Transition from n- to p-Type of Spray Pyrolysis Deposited Cu Doped ZnO Thin Films for NO2 Sensing

The structural, optical, and gas-sensing properties of spray pyrolysis deposited Cu doped ZnO thin films were investigated. Gas response of the undoped and doped films to NO2 (oxidizing) gas shows an increase and decrease in resistance, respectively, indicating p-type conduction in doped samples. The UV-Vis spectra of the films show decrease in the bandgap with increasing Cu concentration in ZnO. The observed p-type conductivity is attributed to the holes generated by incorporated Cu atoms on Zn sites in ZnO thin films. The X-ray diffraction spectra showed that samples are polycrystalline with the hexagonal wurtzite structure and increasing the concentration of Cu caused a decrease in the intensity of the dominant (002) peak. The surface morphology of films was studied by scanning electron microscopy and the presence of Cu was also confirmed by X-ray photoelectron spectroscopy. Seebeck effect measurements were utilized to confirm the p-type conduction of Cu doped ZnO thin films.