Oxide-based Resistive Switching Devices Research Articles

Effect of annealing on forming-free bipolar resistive switching of Gd2O3 thin films

Oxide-based resistive switching devices are a promising candidate for non-volatile memory applications. Here, we present a Gd2O3-based non-volatile memory device deposited using e-beam evaporation. In order to know the effect of annealing on the device performance, the as-grown sample was annealed at 500 °C. The capacitance (C) – voltage (V) and conductance (G) – voltage (V) measurements were investigated in the range of 200 kHz to 1 MHz. It was observed that the C-V and G-V changes with frequency due to the presence of interface states. The effect of series resistance (Rs) and interface state density (Dit) was also investigated for both devices. Furthermore, the resistive switching was recorded at± 5 V. Both devices exhibit bipolar resistive switching with charge conduction mechanism ascribed to ohmic, space charge limited conduction, and trap-filled space charge limited conduction. Consequently, the annealed device displayed an improved resistance ratio of 102 and a stable endurance of up to 1000 cycles with better data retention of over 103 s.

Investigating unipolar switching in Niobium oxide resistive switches: Correlating quantized conductance and mechanism

Memory devices based on resistive switching (RS) have not been fully realised due to lack of understanding of the underlying switching mechanisms. Nature of ion transport responsible for switching and growth of conducting filament in transition metal oxide based RS devices is still in debate. Here, we investigated the mechanism in Niobium oxide based RS devices, which shows unipolar switching with high ON/OFF ratio, good endurance cycles and high retention times. We controlled the boundary conditions between low-conductance insulating and a high-conductance metallic state where conducting filament (CF) can form atomic point contact and exhibit quantized conductance behaviour. Based on the statistics generated from quantized steps data, we demonstrated that the CF is growing atom by atom with the applied voltage sweeps. We also observed stable quantized states, which can be utilized in multistate switching.

Patching of Lattice Defects in Two-Dimensional Diffusion Barriers

Two-dimensional crystals offer promise as diffusion barriers that can also facilitate electronic conduction through the barrier plane via tunneling. We present barriers in which crystal imperfections are patched, leaving the pristine regions of the crystal exposed and able to both prevent diffusion and allow electronic conduction. This is accomplished by atomic layer deposition, where nucleation of patch material is inhibited on the pristine crystal and promoted elsewhere. Demonstrations of the effectiveness of this technique are performed in the contexts of sulfur diffusion control in photovoltaic kesterite devices and oxygen diffusion control in oxide-based resistive switching devices.

Synthesis of large-area multilayer hexagonal boron nitride sheets on iron substrates and its use in resistive switching devices

Hexagonal boron nitride (h-BN) is an attractive insulating material for nanoelectronic devices due to its high reliability as dielectric and excellent compatibility with other two dimensional (2D) materials (e.g. graphene, MoS2). Multilayer h-BN stacks have been readily grown on Cu and Pt substrates via chemical vapor deposition (CVD) approach, confirming its potential for wafer scale integration. However, the growth of h-BN on other substrates needs to be also achieved in order to expand the use of this material. Recently, the CVD growth of monolayer h-BN on Fe substrates was reported, but it just focused on material structure characterization. Here we present the first fabrication of electronic devices using multilayer h-BN dielectric stacks grown on Fe foils. We fabricate and characterize resistive switching (RS) devices based on Au/Ag/h-BN/Fe nanojunctions, and observe the coexistence of both volatile and non-volatile RS depending on the electrode to which the bias is applied. The characteristics measured agree well with those simulated via SIM2RRAM software and QPC modeling, and the cycle-to-cycle variability is slightly lower than that of transition metal oxide based RS devices.

Exploring oxygen-affinity-controlled TaN electrodes for thermally advanced TaOx bipolar resistive switching

Recent advances in oxide-based resistive switching devices have made these devices very promising candidates for future nonvolatile memory applications. However, several key issues remain that affect resistive switching. One is the need for generic alternative electrodes with thermally robust resistive switching characteristics in as-grown and high-temperature annealed states. Here, we studied the electrical characteristics of Ta2O5−x oxide-based bipolar resistive frames for various TaNx bottoms. Control of the nitrogen content of the TaNx electrode is a key factor that governs variations in its oxygen affinity and structural phase. We analyzed the composition and chemical bonding states of as-grown and annealed Ta2O5−x and TaNx layers and characterized the TaNx electrode-dependent switching behavior in terms of the electrode’s oxygen affinity. Our experimental findings can aid the development of advanced resistive switching devices with thermal stability up to 400 °C.

(Invited) Atomistic Simulations for Understanding Microscopic Mechanism of Amorphous-Tantalum-Oxide-Based Resistive Switching Devices

Among oxide-based resistive switching devices, amorphous-TaOx based ones show good endurance and switching performances. Their switching behaviors are understood based on the formation/rupture of conductive filaments (CFs). However, the atomistic structures of CFs and their microscopic formation/rupture processes have not been sufficiently clarified yet. In this talk, we present the results of our atomistic simulations (most of which are with the density functional theory (DFT)) and discuss several aspects related to the nature and formation processes of CFs as follows. 1) CFs in the Cu/amorphous-Ta2O5/Pt heterostructure: Our simulations reveals that single Cu chains in Ta2O5 cannot work as conductive filaments (CFs), while Cu nanowires with a diameter of three atoms or larger can work as CFs. The stability of the Cu nanowires has been checked by ab initio molecular dynamics [1]. 2) CFs in the Pt/TaOx/Pt heterostructure with x < 2.5: Our results suggest that not O vacancy chain but the Ta-Ta bonding mainly contributes to the CFs [2].3) Difference in the atomistic features between Cu/ Ta2O5 and Pt/Ta2O5 interfaces: In the former, considerable portion of interface Cu atoms tend to migrate to the amorphous Ta2O5 layer, while similar behavior is not seen in the latter [3].4) Ion migration behaviors in TaOx: Our results of ab initio molecular dynamics and nudged elastic band calculations show that the diffusion coefficient in TaOx strongly depends on x [3]. 5) Construction of neural network interatomic potentials: We constructed interatomic potential combining the DFT calculations and neural network for simulations of Cu migration behavior in amorphous-Ta2O5 to achieve computation speed and reliability simultaneously. The pathways and barrier energies for Cu diffusion calculated using the NN potential agree well with those obtained from DFT calculations [5]. The final part of the present works was partly supported by the Support Program for Starting up Innovation hub from Japan Science and Technology Agency (JST), and CREST-JST, Japan.[1] B. Xiao, T. Gu, T. Tada and S. Watanabe, J. Appl. Phys. 115, 034503 (2014).[2] B. Xiao and S. Watanabe, Nanoscale 6, 10169 (2014).[3] B. Xiao and S. Watanabe, ACS Appl. Mater. Interfaces 7, 519 (2015).[4] B. Xiao and S. Watanabe, Extended abstract of SSDM2014 (September 8-11, 2014, Tsukuba, Japan).[5] W. Li, Y. Ando and S. Watanabe, submitted.

Direct Evidence for the Role of Oxygen Vacancy Configuration for Oxide-Based Resistive Switching Devices

The miniaturization of standard silicon-based memory technologies reaches physical limits in terms of size and power dissipation. Novel memory and computing architectures based on resistive switches are promising for future electronics beyond Von Neumann computing architecture. In oxide-based resistive switches, the memristance is associated with the movement of ionic carriers through oxygen vacancies in an oxide film at high electric field strength via structural defect modifications that are still poorly understood. Here, we newly employ an oxide solid solution by systematic extrinsic doping to probe the role of oxygen vacancy configuration either set as free, immobile or clustered to directly answer the fundamental question how the mobility of oxygen vacancies affects the switching characteristics of the devices at high electric fields. Till date strategies to tailor systematically oxygen vacancy concentration and configuration for oxide-based resistive switches are still to be explored. For this, we design resistive switching devices of Ce1-xGdxO2-y as a novel resistive switching oxide constituent. The active oxide layer of the devices, consisting of a 500 nm thick CeO2 film with Gd3+ doping concentrations of 3 to 30 mol% deposited by Pulsed Laser Deposition (PLD), is sandwiched between micro-fabricated Pt-crosspoint electrodes. Cyclovoltammetry measurements show a strong dependence of the ROFF/RON -resistance ratio on the effective gadolinia concentration per bit. At low gadolinia doping concentration < 10 mol%, a purely capacitive behaviour prevails. By increasing up to 20 mol% a significant hystereis in the current-voltage profile is measured with resistance ROFF/RON -ratios up to 100. Turning to high gadolinia concentrations from 20 to 30 mol%, decreases the hysteretic current-voltage profile. To analyse the structural changes related to the oxygen vacancy concentration and configuration, changes opposed by the extrinsic gadolinia dopant in the ceria lattice of the resistive switching devices, we turn to Raman Spectroscopy. From these results we conclude that for moderate gadolinium concentrations up to 20 mol% the concentration of mobile oxygen vacancies steadily increases and is available as "free" carriers for the resistive switches; in this state the oxide is in a fluorite structure type. Further increase in doping, for which we see a reduction of switching characteristics, leads to the formation of oxygen vacancy clusters in the structure, which are simply as a defect configuration "immobile". For this state we confirm a phase change to a more bixbyite structure. Unequivocally the experiments together with the defect chemical model, show that when the vacancies are set as "free", a maximum in memristance is found for 20 mol% Gd3+ doping, which clearly coincides with the maximum in ionic conductivity. In contrast, for higher gadolinia concentration the oxide exhibits only minor memristance, which originates from the decrease in structural symmetry leading to the formation of "immobile" oxygen defect clusters, thereby reducing the density of mobile ionic carriers available for resistive switching. A clear correlation between memristance and oxygen vacancies’ mobility was seen in this defect chemical model experiment on gadolinium doped ceria device structures. Extrinsic doping allows to tune the configuration, concentration and mobility of the oxygen vacancies in a much more systematic and controlled manner than what can be achieved through oxygen-deficient deposition or annealing steps as being the state-of-the-art in resistive switches. The research demonstrates guidelines for engineering of the oxide`s solid solution series to set the configuration of its oxygen vacancy defects and their mobility to tune the memristance for future nonvolatile memory and logic applications.

In situ transmission electron microscopy of resistive switching in thin silicon oxide layers

Silicon oxide-based resistive switching devices show great potential for applications in nonvolatile random access memories. We expose a device to voltages above hard breakdown and show that hard oxide breakdown results in mixing of the SiOx layer and the TiN lower contact layers. We switch a similar device at sub-breakdown fields in situ in the transmission electron microscope (TEM) using a movable probe and study the diffusion mechanism that leads to resistance switching. By recording bright-field (BF) TEM movies while switching the device, we observe the creation of a filament that is correlated with a change in conductivity of the SiOx layer. We also examine a device prepared on a microfabricated chip and show that variations in electrostatic potential in the SiOx layer can be recorded using off-axis electron holography as the sample is switched in situ in the TEM. Taken together, the visualization of compositional changes in ex situ stressed samples and the simultaneous observation of BF TEM contrast...

Design and Application of Oxide-Based Resistive Switching Devices for Novel Computing Architectures

Resistive switching behaviors of oxide-based resistive random access memory (RRAM) and the applications for the data storage and computing systems have been widely studied. In this paper, the critical issues correlated with the applications of oxide-based RRAM are addressed. The physical mechanism and models of oxide-based RRAM are first introduced. Then the optimization design methodology of devices and operation schemes is discussed. Finally, a novel architecture for unifying memory/computing system is presented.

Ultra-low-energy three-dimensional oxide-based electronic synapses for implementation of robust high-accuracy neuromorphic computation systems.

Neuromorphic computing is an attractive computation paradigm that complements the von Neumann architecture. The salient features of neuromorphic computing are massive parallelism, adaptivity to the complex input information, and tolerance to errors. As one of the most crucial components in a neuromorphic system, the electronic synapse requires high device integration density and low-energy consumption. Oxide-based resistive switching devices have been shown to be a promising candidate to realize the functions of the synapse. However, the intrinsic variation increases significantly with the reduced spike energy due to the reduced number of oxygen vacancies in the conductive filament region. The large resistance variation may degrade the accuracy of neuromorphic computation. In this work, we develop an oxide-based electronic synapse to suppress the degradation caused by the intrinsic resistance variation. The synapse utilizes a three-dimensional vertical structure including several parallel oxide-based resistive switching devices on the same nanopillar. The fabricated three-dimensional electronic synapse exhibits the potential for low fabrication cost, high integration density, and excellent performances, such as low training energy per spike, gradual resistance transition under identical pulse training scheme, and good repeatability. A pattern recognition computation is simulated based on a well-known neuromorphic visual system to quantify the feasibility of the three-dimensional vertical structured synapse for the application of neuromorphic computation systems. The simulation results show significantly improved recognition accuracy from 65 to 90% after introducing the three-dimensional synapses.

A Novel Operation Scheme for Oxide-Based Resistive-Switching Memory Devices to Achieve Controlled Switching Behaviors

A new operation scheme on oxide-based resistive-switching devices [resistive random access memory (RRAM)] is proposed to improve the controllability of switching processes in order to achieve an improved memory performance. The improved device-to-device and cycle-to-cycle uniformity, reduced RESET current, and adjustable R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HRS</sub> /R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LRS</sub> ratio are demonstrated in the HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> -based RRAM devices by using the new operation scheme, indicating the validity of the new operation scheme. The physical mechanism accounting for the new operation scheme effect is discussed.