Theory of electronic properties of amorphous silicon-carbon alloys: Effects of short-range disorder (original) (raw)
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Philosophical Magazine B-Physics of Condensed Matter Statistical Mechanics Electronic Optical and Magnetic Properties, 2000
We study by means of first-principles pseudopotential method the coordination defects in a-Si and a-Si:H, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the "electron localization function" (ELF) allows to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly three-fold (T3) and five-fold (T5) coordinated defects. We found that electronic states in the gap can be associated to both kind of defects, and that in both cases the interaction with hydrogen can reduce the density of states in the gap.
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.
Journal of Non-crystalline Solids, 1989
The compositional dependence of the density of states near the Fermi level of hydrogenated amorphous silicon-carbon alloys from silane-propane mixtures was examined. Optical and electrical measurements reveal two distinct regions of behaviour: a low propane concentration regime and a high propane concentration regime. We propose that at low propane concentration there is only a small carbon contribution to the formation of the silicon-based lattice and at high propane concentration a new structure is formed. 0022-3093/89/$03.50 © Elsevier Science Publishers B.V.
Structural, electronic and energetic properties of silicon carbon alloys
Physica B: Condensed Matter, 2007
Intrinsic interstitials in GaAs are characterized by a remarkable formation energy that makes them unlikely to be present in as-grown materials and therefore commonly neglected. However, the role of interstitials must be considered in implanted GaAs, where collision cascades by energetic ions produce a large amount of these point defects. This paper reports on semiempirical tight-binding molecular dynamics simulations of interstitial defects in GaAs. The adopted parametrization has been initially applied to the simulation of isolated selfinterstitials, on the basis of previous state of the art density-functional theory results, showing good performances. Then di-interstitial properties have been addressed, showing that self-interstitials have a remarkable tendency to aggregate in bigger structures. The binding energy of these clusters has been calculated on the basis of a simple algebraic model that does not require the calculation of the formation energy of the compound. This work will lay the ground work for a subsequent study of larger aggregates.
The configurational energy gap between amorphous and crystalline silicon
Physica Status Solidi - Rapid Research Letters, 2011
In contrast to glasses, whose lower energy states can be accessed by either cooling the liquid at lower rates or by thermal annealing [1], the energy of amorphous tetrahedral semiconductors must be lowered by thermal annealing . Among them, amorphous silicon (a-Si) has been extensively studied due to its technological relevance and because it is usually taken as a model material for covalent amorphous networks.
Amorphous silicon studied by ab initio molecular dynamics: Preparation, structure, and properties
Physical review. B, Condensed matter, 1991
We present a first-principles molecular-dynamics study of pure amorphous silicon obtained by simulated quench from the melt. A cooling rate of 10' K/s is sufficient to recover a tetrahedral network starting from a well-equilibrated metallic liquid having average coordination larger than 6. Dramatic changes in physical properties are observed upon cooling. In particular, a gap forms in the electronic spectrum, indicating a metal-to-semiconductor transition. The as-quenched structure has average coordination very close to 4, but contains several coordination defects as well as a large fraction of distorted bonds. Subsequent annealing reduces the amount of strain and the number of defects present in our system. The average structural, dynamical, and electronic properties of our sample are in good agreement with the available experimental data. We report a detailed analysis of the structural relaxation processes accompanying annealing and compare our findings with recent experiments.
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.
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.
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.
The changes of short range ordering in amorphous silicon–carbon alloys by thermal annealing
Thin Solid Films, 1998
Amorphous Si C :H and Si C :H alloys were deposited by magnetron sputtering onto a non-heated substrate, using benzene 0.9 0.1 0.7 0.3 vapour as a source of carbon atoms. The specimens were exposed to sequential isochronal thermal annealing, up to 10508C, in a vacuum chamber, followed by IR and Raman spectroscopy measurements. The influence of the thermal treatment on the structural ordering was monitored by the evolution of the intensity and shape of the characteristic bands corresponding to the Si-H, Si-C, C-C and Si-Si bonds. At low temperatures, up to 4008C, the most pronounced features are accompanied by hydrogen evolution, the appearance of new Si-C bonds and an increase of disorder in the material. Above 6008C, the structural ordering begins and between 800 and 10008C the crystalline Si-C and C-C phases appear. The results are discussed using a model of continuous random network of silicon and carbon atoms interrupted by voids. q 1998 Elsevier Science S.A.