On the initial oxidation of iron: Quantification of growth kinetics by the coupled-currents approach

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Abstract
Polycrystalline iron was oxidized at pO2=10−4 Pa and at temperatures ranging from 300 to 500 K. Ellipsometry was used for monitoring the oxide-film thickness as a function of time. The oxidation kinetics were described quantitatively by application of the model due to Fromhold and Cook by considering coupled currents of cations and electrons. At the lower temperatures tunneling is the dominant electron transport mechanism and an excellent agreement of experimental and calculated oxidation kinetics was obtained by adopting a time-dependent difference of the work functions of the metal-oxide and the oxide-oxygen interfaces. At the higher temperatures the experimental kinetics can be described quantitatively for a film thickness up to about 3 nm. Above this thickness electron transport becomes dominated by thermal emission rather than by tunneling. To investigate the influence of the surface pretreatment on the oxidation kinetics a sample was oxidized at pO2=10−4 Pa at 330 K, both after sputter cleaning and after sputter cleaning plus subsequent annealing at 700 K. The difference in initial oxidation rate observed after the two pretreatments was discussed in terms of corresponding differences in the work functions.