Papers by Sridhar Chandrasekaran
Applied Physics Letters
An excessive unintentional out-diffuses In atoms into the switching layer is a potential threat t... more An excessive unintentional out-diffuses In atoms into the switching layer is a potential threat to the switching stability of memristor devices having indium tin oxide (ITO) as the electrode. We suggest that the physical factor (bombardment of Ar ions and bombardment induced localized heat during ZnO deposition) and chemical factor (bonding dissociation energy, point defects, and bond length of atoms) responsible for promoting the out-diffusion. The In atom acts as dopant in the ZnO lattice that degenerates the ZnO insulative behavior. Furthermore, the In ions take part in the conduction mechanism where they may compete with other mobile species to form and rupture the filament, and hence, deteriorate the switching performance. We propose a facile UV/O3 (UVO) treatment to mitigate such damaging effects. The device fabricated on the UVO-treated ITO substrate exhibits significant switching parameter improvement than that of the device manufactured on untreated ITO. This work delivers an insight into the damaging effect of out-diffusion and auto-doping processes on the reliability of memristor devices.
Synaptic behaviour of TiO x /HfO2 RRAM enhanced by inserting ultrathin Al2O3 layer for neuromorphic computing
Semiconductor Science and Technology, 2021
The synaptic linearity of resistive random-access memory (RRAM) based on TiO x /HfO2 improved by ... more The synaptic linearity of resistive random-access memory (RRAM) based on TiO x /HfO2 improved by inserting an ultrathin Al2O3 layer is investigated. A gradual bipolar switching with a positive set and a negative reset is observed for devices with an Al2O3 layer after an electroforming process. The devices with a 1 nm Al2O3 layer exhibit acceptable reliability with >400 cycles DC endurance with no decrement of the on/off ratio after 104 sec. A remarkable enhancement in the synaptic linearity of potentiation 2.15 and depression 1.52 is achieved in this device. The conduction mechanisms at different current regions of the optimized device are studied. The presence of the Al2O3 layer is confirmed by x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy analyses. About 42% of the oxygen vacancy concentration calculated from the XPS spectra is responsible for the synaptic properties. This synaptic RRAM structure is suitable for upcoming neuromorphic computing devi...
Memristors [Working Title], 2021
The capability of memristor devices to perform weight changes upon electrical pulses mimics the a... more The capability of memristor devices to perform weight changes upon electrical pulses mimics the analogous firing mechanism in biological synapses. This capability delivers the potential for neuromorphic computing and pushes renewed interests in fabricating memristor with analog characteristics. Nevertheless, memristors could often exhibit digital switching, either during the set, reset, or both processes that degenerate their synaptic capability, and nanodevice engineers struggle to redesign the device to achieved analog switching. This chapter overviews some important techniques to transform the switching characteristics from digital to analog in valence change and electrochemical metallization types memristors. We cover physical dynamics involving interfacial diffusion, interfacial layer, barrier layer, deposition, and electrode engineering that can induce digital-to-analog switching transformation in memristor devices.
Conduction mechanism of Co-doped ZnO transparent memristive devices
IOP Conference Series: Materials Science and Engineering, 2021
The Co dopant substitutes the Zn atomic position in the hexagonal crystal lattice and generates a... more The Co dopant substitutes the Zn atomic position in the hexagonal crystal lattice and generates acceptor defects. These defects play significant role in modulating the conduction mechanism of the memristive device. The devices without Co dopant have high concentration of donor defects so that the electron can flow easily through hopping these donor defects; henceforth, only weak filaments can be formed during the set process. Meanwhile, the increase of the acceptor defects in the films enhances the film resistivity. This acceptor defects also contribute to an increase of barrier height at the electrode/dielectric interface where the electrons require higher energy to overcome this barrier and, eventually, induce the formation of strong filaments during the set process.
Flexible Ta2O5/WO3-Based Memristor Synapse for Wearable and Neuromorphic Applications
IEEE Electron Device Letters, 2021
In this letter, Ta<sub>2</sub>O<sub>5</sub>/WO<sub>3</sub> do... more In this letter, Ta<sub>2</sub>O<sub>5</sub>/WO<sub>3</sub> double-layer wearable memristor synapse has excellent recognition accuracy (97%) for just 12 epochs compared to the single-layer device (83%). The insertion of an ultra-thin WO<sub>3</sub> layer modulates the oxygen vacancy distribution in Ta<sub>2</sub>O<sub>5</sub> and induces digital-to-analog switching behavior. Excellent AC endurance of (>10<sup>9</sup> cycles) under 2 mm extreme bending, a rapid speed (25 ns), reliable bending endurance for 10<sup>4</sup> cycles with 4 mm bending, stable retention (>10<sup>6</sup> s) up to 200°C, and water-resistant behavior are achieved. The potentiation, and depression having outstanding nonlinearity (0.64) is obtained. The Ta<sub>2</sub>O<sub>5</sub>/WO<sub>3</sub> design is a promising candidate for wearable neuromorphic applications due to its wearability, flexibility, lightweight, low cost and environmental friendly fabrication.
Barrier Layer Induced Switching Stability in Ga:ZnO Nanorods Based Electrochemical Metallization Memory
IEEE Transactions on Nanotechnology, 2020
The effect of the TiW barrier layer on the switching properties of Ga-doped ZnO (GZO) nanorods ba... more The effect of the TiW barrier layer on the switching properties of Ga-doped ZnO (GZO) nanorods based on Electrochemical Metallization Memory is investigated. Vertically well-aligned and uniform GZO nanorods having a diameter of approximately 35 nm are hydrothermally grown on a seeding layer of ZnO deposited on indium tin oxide (ITO) coated glass substrate, to fabricate Cu/TiW/nanorods/ITO/Glass devices. The remarkable enhancement in the memory window (on/off ratio) is achieved in the 5 nm TiW barrier layer embedded device. This device exhibits endurance of more than 103 cycles and a large memory window of ∼103. The conduction mechanism at different current regions is studied, and it is found that Schottky emission is dominated in the low field region. The TiW barrier layer helps to retain the Cu ions and control the Cu ions diffusion, hence control the filament growth into the resistive layer, confirmed from the X-ray photoelectron spectroscopy (XPS) analysis. This device is suitable for the future low power non-volatile memory devices.
Evaluating gallium-doped ZnO top electrode thickness for achieving a good switch-ability in ZnO2/ZnO bilayer transparent valence change memory
Journal of Electroceramics, 2021
Gallium doped ZnO (GZO) top electrode thickness dependence of resistive switching characteristic ... more Gallium doped ZnO (GZO) top electrode thickness dependence of resistive switching characteristic of GZO/ZnO2/ZnO/ITO transparent valence change memory device is investigated. The thickness of the GZO top electrode modulates the resistance of the pristine device. Devices made with thicker GZO layer have higher leakage current; thus, require higher current compliance. An excessively high current compliance leads to a device breakdown upon reset process. Conversely, a very low current compliance may form a tiny conducting filament and is difficult to rejuvenate after the rupture; thus, its cycle-to-cycle characteristic shows a decaying behavior. Nevertheless, transparent valence change devices with a stable endurance and sufficient memory window that operate at a moderate level of current compliance are successfully fabricated by employing an appropriate thickness of the top electrode. We suggest that a good switch-ability of transparent valence change memory devices are strongly affected by the thickness of the top electrode.
Suboxide interface induced digital-to-analog switching transformation in all Ti-based memristor devices
Applied Physics Letters, 2020
Oxidation of TiN is a diffusion-limited process due to the high stability of the TiN metallic sta... more Oxidation of TiN is a diffusion-limited process due to the high stability of the TiN metallic state at the TiN/TiO2 junction. Hence, the TiN/TiO2/TiN device being the inability to form a suitable interfacial layer results in the exhibition of abrupt current (conductance) rise and fall during the set (potentiation) and reset (depression) processes, respectively. Interfacial engineering by depositing Ti film served as the oxygen gettering material on top of the TiO2 layer induces a spontaneous reaction to form a TiOx interfacial layer (due to the low Gibbs free energy of suboxide formation). Such an interface layer acts as an oxygen reservoir that promotes gradual oxidation and reduction during the set and reset processes. Consequently, an excellent analog behavior having a 2-bit per cell and robust epoch training can be achieved. However, a thick interfacial layer may degrade the switching behavior of the device due to the high internal resistance. This work suggests that interfacial engineering could be considered in designing high-performance analog memristor devices.
Nanotechnology, 2020
Surface oxidation employing neutral oxygen irradiation significantly improves the switching and s... more Surface oxidation employing neutral oxygen irradiation significantly improves the switching and synaptic performance of ZnO-based transparent memristor devices. The endurance of the as-irradiated device is increased by 100 times, and the operating current can be lowered by 10 times as compared with the as-deposited device. Moreover, the performance-enhanced device has an excellent analog behavior that can exhibit 3-bits per cell nonvolatile multistate characteristics and perform 15 stable epochs of synaptic operations with highly linear weight updates. A simulated artificial neural network comprising 1600 synapses confirms the superiority of the enhanced device in processing a 40 × 40 pixels grayscale image. The irradiation effectively decreases the concentration of oxygen vacancy donor defects and promotes oxygen interstitial acceptor defects on the surface of the ZnO films, which consequently modulate the redox process during rupture and rejuvenation of the filament. This
IEEE Journal of the Electron Devices Society, 2020
To move towards a new generation powerful computing system, brain-inspired neuromorphic computing... more To move towards a new generation powerful computing system, brain-inspired neuromorphic computing is expected to transform the architecture of the conventional computer, where memristors are considered to be potential solutions for synapses part. We propose and demonstrate a novel approach to achieve remarkable improvement of analog switching linearity in TaN/Ta/TaO x /Al 2 O 3 /Pt/Si memristors by varying Al 2 O 3 layer thickness. Presence of the Al 2 O 3 layer is confirmed from the Auger Electron Spectroscopy study. Good analog switching ratio of about 100× and superior switching uniformity are observed for the 1 nm Al 2 O 3 based device. Multilevel capability of the memristive devices is also explored for prospective use as a synapse. More than 10 4 and 4 × 10 4 cycles nondegradable dc and ac endurances, respectively, alongwith 10 4 second retention are achieved for the optimized device. Improved linearities of 2.41 and −2.77 for potentiation and depression, respectively are obtained for such 1 nm Al 2 O 3-based devices. The property of gradual resistance changed by pulse amplitudes confirms that the TaO x memristors can be potentially used as an electronic synapse.
Japanese Journal of Applied Physics, 2018
The effects of varying the thickness of the TiW barrier layer on the switching characteristics of... more The effects of varying the thickness of the TiW barrier layer on the switching characteristics of a ZrO 2-based electrochemical metallization memory (ECM) device were investigated. The thickness of the TiW barrier layer may alter the resistive switching characteristics of Cu/TiW/ZrO 2 /TiN ECM devices. Devices made without a TiW barrier layer exhibit unstable cycle-to-cycle behavior. The switching stability of ZrO 2 devices improves after inserting a TiW layer. However, the insertion of TiW beyond critical thickness leads to switching degradation. We suggest that an appropriate TiW barrier layer thickness is necessary for achieving good switching performance.
IEEE Transactions on Electron Devices, 2019
The synaptic plasticity of indium tin oxide (ITO)/ZnO/ITO highly transparent (more than 88%) anal... more The synaptic plasticity of indium tin oxide (ITO)/ZnO/ITO highly transparent (more than 88%) analog switching resistive memory device is investigated. Highly stable analog switching behavior for more than 2500 cycles with a good memory window of approximately two orders makes it suitable for synapse application. The synaptic response is investigated by applying identical electrical pulses. The potentiation and depression of the device used the conventional identical single-pulse scheme to perform high nonlinearity (0.83) and decaying training epochs. However, the linearity and the training epochs are improved to 0.44 by implementing the identical double-pulse scheme. The proposed double-pulse scheme offers a broad dynamic range (200) having 320 conductance states. This invisible structure and double-pulse scheme can be highly suitable for the neuromorphic computing devices.
APL Materials, 2019
Paper published as part of the special topic on Emerging Materials in Neuromorphic Computing ARTI... more Paper published as part of the special topic on Emerging Materials in Neuromorphic Computing ARTICLES YOU MAY BE INTERESTED IN Multilevel HfO 2-based RRAM devices for low-power neuromorphic networks
Improving linearity by introducing Al in HfO2 as a memristor synapse device
Nanotechnology, 2019
Artificial synapse having good linearity is crucial to achieve an efficient learning process in 1... more Artificial synapse having good linearity is crucial to achieve an efficient learning process in 19 neuromorphic computing. It is found that the synaptic linearity can be enhanced by engineering 20 the doping region across the switching layer. The nonlinearity of potentiation and depression of 21 the pure device is 36% and 91%, respectively; meanwhile, the nonlinearity after doping can be 22 suppressed to be 22% (potentiation) and 60% (depression). Henceforth, the learning accuracy 23 of the doped device is 91% with only 13 iterations; meanwhile, the pure device is 78%. Detailed 24 conduction mechanism to understand this phenomenon is proposed.
Extended Abstracts of the 2017 International Conference on Solid State Devices and Materials, 2017
The switching properties of the Ta5Si3 based CBRAM device are investigated for nonvolatile memory... more The switching properties of the Ta5Si3 based CBRAM device are investigated for nonvolatile memory applications. The resistive switching properties can be improved by inserting a thin Al2O3 layer between the bottom electrode and Ta5Si3 layer. The Ta5Si3/Al2O3 double layer device with the 1 nm thin Al2O3 layer exhibits excellent memory performances, such as stable DC endurance up to 10 4 cycles during the test without degradation, good retention ability (>10 5 s) at a temperature of 130 0 C with more than 10 2 resistance ratio.
Nanoscale Research Letters, 2018
The impact of peroxide surface treatment on the resistive switching characteristics of zinc perox... more The impact of peroxide surface treatment on the resistive switching characteristics of zinc peroxide (ZnO 2)-based programmable metallization cell (PMC) devices is investigated. The peroxide treatment results in a ZnO hexagonal to ZnO 2 cubic phase transformation; however, an excessive treatment results in crystalline decomposition. The chemically synthesized ZnO 2 promotes the occurrence of switching behavior in Cu/ZnO 2 /ZnO/ITO with much lower operation current as compared to the Cu/ZnO/ITO (control device). However, the switching stability degrades as performing the peroxide treatment for a longer time. We suggest that the microstructure of the ZnO 2 is responsible for this degradation behavior and fine tuning on ZnO 2 properties, which is necessary to achieve proper switching characteristics in ZnO 2-based PMC devices.
Thin Solid Films, 2018
The effect of TiW metal barrier layer thickness on voltage-current characteristics of the Cu/TiW/... more The effect of TiW metal barrier layer thickness on voltage-current characteristics of the Cu/TiW/ZrO 2 /TiN conductive bridge random access memory device was systematically investigated. The change of reset behavior from abrupt decrease to gradual decrease with increasing TiW thickness was observed. Electronic conduction during the forming process was also analyzed to obtain detailed information about the effect of TiW layer thickness on the nature of the conduction phenomenon. The temperature coefficient of resistance of the conductive filament confirms that an electro-chemical metallization (ECM) based conduction was observed in the devices made with a thinner TiW layer. On the other hand, valence change memory (VCM) based conduction was observed with a thick TiW layer. A conduction mechanism is proposed to explain the ECM to VCM conduction transformation phenomenon.
Semiconductor Science and Technology, 2017
This contribution is meant to provide a brief overview of current soil-related issues within the ... more This contribution is meant to provide a brief overview of current soil-related issues within the Maltese Islands. The major threats to the islands' soil resources are identified and discussed within a geographical/agricultural context. The second part of the paper outlines the most significant measures, which are addressing such issues and discusses some broad outcomes. A concluding section identifies the needs for intervention.
Journal of Clean Energy Technologie, 2018
In this study a simple one step hydrothermal method has been utilized to synthesize mesoporous Ni... more In this study a simple one step hydrothermal method has been utilized to synthesize mesoporous NiFe 2 O 4 nanoparticles. The prepared NiFe 2 O 4 (NFO) is highly crystalline and possesses homogenously distributed mesopore. The structural analysis, thermal stability and morphological studies of mesoporous NFO nanoparticles are performed using XRD, TGA, TEM, FE-SEM and surface area analyzer. The supercapacitive behavior of synthesized nanomaterial is investigated in a three-electrode configuration cell with 2 M KOH electrolyte using cyclic voltammetry, galvanostatic charge-discharge cycling and electrochemical impedance spectroscopy. The mesoporous crystalline NFO nanoparticles (diameter 10-15 nm) intuitively possess interesting structural advantages, such as high surface area (148 m 2 .g-1), fast electron and ion transport. As a result, it exhibits very good specific capacitance (1040 F.g-1 at 1 A.g-1) and good cycling stability (~70 % up to 500 cycles at 2 A.g-1) when examined as electrode material for high performance electrochemical supercapacitor applications. Index Terms-Mesoporous, NiFe 2 O 4 , Nano particles, Supercapacitor.
Peroxide induced volatile and non-volatile switching behavior in ZnO-based electrochemical metallization memory cell
Nanotechnology, 2017
We explore the use of cubic-zinc peroxide (ZnO2) as a switching material for electrochemical meta... more We explore the use of cubic-zinc peroxide (ZnO2) as a switching material for electrochemical metallization memory (ECM) cell. The ZnO2 was synthesized with a simple peroxide surface treatment. Devices made without surface treatment exhibits a high leakage current due to the self-doped nature of the hexagonal-ZnO material. Thus, its switching behavior can only be observed when a very high current compliance is employed. The synthetic ZnO2 layer provides a sufficient resistivity to the Cu/ZnO2/ZnO/ITO devices. The high resistivity of ZnO2 encourages the formation of a conducting bridge to activate the switching behavior at a lower operation current. Volatile and non-volatile switching behaviors with sufficient endurance and an adequate memory window are observed in the surface-treated devices. The room temperature retention of more than 104 s confirms the non-volatility behavior of the devices. In addition, our proposed device structure is able to work at a lower operation current among other reported ZnO-based ECM cells.
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Papers by Sridhar Chandrasekaran