Electronic States in Vitreous Selenium

The quantum efficiency for photoproduction of electrons and holes in vitreous selenium has been measured at room temperature for photon energies between 2.0 and 3.1 eV. Electrons and holes are found to be created in pairs with energy-dependent quantum efficiencies varying from an assumed value of unity near 3 eV to 2×${10}^{-4\mathrm{}}$ at 2 eV. A general relation between quantum efficiencies and optical properties is developed which, along with conventional band theory, explains the optical properties of vitreous selenium in terms of two different types of optical transitions. One type, assumed to have unit quantum efficiency, is interpreted in terms of direct allowed transitions between valence and conduction bands which are separated by an energy gap of 2.53 eV. Energetically shallow tails at the band edges resemble shallow traps which have been observed in transport measurements. The other type of optical transition, assumed to occur with zero quantum efficiency, is attributed to intrinsic exciton absorption. The shape of this absorption band closely resembles that expected for the two-lattice-mode model of phonon-assisted transitions and accounts for the absorption edge of vitreous selenium, which obeys Urbach's empirical rule. The observed generation of mobile electrons alone at photon energies less than 2 eV is assigned to transitions from filled states just below the Fermi level to the conduction band. Several authors have proposed such a model to account for the absorption edge, but the small observed quantum efficiency for these transitions suggests that they contribute very little to the total absorption.