IN-SITU INFRARED STUDY OF THE INTERFACIAL LAYER DURING THE ANODIC-DISSOLUTION OF A SILICON ELECTRODE IN A FLUORIDE ELECTROLYTE

The anodic dissolution of p-Si in fluoride media has been studied, usi ng in-situ infrared spectroscopy, for various compositions of the elec trolyte (fluoride concentration and pH). The interfacial layer present in the electropolishing regime has been investigated as a function of potential. At potentials E < 1-2 V-SCE, this layer mainly consists of a wet oxide or hydroxide. A well-defined oxide appears only above E s imilar to 2 V-SCE, beyond a second current maximum. The thickness of t he oxide layer is in the range 0-100 Angstrom, and increases with incr easing potential. The infrared spectra of the oxide layer reveal a min imum structural disorder of the oxide in the mid-region of the second current plateau. The potential range of this optimum oxide perfection increases when going to electrolytes giving rise to lower current dens ities. Incorporation of ions from the electrolyte may occur in the low -potential range. Potential-modulated infrared spectroscopy reveals a large accumulation of holes for E > 2 V-SCE. This indicates that, in t his potential range, the limiting step for anodic current flowing is t he availability of sites for hole transfer into the oxide layer. On th e other hand, the weak hole accumulation which is observed for E < 2 V -SCE indicates a large density of hole-acceptor sites in the wet oxide layer. Finally, the current oscillations observed in the far anodic r egion (E > 3 V-SCE) are found to be associated with an oscillation of the oxide thickness, which may reach an amplitude of the order of 30 A ngstrom.