Browsing by Subject "Transition metal dichalcogenides"
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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 Functionality enhancement of two-dimensional transition metal dichalcogenide-based transistors(2019-08-15) Rai, Amritesh; Banerjee, Sanjay; Akinwande, Deji; Register, Leonard F; Sreenivasan, S. V.; Wang, YaguoAtomically thin molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂), members of the transition metal dichalcogenide family, have emerged as prototypical two-dimensional semiconductors with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors, the mechanically flexible and transparent nature of 2D MoS₂ and WSe₂ make them even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of these materials can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to their contact, doping and mobility engineering must be overcome. This dissertation reviews the important technologically relevant properties of semiconducting 2D TMDs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS₂ and WSe₂-based devices. Finally, this dissertation provides a comprehensive insight into two novel and promising engineering solutions that can be employed to address the all-important issues of contact resistance, controllable and area-selective doping, and charge carrier mobility enhancement (electrons in MoS₂ and holes in WSe₂) in these devices. Specifically, this work sheds light upon the interfacial-oxygen-vacancy mediated n-doping of MoS₂ by high-κ dielectrics, such as HfO₂, Al₂O₃ and TiO₂, using detailed experimental characterizations and theoretical calculations. This n-doping effect on MoS₂ by high-κ dielectrics is proposed as a mechanism responsible for the performance enhancement observed in MoS₂ devices upon encapsulation in high-κ dielectric environments. This work also sheds light upon the band structure engineering and p-doping of layered WSe₂ using a simple and facile one-step chemical functionalization technique utilizing ammonium sulfide solution. Detailed experimental and theoretical studies once again reveal the underlying mechanism responsible for the p-doping in WSe₂ after chemical treatment. Results show that the doping techniques presented in this dissertation can easily be adapted to obtain high-performance FETs based on 2D MoS₂ and WSe₂. Finally, some future research directions, based on the work presented in this dissertation, are highlighted.Item Piezoelectricity and flexoelectricity in 2D transition metal dichalcogenides(2018-05-01) Brennan, Christopher James; Yu, Edward T.; Lu, Nanshu; Banerjee, Sanjay K; Akinwande, Deji; Huang, RuiTwo-dimensional materials are only on to a few atoms thick, making them the thinnest possible material known to man. Their combination of electrical, optical, and mechanical properties allows for unique electrical devices with a wide range of future applications, from being a post-silicon material option, creating high-speed communication systems, allowing the advancement of flexible electronics, and even creating transparent electronics. Among their amazing characteristics is the coupling of electrical and mechanical properties. Although not unique to 2D materials, electromechanical coupling could be used in 2D materials to create a class of sensors, actuators, and energy harvesters at a scale not previously possible. Specifically, 2D materials could be utilized in flexible, wearable electronics as an energy harvester to convert the motion of the body into electrical energy. In this dissertation, the electromechanical coupling properties known as piezoelectricity and flexoelectricity are studied in 2D materials both to advance the development of 2D materials in general, and to improve the understanding of the relatively novel effect of flexoelectricity. This work focuses on a class of 2D materials known as transition metal dichalcogenides (TMDs), which are semiconducting and intrinsically piezoelectric. To begin, the adhesion between the TMDs and soft substrates is studied. Soft substrates could be used in flexible and wearable electronic systems, so adhesion of TMDs to soft substrates is important. It was found that the adhesions between the TMD molybdenum disulfide and polydimethylsiloxane is roughly 18 mJ m⁻². Next, the out-of-plane electromechanical coupling of molybdenum disulfide and other TMDs was studied. Piezoelectric theory predicts that there should be zero out-of-plane response, but a signal is measured in all TMDs, suggesting the presence of flexoelectricity. The measured effective out-of-plane piezoelectric response is on the order of 1 pm V⁻¹ and the estimated flexoelectric response is on the order of 0.05 nC m⁻¹. Additionally, it was found that the magnitude of the out-of-plane electromechanical response of different TMDs roughly follows a trend predicted by a simple model of flexoelectricity. The work presented in this dissertation provides the first experimental evidence of a flexoelectric effect present in 2D TMDs.Item Thin film transistors based on transition metal dichalcogenides for spintronics and logic circuits(2023-06-23) Li, Xintong, Ph. D.; Incorvia, Jean Anne; Akinwande, Deji; Yu, Edward; Dodabalapur, Ananth; Li, XiaoqinThin film transistors (TFTs) based on two-dimensional (2D) van der Waals materials have attracted significant interest in the post-Moore era due to their unique properties, such as high carrier mobility, ultra-thin thickness, tunable bandgap, reduced short channel effects, and novel physics such as Moiré pattern and spin and valley Hall effects (SVHE). These properties have the potential to enable faster, more efficient logic and analog circuits, memory, optoelectronics and spintronics. TFTs based on transition metal dichalcogenides (TMDs) are popular since these materials possess proper thickness-dependent band structures, high mobility, and abundant novel physics. In this dissertation, the TFTs mainly based on tungsten diselenide (WSe₂) are fabricated and studied. First, the SVHE in monolayer WSe₂ TFTs is experimentally measured, and key parameters are extracted from the measurements. Kerr rotation (KR) measurements show the spatial distribution of the SVHE at different temperatures, its persistence up to 160 K, and that it can be electrically modulated via gate and drain bias. A spin/valley drift and diffusion model together with a reflection measurement and a four-port electrical measurement is used to interpret the KR data. The spin/valley lifetime, mean free path and polarization are calculated. These are important steps towards the potential application of spintronics and valleytronics based on TMD materials. Then the potential of TMD based TFTs in logic circuits is studied, which is a more practical application. New ambipolar dual-gate TFTs based on WSe₂ are fabricated and show near-ideal performances, including a high on-off ratio, low off-state current, ideal subthreshold swing (SS), and negligible hysteresis. For the first time, cascadable logic gates based on ambipolar TMD transistors are then demonstrated with fewer transistors than CMOS and minimal static power consumption, including inverters, XOR, NAND, NOR, and buffers made by cascaded inverters. Then a thorough study of the behavior of both gates is conducted, which has previously been lacking. The large noise margin enables the implementation of VT-drop circuits, a type of logic with reduced transistor number and simplified circuit design. Finally, the speed performance of the VT-drop and other circuits built by dual-gate devices are qualitatively analyzed.Item Transition metal dichalcogenide MoSe2 nanostructures(2016-12-13) Chen, Yuxuan, 1986-; Shih, Chih-Kang; de Lozanne, Alejandro; Fiete, Gregory A; Niu, Qian; Shi, LiTransition metal dichalcogenides (TMDs) are a family of van der Waals (vdW) layered materials exhibiting unique electronic, optical, magnetic, and transport properties. Their technological potentials hinge critically on the ability to achieve controlled fabrication of desirable nanostructures. Here I present three kinds of nanostructures of semiconducting TMD MoSe₂, created by molecular beam epitaxy (MBE) and characterized by scanning tunneling microscopy and spectroscopy (STM/STS). The three kinds of nanostructures are two-dimensional (2D) nanoislands, quasi one-dimensional (1D) nanoribbons, and heterostructures. The successful growth of 2D nanoislands lays the foundation for the preparation of the other two structures. By properly controlling the substrate temperature and Se over-pressure, the MoSe₂ atomic layers undergo a dramatic three-stage shape transformation: from fractal to compact 2D nanoislands, and eventually to nanoribbons, in stark contrast to the traditional two-stage growth behaviour involving only the transformation from the fractal to compact regime. Experimentally, it is found that the Se:Mo flux ratio during MBE growth plays a central role in controlling the nanoribbon formation. Theoretically, first-principles calculations show that the abundance/deficiency of extra Se atoms at different island edges significantly modifies the relative step energies between zigzag and armchair edges, which in turn impacts the island shape evolution during nonequilibrium growth. The successful preparation of MoSe2/hBN/Ru(0001) heterostructure is a demonstration that MBE technique is suitable for fabricating vdW heterostructures. Surprisingly, we found that the quasi-particle gap of the MoSe₂ on hBN/Ru is about 0.25 eV smaller than those on graphene or graphite substrates. We attribute this result to the strong interaction between hBN/Ru which causes residual metallic screening from the substrate. The surface of MoSe₂ exhibits Moiré pattern that replicates the Moiré pattern of hBN/Ru. In addition, the electronic structure and the work function of MoSe₂ are modulated electrostatically with an amplitude of ~ 0.13 eV. Most interestingly, this electrostatic modulation is spatially in phase with the Moiré pattern of hBN on Ru(0001) whose surface also exhibits a work function modulation of the same amplitudeItem Two-dimensional coherent spectroscopy of monolayer transition metal dichalcogenides(2015-08) Dass, Chandriker Kavir; Li, Elaine; Downer, Michael; Lai, Keji; Shih, Chih-Kang; Tutuc, EmanuelTwo-dimensional semiconductors have long been studied for their unique optical and electronic properties, but with the work of Novoselov and Geim on van der Waals materials, two-dimensional semiconductors have seen a surge of renewed interest. This dissertation focuses on monolayer transition metal dichalcogenides (TMDCs), a class of two-dimensional materials that can easily be fabricated by mechanical exfoliation, much like graphene. In their bulk form, these materials have indirect band gaps, but transition to direct gap semiconductors in the monolayer limit. The band-edge optical response of TMDCs, like WSe₂ and MoS₂, is dominated by exciton absorption occurring at the ±K-points of the Brillouin zone. Because of the unique electronic structure of these materials, these two points form distinct valleys in the band structure which can be exploited to produce valley polarization. Exciton quantum dynamics are characterized by two fundamental parameters, one of which is the dephasing rate, γ, which describes quantum dissipation arising from the interaction of the excitons with their environment (i.e. other excitons, impurities, etc…). This dissertation focuses on measuring the fundamental property of dephasing time (which is inversely proportional to the dephasing rate and homogeneous linewidth) in monolayer WSe₂ through the use of two-dimensional coherent spectroscopy. Our measurements have revealed a homogeneous linewidth consistent with dephasing times in the sub-picosecond regime. We also characterize the role of exciton-exciton and exciton-phonon interactions, on the homogeneous linewidth, through excitation density and temperature dependent studies. These studies have revealed strong many-body effects and nonradiative population relaxation as the primary dephasing mechanisms. Microscopic calculations show that in perfect crystalline samples of monolayer TMDCs, the radiative lifetimes are also in the sub-picosecond regime due to the large oscillator strengths inherent in these materials. This result is consistent with the short dephasing times found experimentally.