Optical properties of TlGa_xIn_(1-x)Se_2-layered mixed crystals (0.5 ≤ x ≤ 1) by spectroscopic ellipsometry, transmission, and reflection measurements

摘要

The layered semiconducting TlGa_xIn_(1-x)Se_2-mixed crystals (0.5 ≤ x ≤ 1) were studied for the first time by spectroscopic ellipsometry measurements in the 1.2-6.2 eV spectral range at room temperature. The spectral dependence of the components of the complex dielectric function, refractive index, and extinction coefficient were revealed using an optical model. The interband transition energies in the studied samples were found from the analysis of the second-energy derivative spectra of the complex dielectric function. The effect of the isomorphic cation substitution (indium for gallium) on critical point energies in TlGa_xIn_(1-x)Se_2 crystals was established. Moreover, the absorption edge of TlGa_xIn_(1-x)Se_2 crystals have been studied through the transmission and reflection measurements in the wavelength range of 500-1100 nm. The analysis of absorption data revealed the presence of both optical indirect and direct transitions. It was found that the energy band gaps decrease with the increase of indium content in the studied crystals.