A BLUE SHIFT FOLLOWED BY A RED SHIFT OF THE PHOTOLUMINESCENCE SPECTRA FOR POROUS SILICON TREATED ALTERNATIVELY BY CHEMICAL ETCHING AND LASER ILLUMINATION

We have treated porous silicon (PS) alternatively by chemical etching and laser illumination in air, and repeated the process many times without interruption on the same specimen. During the processing the photoluminescence (PL) spectra of the PS specimen exhibited a new phenomenon: first a blue shift, then a red shift. At the same time, both the peak intensity and full width at half maximum of the PL spectra also showed transitions, i.e. increased until the PL peak position reached a maximum energy and then decreased. In comparison with the above treatments, the PL spectra of a PS, which was treated only once by chemical etching just after the PS was formed and then by continuous laser illumination, exhibited a smaller blue shift and no transition from blue shift to red shift. Moreover, by fixing the time for which the PS is illuminated , it is found that the energy position of the PL peak is not dependent on the chemical etching time. The above mentioned facts can be tentatively explained on the basis of the quantum confinement effect of carriers.