Excitons and band structure of highly anisotropic GaTe single crystals

Abstract

Exciton characteristics of GaTe single crystals grown by vapor-phase transport were studied by optical measurements. A hydrogenlike exciton series up to $n=4$ was clearly observed in the absorption spectra at 2 K. In the $n=1$ exciton energy region three types of exciton lines were found. By analyzing microphotoluminescence and micro-Raman-scattering spectra on the basis of group theory, it was clarified that these exciton lines are not due to different polytypes but to intrinsic exciton states. Furthermore, optical-absorption spectra in a magnetic field at 4.2 K were measured. In the Voigt configuration, one and two components for $E\ensuremath{\parallel}b$ and $E\ensuremath{\perp}b$ polarizations, respectively, were observed in the $n=1$ and 2 exciton lines. These magnetic-field dependencies cannot be interpreted on the basis of the previously proposed L-S coupling regime. The electronic band structure of GaTe was studied by the ab initio tight-binding linear muffin-tin orbitals method. It was found that GaTe is a direct-gap semiconductor and that the band edge is located at an M point of the Brillouin zone. From a comparison of exciton absorption spectra and the calculated band structure, the existence of the three types of excitons was interpreted from the viewpoint of $j\ensuremath{-}j$ coupling. Our model calculation was also able to explain the Zeeman splitting and the diamagnetic shift of the exciton peak energies.