Thomas Köhler

Computational approach for structure design and prediction of optical properties in amorphous TiO2 thin-film coatings

We have investigated the structural and electronic properties of amorphous TiO2 using molecular dynamics (MD) simulations based on ab initio density functional theory, a numerically efficient density-functional-based tight-binding approach and classical many-body potentials. The lower level approximations are successively validated by the higher level ones through comparison of the calculated structural and electronic properties. The classical results reproduce all relevant structural features of a-TiO2 as obtained by quantum-mechanical simulation and reproduce the experimentally observed reduced radial distribution function. This gives convincing justification for the use of classical MD in the simulation of ion beam sputtering synthesis of large-area amorphous thin films. Cross-correlation of electronic data with the statistics of disorder-induced under- and over-coordination is derived as a basis for evaluating the optical quality of thin-film coatings.

Stability of silicon carbide structures: from clusters to solid surfaces

We present a density-functional based non-orthogonal tight-binding (DF–TB) Hamiltonian in application to silicon carbide. The Kohn-Sham orbitals of the system are represented by a linear combination of atomic orbital (LCAO) equation with respect to a minimal basis of the localized valence electron orbitals of all atoms. Within a two-centre approach all Hamiltonian and overlap matrix elements are derived in a parameter-free way via the construction of pseudo-atomic orbitals and potentials by self-consistent single-atom calculations using the local-density approximation (LDA). This is in favour of a tabulation of the corresponding Slater–Koster integrals vs. distance. Making use of a non-self-consistent solution of the Kohn-Sham equations for the many-atom structure and an adjustment of the universal short-range repulsive two-particle potentials with respect to self-consistent field (SCF)–LDA results in the method becoming sufficiently accurate to obtain the total energy of all-scale silicon carbide structures and this is transferable and efficient for predictive molecular-dynamics simulations. We present results for the energetic stability and properties of various microclusters and molecules including interactions with hydrogen. We give proof of the stability of the solid-state modifications and calculate the vibrational density of states for the most stable zinc blende structure. In addressing further applications to surface properties, we discuss the (1 × 1) reconstruction of the (110) SiC surface.

The atomic structure of ternary amorphous TixSi1−xO2 hybrid oxides

Atomic length-scale order characteristics of binary and ternary amorphous oxides are presented within the framework of ab initio theory. A combined numerically efficient density functional based tight-binding molecular dynamics and density functional theory approach is applied to model the amorphous (a) phases of SiO2 and TiO2 as well as the amorphous phase of atomically mixed TixSi1-xO2 hybrid-oxide alloys over the entire composition range. Short and mid-range order in the disordered material phases are characterized by bond length and bond-angle statistics, pair distribution function analysis, coordination number and coordination polyhedra statistics, as well as ring statistics. The present study provides fundamental insights into the order characteristics of the amorphous hybrid-oxide frameworks formed by versatile types of TiOn and SiOm coordination polyhedra. In a-SiO2 the fourfold crystal coordination of Si ions is almost completely preserved and the atomic structure is widely dominated by ring-like mid-range order characteristics. In contrast, the structural disorder of a-TiO2 arises from short-range disorder in the local coordination environment of the Ti ion. The coordination number analysis indicates a large amount of over and under-coordinated Ti ions (coordination defects) in a-TiO2. Aside from the ubiquitous distortions of the crystal-like coordinated polyhedra, even the basic coordination-polyhedra geometry type changes for a significant fraction of TiO6 units (geometry defects). The combined effects of topological and chemical disorder in a-TixSi1-xO2 alloys lead to a continuos increase in both the Si as well as the Ti coordination number with the chemical composition x. The important roles of intermediate fivefold coordination states of Ti and Si cations are highlighted for ternary a-TixSi1-xO2 as well as for binary a-TiO2. The continuous decrease in ring size with increasing Ti content reflects the progressive loss of mid-range order structure characteristics and the competing roles of network forming and network modifying SiOm and TiOn units in the mixed hybrid oxides.

Plasma and optical thin film technologies

The PluTO project is aimed at combining thin-film and plasma technologies. Accordingly, the consortium comprises experts in optical coating (Laser Zentrum Hannover, Fraunhofer IOF) and such in plasma technology (INP Greifswald, Ruhr University of Bochum RUB). The process plasmas available, especially the sheath layers, will be thoroughly characterized by means of special probes, so that the types, numbers and energies of the particles participating in the coating formation processes can be determined comprehensively in every detail for the first time. The data thus obtained will provide a basis for a numerical modelling of layer growth at atomic scale (Bremen Center for Computational Materials Science BCCMS). The results are expected to deepen the understanding of the physical mechanisms responsible for the influence of plasma action on the layer properties. In parallel, suitable tools for process monitoring will be identified and made available. Some first results have already been achieved which prove the viability of the approach.