In this study, resistive switching (RS), polarization switching, and charge distribution under DC bias in undoped ZnO thin films are studied by applying scanning probe microscopy (SPM) techniques on the same location. The techniques include Piezoresponce Force Microscopy, Kelvin Probe Force Microscopy, and Conductive Atomic Force Microscopy. The effects of oxygen partial pressure during the film deposition are also investigated. The results show that high resistance state (HRS) is accompanied by the polarization switching and charges storage. By comparing the SPMs results from the same location, it is found that the oxygen partial pressure during film deposition is an important factor over the holes injection during the poling processes in the HRS. On the other hand, the low resistance state (LRS) may be dominated by the electrons injection. Based on these findings, the energy band diagrams in the Pt-tip/ZnO-film/Pt-bottom-electrode structure with the applications of the external biases are illustrated schematically. This study also proposes a more persuasive mechanism of RS in ZnO films.

ZnO thin films were produced by argon plasma assisted electron beam vacuum evaporation and d.c. magnetron sputtering deposition techniques. ZnO films are used in solar cells as transparent contact in heterojunction cells, and can be deposited on a variety of substrates by different techniques, including electron beam deposition and sputtering and laser ablation. ZnO thin films were prepared for photovoltaic applications and the structural properties were studied. The results showed that the sputtering and the vacuum evaporation techniques resulted, respectively, in a textured ZnO and ZnO plus Zn mixed phases. The annealing of the vacuum evaporation ZnO films resulted in films with high crystallinity.