Optical properties and bipolar resistive switching of ZnO thin films deposited via DC magnetron sputtering

This work presents a study of the resistive switching behavior of ZnO thin films deposited via DC magnetron sputtering. Thin films were deposited on soda-lime glass and Ti foils, varying ZnO target power, between 80 W and 120 W, and substrate temperature, between 293 K and 423 K. All samples were submitted to annealing processes in situ at 473 K for 2 h. XRD measurements allowed identifying the ZnO wurtzite phase, while AFM micrographs showed the surface formation characterized by small, elongated grains of approximately 80 nm wide. Optical properties were determined by the Swanepoel method using transmittance spectra in the range of 300 nm to 1100 nm. The band-gap obtained was about 3.28 eV for the ZnO thin film, and the refractive index and extinction coefficient were calculated. Modification of the transport mechanism was associated with high defect concentration in the samples and contributed to resistive changes. I-V curves were performed in the -1 V-1 V range and showed a bipolar switching behavior in all samples. A correlation between the variation of synthesis parameters, structural characterization, and shape of bipolar switching effect was evidenced through a change in the resistance states and thickness of thin films. This research provides an insight into the fabrication and control of the resistive switching behavior for resistive random access memories.