Effect of contact with air on the photoluminescence spectrum of porous silicon

A study has been made of the transformation of photoluminescence (PL) spectra of porous silicon (PS) induced by its ageing, including the early stages of contact with air. The sample was prepared under conditions that minimized this contact, and spectral measurements were carried out in a high vacuum or in liquid nitrogen. The PS PL spectra obtained under continuous measurement in high vacuum are always dominated by one emission band of PS nanoelements, which shifts toward shorter wavelengths with ageing by 150 nm. At 80 K, the band intensity is considerably higher than at 300 K, and this difference grows with ageing. Exposure of a sample to air for a few tens of seconds is long enough to strongly transform its time-resolved PL spectra, which is evidence of a change in the sample surface. The effect of immersion of PS samples in liquid nitrogen on PL spectra is associated not only with their cooling, but also with the field of adsorbed nitrogen molecules, whose influence becomes weaker with increasing thickness of the oxidized near-surface layer. The variation of the spectral properties and kinetics of the long-wavelength PS PL band with temperature, medium (liquid nitrogen or vacuum), and exposure time suggest that these factors affect carrier migration between silicon nanoelements.