Nanoacademic Technologies Inc.

Brossard, Canada

Nanoacademic Technologies Inc.

Brossard, Canada
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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.

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.

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).

Zhang L.,Shanxi University | Zhang L.,McGill University | Gong K.,McGill University | Gong K.,University of Science and Technology Beijing | And 4 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

We report first principles investigations of the anomalous Hall effect (AHE) in Fe and Ni four-probe crossbar nanostructures where the boundary scattering is a very important mechanism for the predicted AHE resistance. The results allow us to understand nanoscopic AHE physics in terms of how quantized channels are scattered by boundaries to contribute to the AHE resistance, the spin orbit interaction (SOI) versus boundary scattering, the spin texture of the scattered channels, and the symmetry of the scattering matrices. From individual transport channels we find that electrons are pushed in the transverse direction deep inside the incoming probe by the SOI, and strongly scattered by the crossbar boundaries to negotiate corners and enter into voltage probes. The combined SOI and boundary scattering lead to opposite signs of the AHE resistance for Fe and Ni. The spin texture is largely collinear deep inside the probes, but is complicated after boundary scattering. The calculated AHE resistance satisfies an Onsager-like symmetry relation. ©2016 American Physical Society.

Shi Q.,McGill University | Shi Q.,Nanoacademic Technologies Inc. | Guo H.,McGill University | Guo H.,Nanoacademic Technologies Inc. | And 4 more authors.
Physical Review Applied | Year: 2015

Effects of disorder scattering critically influence quantum-transport properties of nanostructures both fundamentally and practically. In this work, we report a theoretical analysis of the important issue of device-to-device quantum-transport variability (DDV) induced by random configurations of discrete dopants. Instead of calculating many impurity configurations by brute force, which is practically impossible to accomplish from first principles, here we use a state-of-the-art atomistic technique where the configurational average is carried out analytically, thereby, DDV can be predicted for any impurity concentration. The DDV we quantitatively analyze is the off-state tunnel conductance variability in Si nanosized field-effect transistor channels with channel lengths ranging from 6.5 to 15.2 nm doped with different concentrations of boron impurity atoms. The variability is predicted by varying the doping concentration, channel length, and the doping positions. We find that doping away from the source or drain contacts of the channel very significantly reduces variability, and doping close to the source or drain produces a nonintuitive outcome of increasing variability. The physics is understood by analyzing the microscopic details of the potential profile in the tunnel barrier. Finally, we organize the ab initio data by a Wentzel-Kramers-Brillouin model. © 2015 American Physical Society.

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.

Silva F.W.N.,Federal University of Ceará | Costa A.L.M.T.,Federal University of Ceará | Liu L.,NanoAcademic Technologies Inc. | Barros E.B.,Federal University of Ceará
Nanotechnology | Year: 2016

The effects of edge vacancies on the electron transport properties of zigzag MoS2/WSe2 nanoribbons are studied using a density functional theory (DFT)-based tight-binding model with a sp3d5 basis set for the electronic structure calculation and applying the Landauer-Büttiker approach for the electronic transport. Our results show that the presence of a single edge vacancy, with a missing MoS2/WSe2 triplet, is enough to suppress the conductance of the system by almost one half for most energies around the Fermi level. Furthermore, the presence of other single defects along the same edge has little effect on the overall conductance, indicating that the conductance of that particular edge has been strongly suppressed by the first defect. The presence of another defect on the opposite edge further suppresses the quantum conductance, independently of the relative position between the two defects in opposite edges. The introduction of other defects cause the suppression to be energy dependent, leading to conductance peaks which depend on the geometry of the edges. The strong conductance dependence on the presence of edge defects is corroborated by DFT calculations using SIESTA, which show that the electronic bands near the Fermi energy are strongly localized at the edge. © 2016 IOP Publishing Ltd.

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.

Maassen J.,Purdue University | Harb M.,McGill University | Michaud-Rioux V.,McGill University | Zhu Y.,Nanoacademic Technologies Inc. | Guo H.,McGill University
Proceedings of the IEEE | Year: 2013

In the past two decades, significant progress has been achieved in the large-scale fabrication of nanostructures where quantum transport properties of charge and spin are closely coupled to the discreteness of the device material. Multitudes of emerging device concepts and new materials with interesting application potential have been discovered. In order to understand the experimental data and device physics of nanoelectronics, an important task is to develop appropriate theoretical formalisms and associated modeling tools which are capable of making quantitative and material specific predictions of device characteristics without any phenomenological parameters. Here we review the atomistic modeling method based on carrying out density functional theory (DFT) within the nonequilibrium Green's function (NEGF) formalism. Since its original implementation ten years ago, the NEGF-DFT technique has emerged as a very powerful and practically very useful method for predicting nonlinear and nonequilibrium quantum transport properties of nanoelectronics. Recent new developments concerning nonequilibrium disorder scattering will also be presented. Large-scale and scalable computations have allowed NEGF-DFT to model Si structures reaching the present day realistic channel sizes. © 1963-2012 IEEE.

Zhao Y.,McGill University | Zhao Y.,Sichuan Normal University | Hu Y.,McGill University | Liu L.,Nanoacademic Technologies Inc. | And 2 more authors.
Nano Letters | Year: 2011

We report density functional theory analysis of the electronic and quantum transport properties of Bi2Se3 topological insulator, focusing on the helical surface states at the Fermi level EF. The calculated Dirac point and the tilt angle of the electron spin in the helical states are compared quantitatively with the experimental data. The calculated conductance near EF shows a V-shaped spectrum, consistent with STM measurements. The spins in the helical states at EF not only tilts out of the two-dimensional plane, they also oscillate with a 3-fold symmetry going around the two-dimensional Brillouin zone. The helical states penetrate into the material bulk, where the first quintuple layer contributes 70% of the helical wave functions. © 2011 American Chemical Society.

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