Investigation of WO3/ZnO thin-film heterojunction-based Schottky diodes for H2 gas sensing

A comparative study of Schottky diode hydrogen gas sensors based on Pd/WO3/Si and Pd/WO3/ZnO/Si structure is presented in this work. Atomic force microscopy and X-ray photoelectron spectroscopy reveal that the WO3 sensing layer grown on ZnO has a rougher surface and better stoichiometric composition than the one grown on the Si substrate. Analysis of the I–V characteristics and dynamic response of the two sensors when exposed to different hydrogen concentrations and various temperatures indicate that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10 times higher sensitivity, and shorter response and recovery times (105 s and 25 s, respectively) towards 10,000-ppm H2/air at 423 K. Study on the energy band diagram of the diode suggests that the barrier height is modulated by the WO3/ZnO heterojunction, which could be verified by the symmetrical sensing properties of the Pd/WO3/ZnO/Si gas sensor with respect to applied voltage.