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Electronic transport and the origin of Urbach tails in amorphous silicon

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Electronic transport and the origin of Urbach tails in amorphous silicon D. A. Drabold, Ohio University Recently, we have studied the structure of the electron states in large and realistic models of a-Si. We have explored the nature of the electronic localized-delocalized (Anderson) transition by direct calculation, and have found that important aspects of this transition are universal1 (similar in different materials, and even different physical quantities, such as classical vibrations). The valence (conduction) band tail states are shown to derive from short (long) bonds. What is more, we find that one-dimensional filamentary structures (connected short bonds and connected long bonds) exist in good quality relaxed network models, and these filaments are responsible for the observed exponential (Urbach) edges observed in the material2. We also briefly report on transport calculations using the Kubo formula and a simple thermal averaging procedure, and show that the temperature dependence of the electrical conductivity and the Meyer-Neldel relation can be obtained from ab initio simulations on suitable structural models of the material3.

1 J. J. Ludlam, S. N. Taraskin, S. R. Elliott and D. A. Drabold, Universal features of eigenstates in disordered systems , J. Phys. Cond. Matter 17 L321 (2005).

2 Y. Pan, F. Inam, M. Zhang and D. A. Drabold, Atomistic origin of Urbach tails in amorphous silicon, Phys. Rev. Lett. 100 206403 (2008).

3 T. A. Abtew, M. Zhang and D. A. Drabold, Ab initio estimate of the temperature dependence of electrical conductivity in a model disordered material: a-Si:H Phys. Rev. B 76 045212 (2007).

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