Zinc Oxide and IGZO Bi-layer Synaptic Device with Highly Linear Long-term Potentiation and Depression Characteristics

The electrical properties, resistive switching behavior, and long-term potentiation/depression (LTP/LTD) in a single indium-gallium-zinc-oxide (IGZO) and bi-layer IGZO/ZnO memristors were investigated for synapse application. The use of oxide bi-layer memristor, in particular, improved electrical properties such as stability, reliability of memristors, and increase in the synaptic weight states. Bi-layer IGZO/ZnO memristors had a set voltage of 0.9 V, and reset voltage around -0.7 V, resulting in low-power consumption for neuromorphic systems. The oxygen vacancies in X-ray photoelectron spectroscopy analysis played a role in the modulation of the high-resistance state (HRS) (oxygen-deficient) and the low-resistance state (oxygen-rich) region. The VRESET of bi-layer IGZO/ZnO memristors was lower than that of a single IGZO, which implied that oxygen vacancy filaments could be easily ruptured due to the higher oxygen vacancy peak HRS layer. The nonlinearity of LTP and LTD characteristics in a bi-layer IGZO/ZnO memristor was 6.77% and 11.49%, respectively, compared to those of 20.03% and 51.1% in a single IGZO memristor, respectively. Therefore, the extra ZnO layer in the bi-layer memristor with IGZO was potentially significant and essential to achieve a small set voltage and a reset voltage, and the switching behavior to form the conductive path.

Modelling and Evolution of Conducting Filament in TiOx Memristor Using Conducting Atomic Force Microscopy

Conducting filament evolution in TiOx based resistive switching memory fabricated by simple oxidation of Ti film is investigated. Formation of titanium oxide is confirmed from the X-ray diffraction study. Forming is required to initiate the switching process. A bipolar analog switching is observed with a positive set and negative reset voltage. The switching properties in TiOx layer owing to the formation of conducting filament is confirmed from the conducting atomic force micrograph at different bias voltage. A significant change in surface topography as a filament formation during set and reset is presented. Conduction mechanism inside the device at various voltage and effect of tunnel width on current is studied. The effective tunnel width of conduction filament and related parameters for device using device modelling (Threshold Adaptive Memristor model) is studied. The device can be used for synaptic applications.

Impacts of LaOx Doping on the Performance of ITO/Al2O3/ITO Transparent RRAM Devices

Fully transparent ITO/LaAlO3/ITO structure RRAM (resistive random access memory) devices were fabricated on glass substrate, and ITO/Al2O3/ITO structure devices were set for comparison. The electrical characteristics of the devices were analyzed by Agilent B1500A semiconductor analyzer. Compared with the ITO/Al2O3/ITO RRAM devices, the current stability, SET/RESET voltage distribution, and retention characteristic of the ITO/LaAlO3/ITO RRAM devices have been greatly improved. In the visible light range, the light transmittance of the device is about 80%, that of the LaAlO3 layer is about 95%, the on-off ratio of the device is greater than 40, and the data retention time is longer than 10,000 s. The devices have great optical and electrical properties and have huge application potential as fully transparent RRAM devices.

Electron-beam-irradiated rhenium disulfide memristors with low variability for neuromorphic computing

State-of-the-art memristors are mostly formed by vertical metal–insulator–metal (MIM) structure, which rely on the formation of conductive filaments for resistive switching (RS). However, owing to the stochastic formation of filament, the set/reset voltage of vertical MIM memristors is difficult to control, which results in poor temporal and spatial switching uniformity. Here, a two-terminal lateral memristor based on electron-beam-irradiated rhenium disulfide (ReS2) is realized, which unveils a resistive switching mechanism based on Schottky barrier height (SBH) modulation. The devices exhibit a forming-free, stable gradual RS characteristic, and simultaneously achieve a small transition voltage variation during positive and negative sweeps (6.3%/5.3%). The RS is attributed to the motion of sulfur vacancies induced by voltage bias in the device, which modulates the ReS2/metal SBH. The gradual SBH modulation stabilizes the temporal variation in contrast to the abrupt RS in MIM-based memristors. Moreover, the emulation of long-term synaptic plasticity of biological synapses is demonstrated using the device, manifesting its potential as artificial synapse for energy-efficient neuromorphic computing applications.

Amorphized length and variability in phase-change memory line cells

The dimensions of amorphized regions in phase-change memory cells are critical parameters to design devices for different applications. However, these dimensions are difficult to be determined by direct imaging. In this work, the length of amorphized regions in multiple identical Ge2Sb2Te5 (GST) line cells was extracted from electrical measurements. After each cell was programmed to an amorphous state, a sequence of increasing-amplitude post-reset voltage pulses separated by low-amplitude read DC sweeps was applied. When a post-reset voltage pulse with sufficient amplitude was applied to a given cell, the measured current and the post-pulse resistance increased drastically, indicating that the cell re-amorphized after threshold switching, melting, and quenching. The amorphized length was calculated using the measured voltage at which the threshold switching occurred and the expected drifted threshold field at that time. The measured threshold voltage values and, hence, the extracted amorphized length, generally increase linearly with the programmed resistance levels. However, significant variability arises from the intrinsically unique crystallization and amorphization processes in these devices. For example, cells programmed to an amorphous resistance of approx. 50 MΩ show threshold voltage values of 5.5–7.5 V, corresponding to amorphized length values of 290–395 nm. This unpredictable programming feature in phase-change memory devices can be utilized in hardware security applications.

Emulation of Biological Synapse Characteristics from Cu/AlN/TiN Conductive Bridge Random Access Memory

Here, we present the synaptic characteristics of AlN-based conductive bridge random access memory (CBRAM) as a synaptic device for neuromorphic systems. Both non-volatile and volatile memory are observed by simply controlling the strength of the Cu filament inside the AlN film. For non-volatile switching induced by high compliance current (CC), good retention with a strong Cu metallic filament is verified. Low-resistance state (LRS) and high-resistance state (HRS) conduction follow metallic Ohmic and trap-assisted tunneling (TAT), respectively, which are supported by I–V fitting and temperature dependence. The transition from long-term plasticity (LTP) to short-term plasticity (STP) is demonstrated by increasing the pulse interval time for synaptic device application. Also, paired-pulse facilitation (PPF) in the nervous system is mimicked by sending two identical pulses to the CBRAM device to induce STP. Finally, potentiation and depression are achieved by gradually increasing the set and reset voltage in pulse transient mode.

Devices with Tuneable Resistance Switching Characteristics Based on a Multilayer Structure of Graphene Oxide and Egg Albumen

We used graphene oxide (GO) and egg albumen (EA) to fabricate bipolar resistance switching devices with indium tin oxide (ITO)/GO/EA/GO/Aluminum (Al) and ITO/EA/Al structures. The experimental results show that these ITO/GO/EA/GO/Al and ITO/EA/Al bio-memristors exhibit rewritable flash memory characteristics. Comparisons of ITO/GO/EA/GO/Al devices with 0.05 ωt %, 0.5 ωt %, and 2 ωt % GO in the GO layers and the ITO/EA/Al device show that the ON/OFF current ratio of these devices increases as the GO concentration decreases. Among these devices, the highest switching current ratio is 1.87 × 103. Moreover, the RESET voltage decreases as the GO concentration decreases, which indicates that GO layers with different GO concentrations can be adopted to adjust the ON/OFF current ratio and the RESET voltage. When the GO concentration is 0.5 ωt %, the device can be switched more than 200 times. The retention times of all the devices are longer than 104 s.