Surface and bulk photoresponse of crystallineAs2S3

The steady-state photoresponse of the layer-structure crystal ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ is reported as a function of wavelength, light intensity, and electric field strength, using aqueous salt solutions as transparent electrodes. The photocurrent spectrum follows the edge-absorption spectrum for photon energies near and above the optical bandgap (2.7 eV), but not for longer wavelengths. In the long-wavelength photoresponse, a unique interference effect is observed which identifies the underlying mechanism as photoinjection of electrons from surface states. For bulk absorbed light, the photocarrier generation efficiency is in the range of 0.1 to 1.0, is ambipolar, is independent of light intensity, and shows a field dependence consistent with the Onsager model. From Onsager theory a thermalization length for hot carriers is calculated to be 40 Å. The dark resistivity perpendicular to the layers is measured to be 3 × ${10}^{15}$ Ω cm.