Two kinds of enhancement in photoluminescence by chemical etching of porous silicon

We investigated the effect of chemical etching of porous silicon in a solution of HF acid and ethanol. The first etching for 20 s enhanced photoluminescence and shifted to the higher energy. The successive etching enhanced photoluminescence further but peak position was not changed. Photoacoustic spectra evidenced that the first etching induced a blue shift while the successive etching diminished the density of defect levels distributed under 1.3 eV.