Experimental and theoretical study of band structure of InSe and In1-xGaxSe (x<0.2) under high pressure:: Direct to indirect crossovers -: art. no. 125330

This paper reports on the pressure dependence of the absorption edge of indium selenide and In1-xGaxSe alloys (x<0.2) up to the pressure at which precursor effects of the phase transition prevent further transmission measurements. The absorption edge could be divided into three components exhibiting different pressure coefficients: one corresponding to a direct transition that could be analyzed through the Elliot-Toyozawa theory, and two supplementary edges with quadratic dependence on the photon energy. The first component is attributed to the direct transition at the Z point of the rhombohedral Brillouin zone. One of the quadratic absorption edges red shifts under pressure and can be attributed to a Z-B indirect transition. The direct transition and the second quadratic edge have a very similar pressure evolution, with a lower pressure coefficient for the latter. This second quadratic edge can be attributed to an indirect transition from an additional maximum of the valence band, which becomes the absolute one under pressure, to the Z conduction-band minimum. Both assignments are in agreement with density-functional theory band-structure calculations also reported in the paper. The three transitions show nonlinear behavior under pressure, which is attributed to the nonlinear evolution of the uppermost valence band at the Z point at low pressures. Above 1 GPa, the three transitions behave almost linearly, with pressure coefficients (in InSe) of 64, 42, and -22 meV/GPa for the direct and indirect transitions, respectively. The strength of the direct transition is shown to be related to the change under pressure of the spin-orbit coupling of the uppermost Se p(z) conduction band with lower lying Se p(x)-p(y) bands. The evolution of the exciton width is also discussed and shown to be governed by the increase of intervalley scattering as a consequence of direct to indirect band-gap crossovers.