MgHx 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 MgH2 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 MgH2 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 MgHx thin film, the absorption edge at 2.0 eV disappears and the resistivity decreases to values normally observed for ex-situ hydrogenated films.@en

Mg₂NiH₄ thin films have been prepared by activated reactive evaporation in a molecular beam epitaxy system equipped with an atomic hydrogen source. The optical reflection spectra and the resistivity of the films are measured in situ during deposition. In situ grown Mg₂NiH ₄ appears to be stable in vacuum due to the fact that the dehydrogenation of the Mg₂NiH₄ phase is kinetically blocked. Hydrogen desorption only takes place when a Pd cap layer is added. The optical band gap of the in situ deposited Mg₂NiH₄ hydride, 1.75 eV, is in good agreement with that of Mg₂NiH₄ which has been formed ex situ by hydrogenation of metallic Pd capped Mg₂Ni films. The microstructure of these in situ grown films is characterized by a homogeneous layer with very small grain sizes. This microstructure suppresses the preferred hydride nucleation at the film/substrate interface which was found in as-grown Mg₂Ni thin films that are hydrogenated after deposition.

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