Nonvolatile resistive switching behaviors in metal-insulator-metal structures with titanium and cobalt oxide materials

Abstract

ABSTRACT This doctoral dissertation describes an experimental study on resistively switching behaviors and switching mechanisms of the titanium dioxide (TiO2) and cobalt oxide (CoO, Zn:CoO) thin films.

At first, the TiO2 thin films for resistive switching characteristics were successfully prepared on Pt electrode under various conditions by using a rf magnetron sputtering system. The TiO2 thin film shows reversible resistive switching behaviors between the high resistance state (HRS) and the low resistance state (LRS). Both unipolar resistive switching and bipolar resistive switching are found to be observed in TiO2 depending on the electrode material. The current-voltage characteristics of resistive switching were investigated by dc voltage sweep, temperature dependence and pulsed signal measurement. The reproducible switching behavior was found to be strongly depending on the crystalline structure of TiO2 thin film. Pt/TiO2/Pt structures only exhibited an unipolar switching behavior, while TiN/TiO2/TiN structures only displayed a bipolar switching behavior. Switching characteristics with electrode materials depend upon the oxygen-related electrochemical reaction (i.e. redox effect) during electroforming process. In addition, unipolar resistive switching properties were investigated in a polycrystalline TiO2 film with a WN barrier layer sandwiched between the Pt electrodes. The dc voltage-current characteristics of the TiO2 memory cell with a WN barrier layer demonstrated enhanced switching compared to those of the memory cell without WN barrier layers. Resistance switching in the cell with WN inserted was repetitively observed to be a strong function of the amplitude and the width of the voltage pulse. This stable switching property is expected to originate from the decrease in the number of oxygen vacancies at the interfaces between the Pt electrode and the TiO2 layer due to the inserted WN buffer layer. Second, The Pt/CoO/Pt tri-layer exhibited a reproducible and stable unipolar switching property under dc sweeping voltage and short bias pulse voltage. In order to investigate the role of oxygen reduction in metal-deficient CoO layer, resistive switching property of the post-annealed CoO thin film was compared with those of the as-deposited CoO thin film. The analysis of the x-ray photoemission spectroscopy displayed larger reproducible resistance switching and the decreasing of current level in post-annealed CoO thin film. It might be explained due to reduction of the oxygen stoichiometry without phase transformation of the CoO. In addition, the observation of stable switching property in post-annealed CoO is considered to originate from the decrease of Co vacancies in CoO layer, due to the variation of the oxygen stoichiometry. Finally, The resistive switching behavior in Pt/CoO/Pt and Pt/Zn:CoO/Pt have been compared under dc sweeping voltage. Two types of metal-oxide-metal (MOM) structure were prepared by a radio frequency magnetron sputtering using a Co metal target and Zn doped cobalt oxide target at each sample. In order to investigate the effect of doped Zn atoms on resistive switching property, structural and transport property of CoO and Zn:CoO films were analyzed by x-ray diffraction, x-ray photoemission spectroscopy, and hall measurement. The typical I-V characteristics of the CoO films were similar to those of the Zn:CoO samples. However, the on and off state current of Zn:CoO sample is lower than the CoO sample with same current compliance condition of set process. The lowering of on and off state current in Zn:CoO film is expected to originate from the decrease of Co vacancy in CoO layer.