3 years ago

Amorphous chalcogenides as random octahedrally-bonded solids: II. Topological midgap electronic states and the Urbach tail in pnictogen-chalcogenide glasses.

Alexey Lukyanov, Vassiliy Lubchenko

We report the electronic density of states for computationally-generated bulk samples of amorphous chalcogenide alloys As$_{x}$Se$_{100-x}$, which belong to the technologically important family of phase-change materials. The samples were generated using a novel algorithm reported in the first paper of the series. The calculated density of states is in good agreement with experiment, including the trend of the mobility gap with arsenic content. The sample-to-sample variation in the band edges is quantitatively consistent with the width of the exponential (Urbach) tail of band-edge states observed in experiment. Importantly, our samples consistently exhibit very deep-lying midgap electronic states that are delocalized in one direction significantly more than what would be expected for a deep impurity or defect state. These properties are consistent with those of the topological midgap electronic states that have been proposed by Zhugayevych and Lubchenko as an explanation for several puzzling opto-electronic anomalies observed in the chalcogenides, including light-induced midgap absorption and ESR signal, and anomalous photoluminescence. In a complement to the traditional view of the Urbach states as a generic consequence of disorder in atomic positions, the present results suggest these states can be also thought of as intimate pairs of topological midgap states that cannot recombine. Finally, samples with an odd number of electrons exhibit neutral, spin $1/2$ midgap states as well as polaron-like configurations that consist of a charge carrier bound to an intimate pair of midgap states; the polaron's identity---electron or hole---depends on the preparation protocol of the sample.

Publisher URL: http://arxiv.org/abs/1710.04362

DOI: arXiv:1710.04362v2

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