A first principles analysis of the effect of hydrogen concentration in hydrogenated amorphous silicon on the formation of strained Si-Si bonds and the optical and mobility gaps
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Date
2014-05-28
Authors
Legesse, Merid
Nolan, Michael
Fagas, GĂorgos
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AIP Publishing
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Abstract
In this paper, we use a model of hydrogenated amorphous silicon generated from molecular dynamics with density functional theory calculations to examine how the atomic geometry and the optical and mobility gaps are influenced by mild hydrogen oversaturation. The optical and mobility gaps show a volcano curve as the hydrogen content varies from undersaturation to mild oversaturation, with largest gaps obtained at the saturation hydrogen concentration. At the same time, mid-gap states associated with dangling bonds and strained Si-Si bonds disappear at saturation but reappear at mild oversaturation, which is consistent with the evolution of optical gap. The distribution of Si-Si bond distances provides the key to the change in electronic properties. In the undersaturation regime, the new electronic states in the gap arise from the presence of dangling bonds and strained Si-Si bonds, which are longer than the equilibrium Si-Si distance. Increasing hydrogen concentration up to saturation reduces the strained bonds and removes dangling bonds. In the case of mild oversaturation, the mid-gap states arise exclusively from an increase in the density of strained Si-Si bonds. Analysis of our structure shows that the extra hydrogen atoms form a bridge between neighbouring silicon atoms, thus increasing the Si-Si distance and increasing disorder in the sample.
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Keywords
Amorphous silicon , Atoms , Chemical bonds , Crystal atomic structure , Dangling bonds , Electronic properties , Hydrogen , Hydrogenation , Molecular dynamics , Undersaturation , Dangling bonds , Amorphous semiconductors , Optical saturation , Carrier mobility , Bond formation
Citation
Legesse, M., Nolan, M. and Fagas, G. (2014) 'A first principles analysis of the effect of hydrogen concentration in hydrogenated amorphous silicon on the formation of strained Si-Si bonds and the optical and mobility gaps', Journal of Applied Physics, 115, 203711. http://scitation.aip.org/content/aip/journal/jap/115/20/10.1063/1.4880395
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© 2014 AIP Publishing LLC. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in M. Legesse et al., J. Appl. Phys. 115, 203711 (2014) and may be found at http://dx.doi.org/10.1063/1.4880395