Durjoy Dev

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Papers by Durjoy Dev

Semiconductor-to-metal transition in atomic layer deposition (ALD) of VO2 films using VCl4 and water

Applied Physics Letters, 2021

The semiconductor-to-metal transition of vanadium dioxide (VO2) films is studied using temperatur... more The semiconductor-to-metal transition of vanadium dioxide (VO2) films is studied using temperature-dependent Raman, optical, and electrical measurements. The VO2 films are deposited via an atomic layer deposition (ALD) process using alternate pulses of vanadium tetrachloride and H2O at 350 °C. A growth rate of 0.021 nm/cycle and a thickness of 33 nm of VO2 are obtained for all films studied. The phase of the film is determined using x-ray diffraction. The as-deposited films are amorphous and are transformed to the monoclinic phase with a post-deposition, forming gas anneal at temperatures ≥ 500 °C for 60 min. The purity of the films is determined using x-ray photoelectron spectroscopy and no evidence of residual chlorine is detected. The temperature-dependent Raman Ag mode of the monoclinic VO2 phase is observed to monotonically decrease from 25 °C to 78 °C; where no evidence of the Ag peak is observed in the film beyond 68 °C. The refractive index and extinction coefficient extract...

Linear Weight Update in MoS2/Graphene Memristive Synapses for Unsupervised Learning

2019 Device Research Conference (DRC), 2019

Memristive synaptic devices are considered one of the most promising candidates for brain-inspire... more Memristive synaptic devices are considered one of the most promising candidates for brain-inspired neuromorphic computing, owing to their decreased complexity and nanoscale footprint compared to conventional complementary metal oxide semiconductor (CMOS) circuitry[1], [2]. However, the non-linearity and asymmetry in weight update observed in most memristive synapses due to the difference in the long-term potentiation and depression characteristics makes it difficult to use these devices for unsupervised learning applications[1]. Previously, linearity in synaptic weight update has been engineered using non-identical input voltage pulsing scheme[3]. However, not many reports on linear synaptic weight update using identical input voltage pulses exist in literature [4]. In this work, we present large-area chemical vapor-deposited (CVD) $\mathrm{MoS}_{2}$ /graphene memristive synapses which exhibit linear weight update using identical input voltage pulses. These synaptic devices also exh...

(Invited) Semiconductor-to-Metal Transition in Atomic Layer Deposition (ALD) of VO2 Films Using VCl4 and Water

ECS Meeting Abstracts

Artificial Neuron using Ag/2D-MoS2/Au Threshold Switching Memristor

2019 Device Research Conference (DRC)

Neuromorphic computing comprises of systems that are based on the human brain or artificial neura... more Neuromorphic computing comprises of systems that are based on the human brain or artificial neural networks, with the promise of creating a brain inspired ability to learn and adapt, but technical challenges, such as developing an accurate neuroscience model of the functionality of the brain to building devices to support these models, are significantly hindering the progress of neuromorphic systems. This has paved the way for artificial neural networks (ANN) to meet these criteria. The memristor has become an emerging candidate to realize ANN through emulation synapse and neuron behavior. In this work, we are fabricating an Ag/MoS2/Au threshold switching memristor (TSM), to emulate four critical behaviors of neurons-all-or-nothing spiking, thresholddriven firing, post firing refractory period and stimulus strength-based frequency response. We will also test the innate stochastic behavior of these devices to see if they are voltage dependent, making them a possible application in the integrate and fire neuron. Continuing to emulate biological synapses using memristors can help solve many optimization and machine learning problems, which in turn, can make electronics as energy-efficient as our brain.

$Figure 4: Output spike of artificial neuron showing the refractory and integration period of the neuron [5]. visualized in Figure 4.$

$and post-firing refractory period of a neuron.$

$Figure 16b. This demonstrates the all-or-nothing and post refractory behavior of a biological$

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Artificial Nociceptor Using 2D MoS2 Threshold Switching Memristor

IEEE Electron Device Letters

An artificial nociceptor realized with a single 2D MoS2-based memristor device is demonstrated in... more An artificial nociceptor realized with a single 2D MoS2-based memristor device is demonstrated in this work. The threshold switching memristor (TSM) device exhibits volatile resistance switching characteristics with low threshold voltage and a high ON-OFF ratio of 106. The Au/MoS2/Ag TSM device imitates a nociceptor, a special receptor of a sensory neuron that can detect noxious stimulus and transfer the signal to the central nervous system for preventive actions. The single device exhibits all key features of nociceptors including threshold, relaxation, “no adaptation” and sensitization phenomena of allodynia and hyperalgesia depending on the strength, duration, and repetition of the external stimuli. This work indicates applicability of this device in artificial sensory alarm systems for humanoid robots.

Optoelectronic synapse using monolayer MoS2 field effect transistors

Scientific Reports

Optical data sensing, processing and visual memory are fundamental requirements for artificial in... more Optical data sensing, processing and visual memory are fundamental requirements for artificial intelligence and robotics with autonomous navigation. Traditionally, imaging has been kept separate from the pattern recognition circuitry. Optoelectronic synapses hold the special potential of integrating these two fields into a single layer, where a single device can record optical data, convert it into a conductance state and store it for learning and pattern recognition, similar to the optic nerve in human eye. In this work, the trapping and de-trapping of photogenerated carriers in the MoS2/SiO2 interface of a n-channel MoS2 transistor was employed to emulate the optoelectronic synapse characteristics. The monolayer MoS2 field effect transistor (FET) exhibits photo-induced short-term and long-term potentiation, electrically driven long-term depression, paired pulse facilitation (PPF), spike time dependent plasticity, which are necessary synaptic characteristics. Moreover, the device’s...

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Two-Dimensional Near-Atom-Thickness Materials for Emerging Neuromorphic Devices and Applications

iScience

Two-dimensional (2D) layered materials and their heterostructures have recently been recognized a... more Two-dimensional (2D) layered materials and their heterostructures have recently been recognized as promising building blocks for futuristic brain-like neuromorphic computing devices. They exhibit unique properties such as near-atomic thickness, dangling-bond-free surfaces, high mechanical robustness, and electrical/optical tunability. Such attributes unattainable with traditional electronic materials are particularly promising for high-performance artificial neurons and synapses, enabling energy-efficient operation, high integration density, and excellent scalability. In this review, diverse 2D materials explored for neuromorphic applications, including graphene, transition metal dichalcogenides, hexagonal boron nitride, and black phosphorous, are comprehensively overviewed. Their promise for neuromorphic applications are fully discussed in terms of material property suitability and device operation principles. Furthermore, up-to-date demonstrations of neuromorphic devices based on 2D materials or their heterostructures are presented. Lastly, the challenges associated with the successful implementation of 2D materials into large-scale devices and their material quality control will be outlined along with the future prospect of these emergent materials.

format_quoteThe human brain utilizes significantly lower power (20 W) for complex tasks compared to conventional computers, which operate at much higher levels.format_quote

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2D MoS2-Based Threshold Switching Memristor for Artificial Neuron

IEEE Electron Device Letters

In this work, we use a two-terminal 2D MoS2-based memristive device to emulate an artificial neur... more In this work, we use a two-terminal 2D MoS2-based memristive device to emulate an artificial neuron. The Au/MoS2/Ag device exhibits volatile resistance switching characteristics with a low threshold voltage and a high ON-OFF ratio of 106, originating from an Ag diffusion-based filamentary process. The leaky integrate-and-fire neuron implemented with this device successfully emulates the key characteristics of a biological neuron.

Thickness-Independent Semiconducting-to-Metallic Conversion in Wafer-Scale Two-Dimensional PtSe2 Layers by Plasma-Driven Chalcogen Defect Engineering

ACS Applied Materials & Interfaces

Platinum diselenide (PtSe2) is an emerging class of two-dimensional (2D) transition metal dichalc... more Platinum diselenide (PtSe2) is an emerging class of two-dimensional (2D) transition metal dichalcogenide (TMD) crystals recently gaining substantial interests owing to its extraordinary properties absent in conventional 2D TMD layers. Most interestingly, it exhibits thickness-dependent semiconducting-to-metallic transition; i.e., thick 2D PtSe2 layers, which are intrinsically metallic, become semiconducting with their thickness reduced below a certain point. Realizing both semiconducting and metallic phases within identical 2D PtSe2 layers in a spatially well-controlled manner offers unprecedented opportunities towards atomically thin tailored electronic junctions, unattainable with conventional materials. In this study, beyond this thickness-dependent intrinsic semiconducting-to-metallic transition of 2D PtSe2 layers, we demonstrate that controlled plasma irradiation can "externally" achieve such tunable carrier transports. We grew wafer-scale very thin (a few nm) 2D PtSe2 layers by a chemical vapor deposition (CVD) method and confirmed their intrinsic semiconducting properties. We then irradiated the material with argon (Ar) plasma, which was intended to make it more semiconducting by thickness reduction. Surprisingly, we discovered a reversed transition of semiconducting-to-metallic, which is opposite to the prediction concerning their intrinsic thickness-dependent carrier transports. Through extensive structural and chemical characterization, we identified that the plasma irradiation introduces a large concentration of near-atomic defects and selenium (Se) vacancies in initially stoichiometric 2D PtSe2 layers. Furthermore, we performed density functional theory (DFT) calculation and clarified that the bandgap energy of such defective 2D PtSe2 layers gradually decreases with increasing defect concentration and dimensions, accompanying a large number of mid-gap energy states. This corroborative experimental and theoretical study decisively verify the fundamental mechanism for this externally controlled semiconducting-to-metallic transition in large-area CVD-grown 2D PtSe2 layers, greatly broadening their versatility for futuristic electronics.

Ultrasensitive and ultrathin phototransistors and photonic synapses using perovskite quantum dots grown from graphene lattice

Science Advances

Organic-inorganic halide perovskite quantum dots (PQDs) constitute an attractive class of materia... more Organic-inorganic halide perovskite quantum dots (PQDs) constitute an attractive class of materials for many optoelectronic applications. However, their charge transport properties are inferior to materials like graphene. On the other hand, the charge generation efficiency of graphene is too low to be used in many optoelectronic applications. Here, we demonstrate the development of ultrathin phototransistors and photonic synapses using a graphene-PQD (G-PQD) superstructure prepared by growing PQDs directly from a graphene lattice. We show that the G-PQDs superstructure synchronizes efficient charge generation and transport on a single platform. G-PQD phototransistors exhibit excellent responsivity of 1.4 × 108 AW–1 and specific detectivity of 4.72 × 1015 Jones at 430 nm. Moreover, the light-assisted memory effect of these superstructures enables photonic synaptic behavior, where neuromorphic computing is demonstrated by facial recognition with the assistance of machine learning. We ...

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Electronic synapses with near-linear weight update using MoS2/graphene memristors

Applied Physics Letters

Emulating the human brain's circuitry composed of neurons and synapses is an emerging area of res... more Emulating the human brain's circuitry composed of neurons and synapses is an emerging area of research on mitigating the "von Neumann bottleneck" in present computer architectures. The building block of these neuromorphic systemsthe synapse is commonly realized with oxide-based or phase change material-based devices, whose operation is limited by high programming currents and high reset currents. In this work, we have realized non-volatile resistive switching MoS2/graphene devices that exhibit multiple conductance states at low operating currents. The MoS2/graphene devices

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2D/3D Schottky-Junction Photovoltaic Devices Realized by the Direct CVD Growth of vdW 2D PtSe2 Layers on Silicon

ACS Applied Materials & Interfaces

Two-dimensional (2D) platinum diselenide (PtSe2) layers are a new class of near atom-thick 2D cry... more Two-dimensional (2D) platinum diselenide (PtSe2) layers are a new class of near atom-thick 2D crystals in a van der Waals (vdW) layered crystallinity, similar to previously explored many other 2D transition metal dichalcogenides (2D TMDs). They exhibit distinct advantages over conventional 2D TMDs for electronics and optoelectronics applications such as metallic-to-semiconducting transition, decently high carrier mobility, and low growth temperature. Despite such superiority, much of their electrical properties have remained mostly unexplored leaving their full technological potential far from being realized. Herein, we report a new type of 2D/3D Schottky junction device based on metallic 2D PtSe2 layers integrated on Si wafers. We directly grew 2D PtSe2 layers of controlled orientation and carrier transport characteristics via a low-temperature CVD process and investigated 2D PtSe2/Si Schottky junction properties. We unveiled a comprehensive set of material parameters which decisively confirm the presence of excellent Schottky junctions, i.e., high current rectification, small ideality factor, and temperature-dependent variation of Schottky barrier heights. Moreover, we observed strong photovoltaic effects in the 2D PtSe2/Si Schottky junction devices and extended them to realize flexible photovoltaic devices. This study is believed to broaden significantly the versatility of 2D PtSe2 layers in practical and futuristic electronic devices.

Artificial Neuron using Vertical MoS2/Graphene Threshold Switching Memristors

Scientific Reports

With the ever-increasing demand for low power electronics, neuromorphic computing has garnered hu... more With the ever-increasing demand for low power electronics, neuromorphic computing has garnered huge interest in recent times. Implementing neuromorphic computing in hardware will be a severe boost for applications involving complex processes such as image processing and pattern recognition. Artificial neurons form a critical part in neuromorphic circuits, and have been realized with complex complementary metal-oxide-semiconductor (CMOS) circuitry in the past. Recently, metal-insulatortransition materials have been used to realize artificial neurons. Although memristors have been implemented to realize synaptic behavior, not much work has been reported regarding the neuronal response achieved with these devices. In this work, we use the volatile threshold switching behavior of a vertical-MoS 2 /graphene van der Waals heterojunction system to produce the integrate-and-fire response of a neuron. We use large area chemical vapor deposited (CVD) graphene and MoS 2 , enabling large scale realization of these devices. These devices can emulate the most vital properties of a neuron, including the all or nothing spiking, the threshold driven spiking of the action potential, the post-firing refractory period of a neuron and strength modulated frequency response. These results show that the developed artificial neuron can play a crucial role in neuromorphic computing.

format_quoteThreshold switching in memristors offers a natural realization for artificial neurons, addressing limitations of CMOS circuitry.format_quote

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Total-Ionizing-Dose Response of MoS2 Transistors with ZrO2 and h-BN Gate Dielectrics

IEEE Transactions on Nuclear Science

The total-ionizing-dose response of few layer MoS2 transistors with ZrO2 or h-BN gate dielectrics... more The total-ionizing-dose response of few layer MoS2 transistors with ZrO2 or h-BN gate dielectrics is investigated under various bias conditions. Defects in MoS2 and surrounding dielectric layers significantly affect radiation-induced trapping. For devices with ZrO2 dielectrics, much larger negative Vth shifts and peak transconductance degradation are observed for irradiation under negative and ground bias than under positive bias. The h-BN devices exhibit positive threshold voltage shifts under negative-bias irradiation. For both ZrO2 and h-BN passivated devices, the peak transconductance degradation results from charge trapping at the surface of the MoS2 or in nearby oxides. Changes in defect energy distributions of MoS2 FETs during X-ray irradiation are characterized via temperature-dependent low-frequency noise measurements. Density functional theory calculations are performed to provide insight into the pertinent defects.

High quality gate dielectric/MoS2 interfaces probed by the conductance method

Applied Physics Letters

Two-dimensional materials provide a versatile platform for various electronic and optoelectronic ... more Two-dimensional materials provide a versatile platform for various electronic and optoelectronic devices, due to their uniform thickness and pristine surfaces. We probe the superior quality of 2D/2D and 2D/3D interfaces by fabricating molybdenum disulfide (MoS2)-based field effect transistors having hexagonal boron nitride (h-BN) and Al2O3 as the top gate dielectrics. An extremely low trap density of ~710 10 states/cm 2-eV is extracted at the 2D/2D interfaces with h-BN as the top gate dielectric on the MoS2 channel. 2D/3D interfaces with Al2O3 as the top gate dielectric and SiOx as the nucleation layer exhibit trap densities between 710 10 and 10 11 states/cm 2-eV, which is lower than previously reported 2D-channel/high--dielectric interface trap densities. The comparable values of trap time constants for both interfaces imply that similar types of defects contribute to the interface traps. This work establishes the case for van der Waals systems where the superior quality of 2D/2D and 2D/high- dielectric interfaces can produce high performance electronic and optoelectronic devices. iv ACKNOWLEDGMENTS I would like to thank my advisor Dr. Tania Roy for her unwavering trust and support which always encouraged me to work better. I would like to thank Dr. Reza Abdolvand and Dr. Jiann Shiun Yuaan for being on my committee and for their valuable insights in my research. I would like to thank my colleagues Hirokjyoti kalita, Sonali Das, Durjoy Dev and Nitin Choudhary for their support throughout my Masters. I also would like to thank Teja Kothamsu from Keysight Technologies for helping me in the measurements setup. This work would not have been possible if it wasn't for the support of my friends-Swathi, Nandakishore, Kishan, Archana and Vyshakh. Thanks for believing in me and helping me throughout this journey. Last but not the least I would like to thank my dad Krishnaprasad, brother Akshay, uncle Kanthareddy and my entire family for their never-ending love and support. v

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A theoretical approach for the dislocation reduction of wurtzite In<inf>x</inf>Ga<inf>1−x</inf>N/GaN heteroepitaxy

2012 7th International Conference on Electrical and Computer Engineering, 2012

This paper reports the numerical estimation of misfit dislocation in wurtzite In x Ga 1-x N/GaN h... more This paper reports the numerical estimation of misfit dislocation in wurtzite In x Ga 1-x N/GaN heterostructure. The misfit strain increases with the increase of lattice mismatch for higher In content. The exponential grading technique has been modelled for the reduction of dislocation. An energy balance model has been modified to evaluate the misfit dislocation density. The dislocation density has been reduced from 68.07*10^9 cm -2 to 0.288*10^6 cm -2 which is tapered in nature and help to promote annihilation and coalescence reactions between threading dislocations without unwanted tangling and pinning events. Finally, a comparative analysis has also been shown between without graded layer having constant composition and exponentially graded layer In 0.2 Ga 0.8 N/GaN.

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Misfit Dislocation Reduction of InxGa1-xN/GaN Heteroepitaxy Using Graded Layer

Advanced Materials Research, 2012

The performances of heterostructural devices are often limited by misfit dislocation. In this pap... more The performances of heterostructural devices are often limited by misfit dislocation. In this paper, a theoretical approach for misfit dislocation reduction of wurtzite InxGa1-xN/GaN is presented. The linear and exponential grading techniques have been modeled for the reduction of dislocation. An energy balance model has been taken into consideration and modified for wurtzite structure to evaluate the misfit dislocation density. The value of misfit dislocation has been reduced from 7.112×1010 cm-2 to 6.19×106 cm-2 and 7.039×1010 cm-2 to 6.121×106 cm-2 at the plane 1/3<> {} and 1/3<>{} respectively for linear grading. In case of exponential grading the dislocation density has been reduced to 2.762×105 cm-2 for both slip systems. Because of tapered grading coefficient a tapered dislocation profile has been reported in case of exponential grading technique. Finally, a comparative study has been shown among without graded, linear and exponential grading.

Improved performance of GaAsSb/AlGaAs nanowire ensemble Schottky barrier based photodetector via in-situ annealing

Nanotechnology, Jan 13, 2018

In this work, we report on the p-i GaAsSb/AlGaAs nanowires (NWs) ensemble device exhibiting good ... more In this work, we report on the p-i GaAsSb/AlGaAs nanowires (NWs) ensemble device exhibiting good spectral response up to 1.1 µm with a high responsivity of 311 A/W, an external quantum efficiency of 6.1x104 %, and a detectivity of 1.9x1010 Jones at 633 nm. The high responsivity of the NWs has been attributed to in-situ post-growth annealing of GaAsSb axial NWs in the ultra-high vacuum. The enabling growth technology is molecular beam epitaxy for the Ga-assisted epitaxial growth of these NWs on Si (111) substrates. Room temperature Raman spectra, as well as temperature dependent micro-photoluminescence (μ-PL) peak analysis indicated suppression of band tail states and non-radiative channels due to annealing. A similar improvement in in-situ annealed p-i GaAsSb NW ensemble with an AlGaAs passivating shell was inferred from a reduction in the Schottky barrier height as well as the NW resistance compared to the as-grown NW ensemble. These results demonstrate in-situ annealing of nanowir...

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