Testing the Cabrera–Mott Oxidation Model for Aluminum under Realistic Conditions with Near-Ambient Pressure Photoemission

Using the nascent band theory of solids, Cabrera and Mott designed in the late 1940s a model for the low-temperature oxidation of metals that still stands today as a landmark. The core assumption is that an electric field set up in the growing oxide at thermodynamic equilibrium drives the transport of the ionic species responsible for the oxidation process. The existence of an electrostatic potential has long been sought experimentally by in situ measurement of the work function changes in the presence of gaseous O-2. Here, we demonstrate that the work function measurement is insufficient to test the model. Instead, the oxide band structure characteristics (surface dipole nerg)r barrier and band bending) should be followed. We exemplify this for the paradigmatic case of the A1(111) surface oxidation at room temperature using near -ambient pressure X-ray photoemission spectroscopy (operated up to a pressure of mbar). Using an in situ spectroscopic tool, we monitor the oxide growth in real time and obtain detailed energetic information on the rnetal/oifide/gas system. This allows us to validate the central hypothesis of the Cabrera-Mott model (i.e., the existence of the Cabrera-Mott potential). The original assumption that oxygen anions are adsorbed at the oride/gas interface is also discussed. The concept of "realistic conditions" also means that the issue of water coadsorption (inherent to near-ambient O-2 conditions) is addressed. The specific consequences of the Cabrera-Mott regime of oxidation are also discussed with respect to the functioning of aluminum-based superconducting qubits. The in situ, real-time spectroscopic methodology used here is effective and can be generalized far beyond the specific case of aluminum oxidation.