Xiaobing Yan

Superior resistive switching memory and biological synapse properties based on a simple TiN/SiO2/p-Si tunneling junction structure

In this study, a simple TiN/SiO2/p-Si tunneling junction structure was fabricated via thermal oxidation growth on a Si substrate annealed at 600 °C. After electroforming, the number of cycle times for the SiO2-based tunneling junction device can reach an order of magnitude of greater than 105. The resistances at low and high resistance states and the threshold voltage of the device fluctuated in a very narrow range. More interestingly, excitatory and inhibitory postsynaptic current phenomena (EPSC and IPSC) were observed during the pulse mode measurements, indicating that the device can be used in biological synapse applications. At different measurement temperatures and electric fields, direct, Fowler–Nordheim, and trap-assisted tunneling were responsible for the intrinsic conductance mechanism of the device before and after electroforming. This study provides a convenient approach to prepare simple tunneling junction structures for resistive random access memory applications with superior properties.

Roles of grain boundary and oxygen vacancies in Ba0.6Sr0.4TiO3 films for resistive switching device application

Oxygen vacancies are widely thought to be responsible for resistive switching (RS) effects based on polycrystalline oxides films. It is also well known that grain boundaries (GB) serve as reservoirs for accumulating oxygen vacancies. Here, Ar gas was introduced to enlarge the size of GB and increase the quantity of oxygen vacancies when the Ba0.6Sr0.4TiO3 (BST) films were deposited by pulse laser deposition technique. High resolution transmission electron microscopy images show that an amorphous region GB with large size appears between two lattice planes corresponding to oxygen vacancies defects in the Ar-introduced BST. And we propose that the conduction transport of the cell was dominantly contributed from not ions migration of oxygen vacancies but the electrons in our case according to the value of activation energies of the films.

Tristate electrochemical metallization memory based in the hydrogenated nanocrystalline silicon films

The hydrogenated nanocrystalline silicon (nc-Si:H) films have been fabricated as resistive switching medium by radio frequency plasma enhanced chemical vapor deposition technology. The constructed Ag/nc-Si:H/Pt structure exhibits stable three nonvolatile resistance states. Tristate resistive states with large ratio 102 and 105, less variation of resistance, and long retention exceeding 2.3 × 105 s are observed in Ag/nc-Si:H/Pt stack. The temperature dependence of high resistance state (HRS) and intermediate resistance state (IRS) both show semiconductor behavior, and the temperature dependence of low resistance state (LRS) represents metallic property. Fitting results demonstrated that the conduction mechanism of HRS, IRS, and LRS showed space charge limited conduction (SCLC), tunneling, and ohmic characteristics, respectively. The discrete Ag filament with Si nanocrystalline and complete Ag filament is proposed to be responsible for the performance IRS and LRS. We supposed that the Ag+ ions prefer to be ...

Flexible memristors as electronic synapses for neuro-inspired computation based on scotch tape-exfoliated mica substrates

Flexible memristor devices based on plastic substrates have attracted considerable attention due to their applications in wearable computers and integrated circuits. However, most plastic-substrate memristors cannot function or be grown in high-temperature environments. In this study, scotch-tape-exfoliated mica was used as the flexible memristor substrate in order to resolve these high-temperature issues. Our TiN/ZHO/IGZO memristor, which was constructed using a thin (10 μm) mica substrate, has superior flexibility and thermostability. After bending it 103 times, the device continues to exhibit exceptional electrical characteristics. It can also be implemented for transitions between high and low resistance states, even in temperatures of up to 300 °C. More importantly, the biological synaptic characteristics of paired-pulse facilitation/depression (PPF/PPD) and spike-timing-dependent plasticity (STDP) were observed through applying different pulse measurement modes. This work demonstrates that flexible memristor devices on mica substrates may potentially allow for the realization of high-temperature memristor applications for biologically-inspired computing systems.

Monoclinic Lu2–xSmxWO6-Based White Light-Emitting Phosphors: From Ground–Excited-States Calculation Prediction to Experiment Realization

Through ground state and constrained density function calculations, Sm3+ ions luminescence in self-activated monoclinic Lu2WO6 was originated from intra 4f → 4f transitions, not inter 5d → 4f transitions. Theoretically the white luminescence was obtained by combining red and blue-green emissions of 4f energy levels and W-O charge transfer transitions. Experimentally, pure and Sm3+ doping Lu2WO6 powders were synthesized using solid phase reaction calcined in air atmosphere. By the analysis of X-ray photoelectron spectroscopy and Rietveld refinement, element Sm charge state was trivalent, and Sm3+ doping was concentration-dependent selectively doping in three Lu sites. With the increase of Sm3+ concentrations, the color coordinates changed gradually from blue (0.17, 0.17) through white light (0.33, 0.25) toward orange (0.44, 0.32) in the visible spectral under 325 nm excitation. On the basis of the theoretical prediction and experimental preparation, a white emission LED lamp was produced using a 365 nm ultraviolet chip and Lu1.99Sm0.01WO6 phosphor. The present design method can be applied to select excellent activators from a large number of rare-earth (Re) ions like Sm3+ and Eu3+/2+ or non-Re ions like Bi3+ and Mn4+ in various matrixes.

Highly improved performance in Zr0.5Hf0.5O2 films inserted with graphene oxide quantum dots layer for resistive switching non-volatile memory

Resistive memory (RRAM) based on a solid–electrolyte insulator is a type of critical nanoscale device with promising potential in non-volatile memory, analog circuits and neuromorphic synapse applications. However, the random nature of the nucleation and growth of the conductive filaments (CFs) causes instability of the switching parameter, which is a major obstacle for RRAM performance improvement. Herein, we report a novel approach to resolve this challenge by inserting graphene oxide quantum dots (GOQDs) in Zr0.5Hf0.5O2 (ZHO) films. The Ag/ZHO/GOQDs/ZHO/Pt stacked device exhibited a reversible bipolar resistive switching (RS) behavior under a direct current (DC) sweeping voltage. The device with GOQDs exhibited better performance than the device without GOQDs with characteristics such as reduced threshold voltage, uniform distribution of set and reset voltage, robust retention, fast switching speed and low switching power. The underlying RS mechanism of RRAM was ascribed to the formation and rupture of the nanoscale CFs inside the solid–electrolyte oxide layer. The GOQDs could guide the CF nucleation and growth direction to provide a superior uniformity of RS properties and shorten the effective distance of Ag+ motion through enhancing the local electric field on the GOQD sites. The overall device performance of the GOQDs-inserted memristor has the potential to open up a new route to improve the reliability of oxide-based RRAM, which could significantly accelerate their existing applications.

Impacts of annealing temperature on charge trapping performance in Zr0.5Hf0.5O2 for nonvolatile memory

In this study, Zr0.5Hf0.5O2 films were fabricated on Si substrate and were annealed at different temperatures by rapid thermal annealing (RTA) process. The charge trapping memory devices based on Zr0.5Hf0.5O2/SiO2/Si simple structure were investigated in detail. The memory device annealing at 690 °C shows the best property with a memory window of 5.6 V under ±12 V sweeping voltages in its capacitance-voltage curve and a better retention property. The high resolved transmission electron microscopy shows the generated SiO2 working as tunneling layer after RTA process, whose thickness increases with the rise of temperature. Combined with the TEM results, the photoluminescence spectrum and in situ angle resolved photoemission spectroscopy results further verify that oxygen vacancies and inter-diffusion layer also play a crucial role in charge trapping performance. This work provides direct insights for the charge trapping mechanisms based on high-k Zr0.5Hf0.5O2 films devices.

A metal/Ba0.6Sr0.4TiO3/SiO2/Si single film device for charge trapping memory towards a large memory window

In this study, we present a metal/Ba0.6Sr0.4TiO3/SiO2/Si (MBOS) structure for charge trapping memory, where the single Ba0.6Sr0.4TiO3 film acts as the blocking layer and charge trapping layer. This MBOS device structure demonstrates excellent charge trapping characteristics, a large memory window up to 8.4 V under an applied voltage of ±12 V, robust charge retention of only 4% charge loss after 1.08 × 104 s, fast switching rate, and great program/erase endurance. These attractive features are attributed to the high density of defect states in the Ba0.6Sr0.4TiO3 film and its inter-diffusion interface with SiO2. The properties of defect states in the Ba0.6Sr0.4TiO3 film are investigated through measurements of photoluminescence and photoluminescence excitation spectroscopy. The energy levels of these defect states are found to be distributed between 2.66 eV and 4.05 eV above the valence band. The inter-diffusion at the Ba0.6Sr0.4TiO3/SiO2 interface is observed by high-resolution transmission electron micros...

Artificial electronic synapse characteristics of a Ta/Ta2O5-x/Al2O3/InGaZnO4 memristor device on flexible stainless steel substrate

In this work, we fabricate and report a flexible memristor device with the structure of Ta/Ta2O5-x/Al2O3/InGaZnO4 on a stainless steel substrate, which is robust in emulating the bio-synapse function and anti-pull capacity. The I-V curves show that this device has excellent stability and uniformity in 100 sweep cycles. When applying stimulation voltage pulses, the device conductance is adjusted gradually and can still be modulated after 1000 times of bending. Furthermore, this device demonstrates essential synaptic behaviors, including short-term plasticity, long-term plasticity, and short-term to long-term transition. In addition, under a tension of 200 N, the I-V characteristics have no obvious degeneration and the conduction of the device can still be modulated under pulse trains. The flexible Ta/Ta2O5-x/Al2O3/InGaZnO4 memristor can be a promising candidate for neuromorphic computing applications.

A radiation-hardening Ta/Ta2O5-x/Al2O3/InGaZnO4 memristor for harsh electronics

In this work, the electrical characteristics of Ta/Ta2O5-x/Al2O3/InGaZnO4 memristor devices under radiation are studied. The measured I-V curves indicate that this type of device has excellent stability and uniformity after radiation with a total ionization dose of 59.5 krad. The electrical properties of this post-irradiation memristor change slightly at a high temperature of 200 °C. These features enable our fabricated memristor devices operate as electronic (or artificial) synapses for neuromorphic computing or artificial intelligence in harsh electronics. The conductance of the device can be adjusted continuously like the synaptic weight, which lays the foundation for the electronic synapse. The temperature dependence of I-V characteristics before and after radiation is in good agreement with the hopping conduction mechanism. The activation energy is lower and the trap spacing is shorter after a total ionization dose of 59.5 krad irradiation. Moreover, the existence of oxygen vacancies is observed by XPS (X-ray photoelectron spectroscopy). The highly stable nature of this Ta/Ta2O5-x/Al2O3/InGaZnO4 memristor device under radiation indicates its great potential in harsh electronics for aerospace, nuclear, and military applications.In this work, the electrical characteristics of Ta/Ta2O5-x/Al2O3/InGaZnO4 memristor devices under radiation are studied. The measured I-V curves indicate that this type of device has excellent stability and uniformity after radiation with a total ionization dose of 59.5 krad. The electrical properties of this post-irradiation memristor change slightly at a high temperature of 200 °C. These features enable our fabricated memristor devices operate as electronic (or artificial) synapses for neuromorphic computing or artificial intelligence in harsh electronics. The conductance of the device can be adjusted continuously like the synaptic weight, which lays the foundation for the electronic synapse. The temperature dependence of I-V characteristics before and after radiation is in good agreement with the hopping conduction mechanism. The activation energy is lower and the trap spacing is shorter after a total ionization dose of 59.5 krad irradiation. Moreover, the existence of oxygen vacancies is observed by X...