Morphologies and photoluminescence of porous silicon under different etching and oxidation conditions

It is found from scanning electron microscopy that the porous silicon samples fabricated under different conditions exhibit different morphologies. Some of them show islands or cracks, but the rest appear to be smooth. It is proposed that the formation of the islands and the cracks depends on the porosity of the porous silicon samples and result from the shrinkage of the porous structure. The photoluminescence spectra of the porous silicon samples also depend on the etching conditions. The peak position of the photoluminescence shifts to higher energy with increasing the porosity of the samples or by rinsing the samples in deionized water for a long time. However, in certain situations the photoluminescence peak shifts to lower energy after the samples are placed in air for a couple of days. It is proposed that not only the quantum size effect but also the transition between oxide levels contribute to the photoluminescence of the porous silicon.