microstructure and performance. Challenges related to both seeded and non-seeded growth conditions are addressed. Seeding with pre-defined silicon crystals are a relatively simple way of limiting random nucleation, and tuning the final microstructure of the ingot. The microstructure of the ingot will, to a large extent, reflect the microstructure of the seed particles, meaning that monocrystalline seeds lead to a near-monocrystalline, or mono-like, microstructure, while fine-grained multicrystalline seeds lead to the more refined high performance multicrystalline microstructure. This work reveals that the main challenges with the mono-like method are i) the parasitic grain structure developing from the periphery of the ingot and ii) dislocation sources appearing between multiple seed crystals. While dislocations generated at stresspoints can be minimized by proper seed preparation, and by introducing small gaps between individual seeds, the dislocation structures generated due to seed misorientation can become very detrimental for the final material performance. The magnitude and complexity of the misorientation appears to be determinative in the advance of these dislocation structures, and a better understanding of the mechanisms governing the dislocation behaviour at the seed junctions is needed. While the mono-like method targets to eliminate both grain boundaries and dislocations, the high performance method utilize certain properties of random angle grain boundaries to terminate the propagation of dislocation clusters. Seeding of this type of material is therefore usually done with very fine-grained seeds that contains a very high fraction of random angle grain boundaries. This part of the work focuses on investigating the microstructural differences between ingots seeded with polysilicon chips and fluidized bed reactor granules. The work suggests that larger and more uniformly shaped seeds may be more suitable for seeding purposes, as they contain less morphological extremities for stress and dislocation generation, and are less prone to microstructural coarsening. Nevertheless, due to the high density of grain boundaries, further performance improvements to this type of material can mainly be realized through advances in impurity control and/or post-processing. Non-seeded growth methods are mainly governed by the nucleation conditions during the earliest stages of solidification, i.e. the nucleation stage. In this work, samples cut from the bottom of ingots are investigated in terms of locating possible nucleation sites, and mechanisms active during this stage. By correlating experimental observations with the free growth model, we posit that nucleation takes place preferably on β-Si3N4 particles, due to their high compatibility with silicon and their large size. The clear signs of dendritic growth indicate that dendrites play an important role also during directional solidification of multicrystalline silicon, and should be avoided if a finer grain structure is desired. Based on this study, it is also suggested that proper engineering of Si3N4 particles, together with certain cooling parameters, may prove useful for achieving different microstructures.