Quenching of Porous Silicon Photoluminescence by Deposition of Metal Adsorbates.

Luminescent porous silicon is formed by anodization of silicon in HF acid in the dark and then under UV illumination. When illuminated with UV, samples exhibit a bright orange photoluminescence with a quantum efficiency of order a few percent. Porous silicon is characterized as a very complex network of interconnecting pores which range in size from 1 nm to 100 nm. It has a surface area of order 500 m^2/cm ^3. The mechanism responsible for efficient luminescence of porous silicon is unknown, although there is a growing consensus that quantum confinement is necessary element to any successful explanation. We have attempted to increase understanding in porous silicon by a series of experiments in which luminescence is quenched. We have performed experiments in which the luminescence was quenched by immersion in metal ion solutions. In these experiments, we found that those ions solutions which caused quenching required a metal cation with a positive standard reduction potential. In this case, the metal ion was electrochemically deposited onto the surface of the porous silicon. Numerous Auger electron spectroscopy measurements indicate that the metal is present in the porous silicon at significant concentrations (~5-15%) for those samples in which the porous silicon was removed from solution as soon as the photoluminescence had been quenched as observed by eye. SEM measurements show no significant topographic differences between luminescent and quenched porous silicon samples. We have performed experiments in which metals were evaporated onto the surface of porous silicon. We have found for experiments involving evaporation of metal onto the surface of luminescent porous silicon that only copper metal evaporated onto p-type porous silicon has caused quenching of photoluminescence. Copper was evaporated onto p- and n-type porous silicon, but only copper evaporated onto p-type porous silicon caused quenching of the photoluminescence. Significant diffusion was noted only for copper evaporated onto p-type porous silicon; thus, the presence of copper in porous silicon appears sufficient to quench photoluminescence in porous silicon.