Temperature Dependence of the Band Gap and Exciton Photoreflectance in Layered Gallium Telluride

Among Group III-A metal monochalcogenides, gallium telluride (GaTe) is one of the less studied materials in terms of applications and optical characterization. For the temperature dependence of the energy transitions in GaTe, photoluminescence (PL) spectroscopy is commonly used, and photomodulated reflectance (PR) is yet to be reported. In this work, layered monoclinic GaTe single crystals were synthesized by the Bridgman technique and used for the investigation of the conduction band (CB) edge and free-exciton (FX) state transitions using PR spectroscopy. Both energy transitions (i.e., absorption and emission) were present at room temperature at 1.656 and 1.647 eV for the CB edge transition (≡E_g) and for the FX state transition, respectively, and show a blueshift at cryogenic temperatures that can be fitted with Varshni's equation. The estimated E₍₀₎ is 1.794 eV for E_g and 1.776 eV for the FX transitions at 0 K. The energy of the FX state transition is ∼18 meV lower than that of the band gap (E_g) at 0 K. PL spectroscopy confirms that the PL emission is only the FX state transition that is lower than E_g. The temperature-induced band-gap shifting is related to performing temperature-dependent photodetector experiments using various incident light wavelengths. At 80 K, the responsivity of the single-crystal GaTe photodetector to the energies of wavelengths (735 and 845 nm) smaller than E_g is relatively smaller than that to 630 nm incident light. This indicates that the low-temperature band-gap shift plays a role in applications of GaTe in optoelectronics.