Electrical and photoelectric properties of GeS layered crystals grown by different techniques

Thermally stimulated currents and anisotropic electrical conductivity are studied in GeS layered crystals prepared by the Bridgman-Stockbarger, Pizzarello, and sublimation techniques. All the crystals are p-type, regardless of the growth technique, owing to the presence of Ge vacancies. The conductivity anisotropy in the melt-grown crystals is high compared to the vapor-grown GeS. The anisotropy rises exponentially with temperature. The concentrations and ionization energies of traps in GeS crystals are determined from thermally stimulated current curves. The spectral response of the photocurrent through the crystals prepared by sublimation, whose structural perfection is higher than that of the melt-grown crystals, is governed by the spectral dependence of the absorption coefficient for alpha d much less than 1 (near-edge region) and by the spectral dependence of reflectivity for alpha d > 1 (high-alpha region). Regardless of the growth technique, the 293-K photocurrent spectra of GeS crystals show strongly polarized peaks at 1.65 (E parallel to a) and 1.78 eV (E parallel to b), which are due to the Lambda(1)(v) --> Lambda(1)(c) and Lambda(2)(v) --> Lambda(2)(c) optical transitions. The low-temperature photoresponse at hv < 1.7 eV is due to absorption by Si impurity.