A Study on Resistive Switching Devices Based on Halide Perovskite Materials for High-Performance Memory Applications

Abstract

The demand for next-generation memory devices with high density and fast switching speed has increased to replace conventional memory devices. Among various next-generation memory devices, resistive switching memory (RSM) devices, which can store information using resistance change of the active layer, are promising candidates for future memory devices due to advantages such as scalability, fast switching, stable retention, and low power consumption. Also, RSM devices can be applied to three-dimensional (3D) memory architectures due to their simple metal/insulator/metal structure. The RSM devices have been widely researched using oxide materials, but there are still requirements to be improved such as high-temperature process, high energy consumption, and flexibility. Recently, halide perovskite (HP) materials have begun to be used in RSM. The hysteresis in current-voltage characteristics based on defect and/or ion migration enables the use of HP as an active layer for RSM. The HP-based RSM devices have been investigated and demonstrated excellent advantages in terms of low voltage operation and high on/off ratio. However, HP-based memory devices still suffer from several problems that need to be solved for practical applications. This dissertation focuses on the development of RSM devices using HP materials. I research HP-based RSM devices from the step of selecting the optimal HP material to the step for improvement of the device structure. To find optimal HP materials and fabrication processes for RSM devices, various HP materials and deposition processes including sequential vapor deposition and solution process are conducted. Also, I evaluate the feasibility of various candidates for the HP-based RSM devices, conduct in-depth studies including electrical characteristics and device reliability tests, and demonstrate the potential for HP-based RSM devices for high-density memory applications. Additionally, I suggest the possibility of HP as a photonic synaptic device. These works provide a promising route to implement HP-based RSM devices and to improve the device characteristics for high-performance RSM devices. In chapter 1, I briefly introduce the background for these researches for the development of HP-based RSM devices. In chapter 2, I report RSM devices using various HP materials, structures, processes, and device structures to improve the performance of RSM devices such as endurance, switching speed, and stability. I develop HP-based RSM devices from the stage of selecting the optimal HP material for memory devices to the step of introducing an improved device structure. To form the active layers based on HP, various deposition processes including the spin-coating process and sequential vapor deposition are utilized. By evaluating the memory characteristics such as switching speed, endurance, reliability, and stability of the HP-based RSM, I evaluate the possibility of HP materials as memory applications. First, the lead-free HP materials of Cs3Sb2I9 with a dimer structure are introduced for RSM. The Cs3Sb2I9 has low ion migration barrier, which enables fast switching operation in RSM devices based on the filament formation by ion migration. The RSM device based on dimer-Cs3Sb2I9 has an ultrafast switching speed compared to a device using layer-Cs3Sb2I9. Furthermore, I report multilayer HP-based memory devices which have an Ag-doped ZnO layer on HP. Ag-doped ZnO layer is used as an Ag ion reservoir for filament formation in HP, and this reservoir enables control of filament formation. By adjusting the Ag concentration in the Ag-doped ZnO layer, the controlled filament composed of Ag can be formed; as a result, the memory device has excellent endurance. In chapter 3, I report HP-based optical synapses using the combined structure with an HP with indium zinc tin oxide (IZTO) to perceive and memorize optical information for long-term storage in neuromorphic visual systems. The device structure where transparent IZTO on HP allows optical absorption by the HP and protects it from moisture in ambient air. Essential synaptic functions were emulated by optical stimuli. Optical absorption of HP leads to the generation of photo-excited carriers, and the band alignment between IZTO and HP facilitates the spatial separation of photo-excited carriers that serves as the basis of optically-meditated charge trapping, which enables emulation of synaptic functions. The potentiation/depression characteristics, which are one of the essential synaptic functions, are realized by applying consecutive optical signals under multiple wavelengths with a large dynamic range. These results demonstrate the feasibility of HP-based optical synapses for neuromorphic visual systems.