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

摘要

Magnesium nickel hydride films have earlier been suggested for severaloptoelectronic applications, but the optical properties and band gap have notbeen firmly established. In this work, the dielectric functions and the opticalband gaps of thin films of Mg2NiH4 have been determined experimentally fromoptical modeling using spectroscopic ellipsometry and spectrophotometry in thephoton energy range between 0.7 and 4.2 eV. Samples were prepared by reactivesputtering, resulting in a single-layer geometry that could easily be studiedby ellipsometry. Crystalline samples were prepared by annealing the amorphousfilms ex-situ. The resulting films remained in the high temperature cubicMg2NiH4 structure even after cooling to room temperature. Tauc analysis of thedielectric function shows that Mg2NiH4 exhibits a band gap of 1.6 eV foramorphous structure films and 2.1 eV for its cubic crystalline structure.