Unified simulation of transport and luminescence inoptoelectronic nanostructures

Computer simulation of microscopic transport and light emission in semiconductor nanostructures is often restricted to an isolated system of a single quantum well, wire or dot. In this work we report on the development of a simulator for devices with various kinds of nanostructures which exhibit quantization in different dimensionalities. Our approach is based upon the partition of the carrier densities within each quantization region into bound and unbound populations. A bound carrier is treated fully coherent in the directions of confinement, whereas it is assumed to be totally incoherent with a motion driven by classical drift and diffusion in the remaining directions. Coupling of the populations takes place through electrostatics and carrier capture. We illustrate the applicability of our approach with a well-wire structur

tdkp/AQUA: Unified modeling of electroluminescence in nanostructures

This article summarizes the capabilities of the optoelectronic simulation framework tdkp/AQUA aimed at the description of electroluminescence in semiconductor nanostructures such as light-emitting diodes. tdkp is a stand-alone finite-element software able to accurately calculate strain, built-in fields due to spontaneous and piezoelectric polarization, bound quantum states using k · p theory, gain and luminescence spectra in zero- to three-dimensional structures. AQUA calculates transport through nanostructures using a model which accounts for the distinct behaviour of carriers confined to active regions and unconfined carriers. Furthermore, it computes electroluminescence spectra via a self-consistent coupling of the confined carriers to quantum-mechanical calculations using tdkp. Two examples are presented which highlight the versatility and generality of the developed framewor

Operator ordering, ellipticity and spurious solutions in k · p calculations of III-nitride nanostructures

We analyze the ellipticity of the standard k · p wurtzite model for the symmetrized and the Burt-Foreman operator ordering. We find that for certain situations the symmetrized Hamiltonian is unstable, leads to unplausible results and can cause spurious solutions. We show that the operator ordering in wurtzite must be completely asymmetric to be stable. The asymmetric operator ordering is elliptic and consequently no spurious solutions are obtained. Therefore we recommend the use of a complete asymmetric operator ordering for nitride device simulatio

Reliable k ⋅ p band structure calculation for nanostructures using finite elements

The k⋅p envelope function method is a popular tool for the study of electronic properties of III-V nanostructures. The equations are usually transferred to real-space and solved using standard numerical techniques. The powerful and flexible finite element method was seldom employed due to problems with spurious solutions. The method would be favorable for the calculation of electronic properties of large strained nanostructures as it allows a flexible representation of complex geometries. In this paper, we show our consistent implementation of the k⋅p envelope equations for nanostructures of any dimensionality. By including Burt-Foreman operator ordering and ensuring the ellipticity of the equations, we are able to calculate reliable and spurious solution free subband structures for the standard k⋅p 4×4, 6×6 and 8×8 models for zinc-blende and wurtzite crystals. We further show how to consistently include strain effects up to second order by means of the Pikus-Bir transformation. Finally, we analyze the performance of our implementation using benchmark example

Computational study of an InGaN/GaN nanocolumn light-emitting diode

A comprehensive three-dimensional analysis of the operation of an In0.4Ga0.6N/GaN nanocolumn light-emitting diode is presented. Focus is put on the investigation of the nature and location of the emitting states. Calculations of strain and polarization-induced internal fields show that the strong lateral dependence of the potential gives rise to states confined to the periphery and to the center of the nanocolumn. However, lateral confinement of states near the column center is weak such that a quantum-well-like treatment of the remaining bound states seems appropriate where coherence is lost in the lateral directions. Within this picture, a coupled and self-consistent three-dimensional simulation of carrier transport and luminescence is presented, thus accounting for screening and lateral transport effects. Results are compared to a planar quantum-well device

Möglichkeiten und Maßnahmen zur Wahrung und Steigerung der Wettbewerbsfähigkeit der Baden-Württembergischen Wertpapierbörse zu Stuttgart

Die Börsenstrukturen befinden sich international in einer Phase dynamischer Veränderungen. Der härter gewordene Wettbewerb hinterläßt bei den großen nationalen Börsen tiefe Spuren und führt zu Anpassungsmaßnahmen, die vor allem in der technischen Neuorganisation des Börsenhandels und in der Erweiterung der Produktpalette ihren Ausdruck finden. Regionalbörsen wie die Baden-Württembergische Wertpapierbörse zu Stuttgart sind dabei einem noch stärkeren Wettbewerbsdruck ausgesetzt, da neben die internationale Konkurrenz noch die Konkurrenz der deutschen Börsen untereinander tritt. Das vorliegende Gutachten, das vom Staatsministerium Baden-Württemberg in Auftrag gegeben wurde, widmet sich speziell der Erarbeitung von Vorschlägen, die der Wahrung und Steigerung der Wettbewerbsfähigkeit der Stuttgarter Wertpapierbörse dienen

Möglichkeiten und Maßnahmen zur Wahrung und Steigerung der Wettbewerbsfähigkeit der Baden-Württembergischen Wertpapierbörse zu Stuttgart

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The Nanoelectronic Modeling Tool NEMO 5: Capabilities, Validation, and Application to Sb-Heterostructures

Modeling and simulation take an important role in the exploration and design optimization of novel devices. As the downscaling of electronic devices continues, the description of interfaces, randomness, and disorder on an atomistic level gains importance and continuum descriptions lose their validity. Often a full-band description of the electronic structure is needed to model the interaction of different valleys and nonparabolicity effects. NEMO 5 [1] is a modeling tool that addresses these issues and is able to provide insight into a broad range of devices. It unifies the capabilities of prior projects: multiscale approaches to quantum transport in planar structures in NEMO-IO [2], multimillion-atom simulations of strain and electronic structure in NEMO-30 [3] and NEMO-30-Peta [4], and quantum transport in nonplanar structures in OMEN [5]. NEMO 5 aims at becoming a community code whose structure, implementation, resource requirements and license allow experimental and theoretical researchers in academia and industry alike to use and extend the tool

Une analyse écologique orientée vers des processus d’un système riverain dynamique: la rivière Allier aval (France)

International audienceRiparian ecosystems are highly dynamic ecosystems subjected to hydrogeomorphological processes. Their geomorphological, sedimentological and hydrological heterogeneity makes them one of the richest ecosystems in terms of species diversity. In addition, riparian zones also provide numerous ecosystem functions and services to society. In the European context, the lower river Allier (France) is one of the last remaining rivers with laterally dynamic sections. Its historical evaluation has shown repeated river bed displacements during the last century. Spatio-temporal processes on a highly mobile river section of the lower river Allier have been studied analysing the mosaic of vegetation types and successional phases as well as physical habitat parameters. Understanding riparian ecosystem functioning and evolution in natural or nearly natural systems is essential for river restoration practices in highly degraded rivers. This understanding may lead to the establishment of better sustainable river rehabilitation targets that consider societal needs and natural processes.Les écosystèmes riverains sont des écosystèmes dynamiques soumis aux processus hydrogéomorphologiques avec une forte hétérogénéité géomorphologique, sédimentologique et hydrologique et de nombreux services écosystémiques rendus à la société. Ils font partie des écosystèmes les plus riches en termes de diversité des espèces. Dans le contexte européen, la rivière Allier aval est une des dernières rivières avec des sections à forte mobilité latérale. L'analyse historique a démontré cette forte mobilité latérale des chenaux durant le siècle dernier. Des processus spatio-temporels d'une section de rivière très mobile ont été étudiés à travers l'analyse de la mosaïque des types de végétation, les phases de succession, ainsi que les paramètres physiques des habitats. Comprendre le fonctionnement et l'évolution des écosystèmes riverains dans des systèmes naturels ou proche de l'état naturel est essentiel pour la restauration des cours d'eau très dégradées, afin de permettre la mise en place de stratégies de réhabilitions durables qui considèrent les besoins de la société et les processus naturels