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

TL;DR

This study experimentally determines the dielectric functions and optical band gaps of amorphous and cubic crystalline Mg2NiH4 thin films, revealing distinct band gap values for each structure using spectroscopic ellipsometry and spectrophotometry.

Contribution

It provides the first experimental measurements of optical properties and band gaps for Mg2NiH4 thin films in both amorphous and crystalline forms.

Findings

Amorphous Mg2NiH4 has a band gap of 1.6 eV.

Crystalline Mg2NiH4 has a band gap of 2.1 eV.

Films remain in cubic structure after annealing.

Abstract

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 Mg2NiH4 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 amorphous films ex-situ. The resulting films remained in the high temperature cubic Mg2NiH4 structure even after cooling to room temperature. Tauc analysis of the dielectric function shows that Mg2NiH4 exhibits a band gap of 1.6 eV for amorphous structure films and 2.1 eV for its cubic crystalline structure.