Nitrogen dioxide induced conductivity switching in ZnO thin film

The spin coated ZnO thin films were synthesized and tested for O2 and NO2 gas detection. It exhibited an increase in resistance i.e. n-type behavior for O2 exposure (5-20%) and decrease in resistance i.e. p-type behavior for NO2 (2-40 ppm) exposure at operating temperatures of 250 °C and 300 °C in N2 atmosphere (0.4 ± 0.03 mbar). The origin of this aberrant behavior for NO2 exposure was attributed to the participation of defects in NO2 sensing characteristics in light of the data obtained from photoluminescence and Raman spectroscopy studies demonstrating the unique defect morphology of ZnO thin film. The underlying mechanism is discussed on the basis of conclusions drawn from characterization techniques and subsisting knowledge in the literature. The NO2 gas sensing is explained in terms of two competing mechanisms: one is change in charge carrier density on gas exposure as explained by surface depletion model and another is due participation of defects (oxygen vacancies) in gas sensing characteristics.