Effects of S-doping on the electronic transition, band gap, and optical absorption of GaSe_1−xS_x single crystals

The intrinsic evolutions of electronic transition and the band gap of GaSe_1−xS_x solid solution single crystals (x = 0, 0.133, and 0.439) grown for nonlinear optical applications have been systemically investigated by using spectroscopic ellipsometry and first-principle calculations. Five interband electronic transitions E₁, E₂, E₃, E₄, and E₅ have been obtained by fitting the second derivatives of the complex dielectric functions and the physical origins were explained with the aid of theoretical calculations. It is found that the interband electronic transition energy E₂, E₃, and E₄ show a blueshift trend from 3.457 eV, 3.736 eV, and 4.810 eV at x = 0 to 3.786 eV, 4.628 eV, and 5.086 eV at x = 0.439, respectively. This is because the larger Se atoms are replaced by smaller S atoms in GaSe_1−xS_x. The experimental band gap of GaSe_1−xS_x is increased from 1.908 eV at x = 0 to 2.081 eV at x = 0.439. Moreover, in order to verify the influences of S-doping on the band gap of GaSe_1-xS_x, we performed the first-principle calculations based on the density-functional theory. The theoretical results also confirm that the band gap energy increases from 2.085 eV at x = 0 to 2.15 eV at x = 0.439, which is in good agreement with the experiment results.