Origin of ultraviolet photovoltaic effect in Fe₃O₄ thin films

We have investigated the transport and ultraviolet photovoltaic properties of Fe_3O_4 thin films grown on glass substrates by facing-target sputtering technique. The nonlinear dependence of current-density on voltage suggests that the transport process is most likely the tunnelling process and grain boundaries act as barriers. Furthermore, nonequilibrium electron-hole pairs are excited in the grains and grain boundary regions for Fe_3O_4 film under ultraviolet laser, since the energy gap of Fe_3O_4 is smaller than the ultraviolet photon energy. And then the built-in electric field near the grain boundaries will separate carriers, leading to the appearance of an instant photovoltage.