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Research findings by Dr. Julia Medvedeva, professor of physics, together with her colleagues from Northwestern University, have recently been published in Proceedings of the National Academy of Sciences and the Journal of Materials Chemistry C. She will also present her work at Display Week 2021, the International Symposium and Exhibition by the Society of Information Display in San Jose, California, in May.
For decades, hydrogen has been known to play a key role in defect passivation, doping, crystallization, and improved transport properties in Si-based semiconductors. Despite a tremendous progress, hydrogen remains a mysterious ion in materials chemistry and physics.
Complex oxides that consist of multiple post-transition metals have recently become competitive with silicon as the active transistor layer in large area displays. As the billion-dollar display industry moves forward, the amorphous phase of the complex oxides is favored both for flexible, high-resolution and low-cost displays. However, the parameter space for such disordered materials is too large for empirical sample-by-sample experimental approaches, and further progress depends on microscopic understanding of the complex structure-property relationships in these materials.
To untangle the multidimensional parameter space of amorphous conducting oxides, Medvedeva uses computationally intensive ab-initio molecular dynamics simulations combined with accurate density- and hybrid-functional calculations and analyzes the results using statistical time- and temperature-dependent tools developed in her group.
Revealing the microscopic behavior of hydrogen in amorphous oxide semiconductors presents a formidable problem. Weak metal-oxygen bonding, ionic in nature, renders strong structural disorder even in the short, nearest-neighbor range of the amorphous oxide, dramatically increasing the number of possible locations for H defects in the structure that determines the concentration, stability and diffusion of hydrogen and controls the resulting electrical properties of H-doped oxides.
To address the problem, Medvedeva ran hundreds of computer simulations that have been performed using the Foundry, S&T’s high-performance computer cluster. The cluster is funded by a large Major Research Instrumentation grant from the National Science Foundation. Medvedeva serves as co-principal investigator for the grant.
The results of her studies have revealed a stark difference between H-passivation in covalent Si-based semiconductors and the ionic metal oxides: remarkably, hydrogen doping triggers an extended bond reconfiguration in the oxide and this structural rearrangement results in energy gains that outweigh passivation of dangling O–p-orbitals.