Thickness-dependent photovoltaic effect in monocrystalline lithium niobate films of nanoscale thickness

We report that the photovoltaic (PV) effect in nanoscale monocrystalline lithium niobate (LN) films originates from the Schottky barrier formed at the electrode-LN interface, different from the bulk PV effect reported previously in the bulk crystal. The Schottky barrier height and the depletion layer width at the $\mathrm{Cr}$-LN interface was measured to be 1.15 eV and 16 nm, respectively. The open-circuit voltage and the power conversion efficiency of the PV devices based on LN films were determined to be 0.8 V and 0.1%, respectively, at 325 nm. It was found the PV properties are film-thickness-dependent, mainly due to the increasing contribution of the polarization-induced asymmetric conduction from the bulk layer with the increase of LN film thickness. Furthermore, direct tunneling of electrons was observed to become the dominant conduction mechanism in ultrathin LN films (much less than 16 nm). These results could be useful for developing integrated optoelectronic devices, such as PV devices and photodetectors based on LN films.