Structure and Optoelectronic Anomalies of Amorphous Chalcogenides

Date

2017-08

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Abstract

We develop a computationally-efficient algorithm for generating high-quality structures for amorphous materials exhibiting distorted octahedral coordination. A spatially uniform, chemically motivated aperiodic parent structure is generated, which is subsequently geometrically optimized using quantum-chemical force fields; by varying the extent of geometric optimization of the parent structure, one can partially control the degree of octahedrality in local coordination and the strength of secondary bonding. Importantly, the algorithm allows one to control the number of homo-nuclear and hetero-nuclear bonds and, hence, effects of the mixing entropy. The present methodology is applied to the archetypal chalcogenide alloys AsxSe1−x. We find that local coordination in these alloys interpolates between octahedral and tetrahedral bonding but in a non-obvious way; it exhibits bonding motifs that are not characteristic of either extreme. We consistently recover the first sharp diffraction peak (FSDP) in our structures and argue that the corresponding mid-range order stems from the charge density wave formed by regions housing covalent and weak, secondary interactions. The present results are consistent with the dependence of the FSDP on pressure and temperature and its correlation with the photodarkening and the Boson peak. They also suggest that the position of the FSDP can be used to infer the effective particle size relevant for the configurational equilibration in covalently-bonded glassy liquids, where identification of the effective rigid molecular unit is ambiguous. We find the electronic density of states (eDOS) of present samples 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 exponential, Urbach tail of band-edge states observed in experiment. We have obtained computational evidence, for the first time, of very deep-lying midgap states in bulk samples. We argue that these deep states are the topological midgap states that have been implicated in several opto-electronic anomalies including light induced midgap absorption and ESR signal, and anomalous photoluminescence. Our results suggest that the localized Urbach states can be thought of as intimate pairs of topological midgap states that cannot recombine.

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Keywords

Amorphous chalcogenides, Midgap states, Photodarkening, FSDP

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