Ellipsometric studies of the dielectric function of SnSe and a simple model of the electronic structure and the bonds of the orthorhombic IV-VI compounds

The dielectric function of the orthorhombic SnSe has been measured ellipsometrically in the 1.26–5.6-eV photon-energy region. The second derivatives, with respect to frequency, of the real and imaginary parts of the dielectric function are obtained numerically from the measured spectra for the electric vector E parallel to the a and b axes and used to determine the critical-point parameters. The critical-point parameters are found to show features similar to those of GeS. The effect of the air exposure on both the dielectric function and the ellipsometric angles, as well as the structure of the surface, have been studied by spectroscopic and ‘‘null’’ ellipsometry and electron microscopy, respectively. The results indicate the formation of an amorphous overlayer after cleavage. A simple model for the electronic structure of the IV-VI orthorhombic compounds is presented. The energy bands and the density of states of the model are calculated and compared with the known band structure and density of states. The bonding is found to be provided from p orbitals forming σ saturated bonds. The dielectric constant ${\epsilon }_0$ is calculated for all the orthorhombic IV-VI compounds using this simple model. It is found that the theoretical estimates are close to the experimental values and follow the experimental trend.