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.
We report on photoluminescence (PL) measurements under pressure on p-type N-doped InSe at 10 K and on n-type Si-doped InSe at room temperature. Low-temperature PL of N-doped InSe is dominated by a band-to-acceptor peak. From the pressure dependence of the ionization energy of the N related shallow acceptor, the pressure change of the hole effective mass is estimated through the Gerlach-Pollmann model for hydrogenic levels in uniaxial crystals and discussed in the framework of a k Á p model. Room temperature PL in Si-doped InSe is dominated by a band-to-band peak exhibiting a pressure shift in agreement with previous works. This PL peak has been measured up to 7 GPa and a steep reversible decrease of its intensity has been observed above 4 GPa. This decrease has been interpreted as a supplementary evidence of a direct-to-indirect gap crossover, already observed in other layered semiconductors.
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