22 research outputs found

    Electronic and Thermoelectric Properties of Few-Layer Transition Metal Dichalcogenides

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    The electronic and thermoelectric properties of one to four monolayers of MoS2_{2}, MoSe2_{2}, WS2_{2}, and WSe2_{2} are calculated. For few layer thicknesses,the near degeneracies of the conduction band KK and Σ\Sigma valleys and the valence band Γ\Gamma and KK valleys enhance the n-type and p-type thermoelectric performance. The interlayer hybridization and energy level splitting determine how the number of modes within kBTk_BT of a valley minimum changes with layer thickness. In all cases, the maximum ZT coincides with the greatest near-degeneracy within kBTk_BT of the band edge that results in the sharpest turn-on of the density of modes. The thickness at which this maximum occurs is, in general, not a monolayer. The transition from few layers to bulk is discussed. Effective masses, energy gaps, power-factors, and ZT values are tabulated for all materials and layer thicknesses

    Structure and Dielectric Properties of Amorphous High-kappa Oxides: HfO2, ZrO2 and their alloys

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    High-κ\kappa metal oxides are a class of materials playing an increasingly important role in modern device physics and technology. Here we report theoretical investigations of the properties of structural and lattice dielectric constants of bulk amorphous metal oxides by a combined approach of classical molecular dynamics (MD) - for structure evolution, and quantum mechanical first principles density function theory (DFT) - for electronic structure analysis. Using classical MD based on the Born-Mayer-Buckingham potential function within a melt and quench scheme, amorphous structures of high-κ\kappa metal oxides Hf1x_{1-x}Zrx_xO2_2 with different values of the concentration xx, are generated. The coordination numbers and the radial distribution functions of the structures are in good agreement with the corresponding experimental data. We then calculate the lattice dielectric constants of the materials from quantum mechanical first principles, and the values averaged over an ensemble of samples agree well with the available experimental data, and are very close to the dielectric constants of their cubic form.Comment: 5 pages, 4 figure

    Thermoelectric properties of Bi2Te3 atomic quintuple thin films

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    Motivated by recent experimental realizations of quintuple atomic layer films of Bi2Te3,the thermoelectric figure of merit, ZT, of the quintuple layer is calculated and found to increase by a factor of 10 (ZT = 7.2) compared to that of the bulk at room temperature. The large enhancement in ZT results from the change in the distribution of the valence band density of modes brought about by the quantum confinement in the thin film. The theoretical model uses ab initio electronic structure calculations (VASP) with full quantum-mechanical structure relaxation combined with a Landauer formalism for the linear-response transport coefficients.Comment: 4 figures, submitted to AP

    A generic tight-binding model for monolayer, bilayer and bulk MoS2

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    Molybdenum disulfide (MoS2) is a layered semiconductor which has become very important recently as an emerging electronic device material. Being an intrinsic semiconductor the two-dimensional MoS2 has major advantages as the channel material in field-effect transistors. In this work we determine the electronic structure of MoS2 with the highly accurate screened hybrid functional within the density functional theory (DFT) including the spin-orbit coupling. Using the DFT electronic structures as target, we have developed a single generic tight-binding (TB) model that accurately produces the electronic structures for three different forms of MoS2 - bulk, bilayer and monolayer. Our TB model is based on the Slater-Koster method with non-orthogonal sp3d5 orbitals, nearest-neighbor interactions and spin-orbit coupling. The TB model is useful for atomistic modeling of quantum transport in MoS2 based electronic devices.Comment: 4 pages, 2 figures, 3 table

    Direct tunneling through high-κ\kappa amorphous HfO2_2: effects of chemical modification

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    We report first principles modeling of quantum tunneling through amorphous HfO2_2 dielectric layer of metal-oxide-semiconductor (MOS) nanostructures in the form of n-Si/HfO2_2/Al. In particular we predict that chemically modifying the amorphous HfO2_2 barrier by doping N and Al atoms in the middle region - far from the two interfaces of the MOS structure, can reduce the gate-to-channel tunnel leakage by more than one order of magnitude. Several other types of modification are found to enhance tunneling or induce substantial band bending in the Si, both are not desired from leakage point of view. By analyzing transmission coefficients and projected density of states, the microscopic physics of electron traversing the tunnel barrier with or without impurity atoms in the high-κ\kappa dielectric is revealed.Comment: 5 pages, 5 figure

    Band offset of GaAs/AlxGa1-xAs heterojunctions from atomistic first principles

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    Using an atomistic first principles approach, we investigate the band offset of the GaAs/AlxGa1-xAs heterojunctions for the entire range of the Al doping concentration 0<x<=1. We apply the coherent potential approach to handle the configuration average of Al doping and a recently proposed semi-local exchange potential to accurately determine the band gaps of the materials. The calculated band structures of the GaAs, AlAs crystals and band gaps of the GaAs/AlxGa1-xAs alloys, are in very good agreement with the experimental results. We predict that valence band offset of the GaAs/AlxGa1-xAs heterojunction scales with the Al concentration x in a linear fashion as VBO(x)~0.587 x, and the conduction band offset scales with x in a nonlinear fashion. Quantitative comparisons to the corresponding experimental data are made.Comment: 4 pages, 3 figure

    Electronic structures of III-V zinc-blende semiconductors from atomistic first principles

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    For analyzing quantum transport in semiconductor devices, accurate electronic structures are critical for quantitative predictions. Here we report theoretical analysis of electronic structures of all III-V zinc-blende semiconductor compounds. Our calculations are from density functional theory with the semi-local exchange proposed recently [F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401 (2009)], within the linear muffin tin orbital scheme. The calculated band gaps and effective masses are compared to experimental data and good quantitative agreement is obtained. Using the theoretical scheme presented here, quantum transport in nanostructures of III-V compounds can be confidently predicted.Comment: 4 pages, 2 figure

    Self -consistent semi-empirical *transport models for molecular conductors

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    In this study we focus in the development of transport models for molecular conductors using only semi-empirical methods but with a rigorous self-consistent approach. In our models, an Extended Hückel Theoretical (EHT) treatment of the molecular chemistry is combined with a Non-equilibrium Green\u27s function (NEGF) treatment of quantum transport. In our first model (Hückel I-V 2.0), a simple charging scheme is used for the description of the self-consistent potential where the potential profile across the molecule is assumed to be flat. In the next stage of our study, the self-consistent potential is modified by CNDO (complete neglect of differential overlap) with the electrostatic effects of metallic leads (bias and image charges) included through a 3-D finite element method (FEM). This new model (Hückel I-V 3.0) takes into account the spatial profile of the potential inside the molecule by incorporating both screening and charging effects. We apply this model to investigate recent experimental results on alkane dithiol molecules obtained from nanopore set-up and observe excellent agreement. We also present a study on single molecule transistors and identify electronic properties that control their performance by comparing the transistor action of two different types of molecules. Finally, we successfully explain and match experimental data on I-V asymmetry recently observed in a break junction set-up. It is shown that the asymmetry in the I-V is induced by charging effects