Pengxiao Sun

Thermoelectric Seebeck effect in oxide-based resistive switching memory

Reversible resistive switching induced by an electric field in oxide-based resistive switching memory shows a promising application in future information storage and processing. It is believed that there are some local conductive filaments formed and ruptured in the resistive switching process. However, as a fundamental question, how electron transports in the formed conductive filament is still under debate due to the difficulty to directly characterize its physical and electrical properties. Here we investigate the intrinsic electronic transport mechanism in such conductive filament by measuring thermoelectric Seebeck effects. We show that the small-polaron hopping model can well describe the electronic transport process for all resistance states, although the corresponding temperature-dependent resistance behaviours are contrary. Moreover, at low resistance states, we observe a clear semiconductor–metal transition around 150 K. These results provide insight in understanding resistive switching process and establish a basic framework for modelling resistive switching behaviour.

Atomic View of Filament Growth in Electrochemical Memristive Elements

Memristive devices, with a fusion of memory and logic functions, provide good opportunities for configuring new concepts computing. However, progress towards paradigm evolution has been delayed due to the limited understanding of the underlying operating mechanism. The stochastic nature and fast growth of localized conductive filament bring difficulties to capture the detailed information on its growth kinetics. In this work, refined programming scheme with real-time current regulation was proposed to study the detailed information on the filament growth. By such, discrete tunneling and quantized conduction were observed. The filament was found to grow with a unit length, matching with the hopping conduction of Cu ions between interstitial sites of HfO2 lattice. The physical nature of the formed filament was characterized by high resolution transmission electron microscopy. Copper rich conical filament with decreasing concentration from center to edge was identified. Based on these results, a clear picture of filament growth from atomic view could be drawn to account for the resistance modulation of oxide electrolyte based electrochemical memristive elements.

Thermal crosstalk in 3-dimensional RRAM crossbar array.

High density 3-dimensional (3D) crossbar resistive random access memory (RRAM) is one of the major focus of the new age technologies. To compete with the ultra-high density NAND and NOR memories, understanding of reliability mechanisms and scaling potential of 3D RRAM crossbar array is needed. Thermal crosstalk is one of the most critical effects that should be considered in 3D crossbar array application. The Joule heat generated inside the RRAM device will determine the switching behavior itself, and for dense memory arrays, the temperature surrounding may lead to a consequent resistance degradation of neighboring devices. In this work, thermal crosstalk effect and scaling potential under thermal effect in 3D RRAM crossbar array are systematically investigated. It is revealed that the reset process is dominated by transient thermal effect in 3D RRAM array. More importantly, thermal crosstalk phenomena could deteriorate device retention performance and even lead to data storage state failure from LRS (low resistance state) to HRS (high resistance state) of the disturbed RRAM cell. In addition, the resistance state degradation will be more serious with continuously scaling down the feature size. Possible methods for alleviating thermal crosstalk effect while further advancing the scaling potential are also provided and verified by numerical simulation.

Investigation on the RESET switching mechanism of bipolar Cu/HfO2/Pt RRAM devices with a statistical methodology

The RESET switching of bipolar Cu/HfO2/Pt resistance random access memory (RRAM) is investigated. With a statistical methodology, we systematically analyze the RESET voltage (VRESET) and RESET current (IRESET). VRESET shows a U-shape distribution as a function of RON according to the scatter plot of the raw experimental data. After data correction by a series resistance (RS), VRESET is nearly constant, while IRESET decreases linearly with RCF. These behaviours are consistent with the thermal dissolution model of RESET. Moreover, the IRESET and VRESET distributions are strongly affected by the RON distribution. Using a ?resistance screening? method, the IRESET and VRESET distributions are found to be compatible with the Weibull distribution model. The Weibull slopes of the VRESET and IRESET distributions are independent of RCF, indicating that the RESET point corresponds to the initial phase of conductive filament (CF) dissolution, according to our cell-based model for the unipolar RESET of RRAM devices. The scale factor of the VRESET distributions is roughly constant, while that of the IRESET distributions scale with 1/RCF. Accordingly, the RESET switching of the HfO2-based solid electrolyte memory is compatible with the thermal dissolution mechanism, improving our understanding on the physics of resistive switching of RRAM devices.

A novel method of identifying the carrier transport path in metal oxide resistive random access memory

Characterization of defect energy levels is of crucial importance to understand the carrier transport and conduction mechanism of a conducting filament. Currently, it is difficult to probe the defect energy level of a conducting filament in random access memory (RRAM) by experiment. Based on the activation energy of carrier transport from the first-principles calculations, we present a physical model correlating macroscopic I–V characteristics with material microstructure to analyze the defect energy level of a conducting filament in metal oxide RRAM. The carrier transport path in the conducting filament can be specially extracted using the defect energy level.

Charge carrier hopping transport based on Marcus theory and variable-range hopping theory in organic semiconductors

Charge carrier hopping transport is generally taken from Miller-Abrahams and Marcus transition rates. Based on the Miller-Abrahams theory and nearest-neighbour range hopping theory, Apsley and Hughes developed a concise calculation method (A-H method) to study the hopping conduction in disordered systems. Here, we improve the A-H method to investigate the charge carrier hopping transport by introducing polaron effect and electric field based on Marcus theory and variable-range hopping theory. This improved method can well describe the contribution of polaron effect, energetic disorder, carrier density, and electric field to the charge carrier transport in disordered organic semiconductor. In addition, the calculated results clearly show that the charge carrier mobility represents different polaron effect dependence with the polaron activation energy and decreases with increasing electric field strength for large fields.

A unified physical model of Seebeck coefficient in amorphous oxide semiconductor thin-film transistors

A unified physical model for Seebeck coefficient was presented based on the multiple-trapping and release theory for amorphous oxide semiconductor thin-film transistors. According to the proposed model, the Seebeck coefficient is attributed to the Fermi-Dirac statistics combined with the energy dependent trap density of states and the gate-voltage dependence of the quasi-Fermi level. The simulation results show that the gate voltage, energy disorder, and temperature dependent Seebeck coefficient can be well described. The calculation also shows a good agreement with the experimental data in amorphous In-Ga-Zn-O thin-film transistor.

Carrier-transport-path-induced switching parameter fluctuation in oxide-based resistive switching memory

A key challenge in resistive switching memory is to reduce the switching parameter fluctuation, which always affects the stability and reliability of an RS device. Numerous methods have been carried out for improving the fluctuation of the switching parameter. However, because the physical nature of the switching parameter fluctuation is, to date, not well understood, a universal identification of the switching parameter fluctuation still has not be achieved. Based on the activation energy of carrier transport from the first-principles calculations, we present a physical model. This proposed model is considering the macroscopic fluctuating I–V curve and material microstructure to analyze the characteristics of carrier transport and the origin of switching parameter fluctuations. The proposed model may specially identify the defect energy level and quantify the distribution of the switching parameter. The model provides possible clues for improving the uniformity of the switching parameter as well.

Contact Length Scaling in Staggered Organic Thin-Film Transistors

In this letter, the contact length (source and drain electrodes length) scaling potential in staggered organic thin-film transistors (OTFTs) was studied. We performed the effect of gate-source voltage, channel length, semiconductor film thickness, mobility, material disorder, anisotropy of mobility, and Schottky barrier between organic semiconductor and source/drain electrodes on the contact length downscaling, and found that the contact resistance do not increase until the contact length is scaled down to sub-500 nm. Importantly, the cutoff frequency (fT) of OTFTs was described and it was found that fT would increase with the decrease of contact length and the highest fT could be obtained with contact length in the range of sub-100 nm with Ohmic contacts. This letter showed excellent contact length scaling potential of OTFTs and provided guidelines for the design of high frequency, low-cost, and printable OTFTs.

Thermal effect on endurance performance of 3-dimensional RRAM crossbar array*

Three-dimensional (3D) crossbar array architecture is one of the leading candidates for future ultra-high density nonvolatile memory applications. To realize the technological potential, understanding the reliability mechanisms of the 3D RRAM array has become a field of intense research. In this work, the endurance performance of the 3D 1D1R crossbar array under the thermal effect is investigated in terms of numerical simulation. It is revealed that the endurance performance of the 3D 1D1R array would be seriously deteriorated under thermal effects as the feature size scales down to a relatively small value. A possible method to alleviate the thermal effects is provided and verified by numerical simulation.