Optical, structural, and electrical properties of Mg_2NiH_4 thin films in situ grown by activated reactive evaporation

摘要

Mg_2NiH_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_2NiH_4 appears to be stable in vacuum due to the fact that the dehydrogenation of the Mg_2NiH_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_2NiH_4 hydride, 1.75 eV, is in good agreement with that of Mg_2NiH_4 which has been formed ex situ by hydrogenation of metallic Pd capped Mg_2Ni 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_2Ni thin films that are hydrogenated after deposition.