Publications
Publications and papers from the nano memory resistors project team
close
Pulse-induced resistive and capacitive switching in TiO2 thin film devices
Authors:
  Iulia Salaoru, Ali Khiat, Qingjiang Li, Radu Berdan, and Themistoklis Prodromakis. Published by: Applied Physics Letters 103, 233513 (2013); doi: 10.1063/1.4840316

In this study, we exploit the non-zero crossing current–voltage characteristics exhibited by nanoscale TiO2 based solid-state memristors.

close
A Memristor SPICE Model Accounting for Volatile Characteristics of Practical ReRAM
Authors:
  Radu Berdan, Chuan Lim, Ali Khiat, Christos Papavassiliou. Published by: IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 1, JANUARY 2014

Realizing large-scale circuits utilizing emerging nanoionic devices known as memristors depends on the accurate modeling of their behavior under a wide range of biasing conditions.

close
Resistive switching characteristics of indium-tin-oxide thin film devices
Authors:
  Ali Khiat, Iulia Salaoru, and Themistoklis Prodromakis. Published by: Phys. Status Solidi A, 1–6 (2014) / DOI 10.1002/pssa.201330646

We demonstrate that indium–tin-oxide (ITO), when used as an active core material in metal–insulator–metal type devices, facilitates resistive switching. We fabricated devices both on silicon as well as quartz wafers, to demonstrate transparent devices.

close
Origin of the OFF state variability in ReRAM cells
Authors:
  Salaoru, I., Khiat, A., Berdan, R., Quingjian, L., Papavassiliou, C. and Prodromakis, T. Published by: Journal of Physics D: Applied Physics

This work exploits the switching dynamics of nanoscale Resistive Random Access Memory (ReRAM) cells with particular emphasis on the origin of the observed variability when consecutively cycled/programmed at distinct memory states.

close
Applications of solid-state memristors in tunable filters
Authors:
  Wizenberg, R., Khiat, A., Berdan, R., Papavassiliou, C. and Prodromakis, T. Published by: IEEE ISCAS-CAS-FEST 1-5 June 2014, Melbourne, Australia

In this paper we present a practical approach to employ solid-state TiO2 memristors as tunable loads in filter configurations.

close
Memristors as synapse emulators in the context of event-based computation
Authors:
  Serb, A., Berdan, R., Khiat, A., Shari, L., Vasilaki, E., Papavassiliou, C. and Prodromakis, T. Published by: IEEE ISCAS-CAS-FEST 1-5 June 2014, Melbourne, Australia

This paper examines the widespread Biolek and Pershin models of the memristor in order to find out whether they support STDP in an event-based computation environment.

close
Qualitative SPICE modeling accounting for volatile dynamics of TiO2 memristors
Authors:
  Berdan, R., Khiat, A., Papavassiliou, C. and Prodromakis, T. Published by: IEEE ISCAS-CAS-FEST 1-5 June 2014, Melbourne, Australia

We propose a SPICE model that describes qualitatively real memristor device operation.

close
Origin of stochastic resistive switching in devices with phenomenologically identical initial states
Authors:
  Qingjiang, Li, Khiat, A., Salaoru, I., Hui, X. and Prodromakis, T. Published by: IEEE ISCAS-CAS-FEST 1-5 June 2014, Melbourne, Australia

Nanoscale resistive switching devices are nowadays widely employed in applications of storage, logic and computing.

close
A versatile, low-cost platform for testing large ReRAM cross-bar arrays
Authors:
  Serb, A., Berdan, R., Khiat, A., Papavassiliou, C. and Prodromakis, T. Published by: IEEE ISCAS-CAS-FEST 1-5 June 2014, Melbourne, Australia

We demonstrate a testing platform that allows the manipulation of memristor cross-bar arrays by use of little more than a computer with MATLAB, an mBED and some external components mounted on a PCB.

close
Memory Impedance in TiO2 based Metal-Insulator-Metal Devices
Authors:
  Qingjiang Li, Ali Khiat, Iulia Salaoru, Christos Papavassiliou, and Themistoklis Prodromakis. Published by: Scientific Reports 2014

Here we demonstrate that TiO2-based metal-insulator-metal devices are more than just a memory-resistor. They possess resistive, capacitive and inductive components that can concurrently be programmed; essentially exhibiting a convolution of memristive, memcapacitive and meminductive effects.

close
Stochastic switching of TiO2-based memristive with identical initial memory states
Authors:
  Qingjiang Li, Ali Khiat, Iulia Salaoru, Hui Xu and Themistoklis Prodromakis. Published by: Nanoscale Research Letters 2014, 9:293

In this work, we show that identical TiO2-based memristive devices that possess the same initial resistive states are only phenomenologically similar as their internal structures may vary significantly, which could render quite dissimilar switching dynamics.

close
Coexistence of Memory-Resistance and Memory-Capacitance in TiO2 Solid State Devices
Authors:
  Iulia Salaoru, Qingjiang Li, Ali Khiat and Themistoklis Prodromakis. Published by: Nanoscale Research Letters, September 2014.

This work exploits the coexistence of both resistance and capacitance memory effects in TiO2 based two terminal cells.

close
X-ray Absorption Spectroscopy Study of TiO2-x Thin Films for Memory Applications
Authors:
  D. Carta, G. Mountjoy, A. Regoutz, A. Khiat, A. Serb, T. Prodromakis. Published by American Chemical Society, Journal of Physical Chemistry C, 119, 8, 4362, (2015)

X-ray absorption spectroscopy (XAS) at the Ti K-edge was used to characterize the atomic-scale structure of a TiO2-x thin film before and after annealing and for the first time after inclusion in a metal-insulator-metal device based on a Cr/Pt/TiO2-x/Pt stack.

close
Conductive Atomic Force Microscopy Investigation of Switching Thresholds in Titanium Dioxide Thin Films
Authors:
  Maria Trapatseli, Daniela Carta, Anna Regoutz, Ali Khiat, Alexander Serb, Isha Gupta, and Themistoklis Prodromakis, Journal of Physical Chemistry C, 119, 21, 11958, (2015).

In this work, we have used conductive atomic force microscopy (C-AFM) to identify the resistive switching thresholds of titanium dioxide thin films deposited on Si/SiO2/Ti/Pt stacks to be used in memory devices. By performing a set of reading/writing voltage scans over pristine areas of the thin films, we have identified the critical thresholds, which define a reversible operation (soft-breakdown, SB) via localised changes in electrical resistance across the film and an irreversible operation (hard-breakdown, HB) that includes both changes in local electrical resistance and thin film topography.

close
An amorphous TiO2 metal insulator metal selector device for ReRAM crossbar arrays with tunable voltage margin
Authors:
  Simone Cortese, Ali Khiat, Daniela Carta, Mark E. Light and Themis Prodromakis, Applied Physics Letters, 108, 033505 (2016); http://dx.doi.org/10.1063/1.4940361

In this study we present a selector device based on a nickel/sub-stoichiometric amorphous titanium dioxide film with an high Voltage Margin of 3 V in order to effectively tackle ReRAM crossbar arrays sneak currents. Its unique feature lies in the possibility of tuning of the voltage margin by simple device annealing steps,up to 40, which resulted in a improved Voltage Margin of 1.4 V without compromising the device behaviour in terms of Current and non linearity.

close
A TiO2-based Volatile Threshold Switching Selector Device with 10^7 non linearity and sub 100 pA Off Current
Authors:
  Simone Cortese, Maria Trapatseli, Ali Khiat and Themistoklis Prodromakis, Proceedings of VLSI-TSA, Taiwan, April 2016

An highly non-linear two-terminal selector device is proposed: the device is 5 nm thick TiO2-based selector which exploits a volatile threshold resistive switching, so far unreported for this material. The device shows a current density up to 100 kA/cm2, 10^7 current non-linearity and a 4 V voltage margin, the highest reported for TiO2-based selectors and sub 100 pA off current.

close
A cell classifier for RRAM process development
Authors:
  Isha Gupta, Alexantrou Serb, Radu Berdan, Ali Khiat, Anna Regoutz, and Themis Prodromakis. IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 62, NO. 7, JULY 2015

In this brief, we develop and experimentally test an algorithm for identifying promising devices within an “array under study.”The objective is to identify devices that switch between highly distinct resistive states, i.e., a low resistive state (LRS) and a high resistive state (HRS) when subjected to a given biasing protocol. No underlying assumption is made as to what levels of resistance should constitute a digital 1 or 0, making the proposed algorithm a particularly effective process control tool, particularly in immature technologies.

close
Practical Operation Consideration for Memristive Integrating Sensors
Authors:
  I.Gupta, A.Serb, A. Khiat, T. Prodromakis. International Symposium on Circuits and Systems, 2016

In this work, we study the effect of different sets of operational parameters (pre-amplification i.e gain and offset, sampling rate) on the response of a typical TiOx MIS device to a reference pre-processed neural recording. Key performance metrics, such as number of spikes detected are measured and important conclusions and limitations on practical MIS operation are drawn.

close
Towards a memristor-based spike-sorting platform
Authors:
  I. Gupta, A. Serb, A. Khiat and T. Prodromakis, Published by: Proceedings of the IEEE Biomedical Circuits and Systems (BioCAS) Conference, Oct 2016.

We present a new approach for performing spikesorting through a memristor-based, neural-signal processing platform. We have previously shown that the inherent threshold property of the memristor allows spike-detection through nonvolatile resistive state transition. Here, a test memristive device is subjected to a neural recording and the periodically recorded resistive state changes are mapped to the amplitude of the spiking events. It is found that the resistive state changes can be differentiated into clusters, where each cluster can be mapped to a range of spiking events in the input neural waveform, thus indicating the address of source neuron.

close
Volatile Resistive Switching in Ni/TiO2/Ni stacks for Access Devices Applications
Authors:
  S. Cortese, A. Khiat and T. Prodromakis. Publiehsed by: UK Semiconductor, July 2016.

Resistive switching and resistive random access memories have attracted huge interest for next generation nonvolatile memory applications, also thought to be able to overcome flash memories limitations when arranged in crossbar arrays. A cornerstone of their potential success is that the toggling between two distinct resistance states, usually a High Resistive State (HRS) and a Low Resistive State (LRS), is an intrinsic non-volatile phenomenon with the two states being thermodynamically stable. TiO2 is one of the most common materials known to support non-volatile RS. In this paper, we report a volatile resistive switching in a titanium dioxide thin film sandwiched by two nickel electrodes. The aim of this work is to understand the underlying physical mechanism that triggers the volatile effect, which is ascribed to the presence of a NiO layer at the bottom interface. The NiO layer alters the equilibrium between electric field driven filament formation and thermal enhanced ion diffusion, resulting in the volatile behaviour. Although the volatility is not ideal for non-volatile memory applications, it shows merit for access devices in crossbar arrays due to its high LRS/HRS ratio, which are also briefly discussed.

close
Live Demonstration: Characterization of RRAM Crossbar Arrays at a Click of a Button
Authors:
  R. Berdan, A. Serb, A. Khiat, C. Papavassiliou and T. Prodromakis. Published by: Proceedings of the IEEE International Symposium on Circuits and Systems, May 2016

close
An Ultra-Low Voltage RRAM Read-Out Technique Employing Dithering Principles
Authors:
  J. Xing, A. Serb and T. Prodromakis. Published by: Proceedings of the IEEE International Symposium on Circuits and Systems, May 2016

close
HfO2-Based Memristors for Neuromorphic Applications
Authors:
  E. Covi, S. Brivo, A. Serb, T. Prodromakis, M. Fanciulli and S. Spiga. Published by: Proceedings of the IEEE International Symposium on Circuits and Systems, May 2016.

close
EU COST Action IC1401 – Pushing the Frontiers of Memristive Devices to Systems
Authors:
  D. Biolek, S. Cararra, E. Chicca, F. Corinto, J. Georgiou, B. Linares-Barranco, T. Prodromakis, S. Spiga and R. Tetzlaff. Published by: MELECON Mediterranean Electrotechnical Conference, April 2016

close
Gradual set dynamics in HfO2-based memristor driven by sub-threshold voltage pulses
Authors:
  S. Brivio, E. Covi, A. Serb, T. Prodromakis, M. Fanciulli, and S. Spiga. Published by: MEMRISYS 2015

close
Easily integrable Metal-Insulator-Metal (MIM) Selector Devices for High Density ReRAM Crossbar Arrays
Authors:
  S18. S. Cortese, A. Khiat, T. Prodromakis. Publishe by: International Workshop on Advances in ReRAM: Materials & Interfaces, Oct 2015

close
Switching mechanisms in microscale Memristors
Authors:
  T. Prodromakis, K. Michelakis and C. Toumazou. Published by: IET Electronic Letters, vol. 46, no. 1, pp. 63-65, 2010

It is only very recently that the memristor, the fourth missing passive element, was discovered, as technological advances and the scaling- down to nanometre dimensions in particular resulted in clearly evident and measurable memristance. At the nanoscale, these devices exhibit variable resistive behaviour, which can be applied in switching networks and memory. Experimental evidence is provided that micro- metre-size memristors are viable with practical R OFF / R ON ratios.

close
Cost-effective fabrication of nanoscale electrode memristors with reproducible electrical response
Authors:
  K. Michelakis, T. Prodromakis and C. Toumazou. Published by: IET Micro and Nano Letters, vol. 5, no. 2, pp. 91-94, 2010

This Letter aims to promote basic research into memristors, which will help provide theorists with much- needed reliable benchmarks and will also aid the technology progress. It addresses an information gap presently in the literature on simple microfabrication techni ques for the realisation of such devices. Consequently, a fabrication method is reported for implementing the full active-material stack, requiring a single lithography and evaporation step. A cost-effective technique that can r eliably shrink device lateral dimensions towards the nanoscale is also demonstrated. Experimental results confirm the suitability of the proposed methods for fabricating memristors of varying dimensions that exhibit consistent electrical characteristics

close
A Versatile Memristor Model With Non-linear Dopant Kinetics
Authors:
  T. Prodromakis, B.P. Peh, C. Papavassiliou and C. Toumazou. Published by: IEEE Transactions on Electron Devices, vol. 58, no. 9, 2011

The need for reliable models that take into account the nonlinear kinetics of dopants is nowadays of paramount importance, particularly with the physical dimensions of electron devices shrinking to the deep nanoscale range and the development of emerging nanoionic systems such as the memristor. In this paper, we present a novel nonlinear dopant drift model that resolves the boundary issues existing in previously reported models that can be easily adjusted to match the dynamics of distinct memristive elements. With the aid of this model, we examine switching mechanisms, current-voltage characteristics, and the collective ion transport in two terminal memristive devices, providing new insights on memristive behavior.

close
Biomimetic Model of the Outer Plexiform Layer by Incorporating Memristive Devices
Authors:
  A. Gelencser, T. Prodromakis, C. Toumazou and T. Roska. Published by: Phys. Rev. E., vol. 85, Apr 2012

In this paper we present a biorealistic model for the first part of the early vision of processing by incorporating memristive nanodevices. The architecture of the proposed network is based on the organization and functioning of the outer plexiform layer (OPL) in the vertebrate retina. We demonstrate that memristive devices are indeed a valuable building block for neuromorphic architectures, as their highly nonlinear and adaptive response could be exploited for establishing ultradense networks with dynamics similar to that of their biological counterparts. We particularly show that hexagonal memristive grids can be employed for faithfully emulating the smoothing effect occurring in the OPL to enhance the dynamic range of the system. In addition, we employ a memristor-based thresholding scheme for detecting the edges of grayscale images, while the proposed system is also evaluated for its adaptation and fault tolerance capacity against different light or noise conditions as well as its distinct device yields.

close
High Precision Analogue Memristor Tuning
Authors:
  R. Berdan, T. Prodromakis and C. Toumazou. Published by: IET Electronic Letters

A report is presented on the operation of an analogue programming circuit for accurately setting the state of a memristor. The circuit exploits the dynamic modulation of resistance under a constant DC bias while real-time measurements of the memristance are performed using an AC signal. The circuit employs feedback for converging the state of a device at any required level within a decade. This allows the memristor to act as an analogue potentiometer, with its resistance corresponding to an input analogue voltage. This implementation was tested with the HP memristor model revealing an accuracy of less than 0.4% (8 bit precision) in relation to the full dynamic range.

close
Two Centuries of Memristors
Authors:
  T. Prodromakis, C. Toumazou and L.O. Chua. Publieshed by: Nature Materials, vol. 11, no 6, pp. 478-481, Jun 2012

Memristors are dynamic electronic devices whose nanoscale realization has led to considerable research interest. However, their experimental history goes back two centuries.

close
Memristive Devices as Parameter Setting Elements in Programmable Gain Amplifiers
Authors:
  R. Berdan, T. Prodromakis, I. Salaoru, A. Khiat and C. Toumazou. Published by: Applied Physics Letters, vol. 101, 243502, 2012

In this paper, we investigate the AC performance of a variable gain amplifier that utilizes an in-house manufactured memristor as a gain setting element. Analysis includes frequency and phase responses as the memristor is programmed at different resistive states. A TiO2-based solid-state memristor was employed in the feedback branch of an inverting voltage amplifier and was programmed externally. We have also observed indications of memcapacitive effects and a correlation with resistive states is presented. We demonstrate that our TiO2 memristive devices, although possessing relatively low ROFF/RON switching ratios (?10), are versatile and can be used reliably in programmable gain amplifiers.

close
STDP and STDP Variations with Memristors
Authors:
  T. Serrano-Gotarredona, T. Masquelier, T. Prodromakis, G. Indiveri, and B. Linares-Barranco. Published by: Frontiers in Neuroscience, vol. 7, no. 2, 2013

In this paper we review several ways of realizing asynchronous Spike-Timing-Dependent-Plasticity (STDP) using memristors as synapses. Our focus is on how to use individual memristors to implement synaptic weight multiplications, in a way such that it is not necessary to (a) introduce global synchronization and (b) to separate memristor learning phases from memristor performing phases. In the approaches described, neurons fire spikes asynchronously when they wish and memristive synapses perform computation and learn at their own pace, as it happens in biological neural systems. We distinguish between two different memristor physics, depending on whether they respond to the original “moving wall” or to the “filament creation and annihilation” models. Independent of the memristor physics, we discuss two different types of STDP rules that can be implemented with memristors: either the pure timing-based rule that takes into account the arrival time of the spikes from the pre- and the post-synaptic neurons, or a hybrid rule that takes into account only the timing of pre-synaptic spikes and the membrane potential and other state variables of the post-synaptic neuron. We show how to implement these rules in cross-bar architectures that comprise massive arrays of memristors, and we discuss applications for artificial vision.

close
Resistive switching of oxygen enhanced TiO2 thin-film devices
Authors:
  I. Salaoru, T. Prodromakis, A. Khiat and C. Toumazou. Published by: Applied Physics Letters, vol. 102, 013506, 2013

In this work, we investigate the effect of oxygen-enhanced TiO2 thin films on the switching dynamics of Pt/TiO2/Pt memristive nanodevices. We demonstrate that such devices can be used as resistive random access memory (RRAM) cells without required electroforming. We experimentally demonstrate that devices based on TiO2 films fabricated via sputtering with partial pressures of Ar/O2 6/6 sccm and 2/10 sccm show OFF/ON ratios of six and two orders of magnitude, respectively. Additionally, it was found that a lower O2 flow during sputtering of TiO2 allows for lower energy requirements for switching the devices from a high to low resistive state.

close
A Proposal for Hybrid Memristor-CMOS Spiking Neuromorphic Learning Systems
Authors:
  T Serrano-Gotarredona, T. Prodromakis, B Linares-Barranco. Published by: IEEE Circuits and Systems Magazine, vol. 12, no. 2, pp. 74-88, May 2013

Recent research in nanotechnology has led to the practical realization of nanoscale devices that behave as memristors, a device that was postulated in the seventies by Chua based on circuit theoretical reasonings. On the other hand, neuromorphic engineering, a discipline that implements physical artifacts based on neuroscience knowledge, has related neural learning mechanisms to the operation of memristors. As a result, neuro-inspired learning architectures can be proposed that exploit nanoscale memristors for building very large scale systems with very dense synaptic-like memory elements. At present, the deep understanding of the internal mechanisms governing memristor operation is still an open issue, and the practical realization of very large scale and reliable ?memristive fabric? for neural learning applications is not a reality yet. However, in the meantime, researchers are proposing and analyzing potential circuit architectures that would combine a standard CMOS substrate with a memristive nanoscale fabric on top to realize hybrid memristor-CMOS neural learning systems. The focus of this paper is on one such architecture for implementing the very well established Spike-Timing-Dependent-Plasticity (STDP) learning mechanism found in biology. In this paper we quickly review spiking neural systems, STDP learning, and memristors, and propose a hybrid memristor-CMOS system architecture with the potential of implementing a large scale STDP learning spiking neural system. Such architecture would eventually allow to implement real-time brain-like processing learning systems with about neurons and synapses on one single Printed Circuit Board (PCB).

close
Integration of nanoscale memristor synapses in neuromorphic computing architectures
Authors:
  G. Indiveri, B. Linares-Barranco, R. Legenstein, G. Deligeorgis and T. Prodromakis. Published by: Nanotechnology, 24, 384010, 2013

Conventional neuro-computing architectures and artificial neural networks have often been developed with no or loose connections to neuroscience. As a consequence, they have largely ignored key features of biological neural processing systems, such as their extremely low-power consumption features or their ability to carry out robust and efficient computation using massively parallel arrays of limited precision, highly variable, and unreliable components. Recent developments in nano-technologies are making available extremely compact and low power, but also variable and unreliable solid-state devices that can potentially extend the offerings of availing CMOS technologies. In particular, memristors are regarded as a promising solution for modeling key features of biological synapses due to their nanoscale dimensions, their capacity to store multiple bits of information per element and the low energy required to write distinct states. In this paper, we first review the neuro- and neuromorphic computing approaches that can best exploit the properties of memristor and scale devices, and then propose a novel hybrid memristor-CMOS neuromorphic circuit which represents a radical departure from conventional neuro-computing approaches, as it uses memristors to directly emulate the biophysics and temporal dynamics of real synapses. We point out the differences between the use of memristors in conventional neuro-computing architectures and the hybrid memristor-CMOS circuit proposed, and argue how this circuit represents an ideal building block for implementing brain-inspired probabilistic computing paradigms that are robust to variability and fault tolerant by design.

close
A μ-controller-based system for interfacing selector-less RRAM crossbar arrays
Authors:
  R. Berdan, A. Serb, A. Khiat, A. Regoutz, C. Papavassiliou, and T. Prodromakis. Published by: IEEE Transactions on Electron Devices, vol. 62, no. 7, 2015.

Selectorless crossbar arrays of resistive randomaccess memory (RRAM), also known as memristors, conduct large sneak currents during operation, which can significantly corrupt the accuracy of cross-point analog resistance (Mt) measurements. In order to mitigate this issue, we have designed, built, and tested a memristor characterization and testing (mCAT) instrument that forces redistribution of sneak currents within the crossbar array, dramatically increasing Mt measurement accuracy. We calibrated the mCAT using a custom-made 32 × 32 discrete resistive crossbar array, and subsequently demonstrated its functionality on solid-state TiO2-x RRAM arrays, on wafer and packaged, of the same size. Our platform can measure standalone Mt in the range of 1 kΩ to 1 MΩ with <;1% error. For our custom resistive crossbar, 90% of devices of the same resistance range were measured with <;10% error. The platform's limitations have been quantified using large-scale nonideal crossbar simulations.

close
“Implementation of a spike-based perceptron learning rule using TiO2−x memristors
Authors:
  H. Mostafa, A. Khiat, A. Serb, C. Mayr, G. Indiveri, T. Prodromakis. Published by: Frontiers in Neuroscience, vol. 9, no. 357, 2015.

Synaptic plasticity plays a crucial role in allowing neural networks to learn and adapt to various input environments. Neuromorphic systems need to implement plastic synapses to obtain basic “cognitive” capabilities such as learning. One promising and scalable approach for implementing neuromorphic synapses is to use nano-scale memristors as synaptic elements. In this paper we propose a hybrid CMOS-memristor system comprising CMOS neurons interconnected through TiO2−x memristors, and spike-based learning circuits that modulate the conductance of the memristive synapse elements according to a spike-based Perceptron plasticity rule. We highlight a number of advantages for using this spike-based plasticity rule as compared to other forms of spike timing dependent plasticity (STDP) rules. We provide experimental proof-of-concept results with two silicon neurons connected through a memristive synapse that show how the CMOS plasticity circuits can induce stable changes in memristor conductances, giving rise to increased synaptic strength after a potentiation episode and to decreased strength after a depression episode.

close
Practical determination of individual element resistive states in selectorless RRAM arrays
Authors:
  A. Serb, W.R. White, C. Papavassiliou and T. Prodromakis. Published by: IEEE Trans. CAS-I, vol. 62, no. 11, 2015

Three distinct methods of reading multi-level cross-point resistive states from selector-less RRAM arrays are implemented in a physical system and compared for read-out accuracy. They are: the standard, direct measurement method and two methods that attempt to enhance accuracy by computing cross-point resistance on the basis of multiple measurements. Results indicate that the standard method performs as well as or better than its competitors. SPICE simulations are then performed with controlled amounts of non-idealities introduced in the system in order to test whether any technique offers particular resilience against typical practical imperfections such as crossbar line resistance. We conclude that even though certain non-idealities are shown to be minimized by different circuit-level read-out strategies, line resistance within the crossbar remains an outstanding challenge.

close
An RRAM biasing parameter optimiser
Authors:
  A. Serb, A. Khiat, and T. Prodromakis. Published by: IEEE Trans. on Electron Devices, vol. 62, no.11, 2015.

Research on memory devices is a highly active field, and many new technologies are being constantly developed. However, characterizing them and understanding how to bias for optimal performance are becoming an increasingly tight bottleneck. Here, we propose a novel technique for extracting biasing parameters, conducive to desirable switching behavior in a highly automated manner, thereby shortening the process development cycles. The principle of operation is based on: 1) applying variable amplitude, pulse-mode stimulation on a test device in order to induce switching multiple times; 2) collecting the data on how pulsing parameters affect the device’s resistive state; and 3) choosing the most suitable biasing parameters for the application at hand. The utility of the proposed technique is validated on TiO x -based prototypes, where we demonstrate the successful extraction of biasing parameters that allow the operation of our devices both as multistate and binary resistive switches.

close
Role and optimization of the active oxide layer in TiO2-based RRAM
Authors:
  A. Regoutz, I. Gupta, A. Serb, A. Khiat, F. Borgatti, T.-L. Lee, C. Schlueter, P. Torelli, B. Gobaut, M. Light, D. Carta, S. Pearce, G. Panaccione, T. Prodromakis. Published by: Advanced Functional Materials, doi:10.1002/adfm.201503522, 2015

TiO2 is commonly used as the active switching layer in resistive random access memory. The electrical characteristics of these devices are directly related to the fundamental conditions inside the TiO2 layer and at the interfaces between it and the surrounding electrodes. However, it is complex to disentangle the effects of film “bulk” properties and interface phenomena. The present work uses hard X-ray photoemission spectroscopy (HAXPES) at different excitation energies to distinguish between these regimes. Changes are found to affect the entire thin film, but the most dramatic effects are confined to an interface. These changes are connected to oxygen ions moving and redistributing within the film. Based on the HAXPES results, post-deposition annealing of the TiO2 thin film was investigated as an optimisation pathway in order to reach an ideal compromise between device resistivity and lifetime. The structural and chemical changes upon annealing are investigated using X-ray absorption spectroscopy and are further supported by a range of bulk and surface sensitive characterisation methods. In summary, it is shown that the management of oxygen content and interface quality is intrinsically important to device behavior and that careful annealing procedures are a powerful device optimisation technique.

close
Emulating short-term synaptic dynamics with memristive devices
Authors:
  R.Berdan, E. Vasilaki, A. Khiat, G. Indiveri, A. Serb, and T. Prodromakis. Published by: Scientific Reports, 6, 18639, 2016

Neuromorphic architectures offer great promise for achieving computation capacities beyond conventional Von Neumann machines. The essential elements for achieving this vision are highly scalable synaptic mimics that do not undermine biological fidelity. Here we demonstrate that single solid-state TiO2 memristors can exhibit non-associative plasticity phenomena observed in biological synapses, supported by their metastable memory state transition properties. We show that, contrary to conventional uses of solid-state memory, the existence of rate-limiting volatility is a key feature for capturing short-term synaptic dynamics. We also show how the temporal dynamics of our prototypes can be exploited to implement spatio-temporal computation, demonstrating the memristors full potential for building biophysically realistic neural processing systems.

close
Spatially resolved TiOx phases in RRAM conductive nanofilaments using soft X-ray spectromicroscopy
Authors:
  D. Carta, A. Hitchcock, P. Guttmann, A. Regoutz, A. Khiat, A. Serb, I. Gupta, and T. Prodromakis. Published by: Scientific Reports, 6, 21525, 2016

Reduction in metal-oxide thin films has been suggested as the key mechanism responsible for forming conductive nanofilaments within solid-state memory devices, enabling their resistive switching capacity. The quantitative spatial identification of such filaments is a daunting task, particularly for metal-oxides capable of exhibiting multiple phases as in the case of TiOx. Here, we spatially resolve and chemically characterize distinct TiOx phases in localized regions of a TiOx-based memristive device by combining full-field transmission X-ray microscopy with soft X-ray spectroscopic analysis that is performed on lamella samples. We particularly show that electrically pre-switched devices in low-resistive states comprise reduced disordered phases with O/Ti ratios close to Ti2O3 stoichiometry that aggregate in a ~ 100 nm filamentary region electrically conducting the top and bottom electrodes of the devices. We have also identified crystalline rutile and orthorhombic-like TiO2 phases in the region adjacent to the filament, suggesting that the temperature increases locally up to 1000 K, validating the role of Joule heating in resistive switching. Contrary to previous studies, our approach enables to simultaneously investigate morphological and chemical changes in a quantitative manner without incurring difficulties imposed by interpretation of electron diffraction patterns acquired via conventional electron microscopy techniques.

close
An FPGA-based instrument for en-masse RRAM characterisation with ns pulsing resolution
Authors:
  J. Xing, A. Serb, A. Khiat, R. Berdan, X. Hui and T. Prodromakis. Published by: IEEE Trans. CAS-I, vol. 63, no. 6, 2016.

An FPGA-based instrument with capabilities of on-board oscilloscope and nanoscale pulsing (70 ns @ ±10 V) is presented, thus allowing exploration of the nano-scale switching of RRAM devices. The system possesses less than 1% read-out error for resistance range between 1 kΩ to 1 MΩ, and demonstrated its functionality on characterizing solid-state prototype RRAM devices on wafer; devices exhibiting gradual switching behavior under pulsing with duration spanning between 30 ns to 100 μs. The data conversion error-induced degradation on read-out accuracy is studied extensively and verified by standard linear resistor measurements. The integrated oscilloscope capability extends the versatility of our instrument, rendering a powerful tool for processing development of emerging memory technologies but also for testing theoretical hypotheses arising in the new field of memristors.

close
Experimental study of gradual/abrupt dynamics of HfO2-based memristive devices
Authors:
  S. Brivio, E. Covi, A. Serb, T. Prodromakis, M. Fanciulli, and S. Spiga. Applied Physics Lett., in press

close
On the Origin of Resistive Switching Volatility in Ni/TiO2/Ni stacks
Authors:
  S. Cortese, M. Trapatseli, A. Khiat and T. Prodromakis. Published by: Journal of Applied Physics, 120, 065104, 2016

Resistive switching and resistive random access memories have attracted huge interest for next generation nonvolatile memory applications, also thought to be able to overcome flash memories limitations when arranged in crossbar arrays. A cornerstone of their potential success is that the toggling between two distinct resistance states, usually a High Resistive State (HRS) and a Low Resistive State (LRS), is an intrinsic non-volatile phenomenon with the two states being thermodynamically stable. TiO2 is one of the most common materials known to support non-volatile RS. In this paper, we report a volatile resistive switching in a titanium dioxide thin film sandwiched by two nickel electrodes. The aim of this work is to understand the underlying physical mechanism that triggers the volatile effect, which is ascribed to the presence of a NiO layer at the bottom interface. The NiO layer alters the equilibrium between electric field driven filament formation and thermal enhanced ion diffusion, resulting in the volatile behaviour. Although the volatility is not ideal for non-volatile memory applications, it shows merit for access devices in crossbar arrays due to its high LRS/HRS ratio, which are also briefly discussed.

close
Real-time encoding and compression of neuronal spikes by metal-oxide memristor
Authors:
  I. Gupta, A. Serb, A. Khiat, R. Zeitler, S. Vassanelli, and T. Prodromakis. Nature Communications, in press.

Advanced brain - chip interfaces with numerous recording sites bear great potential for investigation of neuroprosthetic applications. The bottleneck towards achieving a n efficient bio - electronic link is the real - time processing of neuronal signals, which imposes excessive requirements on bandwidth, energy and computation capacity. Here, we present a unique co ncept where the intrinsic properties of memristive devices are exploited to compress information on neural spikes in real time . W e demonstrate that the inherent voltage thresholds of metal - oxide memristors can be used for discriminating recorded spiking events from background activity and without resorting to computationally heavy off - line processing . W e prove that information on spike amplitude and frequency can be transduced and stored in single devices as non - volatile resistive state transitions. Finally, we show that a memristive device array allows for efficient data compression of signals recorded by a multielectrode array, demons trating the technology’s potential for building scalable, yet energy - efficient on - node processors for brain - chip interfaces.

close
Unsupervised learning in probabilistic neural networks with multi-state metal-oxide memristive synapses
Authors:
  A. Serb, J. Bill, A. Khiat, R. Berdan, R. Legenstein and T. Prodromakis. Nature Comms, in press.

close
Improving detection accuracy of memristor-based bio-signal sensing platform
Authors:
  I. Gupta, A. Serb, A. Khiat, and T. Prodromakis. IEEE Trans. on Biomedical Circuits and Systems, in press

Recently a novel neuronal activity sensor exploiting the intrinsic thresholded integrator capabilities of memristor devices has been proposed. Extracellular potentials captured by a standard bio-signal acquisition platform are fed into a memristive device which reacts to the input by changing its resistive state (RS) only when the signal ampitude exceeds a threshold. Thus, significant peaks in the neural signal can be stored as non-volatile changes in memristor resistive state whilst noise is effectively suppressed. However, as a memristor is subjected to increasing numbers of supra-threshold stimuli during practical operation, it accumulates (RS) changes and eventually saturates. This leads to severely reduced neural activity detection capabilities. In this work we explore different signal processing and memristor operating procedure strategies in order to improve the detection rate of significant neuronal activity events. We analyse the data obtained from a single-memristive device biased with a reference neural recording and observe that performance can be improved markedly by a) increasing the frequency at which the memristor is reset to an initial resistive state where it is known to be highly responsive, b) appropriately preconditioning the input waveform through application of gain and offset in order to optimally exploit the intrinsic device behaviour. All results are validated by benchmarking obtained spike detection performance against a state-of-the-art template matching system utilising computationally-heavy, multi-dimensional, principal component analysis.

close
Engineering the switching dynamics of TiO2 RRAM with Al doping
Authors:
  M. Trapatseli, A. Khiat, S. Cortese, A. Serb, D. Carta and T. Prodromakis. Published by: Journal of Applied Physics, 120, 025108, 2016.

close
Investigation of the switching mechanism in TiO2-based RRAM: a two-dimensional EDX approach
Authors:
  D.Carta, I. Salaoru, A. Regoutz, A. Khiat, C. Mitterbauer, N.M. Harrison and T. Prodromakis. Published by: ACS Applied Materials and Interfaces, 10.1021/acsami.6b04919, 2016

The next generation of nonvolatile memory storage may well be based on resistive switching in metal oxides. TiO2 as transition metal oxide has been widely used as active layer for the fabrication of a variety of multistate memory nanostructure devices. However, progress in their technological development has been inhibited by the lack of a thorough understanding of the underlying switching mechanisms. Here, we employed high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with two-dimensional energy dispersive X-ray spectroscopy (2D EDX) to provide a novel, nanoscale view of the mechanisms involved. Our results suggest that the switching mechanism involves redistribution of both Ti and O ions within the active layer combined with an overall loss of oxygen that effectively render conductive filaments. Our study shows evidence of titanium movement in a 10 nm TiO2 thin-film through direct EDX mapping that provides a viable starting point for the improvement of the robustness and lifetime of TiO2-based resistive random access memory (RRAM).

close
High Density Crossbar Arrays with Sub- 15 nm Single Cells via Liftoff Process Only
Authors:
  A.Khiat, P. Ayliffe and T. Prodromakis. Published by: Scientific Reports, 6, 32614, 2016.

Emerging nano-scale technologies are pushing the fabrication boundaries at their limits, for leveraging an even higher density of nano-devices towards reaching 4F2/cell footprint in 3D arrays. Here, we study the liftoff process limits to achieve extreme dense nanowires while ensuring preservation of thin film quality. The proposed method is optimized for attaining a multiple layer fabrication to reliably achieve 3D nano-device stacks of 32 × 32 nanowire arrays across 6-inch wafer, using electron beam lithography at 100 kV and polymethyl methacrylate (PMMA) resist at different thicknesses. The resist thickness and its geometric profile after development were identified to be the major limiting factors, and suggestions for addressing these issues are provided. Multiple layers were successfully achieved to fabricate arrays of 1 Ki cells that have sub- 15 nm nanowires distant by 28 nm across 6-inch wafer.