Direct to Indirect Crossover in III-VI Layered Compounds and Alloys under Pressure

The pressure dependence of the optical absorption edge of In1—xGaSe (0 < x < 0.2) and GaTe has been investigated in order to determine the direct to indirect crossover pressure and the energy difference between the absolute and subsidiary minima of the conduction band at ambient pressure. In the In1—xGaSe alloy, the crossover pressure decreases with increasing Ga proportion. For InSe, from the extrapolation to x = 0 the band crossover is found to occur at 4.3 GPa and the subsidiary minimum of the conduction band is located, at ambient pressure, (0.32 ± 0.02) eV above the absolute minimum. In addition, the energy difference between the conduction band minima is shown to decrease linearly with pressure, in agreement with previous transport results in InSe and optical results in GaSe. As regards GaTe, the behaviour of the absorption edge is similar to that of the other III–VI compounds, which suggests that, in spite of the different crystal structure, the electronic states close to the band-gap have the same character.