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Mishra V.,University of California at Berkeley | Smith S.,University of California at Berkeley | Liu L.,Nanoacademic Technologies Inc. | Zahid F.,University of Hong Kong | And 3 more authors.
IEEE Transactions on Electron Devices | Year: 2015

Full-band ballistic quantum transport calculations were used to study the screening effects in ultrashort-channel few-layer MoS2 transistors. A large density of states resulted in small screening lengths while inhibiting direct source-to-drain tunneling. Short-channel effects were observed even for the structurally confined 2-D transistors resulting in degraded electrostatic control. Electron confinement effects were also observed in the OFF-state in multilayered devices. © 1963-2012 IEEE. Source


Zhang L.,Hong Kong University of Science and Technology | Zahid F.,University of Hong Kong | Zhu Y.,Nanoacademic Technologies Inc. | Liu L.,Nanoacademic Technologies Inc. | And 4 more authors.
IEEE Transactions on Electron Devices | Year: 2013

We report parameter-free first principle atomistic simulations of quantum transport in Si nanochannels under uniaxial strain. Our model is based on the density functional theory (DFT) analysis within the Keldysh nonequilibrium Green's function (NEGF) formalism. By employing a recently proposed semi-local exchange along with the coherent potential approximation we investigate the transport properties of two-terminal Si nanodevices composed of large number of atoms and atomic dopants. Simulations of the two-terminal device based on the NEGF-DFT are compared quantitatively with the traditional continuum model to establish an important accuracy benchmark. For bulk Si crystals, we calculated the effects of uniaxial strain on band edges and effective masses. For two-terminal Si nanochannels with their channel length of {\sim}{10}~{\rm nm}, we study the effects of uniaxial strain on the electron transport. With 0.5% uniaxial tensile strain, the conductance along [110] direction is increased by {\sim}{8\%} and that along [001] is increased by {\sim}{2\%}, which are comparable with the other reported results. This paper qualitatively and quantitatively shows the current capability of first principle atomistic simulations of nanoscale semiconductor devices. © 1963-2012 IEEE. Source


Zhu Y.,Nanoacademic Technologies Inc. | Liu L.,Nanoacademic Technologies Inc. | Guo H.,Nanoacademic Technologies Inc. | Guo H.,McGill University
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

Due to random dopant fluctuations, the device-to-device variability is a serious challenge to emerging nanoelectronics. In this work we present theoretical formalisms and numerical simulations of quantum-transport variability, based on the Green's function technique and the multiple-scattering theory. We have developed a general formalism using the diagrammatic technique within the coherent-potential approximation that can be applied to a wide range of disorder concentrations. In addition, we have developed a method by using a perturbative expansion within the low-concentration approximation that is extremely useful for typical nanoelectronic devices having low dopant concentration. Applying both formalisms, transport fluctuations due to random impurities can be predicted without lengthy brute force computation of ensembles of device structures. Numerical implementations of the formalisms are demonstrated using both tight-binding models and first-principles models. © 2013 American Physical Society. Source


Zhu Y.,Nanoacademic Technologies Inc. | Liu L.,Nanoacademic Technologies Inc. | Guo H.,Nanoacademic Technologies Inc. | Guo H.,McGill University
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

Since any realistic electronic device has some degree of disorder, predicting disorder effects in quantum transport is a critical problem. Here, we report the theory of nonequilibrium coherent potential approximation (NECPA) for analyzing disorder effects in nonequilibrium quantum transport of nanoelectronic devices. The NECPA is formulated by contour-ordered nonequilibrium Green's function where the disorder average is carried out within the coherent potential approximation on the complex-time contour. We have derived a set of rules that supplement the celebrated Langreth theorem and, as a whole, the generalized Langreth rules allow us to derive NECPA equations for real-time Green's functions. The solution of NECPA equations provides the disorder-averaged nonequilibrium density matrix as well as other relevant quantities for quantum transport calculations. We establish the excellent accuracy of NECPA by comparing its results to brute force numerical calculations of disordered tight-binding models. Moreover, the connection of NECPA equations which are derived on the complex-time contour to the nonequilibrium vertex correction theory which is derived on the real-time axis is made. As an application, we demonstrate that NECPA can be combined with density functional theory to enable analysis of nanoelectronic device physics from atomistic first principles. © 2013 American Physical Society. Source


Zahid F.,University of Hong Kong | Liu L.,Nanoacademic Technologies Inc. | Zhu Y.,Nanoacademic Technologies Inc. | Wang J.,University of Hong Kong | Guo H.,McGill University
AIP Advances | Year: 2013

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 structures 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. © 2013 © 2013 Author(s). Source

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