Tailoring of rhenium oxidation state in ReOx thin films during reactive HiPIMS deposition process and following annealing
Bulk rhenium trioxide (ReO3) has an unusually high electrical conductivity and, being nanosized, has promising catalytic properties. However, the production of pure ReO3 thin films is challenging due to the difficulty to stabilize rhenium in a 6+ oxidation state. Here we present a novel approach for the deposition of ReOx (x = 1.6-2.9) thin films using reactive high power impulse magnetron sputtering (r-HiPIMS) from a metallic rhenium target in a mixed Ar/O2 atmosphere. The thin films were deposited in the gas-sustained self-sputtering regime, observed during r-HiPIMS process according to current waveforms. The influence of the substrate temperature, the oxygen-to-argon flow ratio and post-annealing at 250 {\deg}C in the air for 3 h on the properties of the films were studied. The as-deposited films have an X-ray amorphous structure (a-ReOx) when deposited at room temperature while a nano-crystalline \b{eta}-ReO2 phase when deposited at elevated temperatures (150 or 250 {\deg}C). The amorphous a-ReOx can be converted into the crystalline ReO3 with a lattice parameter of 3.75 {\AA} upon annealing in the air. The surface morphology of the films is dense without detectable voids when elevated substrate temperatures are used. Various Re oxidation states are observed on the surface of the films in different ratios depending on the deposition parameters. All samples exhibit electrical resistivity on the order of 10-3 Ohmxcm and optical properties typical for thin metallic films.