  We report an investigation of growth kinetics and transition from thin to
thick films during metal oxidation. In the thin film limit (< 20 nm), Cabrera
and Mott's theory is usually adopted by explicitly considering ionic drift
through the oxide in response to electric fields, where the growth kinetics
follow an inverse logarithmic law. It is generally accepted that Wagner's
theory, involving self-diffusion, is valid only in the limit of thick film
regime and leads to parabolic growth kinetics. Theory presented here unifies
the two models and provides a complete description of oxidation including the
transition from thin to thick film. The range of validity of Cabrera and Mott's
theory and Wagner's theory can be well defined in terms of the Debye-Huckel
screening length. The transition from drift-dominated ionic transport for thin
film to diffusion-dominated transport for thick film is found to strictly
follow the direct logarithmic law that is frequently observed in many
experiments.
