EVOLUTION OF PHOTOLUMINESCENCE IN LASER ILLUMINATING POROUS SILICON

Exposed to strong laser in air, the photoluminescence (PL) peak positi on of porous silicon (PS) shifts first to blue side followed by a pinn ing at a constant wave-length, while PL intensity degrades rapidly fir st, then increases slowly. Fourier transform infrared absorption spect ra show a continuous enhancement of vibrations related to oxygen and a continuous attenuation of vibrations related to hydrogen with exposur e time. A new physical model on light emission of PS, which says that incident light excites electron-hole pairs in nanoscale silicon units (NSUs) and the electron-hole pairs diffuse outwards and recombine thro ugh the luminescence centers (LCs) outside NSUs, can explain the exper imental results successfully. We think the main LCs are hydrides, such as polysilanes or Si-H bonds, in as-prepared PS samples but transfer tinder laser illumination to be point defects or impurities in SiO2 la yers which cover the NSUs.