Browsing by Subject "MoS2"
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Item Chemical vapor deposited two-dimensional material based high frequency flexible field-effect transistors(2018-06-20) Park, Saungeun; Akinwande, Deji; Banerjee, Sanjay K.; Shi, Li; Register, Leonard F.; Dodabalapur, AnanthFlexible nanoelectronics have attracted great attention due to interesting concepts such as wearable electronics and internet of things, which requires high speed and low power consumption flexible smart system with functions ranging from sensing, computing to wireless communicating. In this dissertation, transparent and solution processable nanoscale polyimide film for highly flexible gate dielectrics is demonstrated by in-situ opto-electro-mechanical measurement and utilized for two-dimensional nanomaterials based field-effect transistors (FETs). Graphene thin film transistor with the nanoscale polyimide dielectric on flexible glass is operated in extremely high frequency regime and shows the highest experimental saturation velocity (~8.4 × 10⁶ cm/s) in any materials in any flexible transistors. Molybdenum disulfide (MoS₂) based transistors with embedded gate structure on rigid substrate are demonstrated with enhancement mode operation, ON/OFF ratio over 10⁸, the highest transconductance (~ 70 µS/µm) and saturation velocity (~1.8 × 10⁶ cm/s). CVD MoS₂ FETs on flexible plastic substrates are also demonstrated, showing enhancement mode operation, ON/OFF radio over 10¹⁰ and transconductance (~6 µS/µm). The flexible CVD MoS₂ transistors with embedded gate structure were employed to study effects of substoichiometric doping by HfO [subscript 2-x]. After the doping layer, the flexible MoS₂ transistors show ×8 higher source-drain current density as well as more than ×2 mobility improvements. For the another first demonstration, GHz operation and flexibility of graphene and MoS₂ based FETs are realized on commercial available paper substrates, which indicates flexible two-dimensional material based nanoelectronics can be implemented on paper substrates for systems, sensors, and Internet of Things.Item CVD MoS₂ for high speed devices and circuits(2018-05-03) Sanne, Atresh Murlidhar; Banerjee, Sanjay; Akinwande, Deji; Register, Leonard F; Sreenivasan, S.V.; Rao, RajeshTwo-dimensional layered materials (2DLMs) have been widely studied as a potential alternative to the complementary metal-oxide semiconducting field-effect transistor (CMOS FET) "switch." The atomically thin body of 2DLMs lends itself to improved electrostatic gate control, leading to a suppression of the short channel effects which limit the scalability of CMOS devices. While many experiments have examined 2DLMs as a low power solution for aggressively scaled digital devices, their feasibility study for use in high speed radio frequency (RF) devices and circuits is still in its infancy. Current technological trends such as the Internet of Things (IoT) and 5G communication have increased the demand for novel high speed devices to serve next-generation circuits and systems. Graphene, as a 2DLM, has garnered significant interest for its use in high speed radio frequency (RF) devices and circuits. A carrier mobility greater than 10,000 cm²/Vs, ambipolar transport, and excellent thermomechanical stability has afforded graphene cutoff frequencies greater than 400 GHz. Multi-transistor integrated circuits, including a fully integrated RF receiver have been demonstrated using graphene. However, graphene poses a limitation in high speed operation in that the Dirac cone band structure results in a zero bandgap, leading to semi-metallic transport behavior. As a result, graphene field-effect transistors (GFETs) exhibit a low ION/IOFF ratio and non-saturating output behavior. This translates to FETs showing reduced power and voltage gains, hindering the realization of high performance amplifiers, mixers, and other RF circuit elements. Another class of 2DLMs has generated renewed interest for its potential to replace silicon as the next-generation CMOS "switch." Transition metal dichalcogenides (TMDs) is a family of 2DLMs with the general chemical formula MX₂ (M = metal, X = chalcogen). Of the class of TMDs, molybdenum disulfide (MoS₂) is of special interest. With its thickness-dependent electronic properties, MoS₂ has been considered for applications in the fields of opto-electronics, flexible electronics, spintronics, and coupled electro-mechanics. Its single layer direct bandgap of ~1.8 eV allows for high ION/IOFF metal-oxide semiconducting FETs. More relevant for RF applications, theoretical studies predict MoS₂ can afford saturation velocities greater than 3×10⁶ cm/s. While the mobility of MoS₂ is lower than that of graphene, the intrinsic bandgap in MoS₂ has shown voltage gains, A [subscript v] = g [subscript m] /g [subscript ds], greater than 30. Thus far, most of the studies of graphene and MoS₂ have utilized crystalline exfoliated layers, which provide a convenient high quality source of material for laboratory experiments. However, for industrial scale applications, the mechanical cleavage process is not scalable and, thus far, there have been few studies on large area chemical vapor deposited (CVD) MoS₂ RF FETs. In this dissertation, the initial efforts to utilize CVD MoS₂ for RF FETs are presented. The RF figures-of-merit transit frequency, fT, and maximum frequency of oscillation are measured for CVD MoS₂. The effects of different substrates and superstrates on MoS₂ are investigated. In order to improve the cutoff frequencies, a combination of channel length scaling and device geometry modifications are applied. Simple RF circuits are demonstrated experimentally using CVD MoS₂ FETs. Additionally larger circuit building blocks are simulated using experimental data. The goal of this work is to provide a baseline of the RF performance achievable using CVD MoS₂. Hopefully, this work will motivate future studies directing MoS₂ towards industrial electronic applications.Item Electron irradiation-induced defects for reliability improvement in monolayer MoS2-based conductive-point memory devices(2022-05-09) Wu, Xiaohan; Gu, Yuqian; Ge, Ruijing; Serna, Martha I.; Huang, Yifu; Lee Jack C.; Akinwande, DejiMonolayer molybdenum disulfide has been previously discovered to exhibit non-volatile resistive switching behavior in a vertical metal-insulator-metal structure, featuring ultra-thin sub-nanometer active layer thickness. However, the reliability of these nascent 2D-based memory devices was not previously investigated for practical applications. Here, we employ an electron irradiation treatment on monolayer MoS2 film to modify the defect properties. Raman, photoluminescence, and X-ray photoelectron spectroscopy measurements have been performed to confirm the increasing amount of sulfur vacancies introduced by the e-beam irradiation process. The statistical electrical studies reveal the reliability can be improved by up to 1.5× for yield and 11× for average DC cycling endurance in the devices with a moderate radiation dose compared to unirradiated devices. Based on our previously proposed virtual conductive-point model with the metal ion substitution into sulfur vacancy, Monte Carlo simulations have been performed to illustrate the irradiation effect on device reliability, elucidating a clustering failure mechanism. This work provides an approach by electron irradiation to enhance the reliability of 2D memory devices and inspires further research in defect engineering to precisely control the switching properties for a wide range of applications from memory computing to radio- frequency switches.Item Electronic properties and electron-electron interaction effects in transition metal dichalcogenides(2018-08-09) Larentis, Stefano; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K; MacDonald, Allan H; Register, Leonard F; Shi, LiTransition metal dichalcogenides (TMDs) are a new class of two-dimensional layered materials characterized by a MX₂ chemical formula, where M (X) stands for a transition metal (chalcogen). MoS₂, MoSe₂ and MoTe₂ are semiconducting TMDs, which at the monolayer limit possess bandgaps >1 eV, rendering them attractive as possible channel material for scaled transistors. The bandstructures of monolayers feature coupled spin and valley degrees of freedom, thanks to large spin-orbit interaction, and large effective masses (m*), suggesting that electron-electron interaction effects are expected to be important in these semiconductors. In this dissertation we discuss the fabrication and electrical characterization of TMD-based electronic devices, with a focus on their electronic properties, including scattering mechanisms contributing to the mobility, carriers' effective mass, band offset in heterostructures, electronic compressibility, and spin susceptibility. We begin studying the four-point field-effect mobilities of few-layers MoS₂, MoSe₂ and MoTe₂ field effect transistors (FETs), in top-contact, bottom-gate architectures. Using hexagonal boron-nitride dielectrics, we fabricate FETs with an improved bottom-contact, dual-gate architecture to probe transport at low temperatures in monolayer MoS₂, and mono- and bilayer MoSe₂. From conductivity and carrier density measurements we determine the Hall mobility, which shows strong temperature dependence, consistent with phonon scattering, and saturates at low temperatures because of impurity scattering. High mobility MoSe₂ samples probed in perpendicular magnetic field, at low temperatures show Shubnikov-de Haas oscillations. Using magnetotransport we probe carriers in spin split bands at the K point in the conduction band and extract their m* = 0.8m [subscript e]; m [subscript e] is the bare electron mass. Quantum Hall states emerging at either odd or even filling factors are explained by a density dependent, interaction enhanced Zeeman splitting. Gated graphene-MoS₂ heterostructures reveal a saturating electron branch conductivity at the onset of MoS₂ population. Magnetotransport measurements probe the graphene electron density, which saturates and decreases as MoS₂ populates, a finding associated with the negative compressibility of MoS₂ electrons, modeled by a decreasing chemical potential, where many-body contributions dominate. Using a multi-gate architecture in monolayer MoTe₂ FETs, that allows for independent contact resistance and threshold voltage tuning, we integrate reconfigurable n- and p-FETs, and demonstrate a complementary inverter.Item Experimental investigations of energy carrier interactions with atomic disorders and artificial long-range orders(2022-10-06) Smith, Brandon Paul; Shi, Li, Ph. D.; Orbach, Raymond L; Akinwande, Deji; Bank, Seth R; Wang, YaguoThe field of nanoscale energy transport, conversion, and storage is at an exciting time with next-generation devices manipulating discrete energy carriers, e.g. phonons, photons, and electrons, in confined dimensions arriving closer to commercialization, such as solid-state flexible electronics and optoelectronics utilizing one dimension (1D) and two dimensional (2D) nanomaterials. The transport dynamics of quasiparticles and their coupling are modified, notably in low dimensional nanomaterials, with the inclusion of disorder and artificial long-range order. Through this lens, it is possible to probe interesting physics and draw out intrinsic properties of the nanomaterials. This is especially important for electronic systems and energy conversion & storage devices where heat generation and dissipation within nano- and microscale locations of nonequilibrium impedes continued advancement. This thesis examines outstanding questions concerning nanoscale thermal and thermoelectric transport in low-dimensional materials to further understanding of crystal disorder and artificial long-range order. Specifically, the material systems investigated are alloy disorder and surface roughness in semiconducting silicon germanium (SiGe) nanowires, microscale rippling in layered molybdenum disulfide (MoS2) flakes, intra- and interlayer interactions in bulk and monolayer MoS2, and artificially created, long-range domain walls in twisted bilayer graphene (TBG). The fundamental questions are addressed through electrothermal, optothermal, and scanning probe metrology techniques. First, eight-probe thermal conductivity measurements of SiGe nanowires show that alloying suppresses thermal transport, and the mean-free-paths of low-frequency phonons are suppressed by diffuse surface roughness scattering in nanowires. The diffuse surface scattering results in length-independent thermal conductivity for lengths over two micrometers. Similarly, four-probe thermal conductivity measurements reveal that microscale ripples have negligible effects on phonon transport in 2D layers as the ripple wavelengths and curvatures are much larger than the phonon mean free paths and wavelengths. The peak thermal conductivity is found to increase with decreasing Raman scattering intensity in the frequency range with vanishing phonon density of states in MoS2 indicating an important role of point defect scattering. In addition, this dissertation presents an experimental effort to employ micro-Raman spectroscopy to investigate local nonequilibrium among different phonon polarizations in MoS2 inside the focused laser spot. It also describes an exploration of ultra-high vacuum scanning probe microscopy for probing the local thermoelectric property of twisted bilayer graphene moiré superstructures.Item Fabrication and characterization of TMD FET architectures for increased functionality(2020-12-09) Rodder, Michael Allen; Dodabalapur, Ananth, 1963-; Tutuc, Emanuel; Banerjee, Sanjay K; Li, Xiaoqin; Yu, Edward TThe discovery of ultra-thin, van der Waals bound semi-metal (graphene), transition metal dichalcogenide (TMD) semiconductors, and insulator (h-BN), obtained via mechanical exfoliation and stacked cleanly onto one another via dry-transfers, leads naturally to the study of field effect transistors (FETs) made from these 2D materials. In this dissertation, we fabricate various conventional and newly designed field-effect transistor (FET) architectures comprised of 2D materials (for logic, memory, or synaptic applications) and report on electrical characteristics. The 2D materials used in fabrication of our FET architectures include materials for the channel region (MoS2 or WSe2), gate dielectric (h-BN), or the gate (graphene). We begin studying the FET properties of a simple 2D FET architecture (demonstrated with a MoS2 channel) which could augment Si, namely a 2D FET structure utilizing contact gating to reduce parasitic source-drain series resistance (RSD). We show that if mobility and threshold voltage (VT) are well-extracted, then this contact-gated 2D FET structure can still be easily modeled with basic FET equations, such that e.g. a conventional circuit design algorithm could implement the contact-gated 2D FET as easily as a conventional Si FET. Since contact gating is a basic feature of our 2D FET architectures, we next fabricate and characterize two novel contact-gated 2D FET architectures, with the goal being to reduce RSD, while maintaining as thin-as-possible channel layer for electrostatics and/or small subthreshold slope, for improved overall performance. The first FET architecture (single-gated; demonstrated with a WSe2 channel) improves hole injection into a single layer WSe2 channel by use of multilayer WSe2 (with smaller bandgap compared to single layer WSe2) only underneath the source (hole injecting) metal contact. The smaller bandgap multilayer WSe2 results in a smaller Schottky barrier at the metal-WSe2 interface, reducing contact resistance (RC) and thus reducing overall RSD. The second FET architecture (double-gated; demonstrated with MoS2) reduces RSD by using a MoS2 channel layer both above and below the source/drain metal contacts so that both the top and bottom gate electrostatically dope the contact regions. This reduces RSD by ~factor of 2x, and further improves the gating symmetry in MoS2 DGFETs, thus allowing for circuit design flexibility. We then move on to the fabrication of a significantly different FET architecture (i.e. not focused on a contact-gated FET architecture) that could further augment Si technology. In particular, we fabricate and characterize a device, a double-gate MoS2 field-effect transistor (FET) with hexagonal boron nitride (h-BN) gate dielectrics and a multi-layer graphene floating gate (FG), in multiple operating conditions to demonstrate logic, memory, and synaptic applications, beyond that which could be demonstrated in a single Si-FET architecture. In our work, we noted that some of our fabricated devices exhibited a particular gate-bias-dependent kinking in I-V characteristics, which required explanation. The final chapter of this thesis thus formulates a phenomenological model, accounting for interface states at metal-semiconductor contacts, to explain the I-V kinking. The model highlights that 1) metal-semiconductor interface states need to be accounted for when modeling MoS2 FETs, and 2) the importance of forming metal-semiconductor interfaces with low interface state density to avoid I-V kinks which are detrimental for analog applications.Item Molybdenum Disulfide Nanodisks For Photoelectrochemical Hydrogen Evolution(2019-05-01) Tekell, Marshall; Fan, DongleiCurrent strategies of energy production and conversion continue to emit CO2 at a rate that is extremely likely to warm the planet 1.5°C by 2052, and energy sourced from renewables needs to increase 95% by 2050 in the most relaxed reductions emissions scenarios. Photoelectrochemical (PEC) hydrogen evolution from MoS2/p-Si is introduced as a technology that can directly convert solar energy to chemical energy without the use of rare earth metals. The mechanisms of both hydrogen evolution from 2H-MoS2 edge sites and semiconductor photocatalysis are discussed. Colloidal lithography using masks of polystyrene (PS) nanospheres, electron-beam deposition, and chemical vapor deposition were used to control the diameter (200–500 nm), thickness (1.5–9.5 nm) and areal density (1.8–20.9%) of MoS2 nanodisks on p-Si, which were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, and image processing techniques. The PEC performances of bare p-Si and MoS2/p-Si were analyzed using linear scan voltammetry (LSV) and chronoamperometry (CA). Replacing Ga-In with Au as the back contact for p-Si in PEC testing was found to reduce the magnitude of the overpotential at -10 mA cm-2 from -593 to -390 mV due to Schottky barrier removal. 200 nm MoS2 and 500 nm MoS2 nanodisks on p-Si further decreased the overpotential at 10 mA cm-2 from -390 mV to -234 and -172 mV, respectively, and produced short-circuit currents of -0.45 mA cm-2 and -0.80 mA cm-2, respectively. The stability of MoS2/p-Si photocathode performance was found to depend on the thickness of e-beam deposited Mo, with a 31 and 134 mV decrease in overpotential measured for 500 nm MoS2 nanodisks produced from 1 nm and 4 nm Mo, respectively. Finally, the in situ observation of hydrogen evolution from bare p-Si was demonstrated, and images were collected bubbles on the microscale. Future work involves optimizing the thickness of MoS2 to meet state-of-the-art performance parameters and investigating the conditions under which the growth of insulating SiO2 affects photocathode performance.