The dielectric functions and optical band gaps of thin films of amorphous and cubic crystalline Mg _{~ 2}NiH _{~ 4}

Magnesium nickel hydride films have earlier been suggested for several optoelectronic applications, but the optical properties and band gap have not been firmly established. In this work, the dielectric functions and the optical band gaps of thin films of Mg ₂NiH ₄ have been determined experimentally from optical modeling using spectroscopic ellipsometry and spectrophotometry in the photon energy range between 0.7 and 4.2 eV. Samples were prepared by reactive sputtering, resulting in a single-layer geometry that could easily be studied by ellipsometry. Crystalline samples were prepared by annealing the as-deposited amorphous films ex-situ. The resulting films remained in the high temperature cubic Mg ₂NiH ₄ structure even after cooling to room temperature. Tauc analysis of the dielectric functions shows that Mg ₂NiH ₄ films exhibit a band gap of 1.6 eV for the amorphous structure and 2.1 eV for the cubic crystalline structure.