Nucleation and growth of multicrystalline silicon
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This work has focused on the nucleation process of multicrystalline silicon (mc-Si) for photovoltaic (PV) applications during solidification in α-Si3N4 coated crucibles, the typical crucible coating used commercially. Understanding the nucleation process is an important step to improve the ingot quality as it can greatly influence the grain structure and the dislocation density, both of which are important factors for the efficiency of a Si solar cell. Previous work on nucleation in mc-Si suggested that the nucleation occurred on particles in contact with the Si melt. Investigations of bottom cuts of mc-Si ingots showed mainly two types of particles: α- and β-Si3N4. The α-phase particles originate from the coating, while the β-phase is created during the solidification process by a phase transformation of the α-phase. The observed particles are both faceted and hexagonal, but differ significantly in size, with β-Si3N4 being up to a factor of 50 larger than α-Si3N4 in industrial size ingots. It was also observed that grain boundaries extended from clusters of β-Si3N4 particles, indicating that nucleation occurred on particles in these clusters. By thermal analysis and in-situ observations of the solidification process it was determined that the dominant nucleation sites are the β-Si3N4 particles, and that the nucleation undercooling is dependent on the particle size. The experimental results were explained using athermal nucleation theory, which states that the nucleation will occur at low temperatures and that the nucleation undercooling is inversely proportional to the size of the nucleation substrate. A consequence of the athermal nucleation theory is that the nucleation will first occur on the largest particles, and subsequent nucleation will occur on smaller and smaller particles until the nucleation stops. This is likely why the nucleation occurs mainly on β-Si3N4, as these particles are significantly larger than the α-phase particles. It was shown conclusively that β-Si3N4 particles can nucleate Si by studying the orientation relationship between a β-Si3N4 nucleation site and Si using transmission electron microscopy (TEM). An epitaxial relationship between the two phases was found by analysis of diffraction patterns. The β-Si3N4 particle was identified as a nucleation site based on grain boundaries extending from it. It was observed that Si could nucleate on different facets on the same particle. The grain boundaries was in one case identified as Σ3 twin boundaries, and based on the orientation relationship and particle geometry, β-Si3N4 particles were discussed as a source of the formation of twins in mc-Si.