Porous silicon studied by SiL23 soft X-ray emission

SiL emission spectra, along with OK and CK emission, have been studied in many films of porous silicon (por-Si) when excited by direct electron bombardment. It is confirmed that there is a high oxygen content in the films, but only a small amount is present as oxide in fresh films. The data are consistent with much initial carbon and oxygen being present as CO2, and with the CO2 arising from adsorption from the atmosphere. After standing in air for over 30 days, oxide peaks appear, and after 4 months, the valence band resembles that of furnace-oxidised Si. There is evidence that the oxide grows in from the surface. The valence band emission profiles vary significantly among fresh samples, but are more uniform after annealing at 250°C, a procedure known to improve scanning electron micrograph sharpness. The profiles feature peaks that are only weakly prominent, which is characteristic of disordered material, but with little indication of amorphous Si.