A Korean research team has identified how oxygen vacancy defects degrade the performance of oxide semiconductors used in smartphone and TV displays. They found that the electrical properties of the defects are determined not by the overall atomic density of the semiconductor but by the distances between metal atoms around the defects.
Ulsan National Institute of Science and Technology (UNIST) said on the 19th that a research team led by Graduate School of Semiconductor Materials and Components Professor Jeong Chang-uk verified this through theoretical calculations.
Indium gallium zinc oxide (IGZO) is easy to form into thin films at low temperatures and is used as the semiconductor material for thin-film transistors (TFTs) that drive smartphone and TV screens. However, if a defect occurs in which the site that should be occupied by an oxygen atom is left vacant during manufacturing, the current flow and operating voltage can change, destabilizing device performance.
Two electrons remain at the site missing oxygen. Depending on whether these electrons are trapped near the defect or spread across the entire film, the device's threshold voltage and electrical properties differ. The team confirmed that when specific metal atoms near an oxygen vacancy move closer together, electrons are trapped around the defect, and when the distance increases, electrons spread throughout the film. The "defect level," the energy at which electrons remain in the defect, becomes lower and deeper as electrons are more strongly bound to the vacancy, making it harder for them to escape.
The study also explains the contrasting effects of annealing and compression on oxide semiconductors. Previously, it was thought that once overall atomic density increased through annealing, electrons trapped in defects would delocalize, but when the film was compressed to raise density, electrons were instead observed to become trapped near the defects.
The team concluded that the atomic arrangement around defects is more important than overall density. Annealing and tensile stress that stretches the film widen the spacing between certain metal atoms, raise the defect level, and make it easier for electrons to spread across the film. Conversely, compression that brings atoms closer can trap electrons around the defects.
The team used density functional theory and configuration coordinate analysis to calculate atomic and electronic states, along with first-principles molecular dynamics simulations to reproduce atomic motion over time. Jeong said, "Oxygen vacancy defects are hard to avoid in oxide semiconductors, but we proved that the electrical role of defects can be tuned through process conditions," adding, "By designing annealing conditions or the stress applied to thin films, we can jointly control threshold voltage, on-off current characteristics, and reliability."
The findings were published on Feb. 23 in Chemistry of Materials, an international journal of the American Chemical Society (ACS).