Growth of Self‐Passivating Oxide Layers on Aluminum—Pressure and Temperature Dependence

Abstract

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 O2 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 high‐temperature 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.