| Summary: | 博士 === 長庚大學 === 電子工程學系 === 100 === In this study, conductive bridging random access memory (CBRAM) devices using different GexSe1-x (x=0.2–0.5) solid–electrolytes in an Al/Cu/GexSe1-x/W structure have been investigated. All deposited materials, via filling, and crossbar structure are studied by HRTEM, EDX, and XPS analyses. Bipolar resistive switching memory characteristics can be controlled by controlling different compositions of the solid–electrolytes (GexSe1-x; x=0.2–0.5). Larger SET voltage (>0.5 V), stable endurance (>10^5 cycles), and stable retention (>48 hrs) at 85^oC at a current compliance (CC) of 300 µA are obtained for higher Ge content (≥40%) devices. The crystalline Cu nanofilament with a diameter of ~11 nm in the Ge0.4Se0.6 solid–electrolyte using a Cu/Ge0.4Se0.6/W device under SET is also observed by HRTEM. To understand the switching mechanism indirectly, different electrodes such as Al, Cu, W, TiN, and IrOx have also been studied. It is found that, if one of the electrodes is Cu in an Al/Cu/GexSe1-x/W structure then the bipolar resistive switching phenomena with a small operation voltage of approximately ±1V are observed. The Al/Cu/Ge0.5Se0.5/W resistive switching memory device that was not reported in the literature could be operated at low CC of 1 nA with a small operation voltage of ±1V and low SET/RESET power of 0.61 nW/6.4 pW is obtained. Furthermore, an extra formation voltage is not required for all Al/Cu/GexSe1-x/W memory devices.
To obtain a better performance and CMOS compatibility, the CBRAM device in a new Cu/GeOx/W structure has also been designed. The switching mechanism occurs because of the lower barrier height for hole injection rather than electron injection. Therefore, Cu ions, as a positive charge, migrate before initiating growth at the GeOx/W interface and dissolving at the GeOx/Cu interface. It is noted that repeatable unipolar/bipolar resistive switching memory characteristics with CCs of 1nA–10 mA are observed. This memory device has a good data retention of >10^4 s with a large HRS/LRS ratio of 10^2–10^5 for bipolar mode under CCs of 1 nA–50 µA, and >10^9 for unipolar mode under CC of >100 µA.
To further improve the resistive switching characteristics, novel CBRAM device in an Al/Cu/Ge0.2Se0.8/TaOx/W structure has been fabricated. Improved resistive switching parameters are obtained compared to those of a single Ge0.2Se0.8 and TaOx switching layer. Extrapolated, long program/erase endurance of >10^6 cycles, attributed to the Al/Cu/Ge0.2Se0.8/TaOx/W structure design, is observed. This resistive switching memory structure shows extrapolated 10 years data retention with a resistance ratio of >10 at a low CC of 0.1 µA at 50^oC. In addition, a Ti nanolayer at the Cu/TaOx interface in an Al/Cu/Ti/TaOx/W structure has been also investigated for CMOS compatible in future. By using a new approach, the nanoscale diameter of Cu filament decreases from 10.4 to 0.17 nm as the CC decreased from 500 to 0.1 µA, resulting in a large predicted memory size of 7.6 T–28 Pbit/sq. in.
Beside the CBRAM devices using different solid–electrolytes, a resistive random access memory (RRAM) in an W/GeOx:WOx/W structure has also been studied. Enhanced performances in terms of resistance ratio (>30), cycles of >10^6 with a large Vread of +/-1V, data retention (>10^5 s) at 85^oC, and uniformity of the W/GeOx:WOx/W RRAM devices are observed as compared to the W/WOx/W devices. The SET/RESET states are observed owing to the oxygen ions migration through the WO3 nanocrystals boundaries (density ~2×10^12/cm2), which is also confirmed by HRTEM images for the pristine and stressed RRAM devices.
To obtain a high–density low power non–volatile memory, a crossbar architecture using CBRAM device in an Al/Cu/GeOx/W structure is designed. Superior resistive switching memory performances in terms of high resistance ratio (>10^4), long endurance of >10^5 cycles, excellent data retention (>500 hrs) at 85^oC for CC of 50 µA, and excellent scalability potential are observed for the Al/Cu/GeOx/W CBRAM devices as compared to the Al/GeOx/W RRAM devices. This study is important for future high–density low power 3D architecture.
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