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theses

Recently, resistive switching (RS) memory devices have attracted increasing attentions due to their potential applications in the next-generation nonvolatile memory. Zinc Oxide (ZnO) - based RS devices possess promising features, such as well-controlled switching properties by in-situ doping and alloying, low-temperature fabrication processes, superior radiation hardness, and low cost. The goal of the research is to study the feasibility of using the transitional metal (TM) doped ZnO for making RS devices. The Fe-doped ZnO (FeZnO) is used to make the bipolar and unipolar RS. The FeZnO is grown through MOCVD. Fe is a deep-level donor in ZnO, and Fe doping leads to better device thermal stability and larger value at the high resistance state (HRS) for switching. For the Ag/FeZnO/Pt bipolar RS structures, the ratio of the HRS over the low resistance state (LRS) of 3.8×10^2 is achieved. The dominant conduction mechanisms are attributed to the Poole-Frenkel emission at the HRS and Ohmic behavior at the LRS, respectively. A FeZnO/MgO bi-layer (BL) is used to replace the FeZnO single layer (SL) to form an Ag/FeZnO/MgO/Pt bipolar RS structure. This BL device demonstrates a higher RHRS/RLRS ratio (~106) than the SL counterpart. For the Au/FeZnO/MgO/Pt unipolar RS device, the RHRS/RLRS ratio of 2.4×10^6 at 1V is achieved. In order to reduce the processing temperature, the Ni-doped ZnO/MgO BL structure is adapted to make the RS devices using the sputtering - MOCVD hybrid deposition. The Ni doping enhances the compensation of oxygen deficiency in ZnO, resulting in larger HRS values. By controlling the compliance current during the “SET” process, three different reversible RS modes, i.e. threshold switching, volatile switching, and memory switching are obtained. Compared with the memory switching, the volatile switching possesses lower power consumption and better HRS stability. Furthermore, the different compliance currents lead to the different LRS values, which could be used for the multi-level per storage cell applications. The electrical characteristics and microstructure analysis suggest that the compliance current setting affects the formation and rupture of the metallic filaments, leading to the conversion of different switching modes. The FeZnO switching resistor (R) is vertically integrated with a ZnO diode (D) to form the 1D1R structure, which overcomes the cross-talk in the 1R-based crossbar switching matrix. The 1D1R exhibits high RHRS/RLRS ratio, excellent rectifying characteristics, and robust retention. The new ZnO RS technology presents great impact on the future classes of memory devices for applications such as switching matrix, multi-level storage, and 3D non-volatile memory architecture.

ETD_5713

Ph.D.

Includes bibliographical references

by Yang Zhang

doi:10.7282/T3FN17V5

ETD doctoral

The author owns the copyright to this work.