The pressure dependence of the optical absorption edge of In 1± ±x Ga x Se (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 In 1± ±x Ga x Se 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 AE 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.
In this work we report on the pressure dependence of the low-frequency dielectric constant parallel to the c-axis (e || ) in GaS, GaSe, and InSe as obtained from direct capacitance measurements. A large increase of e || with pressure has been observed. The pressure change of the lattice polarizability along the c-axis is calculated in the framework of a rigid-ion model from the change of the angle of the anion±cation bond with respect to the layer plane, which results in a slight increase of the lattice contribution. Consequently, the pressure behaviour of e || is proposed to arise from the large increase of the electronic polarizability along the c-axis. This is explained through a decrease of the Penn gap, whose energy and pressure coefficients are shown to scale with those of the indirect gap in these compounds. A supplementary and reversible step-increase of e || has been observed at 1.6 GPa in GaS. This increase has been associated to a phase transition that was reported by other authors.
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