Effects of annealing on the structure of ion beam sputtered amorphous tantalum oxide: Oxygen-17 NMR spectra and relaxation times

Unlike well-studied network-structured oxide glasses such as silicates and borates, the structures of amorphous films of heavy transition metal oxides such as tantalum oxide (Ta2O5) are poorly known, in spite of their importance in applications such as the multilayer mirror coatings in optical interferometers, such as the Laser Interferometer Gravitational Wave Observatory (LIGO). 17O MAS NMR can provide new and unique structural information about these materials, but requires isotopic enrichment. For optical coatings formed by ion-beam sputtering (IBS), annealing at temperature up to 600 °C is known to improve properties such as optical absorption and thermal noise. In this paper we report NMR results on tantala films deposited by IBS in an 17O-enriched environment, which allows the structure to be examined by NMR for the first time without an added step of isotope exchange at elevated temperature. Subsequent annealing at temperatures from 300 to 600 °C shows only subtle changes in the proportions of two- and three-coordinated oxygen anions and in bond distances, which may be related to minor structural relaxation and property changes. In contrast, the nuclear spin-relaxation rates increase dramatically on annealing in oxygen gas, suggesting the progressive removal of defects in the as-grown films that contain paramagnetic, unpaired electron spins. EPR spectra qualitatively confirm this observation. The most likely candidate for this effect is the oxidation of lower-valent tantalum cations (e.g. Ta4 +) and thus the removal of a slight oxygen deficit present in the as-deposited films. This may lead to reduction in optical absorption and possibly to improvements in other physical properties during annealing.