Study of the hydride forming process of in-situ grown MgH 2 thin films by activated reactive evaporation

Abstract

MgH x thin films are grown by activated reactive evaporation in a Molecular Beam Epitaxy system fitted with an atomic hydrogen source. During deposition the electrical and optical properties are measured in-situ. The structural properties are determined ex-situ by Atomic Force Microscopy. These measurements confirm the growth of the MgH 2 phase, however the presence of 10 vol.% of metallic Mg cannot be prevented. The metallic Mg grains cause an optical absorption edge at 2.0 eV, which has a completely different origin than the observed band gap of MgH 2 at 5.6 eV. The observed optical spectra can be modelled using an effective medium theory. The Mg hydride films are electrically insulating despite the presence of metallic Mg particles. Upon re-hydrogenation of a de-hydrogenated in-situ grown MgH x thin film, the absorption edge at 2.0 eV disappears and the resistivity decreases to values normally observed for ex-situ hydrogenated films.