Abstract
ZnO thin films were made to grow on soda lime glass substrate through a two-step technique. The successive ionic layer adsorption and reaction method (SILAR) at room temperature was employed by varying the number of SILAR cycles from 50 to 125 in steps of 25 on a ZnO seed layer deposited through nebulizer assisted spray pyrolysis technique (NSP). The characteristics of the two-step deposited films were studied with a quest for improvement when compared to ZnO thin films prepared through single step chemical processing techniques. All the prepared films are observed to be polycrystalline, with hexagonal crystal structure and preferential orientation of grains along (002) direction as per XRD analysis. The size of the crystallites grown along the (002) plane seems to increase with the number of dip cycles, reaching a maximum at 100 dip cycles. Furthermore increase in the film thickness causes the crystallite size to decrease. The 940 nm thick film had the lowest strain and dislocation density values, affirmed through structural characteristics. The surface morphology of the two-step processed ZnO thin films is observed to change with the film thickness as evidenced from HRSEM observations. The average elemental percentage ratio of Zn and O atoms in the deposited ZnO films was estimated using EDAX. Optical tests show a decrement in the band gap energy from 3.30 to 2.80 eV as the dip cycle is varied. The film with better characteristic properties is used to detect the presence of ammonia gas. This study shows that there is a definite variation in the ammonia detecting capability of the gas sensor when the active layer is produced through suggested two-step process. According to the impedance spectroscopy investigation, the grain boundary resistance reduces as ammonia concentration rises up to 250 ppm, with a maximum response observed for the film deposited with 100 dip cycles.