M. Zubkins, E. Strods, I. Aulika, J. Gabrusenoks, V. Vibornijs, G. Chikvaidze, L. Bikse, K. Kundzins, E. Letko, A. Sarakovskis, J. Purans , University of Latvia, Riga, Latvia
Yttrium oxyhydride (YHO, mixed-anion compound) thin films have been shown to be a promising smart material with a reversible photochromic effect – photochromic contrast ΔT_vis up to ~50% induced by UV/blue light. However, neither the precise structure of YHO nor the responsible mechanism of photochromism is fully understood. The synthesis of YHO films is based on the oxidation of deposited yttrium hydride (YH₂) in ambient conditions. The sputtering pressure and the thickness are the most crucial parameters that influence the photochromic properties. We report on the synthesis details as well as optical, structural, and photochromic properties of the YHO thin films deposited by both reactive pulsed-DC magnetron sputtering and HiPIMS. Large area roll-to-roll deposition is also presented. Since the optical absorbance is closely related to the phase and chemical composition of the films, in situ transmittance measurements during and after deposition are performed to investigate the oxidation in more detail. XRD, SEM, FTIR, XPS, spectroscopic ellipsometry (SE), UV-Vis-NIR spectroscopy, and photchromic activity measurements have been performed on the produced films. To obtain metallic YH₂ films with a low oxygen content, a low sputtering pressure is required; otherwise, the films are already partially transparent during the deposition. The post-oxidation is faster when higher deposition pressures are used. This is due to the more porous growth of the microstructure at higher pressures observed in the surface and cross-section images of the films. SE is used to determine the optical constants throughout the film thickness. The films exhibit a refractive index gradient perpendicular to the substrate surface, which is related to the porosity and variation of the chemical composition. FTIR spectroscopy was exploited to study the vibrational properties of YHO in both clear, dark, and isotopically exchanged YDO films. The detected vibration bands are relatively wide due to the disordered structure and small crystallite size. Both experiment and theory were used to interpret the spectra.