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

The anodic dissolution of single-crystal p-Si in fluoride media in a l arge range of pH and nominal fluoride concentration c(F) has been stud ied using a rotating-disc electrode. The process is found to be under mixed control, and the measured current is split into kinetic and diff usional contributions. A zero diagram for both kinetic and diffusion c urrents as a function of c(F), pH and electrode potential is construc ted. A critical concentration c(F) is introduced to define the c(F) v alue at which kinetic and diffusion contributions are the same: while values higher than C-F lead to a diffusion-controlled process, kineti c control becomes dominant at values lower than c(F). The results als o 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 tetr avalent dissolution scheme leading to a SiF62- reaction product.