Excitons and band structure of highly anisotropic GaTe single crystals

Exciton characteristics of GaTe single crystals grown by vapor-phase transport were studied by optical measurements. A hydrogenlike exciton series up to (formula presented) was clearly observed in the absorption spectra at 2 K. In the (formula presented) 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 (formula presented) and (formula presented) polarizations, respectively, were observed in the (formula presented) 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 (formula presented) coupling. Our model calculation was also able to explain the Zeeman splitting and the diamagnetic shift of the exciton peak energies.