First-principles calculation of the structural, electronic, elastic, and optical properties of sulfur-doping ε-GaSe crystal

The structural, electronic, mechanical properties, and frequency-dependent refractive indexes of GaSe_1-xS_x (x=0, 0.25, and 1) are studied by using the first-principles pseudopotential method within density functional theory. The calculated results demonstrate the relationships between intralayer structure and elastic modulus in GaSe_1-xS_x (x=0, 0.25, and 1). Doping of ε-GaSe with S strengthens the Ga-X bonds and increases its elastic moduli of C₁₁ and C₆₆. Born effective charge analysis provides an explanation for the modification of cleavage properties about the doping of ε-GaSe with S. The calculated results of band gaps suggest that the distance between intralayer atom and substitution of S_Se, rather than interlayer force, is a key factor influencing the electronic exciton energy of the layer semiconductor. The calculated refractive indexes indicate that the doping of ε-GaSe with S reduces its refractive index and increases its birefringence.