Growth-controlled photochromism in yttrium oxyhydride thin films deposited by HiPIMS and pulsed-DC magnetron sputtering

TL;DR

This study compares yttrium oxyhydride thin films produced by HiPIMS and pulsed-DCMS, revealing how deposition methods influence their microstructure and photochromic properties, with implications for optimizing photochromic device performance.

Contribution

It provides a detailed comparison of the effects of HiPIMS and pulsed-DCMS deposition techniques on the properties of yttrium oxyhydride thin films, highlighting the role of growth conditions and microstructure.

Findings

Pulsed-DCMS films show higher photochromic contrast than HiPIMS films.

HiPIMS films are more polycrystalline with random orientation, while pulsed-DCMS films have <100> preferred orientation.

Deposition method affects oxygen-to-hydrogen ratio and photochromic performance.

Abstract

The present study investigates photochromic oxygen-containing yttrium hydride (YHO) thin films deposited by reactive high power impulse magnetron sputtering (HiPIMS) and compares their photochromic, optical, and structural properties with those of films synthesized by reactive pulsed direct current magnetron sputtering (pulsed-DCMS). Optical emission spectroscopy reveals that, unlike pulsed-DCMS where Ar$^{+}$ ions dominate, HiPIMS discharges are characterised by strong Y$^{+}$ emission, evidencing high yttrium ionisation and substantial self-sputter recycling. The critical working pressure (P$_c$) required to obtain transparent and photochromic films is higher for HiPIMS (Pc $\approx$ 1.0 Pa) than for pulsed-DCMS (Pc $\approx$ 0.5 Pa). Although films deposited near Pc exhibit similar solar transmittance (~72 %) and lattice parameters (5.38--5.39 \r{A}), the pulsed-DCMS film shows a substantially higher relative photochromic contrast (34 %) and a lower optical band gap (2.70 eV) compared with the HiPIMS film (9 % contrast and 2.94 eV). This difference is partly attributed to a lower oxygen-to-hydrogen atomic ratio in the pulsed-DCMS film. Structurally, HiPIMS films are largely polycrystalline with random out-of-plane crystallographic orientation, whereas pulsed-DCMS films exhibit a pronounced <100> out-of-plane preferred orientation. These results demonstrate that, beyond composition, thin-film growth conditions and microstructure play a crucial role in governing the photochromic performance of YHO.