Characterization of the Silicon / Fluoride Solution Interface by In-Situ Microwave Reflectivity

Etching of silicon in aqueous fluoride solutions can lead to almost atomically flat surfaces with a low density of surface states and recombination centers. The final quality of the surface, however, is strongly dependent on the solution composition and pH. We have performed electrochemical impedance spectroscopy in combination with potential modulated microwave reflectance spectroscopy (PMMRS) to elucidate the processes occurring at the surface during etching. PMMRS is a novel technique that only probes the free carriers in the conduction and valence bands and is, under certain conditions, not affected by processes involving electrically active surface states or charge transfer. This unique feature allows us to separate the energetics of the semiconductor from surface processes. Microwave reflectivity (δR) versus potential curves in HF solutions demonstrate the variation of the flatband potential as a function of pH. The AR response in the narrow potential region around the flatband potential and at more negative potentials is also discussed.