Ultraviolet photovoltaic characteristic of MgB₂ thin film

Fast photoelectric effects have been observed in MgB₂ thin film fabricated by chemical vapour deposition. The rise time was \sim 10 ns and the full width at half-maximum was \sim 185 ns for the photovoltaic pulse when the film was irradiated by a 308 nm laser pulse of 25 ns in duration. X-ray diffraction and the scanning electron microscope revealed that the film was polycrystalline with preferred c-axis orientation. We propose that nonequilibrium electron--hole pairs are excited in the grains and grain boundary regions for MgB₂ film under ultraviolet laser and then the built-in electric field near the grain boundaries separates carriers, which lead to the appearance of an instant photovoltage.