New type of charged defect in amorphous chalcogenides

Area of Science:

• Materials Science
• Solid State Physics
• Computational Chemistry

Background:

• Chalcogenide glasses, like amorphous arsenic sulfide, are known to exhibit unique electronic and optical properties.
• Coordination defects are often implicated in the behavior of these disordered materials.
• Understanding defect structures is crucial for explaining phenomena like photodarkening.

Purpose of the Study:

• To investigate the atomic and electronic structure of amorphous arsenic sulfide models using computational simulations.
• To identify and characterize novel defect structures beyond previously proposed charged coordination defects.
• To explore the role of electronic excitation in structural transformations and their potential link to photodarkening.

Main Methods:

• Density-functional-based tight-binding (DFTB) simulations were employed.
• A series of amorphous arsenic sulfide models were systematically analyzed.
• Electronic excitation effects, specifically HOMO-to-LUMO transitions, were simulated.

Main Results:

• A novel defect pair, [As(4)](-)-[S(3)](+), was identified, featuring a fourfold coordinated arsenic and a threefold coordinated sulfur.
• Valence-alternation pairs ([S(3)](+)-S-1) were observed to transform into the novel [As(4)](-)-[S(3)](+) pairs upon electronic excitation.
• This structural transformation was accompanied by a reduction in the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) band gap.

Conclusions:

• The newly discovered [As(4)](-)-[S(3)](+) defect pair plays a significant role in the electronic structure of amorphous arsenic sulfide.
• The transformation of valence-alternation pairs into [As(4)](-)-[S(3)](+) defects under electronic excitation is a key finding.
• These findings provide a potential microscopic mechanism for the photodarkening phenomenon observed in chalcogenide glasses.