The photoconductivity of pure and Cu-doped layered gallium monosulfide (GaS) single crystals, and of Ni-GaS(Cu)-In and GaS(Cu)-ZnO structures, is investigated. The activation energies of surface states were found as 0.10 eV, 0.40 eV and 17 meV, similar to 400 meV for GaS and GaS(Cu), respectively. Cu acceptor levels are localized at 0.44 eV and 0.52 eV above the valence band of GaS. ZnO-GaS(Cu) heterojunctions show remarkable photosensitivity in the wavelength range of 250-700 nm. (c) 2008 Elsevier Ltd. All rights reserved.