G. Penazzi

Non-equilibrium Green's functions in density functional tight binding: method and applications

We present a detailed description of the implementation of the non-equilibrium Greens function (NEGF) technique on the density-functional-based tight-binding (gDFTB) simulation tool. This approach can be used to compute electronic transport in organic and inorganic molecular-scale devices. The DFTB tight-binding formulation gives an efficient computational tool that is able to handle a large number of atoms. NEGFs are used to compute the electronic density self-consistently with the open-boundary conditions naturally encountered in quantum transport problems and the boundary conditions imposed by the potentials at the contacts. The efficient block-iterative algorithm used to compute the Greens functions is illustrated. The Hartree potential of the density-functional Hamiltonian is obtained by solving the three-dimensional Poisson equation. A scheme to treat geometrically complex boundary conditions is discussed, including the possibility of including multiterminal calculations.

The Multiscale Paradigm in Electronic Device Simulation

In this paper, we present a framework for the simulation of electronic devices based on a multiscale and multiphysics approach. A formal description is provided that includes both multiscale and multiphysics problems and which can be linked to already established multiscale methods. We present a set of simulations of an AlGaN/GaN nanocolumn based on a multiscale coupling between atomistic descriptions and continuous media models, illustrating the application of such a multiscale approach to electronic device simulation.

TiberCAD: Towards multiscale simulation of optoelectronic devices

Due to the downscaling of semiconductor device dimensions and the emergence of new devices based on nanostructures, CNTs and molecules, the classical device simulation approach based on semi-classical transport theories needs to be extended towards a quantum mechanical description. We present a simulation environment designed for multiscale and multiphysics simulation of electronic and optoelectronic devices with the final aim of coupling classical with atomistic simulation approaches.

Optoelectronic and transport properties of nanocolumnar InGaN/GaN quantum disk LEDs

In this work we use the multi-scale software tool TiberCAD to study the transport and optical properties of InGaN quantum disk (QD) - based GaN nanocolumn p-i-n diode structures. IV characteristics have been calculated for several values of In concentration in the QD and of nanocolumn width. Strain maps show a clear relaxation effect close to the column boundaries, which tends to vanish for the larger columns. Effects of strain and polarization fields on the electron and hole states in the QD are shown, together with the dependence of optical emission spectra on geometrical and material parameters.

Multiscale thermal modeling of GaN/AlGaN quantum dot LEDs

In this work we develop a multiscale model to investigate the self-heating in nanodevices. The scheme splits up the simulation region in two domains: the micro domain, modeled by the phonon Boltzmann Transport Equation (BTE) and the macro domain, where the heat transport is calculated within the Fourier model. Appropriate boundary conditions match the two domains. The multiscale method is applied to a GaN/AlGaN quantum dot LED. We find out that the maximum temperature is about 334 K. A comparison with the temperature profile given by the BTE and Fourier model is provided. Finally, the effect of the temperature on the optical spectrum is investigated.

Simulations of Optical Properties of a GaN Quantum Dot Embedded in a AlGaN Nanocolumn within a Mixed FEM/atomistic Method

Calculations of optoelectronic properties of a GaN quantum dot embedded in an AlGaN nanocolumn are presented, using the TiberCAD simulator. The calculations emphasize the role of the growth direction in determining the quantum efficiency of such light emitting devices. Multiband kldrp is used, with corrections from drift diffusion and strain calculations. Results are discussed using an empirical tight binding method, with the same macroscopic corrections as for kldrp, implemented in TiberCAD framework itself.

Multiscale Atomistic Simulations of High-k MOSFETs

We report on a multiscale simulation approach that includes both macroscopic drift-diffusion current model and atomistic quantum tunneling model. The models are solved together in a self-consistent way inside a single simulation package. We compare the high-K gates based on HfO2 and ZrO2 with a SiO2 gate of the same equivalent thickness and show the effect of the tunneling current on transistor performance.

Efficient Green’s Function Algorithms for Atomistic Modeling of Si Nanowire FETs

Atomistic simulations of transport properties of an ultra-scaled Silicon nanowire (SiNW) field-effect transistor (FETs) in a Gate-All-Around configuration are reported. The calculations have been obtained using a semi-empirical tight-binding representation of the System Hamiltonian based on first-principles density functional theory (DFT). An efficient non-equilibrium Green’s functions (NEGF) scheme has been implemented in order to compute self-consistently the charge density and the electrostatic potential in the SiNW Channel.

Electronic and transport properties of GaN/AlGaN quantum dot-based p-i-n diodes

Quantum dot (QD) systems based on III-nitride have recently shown to be very promising nanostructures for high-quality light emitters. In this work, electronic and transport properties of AlN/GaN QDs are investigated by means of the TIBERCAD software tool, which allows both a macroscopic and an atomistic approach, with the final aim to couple them in a multiscale simulation environment.

Atomic level simulation of permittivity of oxidized ultra-thin si channels

We investigate the applicability of density functional tight binding (DFTB) theory [1], [2], coupled to nonequilibrium Green functions (NEGF), for atomistic simulations of ultra-scaled electron devices, using the DFTB+ code [3], [4]. In the context of ultra-thin silicon-on-insulator (SOI) transistors we adopt atomic models that include not only the Si channel, but also the interfacial SiO2, and look at the change of electronic, dielectric and transport properties as Si film thickness is reduced to less than 1 nm. We build on our previous reports [5]-[7], and draw a comparison against a hydrogen-passivated model.