Quaternary Tl2CdGeSe4 selenide: Electronic structure and optical properties of a novel semiconductor for potential application in optoelectronics

Highlights: • Electronic structure of Tl₂CdGeSe₄ crystal was studied by XPS. • Tl₂CdGeSe₄ possesses p-type of electrical conductivity. • Energy band gap of Tl₂CdGeSe₄ is equal to 1.52eV at room temperature. • Calculations within TB-mBJ+SOC+U model are in agreement with XPS measurements. • Main optical properties of Tl₂CdGeSe₄ are calculated from first principles. We report on a coupled experimental and theoretical study of electronic and optical properties of quaternary Tl₂CdGeSe₄ selenide. The experimental results reveal p-type electrical conductivity, the band gap energy of E_g=1.52eV for this compound and its non-direct nature. The latter experimental findings are confirmed theoretically by first-principles calculations based on density functional theory (DFT). The DFT calculations present that Se 4p states give principal contributions to the top of the valence band of Tl₂CdGeSe₄, whereas the central portion of the band is composed from Se 4p, Ge 4p and Cd 5s states, its lower portion is due to the input of Tl 6s states and the band bottom is dominated by Ge 4s states with smaller Se 4p states contributions. To verify the DFT calculations the XPS spectrum of valence electrons was measured for Tl₂CdGeSe₄ crystal and XPS core-level spectra were studied to evaluate charge states of the composing atoms and features of the chemical bonding. The present theoretical data of calculations of the principal optical constants reveal that Tl₂CdGeSe₄ selenide is a very promising semiconductor for optoelectronics. The Tl₂CdGeSe₄ crystal was found to be rather stable with respect to treatment its surface with middle-energy Ar⁺ions. The crystal surface of Tl₂CdGeSe₄ demonstrates comparatively low hygroscopicity.