Electronic and spintronic devices using two-dimensional materials

179 p. El contenido del capítulo 8 está sujeto a confidencialidadEver since in 2004 atomically-thin two-dimensional van der Waals materials became available to the scientific community, at the reach of manual microexfoliation techniques, their implementation in novel device structures and concepts promised disruptive new applications and motivated research in a vast range of fields.Confined to the thinnest possible thickness, electrons in these materials exhibit a plethora of electronic properties, from semiconducting MoS2, to superconductor NbSe2, dielectric BN, and, jack-of-all trades, graphene.In this thesis, we explore fundamental and applied aspects of chemical vapor deposition (CVD) graphene, MoS2, and WSe2 using electronic device structures that use them as transporting channel, namely field-effect transistors (FETs), Hall bars, and diodes.MoS2 is a n-type semiconducting 2D vdW that complements one of the weak aspects of graphene-based transistors, which is the small ratio between the maximum current output and of the minimum current output of the transistors. Using MoS2 we identify an electron doping constraint for performing stable magnetotransport measurements, and we investigate the origins of the strong current fluctuations of the FETs. We study the low-frequency noise (LFN) of the current output of devices made with different layer thicknesses, and use the strong light-matter interactions of MoS2 to employ photodoping techniques together with the electrostatic gating to dope the channel. By converging all these conditions, we are able to discern the mechanism behind the different types of LFN noise reported in literature for MoS2, while at the same time identifying a LFN crossover driven by photodoping.With p-type semiconducting WSe2 we optimize the electron and hole transport properties of ambipolar FETs by considering BN as a top and bottom interface substrate and encapsulation layer, respectively. By doing so, we areable to address to some extent the strong hysteretic effects that adversely affect the operation of WSe2 FETs on oxide substrates, and improve the overall device performance.The versatility of CVD graphene allows us to do both applied and fundamental studies, both related to spintronics and electronics.The unique properties of graphene make it a core material in the search of full-electrical approaches to generate, transport, and detect spin currents without the use of magnetic elements. Using a Hall-bar shaped sample, non-local signals in graphene have been demonstrated to be associated with spin transport. In our case, we use the large area availability of CVD graphene to study non-local effects in an unlikely scenario for the transport of spins. We study the non-local signals of millimeter sized Hall-bars of CVD graphene, and by doing a systematic study as a function of device scale, from macro-to-microscale we identify a mechanism that cannot be connected with spin diffusion that also leads to large signals. By evaluating the microscopic details of the samples, and the different effects observed, we propose a mechanism mediated by grain boundaries to drive such effects.In a more applied manner, we use CVD graphene for two other types of devices. First, we study the use of graphene as an electrode material for lateral and vertical field-effect transistors that operate using organic channels, and determine that the low density of states of graphene allows for unscreened electric fields to reach the organic layer and enable the transistor operation in the vertical geometry.The second applied study is the large-scale fabrication of diodes using CVD graphene. Benefiting from the ultra-thin cross section of graphene, and using a lateral geometry we demonstrate the reliable fabrication of lateral metal/insulator/graphene diodes. The time constants determined from the direct-current analysis place the operation of the fabricated devices in the THz range. Additionally, the material combination considered enabled large current densities based on field-emission processes.CICnanoGUNE : nanoscience cooperative research cente

Spin transport in graphene/transition metal dichalcogenide heterostructures

Since its discovery, graphene has been a promising material for spintronics: its low spin-orbit coupling, negligible hyperfine interaction, and high electron mobility are obvious advantages for transporting spin information over long distances. However, such outstanding transport properties also limit the capability to engineer active spintronics, where strong spin-orbit coupling is crucial for creating and manipulating spin currents. To this end, transition metal dichalcogenides, which have larger spin-orbit coupling and good interface matching, appear to be highly complementary materials for enhancing the spin-dependent features of graphene while maintaining its superior charge transport properties. In this review, we present the theoretical framework and the experiments performed to detect and characterize the spin-orbit coupling and spin currents in graphene/transition metal dichalcogenide heterostructures. Specifically, we will concentrate on recent measurements of Hanle precession, weak antilocalization and the spin Hall effect, and provide a comprehensive theoretical description of the interconnection between these phenomena.Comment: 21 pages, 11 figures. This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters, copyright\c{opyright}American Chemical Society after peer review. To access the final edited and published work see http://pubs.rsc.org/en/Content/ArticleLanding/2018/CS/C7CS00864

Proximitized Materials

Advances in scaling down heterostructures and having an improved interface quality together with atomically-thin two-dimensional materials suggest a novel approach to systematically design materials. A given material can be transformed through proximity effects whereby it acquires properties of its neighbors, for example, becoming superconducting, magnetic, topologically nontrivial, or with an enhanced spin-orbit coupling. Such proximity effects not only complement the conventional methods of designing materials by doping or functionalization, but can also overcome their various limitations. In proximitized materials it is possible to realize properties that are not present in any constituent region of the considered heterostructure. While the focus is on magnetic and spin-orbit proximity effects with their applications in spintronics, the outlined principles provide also a broader framework for employing other proximity effects to tailor materials and realize novel phenomena.Comment: Invited Review to appear in Materials Today, 28 pages, 22 figure

Physical and Electronic Properties of Nanoscale 2D Materials

There is a great push towards reducing the size scale of both electronic components and machines. Two dimensional materials, such as graphene, are ideal candidates towards this push, as they are naturally atomically thin. In the case of nanoscale machines, the mechanical properties of the material surfaces become increasingly important. The use of laminar materials, such as graphene and MoS2, to modify the surface properties, yet maintain nanoscale topographical features, are very attractive. Towards this goal, we have investigated the surface properties of MoS2 at the nanoscale using Lateral Force Microscopy (LFM). In these investigations, we measure periodic frictional features with periodicity of ~ 4 nm. Ultrashort devices that incorporate atomically thin components have the potential to be the smallest electronics. Such extremely scaled devices are expected to show ballistic nonlinear behavior that could make them tremendously useful for ultra fast electronic applications. We report nonlinear electron transport in ultrashort channel graphene devices. We observe this nonlinear response up to room temperature, with zero applied magnetic field, on a readily accessible oxide substrate. This makes the nanogap technology we utilize of great potential for achieving extremely scaled high-speed atomically thin devices

Two-dimensional materials for electronic applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 205-232).The successful isolation of graphene in 2004 has attracted great interest to search for potential applications of this unique material and other members of the two-dimensional materials family in electronics, optoelectronics and their interface with the biological systems. At this early stage of 2D materials research, many opportunities and challenges co-exist in this area. This thesis addresses the following issues which are crucial for 2D electronics to be successful, focusing on developing graphene for RF electronics and MoS2 for digital applications: (1) Development of some of the first graphene-based devices for high frequency applications; (2) Development of compact physical models for graphene transistors; and (3) Understanding the carrier transit delays in graphene transistors. In addition, this thesis proposes and experimentally demonstrates a completely new concept - Ambipolar Electronics - to take advantage of the unique properties of graphene for RF applications. Based on this new concept, a family of novel applications are developed that can significantly simplify the design of many fundamental building blocks in RF electronics, such as frequency multipliers, mixers and binary phase shift keying devices. In the last part of the thesis, the applications of other emerging 2D materials from the transition metal dichalcogenides family, such as molybdenum disulfide (MoS₂), is also explored for potential application in digital electronics, especially as a new material option for high performance flexible electronics. The future opportunities and potential challenges for the applications of the 2D materials family are also discussed.by Han Wang.Ph.D

Graphene spin circuits and spin-orbit phenomena in van der Waals heterostructures with topological insulators

Spintronics offers an alternative approach to conventional charge-based information processing by using the electron spin for next-generation non-volatile memory and logic technologies. To realize such technologies, it is necessary to develop spin-polarized current sources, spin interconnects, charge-to-spin conversion processes, and gate-tunable spintronic functionalities. The recently emerged two-dimensional (2D) and topological materials represent a promising platform to realize such spin-based phenomena. Due to its small spin-orbit coupling (SOC), graphene was predicted to preserve electron spin coherence for a long time, making it an ideal material for spin communication. In contrast, topological insulators (TIs) have high SOC and develop a nontrivial band structure with insulating bulk but conducting spin-polarized surface states. Combining these materials in van der Waals heterostructures has been predicted to give rise to unique proximity-induced spin-orbit phenomena that may be used for electrical control of spin polarization.In this thesis, we experimentally prove that the large-area chemical vapor deposited (CVD) graphene is an excellent material choice for the realization of robust spin interconnects, which are capable of spin communication over channel lengths exceeding 34 μm. Utilizing such graphene, we realize a spin summation operation in multiterminal devices and employ it to construct a prototype spin majority logic gate operating with pure spin currents. In topological insulators, we electrically detect the spin-momentum locking and reveal how the bulk and surface conducting channels affect the charge-to-spin conversion efficiency. Finally, by combining graphene and TIs in hybrid devices, we confirm the emergence of a strong proximity-induced SOC with a Rashba spin texture in graphene. We further show that in such heterostructures a spin-charge conversion capability is induced in graphene via the spin-galvanic effect at room temperature and reveal its strong tunability in magnitude and sign by the gate voltage. These findings demonstrate the robust performance of graphene as a spin interconnect for emerging spin-logic architectures and present all-electrical and gate-tunable spintronic devices based on graphene-TI heterostructures, paving the way for next-generation spin-based computing

Electronic and spintronic devices using two-dimensional materials

179 p. El contenido del capítulo 8 está sujeto a confidencialidadEver since in 2004 atomically-thin two-dimensional van der Waals materials became available to the scientific community, at the reach of manual microexfoliation techniques, their implementation in novel device structures and concepts promised disruptive new applications and motivated research in a vast range of fields.Confined to the thinnest possible thickness, electrons in these materials exhibit a plethora of electronic properties, from semiconducting MoS2, to superconductor NbSe2, dielectric BN, and, jack-of-all trades, graphene.In this thesis, we explore fundamental and applied aspects of chemical vapor deposition (CVD) graphene, MoS2, and WSe2 using electronic device structures that use them as transporting channel, namely field-effect transistors (FETs), Hall bars, and diodes.MoS2 is a n-type semiconducting 2D vdW that complements one of the weak aspects of graphene-based transistors, which is the small ratio between the maximum current output and of the minimum current output of the transistors. Using MoS2 we identify an electron doping constraint for performing stable magnetotransport measurements, and we investigate the origins of the strong current fluctuations of the FETs. We study the low-frequency noise (LFN) of the current output of devices made with different layer thicknesses, and use the strong light-matter interactions of MoS2 to employ photodoping techniques together with the electrostatic gating to dope the channel. By converging all these conditions, we are able to discern the mechanism behind the different types of LFN noise reported in literature for MoS2, while at the same time identifying a LFN crossover driven by photodoping.With p-type semiconducting WSe2 we optimize the electron and hole transport properties of ambipolar FETs by considering BN as a top and bottom interface substrate and encapsulation layer, respectively. By doing so, we areable to address to some extent the strong hysteretic effects that adversely affect the operation of WSe2 FETs on oxide substrates, and improve the overall device performance.The versatility of CVD graphene allows us to do both applied and fundamental studies, both related to spintronics and electronics.The unique properties of graphene make it a core material in the search of full-electrical approaches to generate, transport, and detect spin currents without the use of magnetic elements. Using a Hall-bar shaped sample, non-local signals in graphene have been demonstrated to be associated with spin transport. In our case, we use the large area availability of CVD graphene to study non-local effects in an unlikely scenario for the transport of spins. We study the non-local signals of millimeter sized Hall-bars of CVD graphene, and by doing a systematic study as a function of device scale, from macro-to-microscale we identify a mechanism that cannot be connected with spin diffusion that also leads to large signals. By evaluating the microscopic details of the samples, and the different effects observed, we propose a mechanism mediated by grain boundaries to drive such effects.In a more applied manner, we use CVD graphene for two other types of devices. First, we study the use of graphene as an electrode material for lateral and vertical field-effect transistors that operate using organic channels, and determine that the low density of states of graphene allows for unscreened electric fields to reach the organic layer and enable the transistor operation in the vertical geometry.The second applied study is the large-scale fabrication of diodes using CVD graphene. Benefiting from the ultra-thin cross section of graphene, and using a lateral geometry we demonstrate the reliable fabrication of lateral metal/insulator/graphene diodes. The time constants determined from the direct-current analysis place the operation of the fabricated devices in the THz range. Additionally, the material combination considered enabled large current densities based on field-emission processes.CICnanoGUNE : nanoscience cooperative research cente

A Review on Mechanics and Mechanical Properties of 2D Materials - Graphene and Beyond

Since the first successful synthesis of graphene just over a decade ago, a variety of two-dimensional (2D) materials (e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical) is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces (adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date, we expect continual growth of interest in the mechanics of other 2D materials beyond graphene

Influence of structural and electrostatic disorder on transport properties of monolayers of two-dimensional semiconductors

Two-dimensional (2D) materials are under intensive investigation recently due to variety of electronic properties, ranging from insulators (h-BN) to semi-metals (graphene), semiconductors (MoS2, WSe2) with wide variability of band-gap and correlated phases (NbSe2, TaS2). Recently, focus of research has been moved from graphene, which is now a well understood material, towards less explored materials, in particular monolayers of semiconducting transition metal dichalcogenides (TMDCs) such as MoS2 with direct band-gap in optical range (1.5 - 2 eV) and potential towards scalable electronics applications immune to short channel effects. This thesis in fact explores materials beyond exfoliated MoS2 with the focus on electrical and structural properties of monolayers of WS2, ReS2 and synthesized by means of chemical vapour deposition (CVD) monolayers of MoS2, MoSe2 and WSe2. Four main chapters discuss the following aspects of these materials. Chapter 4 studies in details transport properties of monolayer WS2 for the first time. We demonstrate wide tuning of transport in this material from insulating regime through localized states to band-like transport with insulator to metal transition (MIT) in between, with mobilities = 50 cm2/Vs at room temperature. In Chapter 5 we employ electrolyte gating for fabrication of the first electric double-layer transistor (EDLT) based on monolayers and multilayers of ReS2. Furthermore, we employ this as a system for variation of electrostatic disorder and find an unusual and strong modulation in conduction band of ReS2. The discovered modulation of conductivity via controllable introduction of electrostatic disorder can help to understand this material as well as other ultrathin 2D systems and to optimize the design of devices based on them. In Chapter 6 we use atomically smooth sapphire for demonstration of the first epitaxial monolayer MoS2 growth. We can control the orientation of single-crystalline grains on the substrate and create large area continuous films, where grain boundaries between the grains have no impact on transport properties in contrast to other reports. Finally, Chapter 7 discusses further advances in CVD growth of TMDCs, with the first demonstration of ambipolar insulator to metal transition in devices, based on monolayers of MoSe2 and WSe2. In conclusion, this thesis establishes connection between transport properties of monolayers of 2D semiconductors and structural or electrostatic disorder. These results are important for both fundamental understanding of transport in two-dimensional materials and practical applications