Hydrogen in aluminum oxide and at the aluminum oxide/aluminum interface: an ab initio thermodynamics and Monte Carlo investigation

Abstract

The thermodynamic stability, wide band gap, and extreme hardness of Al₂O₃ make it indispensable in anti-corrosion coatings, resistive switching devices, and superconducting qubits. However, point defects due to impurities like hydrogen, or intentionally added dopants, can significantly alter the properties of Al₂O₃ coatings and microelectronics. Moreover, Al₂O₃ is often formed by oxidizing aluminum, resulting in an Al₂O₃/Al interface. The interface structure plays a critical role in these applications, but its buried nature makes experimental characterization challenging. This thesis utilizes ab initio statistical thermodynamics to resolve the Al₂O₃/Al interface structure and the effect of point defects on the properties of Al₂O₃ and Al₂O₃/Al interface.

In the first part, we identify dopants that reduce hydrogen permeability of Al₂O₃ coatings for hydrogen pipelines. By utilizing an ab initio thermodynamics model that accounts for the distinct Al₂O₃ formation and pipeline functional conditions, we demonstrate that silicon and titanium dopants improve the properties of Al₂O₃ barriers by eliminating the concentration of protons and trapping hydrogen at aluminum vacancy sites.

Following this, we determine the evolution of the atomic and electronic structure of the Al₂O₃/Al interface during oxide growth and in the presence of hydrogen. Using ab initio Grand Canonical Monte Carlo to explore the interfacial configuration space, we find that the interface is atomically sharp and propagates layer-by-layer into Al. We identify point defects that are crucial to scale growth, and determine electronic structure changes that underpin the self-healing property of Al₂O₃/Al coatings and Schottky barrier height variation in Al₂O₃/Al electronics. We establish that hydrogen promotes the formation of interfacial superabundant aluminum vacancies, which is likely the precursor to blistering of Al₂O₃/Al coatings.

Finally, we focus on the effect of point defects on another important application of Al₂O₃: as an electrolyte in resistive switching devices. We determine that electronegative dopants in Al₂O₃ serve as preferential sites for the formation of conductive oxygen vacancy networks, reducing switching variability of Al₂O₃ resistive switching devices.

The detailed understanding of the effect of ionic and electronic defects on the Al₂O₃ and Al₂O₃/Al interface expand our understanding of this fundamental system. The atomistic insight provides routes to engineering advanced barrier coatings, controllable transistor technologies and resistive switching devices, and noise-free superconducting qubits.