Transition‐metal dichalcogenides (TMDCs) are an important class of two‐dimensional (2D) layered materials for electronic and optoelectronic applications, due to their ultimate body thickness, sizable ...and tunable bandgap, and decent theoretical room‐temperature mobility. So far, however, all TMDCs show much lower mobility experimentally because of the collective effects by foreign impurities, which has become one of the most important limitations for their device applications. Here, taking MoS2 as an example, the key factors that bring down the mobility in TMDC transistors, including phonons, charged impurities, defects, and charge traps, are reviewed. A theoretical model that quantitatively captures the scaling of mobility with temperature, carrier density, and thickness is introduced. By fitting the available mobility data from literature over the past few years, one obtains the density of impurities and traps for a wide range of transistor structures. It shows that interface engineering can effectively reduce the impurities, leading to improved device performances. For few‐layer TMDCs, the lopsided carrier distribution is analytically modeled to elucidate the experimental increase of mobility with the number of layers. From our analysis, it is clear that the charge transport in TMDC samples is a very complex problem that must be handled carefully.
Transition‐metal dichalcogenides (TMDCs) are widely investigated for enhanced characteristics for electronics among next generation semiconductors. The understanding of charge transport in TMDCs is significant for further device applications. Through carefully analyzing the reported high performance MoS2 devices, this review provides a systematic theoretical and experimental path to optimize the device structure and improve device performance.
By combining a high‐κ dielectric substrate and a high density of charge carriers, Coulomb impurities in MoS2 can be effectively screened, leading to an unprecedented room‐temperature mobility of ≈150 ...cm2 V−1 s−1 and room‐temperature phonon‐limited transport in a monolayer MoS2 transistor for the first time.
Using first-principles calculations and the nonequilibrium Green’s function method, we investigate ballistic thermal transport in two-dimensional monolayer phosphorene sheet. A significant ...crystallographic orientation dependence of thermal conductance is observed, with room temperature thermal conductance along zigzag direction being 40% higher than that along armchair direction. Furthermore, we find that the thermal conductance anisotropy with the orientation can be tuned by applying strain. In particular, the zigzag-oriented thermal conductance is enhanced when a zigzag-oriented strain is applied but decreases when an armchair-oriented strain is applied; whereas the armchair-oriented thermal conductance always decreases when either a zigzag- or an armchair-oriented strain is applied. The present work suggests that the remarkable thermal transport anisotropy and its strain-modulated effect in single-layer phosphorene may be used for thermal management in phosphorene-based electronics and optoelectronic devices.
The Kapitza or interfacial thermal resistance at the boundary of two different insulating solids depends on the transmission of phonons across the interface and the phonon dispersion of either ...material. We extend the existing atomistic Green's function (AGF) method to compute the probability for individual phonon modes to be transmitted across the interface. The extended method is based on the concept of the Bloch matrix and allows us to determine the wavelength and polarization dependence of the phonon transmission as well as to analyze efficiently the contribution of individual acoustic and optical phonon modes to interfacial thermal transport. The relationship between the phonon transmission probability and dispersion is explicitly established. A detailed description of the method is given and key formulas are provided. To illustrate the role of the phonon dispersion in interfacial thermal conduction, we apply the method to study phonon transmission and thermal transport at the armchair interface between monolayer graphene and hexagonal boron nitride. We find that the phonon transmission probability is high for longitudinal (LA) and flexural (ZA) acoustic phonons at normal and oblique incidence to the interface. At room temperature, the dominant contribution to interfacial thermal transport comes from the transverse-polarized phonons in graphene (45.5%) and longitudinal-polarized phonons in boron nitride (47.4%).
Molybdenum disulfide is considered as one of the most promising two-dimensional semiconductors for electronic and optoelectronic device applications. So far, the charge transport in monolayer ...molybdenum disulfide is dominated by extrinsic factors such as charged impurities, structural defects and traps, leading to much lower mobility than the intrinsic limit. Here we develop a facile low-temperature thiol chemistry route to repair the sulfur vacancies and improve the interface, resulting in significant reduction of the charged impurities and traps. High mobility >80 cm(2) V(-1) s(-1) is achieved in backgated monolayer molybdenum disulfide field-effect transistors at room temperature. Furthermore, we develop a theoretical model to quantitatively extract the key microscopic quantities that control the transistor performances, including the density of charged impurities, short-range defects and traps. Our combined experimental and theoretical study provides a clear path towards intrinsic charge transport in two-dimensional dichalcogenides for future high-performance device applications.
The combination of high‐quality Al2O3 dielectric and thiol chemistry passivation can effectively reduce the density of interface traps and Coulomb impurities, leading to a significant improvement of ...the mobility and a transition of the charge transport from the insulating to the metallic regime. A record high mobility of 83 cm2 V−1 s−1 (337 cm2 V−1 s−1) is reached at room temperature (low temperature) for monolayer WS2. A theoretical model for electron transport is also developed.
In this paper, we study the problem of wave scattering from finite heterogeneities (in 1- and 2-D) by using the Atomistic Green’s Function (AGF) technique. The application of AGF to classical wave ...scattering problems is novel and it allows us to compute the Green’s function of the scatterers, which is central to understanding the dynamics of the problem and is, in general, difficult to obtain. The AGF method also allows us to efficiently compute the numerically exact transmission and reflection coefficients without the need for any artificial truncating boundaries such as perfectly matched layers or Dirichlet to Neumann (DtN) maps. The technique generates the effective Hamiltonian of the wave scatterer and uses it to compute the numerically exact Green’s function of the scatterer. The formalism presented here is especially suited to scattering problems involving waveguides, phononic crystals, metamaterials, and metasurfaces. To illustrate the utility of the technique, we demonstrate the application of the method to three scattering problems: scattering from a slab (1D), scattering from a finite phononic crystal (1D), and scattering from defects in a waveguide (2D).
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•The Atomistic Green’s Function (AGF) method adopts an open system perspective.•AGF method eliminates the necessity for non-reflecting boundary conditions.•Scattering problems in an infinite domain are addressed by formulating an effective Hamiltonian.•The method remains unconcerned about the complexity of the scatterer.
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