Study of RGS (Ribbon Growth on Substrate) microstructure development

Abstract

Silicon wafer for solar cell applications can be produced at high speed and in one step by the RGS technique. Textured substrates at initial temperature below the melting point of silicon are moved at a constant speed under a silicon bath, providing the driving force for nucleation and crystal growth. Heat extraction is perpendicular to the wafer transport; therefore production rate is decoupled from crystallization velocity. The development of the microstructure is strongly influenced by the thermal-mechanical contact interface between the wafer and the substrate: the summits of the substrate surface texture make discrete contact points with the wafer. It was found that a better contact leads to higher growth velocity, smaller grain size and higher dislocation density. The important parameters influencing the formation of the interface were studied during casting experiments and by bringing silicon droplets in contact with the substrate. It was found that the initial temperature of the substrate is strongly influencing the formation of the thermal contact. Under certain conditions it was found that the crystallization front can form twinned dendritic crystals. This was attributed to a higher driving force for crystal growth than the driving force for nucleation on the substrate.