Optical Characterization of Passive Oxides on Metals

Abstract

In-situ optical detection of the passive oxide films on iron and titanium was reviewed. The optical techniques such as ellipsometry (i.e., reflection of polarized light), Raman spectroscopy, and potential modulation reflectance (PMR) have been successfully applied to the detection in thickness, composition, and semiconducting property, respectively, of the thin passive oxides under the in-situ condition. The growth mechanism of the passive oxide has been discussed from the precise measurement of thickness as a function of potential and time by a three-parameter ellipsometry. From the in-situ Raman spectra, the composition of the passive oxide has been estimated to be Fe₃O₄-γ-Fe₂O₃ for iron and anatase TiO₂ for titanium. From PMR, the Mott-Schottky type plot could be drawn in the passive oxide on iron, which indicates that the formation of the space charge layer can be optically seen. From the PMR spectra, one has evaluated light-absorption edge that may correspond to a band-gap energy between the valence and conduction band.