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 solutions without biasing have been studied by a variety of techniques to investigate the film structure and photoluminescence (PL). The use of ultrathin silicon wafers allows us to perform plan view transmission electron microscopy studies without recourse to thinning procedures that might adversely affect 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 submerged in aqueous HF, (b) in Ar after rinsing in ethanol, and (c) in air after 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 solution. Little correlation is found in Ar or in air. Exposure to air can change the PL spectrum dramatically on a time scale of just seconds. We demonstrate that samples can exhibit essentially identical PL spectra in one medium but have spectra that differ from one another when the samples are placed in a different medium. The PL results indicate that band-to-band recombination cannot explain photon emission under all circumstances, and that surface states must also be involved in radiative processes under those conditions in which the bands are sufficiently separated to allow for the appearance of gap states. (C) 2000 American Institute of Physics. [S0021-8979(00)08617-5].