Memristor materials, mechanisms and devices for unconventional computing

Resume : The electrochemical organic memristor with polyaniline active layer is a stand-along device, designed and realized for the reproduction of some synapse properties in electronic circuits. It was demonstrated the advantageous possibilities of networks built of such organic memristors in tasks of supervised and unsupervised learning. Nevertheless, for practical usage and reproducibility of memristor adaptive networks it is needed to control or predict its properties during the preparation and functioning under different conditions. It was experimentally shown that working principles of the single memristor are based on the drift of lithium ions between the working conducting polymer channel and solid electrolyte. But so far there is no theoretical description of the memristor in action from the point of view of physical and chemical laws. In this work a new theoretical model of the polyaniline memristor device is presented. The developed model of the organic memristor functioning was based on the detailed consideration of possible electrochemical processes occuring in the active zone of this device. Results of the calculation have demonstrated not only the qualitative explanation of the characteristics, observed in the experiment, but also quantitative similarities of the resultant current values. It is shown how the memristor could behave at zero potential difference relative to reference electrode. This improved model can establish a basis for the design and prediction of properties of more complicated circuits and systems (including stochastic ones) based on the organic memristive devices.

Resume : Resistive switching memory (ReRAM) requires high off/on resistance ratio, high endurance at low operation current and low switching voltage. One of the most studied resistive switching (RS) oxides, TiOx fits to these requirements, but suffers from high leakage current through sneak paths. To reduce the total leakage in TiOx based ReRAM devices, a bilayer stack of TiOx and a highly insulating layer like Al2O3 is one of the choices. For very small devices, deposition techniques guaranteeing high thin film quality are needed, therefore atomic layer deposition (ALD) was chosen. A systematic study on TiOx/Al2O3 bilayer structures integrated into cross point TiN/bilayer/Pt devices was carried out for ReRAM application. ALD was used for the deposition of Al2O3 and TiOx thin films, along with the optimization of growth parameters for amorphous Al2O3 films of about 3 to 10 nm thickness. With AFM, XPS and SEM the high qualities of Al2O3 thin layers were verified through low surface roughness, stoichiometric composition and sharp interface. In this contribution, we will discuss the influence of the thicknesses of the Al2O3 and TiOx films on the RS behavior of the bilayer stacks. We found that the variation of TiOx thickness has a significant effect on the HRS/LRS ratio during switching, whereas the change of Al2O3 thickness mainly affects the forming and reset voltage. Bilayer cells with about 3 to 4 nm Al2O3 and 5 to 10 nm TiOx exhibited stable bipolar type RS with resistance ratios of about 10E4 to 10E5. This and other dependencies will be discussed. This work was supported in part by the Deutsche Forschungsgemeinschaft (SFB917), and by the Global Research Laboratory program (2012040157) through the National Research Foundation (NRF) of Korea.

Resume : These past years, the development of neuromorphic computing has been strongly boosted by the large amount of nanoscale memory components that demonstrated the ability to realize the synaptic operation [1]. If each technology presents different pros and cons for the implementation of synaptic functions, a common goal in order to succeed in the hardware realization of neuromorphic systems, will be their integration into high density crossbar and interfacing with a CMOS platform for neuron operation. In this work, we investigate two promising candidates for synapse implementation, namely TiO2 and HfO2 resistive switches, and study engineering solutions for their integration into crossbar. Depending on the stack of layers configuration of the device (i.e. nature, composition and thickness), different switching mechanisms can be realized, from interface switching to filamentary switching, leading to different memory performances that could benefit to synaptic operation (i.e. analog controllability, retention,…). Practically, we show how engineering solutions are a simple tool to tune the forming voltage and to make each technology suitable for crossbar integration. [1] F. Alibart, E. Zamanidoost, D. B. Strukov, Nature Communication, 2072, 4, 2013

Resume : InSbTe alloys are a promising candidate for optical and multi-level cell (MLC) phase change memory device. Among InSbTe alloys, In3SbTe2 (IST) shows multi step switching operation and excellent thermal stability. However, it is tradeoff between higher thermal stability and switching speed because for the thermally stable materials phase transformation needs higher activation energy, which may relate to switching speed. But, there is no report about the switching speed of IST. Then, switching speed test shows that the IST has serious drawback in the set/reset speeds that are slower than those of the GeSbTe (GST) by 1.5~1.7 times. To improve the set/reset switching speed of the IST, Bi atoms are doped into the IST within the maximum range of 5.5 at.%. As a result, the set switching speed of Bi doped IST is 2.6 times as fast as that of the un-doped IST, and the reset speed also shows the same trend. Effect of Bi atoms in the IST raises question that the fast phase transformation is related with the Bi atoms and optical reflectivity. Atomic lattice images are thoroughly investigated and it is found that lattice distortion is generated by the Bi atoms, which increases vacancy concentration although the crystal structure of Bi doped IST is similar to the un-doped IST that is confirmed with x-ray diffractions and Laue patterns. Microscopic analysis of Bi doped IST reveals that inter-planar distance and directions of [001] and [1-1-2] zone axes are changed with the concentration of Bi atoms. In this work, we will discuss the relationship among the vacancy, the activation energy, the reflectivity and the switching speed of Bi-doped IST MLC-phase change memory devices.

Resume : Mimicking synaptic functions using memristor will contribute significantly to the development of bio-inspired computing system. It is very competitive in device technique, computing method and power consumption, etc., for simply using a memristor node to replace a complex circuit having the equivalent functions. This prompts us to realize synaptic functions as many as possible using one memristor and resolve the problems met in integrating memristor arrays. We report our recent works may provide helpful ideas for constructing artificial synapse. We utilized the compounds naturally formed in the metallic electrode/polymer interfaces in these works. In the memristor structure of Ag/PEDOT:PSS/Ta, the Ag filaments formed by the bias induced the synaptic modification, while the interface compound at the PEDOT:PSS/Ta interface constructed p-n junction with PEDOT:PSS. This memristor structure enhanced the resistivity effectively, which is favorable for the memristor integration. We have successfully mimicked spike-rate-dependent-plasticity, spike-time-dependent-plasticity and learning model of ‘stock of contacts’. In the memristor structure of Ti/PEDOT:PSS/Ti, the Ti compound at the interface has contrast switching direction with PEDOT:PSS. The former has lower switching threshold. Thus, the synaptic weight increased with the stimulations under the low strength, but decreased due to the de-doping of PEDOT:PSS under the greatly culminated stimulation strength. These contrast mechanisms prevented the system from losing stability. Moreover, this structure can be expanded to the memristor systems having negative differential resistivity and resolve the competing problem in the memristor integration.

Resume : Recently, self-assembled networks of functionalized nano particles (NPSAN) were proposed as a prototype of innovative devices [1], with switchable NP-NP links. Thanks to the chemical properties of the surface molecules, these would ultimately show memristive behaviour, as building blocks of a logical computing architecture [2]. Inspired by the experiments in [1,2], we describe the atomic structure of large (50 nm) Au NP, coated by a dense monolayer of azobenzene-bithiophene (AzBT) derivatives. We use atomic-scale simulations to address the multi scale problem of computing the electrical response of the NP-NP contact, under different biasing conditions and different switching states of the AzBT molecules. We run Molecular Dynamics simulations, with AzBT molecules linked to the surface of rigid NP, and interdigitated as a function of the NP-NP distance and molecule density. We then can compute the local conductivity of such realistic interacting configurations, by a localised tight-binding decimation method. Finally, simulating the aggregate logic response of such model NP structures is also considered. We outline a divide-and-conquer technique, to calculate the response of multiterminal networks, by recursive Green functions [3]. [1] G. Wendin, et al. Int. J. Unconv. Comp., 8, 325-332 (2012) [2] V. Beiu et al., Procs. of ICNAAM 2012, AIP Conf. Proc. Vol. 1479, pp. 1875-1879 (2012) [3] C. White and A. P. Horsfield, paper I.11 Symp. I, E-MRS 2012 Fall Meeting, Warsaw

Resume : Resistive Random Access Memory (ReRAM) with a structure Au/TiO2-x/Au demonstrated a clear bipolar resistive switching behavior without the necessity of an initial electroforming process. The titanium oxide (TiO2-x) thin film was deposited by reactive RF magnetron sputtering at room temperature in a controlled oxygen/argon ambient1. The high density of oxygen vacancies within the film (induced by the low oxygen content) is essential component for the formation of conducting filaments and demonstration of DC or nanosecond pulsed resistance switching. Conductive Atomic Force Microscopy (C-AFM) on structures without the Au top electrode was then employed in order to investigate the nanoscale electrical properties of the device. In situ current distribution during the SET process revealed possible formation of conducting filaments while DC sweeping bias voltage revealed an OFF/ON switching ratio of about 200. We have also demonstrated that by using C-AFM both a low resistance state and a high resistance state can be written by bipolar voltage application imaged by corresponding patterns on the TiO2-x film, suggesting that oxygen ions movement at the Au coated tip/TiO2-x interface plays a critical role in the rupture of conducting filaments and thus correlating the macroscopic characteristics of our device1 with microscopic origins. Conduction mechanism analysis divulged that trap assisted tunneling governs the conduction behavior of the high resistance state, whereas as Ohm’s interpret well the electrical behavior of low resistance state. 1 P. Bousoulas, I. Michelakaki, and D. Tsoukalas, “Influence of oxygen content of room temperature TiO2-x deposited films for enhanced resistive switching memory performance”, J. Appl. Phys. 115, Article in press, (2014)

Resume : Recently, resistance switching (RS) phenomena in a bulk polymer nanocomposite using Ag nanowires have been reported. However, the RS behavior seems to be unstable, reversible, or irreversible by cases. In this study, in order to solve the aforementioned problems, we propose a RRAM material of novel concept, which is made up of the one-dimensional resistance-switchable fillers in a polymer matrix. A nanocomposite capable of showing a resistance-switching behavior is prepared using novel resistance-switchable fillers embedded in a polymer matrix. The filler in this study employs a conformal passivation layer of highly crystalline TiO2 on surfaces of conductive Ag nanowires to effectively gate electron flows delivered through the conductive core, resulting in an excellent resistance-switching performance. A nanocomposite prepared by controlled mixing of the resistance-switchable nanowires with a polymer matrix successfully exhibited a resistance-switching behavior of highly enhanced reliability and a resistance on/off ratio, along with flexibility due to the presence of nanowires of a tiny amount. The advantages of our approach include a simple and low-cost fabrication procedure along with sustainable performances suitable for a resistance switching random-access-memory application.

Resume : Currently, much attention is being devoted to memory circuit elements (memristors). One of the most promise compounds for applications in memristor devices is TiOx (x≤2). In our work we investigated the electrophysical properties of Pt/TiOx/TiO2/Pt memristor elements. This structure was obtained with the use of pulsed laser deposition technique. The thickness of both TiOx and TiO2 layers was varied in the range from 10 nm to 60 nm. During the measurements we calculated the resistance of structure in low and highly conductive state (Roff and Ron respectively). It was found that the value Roff/Ron of the investigated structure nonmonotonically depends on the thickness of TiOx/TiO2 structure. It must be noted that maximum value Roff/Ron is obtained when the thickness of both TiOx and TiO2 layers equals 30 nm. It was suggested that a broad distribution of stoichiometry index inside the TiOx/TiO2 layers is responsible for observed nonmonotonic dependence. This hypothesis was further provided by the results of Auger spectrometry measurements.

Resume : Among the key issues the semiconductor industry is facing are the power consumption and the die size of the interconnect in modern nanoelectronics. Crossbars of memristive devices based on resistive switching oxides are currently considered as a replacement solution for reconfigurable interconnects in Field Programmable Gate Array (FPGA)-like architectures. Nevertheless, crossbar architectures of memristive devices have significant drawbacks inherent to the large fan-in/fan-out such as power supply limitation and cross-talk between devices when one particular component needs to be addressed. In this work, we propose to evaluate the case of a matrix-based interconnect using oxide-based memristive devices. Such architecture requires smaller connectivity with respect to crossbar, thus potentially leading to a reduced complexity for memristive device operation. The set of required connections in the memristive-devices-based interconnect is defined according to the considered network topology,. e.g. butterfly. The resulting interconnect structure can be configured according to application-specific connectivity requirements.

Resume : Non volatile memories based on switchable resistance cells have emerged in the last years as an alternative to conventional technologies like Si-based flash memories. One of the most promising resistive switching devices is metal-insulator-metal (MIM) structures formed by metal oxides like TiO2 and HfO2. The switching mechanism of these devices depends on both metal electrodes and insulator. In configurations like cross-bar structures, these devices can work together as a high density non volatile memory with a low power consumption compared to current technologies. The switching resistance performance of Pt/HfO2 /Ti atomic-layer deposited (ALD) MIM devices in a cross-bar structure has been characterized. Bipolar resistive switching and non volatile memory effects have been observed. High endurance and reliability, working at low currents current (1µA) and moderate voltages (3V maximum) provides excellent features to act as a non volatile memory. Devices show a high-to-low resistance ratio larger than 103, proving the excellent quality of the device. The formation of an oxygen vacancies channel in the dielectric thanks to an inert cathode (Pt) and an oxidizable anode (Ti) can explain the reversible resistance change of the HfO2 layer, as has been reported. To complete the electrical study, impedance spectroscopy analysis has been carried out, together with physical and chemical characterization of the devices, in agreement with previously reported operation mechanisms.

Resume : Abstract We report various ceria based nanostructures such as; nanorods, nano-dots and nanocubes to demonstrate excellent bipolar resistive switching characteristics. Bottom up approaches, such as electrochemical deposition and solvothermal techniques have been deployed to fabricate these nanostructures. In the first approach, the vertically aligned ZnO (NR) arrays were grown on transparent conductive glass by electrochemical deposition while CeO2 quantum dots (QDs)/ nano-dots were prepared by a solvothermal method. Subsequently, the as-prepared CeO2 QDs were embedded into ZnO NRs array by dip-coating to obtain CeO2-ZnO nano-composite. Interestingly, such a device exhibits excellent resistive switching properties with much higher ON/OFF ratios, better uniformity and stability over the pure ZnO and CeO2 thin films. In a different approach, self-assembled CeO2 nanocubes based resistance switching devices were fabricated by hydrothermal process. The devices were proven to exhibit even more excellent resistive switching performance. The origin of switching behaviour on the basis of filament model and inter cube junctions was presented. Furthermore, indium (In) and Gadolinium (Gd) doped CeO2 nanocubes were also prepared to expand these nanostructures for memristive applications. The present studies demonstrate that ceria nanostructures have the potential for next generation non-volatile memory applications. Biography Mr Adnan Younis received his M.Phil in Physics from Quaid-i-Azam University Islamabad-Pakistan in 2009, where he researched high temperature superconductors in the Material Science laboratory of Dr. Nawazish Ali Khan. He is currently a PhD candidate with Prof. Sean Li and Dr. Dewei Chu in the laboratory of advanced multifunctional and energy materials at school of Material Science and Engineering, University of New South Wales Australia. His research interests involve development of metal oxides with their applications in next generation non-volatile memory devices such as resistive random access memories (RRAM).

Resume : Because of a possibility used as analog memories, resistive RAMs (ReRAMs) are energetically studied. Recently, they are investigated as synapses of artificial neural networks. The stacked film composed of a solid electrolyte and Cu (or Ag) is one of ReRAMs, which are believed to operate by formation and rupture of conductive filament formed in the electrolytes. However, details of the analog operation are still obscure. The combination of Cu/MoOx studied in this work is in this category. To understand some details, the switching operation was investigated dynamically by means of the TEM/STM in-situ observation. Typical results of Cu/MoOx/TiN showed the cyclic bi-polar ReRAM switching inside the TEM instrument. In the Set process, a region with dark contrast grew from the interface between MoOx and TiN. The EDX results indicated that this region contained more Cu than other regions in the MoOx layer. This region was small when the resistance changed suddenly to the low resistance state, and it grew along the Cu top electrode (TE) during subsequent current flow. Just after the weak Reset, no drastic change was recognized in the TEM image. Afterwards, it shrunk to the Cu (TE) layer. The Cu conductive filament contributes to the ReRAM switching, but it does not need to be recognizably thick. The filament became large with high compliance current. The filament position connecting TE and BE was not fixed.