The structure of ion beam sputtered amorphous alumina films and effects of Zn doping: High-resolution 27Al NMR

• Amorphous Al2O3 thin films with 1 to 20% ZnO were deposited by ion beam sputtering.
• Contents of AlO4, AlO5, and AlO6 groups were measured by 27Al NMR on 2–3 mg samples.
• Low-Zn film is mostly AlO4 and AlO5, similar to films made by very different methods.
• High alumina thin films may pass through a quenchable, short-lived, liquid-like state.
• High-Zn film has mostly AlO6 groups, similar to crystalline α-Al2O3 or ZnAl2O4.

Amorphous aluminum oxide thin films have widespread technological applications, potentially including multilayer optical coatings in large-scale instruments such as the Laser Interferometric Gravitational Wave Observatory (LIGO). However, the short-range structures of such materials, especially when other components are added, are poorly known. Here we present high-field, high resolution 27Al MAS NMR data on thin film samples of Zn-doped amorphous alumina deposited by ion beam sputtering (IBS). Samples with about 1% and 10–20% Zn both contain mixtures of four-, five-, and six-coordinated Al. The former is dominated by the lower coordinations and is remarkably similar to that recently reported for pure amorphous alumina formed by very different methods of deposition. A common, initially high-energy, “liquid-like” state may be suggested. In contrast, the structure of an X-ray amorphous high Zn alumina film contains mostly six-coordinated Al, with short range order resembling that of crystalline α-Al2O3 and/or ZnAl2O4 spinel. Here, the role of the dopant cation may be to speed up dynamics of local structural relaxation toward that of the equilibrium crystals.