28 research outputs found
Complex band structure and electronic transmission
The function of nano-scale devices critically depends on the choice of
materials. For electron transport junctions it is natural to characterize the
materials by their conductance length dependence, . Theoretical
estimations of are made employing two primary theories: complex band
structure and DFT-NEGF Landauer transport. Both reveal information on
of individual states; i.e. complex Bloch waves and transmission eigenchannels,
respectively. However, it is unclear how the -values of the two
approaches compare. Here, we present calculations of decay constants for the
two most conductive states as determined by complex band structure and standard
DFT-NEGF transport calculations for two molecular and one semi-conductor
junctions. Despite the different nature of the two methods, we find strong
agreement of the calculated decay constants for the molecular junctions while
the semi-conductor junction shows some discrepancies. The results presented
here provide a template for studying the intrinsic, channel resolved length
dependence of the junction through complex band structure of the central
material in the heterogeneous nano-scale junction.Comment: 7 pages, 6 figure
ATK-ForceField: A New Generation Molecular Dynamics Software Package
ATK-ForceField is a software package for atomistic simulations using
classical interatomic potentials. It is implemented as a part of the Atomistix
ToolKit (ATK), which is a Python programming environment that makes it easy to
create and analyze both standard and highly customized simulations. This paper
will focus on the atomic interaction potentials, molecular dynamics, and
geometry optimization features of the software, however, many more advanced
modeling features are available. The implementation details of these algorithms
and their computational performance will be shown. We present three
illustrative examples of the types of calculations that are possible with
ATK-ForceField: modeling thermal transport properties in a silicon germanium
crystal, vapor deposition of selenium molecules on a selenium surface, and a
simulation of creep in a copper polycrystal.Comment: 28 pages, 9 figure
Semi-Empirical Model for Nano-Scale Device Simulations
We present a new semi-empirical model for calculating electron transport in
atomic-scale devices. The model is an extension of the Extended H\"uckel method
with a self-consistent Hartree potential. This potential models the effect of
an external bias and corresponding charge re-arrangements in the device. It is
also possible to include the effect of external gate potentials and continuum
dielectric regions in the device. The model is used to study the electron
transport through an organic molecule between gold surfaces, and it is
demonstrated that the results are in closer agreement with experiments than ab
initio approaches provide. In another example, we study the transition from
tunneling to thermionic emission in a transistor structure based on graphene
nanoribbons.Comment: 8 pages, 8 figures. Submitted to PR
First-principles investigation of polytypic defects in InP
In this paper we study polytypic defects in Indium Phosphide (InP) using the complementary first-principles methods of density functional theory and non-equilibrium Greens functions. Specifically we study interfaces between the ground state Zincblende crystal structure and the meta-stable Wurtzite phase, with an emphasis on the rotational twin plane defect, which forms due to the polytypic nature of InP. We found that the transition of the band structure across the interface is anisotropic and lasts 7 nm (3.5 nm). Due to this, a crystal-phase quantum well would require a minimal width of 10 nm, which eliminates rotational twin planes as possible quantum wells. We also found that for conducting current, the interfaces increase conductivity along the defect-plane ([112¯]), whereas due to real growth limitations, despite the interfaces reducing conductivity across the defect-plane ([111]), we found that a high degree of polytypic defects are still desirable. This was argued to be the case, due to a higher fraction of Wurtzite segments in a highly phase-intermixed system
QuantumATK: An integrated platform of electronic and atomic-scale modelling tools
QuantumATK is an integrated set of atomic-scale modelling tools developed
since 2003 by professional software engineers in collaboration with academic
researchers. While different aspects and individual modules of the platform
have been previously presented, the purpose of this paper is to give a general
overview of the platform. The QuantumATK simulation engines enable
electronic-structure calculations using density functional theory or
tight-binding model Hamiltonians, and also offers bonded or reactive empirical
force fields in many different parametrizations. Density functional theory is
implemented using either a plane-wave basis or expansion of electronic states
in a linear combination of atomic orbitals. The platform includes a long list
of advanced modules, including Green's-function methods for electron transport
simulations and surface calculations, first-principles electron-phonon and
electron-photon couplings, simulation of atomic-scale heat transport, ion
dynamics, spintronics, optical properties of materials, static polarization,
and more. Seamless integration of the different simulation engines into a
common platform allows for easy combination of different simulation methods
into complex workflows. Besides giving a general overview and presenting a
number of implementation details not previously published, we also present four
different application examples. These are calculations of the phonon-limited
mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model
simulation of lithium ion drift through a battery cathode in an external
electric field, and electronic-structure calculations of the
composition-dependent band gap of SiGe alloys.Comment: Submitted to Journal of Physics: Condensed Matte