Angle-dependent carrier transmission probability in graphene p-n junctions is investigated. Using electrostatic doping from buried gates, p-n junctions are formed along graphene channels that are patterned to form different angles with the junction. A peak in the junction resistance is observed, which becomes pronounced with angle. This angular dependence is observed for junctions made on both exfoliated and CVD-grown graphene and is consistent with the theoretically predicted dependence of transmission probability on incidence angle.

The rapid cadence of MOSFET scaling is stimulating the development of new technologies and accelerating the introduction of new semiconducting materials as silicon alternative. In this context, 2D materials with a unique layered structure have attracted tremendous interest in recent years, mainly motivated by their ultra-thin body nature and unique optoelectronic and mechanical properties. The development of scalable synthesis techniques is obviously a fundamental step towards the development of a manufacturable technology. Metal-organic chemical vapor deposition has recently been used for the synthesis of large area TMDs, however, an important milestone still needs to be achieved: the ability to precisely control the number of layers and surface uniformity at the nano-to micro-length scale to obtain an atomically flat, self-passivated surface. In this work, we explore various fundamental aspects involved in the chemical vapor deposition process and we provide important insights on the layer-dependence of epitaxial MoS film's structural properties. Based on these observations, we propose an original method to achieve a layer-controlled epitaxy of wafer-scale TMDs.

is, graphene, transition metal dichalcogenides (TMDs), [ 2,3 ] topological insulators, [ 4 ] h-BN [ 5 ] and h-AlN, [ 6 ] as well the recent phosphorene, [ 7 ] silicene, [ 8 ] and germanene [ 9 ] provide the ability to control the channel thickness at atomic level. This characteristic translates into improved gate control over the channel barrier and into reduced short-channel effects, thus paving the way toward ultimate miniaturization and new device concepts. Recently, 2D transition metal dichalcogenides, have proven to be promising candidates for electronics and optoelectronic applications. [10][11][12][13][14][15][16] From a pioneering perspective, the availability of TMDs with different work functions and band structures guarantees a great potential for band gap engineering of heterostructures. These systems are fundamentally different and more fl exible than traditional heterostructures composed of conventional semiconductors. In particular, due to the weak interlayer interaction, a TMD molecular layer grows from the beginning with its own lattice constant forming an interface with reduced amount of defects. The relaxed lattice matching condition permits to combine almost any layered material and create artifi cial heterojunctions with designed band alignment. 2D heterostructures

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Electrons in graphene follow unconventional trajectories at PN junctions, driven by their pseudospintronic degree of freedom. Significant is the prominent angular dependence of transmission, capturing the chiral nature of the electrons and culminating in unit transmission at normal incidence (Klein tunneling). We theoretically show that such chiral tunneling can be directly observed from the junction resistance of a tilted interface probed with separate split gates. The junction resistance is shown to increase with tilt in agreement with recent experimental evidence. The tilt dependence arises because of the misalignment between modal density and the anisotropic transmission lobe oriented perpendicular to the tilt. A critical determinant is the presence of edge scattering events that can completely reverse the angle-dependence. The absence of such reversals in the experiments indicates that these edge effects are not overwhelmingly deleterious, making the premise of transport governed by electron 'optics' in graphene an exciting possibility. Despite the exciting physics of chiral electron flow, its measurable signatures have so far been sparse and indirect. Signatures of Klein tunneling were seen [6,7] in the preferential transmission of normally incident carriers predicted in [8]. A more direct measurement was the conductance oscillation in an npn structure [9]. The reflection amplitude undergoes a phase shift of π at normal incidence under the action of a magnetic field, due to the cyclotron bending of the carriers [10]. However, the main underlying physics of the angle dependent electron transmission has not been explicitly measured. Neither has there been a proper model that can capture both the quantum mismatch of spinors over the entire doping regime, as well as diffusive scattering to explore their robustness to impurity and edge scattering events.In this paper we focus on a tilted GPNJ (i) to show that it serves as an explicit signature of chiral tunneling. We show that the junction resistance (similar to the odd resistance shown in other experiments [6,7]) is higher than the non-tilted device (Fig. 1). We argue that this enhancement originates from the chiral nature of graphene electrons which manifests itself through the highly angle dependent transmission characteristics of GPNJ (Fig. 2). The angular transmission lobe, oriented perpendicular to the interface is rotated with the tilt, where fewer transarXiv:1207.6619v1 [cond-mat.mes-hall]

As scaling of conventional silicon-based electronics is reaching its ultimate limit, considerable effort has been devoted to find new materials and new device concepts that could ultimately outperform standard silicon transistors. In this perspective two-dimensional transition metal dichalcogenides, such as MoS2 and WSe2, have recently attracted considerable interest thanks to their electrical properties. Here, we report the first experimental demonstration of a doping-free, polarity-controllable device fabricated on few-layer WSe2. We show how modulation of the Schottky barriers at drain and source by a separate gate, named program gate, can enable the selection of the carriers injected in the channel, and achieved controllable polarity behaviour with ON/OFF current ratios >106 for both electrons and holes conduction. Polarity-controlled WSe2 transistors enable the design of compact logic gates, leading to higher computational densities in 2D-flatronics.

Despite the fact that two-dimensional MoS films continue to be of interest for novel device concepts and beyond silicon technologies, there is still a lack of understanding on the carrier injection at metal/MoS interface and effective mitigation of the contact resistance. In this work, we develop a semi-classical model to identify the main mechanisms and trajectories for carrier injection at MoS contacts. The proposed model successfully captures the experimentally observed contact behavior and the overall electrical behavior of MoS field effect transistors. Using this model, we evaluate the injection trajectories for different MoS thicknesses and bias conditions. We find for multilayer (>2) MoS, the contribution of injection at the contact edge and injection under the contact increase with lateral and perpendicular fields, respectively. Furthermore, we identify that the carriers are predominantly injected at the edge of the contact metal for monolayer and bilayer MoS. Following these insights, we have found that the transmission line model could significantly overestimate the transfer length and hence the contact resistivity for monolayer and bilayer MoS. Finally, we evaluate different contact strategies to improve the contact resistance considering the limiting injection trajectory.

Realizing basic semiconductor devices such as p-n junctions are necessary for developing thin-film and optoelectronic technologies in emerging planar materials such as MoS2. In this work, electrostatic doping by buried gates is used to study the electronic and optoelectronic properties of p-n junctions in exfoliated MoS2 flakes. Creating a controllable doping gradient across the device leads to the observation of the photovoltaic effect in monolayer and bilayer MoS2 flakes. For thicker flakes, strong ambipolar conduction enables realization of fully reconfigurable p-n junction diodes with rectifying current-voltage characteristics, and diode ideality factors as low as 1.6. The spectral response of the photovoltaic effect shows signatures of the predicted band gap transitions. For the first excitonic transition, a shift of >4kBT is observed between monolayer and bulk devices, indicating a thickness-dependence of the excitonic coulomb interaction. Two-dimensional (2-D) crystalline materials have attracted a significant amount of research efforts since the isolation of graphene by micromechanical exfoliation [1,2,3,4]. They show promise in novel electronic and optoelectronic applications, where the low-dimensionality provides ideal electrostatic control for field-effect transistor devices, or large area-to-volume ratio for sensors and photoelectric devices. Among 2-D crystals, MoS2, a transition metal dichalcogenide (TMDC), has received particular attention as channel material for thin-film or flexible electronics [5,6,7] because its mobility is considerably higher than amorphous or polycrystalline materials, and because it can be used in various heterostructures to enable diverse electronic applications [8,9,10,11,12].The most remarkable attributes of MoS2 lie in its bandstructure, which shows a crossover from an indirect bandgap (~1.3 eV) in bulk to a direct one (~1.9 eV) for a monolayer [13,14]. In the monolayer form, MoS2 has been used in optoelectronics [15,16,17], and has been investigated to enable a new class