Firing stability of n-type poly-Si on oxide junctions formed by quartz tube annealing

Passivating contacts formed by poly-Si on oxide (POLO) junctions yield high passivation qualities after an appropriate annealing process at temperatures between 800°C and 1050°C. In today's typical cell process, firing is applied in the end of cell production mainly to form screen printed contacts. Thus, a high stability of the passivation quality against this firing process is required - and also expected since a previous high-temperature process for POLO junction formation implies a much higher thermal budget. However, in this work we found a significant decrease in effective lifetimes of up to 75% for n-type POLO samples with ~ 1.5 nm interfacial oxide at firing temperatures of 620°C to 900°C. This holds without a supply of hydrogen (no capping layers). Experiments with hydrogen-rich dielectric capping layers show, however, that a coating with AlOx:H as opposed to SiNy:H (n = 2.05), can significantly increase the stability of the passivation upon firing. Capacitance-voltage measurements show that the saturation current density correlates to the density of defect states at the SiOx/c-Si interface when varying the firing temperature. Although firing with hydrogen supplying layers such as AlOx:H seems to be viable, our results may indicate that the chemical configuration of the SiOx/Si interface changes from Si-O to Si-H bonds upon firing. If this hypothesis holds true, possible implications on the long-term stability of the passivation quality should be evaluated.