Electron Microscopy Based Characterization of Resistive Switches

Random Access Memory (RRAM) has emerged as a leading candidate for nonvolatile memory storage. RRAM devices typically consist of a metal/insulator/metal (MIM) structure and exhibit switching of the device resistivity state (low-to-high, highto- low) by application of electrical bias. It is now widely accepted that shunting and rupturing of local conductive paths (filaments) directly determines the resistance state. The size and composition of these filaments are very much an open question, but are usually attributed to high local concentrations of oxygen vacancies. Although there has been a huge body of research conducted in this field, the fundamental nature of the conductive path and basic switching/failure mechanisms are still under debate. This is largely due to a lack of structural analysis of existing filament size and composition in actual devices. Since the non-volatile nature and device reliability issues (i.e. retention and endurance) are directly related to the irreversible structural transformations in the device, microstructural characterization is essential for eventual commercialization of RRAM. In this study, I investigated oxygen vacancy defect dynamics under electric filed essential for resistive switching and aim to identify size, location, and chemical nature of the conductive filaments in RRAM devices by using a variety of devices and materials characterization methods: in situ transmission electron microscopy (TEM), highresolution TEM (HRTEM), scanning TEM (STEM)-electron energy loss spectroscopy (EELS), electron holography, rapid thermal annealing (RTA), transient thermometry, and electro-thermal simulation. I adopt an in situ electrical biasing TEM technique to study microstructural changes occurring during resistive switching using a model TiO2-based RRAM device, and confirmed the device is switchable inside of the TEM column. I observed extension and contraction of {011} and {121}-type Wadsley defects, crystallographic shear faults, associated with resistive switching. More specifically, emission and adsorption of oxygen vacancies under different polarity of electrical biases at the fault bounding dislocations were identified. The motion of Wadsley defects was used to track oxygen vacancy migration under electric field. Also, the microstructural changes that occur when the device experiences low electric field (~104 V/cm) was reported, akin to read disturb. Crossbar type RRAM device stacks consisting of TiN/a-HfAlOx/Hf/TiN were investigated to estimate filament size, filament temperature, and its chemical footprint using HRTEM, transient thermometry and numerical simulation. In each of the switched devices, a single crystallite ~ 8-16 nm in size embedded in an amorphous HfAlOx matrix was found. The HfAlOx crystallization temperature (Tc) of 850 K was determined by combining RTA and HRTEM imaging. In parallel, the filament size has been determined by transient thermometry. The temperature profile extracted from these measurements suggested that the peak filament temperature was > 1500 K at the center, with the hot zone (T > Tc = 850 K) extending to a radius of 7 nm around the filament. These results were consistent with the HRTEM observations of the crystallite size. The potential filament location (crystallite) in the switching devices was analyzed by STEM-EELS and identification of the filament chemical nature identification has been attempted.