Kinetics of silicon nitride coatings degradation and its influence on liquid infiltration in PV silicon crystallization processes

Safe implementation of reusable crucibles in the silicon PV industry requires a thorough understanding of the reactions in the system to avoid liquid infiltration and crucible failure at high temperatures. Typically, an oxidized Si3N4-coating layer is applied to the crucibles to avoid wetting. During melting, the coating undergoes a transition in its wettability behavior as a result of oxide depletion. However, much uncertainty still exists about undergoing reactions and their effect on the coating depletion, and hence the wetting kinetics. Here we report on the coating's oxygen depletion mechanisms by applying a novel coating method to avoid conventional oxidation as it causes severe degradation of non-oxide crucibles. By adding colloidal silica to the silicon nitride coating, we (i) control the oxygen concentration in the coating, (ii) avoid the crucible degradation, and (iii) eliminate the pre-oxidation step. Furthermore, a quantification of the coating's oxygen content effect on the depletion rate was presented via an analytical model. These results provided insight into how the coating depletion can hinder the liquid infiltration by forming and stabilizing silicon oxynitride in the coating as evidenced by Raman mapping and thermodynamic calculations. The difference between the rates of liquid infiltration and oxygen depletion was also elucidated in detail.