Photogeneration of charge carriers in orthorhombic sulphur

Abstract

The photogeneration efficiency of electrons and holes has been studied in orthorhombic sulphur crystals using fast excitation light pulses in the range of 2.0 to 6.0 eV. At photon energies below 3.6 eV, electron and hole generation is confined to the surface region. It may be associated with discrete centres and exciton diffusion. At photon energies above 3.6 eV, electron and hole generation rises sharply and tends towards unit quantum efficiency near 4.9 eV. Detailed measurements of the absorption coefficient at photon energies in the range from 2.0 to 3.9 eV and the reflectivity up to photon energies of 6.0 eV have also been made. The results are interpreted by a model in which two components, a photoconductive and a non-photoconductive one contribute to the optical properties. It is suggested that the Gaussian band characterizing the non-photoconductive process is associated with localized excitons which are formed by electronic transitions coupled linearly to the vibrational modes. The photoconductive component of the absorption, as well as photogeneration and reflectivity data place the onset of direct transition near 4.2 eV, and suggest that the previously assigned value of 2.5 eV is incorrect.