The visible photoluminescence (PL) of porous silicon (PS) has been investigated in terms of the behavior of PL excitation and emission spectra. It is shown that for appropriately prepared samples, the excitation and emission spectra become entirely independent of the emission and excitation wavelengths, respectively, while both their bandwidths and the large Stokes shift between them remain unchanged. The result indicates that the broad PL bands of PS are due to homogeneous broadening, and that carrier localization accompanied by a strong electron-phonon coupling plays an important role in the visible luminescence of PS.

Photoluminescence excitation ͑PLE͒ 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 ϳ4-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.

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