The role of the pulsed laser deposition in different growth atmospheres on the gas-sensing properties of ZnO films

ZnO films were fabricated by pulsed laser deposition using two different background atmospheres (argon/vacuum). The gas-sensing properties of these materials against reducing and oxidizing gases were examined. The microstructure and crystal symmetry of the deposited films were studied with X-ray diffraction (XRD), Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman, and Photoluminescence (PL) spectroscopy. The XRD studies revealed that the ZnO films grown in an argon environment are highly textured in the c-axis with a hexagonal crystalline structure. The c-axis is perpendicular to the substrate plane orientation (002) compared to (100) plane orientation, which is developed in a vacuum environment. Usually, this orientation (100) is difficult to obtain. Raman scattering spectra for both types of ZnO films revealed the characteristic E2 (high) mode that is related to the vibration of oxygen atoms in wurtzite ZnO. Moreover, PL spectra showed that a high number of defects appear in both the vacuum and argon-grown ZnO films. XPS data indicated that the O1s peak consists of several components identified as lattice oxygen, oxygen close to defects, and chemisorbed species. Furthermore, gas-sensing properties were investigated for nitrogen dioxide (NO2) at different operating temperatures and concentrations. Although both types of ZnO films have shown a good response towards NO2 at ppb levels, the films prepared under vacuum conditions showed higher responses. This was attributed to differences in crystallinity, microstructure, and the type of defects present in these materials.