On the role of the pore filling medium in photoluminescence from photochemically etched porous silicon

Porous silicon thin films created under laser illumination in fluoride solu tions without biasing have been studied by a variety of techniques to inves tigate the film structure and photoluminescence (PL). The use of ultrathin silicon wafers allows us to perform plan view transmission electron microsc opy studies without recourse to thinning procedures that might adversely af fect the film structure. Supercritically dried samples are compared to air dried samples and clearly demonstrate the deleterious effects of air drying on film structure. PL studies were performed (a) while the sample is subme rged in aqueous HF, (b) in Ar after rinsing in ethanol, and (c) in air afte r rinsing in ethanol. The wavelength of light used to fabricate the film is found to correlate strongly with the peak PL wavelength when measured in s olution. Little correlation is found in Ar or in air. Exposure to air can c hange the PL spectrum dramatically on a time scale of just seconds. We demo nstrate that samples can exhibit essentially identical PL spectra in one me dium but have spectra that differ from one another when the samples are pla ced in a different medium. The PL results indicate that band-to-band recomb ination cannot explain photon emission under all circumstances, and that su rface states must also be involved in radiative processes under those condi tions in which the bands are sufficiently separated to allow for the appear ance of gap states. (C) 2000 American Institute of Physics. [S0021-8979(00) 08617-5].