Study On The Controllability And Mechanism Of Conductive Channel In Carbon-based Resistive Switching Memory Devices

Conventional charge-based memories, such as FLASH memory, will suffer from performance degradation as the scaling limit is approached. In order to satisfy further requirement of information storage,there are many memories have been considered as emerging memories, provide the potential for beyond the present limit of FLASH memories. Among them, resistance random access memory(RRAM) is attracting strong interest due to their potential advantages of simple structure, low power consumption, high speed and long retention, which makes it one of the most promising candidates for next generation non-volatile memory. The RRAM consist of a solid electrolyte layer sandwiched between two electrodes. The resistive switching(RS) mechanism usually can be attributed to the formation/rupture of nanoscale conductive channel in the solid electrolyte layer. However, for RRAM, the detail RS mechanism remains unaddressed, and the device performance still needs improvement. Thus,study on conductive channel, including how to effectively control the conductive channel, what will happen when reducing the conductive channel size and where the conductive channel begins to formation/rupture during RS is very important for further development of RRAM. On the other hand,among the various solid electrolytes,carbon-based materials are heavily investigated as replacement of silicon in future microelectronics, thus carbon-based memory could be a significant complement to the rapid advances in carbon-based electronics. Our works are mainly on the controllability and investigation of the mechanism of conductive channel in carbon RRAM devices. The thesis contains as follows:On the controlliabity of conductive channel size: We employed amorphous carbon(a-C) as the switching layer, Cu and Pt as electrodes. For the Cu/a-C/Pt memory device, nonvolatile/volatile RS behaviors and quantized conductance were demonstrated in the Cu/a-C/Pt memory devices by controlling the CCs(that is, the size of Cu channel). By quantitatively studying the dependence of relaxation time on channel’s size and temperature, the volatile behavior can be well understood within the framework of the Rayleigh instability, where the Cu channel spontaneously dissolves to minimize the surface energy. A detail analysis by solving the steady-state Fourier heat flow equation indicated that the temperature(joule-heating) increase with increasing the conductive channel’s size. With the help of Raman specture,it was found that the permanent decrease of HRS upon increasing the channel’s size was induced by the joule-heating assist sp2-clustering within the a-C thin film.On the controlliabity of conductive channel formation/rupture site: The random nature of the formation and rupture of conductive channel results in large fluctuations of switching parameters. In order to improve the device uniformity, a current pre-stress was applied to the Cu/a-C/Pt memory device. The current stressing treatments may well be to generate conductive sp2 clusters and in turn enhance the local electric field. The enhanced local electric field can control the switching site of conductive channel and provide uniformity of switching properties. On the other hand, we demonstrate a forming-free electrochemical metallization resistive memory device based on the nanoporous thin film. Due to the nanoporous structure of switching layer, Ag atoms can migrate into the film during the Ag electrode evaporation process, resulting in a pre-formed Ag channel inside the switching layer. Further, we employed the nanoporous thin film filled with Ag as nano-sized(which is comparable to the conductive channel) electrode to replace the traditional micro-electrode in a-C memory device. By introducing Ag nano-sized electrode, the device offer improved uniformity of switching parameters compared with micro-electrode device.On the study of conductive channel mechanism: To realize an in situ monitor of conductive channel during the RS process, appropriate semiconductor possessing color change(band gap change) combined with RS is needed. In this work, coplanar Au/Graphene oxide/Au(Au/GO/Au) instead of the common metal-insulator-metal stack structure was prepared by photolithography. For the Au/GO/Au structure, we demonstrate that reduced grapheme oxide(RGO) conductive channel can be formed between the Au electrodes through the use of electric field. The RGO conductive channel is further verified by XPS and Raman characterization. It is observed that the RGO conductive channel apparently started growth from the cathode to the anode by optical microscope. The ambient humidity is found to directly influence the resistive switching process:The RGO conductive channel can be partly re-oxidated by reversing the polarities of the electrodes with the humidity up to more than 80%. Additionally, the RGO conductive channel was found to rupture near the anode of the reset voltage. The result indicates that the presence of water play a significant role in the reduction/oxidation of GO.