A study of anodic films on N-type inp by spectroscopic ellipsometry and atomic force microscopy

The growth of anodic films on n-InP in 1 mol dm^-1 KOH is investigated under potential sweep conditions. At lower potentials a thin surface film is formed and a peak is observed on the current-voltage curve. Ellipsometric measurements show that this film increases in thickness with increasing potential but the observed thickness values are significantly less than the corresponding coulometrically estimated values. This indicates that much of the charge passed is not involved in the formation of a surface film but presumably in the formation of soluble anodic reaction products. Cyclic voltammograms show that a current peak is also observed on the reverse sweep and ellipsometric measurements show that the anodic film thickness also increases during the reverse sweep until the peak potential is reached. Atomic force microscopy (AFM) shows that the surface becomes smoother as the potential is increased. We attribute this to the formation of nuclei at lower potentials, which coalesce as the layer becomes thicker. Electron diffraction and x-ray photoelectron spectroscopy (XPS) analysis show that the surface film is predominantly In₂O₃ with no evidence of InPO₄.