Fabrication and Characterization of ZnO/p-Si and TiO2/p-Si Heterojunctions for Hydrogen Detection—Influence of Pd Functionalization

Nanocrystallyne n-ZnO and p-TiO₂ thin films were deposited on crystalline Si substrate by the simple and low-cost sol-gel method. The surfaces of nanocrystalline ZnO and TiO₂ were modified by PdCl₂ solution to passivate the defect states and to improve the gas sensitivity. Both unmodified and Pd modified sensors with Pd-Ag/n-ZnO/p-Si/Al and Pd-Ag/p-TiO₂/p-Si/Al device structures were exposed to different hydrogen concentrations (0.01%, 0.05%, 0.1%, 0.5%, and 1%) at the optimum biasing voltage and temperature. The effect of gas response was investigated using nitrogen as well as synthetic air as the carrier gas. Both the sensor configurations showed the improved gas response after surface treatment with Pd ions. Surface modified p-TiO₂ sensors recorded higher gas response (78%), response time (3 s), and recovery time (74 s) compared with ZnO sensors under the similar conditions.