Visible electroluminescence from p-type porous silicon in electrolyte solution

Electroluminescence (EL) from p-type porous silicon (p-PS) prepared by anodic oxidation in HF solution for various oxidation times (preparation times) and at various current densities in Na2SO4 aqueous solution was investigated galvanostatically. EL was observed as soon as constant current started to flow. It increased at the beginning, reached a maximum, and then decreased with anodic reaction time. The electrode potential gradually increased with oxidation time from ca. 0.8 V (vs Ag/AgCl) at the beginning to 1.2 V after several hundreds of seconds and increased steeply after EL ceased to be observed. How long EL lasted and the total integrated EL intensity were linearly related to the preparation time. EL spectra were blue-shifted during anodic oxidation, and the rate of the blue-shift was dependent on the preparation condition of the p-PS. The time dependence of the integrated EL intensity had a good correlation with that of the amount of Si-H-n species, which was determined by in situ reflection FT-IR, indicating that the surface Si-H-n acts as an electron injector. Cross sectional SEM observations showed that the thickness of the p-PS layer was linearly related to the preparation time. AFM measurements showed that the surface morphology of the p-PS depended not on the preparation time but on the current density for preparation. On the basis of these results, the mechanism of EL was discussed.