Redox-based Memristive Devices Research Articles

Interfacial Thermal Resistive Switching in (Pt,Cr)/SrTiO3 Devices.

The operation of oxide-based memristive devices relies on the fast accumulation and depletion of oxygen vacancies by an electric field close to the metal-oxide interface. Here, we show that the reversible change of the local concentration of oxygen vacancies at this interface also produces a change in the thermal boundary resistance (TBR), i.e., a thermal resistive switching effect. We used frequency domain thermoreflectance to monitor the interfacial metal-oxide TBR in (Pt,Cr)/SrTiO3 devices, showing a change of ≈20% under usual SET/RESET operation voltages, depending on the structure of the device. Time-dependent thermal relaxation experiments suggest ionic rearrangement along the whole area of the metal/oxide interface, apart from the ionic filament responsible for the electrical conductivity switching. The experiments presented in this work provide valuable knowledge about oxide ion dynamics in redox-based memristive devices.

Electrochemical-Memristor-Based Artificial Neurons and Synapses-Fundamentals, Applications, and Challenges.

Artificial neurons and synapses are considered essential for the progress of the future brain-inspired computing, based on beyond von Neumann architectures. Here, a discussion on the common electrochemical fundamentals of biological and artificial cells is provided, focusing on their similarities with the redox-based memristive devices. The driving forces behind the functionalities and the ways to control them by an electrochemical-materials approach are presented. Factors such as the chemical symmetry of the electrodes, doping of the solid electrolyte, concentration gradients, and excess surface energy are discussed as essential to understand, predict, and design artificial neurons and synapses. A variety of two- and three-terminal memristive devices and memristive architectures are presented and their application for solving various problems is shown. The work provides an overview of the current understandings on the complex processes of neural signal generation and transmission in both biological and artificial cells and presents the state-of-the-art applications, including signal transmission between biological and artificial cells. This example is showcasing the possibility for creating bioelectronic interfaces and integrating artificial circuits in biological systems. Prospectives and challenges of the modern technology toward low-power, high-information-density circuits are highlighted.

Multilayer redox-based HfOx/Al2O3/TiO2 memristive structures for neuromorphic computing

Redox-based memristive devices have shown great potential for application in neuromorphic computing systems. However, the demands on the device characteristics depend on the implemented computational scheme and unifying the desired properties in one stable device is still challenging. Understanding how and to what extend the device characteristics can be tuned and stabilized is crucial for developing application specific designs. Here, we present memristive devices with a functional trilayer of HfOx/Al2O3/TiO2 tailored by the stoichiometry of HfOx (x = 1.8, 2) and the operating conditions. The device properties are experimentally analyzed, and a physics-based device model is developed to provide a microscopic interpretation and explain the role of the Al2O3 layer for a stable performance. Our results demonstrate that the resistive switching mechanism can be tuned from area type to filament type in the same device, which is well explained by the model: the Al2O3 layer stabilizes the area-type switching mechanism by controlling the formation of oxygen vacancies at the Al2O3/HfOx interface with an estimated formation energy of ≈ 1.65 ± 0.05 eV. Such stabilized area-type devices combine multi-level analog switching, linear resistance change, and long retention times (≈ 107–108 s) without external current compliance and initial electroforming cycles. This combination is a significant improvement compared to previous bilayer devices and makes the devices potentially interesting for future integration into memristive circuits for neuromorphic applications.

Subfilamentary Networks and Their Dynamics As Source of Cycle-to-Cycle Variability in Reram Devices

One major obstacle for the implementation of redox-based memristive devices in non-volatile memory or neuromorphic computing is the large cycle-to-cycle and device-to-device variability [1]. Here, we combine different complementary analysis methods such as atomic force microscopy (AFM), thermography and operando photoelectron emission spectroscopy (PEEM) [2] to monitor the filament shape and position in memristive SrTiO3 cells and correlate it to the observed variability of the resistance values. Our operando PEEM analysis revealed that some devices exhibit cycle-dependent variations in the shape of the conductive filament or in the oxygen vacancy distribution at and around the filament [3]. In other cases, even the location of the active filament changes from one cycle to the next. We propose that both effects originate from the coexistence of multiple (sub-)filaments observed by transmission electron microscopy [4] and that the active, current-carrying filament may change from cycle to cycle. Besides this, thermal imaging enabled us to monitor the position of the filament during repeated cycling. During cycling of some devices, we observed a movement of the filament towards the edge of the device that goes along with a drift of the low resistive state. As soon as the filament reaches the boarder, the resistance value stabilizes and remains constant during the following cycles. By modifying the interface at the top electrode, we could engineer the position of the filament and its stability against cycling. Our studies thereby represent the scientific basis, rather than prior purely empirical engineering approaches, for developing stable memristive devices.

Redox-based memristive devices for new computing paradigm

Memristive devices have been a hot topic in nanoelectronics for the last two decades in both academia and industry. Originally proposed as digital (binary) nonvolatile random access memories, research in this field was predominantly driven by the search for higher performance solid-state drive technologies (e.g., flash replacement) or higher density memories (storage class memory). However, based on their large dynamic range in resistance with analog-tunability along with complex switching dynamics, memristive devices enable revolutionary novel functions and computing paradigms. We present the prospects, opportunities, and materials challenges of memristive devices in computing applications, both near and far terms. Memristive devices offer at least three main types of novel computing applications: in-memory computing, analog computing, and state dynamics. We will present the status in the understanding of the most common redox-based memristive devices while addressing the challenges that materials research will need to tackle in the future. In order to pave the way toward novel computing paradigms, a rational design of the materials stacks will be required, enabling nanoscale control over the ionic dynamics that gives these devices their variety of capabilities.

Examining Oxygen Exchange Kinetics of Anomalous Resistive Switching By Optical Tracking in Redox Based Memristive Devices

Anomalous resistive switching in redox based memristive devices has recently gained increased attention due to compelling demonstrations of its origin related to homogenous oxygen exchange with device electrodes [1]. Insights into this mechanism are expected to improve understanding of the origins of the competing switching mechanism in mixed valent metal oxide memristive devices. Previously, open systems were shown to be susceptible to oxygen exchange at triple phase boundaries[2] and to the incorporation of humidity,[3,4] under applied voltages, from the gas environment. These environmental exchanges can result in modified space charge layers and surface defect chemistries that in turn, can lead to modified resistive switching properties. Strategies to mitigate these effects have relied on the addition of an interlayer acting as a source of oxygen defects, also known as an exchange layer [5–7]. While exchange between the switching layer and the interlayer has been identified as the origin of resistance change, fundamental aspects of the exchange kinetics remain unclear. Vacuum based techniques, relying on compositional analysis such as EDS in TEM, have been used to demonstrate changes in chemistry, but provided limited information about the details of the exchange kinetics. Being able to track the dynamics of oxygen migration through the metal oxide semiconductor volume, and exchange at the interface over the entire area of the device, could provide insights into the kinetics and thermodynamic limitations responsible for switching speed and device retention. Mastering these two properties is fundamental for the development of anomalous resistive switching elements with increased stability, faster switching speeds and larger hysteresis. To gain further insight into the oxygen exchange kinetics, we investigated the resistive switching properties of a bilayer oxide system based on PrxCe1-xO2/CeO2 stacks, sandwiched between transparent electrodes. Our study focuses on the model material, PrxCe1-xO2 (PCO), whose defect chemical and transport properties have been previously studied both in bulk and thin film forms[8,9]. We leverage the existence of an interband gap optical absorption, related to the oxidation state of Pr ions, to monitor the changes in the film’s non-stoichiometry. By carefully processing the sandwiched PCO, it is possible to quench in different degrees of oxygen non-stoichiometry to room temperature, as confirmed by optical absorption and Raman spectroscopy. By engineering the defect chemistry of PCO and using CeO2 as an exchange layer, we investigate and correlate the switching dynamics via electrical and optical measurements to gain insight into the speed and the amount of oxygen exchanged between the two layers. [1] D. Cooper, C. Baeumer, N. Bernier, A. Marchewka, C. La Torre, R. E. Dunin-Borkowski, S. Menzel, R. Waser, R. Dittmann, Adv. Mater. 2017, 29, 1. [2] C. Lenser, M. Patt, S. Menzel, A. Köhl, C. Wiemann, C. M. Schneider, R. Waser, R. Dittmann, Adv. Funct. Mater. 2014, 24, 4466. [3] M. Lübben, S. Wiefels, R. Waser, I. Valov, Adv. Electron. Mater. 2017, 1700458, 1700458. [4] E. Sediva, W. J. Bowman, J. C. Gonzalez-Rosillo, J. L. M. Rupp, Adv. Electron. Mater. 2018, 1800566, 1. [5] M. Ismail, Shafqat-Un Nisa, T. Akbar, A. M. Rana, Jinju Lee, S. Kim, 2019, 012101. [6] J. C. Gonzalez-rosillo, R. Ortega-hernandez, B. Arndt, M. Coll, 2019, 1800629, 1. [7] Z. Alamgir, J. Holt, K. Beckmann, N. C. Cady, Semicond. Sci. Technol. 2018, 33. [8] S. R. Bishop, T. S. Stefanik, H. L. Tuller, Phys. Chem. Chem. Phys. 2011, 13, 10165. [9] J. J. Kim, S. R. Bishop, N. Thompson, Y. Kuru, H. L. Tuller, Solid State Ionics 2012, 225, 198.

Memristive Devices Formed By ALD Metal Oxide Growth on a Hafnium Layer – Study of the Interfacial HfO2 Formation

Redox-based memristive devices (ReRAM) are considered for future energy-efficient non-volatile memory. Memristive cells enable high speed data access, low power consumption and high scalability, which is a key requirement for their use as artificial synapses in neuromorphic computing. Applications in networks of up to 106 devices require a scaling of the cell volume to about 103 nm3. For a typical valence-change-mechanism (VCM)-type switching cell with a metal oxide, like HfO2, sandwiched between two metal electrodes of different oxidation enthalpy, this means that the individual layer thickness is in the range of a few nanometers. Therefore, a control of the device properties demands for a control of interfacial reactions during device fabrication. Here, we are going to present a careful analysis of the oxidation of an Hf electrode layer during atomic layer deposition (ALD) of different metal oxides, these are Al2O3, TiO2, and HfO2. The in-situ encapsulation of a reactive metal electrode by means of an ALD metal oxide layer enables a better process control compared to stack structures formed by breaking the vacuum conditions.In this study, plasma and thermal ALD processes were used to grow ultrathin layers of Al2O3, TiO2, and HfO2 on top of Hf metal films, after transfer under UHV conditions. The growth of homogeneous metal oxide films of a few nanometer in thickness enabled a systematic analysis of the HfO2 formation at the Hf/metal oxide interface by means of angle dependent X-ray photoelectron spectroscopy (XPS). The results show that the properties of the interfacial HfO2 layer can be controlled by the metal oxide deposited on top as well as by the process chemistry and deposition temperature. In contrast to the TiO2 film, the Al2O3 layer clearly serves as oxidation barrier. In addition, the chemistry and structure of the HfO2 interfacial layer were analyzed from XPS and transmission electron microscopy analysis.Memristive device were fabricated from the Hf/HfO2/MOx stacks by depositing Pt top electrodes. The initial current of the devices scales with the pad size indicating the high quality of the insulating oxide layers. Consistent with VCM-type memristive devices the cells required an electroforming voltage to enable bipolar-type resistive switching. In this presentation, the trade-off between the two functionalities of oxidation protection versus enabling of a low electroforming voltage will be discussed.

Topotactic Phase Transition Driving Memristive Behavior.

Redox-based memristive devices are one of the most attractive candidates for future nonvolatile memory applications and neuromorphic circuits, and their performance is determined by redox processes and the corresponding oxygen-ion dynamics. In this regard, brownmillerite SrFeO2.5 has been recently introduced as a novel material platform due to its exceptional oxygen-ion transport properties for resistive-switching memory devices. However, the underlying redox processes that give rise to resistive switching remain poorly understood. By using X-ray absorption spectromicroscopy, it is demonstrated that the reversible redox-based topotactic phase transition between the insulating brownmillerite phase, SrFeO2.5 , and the conductive perovskite phase, SrFeO3 , gives rise to the resistive-switching properties of SrFeOx memristive devices. Furthermore, it is found that the electric-field-induced phase transition spreads over a large area in (001) oriented SrFeO2.5 devices, where oxygen vacancy channels are ordered along the in-plane direction of the device. In contrast, (111)-grown SrFeO2.5 devices with out-of-plane oriented oxygen vacancy channels, reaching from the bottom to the top electrode, show a localized phase transition. These findings provide detailed insight into the resistive-switching mechanism in SrFeOx -based memristive devices within the framework of metal-insulator topotactic phase transitions.

Oxygen vacancy dynamics in redox-based interfaces: tailoring the memristive response

Redox-based memristive devices are among the alternatives for the next generation of non-volatile memories, but are also candidates for emulating the behavior of synapses in neuromorphic computing devices. Nowadays it is well established that the motion of oxygen vacancies at the nanoscale is the key mechanism for reversible switching of metal/insulator/metal structures from insulating to conducting, i.e. to accomplish the resistive switching effect. The control of oxygen vacancy dynamics has a direct effect on the resistance changes, and therefore on different properties of memristive devices such as switching speed, retention, endurance and energy consumption. Advances in this direction demand not only experimental techniques that allow the measurement of oxygen vacancy profiles but also theoretical studies that shed light on the mechanisms involved. With these goals in mind we analyze the oxygen vacancy dynamics in redox interfaces formed when an oxidizable metallic electrode is in contact with the insulating oxide. We show how the transfer of oxygen vacancies can be manipulated by the use of different electrical stimulus protocols that allow optimization of device figures such as the ON/OFF ratio or writing energy dissipation. Analytical expressions for both high- and low-resistance states are derived in terms of total oxygen vacancies transferred at the interface. Our predictions are validated with experiments performed in Ti/La1/3Ca2/3MnO3 redox memristive devices.

Anomalous Resistance Hysteresis in Oxide ReRAM: Oxygen Evolution and Reincorporation Revealed by In Situ TEM.

The control and rational design of redox-based memristive devices, which are highly attractive candidates for next-generation nonvolatile memory and logic applications, is complicated by competing and poorly understood switching mechanisms, which can result in two coexisting resistance hystereses that have opposite voltage polarity. These competing processes can be defined as regular and anomalous resistive switching. Despite significant characterization efforts, the complex nanoscale redox processes that drive anomalous resistive switching and their implications for current transport remain poorly understood. Here, lateral and vertical mapping of O vacancy concentrations is used during the operation of such devices in situ in an aberration corrected transmission electron microscope to explain the anomalous switching mechanism. It is found that an increase (decrease) in the overall O vacancy concentration within the device after positive (negative) biasing of the Schottky-type electrode is associated with the electrocatalytic release and reincorporation of oxygen at the electrode/oxide interface and is responsible for the resistance change. This fundamental insight presents a novel perspective on resistive switching processes and opens up new technological opportunities for the implementation of memristive devices, as anomalous switching can now be suppressed selectively or used deliberately to achieve the desirable so-called deep Reset.

Spectromicroscopic insights for rational design of redox-based memristive devices.

The demand for highly scalable, low-power devices for data storage and logic operations is strongly stimulating research into resistive switching as a novel concept for future non-volatile memory devices. To meet technological requirements, it is imperative to have a set of material design rules based on fundamental material physics, but deriving such rules is proving challenging. Here, we elucidate both switching mechanism and failure mechanism in the valence-change model material SrTiO3, and on this basis we derive a design rule for failure-resistant devices. Spectromicroscopy reveals that the resistance change during device operation and failure is indeed caused by nanoscale oxygen migration resulting in localized valence changes between Ti4+ and Ti3+. While fast reoxidation typically results in retention failure in SrTiO3, local phase separation within the switching filament stabilizes the retention. Mimicking this phase separation by intentionally introducing retention-stabilization layers with slow oxygen transport improves retention times considerably.