Abstract

Oxygen vacuums (VO) critically influence the electrical performance and reliability of amorphous oxide semiconductor (AOS) devices, including indium–gallium–zinc oxide (IGZO) thin-film transistors (TFTs). Using first-principles density functional theory and configuration-coordinate analysis, we elucidate a unified mechanism connecting the contrasting behaviors of VO under strain versus annealing. We demonstrate that both mechanical strain and annealing-induced structural relaxation modulate the energetic position and localization character of VO states by altering specific interatomic distances. Compressive strain shortens key metal–metal separations, thereby stabilizing bonding-like localized states, lowering their formation energy, and driving a delocalized-to-localized transition; conversely, tensile strain induces the opposite trend. Along the structural-relaxation pathway, the defect level shifts upward toward the conduction band minimum as critical distances increase, promoting delocalization. Ab initio molecular dynamics simulations further corroborate that higher thermal budgets facilitate this relaxation, thereby enhancing donor activation. Crucially, we resolve the prevailing apparent strain–annealing densification inconsistency: annealing-induced delocalization arises not from increased mass density (as in compressive strain), but specifically from structural relaxation that expands critical interatomic distances and minimizes screened ion–ion repulsion. These findings provide a rigorous atomistic framework for manipulating defect states via strain engineering and thermal processing, offering precise guidelines for optimizing the stability and performance of next-generation AOS electronics.

A new study from UNIST explains what determines the behavior of oxygen vacuums—tiny defects that strongly influence how oxide semiconductors perform. The researchers found that the key is not the material's overall atomic density, as previously thought, but the spacing between neighboring atoms around the defect. The findings offer new guidance for designing oxide semiconductors used in displays and next-generation memory devices.

The study was led by Professor Changwook Jeong of the Graduate School of Semiconductor Materials and Devices Engineering, who used theoretical calculations to uncover how local atomic arrangements control the electrical properties of oxygen vacuum.

Oxygen semiconductors, such as indium–gallium–zinc oxide (IGZO) are widely used in thin-film transistors for smartphones and television displays because they can be processed at relatively low temperatures. During fabrication, however, oxygen vacuums naturally form within the material. These defects can change how electricity flows through a device, affecting its operating voltage, performance, and long-term stability.

The team found that even small changes in the spacing between nearby metal atoms determine whether electrons remain trapped around an oxygen vacancy or move freely through the material. When those atoms move closer together, the electrons stay localized near the defect. As the distance increases, the electrons spread more easily, making electrical conduction more favorable. 

The findings also resolve a long-standing question in oxide semiconductor research. Heat treatment and mechanical compression both make the material denser, yet previous studies showed that they have opposite effects on oxygen vacuums. The team demonstrated that density itself is not the deciding factor. Instead, each process changes the local arrangement of atoms in a different way, leading to different electrical behavior. 

To reach these conclusions, the researchers combined density functional theory, configuration-coordinate analysis, and ab initio molecular dynamics simulations. Together, the calculations revealed a single mechanism that explains how both heat treatment and mechanical strain influence oxygen vacuums, providing a theoretical framework for optimizing oxide semiconductor processing.

“Oxygen vacuums are unavoidable in oxide semiconductors, but this study shows that their electrical behavior can be controlled through processing conditions,” said Professor Jeong. “By tailoring heat treatment and mechanical stress, it should be possible to optimize threshold voltage, switching characteristics, and device reliability at the same time.”

The findings were published in Chemistry of Materials on June 23, 2026. The research was supported by the Nano & Material Technology Development Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (MSIT).

Journal Reference

Hyeongjun Jang, Yoonju Park, Beomjin Park, et al., “A Unified Mechanism for Strain- and Anneal-Induced Oxygen-Vacancy Behavior in Oxide Semiconductors,”   Chem. Mater.,  (2026).