Investigation Of The Resistive Switching Mechanism In The RRAMs Based On Transition Metal Oxides

The current memory technology based on charge storage, such as flash memory, is rapidly approaching to its physical and technical limit because it is increasingly difficult to reliably retain sufficient electrons in these shrinking cells. It's important to find novel nonvolatile memories with high-density, high-speed and low power consume. To overcome the limitations of conventional semiconductor devices based on charge storage, various new nonvolatile-memory(NVM) devices such as, ferroelectric, Polymer, phase change memory and resistance random access memories(RRAM) have been investigated. Especially, resistance random access memory (RRAM) is a reliable and achievable alternative among many substitutes due to its low power consumption, simple fabrication process fast switching speed, excellent size scalability and the potential to be scaled down to sub-100 nm scale. Many materials with resistive switching phenomenon have been investigated including transition metal oxides, perovskite oxides, organic compounds and solid electrolytes films. Up to now, many research works have been focused on resistive switching mechanism, however, the underlying mechanism for the resistive switching effect is still a controvertible problem and further works are needed. In this study, novel non-volatile memory devices with Au/SrTiO3-s(STO)/Pt, Pt/STO/Pt, Ag/STO/Pt, Au/Fe2O3/FTO and Ag/AIMO/Pt sandwich structures have been fabricated by using Pulse Laser Deposition (PLD) and ultrasonic spray pyrolysis (USP) combined with Focused Ion Beam (FIB) techniques, and the resistive switching mechanism of these devices also has been analyzed. The main achievements are summarized as follows:1. Au/STO/Pt non-volatile memory devices have been fabricated by PLD technique. The devices with different polarity electroforming voltage both have a good resistive switching performance, including low threshold voltage for turning on and off the device, good endurance and retention properties. However, the device has different resistive hyteresises polarity, clockwise and anticlockwise, respectively. This implies that the polarity of electroforming voltage impacts the distribution of defects in STO films. The resistive switching mechanism based on STO films was analyzed. According to fitting results of I-V curves at high resistance state (HRS) and low resistance state (LRS), the conduction mechanisms of HRS and LRS were ascribed to P-F emission and tunneling of schottky barrier. A model based on both bulk and interface effects were proposed for the resistive switching effects, and the equivalent circuits of the devices after different polarity of electroforming voltage was analyzed.2. Ag/STO/Pt device was prepared and the resistive mechanism of the device was verified. The resistance ratio between the HRS and LRS can reach the order of magnitude 103-104, which is high enough to distinguish the storage states ("0" or "1"). The positive and negative threshold voltages are 0.35 V and-0.11 V, respectively. The cell can be written and erased for more than 2×104 times without failure. The memory cells can maintain at high and low resistance states for-8×106 s without obvious resistance variation. To confirm the resistive switching mechanism, we fabricated the Ag/AIMO/Pt resistive switching device to investigate the resistive switching mechanism in electrolytes film. The characteristics of resistive switching mechanism based on electrolyte film were summarized, which were similar to the characteristics of Ag/STO/Pt. That combining the XPS results and the temperature dependence of resistance of HRS and LRS in Ag/STO/Pt implies that the resistive switching was attributed to the formation and dissolution of metallic Ag filament.3. Unipolar resistive switching device based in amorphous STO films induced by pulse of voltage was prepared. The positive and negative threshold voltage of device was confined at the range of 0.8-1.5 V and 2.5-3.5 V, respectively. The switching of the device can be repeated over 90 times. The resistance at HRS and LRS of the device can maintain for 1.4×104s. The conduction mechanism of HRS and LRS was Poole-Frenkel emission and Joule heating effects. When RL> Rco (about 6.2Ω), some parts of the conducting channels may become singly connected. When RL< Rco (about 6.2Ω), all of the conducting filaments are multiply connected. The PR increases as the compliance current, size of electrode and resistance at LRS. The relationship between PR and RL can also be fitted by the formulas of PR∝RL-r.4. Two types of bipolar resistive switching (RS) model were revealed, before and after electroforming in theα-Fe2O3 films fabricated by ultrasonic spray pyrolysis on fluorine-doped SnO2 conducting glass. The current-voltage curves of cell before and after electroforming were fitted. The conduction mechanism at high resistance state of the model before electroforming was attributed to Schottky emission. Whereas, that after electroforming can be explained by space-charge-limited conduction. The two types of RS model were originated from interface and bulk effect, respectively. The retention characteristics of the cells with different RS model were studied comparatively and the possible mechanism of retention was ascribed to the deep level of defects and the "ordering" of defects resulted from electroforming process.