LARGE AND IRREGULAR SHIFT OF PHOTOLUMINESCENCE EXCITATION-SPECTRA OBSERVED IN PHOTOCHEMICALLY ETCHED POROUS SILICON

Photoluminescence excitation (PLE) spectra of porous silicon have been studied for samples subjected to postanodization photochemical etchin g in a HF solution. It is shown that the peak energies of the PLE spec tra show a large shift of about 1 eV with increasing etching time. Fur thermore, the manner in which the PLE spectra shift is irregular; they first move toward lower energies, and then back to higher energies wi th increasing etching time. This behavior is interpreted by assuming t wo independent excitation mechanisms competing with each other in lumi nescent Si nanocrystallites: a direct-gap absorption process similar t o the similar to 4-eV absorption in bulk crystalline Si and the direct excitation of localized states in the crystallites. The latter excita tion mechanism is more important in efficiently luminescent samples.