Passivation Effects on Low-Temperature Gettering in Multicrystalline Silicon

Annealing at ≤ 500 °C changes minority carrier lifetime in as-grown multicrystalline silicon substantially. Part of the change arises from internal gettering of impurities, but surface passivation for lifetime measurement results in additional effects. We report experiments that aim to clarify the role of passivation. Long-term annealing (up to 60 h) is performed on silicon nitride passivated multicrystalline silicon, and lifetime and interstitial iron concentrations are monitored at each processing stage. Lifetime in all samples is improved under certain conditions, with improvements always achieved at 400 °C. Increases are pronounced in low-lifetime bottom samples, with improvement by a factor of 2.7 at 400 °C or 3.8 at 500 °C. Important differences are found compared with our previous study with iodine-ethanol passivation. First, as-received lifetime is higher with silicon nitride not due to a substantial difference in surface recombination. Second, while interstitial iron concentrations often initially increase with iodine-ethanol, they tend to reduce with silicon nitride. Third, lifetime in high-lifetime samples reduces substantially with iodine-ethanol but increases with silicon nitride. Secondary ion mass spectrometry reveals high iron concentrations in annealed silicon nitride. Results are discussed in terms of gettering of impurities to, and bulk passivation arising from, silicon nitride films.