Optical properties of oxygen-containing yttrium hydride thin films during and after the deposition

TL;DR

This study investigates the optical properties and composition of oxygen-containing yttrium hydride thin films during and after deposition, revealing how deposition conditions influence transmittance and microstructure.

Contribution

It provides in-situ transmittance measurements and detailed optical constants of YHxOy films, highlighting the effects of deposition pressure and oxidation on film properties.

Findings

Higher deposition pressure increases transmittance due to porosity.

Films show a refractive index gradient related to composition variation.

Porous microstructure promotes oxidation and transparency.

Abstract

The synthesis of the photochromic YHO films is based on the oxidation of deposited yttrium hydride in ambient conditions. The actual state of the films during the deposition process, which is influenced by the deposition pressure and the oxidation caused by the residual gases, is not completely known. We report on the YHxOy thin films deposited by reactive pulsed-DC magnetron sputtering. Since the visible light transmittance is closely related to the phase and chemical composition of the films, in-situ transmittance measurements during and after deposition are performed. Ex-situ spectroscopic ellipsometry is used to determine the optical constants of YHxOy throughout the film thickness. In order to obtain metallic YH2-x films, the densest possible structure with a high deposition rate is required, otherwise the films could already be partially transparent during the deposition. The transmittance is higher if deposition pressure is increased. This is because of the oxidation promoted by more porous growth of the microstructure that is observed at the surface and cross-section images of the films. The films exhibit a refractive index gradient perpendicular to the substrate surface, which is related to the porosity and variation of the chemical composition.