The trade-off between phosphorus gettering and thermal degradation in multicrystalline silicon

Abstract

The bulk recombination lifetime of multicrystalline silicon wafers is found to initially increase with phosphorus gettering, but then, for low resistivity wafers, to decrease after a certain optimum gettering time. This peak is attributed to a trade-off between the competing mechanisms of lifetime improvement through impurity removal, and lifetime reduction due to ‘thermal degradation’. Such thermal degradation is found to be more pronounced in low resistivity samples. The physical cause of the thermal degradation has been attributed by some to the dissociation of impurity precipitates, resulting in a greater concentration of interstitial impurities. However, we find that a variable temperature gettering method, designed to circumvent the deleterious effects of precipitate dissociation, does not result in an increase in recombination lifetime, but instead results in a significant decrease. An observed increase in the dislocation density of low resistivity samples upon high temperature treatment is suggested as an alternative explanation for the thermal degradation effect.