Zoltán Hajnal

Atomistic simulations of complex materials: ground-state and excited-state properties

The present status of development of the density-functional-based tightbinding (DFTB) method is reviewed. As a two-centre approach to densityfunctional theory (DFT), it combines computational efficiency with reliability and transferability. Utilizing a minimal-basis representation of Kohn–Sham eigenstates and a superposition of optimized neutral-atom potentials and related charge densities for constructing the effective many-atom potential, all integrals are calculated within DFT. Self-consistency is included at the level of Mulliken charges rather than by self-consistently iterating electronic spin densities and effective potentials. Excited-state properties are accessible within the linear response approach to time-dependent (TD) DFT. The coupling of electronic and ionic degrees of freedom further allows us to follow the non-adiabatic structure evolution via coupled electron–ion molecular dynamics in energetic particle collisions and in the presence of ultrashort intense laser pulses. We either briefly outline or give references describing examples of applications to ground-state and excited-state properties. Addressing the scaling problems in size and time generally and for biomolecular systems in particular, we describe the implementation of the parallel ‘divide-and-conquer’ order-N method with DFTB and the coupling of the DFTB approach as a quantum method with molecular mechanics force fields.

A Self-Consistent charge density-functional based tight-binding method for predictive materials simulations in physics, chemistry and biology

We outline recent developments in quantum mechanical atomistic modelling of complex materials properties that combine the efficiency of semi-empirical quantum-chemistry and tight-binding approaches with the accuracy and transferability of more sophisticated density-functional and post-Hartree-Fock methods with the aim to perform highly predictive materials simulations of technological relevant sizes in physics, chemistry and biology. Following Harris, Foulkes and Haydock, the methods are based on an expansion of the Kohn-Sham total energy in density-functional theory (DFT) with respect to charge density fluctuations at a given reference density. While the zeroth order approach is equivalent to a common standard non-self-consistent tight-binding (TB) scheme, at second order by variationally treating the approximate Kohn-Sham energy a transparent, parameter-free, and readily calculable expression for generalized Hamiltonian matrix elements may be derived. These matrix elements are modified by a Self-Consistent redistribution of Mulliken Charges (SCC). Besides the usual “band-structure” and short-range repulsive terms the final approximate Kohn-Sham energy explicitly includes Coulomb interaction between charge fluctuations. The new SCC-scheme is shown to successfully apply to problems, where defficiencies within the non-SCC standard TB-approach become obvious. These cover defect calculations and surface studies in polar semiconductors (see M. Haugk et al. of this special issue), spectroscopic studies of organic light-emitting thin films, briefly outlined in the present article, and atomistic investigations of biomolecules (see M. Elstner et al. of this special issue).

Role of oxygen vacancy defect states in the n-type conduction of β-Ga2O3

Based on semiempirical quantum-chemical calculations, the electronic band structure of β-Ga2O3 is presented and the formation and properties of oxygen vacancies are analyzed. The equilibrium geometries and formation energies of neutral and doubly ionized vacancies were calculated. Using the calculated donor level positions of the vacancies, the high temperature n-type conduction is explained. The vacancy concentration is obtained by fitting to the experimental resistivity and electron mobility.

Chalcogen passivation of GaAs(1 0 0) surfaces: theoretical study

Abstract In this work, structural and electronic properties of Se- and S-passivated GaAs(1xa00xa00) surface reconstructions are investigated by density functional theory (DFT) based methods. We have performed total energy minimization of several model geometries of the reconstructed surfaces at different stoichiometry. The common feature is the appearance of a chalcogen layer on top of the Ga terminated surface, forming a Ga-chalcogenid like monolayer. In the case of selenium (Se), monomeric first layer formation is predicted, while in extrem chemically circumstances the sulphur (S) passivated surface can also reconstruct forming S-dimers.

Theoretical study of the luminescent substoichiometric silicon oxides (SiOx)

Density-functional based tight-binding molecular-dynamics and semiempirical molecular orbital calculations are applied to identify the 1.9 eV luminescence center in SiOx. The identification is made possible by an 890 cm−1 vibrational line that is correlated to the luminescence peak. Two possible models are examined: (i) Si6 rings in substituted siloxenes are known to show 1.8 eV emission. A model SiO crystal is built based upon Si rings interconnected by bridging O atoms. The structure is found to be stable up to 1000 K, but the correlated vibration is not observed. (ii) a-SiO is exposed to simulated annealing and the quenched amorphous structure is analysed for occurance of Si rings and the previously proposed non-bridging oxygen hole centers (NBOHCs). Localized stretching vibrations of the NBOHCs are found to be in the experimentally detected frequency range, thus explaining the correlation with the observed and calculated photoluminescence peak.

Simulation of physical properties of the chalcogenide glass As2S3 using a density-functional-based tight-binding method

We have used a density-functional-based tight-binding method in order to create structural models of the canonical chalcogenide glass, amorphous

Tubular structures of germanium

{mathrm{As}}_{2}{mathrm{S}}_{3}.

Interstitial-based vacancy annealing in 4H–SiC

The models range from one containing defects that are both chemical (homopolar bonds) and topological (valence-alternation pairs) in nature to one that is defect-free (stoichiometric). The structural, vibrational, and electronic properties of the simulated models are in good agreement with experimental data where available. The electronic densities of states obtained for all models show clean optical band gaps. A certain degree of electron-state localization at the band edges is observed for all models, which suggests that photoinduced phenomena in chalcogenide glasses may not necessarily be attributed to the excitation of defects of only one particular kind.

Recombination with larger than bandgap energy at centres on the surface of silicon microstructures

In this paper we demonstrate, using density-functional tight-binding theory, that certain classes of germanium-based tubular nanostructures are stable and energetically viable. Specifically, we consider GeH nanotubes. The structures adopted by these nanotubes are very similar to those of previously reported silane and siloxene nanotubes. The Ge-based nanotubes have a semiconducting gap which is dependent on the tube diameter.

Correlation between the luminescence and Raman peaks in quantum-confined systems

Abstract The migration of carbon interstitials through the 4H–SiC lattice and their recombination with vacancies has been investigated theoretically within the self-consistent charge density functional based tight-binding (SCC-DFTB) method. For vacancy–interstitial pairs created by irradiation, the capture radius of silicon and carbon vacancies has been examined, showing that interstitial migration through the otherwise perfect lattice starts getting important for distances larger than four nearest-neighbor atomic distances.