Theory of electronic structure (original) (raw)
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Electronic structure and the nature of electronic states of amorphous silicon
Physics Letters A, 2001
In this Letter we present results of Monte Carlo simulation of a model of amorphous Si using an efficient tight-binding technique which gives high quality, reliable structure of amorphous Si. We present the structural and electronic properties of the model and study the nature of electronic states. The electronic states near the band edges have been found to be localized using participation numbers calculation.
The structure of electronic states in amorphous silicon
Journal of Molecular Graphics and Modelling, 1999
We illustrate the structure and dynamics of electron states in amorphous Si. The nature of the states near the gap at zero temperature is discussed and especially the way the structure of the states changes for energies ranging from midgap into either band tail (Anderson transition). We then study the effect of lattice vibrations on the eigenstates, and find that electronic states near the optical gap can be strongly influenced by thermal modulation of the atomic positions. Finally, we show the structure of generalized Wannier functions for amorphous Si, which are of particular interest for efficient ab initio calculation of electronic properties and forces for first principles dynamic simulation.
Theoretical models for the electronic structures of hydrogenated amorphous silicon
Physical Review B, 1980
Self-consistent-field Xa scattered-wave molecular-orbital calculations have been carried out for silane molecules and clusters which are models for local atomic configurations in hydrogenated amorphous silicon. The results are in good agreement with measured photoelectron and optical spectra and provide insight into the electrical transport properties of this material.
Electronic states and total energies in hydrogenated amorphous silicon
Physical Review B Condensed Matter, 1982
The effects of bulklike and surfacelike surroundings on the electronic density of states of a variety of Si-H bonding conformations in hydrogenated amorphous silicon are examined using the cluster Bethe-lattice approach. Firstly, we discover that two fundamentally different bonding patterns, with different consequences for the doping mechanism, are consistent with ultraviolet photoemission spectroscopy (UPS) data. These are (1) H atoms bonded in microcrystalline regions and (2) clusters of monohydrides (SiH) in a continuous random network. Our results suggest an experiment in which x-ray photoemission spectroscopy and UPS taken together should distinguish between (1) and (2) and hence contribute toward understanding doping. Secondly, by using the calculated densities of states, the energies of a number of conformations and dehydrogenation reactions are calculated with the use of an empirical bond-strength total-energy scheme. Our results agree with results from annealing experiments. We introduce an improvement in the Bethe-lattice method which permits efficient solution of a second-neighbor tight-binding Hamiltonian, and which is valid for ¹h-neighbor interactions. We also estimate the Hubbard U, Stokes shifts, and electronic states associated with neutral and charged dangling bonds.
An attempt is made to highlight the importance of inhomogeneities in hydrogenated amorphous silicon (a-Si:H), in controlling its electronic properties. We note that hydrogen increases the gap in a-Si:H and that hydrogen is distributed inhomogeneously in it. This gives rise to long-range potential fluctuations, which are mostly uncorrelated and usually ignored. These and other such considerations have not only enabled us to gain new insights into the behaviour of a-Si:H in general, but have also allowed us to resolve several unsolved puzzles. Among these are questions like why undoped a-Si:H is n-type, why the creation of dangling bonds upon light soaking (LS) so inefficient, why a-Si:H degrades more upon LS when it is doped, why the reciprocity fails for light-induced degradation, why presence of nanocrystalline silicon improves stability and so on. We provide evidence to support some of our ideas and make suggestions for verifying the others.
Approximate ab initio calculations of electronic structure of amorphous silicon
Physical Review B, 2000
We report on ab initio calculations of electronic states of two large and realistic models of amorphous silicon generated using a modified version of the Wooten-Winer-Weaire algorithm and relaxed, in both cases, with a Keating and a modified Stillinger-Weber potentials. The models have no coordination defects and a very narrow bond-angle distribution. We compute the electronic density-of-states and pay particular attention to the nature of the band-tail states around the electronic gap. All models show a large and perfectly clean optical gap and realistic Urbach tails. Based on these results and the extended quasi-one-dimensional stringlike structures observed for certain eigenvalues in the band tails, we postulate that the generation of model a-Si without localized states might be achievable under certain circumstances.
Configurational and electronic properties of amorphous semiconductors
Brazilian Journal of Physics, 1994
The configurational properties of a-Si, aGe and a-Si l-,C, have been studied by Monte Carlo simulation methods. A special attention is given to the selection of the interatomic potential. The calculated networks for the a-Si and aGe systems are found to be nearly the same with only a small scaling factor of difference for the bond distances and nearly the same bond angles. In the case of a-Si l-,C, we find that a11 C sites a.re 4-fold coordinated, whereas the coordination of Si varies between 3 and 6. The increase of x in a-Si l-,C, increases the amount of 5-fold Si sites and decreases the amount of 4-fold Si sites indicanting an increase of the disorder. With the geometrical structures generated by the Monte Carlo simulation a quantum mechanical investigation is made of the electronic structure of a-Si. Using the INDO method for a cluster "supermolecule" composed of 35 Si atoms saturated with hydrogen atoms the density of states of a-Si is simulated. Configuration interaction calculation is also performed to disciiss the optical absorption spectrum of a-Si.
Structure and physical properties of paracrystalline atomistic models of amorphous silicon
Journal of Applied Physics, 2001
We have examined the structure and physical properties of paracrystalline molecular dynamics models of amorphous silicon. Simulations from these models show qualitative agreement with the results of recent mesoscale fluctuation electron microscopy experiments on amorphous silicon and germanium. Such agreement is not found in simulations from continuous random network models. The paracrystalline models consist of topologically crystalline grains which are strongly strained and a disordered matrix between them. We present extensive structural and topological characterization of the medium range order present in the paracrystalline models and examine their physical properties, such as the vibrational density of states, Raman spectra, and electron density of states. We show by direct simulation that the ratio of the transverse acoustic mode to transverse optical mode intensities I TA /I TO in the vibrational density of states and the Raman spectrum can provide a measure of medium range order. In general, we conclude that the current paracrystalline models are a good qualitative representation of the paracrystalline structures observed in the experiment and thus provide guidelines toward understanding structure and properties of medium-range-ordered structures of amorphous semiconductors as well as other amorphous materials.
Electronic Properties of an Amorphous Solid. II. Further Aspects of the Theory
Physical Review B
A further study is made of the properties of the simple tight-binding Hamiltonian for which Weaire has recently shown that a band gap exists in a tetrahedrally bonded solid regardless of its structure. An exact transformation of the density of states is found which relates it to that generated by a much simpler Hamiltonian, providing, at once, an alternative proof of Weaire's result and a powerful tool for future study of this Hamiltonian. Various generalizations and extensions of the model are discussed. These include the definition of a Hamiltonian appropriate to a compound semiconductor and the generalization of the proof of the existence of a gap to cover this case. The resulting structure-independentformula for the gap, in terms of its homopolar and heteropolar parts, bears a -close resemblance to that used in Phillips's semiempirical theory of tetrahedrally bonded semiconductors.
Electronic and transport properties of hydrogenated amorphous silicon
Physical review. B, Condensed matter, 1985
We have extended previous coherent-potential-approximation calculations of the electronic and transport properties of hydrogenated amorphous silicon (a-Si), in order to examine the effects of fully dispersed hydrogen in a-Si. The present calculation replaces random vacancies in the Si matrix by single H atoms instead of the four-H-atom clusters previously considered. In addition, to eliminate dangling-bond states in the gap we have introduced an ad hoc reconstruction of the lattice around the vacancy by effectively saturating the dangling orbitals with other Si atoms. Our results reinforce previous claims that an understanding of various experiments in a-Si:H can be obtained from first-principles calculations which neglect topological disorder and the precise configuration of the hydrogen atoms. The present calculations lead to an improved agreement with the photoemission and optical absorption data.