In-situ TEM Study of Phase Transition and Interface Reaction Mechanisms of Resistive Switching Devices

Abstract

Future memory device technology has been continuously developing toward a level which requires the precise control of ionic migration, redox reaction and phase transition within nanometer-scale materials and/or across the interface between them. Ultimately, seeing the change of these phenomena in-situ during device operation suggests the unique solution for unambiguous understanding of the operation mechanism and also for further optimization of device performance. In this dissertation, I will report a few case studies in which advanced TEM techniques, such as in-situ TEM and spherical aberration (Cs) corrected STEM-EELS, are used to address these challenging issues. In-situ TEM is referred to as a broad class of experiments whereby the dynamic response of a material to an externally applied electrical stimulus is observed as it happens inside the TEM. Recent advances in TEM instrumentation enables direct observation of the resistive switching processes of various non-volatile memory devices at working conditions in real-time. Here I will report four examples. In the first example, the amorphous-to-crystalline phase transition of Ge-Sb-Te (GST)-based vertical phase change random access memory (PCRAM) was observed by applying DC voltages in TEM, showing that the microstructure of GST, particularly the passive component surrounding the active switching volume, plays a critical role in determining the local temperature distribution and is therefore responsible for cell-to-cell inconsistent switching behaviors. The in-situ TEM results also reveal that the failure of PCRAM occurs via two-step void formation due to the phase separation accelerated in the molten GST by the polarity-dependent atomic migration of constituent elements. In the second, the void nucleation, growth and migration in GST-based confined PCRAM was observed by applying AC voltage pulses in TEM, showing that the void in the cylindrical switching volume is formed during the crystallization of GST at the any interfaces, e.g., grain boundary, GST/spacer interface and GST/electrode interface. It plays a critical role in determining the local temperature distribution and current density in the active switching volume, which is therefore responsible for inconsistent device failures. In-situ TEM results also reveal that the unidirectional migration of void by the multi-pass of set pulses occurs at given current density due to the momentum change of hole carriers driven by electrical wind force. In the third, as a model resistive random access memory (ReRAM), applying the highest level of in situ TEM techniques to practical TiN/Pr0.7Ca0.3MnO3 (PCMO)/Pt junction-based ReRAM devices, direct observations on the interfacial redox reaction driving the resistive switching of real working devices will be presented for the first time. The in-situ TEM results reveal that the growth and dissolution of the amorphous Ti-oxynitride (a-TiOxNy) interfacial layer, and the phase transition of the PCMO are key resistance switches. In addition, the kinetics of the redox reaction is controlled by the effective electric field, defining several important parameters like the critical field for the onset of switching and the saturation field for the electromigration of oxygen ions. Finally, as a model system of the interfacial redox reaction-based ReRAM devices, the Mo/PCMO/Pt junction-based device was chosen to study both the effects of the interfacial reaction layer and chemical distribution on the resistance switching. The in-situ TEM results reveal that the irreversible growth of the amorphous Mo-oxide (a-MoOx) interfacial layer predominantly determines the device resistance during the electroforming process. On the other hand, during the reversible set/reset switching, the device resistance is mainly controlled by the oxygen (or vacancy) concentration within the both a-MoOx interfacial layer and oxygen-deficient PCMO (PCMO3-δ) layer, acting as a series resistor. In this dissertation, a possible model of resistive switching mechanisms will be discussed based on the direct observations of the microstructural evolution, correlated I-V characteristics and also chemical analysis.