Effects of grain boundary on impurity gettering and oxygen precipitation in polycrystalline sheet silicon

Abstract

The effects of grain boundaries (GB) in polycrystalline sheet silicon on impurity gettering and oxygen precipitation were investigated by electron beam induced current (EBIC), deep level transient spectroscopy (DLTS), micro-Fourier-transform infrared spectroscopy (FTIR), and preferential etching/Normaski optical microscopy techniques. Both as-grown and thermally processed wafers were studied. A correlation between GB density and transition metal concentration was quantitatively established by combining DLTS and EBIC studies. It was found that four deep levels arising from Fe–B, Fe–Al, Cr–B, and Fei were present in the as-grown sample, and their concentrations decrease with increasing GB density. GB gettering was further verified by the presence of an EBIC image contrast halo around the GB. Preferential etching also revealed a precipitate density of 2×107 cm−2 on the GB. After processing, a clearly defined oxygen precipitate denuded zone formed around the GB with the interstitial oxygen concentration [Oi] decreased from 14.4 to 2.2×1017 cm−3. Micro-FTIR showed that, for both processed and as-grown samples, more silicon oxynitride appears in the GB than in the intragrain region. Since nitrogen enhances oxygen precipitation, it is likely that nitrogen preferentially precipitated on the GB during the wafer formation process and resulted in a nitrogen depletion zone, where oxygen precipitation was further suppressed and a denuded zone formed.