Research projects


Time-dependent density functional theory

The time-dependent generalisation of the density functional theory (DFT) has been developed intensively for the last decade. It provides an expansion of the DFT-formalism to dynamic phenomenons. The method's relevance is significant due to the numerous important applications such as excitations of atoms, molecules and solids by strong laser radiation or collision with highly charged ions and optical and kinetic properties of solids and nano structures. Despite the initial success there is still a lack of understanding of the dynamic effective electron interaction.

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Hydrodynamic approach on the kinetic theory of electron liquids and dynamic density functional theory

Unlike the established static density functional theory the time-dependent version of this theory is inherently non-local. According to this the construction of the dynamic exchange-correlation-functional is a much more complex task. It is possible to simplify matters by considering the long wave limit of quasi-homogeneous systems. In this case the TDDFT for interacting many-body-Fermi-systems reduces to a combination of elasticity theory and hydrodynamics. This method's straight application is the calculation of collective excitations in an electron liquid, which are embedded in different quantum structures (quantum wells, quantum wires and quantum dots) or enclosed at metallic surfaces.

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Density-functional study of localization in disordered interacting two-dimensional electron systems

Recent systematic studies of the temperature dependence of the resistivity in zero magnetic field in variety of dilute, low-disordered two-dimensional systems (high-mobility silicon MOSFETs, p-SiGe heterostructures, p-GaAs/AlGaAs heterostructures, n-AlAs heterostructures, n-GaAs/AlGaAs heterostructures), demonstrated a possibility of metal-insulator transition at a low critical electron density. This discovery, apparently contradicting to the scaling theory of localization, can neither be explained by interaction effects in clean samples nor by disorder alone, and, despite of the high experimental and theoretical activities, is still far from being completely understood.
In this situation the density functional theory, that, in principle, exactly maps an interacting many-body problem onto an effective non-interacting one, may be considered as an alternative tool.
Under this project we develop the time-dependent density functional theory of the interacting electron system with disorder and study the influence of the weak localization on the behavior of the exchange-correlation potential in impure metals.

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Large-scale calculations of the properties of electronic materials

We study the atomic structure and the energy spectrum of complex electronic materials (such as silicides, SiC-polytypes and quantum dots in silicium) with ab-initio computer simulations on a large scale. In doing so programs with Full-Potential attempt and with Pseudo-Potential attempt complement one another. We develop a qualitative valence force model to interpret the results and to characterise the chemical binding in silicides (PtSi and IrSi). These models shall establish a connection between the mechanical properties (elastic constants) and the electronic structure and bond of silicides. Furthermore we specify diverse defects and doping atom-defect-complexes, which are responsible for compensation and diffusion of doping atoms. We also analyse the radiating recombination of charge carriers in Si-quantum dots theoretically. The essential point is to consider the excitonic effects and to accurately describe the single particle excitation spectrum. The dependance of luminescence specta on the quantum dot size shall also be subject to study.

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SiC Research Group: Theory of SiC doping and graphite overlayers on SiC surfaces


Project description

Theory of doping

A semiconductor's necessary properties for the production of electronic devices are obtained by doping, that is insertion of impurity atoms and thereby manipulation of its electronic characteristics. A high concentration of impurity atoms results in a high conductivity, which is technically required. On the path to technological success of the material there are obstacles such as the low doping efficiency and the appearance of distinctive diffusion effects. Our goal is to obtain a basic understanding of the occuring physical processes during the doping of SiC, by using ab initio methods based on density functional theory. We analyse two fundamental aspects of doping: on the one hand solubility of the impurity atoms (including impurity complexation and impurity atom compensation) and on the other hand the mechanisms of impurity atom diffusion. We want to illuminate the nature of substitutional bound doping atoms such as Boron, Phosphorus and Nitrogen and the deep defects connected with these impurity atoms. Thereby the premises for a microscopic understanding of the central aspects mentioned above are accomplished. By comparing hyperfine data from ENDOR/ESR-spectra and phonon replica from photoluminescence spectra with calculated hyperfine parameters and defect oscillation modes we are able to verify the models for the observed intrinsic and extrinsic defect centers.

Graphite overlayers on SiC surfaces

In the new funding period the interaction of graphite layers on top of SiC surfaces is studied in addtion to the theory of doping. There are strong experimental indications that the graphite aggregates play a key role in the formation of Ohmic contacts on SiC. Ohmic contacts are an important preequisite for the manufacturing of p-n diodes. It was observed that with annealing temperatures above 1000˚C SiC/nickel contacts always show an Ohmic behaviour, although the pure nickel/SiC interface has a Schottky barrier. This significant and technologically important change in the behaviour was recently attributed to the catalytic formation of graphite inclusions at the interface. It is well known that under high temperatures the SiC(0001) surface evaporates silicon, so that thin graphite layers with hiqh quality form. It is the goal of this project to understand the interaction between the graphite layer and the underlying substrate. The structural properties are investigated as well as the electronic interface states by efficient ab initio methods. This information will support the development of Ohmic contacts on SiC as well as the interaction of graphite inclusions with SiC in general, which were recently proposed as nanowires buried within the SiC crystal.

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SFB 292: project A5 (SiC)

SiC is a semiconductor with a large bandgap, whose properties make it especially interesting for high-performance, high-temperature and high-frequency applications. In the scope of this research project we study the microscopic characteristics of this materials based on density functional theory. The main focus is put on the construction of a hierarchy of both intrinsic defects and defects with boron, which is frequently used for doping. Starting from this hierarchy a further analysis of diffusion mechanisms of individual atom sorts is possible. That provides for example a deeper understanding of the diffusion effects, that occur during the doping or the following annealing and which are able to destroy the implanted profiles.

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SFB 292: project A6 (Silicides)

Transition metal silicides show a phase diagram rich in crystal structures and crystal stoichiometries. Furthermore it is possible to grow layer structures on silicium substrate, which are not stable as volume crystals. Due to this plenitude of different phases the silicides show a broud spectrum of material characteristics important for applications, such as metallic conductivity, magnetism or semimetallic behaviour. Silicides are especially interesting because of their compatibility to silicium technology. In the scope of the project at hand we examined the volume- and surface-properies of silicide layers with ab initio methods.

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