Aliovalent Calcium Doping of Yttrium Oxyhydride Thin Films and Implications for Photochromism

To develop an understanding of the photochromic effect
in rare-earth
metal oxyhydride thin films (REH3–2xOx, here RE = Y), we explore the aliovalent
doping of the RE cation. We prepared Ca-doped yttrium oxyhydride thin
films ((CazY1–z)HxOy) by reactive magnetron cosputtering with Ca doping concentrations
between 0 and 36 at. %. All of the films are semiconductors with a
constant optical band gap for Ca content below 15%, while the band
gap expands for compositions above 15%. Ca doping affects the photochromic
properties, resulting in (1) a lower photochromic contrast, likely
due to a lower H– concentration, and (2) a faster
bleaching speed, caused by a higher pre-exponential factor. Overall,
these results point to the importance of the H– concentration
for the formation of a “darkened” phase and the local
rearrangement of these H– for the kinetics of the
process.