Effect of pressure on direct optical transitions of gamma-InSe

We have investigated the effect of hydrostatic pressure on direct optical t ransitions of the layered semiconductor gamma -InSe by photoreflectance (PR ) spectroscopy (T = 300 K). In addition, electro-reflectance (ER) measureme nts were performed at ambient pressure. Six structures are resolved in the ER spectra in the energy range from 1.1 to 3.6 eV. The pressure dependence of four of these structures was determined by PR spectroscopy for pressures up to 8 GPa. In order to assign the features observed above the fundamenta l gap we have carried out band structure calculations for InSe at ambient p ressure using a full-potential linear augmented plane wave method. Based on calculated band gap deformation potentials for different uniaxial strains, we explain the strongly nonlinear pressure dependence of the fundamental b and gap and the absence of such effects for the higher lying transitions wh ich increase almost linearly with pressure.