Trap-limited photovoltage in ultrathin metal oxide layers

Photovoltage signals were observed at ultrathin metal oxide $({\mathrm{TiO}}_2,{\mathrm{Cu}}_2\mathrm{O},\mathrm{}\mathrm{ZnO})/$ metal structures by transient and spectral photovoltage (PV) techniques. The sign, the spectral behavior and the time-dependent relaxation of the PV are determined by the nature of the traps in the metal oxide layers. At lower temperatures, the relaxation of the PV signal in ${\mathrm{TiO}}_2$ layers is controlled by recombination due to the overlap of the wave functions of the spatially separated electrons and holes. At higher temperatures, thermal emission accelerates the recombination process. The Bohr radius of trapped holes, the tail of the exponential approximation of electronic states distribution above the valence band, the density of states at the valence band edge were obtained for ${\mathrm{TiO}}_2$ layers by using the proposed model of trap limited PV. The concept of trap limited PV gives a general tool for the investigation of excess carrier separation in ultrathin metal oxide or semiconductor layers with trap states.