Krisztina Kádas

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.

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.

Electronic structure of doped fourfold coordinated amorphous semiconductors. Midgap states in amorphous carbon

Abstract Midgap states have been studied in phosphorus-doped, tetrahedrally coordinated amorphous carbon by means of the Fragment Self-Consistent Field method (FSCF). It has been found that the midgap energy levels are primarily determined by the relative position of dopants. A simple linear correlation has been obtained between the highest occupied molecular orbital (HOMO) energy of the model clusters and the inverse distance of impurities.

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.

Topologically determined midgap states in amorphous carbon: Five- and sevenfold rings

Abstract Using ab initio quantum chemical calculations, it was shown that the topologically determined energy levels decrease the energy gap at EF. It was demonstrated in examples that the presence or lack of five- and sevenfold rings are not sufficient for the existence or non-existence of electronic states at the Fermi level within tight binding approximation.