  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.
