Browsing by Subject "TMD"
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Item Electron mobility in monolayer WS₂ encapsulated in hexagonal boron-nitride(2021-05-03) Wang, Yimeng, M.S. in Engineering; Tutuc, Emanuel, 1974-We report electron transport measurements in dual-gated monolayer WS₂ encapsulated in hexagonal boron-nitride. Using gated Ohmic contacts which operate from room temperature down to 1.5 K, we measure the intrinsic conductivity and carrier density as a function of temperature and gate bias. An intrinsic electron mobility of 100 cm²/(V·s) at room temperature, and 2,000 cm²/(V·s) at 1.5 K are achieved. The mobility shows a strong temperature dependence at high temperatures, consistent with phonon scattering dominated carrier transport. At low temperature, the mobility saturates due to impurity and long-range Coulomb scattering. First principles calculations of phonon scattering in monolayer WS₂ are in good agreement with the experimental results, showing we approach the intrinsic limit of transport in these 2D layers.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 Engineering two-dimensional materials : discovery, defects, and environment(2021-05-21) Holbrook, Madisen A.; Shih, Chih-Kang; Shi, Li; Li, Xiaoqin; Lai, KejiThe discovery of graphene and its unprecedented properties inspired an extraordinary increase in research progress, launching an era of two-dimensional (2D) electronic materials. These stable crystalline atomic layers enable the design of ultrathin 2D devices by combining different 2D materials as the foundational components. In order to control the properties of these devices, materials with a variety of electronic properties must be available. In this dissertation, we explore three distinct paths to achieve this goal: expanding the library of 2D materials, post synthesis defect engineering, and proximity engineering of the electrostatic environment. First, we report the MBE synthesis and STM/S characterization of a new 2D insulator, honeycomb structure BeO. In addition to determining the atomic structure and density of states, we used moiré pattern analysis to demonstrate the high crystallinity of the BeO and determined the work function modulation across the moiré pattern. We illustrate that the scalable growth, weak substrate interactions, and long-range crystallinity make honeycomb BeO an attractive candidate for future technological applications. The next focus of this work was defect engineering of monolayer WS₂ by UHV annealing. A high concentration of S vacancies was generated by UHV annealing of the WS2, leading to S vacancy defect-defect coupling. Using STM/S we determined that the interaction of nearby S vacancies leads to an increase of deep in-gap states for different divacancy geometries. This indicates that vacancy engineering can be a useful tool to controllably manipulate 2D material electronic properties. Finally, we demonstrate the creation of a nanoscale planar p-n junction within a single monolayer of MoSe₂ by modulating the electronic properties of the underlying substrate. By intercalating Se at the interface of the hBN/Ru substrate, the hBN becomes decoupled from the Ru, changing its conductivity and work function. We find that this change in the electronic landscape tunes the band gap of the overlying MoSe₂, by screening and shifting the MoSe₂ work function. Thus, this dissertation shines a light on the vast opportunities 2D materials provide for exploration of novel approaches to materials engineering, and demonstrates a tool set for manipulating the electronic properties of these fascinating materials.Item Growth optimization of WSe₂ and its sulfurization to WS₂(2019-07-09) Liu, Chison Qishan; Banerjee, SanjayTungsten diselenide has gained much interest within recent years after it was reported to have both p-type and ambipolar transport properties. And because most other transition metal dichalcogenides exhibit n-type transport properties, tungsten diselenide would help to further realize CMOS technology if one could find a more reliable way to synthesize it in large areas with high quality crystallinity. In this report I will be detailing my work on successfully synthesizing WSe₂, its sulfurization into WS₂, and discussing what I’ve observed in both the crystal quality and growth mechanisms. My goal is to provide a better understanding of the growth process in hopes of moving forward with improving future growth recipesItem In-situ real-time spectroscopy platform for monitoring gas adsorption and reactions on 2D materials(2019-12-05) Holt, Milo; Akinwande, Deji; Banerjee, Sanjay; Bank, Seth; Cullinan, Michael; Dodabalapur, AnanthThin film gas sensors are at the center of critical areas of research and innovation, with applications in a wide range of important and fast-evolving fields. From environmental monitoring and medical diagnosis to early warning systems and safety, gas sensors are a vital front end technology already ubiquitous in industry. With development of the Internet of Things (IoT), the need for untethered platforms is accelerating the search for gas sensing materials suitable in low-power applications. 2D materials like graphene and 40+ combinations of the transition metal dichalcogenides (TMDs) show particular promise here, owing to a suite of exceptional dimensional, structural and electronic properties. Specifically with respect to gas sensing, 2D thin films provide the ultimate material dimensions and properties for low-power fast-response devices, at the same time providing an excellent interface for analyte-surface observations. This dissertation concerns the sensing properties and stability of the four prototypical semiconducting TMD combinations of molybdenum (Mo), Tungsten (W), Sulfur (S) and Selenium (Se), with a focus on molybdenum disulfide (MoS₂). This work presents a platform solution for the in-situ real-time (dynamic) capture and study of ammonia (NH₃), water vapor (H₂O) and oxygen (O₂) interactions with exfoliated and synthesiszed TMDs in a controlled environment, at both ambient and elevated temperaturesItem Trion and exciton dynamics in two dimensional semiconductors(2016-06-08) Singh, Akshay Kumar; Li, Elaine; Shih, Chih-Kang; Downer, Michael C; Niu, Qian; Bank, Seth RTwo-dimensional semiconducting systems have become increasingly important for a variety of applications including photo-detectors, high-power transistors and optoelectronics. With the discovery of the indirect-to-direct bandgap transition in atomically thin transition metal dichalcogenide (TMDs’) materials, a plethora of further applications and advances await. Optical properties in these materials are especially interesting to measure, due to presence of spin-valley coupling giving rise to valleytronic applications, and enhanced light emission (and absorption) with applications in optoelectronics. Optical studies in semiconductors near the bandgap primarily relate to the fundamental optical excitation of semiconductors, an exciton (a Coulomb-bound electron-hole pair). If the Coulomb interaction is strong enough, excitons may capture an extra electron or hole, forming charged excitons known as trions. Trions have shown to carry longer-lived spin information, and can drift under an electric field. The interaction between excitons and trions, is thus a technologically important issue in optoelectronics. The purpose of this dissertation is to measure the interactions between excitons and trions in a variety of two-dimensional systems, primarily in the new class of semiconducting two-dimensional materials, TMDs’. The interactions are measured for their character (coherent or incoherent) and dynamics. Utilizing a two-color pump-probe setup we uncover coherent coupling, between excitons and trions in monolayer molybdenum diselenide, an order of magnitude larger than traditional semiconductors (like gallium arsenide). Incoherent relaxation pathways towards trions, are measured via resonant excitation of excitons. A mobility edge within the exciton resonance is uncovered, with applications in quantifying transport properties of materials under study. Further, valley sensitive measurements are carried out on monolayer tungsten diselenide, revealing the long-lived trion spin polarization and ultrafast exciton valley relaxation. The possible spectroscopy feature of biexcitons is discussed in monolayer tungsten diselenide. Finally, measurements are extended to high mobility gallium arsenide quantum well systems, and electron-density dependent spin scattering mechanisms are uncovered. We further discuss the possibility to suppress spin relaxation, via gate voltage, in these gallium arsenide quantum wells.