Conductive Filament Growth Research Articles

Organic Flexible Memristor with Reduced Operating Voltage and High Stability by Interfacial Control of Conductive Filament Growth

Herein, the underlying mechanisms for the growth of conductive filaments (CFs) at a metal–polymer electrolyte interface through ion migration in organic electrochemical metallization (ECM) memristor are presented. It is observed that the free volume of voids (nanopores) in the polymer electrolyte serves as the pathways of metal‐cations whereas the interfacial topography between an active electrode and a polymer electrolyte determines the nucleation sites of the CFs. The growth kinetics of the CFs and the resultant resistive memory are found to vary with the molecular weight of the polymer electrolyte and the metal protrusions at the interface. Our direct observations show that the free volume of voids of the polymer electrolyte, varied with the molecular weight, dictates the ion transport for the growth and the disruption of the CFs. Our organic ECM‐based memristor with a hetero‐electrolyte exhibits high mechanical flexibility, low switching voltages reduced by about three times compared to those of conventional devices, and stable memory retention for longer than 104 s under repeated cycles of bending.

Investigation of resistive switching in PVP and ultra-thin HfOx based bilayer hybrid RRAM

Hybrid CBRAM devices based on PVP/HfOx bilayer were investigated for their switching performance. A reliable and low voltage bipolar resistive switching operation was observed in these devices with repeatability of 300 cycles, Ion/Ioff of 80, set (Vset) and reset (Vreset) voltages of 1.03 V and −0.68 V, retention time of 7200 s, and switching endurance of >2000 cycles in devices with 2.5 wt% PVP concentration. Low device-to-device variation was observed with 10 devices tested showing nearly similar switching parameters. Weibull's distribution of Vset and Vreset voltages have indicated reliable switching performance in devices. The pinholes formation in PVP layer guides the confined growth and dissolution of Ag conductive filament (CF) in the bilayer structure resulting in a stable, reliable and low voltage switching operation. PVP film roughness and pinholes depth have shown downtrend with PVP concentration. During the set process, a CF formed across the electrodes constitutes of Ag atoms, due to pinholes assisted electro-migration of Ag ions. Whereas in the reset process, the diffused Ag+ ions migrate back towards top electrode due to the electrochemical and joule-heating assisted rupture of CF in the PVP/HfOx bilayer structure.

Stress-Induced Resistive Switching in Pt/HfO2/Ti Devices

In the present work, we study the initial SET mechanism of resistive switching (RS) in Pt/HfO2/Ti devices under a static electrical stress and the RS mechanism under a bias sweeping mode with rates of 100 mV/s–300 mV/s. We characterize the thin HfO2 dielectric layer by x-ray photoelectron spectroscopy and x-ray diffraction. These findings show that the layer structure is stoichiometric and nanocrystalline with a crystal diameter of ∼ 14 A. We measure the temporal dependence of the conductive filament growth at different temperatures and for various biases. Furthermore, these devices present stable bipolar resistive switching with a high-to-low resistive state (HRS/LRS) ratio of more than three orders of magnitude. Activation energy E RS ≈ 0.56 eV and drift current parameter V 0 ≈ 0.07 V were determined from the temporal dependence of the initial ‘SET’ process, first HRS to LRS transition [for static electrical stress of V DS = (4.7–5.0 V)]. We analyze the results according to our model suggesting generation of double-charge oxygen vacancies at the anode and their diffusion across the dielectric layer. The double-charge vacancies transform to a single charge and then to neutral vacancies by capturing hot electrons, and form a conductive filament as soon as a critical neutral-vacancy cluster is formed across the dielectric layer.

Guiding the Growth of a Conductive Filament by Nanoindentation To Improve Resistive Switching.

Redox-based memristor devices, which are considered to have promising nonvolatile memory, mainly operate through the formation/rupture of nanoscale conductive filaments. However, the random growth of conductive filaments is an obstacle for the stability of memory devices and the cell-to-cell uniformity. Here, we investigate the guiding effect of nanoindentation on the growth of conductive filaments in resistive memory devices. The nanoindented top electrodes generate an electric field concentration and the resultant precise control of a conductive filament in two typical memory devices, Ag/SiO2/Pt and W/Ta2O5/Pt. The nanoindented cells possess a much larger ON/OFF ratio, a sharper RESET process, a higher response speed, and better cell-to-cell uniformity compared with the conventional cells. Our finding reflects that the use of large-scale nanotransfer printing might be a unique way to improve the performance of resistive random access memory.

Investigation on the Conductive Filament Growth Dynamics in Resistive Switching Memory via a Universal Monte Carlo Simulator

In resistive random access memories, modeling conductive filament growing dynamics is important to understand the switching mechanism and variability. In this paper, a universal Monte Carlo simulator is developed based on a cell switching model and a tunneling-based transport model. Driven by external electric field, the growing process of the nanoscale filament occurring in the gap region is actually dominated by cells’ conductive/insulating switching, modeled through a phenomenological physics-based probability function. The electric transport through the partially formed CF is considered as current tunneling in the framework of the Quantum Point Contact model, and the potential barrier is modulated during cells’ evolution. To demonstrate the validity and universality of our simulator, various operation schemes are simulated, with the simulated I − V characteristics well explaining experimental observations. Furthermore, the statistical analyses of simulation results in terms of Weibull distribution and conductance evolution also nicely track previous experimental results. Representing a simulation scale that links atomic-scale simulations to compact modeling, our simulator has the advantage of being much faster comparing with other atomic-scale models. Meanwhile, our simulator shows good universality since it can be applied to various operation signals, and also to different electrodes and dielectric layers dominated by different switching mechanisms.

I-V Characteristics of NiO Nanowire Based Resistive Change Memory

1. Introduction Resistive switching random access memory (ReRAM) has been expected to a next generation memory due to high-speed response, non-volatility, low power consumption [1]. However, poor reproducibility of a switching voltage hinders the practical application. According to a filament model, many kinds and number of filaments made at the soft breakdown of an insulator are thought to be the origin. In our previous studies, we employed anodic porous alumina (APA) with ordered nanoholes as the insulator of ReRAM because nanoholes are expected to inhibit the growth of conductive filaments spacially. In the result, we succeeded in suppression of the switching voltage variation [2].Looking at the top view of SEM image of APA (Fig. 1), we notice no restriction along the dashed lines. It means that there exist a division indicating a bulk property. In this study, a nanowire based insulator has been focused on to confine the filaments omnidirectionally. 2. Experiment Nickel nanowires were buried into nanoholes of APA using electroplating method, where nickel sulfate was used as an electrolyte solution. The exposed Ni nanowires were obtained by chemical etching the alumina layer. The nanowires were oxidized by oxygen plasma for 1, 3, 5 min. The surface morphology of nanowires was investigated by a field emission scanning electron microscope (FE-SEM). For a contact preparation, Indium solder was used against the top electrode. The bottom electrode was a residual aluminum substrate. The I-Vcharacteristics of NiO nanowires were measured by a standard two-probe method at room temperature, where the current was limited to 1 mA in a SET process guard the device from a complete dielectric breakdown. 3. Results Figure 2 (a) ~ (c) shows the cross-sectional SEM images of NiO nanowires. The typical diameter and the length were about 40 nm and 600 nm, respectively. Significant oxidation time dependence was not found in the shape. We can see clear switching behaviors for all samples. In case of 1 min oxidation, a relatively wide distribution of the switching voltage was measured. The distribution width became smaller in 3 min oxidation and widens again in 5 min oxidation. The relatively high current found in the RESET process in case of 3 min oxidation. It might be due to poor crystallinity of the NiO thin film because oxygen plasma is expected to make the film amorphous by the high kinetic energy of oxygen plasma. Longer oxidation time might help to formation of many filaments. So, this leads to widen the switching voltage distribution again in 5 min oxidation.

Thermodynamic analysis of conductive filaments

We present a thermodynamic theory of the conductive filament growth and dissolution in random access memory describing the observed features of their current-voltage (IV) characteristics. Our theory is based on the self-consisted Fokker-Planck approach reducing the filament kinetics to its thermodynamics. Expressing the observed IV features through material parameters, our results pave a way to device improvements.

Controllable Formation of Nanofilaments in Resistive Memories via Tip‐Enhanced Electric Fields

Resistive random access memory (ReRAM) has a great potential to be the next‐generation non‐volatile memory device. However, the random nucleation and growth of conductive filaments (CFs) in ReRAM causes the low reliability in switching behaviors, leading to difficulties in its practical application. This study demonstrates that manipulating electric fields in ReRAM via a structured electrode can provide the controllable formation of CFs. Ag pyramids that have a high‐quality tip prepared via the template‐stripping method generate highly enhanced electric fields at the tip. Because the tip‐enhanced electric fields can facilitate the ionization of Ag atoms and their migration along the electric fields, the nucleation and growth of CFs occurs predominantly at the tip. The CFs in ReRAM are directly observed using electron microscopy and it is confirmed that the CFs are formed only at the tip. The resulting ReRAM exhibits low and reliable SET/RESET voltages (0.48 V ± 0.02 V and 0.15 V ± 0.06 V, respectively). Moreover, its endurance and retention time are highly improved, compared to those devices that are based on conventional geometry. Thus, this approach can encourage creating high‐performance ReRAM.

Atomic View of Filament Growth in Electrochemical Memristive Elements.

Memristive devices, with a fusion of memory and logic functions, provide good opportunities for configuring new concepts computing. However, progress towards paradigm evolution has been delayed due to the limited understanding of the underlying operating mechanism. The stochastic nature and fast growth of localized conductive filament bring difficulties to capture the detailed information on its growth kinetics. In this work, refined programming scheme with real-time current regulation was proposed to study the detailed information on the filament growth. By such, discrete tunneling and quantized conduction were observed. The filament was found to grow with a unit length, matching with the hopping conduction of Cu ions between interstitial sites of HfO2 lattice. The physical nature of the formed filament was characterized by high resolution transmission electron microscopy. Copper rich conical filament with decreasing concentration from center to edge was identified. Based on these results, a clear picture of filament growth from atomic view could be drawn to account for the resistance modulation of oxide electrolyte based electrochemical memristive elements.

On the direction of the conductive filament growth in valence change memory devices during electroforming

We discuss theoretically the filament growth direction in valence change memory (VCM) devices and show that this direction is determined by kinetic considerations. It is being fixed by the relation between the kinetics of ion transfer at the electrodes and that of ion transfer through the bulk. Considering a filament consisting of donors (oxygen vacancies) the direction of growth is from the cathode when the anode ion kinetics is limited in comparison to fast ion kinetics in the bulk of the oxide. In the opposite case the filament grows from the anode side. The filament may in some cases extend between the two electrodes while in others leave a narrow insulating gap under the anode.

A new approach to investigate conductive anodic filament (CAF) formation

Purpose – This paper aims to describe the development of an approach that uses a flexible substrate to investigate the mechanism of conductive anodic filament (CAF) growth and effect of different material and manufacturing variables. Design/methodology/approach – A new approach using a simulated test vehicle (STV) has been developed to study the CAF phenomena. The STV can be easily built under controlled conditions in the laboratory using different glass fibres and resin powder to investigate the effect of different variables separately on CAF. The advantage of the STV is that CAF can be formed in relatively short period in a controlled way, and CAF growth can be easily identified using a back-lighting under a microscope due to the thin flex material used as the test sample. Findings – STV has been used to investigate a number of effects on CAF formation: different glass fibres, reflow process, acid contamination in drilled holes, desmear process and glass bundle size. The results demonstrate that for finished fibres acid contamination (plating solution) at the electrode was necessary for CAF formation. However, for unfinished glass fibres (loom state and heat cleaned) CAF can be formed without acid contamination. The reflow process significantly increases CAF formation. Running an aggressive desmear process and using large glass fibre bundle also increased CAF formation. Originality/value – This new approach will be of benefit for printed circuit board (PCB) supplier to evaluate CAF performance on different resin systems and glass fibres to provide high CAF resistance quality PCBs. The test period (168 hours) would be much shorter than the traditional CAF testing (1,000 hours).

Detection of the insulating gap and conductive filament growth direction in resistive memories.

Filament growth is a key aspect in the operation of bipolar resistive random access memory (RRAM) devices, yet there are conflicting reports in the literature on the direction of growth of conductive filaments in valence change RRAM devices. We report here that an insulating gap between the filament and the semiconductor electrode can be detected by the metal-insulator-semiconductor bipolar transistor structure, and thus provide information on the filament growth direction. Using this technique, we show how voltage polarity and electrode chemistry control the filament growth direction during electro-forming. The experimental results and the nature of a gap between the filament and an electrode are discussed in light of possible models of filament formation.

Simulation study of conductive filament growth dynamics in oxide-electrolyte-based ReRAM

Monte Carlo (MC) simulations, including multiple physical and chemical mechanisms, were performed to investigate the microstructure evolution of a conducting metal filament in a typical oxide-electrolyte-based ReRAM. It has been revealed that the growth direction and geometry of the conductive filament are controlled by the ion migration rate in the electrolyte layer during the formation procedure. When the migration rate is relative high, the filament is shown to grow from cathode to anode. When the migration rate is low, the growth direction is expected to start from the anode. Simulated conductive filament (CF) geometries and I–V characteristics are also illustrated and analyzed. A good agreement between the simulation results and experiment data is obtained.

A relationship between statistical time to breakdown distributions and pre-breakdown negative differential resistance at nanometric scale

Using an ultra-high vacuum Conductive atomic force microscopy (C-AFM) current voltage, pre-breakdown negative differential resistance (NDR) characteristics are measured together with the time dependent dielectric breakdown (TDDB) distributions of Si/SiON (1.4 and 2.6 nm thick). Those experimental characteristics are systematically compared. The NDR effect is modelled by a conductive filament growth. It is showed that the Weibull TDDB statistic distribution scale factor is proportional to the growth rate of an individual filament and then has the same dependence on the electric field. The proportionality factor is a power law of the ratio between the surfaces of the CAFM tip and the filament's top. Moreover, it was found that, for the high fields used in those experiments, the TDDB acceleration factor as the growth rate characteristic is proportional to the Zener tunnelling probability. Those observations are discussed in the framework of possible breakdown or forming mechanism.

Filament growth dynamics in solid electrolyte-based resistive memories revealed by in situ TEM

Solid electrolyte based-resistive memories have been considered to be a potential candidate for future information technology with applications in non-volatile memory, logic circuits and neuromorphic computing. A conductive filament model has been generally accepted to be the underlying mechanism for the resistive switching. However, the growth dynamics of such conductive filaments is still not fully understood. Here, we explore the controllability of filament growth by correlating observations of the filament growth with the electric field distribution and several other factors. The filament growth behavior has been recorded using in situ transmission electron microscopy. By studying the real-time recorded filament growth behavior and morphologies, we have been able to simulate the electric field distribution in accordance with our observations. Other factors have also been shown to affect the filament growth, such as Joule heating and electrolyte infrastructure. This work provides insight into the controllable growth of conductive filaments and will help guide research into further functionalities of nanoionic resistive memories.

Optimal stoichiometry for nucleation and growth of conductive filaments in HfOx

We present a first principles based investigation into the nucleation and growth of metal-rich precipitates in substoichiometric hafnium dioxide with relevance to applications in resistive switching memories. We identify an optimal HfOx stoichiometry, with x in the range 1.50–1.75, for efficient nucleation and growth of stable Hf-rich clusters which may serve as precursors for the growth of conductive filaments during forming. We also show that filaments with sub-nm diameter posses remarkably metallic character. These results provide invaluable atomistic insight into forming in resistive switching films and demonstrate that precise regulation of stoichiometry is essential in order to ensure uniformity in the nucleation and subsequent growth of a conductive filament during the forming process.

Response to “Comment on Real‐Time Observation on Dynamic Growth/Dissolution of Conductive Filaments in Oxide‐Electrolyte‐Based ReRAM”

In our recently published paper, [ 1 ] we achieved direct visualization of the dynamic growth and dissolution of conductive fi laments (CFs) in Ag (or Cu)/ZrO 2 /Pt systems based on in situ transmission electron microscope (TEM) observations. Furthermore, continuous TEM images clearly showed that the CFs start to grow at the Cu/ZrO 2 (or Ag/ZrO 2 ) interface and begin to dissolve at the ZrO 2 /Pt interface, which is in contrast to the prediction from the electrochemical metallization (ECM) theory developed for the solid-electrolyte-based ReRAM (also referred to a the ECM cell). [ 2 , 3 ] The same direction of CF growth/dissolution had been demonstrated in other ECM cells, including Ag/ α -Si/Pt, [ 4 ] Cu/ZnO/Pt, [ 5 ] and Cu/P3HT:PCBM/ ITO devices. [ 6 ] In our paper, [ 1 ] we suggested that the differences in cation solubility and diffusion coeffi cient between traditional solid-electrolyte and oxide-electrolyte materials accounts for this discrepancy, and based on our suggestion, the metal ions would be reduced and crystallized inside the ZrO 2 -electrolyte system in order to explain the observed results. Valov and Waser made a comment to dispute our explanation based on the following arguments. (1) The EDX analysis presented in our paper cannot identify the chemical valence of Ag (Cu) species inside the CF region. They suggested that the resistive switching (RS) behavior in our device cannot rule out valence change memory (VCM) effects due to oxide materials like ZrO 2 , TiO 2 , HfO 2 , and Ta 2 O 5 which are often used for ECM and VCM cells. (2) The large amount of electrons in the solid materials is not a suffi cient condition to reduce the incoming metal ions inside the solid electrolyte materials. (3) Considering the electric potential distribution, the Cu or Ag CFs should not start to grow at the Cu/ZrO 2 (or Ag/ZrO 2 ) interface and begin to dissolve at the ZrO 2 /Pt interface. At the end of their comment, they presented a hypothesis in which the growth and dissolution of CF in ZrO 2 result from the metal ion doping/

Dynamic Modeling and Atomistic Simulations of SET and RESET Operations in $\hbox{TiO}_{2}$-Based Unipolar Resistive Memory

In this letter, atomistic-level models and simulations of unipolar TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> RRAM are addressed. A dynamic model of SET/RESET is developed based on recent experimental findings, which attributes SET to oxygen vacancy (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sub> ) drift and Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> conductive filament (CF) growth, while RESET is explained by the melting of Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> CF and subsequent (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sub> ) diffusion and recombination. Based on the model, electro-thermal and molecular dynamics simulations are carried out to reproduce the complete switching cycle at an atomistic level.

Preparation and Characteristics of Nanoscale Diamond-Like Carbon Films for Resistive Memory Applications

We propose diamond-like carbon (DLC) as the resistance change material for nonvolatile memory applications. Nanoscale DLC films are prepared by filtered cathodic vacuum arc technique and integrated to W/DLC/W structure devices. The deposited DLC film has a thickness of about 20 nm and high sp3 fraction content. Reversible bistable resistive switching from a high resistance state to a low resistance state, and vice versa, is observed under appropriate unipolar stimulation pulses. High resistance switching ratio (larger than a thousand times) and low level of switching power (about 11 μW) are demonstrated. We propose that the mechanism of the repetitive resistive switching is the growth and breakage of conductive sp2-like filaments in the predominantly sp3-type insulating carbon upon applications of voltage pulses, which is consistent with the experimental results.

Processing and material issues related to lead-free soldering

The European requirement for lead-free electronics has resulted in higher soldering temperature and some material and process changes. Traditional tin-lead solder melts at 183°C, where as the most common lead-free alternatives have a much higher melting temperature—tin-copper (227°C), tin-silver (221°C) and tin-silver-copper (217°C). These have challenged the ingenuity of the materials and process engineers. This chapter will explore some of the issues that have come up in this transition, and which these engineers have understood and addressed. As we enter the lead-free era, we see changes as printed wiring board (PWB) substrates which were designed for lower soldering temperatures are being replaced by newer materials. Factors such as glass transition temperature (T g), decomposition temperature (T d) and coefficient of thermal expansion must be considered. Many electronic components are made for lower peak temperatures than those required by the new solders. Solder flux chemistries are changing to meet the needs of the new metal systems, and cleaning of flux residues is becoming more of a challenge. Finally, there is a potential reliability problem—an increased potential for the growth of conductive anodic filament (CAF), an electrochemical failure mechanism that occurs in the use environment.