The effect of structural characteristics of ZnO and NiO thin films on the performance of NiO/ZnO photodetectors

Material properties play an important role in the fabrication of optoelectronic devices such as photodetectors because it is ultimately reflected in their efficiency and performance. To fabricate a NiO/ZnO heterojunction with better structural properties using a low-cost and uncomplicated deposition process, we studied the influence of NiO and ZnO thickness by taking different volumes of spray solution (5, 10, and 15 ml) on the structural and morphological properties that were investigated using the spray pyrolysis technique. When the films' thickness increased, the crystalline structure of both films improved. The deposited ZnO layers have a hexagonal Wurtzite structure with preferable growth orientations along (002). The NiO X-ray diffraction patterns showed that the films were in cubic phase with orientation (111) and the peak density increased with the film thickness. According to our experimental conditions and XRD results, we suggest that thicker NiO and ZnO are the optimal films to fabricate a NiO/ZnO heterostructure. It is found that Raman and XRD results confirm the formation of NiO/ZnO heterostructure. The morphology of NiO/ZnO is smooth and completely covers the substrate without any pinholes. The further investigation related to the effect of NiO and ZnO thin films' structural properties on NiO/ZnO heterostructure photodetector performance is presented using the simulations. It is worth mentioning that based on the suggested transport models, the results confirm that the origin of the dark current has been attributed to the tunneling and thermionic emission at the interface while bulk defects, leading to the increase Shockley-Read-Hall recombination and generation, control the carrier transport. Furthermore, we studied the effect of Gaussian and tail acceptor/donor defects on the current-voltage (J-V) characteristics and responsivity. The obtained results showed that increasing NiO tail states cause an increase in tunneling current. In contrast, the deep defects density in both ZnO and NiO affects the photodetection characteristics, resulting in a decrease in responsivity and photocurrent when these defects increase.