Surface analysis of the electropolishing layer on Si(111) in ammonium fluoride solution

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The influence of potential and pH on the thickness and composition of the anodic films formed on n-Si(111) in dilute ammonium fluoride solutions is investigated by photoelectron spectroscopy. A combined electrochemistry/surface analysis system with very low contamination levels is used for the investigation of the oxidic layers. XPS data show that the film thicknesses increase with decreasing pH reaching 25 Å at pH 3. A potential dependence measured at lower coverage at a pH of 4.9 reveals changes in thickness and composition by increasing the potential from flatband to +5 V. Fluorine is incorporated into these films in substantial amounts which otherwise consist predominantly of silicon oxide. Silicon fluoride and silicon oxyfluoride are found. We define a current transmissivity, T c, of the layers which shows strong variations with potential and pH. For a pH of 3, T c is about 5×10 2 larger than for pH 4.9. The potential dependence shows a variation of T c by a factor of three and an increase for thicknesses exceeding 5 Å. These observations are explained by a structural transition from a high density pseudomorphic interfacial layer to a more open structure at larger thicknesses (up to 25 Å). The analysis of the UP-spectra shows only little changes of the electron affinity χ with coverage. We obtain χ≅4.15±0.1 eV. A distinct band bending is not detected. From HeII spectra, we determine the valence band offset Δ E v between Si and the oxidic layer to be 5.2 eV.