Grain boundaries and quantum transport in monolayer MoS2

The growing research on two-dimensional materials reveals their exceptional physical properties and enormous potential for future applications and investigation of advanced physics phenomena. They represent the ultimate limit in terms of active channel thickness as they consist only of few atomic layers and hold a great promise for future scaling of electronic devices. Further, their unique band structure provides a platform for the exploration of novel physics in the field of spin and valleytronics. The presence of a large band gap allows fully electrostatic control over doping and current flow, as well as the realization of novel optoelectronic devices. On of the most appealing and readily available 2D material is molybdenum disulphide (MoS2). This thesis is concentrated on the electronic properties of monolayers MoS2 ranging from the investigation of line defects in synthetically grown material to advanced study of the confinement of charge carriers at low temperature as function of magnetic field. A chapter is describing the details of each of the principal experiments. In Chapter 4 we study the properties of epitaxially grown MoS2 monolayers by means of electrical measurements and advanced scanning probe techniques. After confirming the high quality of the grown crystallites, we investigate the predominant types of occurring grain boundaries by means of Kelvin probe force microscopy. Due to the highly oriented growth, we find that the electronic properties of the film are homogeneous even across the line defects. This fact is confirmed by the observation of constant mobility even in devices over large area containing multiple grain boundaries. Chapter 5 presents different approaches for the fabrication of high-quality dual-gated MoS2 field effect transistors (FETs). Dielectric layers deposited by means of atomic layer deposition can guarantee good mobility and low contact resistance, but the observation of low-dimensional effects may be hindered due to the presence of charged impurities. On the other hand the use of hexagonal boron nitride as dielectric and few layer graphene as contact material is shown to yield very high mobility and low contact resistance. Using dual gated FETs with such material configuration, an electrostatically controlled quantum point contact (QPC) is realized in monolayer MoS2 and presented in Chapter 6. We find that the device conductance is quantized in multiples of e^2/h revealing the lifting of spin and valley degeneracies. Further, we perform bias spectroscopy at different magnetic fields and extract subband energy spacing from 0.8 (at 0T) to 2 meV (at 9.8T) and g-factor in the conduction band of 2.12±0.13. Finally, in Chapter 7 we propose several methods aiming the fabrication of a quantum dot in MoS2. The electrostatic definition of a small conductive island, weakly couples to two highly doped charge carrier reservoirs, requires similar advanced engineering of the dielectric material environment. Despite the low stability, we find possible regimes, in which controlled tunneling from and to a well defined quantum dot can be detected. The charging energy measured electrically is in very good agreement with the lithographically defined size of the island. In conclusion, this work provides insights about the electron transport in monolayer MoS2 exploring grain boundaries, dielectric environment and electron confinement. The results contribute to better understanding of its electronic properties.

Electronic Transport in 2D Materials with Strong Spin-orbit Coupling

Artem Pulkin

The thesis describes the computational study of structural, electonic and transport properties of monolayer transition metal dichalcogenides (TMDs) in the stable 2H and the metastable 1T' phases. Several aspects have been covered by the study including the electronic properties of the topological quantum spin Hall (QSH) state in the 1T' monolayer phase as well as the effects of strain, periodic line defects, interfaces and edges of monolayer TMDs. The electronic properties of the bulk monolayer phases were described by the ab-initio density functional theory framework while the electronic and transport properties of 1D defects were calculated using the non-equilibrium Green's function formalism. A specific focus was made on the transport of spin-polarized charge carriers across line defects in the monolayer 2H phase. Subject to energy, pseudomomentum and spin conservation, the size of the transport gap is governed by both bulk properties of a material and symmetries of a line defect. Outside the transport gap energy region, the charge carriers are discriminated with respect to their spin resulting in the spin polarization of the transmitted current. Next, the properties of the metastable monolayer 1T' phase were studied. The presence of a sufficiently large band gap is crucial to observe the QSH phase in the family of materials by probing the topological boundary states. The meV-order band gaps of the 1T' phase of monolayer TMDs were found to be sensitive to materials' lattice constants suggesting the control of the band gap size by strain. In particular, the electronic band structure and the size of the band gap in monolayer 1T'-WSe2 were found to be in agreement with spectroscopy studies. The topologically protected states at the edges of the monolayer 1T' phase as well as at the boundaries between the topological 1T' phase and the trivial 2H phase of monolayer TMDs were studied. Specific atomic structure configurations were suggested to observe experimentally the topological protection of the charge carrier transport against back-scattering. Finally, in the context of lateral semiconducting device engineering, the electronic and transverse transport properties of 2H-1T' phase boundaries as well as the dimerization defects in the 1T' phase were investigated. Both kinds of defects considered exhibit a relatively large transmission probability for the charge carriers crossing the defects. However, the differences between the shapes of bulk bands of the two phases open a sizeable transport gap for charge carriers crossing periodic domain boundaries between the monolayer 2H and 1T' phases. The calculated formation energies of dimerization defects were found to be relatively low suggesting their high concentration in real samples of monolayer 1T'-TMDs. Additionally, the thesis includes studies of magnetic dopants on the surface of Bi2Te3 and atomic vacancies in monolayer 2H-MoSe2 where the electronic properties of point defects were calculated and compared to experimental results. The two possible adsorption sites of Fe on the surface of Bi2Te3 both show a large out-of-plane magnetic anisotropy in agreement with experiments. The calculated local electronic properties of Se vacancies in monolayer 2H-MoSe2 show the presence of in-gap states which are not observed in experiment. Nevertheless, the combination of theoretical and experimental scanning tunneling microscopy images allowed the unambiguous identification of the vacancy defect.

EPFL2017

Charge-transport properties of monolayer MoS2 at the interface with dielectric materials

Simone Bertolazzi

Two-dimensional (2D) semiconductors, consisting of single-sheets of layered transition metal dichalcogenides (TMD), are attracting enormous interest from both fundamental science and technology. Monolayer molybdenum disulfide (MoS2), a typical example from this class of materials, is currently under intense research investigation because its direct band gap, atomic-scale thickness and mechanical flexibility could enable a wide range of novel technological applications, such as low-power flexible/transparent electronics, displays and wearable sensors. Critical to all these applications are the material mechanical strength, the mobility of charge-carriers in the 2D semiconductor and its interaction with the surrounding environment. This thesis describes experimental research conducted on these critical aspects and shows a proof-of-concept device application based on heterostructures of MoS2 and graphene. The main goal of the research was to assess experimentally the potential of monolayer MoS2 for application in flexible electronics. The thesis is based on three papers. The first paper describes the measurements of the in-plane stiffness and breaking strength of free-standing membranes of monolayer MoS2. Nanoindentation experiments were performed with the tip of an atomic force microscope (AFM) to extract the material's Young's modulus (E ~ 270 GPa) and breaking strength (Ïmax ~ 23 GPa). Breaking occurred at maximum internal strain Îµint ~ 11%, which indicates that monolayer MoS2 is suitable for integration on flexible plastic substrates. The second paper represents the core work of this thesis. It explores the charge transport properties of monolayer MoS2 supported by different dielectric substrates, such as thin polymer films of parylene, atomically flat sapphire and 2D sheets of hexagonal boron nitride (h-BN). It was found that substrate surface corrugations do not represent a major limit to charge-carrier mobility in monolayer MoS2, which seems at this stage dominated by impurities and material's defects. Field-effect transistors with mobility ~ 100 cm2/Vs and on/off current ratio Ion/Ioff > 10^5 were successfully integrated on parylene substrates, whose root-mean-square roughness Rq can be more than two times larger than the thickness of the transistor channel itself. Because parylene is also a flexible material, this work showed a viable method for the realization of high-mobility and high performance flexible devices based on 2D semiconductors. Finally, the third paper describes a flash-memory cell fabricated using monolayer MoS2/graphene heterostructures and dielectric layers of HfO2 grown by atomic layer deposition (ALD). The architecture of the device resembles that of a floating-gate transistor. Monolayer MoS2 acts as the transistor channel, graphene electrodes are used to collect and inject the charge carriers, and a piece of multilayer graphene serves as ultrathin charge trapping layer. In this paper, it was shown that graphene contacts to monolayer MoS2 result in ohmic-like current-voltage characteristics at room temperature. Moreover, the use of graphene and 2D semiconductors in flash memory technology was indicated as a potential strategy for further scaling of memory cells and for their integration in flexible electronic devices.

EPFL2015

Spin and Charge transport in Dirac Materials

Alexander Ronald Hoyer

The digitalization of our world is proceeding with every new technical product placed on the market. By making them smarter, e.g. linking them together and equipping them with sensors, a vast mass of data is collected. Processing and storing this information is particularly challenging as it should be done in real time and, ideally with a handheld device. This requires logic devices to be very fast, low on power consumption and rather small. The latter requirement is leading towards a transition from the classical physics domain to a domain where quantum effects like tunneling and interference start to dominate the behavior. Thus for miniaturization to continue, it is necessary to adapt new techniques that work in the quantum regime. One promising approach to achieve all of this is to exploit the spin degree of freedom an electron possesses, opening up the field of spin electronics (spintronics). As first principle operation of such devices has been demonstrated with silicon, new materials may possess attributes that better suite the requirements of spintronics devices. Graphene, a two dimensional carbon allotrope, is such a candidate as it shows low spin orbit coupling, which theoretical allows high spin life times to be achieved. Furthermore the 2D nature of graphene allows to control and tailor the electronic and spin properties by external means such as proximity and chemical functionalization. On this basis low temperature magnetotransport measurements are performed to explore the effect of functionalization of graphene with 4-nitrophenyl diazonium tetrafluoroborate within this thesis. Basic measurements in Hall bar geometry show an introduction of disorder due to the functionalization. Additional weak localization, a quantum effect that gives information on the phase coherence length of the electrons, is observed after functionalization and used to explore the electronic properties of the tailored graphene. To address the spin properties of graphene its spin bath needs to be controlled and manipulated. This is done within this thesis by all electrical means and requires the fabrication of spin valve structures on graphene. Two strategies to inject spin polarized currents into graphene are pursued within this framework. First spin valve measurements on functionalized graphene are promising regarding the manipulation of spin properties. The generation of the spin polarized currents is still a big issue. Traditionally, ferromagnets are used as leads to fulfill this task. Latest studies show that topological insulators (TI), a new class of materials, could be better suited to fulfill this task as they give rise to fully spin polarized currents. Conceptually these materials possess a band gap that is crossed by spin polarized bands. Crystals of nanometer size of Tin(II)telluride, a potential TI, are grown in a PVD process and electrically contacted to look for fingerprints of these unusual states. Charge and spin transport experiments show the presence of two types of charge carriers and a high intrinsic p-doping. Finally, a new 3^He system is set up within this thesis and a concept for low noise measurements is implemented that integrates a home build signal generation and amplification stage.

EPFL2016