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Optical transmission spectroscopy is used to monitor the in situ oxidation of granular aluminum films over a wide range of temperatures and pressures. By fitting the time evolution of the transmission to adequate geometric models, the rising transparency of the sample is converted into oxide growth during reaction. The observed oxidation rates increase steeply with temperature, but depend on pressure only below 100 mbar O 2 and become pressure-independent above. The Al oxidation proceeds in two regimes. The initial fast one is compatible with the Cabrera-Mott mechanism of an electric-field-driven process and self-terminates at 1-2 nm oxide thickness depending on oxidation conditions. The subsequent slow regime exhibits a two times larger activation energy and is relevant only for hightemperature oxidation. It is assigned to the thermally activated transport of Al ions through the emerging oxide layer. In contrast to the initial Cabrera-Mott process, it shows no self-passivation and enables oxide thickening beyond 2 nm.

Polycrystalline copper films with mean grain sizes varying by three orders of magnitude are prepared to correlate morphology and oxidation kinetics of metals. The films are oxidized to cuprous oxide at well‐defined pressure and temperature conditions, monitoring the progress of the reaction by in situ optical transmission spectroscopy. The oxidation kinetics is retrieved by fitting the optical data to a three‐layer Cu2O/Cu2O–Cu/Cu model with the central layer accounting for the inhomogeneity of the oxidation front due to grain boundaries. The analysis reveals the highest oxidation rates for Cu films with finest granularity, demonstrating the decisive role of grain boundaries for Cu mass transport during oxidation. Moreover, reaction rates are found to differ by one order of magnitude for oxidation along the grain boundaries and into the crystalline Cu grains. In fact, slow oxidation into the Cu crystallites is responsible for an incomplete metal‐oxide conversion in the case of coarse‐granular films. The experiments demonstrate the direct interplay between morphology and oxidation kinetics for metals.

The conversion of Cu2O to CuO thin films is probed in situ with UV–vis transmission spectroscopy at various thermodynamic conditions. The obtained spectral series are simulated with different CuO/Cu2O growth models, using the transfer‐matrix method and Bruggeman's mean‐field approach to account for Cu residuals in the oxide matrix. The analysis reveals that Cu2O oxidation proceeds via a flat and homogeneous reaction front, in contrast to polycrystalline copper that preferentially oxidizes along the grain boundaries. The conclusion is supported by the measured activation energy for Cu2O oxidation, which is compatible with Cu diffusion via the bulk oxide and not via line defects. The kinetics of CuO growth is found to follow logarithmic and exponential rate laws at low and high temperature, respectively. While the logarithmic regime is governed by CuO nucleation within the Cu2O phase, the exponential dependence results from self‐heating of the sample in the exothermic reaction. In both cases, traces of unreacted copper are detected in the oxide matrix, giving rise to a characteristic tail in the transmission response.