Research On Resistive Switching Memory Based On Binary Metal Oxides

With the prevalent usage of portable equipments, such as mobile phone, MP3, MP4 player as well as notebook PC, the nonvolatile memory (NVM) plays an important role in the semiconductor industry. The advantage of NVM is that the stored data can be kept for a long time without power supply, and the current NVM mainstream for the semiconductor mass storage is based on the Flash technology. However, with the conventional memories approaching their scaling limits, Flash memory, which is based on the traditional floating gate concept, has encountered serious technical challenges due to the tradeoffs between the high speed, low power consumption and long retention time. It is therefore common practice to explore new memory to compensate these weaknesses. Requirements for a perfect memory would have a high capacity, fast speed, long retention time, low power consumption, and it would be also nonvolatile and scales better than existing technologies. In recent years, some emerging nonvolatile memories, such as ferroelectric random access memory (FRAM), magnetic random access memory (MRAM), phase-change random access memory (PRAM) and resistive random access memory (RRAM), are being enthusiastically studied to achieve these requirements. Among all these choices, RRAM device which shows resistive switching between a high resistance state and a low resistance state has overwhelming advantages of easy fabrication process, simple structure, excellent scalability, fast switching speed, and high integration density. Therefore, more and more research communities focus their attention on RRAM devices. This work focuses on the investigation of resistive switching characteristics in binary metal oxides-based RRAM deives due to the simple structure, easy fabrication process and compatibility with the current complementary metal oxide semiconductor (CMOS) technology. We focus our attention mainly on resistive switching characteristics and resistive switching mechanism of ZrO2,WO3 and TiO2-based RRAM deives.As for the ZrO2-based RRAM deives, we fabricated Cu/ZrO2/Pt and Cu/TiOx-ZrO2/Pt structure devices and investigated the resistive switching characteristics of these two devices. By embedding a thin TiOx layer between ZrO2 and Cu top electrode, stable and reproducible bipolar resistive switching characteristics could be observed. Especially, the set voltage obviously showed decrease compared to that in ZrO2-based RRAM device without TiOx layer. We also fabricated Au/ZrO2/Ag structure device which showed low operation voltages (<1V), high resistance ratio (about 104), fast switching speed (50ns), and reliable data retention (10 years extrapolation at both RT and 85℃). Moreover, the benefits of high yield and multilevel storage possibility made this device promising in the next generation nonvolatile memory applications. The instable reset behavior was observed in Cu/ZrO2:Cu/Pt device, and the mechanism of this instable reset behavior was investigated based on a physical model.We investigated the effect of different top electrodes on the resistive switching characteristics of WO3-based RRAM devices. Compared with the Al/WO3/Pt and Cr/WO3/Pt devices, the Cu/WO3/Pt device had much better resistive switching characteristics, such as good reproducibility, low power consumption and good data retention. In addition, the multilevel storage capability of the Cu/WO3/Pt RRAM deivce was demonstrated by setting different compliance currents during the Set process. A direct measurement by using a dc voltage sweeping mode with a smaller sweeping voltage incremental rate was employed to explore the plausible mechanism of the multilevel storage, which might be attributed to the combination of filaments radial growth and formation of more conductive filaments as applying a higher compliance current.As for the TiO2-based RRAM deives, we reported a simple method to improve uniformity of resistive switching parameters in atomic-layer-deposited titanium oxide film by nitrogen annealing. We also fabricated an Al/TiO/Al RRAM device which is fully compatible with CMOS technology.