S. Kugler

Theory of dopant pairs in four-fold coordinated amorphous semiconductors

Abstract The position of energy levels belonging to dopant pairs was investigated in substitutionally p- or n-type doped four-fold coordinated amorphous carbon and silicon by means of Hartree–Fock ab initio and the Fragment Self-Consistent Field method. Our models contain 45 to 583 carbon or silicon atoms. Boron, phosphorus and nitrogen impurities have been incorporated into the amorphous networks. It has been found that the position of midgap states are primarily determined by the separation of the impurity atoms, and the role of the random network is only subordinate. A general relationship is proposed for the determination of midgap energy levels as a function of the distance between dopant pairs investigated for amorphous carbon and silicon.

On photoinduced volume change in amorphous selenium: Quantum chemical calculation and Raman spectroscopy

Hartree–Fock ab initio Raman spectra calculations and Raman spectroscopic measurements were carried out on amorphous selenium in order to identify the characteristic vibrational mode due to sigma bonds. Variations in measured peak intensity were observed as a consequence of band gap light illumination. Based on our previous theoretical investigations we originate these intensity variations from photoinduced covalent bond breaking.

Unusual atomic arrangements in amorphous silicon

Abstract The existence of small bond angles (like those of triangles and squares) in amorphous silicon networks were studied by the tight-binding molecular dynamics method, by analyzing the statistical data of Si–Si–Si fragments inside large molecules, and also by the Reverse Monte-Carlo simulation method. The influence of small bond angles on the electronic density of states was revealed.

Atomic structure and electronic density of states around the fermi level in amorphous carbon models

Abstract There are topologically determined energy levels concentrated around the Fermi level. Graph theory is used to estimate the density of states near the Fermi level (EF) of amorphous carbon (a-C) structure. Within the ab initio Hartree-Fock calculations these states have lower energies than EF, but they remain the highest occupied states in the electronic density of states.

Structure and photoinduced volume changes of obliquely deposited amorphous selenium

Atomic scale computer simulations on structures and photoinduced volume changes of flatly and obliquely deposited amorphous selenium (a-Se) films have been carried out in order to understand how the properties of chalcogenide glasses are influenced by their preparation method. Obliquely deposited a-Se thin films contain more coordination defects and larger voids than the flatly deposited ones. To model the photoinduced volume changes the electron excitation and hole creation were treated independently within the framework of tight-binding formalism. Covalent and interchain bond breakings and formations were found. The obliquely deposited samples containing voids showed a wide spectrum of photoinduced structural changes in the microscopic level and volume changes in the macroscopic level.

Atomic charge distribution in diamondlike amorphous carbon

Based on semiempirical molecular orbital calculations, we propose an empirical formula for the calculation of net atomic charges in diamondlike amorphous carbon. These charges are in a linear relationship with bond angle distortions involving first and second neighbors. On the basis of the estimated charge fluctuation we predict that the integrated infrared absorption intensity will be within the range of 70000 to 120000 cm-2.

Quantum chemical study of internal rotations in liquid crystal molecules

The rotational barriers of p-azoxy-anisole and dibutyl-phenyl-benzoyloxy-benzoates nematic liquid crystal molecules were calculated by the PCILO and CNDO/2 quantum chemical methods. It was found that motion in the side group is hindered to a given extent. According to chemical evidence the central part of the molecules is supposed to be completely rigid.

Molecular dynamics simulations of amorphous carbon structures

The formations of different kinds of carbon structures have been studied by means of molecular dynamics simulations. A chain-like amorphous structure was obtained on diamond (111) surface with fivefold, sixfold, and sevenfold rings. At the temperature of 3500 K in the bulk region amorphous, graphite- and diamond-like structures were developed depending on the macroscopic density.

Midgap states in nitrogen doped diamond-like amorphous carbon

Ab initio and semiempirical quantum chemical methods have been used to investigate the effect of nitrogen doping in amorphous carbon. We propose a simple relationship for the determination of midgap energy levels in substitutionally doped diamond-like amorphous carbon in function of distances between N atoms.

Impurity levels in phosphorus- and boron-doped amorphous silicon

Abstract The AMI semiempirical quantum-chemical method has been used to investigate phosphorus and boron-doped amorphous silicon. A simple relationship for the determination of midgap energy levels is proposed for substitutionally doped amorphous silicon as a function of distances between dopants.