Photoluminescence Properties of Photochromic Yttrium Hydride Films Containing Oxygen. Issue 8 (6th June 2018) - Montero, José
Abstract : Oxidation of YHx±δ modulates its optical properties by converting it from optically opaque to transparent state to the visible light and electrical properties from metallic conductivity to insulating. The qualitative change of the materials properties are related to oxidation‐induced modulation of electronic properties of the material that will influence the band gap and luminescence properties of the oxygen‐containing yttrium hydride (YHO). This work presents the optical properties, studied by optical spectrophotometry and photoluminescence spectroscopy, of YHO thin films deposited by reactive magnetron sputtering at different H2 /Ar flow ratios between 0.18 and 0.28. The indirect and direct band gap of the YHO photochromic films in their clear state is estimated from optical absorption measurements performed at room temperature, and found in the range between 2.5 and 2.6 eV (indirect allowed transition) and between 3.0 and 3.3 eV (direct allowed transition). The study of photoluminescence (PL) properties is performed in the temperature range between 10 and 300 K. The luminescence yield corresponding to the clear state was found to be extremely low in all the YHO films, and it reduced even further once the films darkened under UV‐light exposure. The PL emission at room temperature covers the entire UV and visible range, comprising two broad bands centered at around 2.2 and 3.1 eV. The latter band became fine‐structured at low‐temperatures (10 K), showing a series of distinct PL peaks at energy positions between 2.8 and 3.4 eV which were independent of the H2 /Ar flow ratio used during deposition. By the comparison of the PL measurements conducted in vacuum with the ones performed in air, it is possible to conclude that the environment plays an important role in the photochromic properties of YHO. The possible intrinsic and extrinsic origin of the structured PL emission is discussed in terms of excitonic features and/or luminescent surface defects. We report that the main optical transitions of the films deposited at different H pressures did not change by variation of temperature or by UV light‐induced color change. Abstract : The indirect and direct band gap of oxygen‐containing yttrium hydride (YHO) films is estimated from optical absorption measurements. The luminescence yield of YHO is found to be extremely low and it further reduces once the films darkened under UV‐light. From photoluminescence measurements in vacuum and air, it is concluded that the environment plays an important role in the optical properties of YHO.