Investigating minority-carrier lifetime in small spherical Si using microwave photoconductance decay

Abstract

We have studied minority-carrier recombination lifetime in small spherical Si (<1mm in diameter) using microwave photoconductance decay (μPCD). It was tested that 0.01molar quinhydrone in methanol solution has the best passivation effect, which results in perfect exponential photoconductance decay curve and the highest average lifetime. However, the average lifetime of single crystalline spheres that were formed by polishing single crystalline Czochralski bulks was only 4.8μs, and that of tear-drop-like spheres that were formed by a dropping method was 2.6μs. These values are significantly lower compared with crystalline Si wafers. It has been found that the low lifetime is induced by the small size, and the spherical geometry does not have influence on the lifetime value. The surface recombination velocity becomes more dominant compared to bulk recombination as the size of sphere becomes smaller. Therefore, the principal of surface equivalent lifetime in the sphere is essentially different from the case of the planar wafer.