Particle size effects on the hydrogen sensing properties of Pd/ZnO Schottky contacts fabricated by sol-gel method

ZnO thin films grown on n-Si substrates using sol-gel spin coating method were annealed in Ar atmosphere at 450 °C, 550 °C and 650 °C temperatures. Three types of Pd/n-ZnO/n-Si/Ti/Al vertical Schottky diodes were fabricated using three types of ZnO films obtained by annealing at the aforementioned three different annealing temperatures for hydrogen gas sensing applications. Using thermal evaporation method, the Pd metals dots were deposited for the Schottky contacts on the annealed ZnO films while Ti and Al were sequentially deposited over the back side of the n-Si for forming the ohmic cathode contact of the diode. The XRD and SEM analyses showed that the structural and surface properties of the ZnO thin films were largely influenced by the annealing temperature. The grain size was observed to be increased with annealing temperature of the ZnO films. The increased grain size at higher annealing temperatures reduces the surface to volume ratio, number of nanoparticles in the ZnO films (and hence the number of Schottky barriers formed between a nanoparticle and Pd) and number of grain boundaries (due to merging of a number of smaller grains into a larger sized grain) which, in turn, affects the hydrogen sensing properties of the devices under consideration. This paper investigates the effects of grain size on the hydrogen sensing property, series resistance and barrier height of the Pd/ZnO film Schottky diodes in details. The highest hydrogen response was observed in the device containing the ZnO film annealed at 450 °C which was attributed to the largest surface to volume ratio owing to the smallest particle size in the ZnO film.