Analysis of photoluminescence and pore morphology of porous silicon

The room temperature, visible light emission from porous silicon is an unexpected and poorly understood phenomenon. The models presently proposed to explain this photoluminescence (PL) are vastly different. Analyses of the surface chemistry and morphology of as-prepared and oxidized porous silicon were performed in this study in order to determine the actual PL mechanism.^ Results from x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) establish that the as-prepared surfaces are H-passivated. The morphological results from atomic force microscopy (AFM) and scanning tunneling microscopy (STM) established the presence of nanoscale wires with nodules in the size range of 1-7 nm, after tip correction. These feature sizes and corresponding PL wavelengths are consistent with theoretical predictions of the associated energy band gaps. In situ STM images were taken before and during UV excitation. There is a decrease in feature width and an increase in feature height between the non-luminescing and luminescing porous silicon images corresponding to an increase in the tunneling current and, therefore, an increase in the number of available charge carriers in the wires. The PL from as-prepared porous silicon, therefore, originates from the wires.^ Porous silicon samples were examined as a function of exposure time in air. XPS data revealed that after 18 hours in air, SiO$\sb2$ was present on the surface thereby eliminating the possibility that the PL is due to siloxene. However, the PL intensity increased and shifted toward the red wavelength region upon air exposure. The morphology consisted of nanoscale wires, but with widths too large to be a quantum effect. The PL from oxidized porous silicon is not due to a size effect but rather an oxygen-related chemical model.^ In conclusion, the PL of porous silicon is dependent upon the post-etching environment. The PL of oxidized porous silicon is associated with an oxygen-related chemical effect and the PL of as-prepared porous silicon is due to quantum confinement.