Yimao Cai

Total Ionizing Dose (TID) Effects on

In this brief, the total ionizing dose (TID) effects of γ rays generated from a 60Co source on the TaOx-based resistive switching memory (resistive random-access memory, RRAM) is investigated. The low-resistance state (LRS) of the RRAM is immune to TID effects, whereas the sensitivity of the high-resistance state (HRS) of RRAM to TID effects depends on the dimensions of the device, including the thickness of the oxide film and the area of the device. The HRS of the device with large area and thick oxide layer is vulnerable to TID effects and has a high probability to change into the LRS. Further investigation found that the lower the high resistance, the higher the failure rate of the RRAM device under TID impact, which indicates that the multilevel cell of RRAM should be carefully designed for space system applications considering the radiation effects. The failure of the HRS under TID is explained by defect generation in the oxide film.

\hbox{TaO}_{x}

Crossbar array architecture is usually used for the high-density integration of the RRAM device. However, the large sneak current in the passive crossbar array limits the increase in the integration density. In this brief, the bipolar TiN/TaOx/Pt RRAM device is proposed as the dynamic selector for the unipolar Pt/TaOx/Pt RRAM device to suppress the sneak current in the crossbar array. The testing results show that the bipolar RRAM can act as a good selector, and the sneak current is reduced by about two orders estimated by the 1/2 Vread voltage scheme. With the suppressed sneak current, the maximum size of the crossbar array with the bipolar RRAM selector can be increased to more than 1 Mb according to the simulation results, indicating that the bipolar RRAM selector has great potential for the high-density memory applications.

-Based Resistance Change Memory

A novel vertical 3D RRAM structure with greatly improved reliability behavior is proposed and experimentally demonstrated through basically compatible process featuring self-localized switching region by sidewall electrode oxidation. Compared with the conventional structure, due to the effective confinement of the switching region, the newly-proposed structure shows about two orders higher endurance (>108 without verification operation) and better retention (>180h@150 °C), as well as high uniformity. Corresponding model is put forward, on the base of which thorough theoretical analysis and calculations are conducted as well, demonstrating that, resulting from the physically-isolated switching from neighboring cells, the proposed structure exhibits dramatically improved reliability due to effective suppression of thermal effects and oxygen vacancies diffusion interference, indicating that this novel structure is very promising for future high density 3D RRAM application.

A New Dynamic Selector Based on the Bipolar RRAM for the Crossbar Array Application

In this letter, organic resistive random access memory (RRAM) devices based on double-layer polychloro-para-xylylene (parylene-C) are fabricated, which show stable bipolar resistive switching behavior, excellent data retention, and high scalability. Moreover, extremely low reset current of sub-20 nA and set current of 0.15 μA are obtained with adequate switching margin for the first time in the field of organic RRAM, almost 105 times lower than that of the single-layer parylene-C cells, exhibiting great potentials for future low-power applications. Possible mechanism for the ultralow operating current of double-layer device is discussed.

Novel Vertical 3D Structure of TaOx-based RRAM with Self-localized Switching Region by Sidewall Electrode Oxidation

Parylene is a Food and Drug Administration (FDA)-approved material which can be safely used within the human body and it is also offers chemically inert and flexible merits. Here, we present a flexible parylene-based organic resistive random access memory (RRAM) device suitable for wearable biomedical application. The proposed device is fabricated through standard lithography and pattern processes at room temperature, exhibiting the feasibility of integration with CMOS circuits. This organic RRAM device offers a high storage window (>10(4)), superior retention ability and immunity to disturbing. In addition, brilliant mechanical and electrical stabilities of this device are demonstrated when under harsh bending (bending cycle >500, bending radius <10 mm). Finally, the underlying mechanism for resistance switching of this kind of device is discussed, and metallic conducting filament formation and annihilation related to oxidization/redox of Al and Al anions migrating in the parylene layer can be attributed to resistance switching in this device. These advantages reveal the significant potential of parylene-based flexible RRAM devices for wearable biomedical applications.

Record Low-Power Organic RRAM With Sub-20-nA Reset Current

A resistive switching device with inherent nonlinear characteristics through a delicately engineered interfacial layer is an ideal component to be integrated into passive crossbar arrays for the suppression of sneaking current, especially in ultra-dense 3D integration. In this paper, we demonstrated a TaOx-based bipolar resistive switching device with a nearly symmetrical bi-directional nonlinear feature through interface engineering. This was accomplished by introducing an ultra-thin interfacial layer (SiO2-x) with unique features, including a large band gap and a certain level of negative heat of oxide formation between the top electrode (TiN) and resistive layer (TaOx). The devices exhibit excellent nonlinear property under both positive and negative bias. Modulation of the inherent nonlinearity as well as the resistive switching mechanism are comprehensively studied by scrutinizing the results of the experimental control groups and the extensive characterizations including detailed compositional analysis, which suggests that the underlying mechanism of the nonlinear behavior is associatively governed by the serially connected metallic conductive filament and Flower-Nordheim tunneling barrier formed by the SiO2-x interface layer. The proposed device in this work has great potential to be implemented in future massive storage memory applications of high-density selector-free crossbar structure.

A flexible organic resistance memory device for wearable biomedical applications

Neuromorphic computing represents an innovative technology that can perform intelligent and energy-efficient computation, whereas construction of neuromorphic systems requires biorealistic synaptic elements with rich dynamics that can be tuned based on a robust mechanism. Here, an ionic-gating-modulated synaptic transistor based on layered crystals of transitional metal dichalcogenides and phosphorus trichalcogenides is demonstrated, which produce a diversity of short-term and long-term plasticity including excitatory postsynaptic current, paired pulse facilitation, spiking-rate-dependent plasticity, dynamic filtering, etc., with remarkable linearity and ultralow energy consumption of ≈30 fJ per spike. Detailed transmission electron microscopy characterization and ab initio calculation reveal two-stage ionic gating effects, namely, surface adsorption and internal intercalation in the channel medium, causing different poststimulation diffusive dynamics and thus accounting for the observed short-term and long-term plasticity, respectively. The synaptic activity can therefore be effectively manipulated by tailoring the ionic gating and consequent diffusion dynamics with varied thickness and structure of the van der Waals material as well as the number, duration, rate, and polarity of gate stimulations, making the present synaptic transistors intriguing candidates for low-power neuromorphic systems.

Modulation of nonlinear resistive switching behavior of a TaOx-based resistive device through interface engineering

Impact of heavy-ion irradiation on the TaOx-based resistive random-access memory (RRAM) is investigated in this work. After Br ion irradiation, the set voltage of TaOx-based RRAM shows slight change, and negative shift of the forming voltage is observed. For the resistance change of TaOx-based RRAM devices induced by irradiation, the resistance of the low resistance state (LRS) illustrates acceptable change, while the resistance of the high resistance state (HRS) is quite sensitive to heavy ion irradation. Some irradiated devices even change from the HRS into the LRS, which results in disappeared memory window and may be attributed to the displacement damage in the dielectric layer. These results indicate that TaOx-based RRAM devices need to be carefully designed for future space applications.

Ion Gated Synaptic Transistors Based on 2D van der Waals Crystals with Tunable Diffusive Dynamics

The program and erase injection current characteristics of a NROM with SiO2, HfO2, LaAlO3 and Al2O3 as the tunnel dielectric, respectively, are studied in this paper. Due to the lower electron and hole energy barriers introduced by LaAlO3, both the program and erase injection current densities of the NROM using LaAlO3 as the tunnel dielectric are increased dramatically. The injection efficiency is also improved significantly, which indicates that the introduction of LaAlO3 can lower the operation voltage of NROM cells. We show that the bit line voltage can be reduced to 3 V for both program and erase operations of NROM cells with LaAlO3 of 5 nm and 8 nm equivalent oxide thickness (EOT). This can greatly reduce the additional circuits to generate high voltages in a nonvolatile memory chip, meanwhile maintaining sufficient program/erase (P/E) performance and reliability. Our study also shows that the drain disturb is alleviated during programming and erasing the NROM cell with the LaAlO3 tunnel dielectric due to the lower operating voltages (VBL = 3 V). Hence a low-voltage low-power NROM flash memory device operation can be achieved by using LaAlO3 as the tunnel dielectric, due to the enhancement of the P/E injection current.

Investigation on the Response of TaO

The flash memory technology meets physical and technical obstacles in further scaling. New structures and new materials are implemented as possible solutions. This paper focuses on two kinds of new flash cells for high density and low power memory applications based on the vertical channel double gate structure. The proposed VD-NROM with dual-nitride-trapping-layer and vertical structure can achieve four-bit-per-cell storage capability. And the proposed VSAS-FG cell benefits the high programming efficiency, low power and high density capability, which can be realized without any additional mask and can achieve the self-alignment of the split-gate channel and the floating-gate. The two novel flash cell structures can be considered as potential candidates for different flash memory applications.