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Recombination velocity of SiNx:H/silicon interfaces and the relationship of insulator charge.

Hydrogenated Silicon nitride (SiNx:H) is used as an anti-reflection coating and passivating medium in crystalline silicon photovoltaic technology. In these studies, we deposited SiNx:H on a variety of silicon wafers by both the PBS™ remote plasma enhanced chemical vapor deposition process and the direct plasma enhanced chemical vapor deposition process. After deposition, the minority-carrier lifetime was measured by the resonance-coupled photoconductive decay (RCPCD) technique. From the lifetime data measured, we were able to calculate the surface recombination velocity. The lifetime measurements were also performed by RCPCD using an adjustable dc white light bias. The latter has a maximum of about 1.65 suns. In high-resisitivity samples, there were dramatic reductions of lifetime under white light bias, whereas more heavily doped wafers showed minimal effects. We performed the capacitance-voltage (CV) measurements on these samples using a mercury probe, and found a substantial density of fixed charge in the SiNx. The composite of data show that the positive charge increased the carrier lifetime in high-resisitivity float-zone wafers due to charge separation at the induced, electrical junction. For more heavily doped wafers (1 – 10 ohm-cm) grown by the CZ technique, the light bias had no effect on the measured lifetime. In summary, charge separation at the SiNx:H /silicon interface produces a large increase in the effective lifetime in high resisitivity samples. Light bias has relatively little effect on doped wafers that are typically used as the base in photovoltaic cells. The interface grown by both PECVD methods has a relatively low recombination velocity.