Research

Research Areas

Small silicon hydrides
their geometries, spectroscopic properties and thermodynamic potentials.

Small unsaturated silicon hydride molecules or cluster with few hydrogen atoms are important species in the gas phase decomposition of silanes (CVD, Chemical Vapor Deposition). CVD processes are the most important source of amorphous hydrogenated silicon, a-Si:H, which is mainly used to prepare thin films of semiconducting material used in microelectronics or solar energy conversion.

  • Computational Thermochemistry of Medium-Sized Silicon Hydrides. G. Katzer, M. C. Ernst, A. F. Sax, and J. Kalcher, J. Phys. Chem. 101 (1997) 3942-3958.
  • Bond Strengthening by Deformation of Bond Angles. G. Katzer, A. F. Sax, J. Kalcher. J. Phys. Chem. A 103 (1999) 7894-7899.

Theoretical Thermochemistry
Gas phase reactions in CVD processes occur at high temperatures. Thermodynamic properties at high temperatures need proper accounting for anharmonic properties of molecules.

We did some work on Theoretical Thermochemistry of small and medium sized silicon hydrides many of them are rather floppy. Therefore, anharmonic vibrations and pseudorotation of floppy cyclic molecules must be included into the calculation of thermodynamic potentials.

  • Beyond the Harmonic Approximation: Impact of Anharmonic Molecular Vibrations on the Thermochemistry of Silicon Hydrides. G. Katzer, A. F. Sax. J. Phys. Chem. A 106 (2002) 7204-7215.
  • Numerical determination of pseudorotation constants. G. Katzer, A. F. Sax. J. Chem. Phys. 117 (2002) 8219-8228.
  • A novel partition function for partially asymmetrical internal rotation. G. Katzer, A. F. Sax. Chem. Phys. Lett. 368 (2002) 473-479.

Gernot Katzer wrote a Web-program Thermo to calculate thermodynamic properties up to 4000 K for about 250 silicon hydride molecules and reactions between them.

Amorphous covalent solids
Amorphous covalent solids are an important class of materials most of them are produced in CVD processes. Macroscopic parameters like pressure, temperature, deposition rate and energy source influence structure and properties of the deposited films in an as yet not completely understood way. Controlling film growth, avoiding the creation of defects in the solid and the production of materials with taylored properties need better understanding of the chemical reactions in the gas phase, at the the surface and in the bulk.

Amorphous hydrogenated silicon, a-Si:H, is the most important amorphous material used in opto-electronic and energy conversion devices. The observed reversible degradation of the material (called the Staebler-Wronski effect) is attributed to defects in the bulk. We investigated the microscopic structure of defects in a-Si:H using QM/MM methods.

  • Distorted silicon hydrides - a comparative study with various density functionals. T. Krüger, A. F. Sax, J. Comput. Chem. 22 (2001) 151-161
  • Oligovalent link atoms in embedding calculations. T. Krüger, A. F. Sax, J. Comput. Chem. 23 (2002) 371-377
  • Microstructure of local defects in amorphous Si:H: A quantum chemical study. T. Krüger, A. F. Sax, Phys. Rev. B 64 (2001) 195201/1-195201/11
  • Quadruple voids in amorphous Si:H. T. Krüger, A. F. Sax, Physica B 308-310 (2001) 155-158
  • Methodological Problems in the Calculations on Amorphous Hydrogenated Silicon, a-Si:H. A. F. Sax, T. Krüger. Lecture Notes on Computer Science 2331 (2002) 950-955
  • Theoretical treatment of silicon clusters. A. F. Sax, in: Eds: P. Jutzi, U. Schubert. Silicon Chemistry (2003), 269-280. Wiley-VCH.
  • Electron trapping by excited microvoids (ETEM)-an explanation of the Staebler-Wronski effect. T. Krüger; A. F. Sax Physica B 353 (2004), 263-277.

Understanding of film growth is the key to a better understanding of properties of amorphous materials. The development of theoretical models of film growth is based on better theoretical methods to describe the structure of bulk material, embedded defects, surface reactions and so on. Use of QM/MM methods for the description of materials like a-Si:H, a-Ge:H or a-(Si,Ge):H needs development of new or modification of existing empirical potentials for the bulk which can be combined with proper quantum theoretical methods. I am interested in the development of such methods and their application to interesting problems.

Structure of polysaccharides
Adsorption, swelling and diffusion are processes which are best described with simulation methods like molecular dynamics or Monte Carlo. Application of such methods to polysaccharides needs computationally cheap force fields for the condensed phase(s). Force fields developed for gas phase species cannot be used for the condensed phase, computational efficiency frequently makes a redesign of force fields necessary. We want to write a model builder for polysaccharides based on a monosaccharide data base and link it to simulation software.

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