Jianhui Zhao

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.

Enhanced dielectric constant and fatigue-resistance of PbZr0.4Ti0.6O3 capacitor with magnetic intermetallic FePt top electrode

Both FePt/PbZr0.4Ti0.6O3(PZT)/Pt and Pt/PZT/Pt ferroelectric capacitors have been fabricated on Si substrates. It is found that up to 109 switching cycles, the FePt/PZT/Pt capacitor, measured at 50 kHz, with polarization decreased by 57%, is superior to the Pt/PZT/Pt capacitor by 82%, indicating that an intermetallic FePt top electrode can also improve the fatigue-resistance of a PZT capacitor. Maximum dielectric constants are 980 and 770 for PZT capacitors with FePt and Pt, respectively. This is attributed to the interface effect between PZT film and the top electrode since the interfacial capacitance of FePt/PZT is 3.5 times as large as that of Pt/PZT interface.

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.

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.

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.

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...

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...

Bistable Capacitance Performance-Induced Ambipolar Charge Injected Based on Ba 0.6 Sr 0.4 TiO 3 by an Inlaid Zr–Hf–O Layer for Novel Nonvolatile Memory Application

In this paper, we present a distinguished hysteresis behavior by a few nanometers inlaid Zr–Hf–O (ZHO) layer between the Ba0.6Sr0.4TiO3(BST) film and the metal Pt electrode. The capacitance–voltage curve shows an insignificant change in the range of 0.3–1 MHz. The excellent retention property showing the difference of high state and low state is estimated as about 23.0% and 12.5% after one year and ten years, which might be caused by the deeper trap in an interdiffusion layer. The possible mechanism is proposed that the BST/ZHO interface exists as an interdiffusion, which could create additional defects and ambipolar charge injection causing the hysteresis behavior due to a postdeposition annealing process. The device can be employed as a promising candidate for applying in novel nonvolatile memory device.

Bipolar transparent resistive switching based in a-IGZO/STO/a-IGZO structure for nonvolatile memory application

In this study, we have fabricated the bipolar transparent resistive random access memory (TRRAM) based in complex oxide amorphous STO films as dielectric layer grown by pulsed laser deposition (PLD) technique, and semiconducting In-Ga-Zn-O films were employed as electrode. The experimental results show that the average transmittance of the device is above 82.7% in the visible region (400-800nm). The device can be repeated more than 260 times, and the high and low resistance state can be held without obvious degradation within 5×104s. The conduction mechanisms at HRS and LRS were both attributed to the space charge limited current. And the RS mechanism is related to trapping and releasing of electrons which impact the current level though the cell for changing the resistance state.