Yuchao Yang

Fully Room-Temperature-Fabricated Nonvolatile Resistive Memory for Ultrafast and High-Density Memory Application

Through a simple industrialized technique which was completely fulfilled at room temperature, we have developed a kind of promising nonvolatile resistive switching memory consisting of Ag/ZnO:Mn/Pt with ultrafast programming speed of 5 ns, an ultrahigh R(OFF)/R(ON) ratio of 10(7), long retention time of more than 10(7) s, good endurance, and high reliability at elevated temperatures. Furthermore, we have successfully captured clear visualization of nanoscale Ag bridges penetrating through the storage medium, which could account for the high conductivity in the ON-state device. A model concerning redox reaction mediated formation and rupture of Ag bridges is therefore suggested to explain the memory effect. The Ag/ZnO:Mn/Pt device represents an ultrafast and highly scalable (down to sub-100-nm range) memory element for developing next generation nonvolatile memories.

Observation of conducting filament growth in nanoscale resistive memories

Nanoscale resistive switching devices, sometimes termed memristors, have recently generated significant interest for memory, logic and neuromorphic applications. Resistive switching effects in dielectric-based devices are normally assumed to be caused by conducting filament formation across the electrodes, but the nature of the filaments and their growth dynamics remain controversial. Here we report direct transmission electron microscopy imaging, and structural and compositional analysis of the nanoscale conducting filaments. Through systematic ex-situ and in-situ transmission electron microscopy studies on devices under different programming conditions, we found that the filament growth can be dominated by cation transport in the dielectric film. Unexpectedly, two different growth modes were observed for the first time in materials with different microstructures. Regardless of the growth direction, the narrowest region of the filament was found to be near the dielectric/inert-electrode interface in these devices, suggesting that this region deserves particular attention for continued device optimization.

Electrochemical dynamics of nanoscale metallic inclusions in dielectrics.

Nanoscale metal inclusions in or on solid-state dielectrics are an integral part of modern electrocatalysis, optoelectronics, capacitors, metamaterials and memory devices. The properties of these composite systems strongly depend on the size, dispersion of the inclusions and their chemical stability, and are usually considered constant. Here we demonstrate that nanoscale inclusions (for example, clusters) in dielectrics dynamically change their shape, size and position upon applied electric field. Through systematic in situ transmission electron microscopy studies, we show that fundamental electrochemical processes can lead to universally observed nucleation and growth of metal clusters, even for inert metals like platinum. The clusters exhibit diverse dynamic behaviours governed by kinetic factors including ion mobility and redox rates, leading to different filament growth modes and structures in memristive devices. These findings reveal the microscopic origin behind resistive switching, and also provide general guidance for the design of novel devices involving electronics and ionics.

Complementary resistive switching in tantalum oxide-based resistive memory devices

Complementary resistive switches (CRS) are considered as a potential solution for the sneak path problem in large-scale integration of passive crossbar resistive memory arrays. A typical CRS is composed of two bipolar memory cells that are connected anti-serially. Here, we report a tantalum-oxide based resistive memory that achieves the complementary switching functionality within a single memory cell. The complementary switching effect is accompanied by switching polarity reversal in different voltage bias regimes. These effects were explained by the redistribution of oxygen vacancies inside the tantalum-oxide layers. The effects of symmetry breaking on bipolar switching and complementary switching were also discussed.

Nanoscale resistive switching devices: mechanisms and modeling

Resistive switching devices (also termed memristive devices or memristors) are two-terminal nonlinear dynamic electronic devices that can have broad applications in the fields of nonvolatile memory, reconfigurable logic, analog circuits, and neuromorphic computing. Current rapid advances in memristive devices in turn demand better understanding of the switching mechanism and the development of physics-based as well as simplified device models to guide future device designs and circuit-level applications. In this article, we review the physical processes behind resistive switching (memristive) phenomena and discuss the experimental and modeling efforts to explain these effects. In this article three categories of devices, in which the resistive switching effects are driven by cation migration, anion migration, and electronic effects, will be discussed. The fundamental driving forces and the stochastic nature of resistive switching will also be discussed.

Oxide heterostructure resistive memory.

Resistive switching devices are widely believed as a promising candidate for future memory and logic applications. Here we show that by using multilayer oxide heterostructures the switching characteristics can be systematically controlled, ranging from unipolar switching to complementary switching and bipolar switching with linear and nonlinear on-states and high endurance. Each layer can be tailed for a specific function during resistance switching, thus greatly improving the degree of control and flexibility for optimized device performance.

Building Neuromorphic Circuits with Memristive Devices

The rapid, exponential growth of modern electronics has brought about profound changes to our daily lives. However, maintaining the growth trend now faces significant challenges at both the fundamental and practical levels [1]. Possible solutions include More Moore?developing new, alternative device structures and materials while maintaining the same basic computer architecture, and More Than Moore?enabling alternative computing architectures and hybrid integration to achieve increased system functionality without trying to push the devices beyond limits. In particular, an increasing number of computing tasks today are related to handling large amounts of data, e.g. image processing as an example. Conventional von Neumann digital computers, with separate memory and processer units, become less and less efficient when large amount of data have to be moved around and processed quickly. Alternative approaches such as bio-inspired neuromorphic circuits, with distributed computing and localized storage in networks, become attractive options [2]?[6].

Oxide Resistive Memory with Functionalized Graphene as Built-in Selector Element

DOI: 10.1002/adma.201400270 transfer was repeated several times (typically ∼3) in order to further improve the conductance of the electrodes, therefore forming a multilayer graphene (MLG) stack. An optical graph of the glass substrate with the MLG fi lm on top is displayed in Figure 1 b, showing high transparency. Subsequently, the MLG fi lm was patterned into bottom electrodes (BEs) with widths of 1–5 μm by photolithography and O 2 plasma etching (Figure 1 a2). Au/Ti metal contacts (B1 and B2) were then deposited onto the two ends of the MLG BE afterwards to minimize the contact resistance to the external probes in electrical measurements (Figure 1 a3). Following BE defi nition, a bilayer oxide structure consisting of an oxygen rich Ta 2 O 5– x layer (∼5 nm) and an oxygen defi cient TaO y layer (∼40 nm) serving as the switching medium of the RRAM devices was successively deposited by radio-frequency (RF) sputtering and reactive sputtering (400 °C, O 2 /Ar = 3%), respectively, without breaking the vacuum (Figure 1 a4). Similar to steps 1–3 in Figure 1 a, multiple monolayer graphene transfer, O 2 plasma etching and Au/Ti metal contacts deposition were conducted again to fabricate the top electrodes (TEs) that complete the crossbar memory structure (Figure 1 a5–7). Finally, a pad opening step was performed through a timed reactive ion etching (RIE) process to remove the Ta 2 O 5– x /TaO y bilayer on top of the bottom contact Au/Ti pads (Figure 1 a8). The sizes of the RRAM devices in this work range from 1–5 μm × 1–5 μm, and during measurements the voltage was applied on the TE with the BE grounded. Figure 1 c shows an optical graph of the as-fabricated chip. Compared with Figure 1 b, the transparency decrease is likely due to the relatively high concentration of metallic components in the TaO y base layer of the switching medium. Since resistive switching in Ta 2 O 5– x /TaO y bilayer devices is driven by internal oxygen vacancy redistribution, the change in deposition sequence should in principle only lead to a reversal of the switching polarity, as has been verifi ed by studies on devices with inert metal electrodes. [ 25 ] However, in the case of devices with MLG electrodes, the stacking sequence of the bilayer was found to be critical in determining the device behavior, as shown in Figure 1 d,e. When the Ta 2 O 5– x layer was deposited fi rst on the graphene BE followed by TaO y deposition, the MLG/TaO y /Ta 2 O 5– x /MLG (top to bottom) devices exhibited conventional bipolar resistive switching characteristics similar to the results obtained with metal electrodes, [ 25,26 ] as shown in Figure 1 d. Such switching behavior can be well understood in the picture of V O exchange between the Ta 2 O 5– x and the V O -rich TaO y layers, [ 8,26,27 ] and the switching polarity with positive SET and negative RESET voltages is a natural result of the device confi guration since the V O reservoir (the oxygen defi cient TaO y layer) sits on top in this case. The linear on-state behavior is also consistent with Ohmic conduction for the V O -based conducting fi laments in Ta 2 O 5– x /TaO y RRAMs. [ 8,26,27 ] Driven by the continuing demand for improved computing capability, the semiconductor industry has been constantly looking for a fast, reliable, scalable yet nonvolatile memory technology. Nanoscale resistive switching devices (RRAMs) have recently generated signifi cant interest as a potential building block for novel data storage and computing applications [ 1–7 ] and have shown exciting performance metrics including endurance of >10 12 , [ 8 ] programming time of <1 ns, [ 9 ] scalability better than 10 nm [ 10 ] and retention of >10 years. [ 8 ] In addition, since RRAM devices do not rely on conventional Si-based transistors, they can potentially be fabricated in three-dimensional (3D) multistack fashion or on diverse substrates for fl exible and/or transparent electronics applications. [ 11,12 ] This is particularly suitable for oxide-based valency-change type devices, where the resistance change is caused by the redistribution of oxygen vacancies inside the switching layer so potentially any inert metal can be used as the electrode material. [ 13 ] To this end, instead of using conventional metal electrodes RRAM devices can be integrated with novel electrode materials such as graphene to take advantage of the excellent electrical, mechanical and optical properties of graphene. [ 14,15 ] Furthermore, graphene can be readily functionalized or modifi ed to exhibit a number of interesting electrical characteristics, [ 16–18 ] and resistive switching effects in some graphene based materials, for example graphene oxide, have also been reported. [ 19–21 ] In this study, we show that RRAM devices can be successfully integrated with graphene electrodes in a compact device structure. More importantly, functionalized graphene electrodes exhibit an intrinsic threshold switching behavior analogous to insulator-metal transition (IMT), and provide a built-in selector element for the RRAM devices that is very desirable for mitigating the sneak path problem in crossbar memory arrays. [ 22,23 ]

Memristive Physically Evolving Networks Enabling the Emulation of Heterosynaptic Plasticity

A nanoscale, solid-state physically evolving network is experimentally demonstrated, based on the self-organization of Ag nanoclusters under an electric field. The adaptive nature of the network is determined by the collective inputs from multiple terminals and allows the emulation of heterosynaptic plasticity, an important learning rule in biological systems. These effects are universally observed in devices based on different switching materials.

Probing nanoscale oxygen ion motion in memristive systems

Ion transport is an essential process for various applications including energy storage, sensing, display, memory and so on, however direct visualization of oxygen ion motion has been a challenging task, which lies in the fact that the normally used electron microscopy imaging mainly focuses on the mass attribute of ions. The lack of appropriate understandings and analytic approaches on oxygen ion motion has caused significant difficulties in disclosing the mechanism of oxides-based memristors. Here we show evidence of oxygen ion migration and accumulation in HfO2 by in situ measurements of electrostatic force gradient between the probe and the sample, as systematically verified by the charge duration, oxygen gas eruption and controlled studies utilizing different electrolytes, field directions and environments. At higher voltages, oxygen-deficient nano-filaments are formed, as directly identified employing a CS-corrected transmission electron microscope. This study could provide a generalized approach for probing ion motions at the nanoscale.