ZnO ultraviolet photodiodes with Pd contact electrodes

ZnO epitaxial films were grown on sapphire (0 0 0 1) substrates by using RF plasma-assisted molecular beam epitaxy (MBE). Metal–semiconductor–metal (MSM) ZnO photodiodes with palladium contact electrodes were then fabricated. With an incident wavelength of 370 nm and an applied bias of 1 V, it was found that maximum responsivity of the Pd/ZnO/Pd MSM photodetectors was 0.051 A W−1, which corresponds to a quantum efficiency of 11.4%. Furthermore, it was found that the time constant of our photodiodes was 24 ms with a three-order decay exponential function. For a given bandwidth of 100 Hz and an applied bias of 1 V, we found that noise equivalent power and corresponding detectivity D∗ were 1.13 × 10−12 W and 6.25 × 1011 cm Hz0.5 W−1, respectively.