Optical, structural, and electrical properties of Mg{sub 2}NiH{sub 4} thin films in situ grown by activated reactive evaporation

Mg{sub 2}NiH{sub 4} 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 Mg{sub 2}NiH{sub 4} appears to be stable in vacuum due to the fact that the dehydrogenation of the Mg{sub 2}NiH{sub 4} 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 Mg{sub 2}NiH{sub 4} hydride, 1.75 eV, is in good agreement with that of Mg{sub 2}NiH{sub 4} which has been formed ex situ by hydrogenation of metallic Pd capped Mg{sub 2}Ni 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 Mg{sub 2}Ni thin films that are hydrogenated after deposition.