Effect of oxygen content on optical, structural, and dielectric properties of Al$_x$Ta$_y$O$_z$$ thin films

TL;DR

This paper investigates how varying oxygen content in Al$_x$Ta$_y$O$_z$ thin films affects their optical, structural, and dielectric properties, revealing that metal-metal bond formation at lower oxygen levels reduces dielectric strength and band gap.

Contribution

It combines experimental deposition and characterization with ab initio calculations to explain the impact of oxygen content on the properties of aluminum tantalum oxide thin films.

Findings

Dielectric strength varies significantly with oxygen content.

Metal-metal bonds form at lower oxygen levels, reducing dielectric strength.

Band gap decreases due to metal-metal bonds, affecting optical absorption.

Abstract

This study reports on the optical, structural, and dielectric properties of aluminum tantalum oxide (Al$_x$Ta$_y$O$_z$) thin films deposited at low temperature on silicon and steel substrates by pulsed direct current reactive magnetron sputtering of a target containing 80 at.% aluminum and 20 at.% tantalum in Ar/O$_2$ atmosphere. Oxygen flow rates ranging from 5.0 to 20 sccm corresponded to O content changes from 57.7 to 69.6 at.% and resulted in large differences in dielectric behavior, from films with no measurable dielectric strength to a dielectric strength of 231 V$\mu$m$^{-1}$, respectively. Ab initio calculations were employed to explain the large property changes, and we show that a decrease in the dielectric strength can be linked to the formation of metal-metal bonds in the material, when the O content is less than what would correspond to a stoichiometric Ta$_2$O$_5$ and Al$_2$O$_3$ mixture. The electronic states corresponding to the metal--metal bonds are located in the band gap close to the top of the valence band, leading to an effective band gap reduction, which is directly supported by X-ray photoelectron spectroscopy valence band measurements and by a broad optical absorption in the visible region.