Effect of stress on electronic structure and optical properties of cubic Ca2Ge

Abstract

In this work, the effect of stress on the electronic structure and optical properties of cubic Ca2Ge was investigated using the first-principles method. It is found that under compressive stress Ca2Ge is a direct-gap semiconductor and the band gap decreases from 0.5477 eV @ 0 GPa to 0.1025 eV @ 15 GPa till 20 GPa, at which point Ca2Ge becomes a semimetal. Under tensile stress, it is otherwise an indirect gap, and the band gap decreases to 0.3178 eV @ −10 GPa till −15 GPa where it is a metal. The dielectric constant increases exponentially under compression, while it increases irregularly but monotonically under strain. The absorption edge shifts in the low-energy direction with increasing compressive stress, while the absorption maximum of Ca2Ge shifts in the high-energy direction before Ca2Ge becomes a metal under tensile stress. The effect on absorption increases with increasing energy, and the absorption edge exhibits strong absorption characteristics under tension. Reflectivity shows that Ca2Ge is a material with low reflectivity. The effects of stress modulation on the electronic structure and optical properties are determined based on the atomic population. Under tensile stress, there is no chemical bonding. When the compressive stress increases to 10 GPa, a chemical bond is formed, and the bond population and length decrease with increasing stress.