Research On Reversible Resistance Switching Of Perovakite Metal Oxides Thin Films At Room Temperature

The semiconductor industry is encountering both technological and fundamental challenges as device features are approaching the sub-100-nm regime. The increasing demand for device scaling, as clearly described by the International Technology Roadmap for Semiconductors (ITRS), is a major issue. To overcome the limitations of conventional semiconductor devices, which are based on charge storage, various new nonvolatile-memory (NVM) devices such as phase-change, polymer, magnetic, and resistance random access memories (ReRAM) have been investigated. With nonvolatile characteristics and a simplified device structure, ReRAM provide a simultaneous solution to the need for low power consumption, fast switching speed, and high density integration, as one of the next-generation nonvolatile memory candidates. Of these novel NVMs, materials with reversible resistance switching at room temperature have become increasingly more attractive for today's semiconductor technology processes. This reversible resistance switching (RS) effect has been observed in many insulating systems, including binary oxides, complex perovskite oxides, sulfides, and organics. To explain the intriguing effect, numerous theoretical models have been proposed. However, most of these models leave unanswered questions. Despite its fundamental importance, our understanding of the underlying physics of the RS effect is still poor. Therefore, one important challenge is the ability to reveal the transport mechanisms of the resistance switching. In this dissertation, in order to clarify how and where resistance switching occurs, we focused on the resistive-switching properties of perovskite oxides at room temperature and on improvements of RS properties by a careful control of its experimental techniques and sysths conditions. The experimental results showed a high potential for nonvolatile memory application and are shown as follows:1. The electric-pulse-induced resistance switching of the Au-La0.67Ca0.33MnO3(LCMO)-FTO (fluorine-doped tin oxide) heterostructures was studied by electrochemical workstation. A distinct current-voltage characteristic of the device with pronounced reproducible, nonlinearity, asymmetry and hysteresis was observed at room temperature. The current-voltage characteristics suggest a Poole-Frenkel (PF) and space-charge-limited current (SCLC) type mechanism controlled by Au/LCMO interface traps. More than two controllable resistance states were obtained by applying DC bias sweep and voltage pulses. The resistance switching behavior shows obvious multilevel resistance switching (MLRS).2. V-doped La0.67Ca0.33MnO3 (LCMO) thin films were prepared on fluorine-doped SnO2 (FTO) conducting glass substrates with a sol-gel technique. The resistance switching properties of Au/V:LCMO/FTO heterostructures investigated by electrochemical workstation showed reproducible resistive switching behaviors at room temperature. The interactions between nonlattice (mobile) oxygen and oxygen vacancies and/or the cationic vacancies contribute to the carrier transport of the LCMO layer sandwiched systems. With proper doping concentration (3% V-doped LCMO), the resistive switching behaviors can be well improved and stabilized. The maximum resistance ratio obtained could be reached up to 700%. The DC-bias effects on hysteresis loops suggest the nonuniform distribution of trapped charges attribute to the multilevel switching. The experimental results showed a high potential for nonvolatile memory application on amorphous substrates.3. Oxide heteroj unctions made of La0.67Ca0.33MnO3 (LCMO) and SrTiO3have been fabricated on fluorine-doped tin oxide (FTO) conducting glass substrates by the pulsed laser deposition (PLD) technique. The nonlinear current-voltage(â… -â…¤) behavior and the electric-pulse-induced resistance switching in the heteroj unctions have been investigated by elecrtrochemical workstation. Using an AC-resistance measurement method, the interface-related resistance switching characteristics was revealed. Significantly, with increasing DC-bias, the large semicircle shrinks, which is a typical characteristic of a Faradic charge-transfer. It was clarified that these resistance changes can be reasonably attributed to the predominant charge and mass transfer occured at this interface due to the local solid electrochemical reaction. These results imply that the migration of the oxygen vacancies could change the distribution of the space charge near the interface, resulting in a change in the interface resistances.4. The Na0.5Bi0.5TiO3 (NBT) thin films sandwiched between Au electrodes and fluorine-doped tin oxide (FTO) conducting glass were deposited using a sol-gel method. Based on electrochemical workstation measurements, reproducible resistance switching characteristics and negative differential resistance were obtained at room temperature. A local impedance spectroscopy measurement of Au/NBT was performed to reveal the interface-related electrical characteristics. The DC-bias-dependent impedance spectra suggested the occurrence of charge and mass transfer at the interface of the Au/NBT/FTO device. It was proposed that the first and the second ionization of oxygen vacancies are responsible for the conduction in the low-and high-resistance states, respectively. With decreasing low frequence values, the large semicircle shrinks, and the impedance change increasing simultaneity. It suggests that several resistance states were obtained by modulating the electric-pulse voltage. It is expected that the resistance switching behavior observed in this work can make it possible to integrate the nonvolatile multilevel memory device on glass substrates. The experimental results showed high potential for nonvolatile memory applications in NBT thin films.