Absorption and luminescence studies of free-standing porous silicon films

Abstract

The result of a combined study of the absorption and photoluminescence (PL) from high-optical-quality free-standing porous Si films is presented. These films, which have a scattering loss less than 10%, allow unambiguous, detailed study of the system’s absorption edge and, in addition enable us to correlate the absorption and emission behavior on the same sample. H-ion scattering and H forward recoil data are used to accurately determine the bulk equivalent thickness to allow extraction of the absorption coefficient. X-ray diffraction, in conjunction with transmission electron microscopy, are utilized to measure the lattice constant and to confirm the crystalline nature of the structure. The combined photoluminescence and absorption studies are used to examine the relative energy shift of the luminescence peak with respect to that of the absorption edge. It is shown that the absorption begins coincident in energy with the photoluminescence peak, rising smoothly over a range of 1–2 eV. More importantly, the absorption is very weak in the region of the PL in comparison to a direct-gap semiconductor, indicative of the intrinsically weak oscillator strength. An effective-mass-based model for an ensemble of Si nanocrystallites is compared to the experimental results. Phonon-assisted transitions dominate the optical processes. The results support the picture that the absorption derives primarily from the interior states of nanocrystalline Si particles of characteristic dimension below 100 Å. The dynamics of the photoluminescence are reexamined and found to be consistent either with recombination between interior electron and hole states, of the Si nanocrystallites, or involving shallow trap states.