IN-SITU SPECTROELECTROCHEMICAL STUDY OF THE ANODIC-DISSOLUTION OF SILICON BY POTENTIAL-DIFFERENCE AND ELECTROMODULATED FT-IR SPECTROSCOPY

The anodic dissolution of p-Si has been investigated by ill situ infra red spectroscopy. The combination of potential-difference and electrom odulated spectroscopies allows for the acquisition of a rather complet e picture of the various regimes of the dissolution, After a review of general principles for studying electrochemical interfaces, a study o f the interfacial oxide layer formed in the electropolishing regime is presented. Quantitative analysis shows that the thickness and quality of the oxide (density and defect content) depend upon electrode poten tial, Free-carrier absorption detected in electromodulated spectra sho ws that the blocking character of the oxide is correlated with the bui ldup of a stoichiometric oxide of low defectivity at sufficiently posi tive potentials, Furthermore, the dynamic response to the modulation r eveals that oxides formed at weak positive potentials interact with el ectrolyte species through electro-induced adsorptions/desorptions on c harged SIGH sites. At more positive potentials, charge is transported across the oxide by charged defects which could be associated with tri coordinated, positively charged SiO species. Finally, results obtained during porous silicon formation at weak positive potentials are prese nted. Potential-difference spectroscopy indicates that the electrode e xhibits a very large specific surface area, and that the surface is co vered by SiH bonds. Electromodulated infrared spectroscopy reveals tha t the SiH species are generated upon anodic current bowing and that th e breaking of these bonds is the rate-limiting step of the anodic reac tion. These unexpected results have given rise to the elaboration of n ew microscopic models for the direct anodic dissolution of silicon in fluoride electrolytes.