Optical Properties of a Two-Dimensional GeTe Layer

Using the first principles calculations, we explored the optical properties of the two-dimensional α-GeTe structure. The 2D monolayer α-GeTe had an indirect band gap of 1.79 eV, and that indirect band gap was still observed in the bilayer although the band gap had decreased to 0.61 eV. We found that the optical band gap was much larger than the electrical band gap, which was due to the indirect band gap feature of the 2D α-GeTe system. In both systems, we found a very large refractive index in the visible range. Particularly, the bilayer system had a refractive index larger than 2.5 for blue light. Furthermore, the bilayer structure was found to have a larger reflectivity at ultraviolet frequencies. Consequently, we propose that the bilayer α-GeTe film can be utilized for potential optical device applications.