Electronic structure and optoelectronic properties of single-crystal Ge(S0.5Se0.5)

Abstract

Ge(S_xSe_1−x) alloys are layered IV–VI semiconductors whose optoelectronic properties can be continuously tuned between the endpoints via the composition, but the fundamental electronic structure, particularly for intermediate alloy compositions, has not been sufficiently explored. In this work, we present a comprehensive experimental and theoretical investigation of the electronic structure of single-crystal Ge(S_0.5Se_0.5). Crystals prepared by a melt-growth technique exhibit high phase purity and crystalline quality, as confirmed by transmission electron microscopy and synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS). The energy band diagram of Ge(S_0.5Se_0.5) is constructed by combining data from XPS, optical absorption, photoreflectance, and photoacoustic spectroscopy. The ionization potential is determined from XPS measurements to be 5.6 ± 0.15 eV. Polarization-dependent absorption and photoreflectance measurements reveal two closely spaced direct optical transitions separated by approximately 0.1 eV, with the lower-energy transition at 1.38 eV corresponding to the dominant optical bandgap. This energy is independently confirmed using photoacoustic spectroscopy. Both optical transitions are highly sensitive to the polarization of the incident light beam, revealing strong anisotropy in the optical properties. First-principles electronic-structure calculations with density functional theory are in reasonable agreement with the valence band spectra measured by SR-XPS and highlight the presence of Ge 4s/4p, Se 3p, and S 2p states at the valence band edge.