| Sustained advances in information technologies over the past decades have given us computers with powerful processing capabilities due to high-speed/high-density nonvolatile memories(NVMs). Nevertheless, conventional nonvolatile memory(such as flash memory) scaling is expected to come up against technical and physical limits in the near future. In order to overcome this problem, some concepts of emerging nonvolatile memory, have been proposed by researchers. Candidates for next-generation nonvolatile memories based on new materials, including phase-change random access memory(RAM), ferroelectric RAM, magnetic RAM, and resistive switching RAM(RRAM). However, among these memories, RRAM, which based on resistive switching phenomenon found in some materials, have attracted a great deal of attention, because of its advantages including simple structure, low power consumption, high on/off ratio, fast read-write speed, long retention time and compatibility with semiconductor process. Various polycrystalline and amorphous transition metal oxides(TMOs), including TiO2, ZrO2, ZnO, and HfO2, have found their applications in RRAMs. However, the resistance switching mechanism of TiO2-based RRAM is clear: the formation and disruption of TinO2n-1(or so-called Magnéli phase) conducting filaments. Hence, our research focus is on the TiO2-based RRAM, research works are summarized as follows:(1) The TiO2 thin film was prepared on Pt/Ti/SiO2/Si substrate using the sol-gel method, and its microstructure has been characterized by corresponding methods. Then, electrical measurements of Pt/TiO2/Pt device were performed using a Keithley 4200-SCS analyzer, the coexistence phenomenon of unipolar and bipolar resistive switching behaviors was observed in the Pt/TiO2/Pt device, and there was no forming process in it. The distribution of switching voltages and resistance values of high-resistance state was discrete, the stability of Pt/TiO2/Pt device requires a further improvement.(2) We synthesized various TiO2 films doped with trivalent(Al and Cr) or bivalent(Cu) elements via the sol-gel method and compared their resistive switching characteristics. In contrast to the pure TiO2 device, Al-, Cr- and Cu-doped devices exhibited some better resistive switching properties, including a lower set voltage and a higher stability of switching voltage and high-resistance state. Especially the Cu-doped sample, acquires a better switching performance in contrast to the trivalent ions doped samples. It was attributed to the bivalent dopant-induced enhancement in oxygen vacancy generation. Furthermore, the dopant-induced decrease in oxygen vacancy formation energy calculated by theoretical method was experimentally supported by results of our work.(3) In addition, we present the effects of an amorphous ZrO2 layer on the TiO2-based resistive switching memory device where the ZrO2 layer plays an important role as a supplementary reservoir of oxygen vacancies. Compared with Pt/TiO2/Pt monolayer device, a remarkably improved uniformity of switching parameters such as switching voltages and resistances in high/low states is demonstrated in the Pt/ZrO2/TiO2/Pt system. After 50-cycle tests, resistances of ~130 kΩ and ~50 Ω at high/low states with a read voltage of 0.5 V can be observed in the bilayer structure device, yielding a stable on/off ratio of ~2600. |
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