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

• Received 20 February 1996

DOI:https://doi.org/10.1103/PhysRevB.53.R13291