KINETIC AND DIFFUSIONAL LIMITATIONS TO THE ANODIC-DISSOLUTION OF P-SI IN FLUORIDE MEDIA

The anodic dissolution of single-crystal p-Si in fluoride media in a large range of pH and nominal fluoride concentration c(F) has been studied using a rotating-disc electrode. The process is found to be under mixed control, and the measured current is split into kinetic and diffusional contributions. A zero diagram for both kinetic and diffusion currents as a function of c(F)*, pH and electrode potential is constructed. A critical concentration c(F)* is introduced to define the c(F) value at which kinetic and diffusion contributions are the same: while values higher than C-F* lead to a diffusion-controlled process, kinetic control becomes dominant at values lower than c(F)*. The results also indicate that, unlike the kinetic current, the diffusion current is pH independent and constant in the potential region beyond the domain of porous Si generation. In these conditions, quantitative analysis of the diffusion current via Levich's relation is consistent with a tetravalent dissolution scheme leading to a SiF62- reaction product.