Correlation of resistance switching and polarization rotation in copper doped zinc oxide (ZnO:Cu) thin films studied by Scanning Probe Microscopy

This paper presents multiple-modes Scanning Probe Microscopy (SPM) studies on characterize the correlation of resistance switching (RS) and polarization rotation (PR) in copper doped ZnO (ZnO:Cu) thin films. Firstly, the bipolar RS behavior is confirmed by conductive Atomic Force Microscopy (c-AFM). The PR with almost 180° phase angle is confirmed by using the Piezoresponse Force Microscopy (PFM) on the same location. In addition, it elucidates that obvious PR behavior can be observed in the sample with increasing Cu concentration by combining Kelvin Probe Force Microscopy (KPFM). Furthermore, it is found that the region with downward polarization has low resistance state (LRS), whereas the region with upward polarization has high resistance state (HRS). Moreover, the Piezoresponse Force Spectroscopy (PFS) and Switching Spectroscopy PFM (SS-PFM) measurements further confirm that the existence of the built-in voltage, V built-in is largest in the ZnO:Cu (8 at.%) film deposited at the oxygen partial pressure of 2 × 10 −4 Torr. The schematic diagrams of energy band diagram with varied built-in field, E built-in , polarization directions and redistributed charges are presented to explain the correlation between RS and PR behavior.