Resistive switching characteristics of binary transition metal oxide and its application to emerging memory device

Abstract

Memory is a device to store digitalized information. In the conventional memory device, the information, ‘0’ and ‘1’, is distinguished whether charge is trapped or not, thus called charge based memory. As a new memory concept, resistance based memory is developed to store the information using resistance of system, whether resistance is high or low. The class of resistive switching phenomena that based on the electrically stimulate that changes the resistance state of system, is generally called resistance switching RAM, or ReRAM. To enhance the memory device performance, it is important to know the underlying mechanism i.e., how the resistance of system can be changed. Though there are number of suggested mechanisms, however, it is still controversial. The investigation of the unclear resistive switching mechanism is a key issue studying resistive switching phenomena. Thus, in this study, some efforts were made to reveal the mystery of resistive switching mechanism. Also, to develop new resistive switching materials/systems for possible application and switching characteristics enhancement, various transition metal oxide was evaluated as new switching materials. Based on those results, finally, emerging resistive switching devices such as transparent, flexible and waterproof devices are developed and characterized.In chapter 2, resistive switching characteristics of hafnium oxide (HfO2) were studied as a new switching material. The HfO2 films were grown by metal organic chemical vapor deposition (MOCVD) at 400 oC using tetrakis-diethylamido-hafnium (Hf(N(C2H5)2)4) as a precursor and oxygen gas as an oxidizing agent. The device fabricated in optimized deposition condition showed reproducible resistance switching behavior. The high resistance ratio was about 104 to 109, which is higher than other comparable materials such as TiO2 and ZrO2. SET and RESET voltages were measured about 0.8 and 1.5 V, respectively, indicating that the device can be operated below 2 V. The bipolar resistance switching behavior was also observed as well as the unipolar switching. These studies provide hafnium oxide as new switching material that shows remarkable performance and valuable information for the investigation of switching mechanisms.In chapter 3 and 4, resistive switching characteristics of simple ZnO thin film device were investigated. The structure shows reproducible and stable unipolar resistive switching after electroforming with high compliance current. The switching was performed regardless of the applied voltage polarity which suggests the switching can be explained by the formation and rupture of conductive pathways inside the film, which are referred to as the filaments. With low compliance current at electroforming, however, bipolar resistive switching was observed. In this state, the switching phenomenon was depended on the voltage polarity. Then the device structure was fabricated on stainless steel (SS) was investigated for its applicability as a flexible resistive random access memory. The device performance was not degraded upon bending, which indicates high potential for flexible ReRAM applications. From these studies, I could propose a switching mechanism based on the filament theory to explain the coexistence of unipolar and bipolar resistive switching and provide a new idea of flexible device fabrication.In chapter 5, I present conceptually new and multifunctional resistive switching devices with water resistance, flexibility and transparency. Most electronic devices are vulnerable to water contact even though they are well passivated. The penetration of a single water droplet can cause an electrical short circuit and even more serious electric shock. The application of superhydrophobic nanostructures on device surfaces efficiently blocks the direct contact of water with electronic components or even unprotected metal interconnections. I have discovered that the devices still work even with when water is poured on the surface of the device. I believe that this smart concept will affect many researchers who work on electronic cells, device integration and especially emerging organic devices.In chapter 6, I present the resistive switching characteristics of tungsten oxide (WOx)-Au core-shell nanowire arrays on W substrates. The nanowire array devices showed stable bipolar resistive switching characteristics based on Schottky barrier height changes, which were induced by redox reactions occurring at the interface between the WOx and the W electrode. Evidence of the redox reaction was investigated using HR-TEM and showed that there was a conversion from smooth to rough interface states after a number of switching cycles. The nanowire structure of our resistive switching devices provides superhydrophobic properties due to the inherent roughness of the surface. The superhydrophobic surface repelled water that was poured over the device such that the device was protected from failure by water contact-driven leakage currents. Moreover, the superhydrophobicity of our device showed high stability even in submerged conditions, as observed by stable resistive switching characteristics. Thus, I present the resistive switching phenomena of WOx nanowires as a novel type of resistive switching nanomaterial and the proof of concept of a waterproof electronic device that can work under water.