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, using in-situ infrared spectroscopy, for various compositions of the electrolyte (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∼2 V _SCE, beyond a second current maximum. The thickness of the oxide layer is in the range 0-100Å, and increases with increasing potential. The infrared spectra of the oxide layer reveal a minimum 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 densities. 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 this potential range, the limiting step for anodic current flowing is the availability of sites for hole transfer into the oxide layer. On the 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 region (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 30Å.