M. Auf der Maur

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.

Modeling and simulation of energetically disordered organic solar cells

The aim of this work is to present a consistent model for simulation of organic solar cells (OPV) with a correct description of mobility, density of state, organic-metal contacts, and exciton. We simulate the photoconversion by means of an integration of the optical and electrical part: light absorption is calculated with a Transfer Matrix Model and the charge transport is computed using Drift Diffusion approach including the effect of energetically disorder materials. Most model parameters are directly taken from experiment. The model is used to study the effect of energetic disordered materials and cell thickness on the performance of the cell in terms of short circuit current, open circuit voltage, and fill factor. Based on the results of this model, it will be possible to design and predict the optimal thickness of OPV toward higher efficiencies.

Trap-assisted tunneling in InGaN/GaN single-quantum-well light-emitting diodes

Based on numerical simulation and comparison with measured current characteristics, we show that the current in InGaN/GaN single-quantum-well light-emitting diodes at low forward bias can be accurately described by a standard trap-assisted tunneling model. The qualitative and quantitative differences in the current characteristics of devices with different emission wavelengths are demonstrated to be correlated in a physically consistent way with the tunneling model parameters.

3-D Simulation and Optimization of Organic Solar Cell With Periodic Back Contact Grating Electrode

In this paper, we report an investigation of the optical and electrical properties of an organic solar cell (OSC) with a back contact grating architecture through 3-D numerical simulations. By using finite-element methods for both optical and transport properties, we have modeled the behavior of OSC with a grating architecture and compared with a conventional planar structure. Based on these optoelectrical simulations, we optimized the back contact grating, obtaining an increment of up to 17.5% in power conversion efficiency with respect to a planar structured OSC. This enhancement is the result of an increase of both short-circuit current and fill factor.

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.

Multiscale Simulation of Electronic and Optoelectronic Devices with TiberCAD

We present the TiberCAD multiscale device simulation software. The scope of the project is a full description of charge transport and optoelectronic properties of devices with embedded active regions of nanometer-scale. We show simulations of a GaN LED that requires modeling of strain, transport of electrons, holes and excitons and device heating.

Effect of alloy fluctuations in InGaN/GaN quantum wells on optical emission strength

In this work we present the effect of compositional fluctuations in InGaN/GaN quantum wells (QWs) on their spontaneous emission properties. We show that random alloy fluctuations lead to fluctuations of both the optical matrix elements and the emission energy and that the two quantities are correlated. A qualitatively different behaviour between flat band QWs and QWs with strong quantum confined Stark effect is found and explained by the localization behaviour of electrons and holes.

Electro-thermo-mechanical simulation of AlGaN/GaN HEMTs

A fully selfconsistent, coupled electro-thermo-mechanical model for nitride-based devices is presented and applied to a high-power AlGaN/GaN High Electron Mobility Transistor (HEMT). The influence of converse piezoelectric effect, thermal stress and of the selfconsistent coupling on the static device characteristics and on the stress distribution in the device is studied.

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.

Influence of random alloy fluctuations in InGaN/GaN quantum wells on LED efficiency

In this work we present how compositional fluctuations in the InGaN alloy in InGaN/GaN quantum wells (QWs) influence the strengths of optical transitions and hence LED efficiency. Using atomistic empirical tight-binding simulations we show that random alloy fluctuations lead to statistical spread of the momentum matrix elements and the emission energy, and that the two quantities are correlated for the ground state transitions. These fluctuations can be related to lateral fluctuations of the electronic states and quantum confined Stark effect. From simulations on statistical sets of random samples for different Indium concentrations, we extract mean “macroscopic” emission spectra. The behaviour of the ground state transition momentum matrix elements with Indium content is found to correlate well with experimentally extracted spontaneous recombination parameters. A systematic reduction of these matrix elements with respect to results obtained without considering alloy fluctuations suggests that standard simulation models based on homogeneous media may overestimate LED efficiency by roughly 10%. A qualitatively different behaviour between flat band QWs and QWs with strong quantum confined Stark effect is found and explained by the localization behaviour of electrons and holes.