S. Z. Rahaman

Excellent resistive memory characteristics and switching mechanism using a Ti nanolayer at the Cu/TaOx interface

Excellent resistive switching memory characteristics were demonstrated for an Al/Cu/Ti/TaOx/W structure with a Ti nanolayer at the Cu/TaOx interface under low voltage operation of ± 1.5 V and a range of current compliances (CCs) from 0.1 to 500 μA. Oxygen accumulation at the Ti nanolayer and formation of a defective high-κ TaOx film were confirmed by high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photo-electron spectroscopy. The resistive switching memory characteristics of the Al/Cu/Ti/TaOx/W structure, such as HRS/LRS (approximately 104), stable switching cycle stability (>106) and multi-level operation, were improved compared with those of Al/Cu/TaOx/W devices. These results were attributed to the control of Cu migration/dissolution by the insertion of a Ti nanolayer at the Cu/TaOx interface. In contrast, CuOx formation at the Cu/TaOx interface was observed in an Al/Cu/TaOx/W structure, which hindered dissolution of the Cu filament and resulted in a small resistance ratio of approximately 10 at a CC of 500 μA. A high charge-trapping density of 6.9 × 1016 /cm2 was observed in the Al/Cu/Ti/TaOx/W structure from capacitance-voltage hysteresis characteristics, indicating the migration of Cu ions through defect sites. The switching mechanism was successfully explained for structures with and without the Ti nanolayer. By using a new approach, the nanoscale diameter of Cu filament decreased from 10.4 to 0.17 nm as the CC decreased from 500 to 0.1 μA, resulting in a large memory size of 7.6 T to 28 Pbit/sq in. Extrapolated 10-year data retention of the Ti nanolayer device was also obtained. The findings of this study will not only improve resistive switching memory performance but also aid future design of nanoscale nonvolatile memory.

Resistive switching memory characteristics of Ge/GeOx nanowires and evidence of oxygen ion migration

The resistive switching memory of Ge nanowires (NWs) in an IrOx/Al2O3/Ge NWs/SiO2/p-Si structure is investigated. Ge NWs with an average diameter of approximately 100 nm are grown by the vapor–liquid-solid technique. The core-shell structure of the Ge/GeOx NWs is confirmed by both scanning electron microscopy and high-resolution transmission electron microscopy. Defects in the Ge/GeOx NWs are observed by X-ray photoelectron spectroscopy. Broad photoluminescence spectra from 10 to 300 K are observed because of defects in the Ge/GeOx NWs, which are also useful for nanoscale resistive switching memory. The resistive switching mechanism in an IrOx/GeOx/W structure involves migration of oxygen ions under external bias, which is also confirmed by real-time observation of the surface of the device. The porous IrOx top electrode readily allows the evolved O2 gas to escape from the device. The annealed device has a low operating voltage (<4 V), low RESET current (approximately 22 μA), large resistance ratio (>103), long pulse read endurance of >105 cycles, and good data retention of >104 s. Its performance is better than that of the as-deposited device because the GeOx film in the annealed device contains more oxygen vacancies. Under SET operation, Ge/GeOx nanofilaments (or NWs) form in the GeOx film. The diameter of the conducting nanofilament is approximately 40 nm, which is calculated using a new method.

Enhanced nanoscale resistive switching memory characteristics and switching mechanism using high-Ge-content Ge0.5Se0.5 solid electrolyte

We demonstrate enhanced repeatable nanoscale bipolar resistive switching memory characteristics in Al/Cu/Ge0.5Se0.5/W, as compared with Al/Cu/Ge0.2Se0.8/W structures, including stable AC endurance (>105 cycles), larger average SET voltage (approximately 0.6 V), excellent data retention (>105 s) at 85°C, and a high resistance ratio (>104) with a current compliance of 8 μA and a small operation voltage of ±1.5 V. A small device size of 150 × 150 nm2 and a Cu nanofilament with a small diameter of 30 nm are both observed by high-resolution transmission electron microscope in the SET state. The GexSe1 − x solid electrolyte compositions are confirmed by both energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The switching mechanism relies on the smaller barrier heights for holes rather than for electrons; the positively charged Cuz+ ions (i.e., holes) migrate through the defects in the GexSe1 − x solid electrolytes during SET/RESET operations. Hence, the Cu nanofilament starts to grow at the Ge0.5Se0.5/W interface, and starts to dissolve at the Cu/Ge0.5Se0.5 interface, as illustrated in the energy band diagrams. Owing to both the higher barrier for hole injection at the Cu/Ge0.5Se0.5 interface than at the Cu/Ge0.2Se0.8 interface and greater thermal stability, the resistive switching memory characteristics of the Al/Cu/Ge0.5Se0.5/W are improved relative to the Al/Cu/Ge0.2Se0.8/W devices. The Al/Cu/Ge0.5Se0.5/W memory device can also be operated with a low current compliance of 1 nA, and hence, a low SET/RESET power of 0.61 nW/6.4 pW is achieved. In addition, a large memory size of 1,300 Pbit/in2 is achieved with a small nanofilament diameter of 0.25 Å for a small current compliance of 1 nA.

Low Power Operation of Resistive Switching Memory Device Using Novel W/Ge0.4Se0.6/Cu/Al Structure

Bipolar resistive switching memory device with a low power operation (200μA/1.3V) in a W/Ge0.4Se0.6/Cu/Al structure has been investigated. A stronger Cu chain formation can be observed by monitoring both the erase voltage and current. The low resistance state (RLow) decreases with increasing the programming current from 1nA to 500μA, which can be useful for multi-level of data storage. This resistive memory device has a large threshold voltage of ~0.5V, good resistance ratio (RHigh/RLow) of 1.6x10 2 , good endurance of >1.5x10 5 cycles, and excellent retention (>11 hours) with a resistance ratio of > 1.3×10 2 at 150 o C can be used in future nonvolatile memories.

Low current (5 pA) resistive switching memory using high-к Ta 2 O 5 solid electrolyte

Bipolar resistive switching memory device using high-к Ta<inf>2</inf>O<inf>5</inf> solid electrolyte in a Cu/Ta<inf>2</inf>O<inf>5</inf>/W structure with the device sizes from 0.2–8µm was investigated. This resistive memory device has a high threshold voltage of 0.75V, high resistance ratio (R<inf>High</inf>/R<inf>Low</inf>) of 3×10³, good endurance of ≫ 10³, and excellent retention at 150°C. The memory device with a low current operation of 5 pA is obtained, for the first time, owing to the Cu metallic chain formation in the high-к Ta<inf>2</inf>O<inf>5</inf> solid electrolyte. The strong Cu chain formation is also confirmed by monitoring both the negative voltage and current observations. The low resistance state (R<inf>Low</inf>) decreases with increasing the current compliance from 5pA to 700µA, which can be useful for future multi-level data storage applications.

Record Resistance Ratio and Bipolar/Unipolar Resistive Switching Characteristics of Memory Device Using Germanium Oxide Solid Electrolyte

The bipolar and unipolar resistive switching characteristics of a memory device using a Cu filament in a new Cu/GeOx/W structure under low-voltage operation ( 100 µA. This memory device has excellent uniformity in SET/RESET voltages, low resistance state/high resistance state (LRS/HRS), long read endurance of >1×105 cycles, and good data retention of >1×104 s with high resistance ratios of >105 in the bipolar mode and >109 in the unipolar mode.

Low current and voltage resistive switching memory device using novel Cu/Ta 2 O 5 /W structure

Low current/voltage (∼10 nA/1.0V) resistive switching memory device in a Cu/Ta<inf>2</inf>O<inf>5</inf>/W structure has been proposed. The low resistance state (R<inf>Low</inf>) of the memory device decreases with increasing the programming current from 10 nA to 1mA, which can be useful for multi-level of data storage. This resistive memory devices have stable threshold voltage, good resistance ratio (R<inf>High</inf>/R<inf>Low</inf>) of 5.3×10⁷, good endurance of ≫10³ cycles, and excellent retention (≫11 hours) with resistance ratio of ≫ 9×10³ can be useful in future non-volatile memory applications.

Improved resistive switching memory characteristics using novel bi-layered Ge 0.2 Se 0.8 /Ta 2 O 5 solid-electrolytes

Novel bi-layered solid electrolytic based resistive switching memory device using Al/Cu/Ge<inf>0.2</inf>Se<inf>0.8</inf>/Ta<inf>2</inf>O<inf>5</inf>/W structure has been investigated for the first time. The tight distribution of resistance states and threshold voltage are achieved as compared to that of single layer Ge<inf>0.2</inf>Se<inf>0.8</inf> solid-electrolyte. Stable endurance of ≫3.5×10⁵ cycles and excellent retention characteristics with a low compliance current of 100 nA are obtained at 85°C. The high resistance state (R<inf>High</inf>) increases with decreasing the device size from 8 µm to 0.2 µm. The low resistance state (R<inf>Low</inf>) is independent with different via diameters. The R<inf>Low</inf> decreases with increasing the compliance currents from 1nA to 1mA, which can be useful for future nanoscale low power consuming nonvolatile memory device applications.

Excellent resistive switching memory: Influence of GeO x in WO x mixture

Influence of GeO_x layer on resistive switching memory performance in a simple and CMOS compatible W/WO_x/GeO_x:WO_x mixture/W structure has been investigated for the first time. All layers are confirmed by both HRTEM and XPS. This memory device has enhanced performance in terms of the resistance ratio, uniformity, and program/erase cycles as compared to W/WO_x/W structure. An excellent read endurance and program/erase cycles of >;10⁶ at large V_read of ±1V are obtained. Furthermore, the memory device exhibits robust data retention at 85°C. This device can be operated as low current as 0.1 μA.

Resistive Switching Memory using High-κ Ta 2 O 5 Films

Many kinds of nonvolatile memory (NVM) devices with the technical limitations of scalability potential, higher switching power, nonvolatility, and reliability, etc have been reported by several groups[1-3]. To overcome those problems, a resistive switching memory device is one of the promising candidates for future nanoscale memories in the semiconductor industry. Different binary oxide memory elements such as NiOx [4], HfOx [5], Cu2O [6], etc have been reported by several groups. It is also expected that the flash memory device can be replaced by resistive switching memory in future. Recently, the resistive switching memory of the electrochemical formation and removal of metallic pathways in the GeSe solid electrolytes have been reported [7]. Due to the GeSe process limitation, the high-κ solid electrolyte can be used for the nanoscale resistive switching memory applications. In this study, we have investigated the resistive switching memory in a Cu/Ta2O5/TiN/Si structure for the first time.