OBSERVATIONS ON THE SURFACE AND BULK LUMINESCENCE OF POROUS SILICON

Using the x-ray excited optical luminescence technique, we have invest igated the soft x-ray induced photoluminescence of porous silicon in t he optical region (200-900 nm) and the Si K-edge x-ray absorption fine structures of porous silicon in the near edge region. It is found tha t while porous silicon prepared at low current density (20 mA/cm2 for 20 min) exhibits a single broad luminescence band, porous silicon prep ared at high current density (200 mA/cm2 for 20 min) exhibits three op tical luminescence channels; i.e., in addition to the broad peak chara cteristic of all porous silicon, there are at least two additional opt ical luminescence channels at shorter wavelengths, one with modest int ensity at approximately 460 nm and the other a weak and very broad pea k at approximately 350 nm. These optical channels have been used to mo nitor the Si K-edge absorption of porous silicon in the near edge stru cture region. Analysis of the data shows that while the band at approx imately 627.5 nm corresponds to the bulk emission, the other channels are of a surface origin. These observations and their implications are discussed.