Effect of gas atmospheres on the interactions between liquid silicon and coated graphite substrates

Abstract

Interactions between the gas atmosphere, liquid silicon, Si3N4 coating, and graphite are of great interest for photovoltaic silicon applications. However, previous studies focus more on the wetting of silicon on coated substrates rather than the coating stability and interaction with furnace atmosphere. Here we report on the coating deoxidation and the liquid silicon behavior at different compositions of nitrogen, carbon monoxide, and argon. In-situ melting experiments were performed with solar grade silicon samples in various gas compositions. The results showed a rapid reaction between molten silicon and carbon monoxide that led to the formation of a silicon carbide layer at the liquid free surface. This layer retains the silicon droplet at the early stage of wetting and prevents silicon infiltration and spreading. Furthermore, CO gas prohibits the self-reduction of SiO2 in the coating and the reduction by graphite. On the other hand, nitrogen accelerates the wetting of silicon as it favors the formation of highly wetted silicon nitride compound at the triple line of the droplet. A slight increase in the decomposition rate of silica content in the coating was observed with the introduction of nitrogen to the furnace. The influence of the morphology and the growth of nitride and carbide layers on the wettability was elucidated in detail. The results were supported by Raman spectroscopy and thermodynamic calculations.