ZnO metal–semiconductor–metal ultraviolet sensors with various contact electrodes

ZnO epitaxial films were grown on sapphire (0 0 0 1) substrates by using RF plasma-assisted molecular beam epitaxy. Metal–semiconductor–metal (MSM) sensors with Ag, Pd and Ni contact electrodes were then fabricated. It was found that barrier height at Ag/ZnO, Pd/ZnO and Ni/ZnO interfaces were 0.736, 0.701 and 0.613 eV, respectively. With an incident wavelength of 370 nm and 1 V applied bias, the maximum responsivity of the Ag/ZnO/Ag, Pd/ZnO/Pd and Ni/ZnO/Ni MSM sensors were respectively 0.066, 0.051 and 0.09 A/W, which corresponds to quantum efficiency of 17.3%, 11.4% and 23.8%. For a given bandwidth of 100 Hz and 1 V applied bias, the noise equivalent power of the fabricated sensors with Ag, Pd and Ni electrodes were estimated to be 6.8×10−13, 1.13×10−12 and 6.4×10−12 W, corresponding to the normalized detectivity (D*) of 1.04×1012, 6.25×1011 and 1.1×1011 cm Hz0.5 W−1.