Real time analysis of switching dynamic behavior in transition metal oxide thin films for resistive switching memory applications

Abstract

Resistive switching random access memory (ReRAM) is considered as a serious candidate for the next generation nonvolatile memory, due to its promising scalability. The ReRAM operation is based on the reversible change between two distinct resistance states, i. e. high resistance state (HRS) and low resistance state (LRS), by applying either voltage or current. The resistive switching (RS) systems have been intensively investigated, but the underlying mechanism is still under the debate. To be implemented in future memory, more vigorous investigations on the materials characteristics responsible for RS behaviors need to be conducted for a better understanding. In this study, the dominant factors determining the RS characteristics were identified via the analysis of circuit noise in DC and AC measurement system. A huge discharging current from the parasitic capacitance in source-meter was generated after reaching the current compliance in uncontrolled manner. This overshooting current made the RS less uniform even though the RS was carried out with same current compliance function. Thus, it should be necessary to eliminate the noise source in system or minimize the damage against the voltage/current spike. For a more practical pulse switching case, the impedance matching problem has to be solved, which can have effect on the RS characteristics. The parasitic impedance components, such as line capacitance and mutual inductance, induced a severe distortion of the input pulse pattern, as well as the time delay until the applied voltage approached the threshold level. The intrinsic capacitance of RS cell, however, inhibits a high speed operation, since the ramping rate of applied voltage was limited by the RC delay time. Besides, the charge dissipation from the intrinsic capacitance makes the conducting filament (CF) to be stronger. These results suggest that the understanding of the parasitic noise signal generated from a peripheral circuit is important to optimize the RS characteristics. In addition, the growth process of CF was examined in the real-time scale via the equivalent circuit model that can demonstrate the overall features of the time-transient switching current. It is important to predict the configuration of CF, which is closely related to the RS characteristics. However, the direct observation of CF is very tricky. In this study, a novel methodology to conjecture the configuration of filament was drawn by converting the change of resistance to the volume fraction of conducting region in RS materials with respect to the time. For the specific RS system, associated with the Magnéli type phase transition, the Johnson-Mehl-Avrami (JMA) type kinetics theory can be applied to analyze the growth of the CF as long as the pulse SET switching was performed under the isothermal condition. To analyze the real-time transient behaviors, the numerical calculation was performed in order to capture the exact time evolution of current that passes through the memory cell during a SET process (i. e. the change in the resistance state from HRS to LRS), based on the PSPICE simulation. Due to the lack of a complete model to embody the ReRAM with conventional circuit elements, the equivalent circuit models for the unipolar RS and the bipolar RS system were designed to examine the resistance change in time-domain. The JMA kinetic growth model used in this study showed that it can be effectively used to determine the geometry of the filaments within the materials studied in this work. In general, the phase transformation based RS systems, such as TiO2 and WO3, show not only the unipolar RS but also the bipolar RS behavior even though the RS cell has a symmetric electrode configuration. This is due to the fact that the parts of the CFs remains even after the RESET switching and play a role as the virtual electrode despite the presence of the Pt electrode. Consequently, the fitting results according to JMA theory clearly exhibit that the evolution process undergoes in the multi-sequential steps. Such is consistent with the changes in growth direction of CF from vertical to horizontal, in unipolar RS mode, whereas the vertical growth occurs without additional nucleation process in a single step in bipolar RS mode. Moreover, the CF growth characteristics were involved in the crystallographic and microstructural properties of the RS materials. For a specific case, the sputtered TiO2 film, which is crystallized as the rutile structure and has a columnar grain, appears to help the RESET region to retain the same structure, which facilitates the rejuvenation of the CF during the subsequent SET step at the same location. This is reflected as the better uniformity and repeatability of switching parameters of the sputtered sample over the PEALD TiO2 sample. In the PEALD TiO2 RS cell, the RESET region may resemble the anatase-like structure, which inevitably induces more random nucleation of the CF during the subsequent SET step. Resulting JMA plot shows that the rejuvenation of the filament is accomplished by the repeated one-dimensional nucleation followed by the two-dimensional growth in PEALD samples, whereas one-dimensional nucleation-free mechanism dominates in the sputtered films. Using this kinetics model, it can be found that the lateral growth of CF, inducing a fast degradation, is controlled by connecting an appropriate series resistor.