DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), such as the density functional based tight binding (DFTB) and the extended tight binding method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than the respective ab initio methods. Based on the DFTB framework, it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green’s functions, and many more. DFTB+ can be used as a user-friendly standalone application in addition to being embedded into other software packages as a library or acting as a calculation-server accessed by socket communication. We give an overview of the recently developed capabilities of the DFTB+ code, demonstrating with a few use case examples, discuss the strengths and weaknesses of the various features, and also discuss on-going developments and possible future perspectives.
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.
White light emitting diodes (LEDs) based on III-nitride InGaN/GaN quantum wells currently offer the highest overall efficiency for solid state lighting applications. Although current phosphor-converted white LEDs have high electricity-to-light conversion efficiencies, it has been recently pointed out that the full potential of solid state lighting could be exploited only by color mixing approaches without employing phosphor-based wavelength conversion. Such an approach requires direct emitting LEDs of different colors, including, in particular, the green-yellow range of the visible spectrum. This range, however, suffers from a systematic drop in efficiency, known as the "green gap," whose physical origin has not been understood completely so far. In this work, we show by atomistic simulations that a consistent part of the green gap in c-plane InGaN/GaN-based light emitting diodes may be attributed to a decrease in the radiative recombination coefficient with increasing indium content due to random fluctuations of the indium concentration naturally present in any InGaN alloy.
The conduction mechanism in carbon nanotube (CNT) polymer nanocomposites is complex, and there has been a considerable amount of work invested in understanding the role of the distribution of the CNTs in the composite and how it influences the conductivity. However, less interest has been devoted to the electron transport across a single CNT−polymer−CNT junction. We present a first atomistic study of the electron transmission through a CNT−polyethylene−CNT junction. The morphology of the junction is described using classical molecular dynamics simulations, and transport properties are calculated within density functional tight binding method. The electron transmission depends noticeably on the CNT−CNT separation and on the consequent polymer wrapping. At CNT−CNT distances shorter than 6 Å, the polyethylene molecules do not penetrate in the space between the CNTs. In this near contact regime, the electron transmission proceeds via direct tunneling between the two CNTs across a vacuum region without relevant contribution from the surrounding polymer. For distances larger than 6 Å, the PE molecules enter into the junction region. The frontier orbitals of the PE molecules in the junction provide localized states, which can couple to the CNT metallic states. This resonance tail increases the electron transmission probability between the CNTs across the junction by several orders of magnitude, thus lowering the effective barrier. The gradual interpenetration of the polymer is resembled in transmission fluctuations. An averaging of the transmission in energy and time along MD trajectories allows a quantitative estimation of the junction resistance and tunneling barrier.
Abstract-We establish the dependence of the permittivity of oxidized ultra-thin silicon films on the film thickness by means of atomistic simulations within the density-functional-based tight-binding theory (DFTB). This is of utmost importance for modeling ultra-and extremely-thin silicon-on-insulator MOSFETs, and for evaluating their scaling potential. We demonstrate that electronic contribution to the dielectric response naturally emerges from the DFTB Hamiltonian when coupled to Poisson equation solved in vacuum, without phenomenological parameters, and obtain good agreement with available experimental data. Comparison to calculations of H-passivated Si films reveals much weaker dependence of permittivity on film thickness for the SiO2-passivated Si, with less than 18% reduction in the case of 0.9 nm silicon-on-insulator. Index Terms-permittivity, atomistic modeling, oxide interface, density-functional tight binding, silicon-on-insulatorIt is well known that the dramatic reduction of the dimensions of the Si channel, e.g. in ultra-thin-body silicon-on-insulators devices, leads to a significant change in the electronic and dielectric properties of Si, particularly at channel thickness below 6 nm [1-3]. A number density-functional theory (DFT) studies applied to hydrogen-passivated Si films suggest that the decrease in permittivity with the decrease of Si-film thickness becomes significant even earlier than the corresponding widening of the fundamental band-gap, and predict 35-45% reduction at around 1 nm [4][5][6]. On the experimental side however, we are aware of only one study of oxidized Si films down to 3.3 nm, and while a qualitative trend is evident, the scatter of the results precludes us from establishing an accurate quantitative picture towards sub-nm Si thickness [7]. We note further that to the best of our knowledge, the permittivity dependence of oxidized Si films has not been modeled ab initio, most likely due to the complexity and cost associated with including the oxide on each side of Si in DFT. However, very recently we A. Pecchia is with University of Rome "Tor Vergata", Rome, Italy. demonstrated that density-functional tight-binding theory (DFTB), with an accurate parameterization, can give us very good description of the electronic properties not only of bulk Si and SiO2, but also of their interface [3], and allows us to explore substantially larger systems, including transport through ETSOI devices [8]. The purpose of this paper is therefore twofold: 1) to evaluate a way of calculating the dielectric constant of thin-films within the framework of DFTB; and 2) to establish the permittivity dependence on the thickness of oxidized ultra-thin Si films.We employ the DFTB+ computer code [9], implementing the self-consistent-charge DFTB, coupled self-consistently to a Poisson solver [10][11][12]. This permits us to apply bias ( ) and find the distribution of potential ( ) and electric field ( ) in the model atomic structures. The atomic models are the SiO2/Si/SiO2/vacuum super-cells with varying ...
Quantum decoherence plays an important role in the charge transport characteristics of molecular wires at room temperature. In this paper we propose a generalization of an electron–phonon dephasing model to non orthogonal LCAO basis. We implemented the model in combination with a density functional-based tight binding (DFTB) theory framework and utilized it to model charge transport characteristics of an anthraquinone (AQ) based molecular wire. We demonstrate a modulation of Quantum Interference (QI) effects compatible with experiments and confirm the robustness of QI signatures with respect to dephasing. An analysis of the spatial localization of the dephasing process reveals that both the QI and the dephasing process are localized in the AQ region, hence justifying the general robustness of the transmission temperature dependence in different AQ-based systems.
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