jp8b05204_si_002.mpg (41.4 MB)
Structural Origin of the Midgap Electronic States and the Urbach Tail in Pnictogen-Chalcogenide Glasses
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posted on 2018-08-02, 00:00 authored by Alexey Lukyanov, Jon C. Golden, Vassiliy LubchenkoWe determine the
electronic density of states for computationally
generated bulk samples of amorphous chalcogenide alloys AsxSe100–x. The samples
were generated using a structure-building algorithm reported recently
by us. Several key features of the calculated density of states are
in good agreement with experiment: The trend of the mobility gap with
arsenic content is reproduced. The sample-to-sample variation in the
energies of states near the mobility gap is quantitatively consistent
with the width of the Urbach tail in the optical edge observed in
experiment. Most importantly, our samples consistently exhibit very
deep-lying midgap electronic states that are delocalized significantly
more than what would be expected for a deep impurity or defect state;
the delocalization is highly anisotropic. 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 optoelectronic anomalies observed in the chalcogenides, including
light-induced midgap absorption and electron spin resonance 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 because of disorder. 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.