Research On Resistive Switching Memory For Ultrafast, High-Density And Low Power Nonvolatile Memory Application

The scaling technology of semiconductor devices following Moore's law is vital to continue the advancement of semiconductor integrated circuit technology. However, the current memory technology based on charge storage, such as flash memory, is rapidly approaching to its physical limit due to the tradeoffs between the high speed, low power operation and long time retention. In order to extend Moore's law for memory applications, several emerging nonvolatile memory technologies have attracted extensive attentions. One of the promising candidates is the resistive random access memory (RRAM) due to its superior characteristics including simple structure, excellent scalability, high density, low power, fast speed, and compatibility with standard CMOS process. This work focuses on how to improve the resistive switching performances of the binary-metal-oxide-(BMO)-based RRAM, mainly including research on doping materials, RRAM device structure and resistive switching mechanism.By doping uniform and homogeneous impurities inside the BMO films, we significantly improve the resistive switching performances of the BMO-based-RRAM. Specifically, the resistive switching characteristics and mechanisms of the three types of Au/Cr/ZrO2:Zr/n+-Si, Au/Cr/ZrO2:Au/n+-Si, Cu/ZrO2:Ti/Pt devices are extensively investigated for nonvolatile memory application. Through the theoretical and experimental analyses of these resistive switching materials, we investigate the impact of doping on the resistive switching behaviors and generate several important guidelines for the improvement of the resistive switching behavior of the BMO films. The resistive switching device based on the formation and annihilation of nanoscale metallic conductive filament (CF) inside solid-electrolyte insulator has attracted considerable attentions due to its potential applications for the nonvolatile memory, analog circuit and neuromorphic systems. However, the poor uniformity of these CF-based RRAM devices is a major obstacle to prevent them from high performance storage applications, due to the nucleation and growth of the CFs are random and difficult to control. To solve this bottleneck problem, we develop a novel approach to control the location and orientation of the CF growth by inserting a metal nanocrystal (NC) layer between the inert bottom electrode and the solid-electrolyte layer. Based on the proposed approach, we fabricate a Ag/ZrO2/Cu NC/Pt structure as a prototype and successfully capture transmission electron microscopy (TEM) images of the CF growth around the NC locations. Supported by the electric field simulation results, this TEM result is the first direct evidence to confirm that CF formation process is controlled by the profile of electric field inside solid-electrolyte layer. This controlled CF growth method can improve the device performance by reducing the randomness of CF growth process, which is verified by the excellent resistive switching behaviors of the prototype such as narrow switching voltages, ultrahigh ROFF/RON ratio, long retention time, and good endurance. This work can help establish the deep understanding the physical switching mechanisms of this type of RRAM.The development of multi-level RRAM devices is an efficient approach to achieve high storage density that is important in the development of the next generation nonvolatile memory. Based on the NC-controlled CF growth method, we fabricate and investigate the Cu/ZrO2/Cu NC/Pt device for multilevel storage memory application. This device exhibits low operating voltage (<1.2 V), high uniformity of resistance switching and 2 bit/cell multilevel storage. By using a DC voltage sweep mode with a very small incremental rate, we obtain the multiple resistance steps of Cu/ZrO2:Cu/Pt device in the on-state. The quantitative analysis of these resistance steps indicates that a number of CFs have been established sequentially between Pt and Cu electrodes through the ZrO2 film, which verify the device multilevel switching mechanism is dominated by multiple CFs formation and annihilation. This investigation on the growth and nature of the multiple CFs in the RRAM device can help establish high-density emerging NVM.