Yichen Fang

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

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.

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

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.

Encapsulation layer design and scalability in encapsulated vertical 3D RRAM

Here we propose a novel encapsulated vertical 3D RRAM structure with each resistive switching cell encapsulated by dielectric layers, contributing to both the reliability improvement of individual cells and thermal disturbance reduction of adjacent cells due to the effective suppression of unwanted oxygen vacancy diffusion. In contrast to the traditional vertical 3D RRAM, encapsulated bar-electrodes are adopted in the proposed structure substituting the previous plane-electrodes, thus encapsulated resistive switching cells can be naturally formed by simply oxidizing the tip of the metal bar-electrodes. In this work, TaO x -based 3D RRAM devices with SiO2 and Si3N4 as encapsulation layers are demonstrated, both showing significant advantages over traditional unencapsulated vertical 3D RRAM. Furthermore, it was found thermal conductivity and oxygen blocking ability are two key parameters of the encapsulation layer design influencing the scalability of vertical 3D RRAM. Experimental and simulation data show that oxygen blocking ability is more critical for encapsulation layers in the relatively large scale, while thermal conductivity becomes dominant as the stacking layers scale to the sub-10 nm regime. Finally, based on the notable impacts of the encapsulation layer on 3D RRAM scaling, an encapsulation material with both excellent oxygen blocking ability and high thermal conductivity such as AlN is suggested to be highly desirable to maximize the advantages of the proposed encapsulated structure. The findings in this work could pave the way for reliable ultrahigh-density storage applications in the big data era.

Self-selection effects and modulation of TaOx resistive switching random access memory with bottom electrode of highly doped Si

In this paper, we propose a TaOx resistive switching random access memory (RRAM) device with operation-polarity-dependent self-selection effect by introducing highly doped silicon (Si) electrode, which is promising for large-scale integration. It is observed that with highly doped Si as the bottom electrode (BE), the RRAM devices show non-linear (>103) I-V characteristic during negative Forming/Set operation and linear behavior during positive Forming/Set operation. The underling mechanisms for the linear and non-linear behaviors at low resistance states of the proposed device are extensively investigated by varying operation modes, different metal electrodes, and Si doping type. Experimental data and theoretical analysis demonstrate that the operation-polarity-dependent self-selection effect in our devices originates from the Schottky barrier between the TaOx layer and the interfacial SiOx formed by reaction between highly doped Si BE and immigrated oxygen ions in the conductive filament area.

Microscopic origin of read current noise in TaOx-based resistive switching memory by ultra-low temperature measurement

TaOx-based resistive random access memory (RRAM) attracts considerable attention for the development of next generation nonvolatile memories. However, read current noise in RRAM is one of the critical concerns for storage application, and its microscopic origin is still under debate. In this work, the read current noise in TaOx-based RRAM was studied thoroughly. Based on a noise power spectral density analysis at room temperature and at ultra-low temperature of 25 K, discrete random telegraph noise (RTN) and continuous average current fluctuation (ACF) are identified and decoupled from the total read current noise in TaOx RRAM devices. A statistical comparison of noise amplitude further reveals that ACF depends strongly on the temperature, whereas RTN is independent of the temperature. Measurement results combined with conduction mechanism analysis show that RTN in TaOx RRAM devices arises from electron trapping/detrapping process in the hopping conduction, and ACF is originated from the thermal activatio...

Influence of selector-introduced compliance current on HfOx RRAM switching operation

The influences of compliance current (CC) introduced by transistor during the forming, set and reset operations on hafnium oxide based RRAM devices are investigated respectively. Experimental results show that CC during forming operation is more critical to RRAM performances than that in set/reset operations, indicating that the suppression of current overshoot issue is more important during forming process. The impacts of CC on oxygen ions immigration during resistive switching can be responsible for the different influences on devices in set/reset and forming operation.

Flexible Polymer Device Based on Parylene-C with Memory and Temperature Sensing Functionalities

Polychloro-para-xylylene (parylene-C) is a flexible and transparent polymer material which has excellent chemical stability and high biocompatibility. Here we demonstrate a polymer device based on single-component parylene-C with memory and temperature sensing functionalities. The device shows stable bipolar resistive switching behavior, remarkable storage window (>104), and low operation voltages, exhibiting great potential for flexible resistive random-access memory (RRAM) applications. The I-V curves and conductive atomic force microscopy (CAFM) results verify the metallic filamentary-type switching mechanism based on the formation and dissolution of a metal bridge related to the redox reaction of the active metal electrode. In addition, due to the metallic properties of the low-resistance state (LRS) in the polymer device, the resistance in the LRS exhibits a nearly linear relationship at the temperature regime between 25 °C and 100 °C. With a temperature coefficient of resistance (TCR) of 2.136 × 10−3/°C, the device is also promising for the flexible temperature sensor applications.

TaOx based memristors with recessed bottom electrodes and built-in ion concentration gradient as electronic synapses

Inspired by the computing architecture of human brain, neuromorphic computing promises massively parallel, energy efficient and fault tolerant computation compared with conventional von Neumann approaches. In order to achieve this ambitious goal, one of the crucial tasks is to make devices that can emulate the functions of biological synapses at the physical level. Here we fabricated a novel Pt/TaOx/Ta memristor with recessed Ta bottom electrodes, where the TaOx film was formed by thermal oxidation of the bottom electrode and thus had a constantly ascending concentration of oxygen vacancies from the Pt/TaOx interface to the TaOx/Ta interface. Such cell geometry and ion distribution profile gave rise to highly incremental and uniform resistive switching that emulated the potentiation or depression process of synapses. Furthermore, important synaptic learning rules, such as spike timing dependent plasticity, could also be implemented by these devices, making them well suited for electronic synapses in neuromorphic systems.

Associative learning based on symmetric spike time dependent plasticity

Spike-timing-dependent-plasticity (STDP) is an important learning rule in organisms. In the application of neural computation, it is meaningful to apply symmetric STDP to associative learning. In this paper, an electronic synapse with symmetric STDP features was demonstrated. Meanwhile, taking Pavlovs experiment as an example, a model of neural network was built with this electronic synapse and successfully simulated the Pavlovs experiment, indicating the proposed symmetric STDP synaptic circuit can mimic the working principle of associative learning.

A tantalum oxide memristor for artificial synapse applications

Memristor has attracted significant attention in recent years because of its capability to act as artificial synapse in neuromorphic systems. In this paper, a memristor is demonstrated based on tantalum oxide. Multiple resistance states can be achieved. The resistance switching mechanism of continuous sets and resets is discussed. The essential synaptic behaviors of potentiation and depression are achieved.