IAS / School of Science Joint Lecture - A Journey to Defect Science and Engineering

Abstract

A defect in a material is one of the most important concerns when it comes to modifying and tuning the properties and phenomena of materials. The speaker will review his study of defects over the course of his professional career, reflecting on his journey through the history of this research as he prepares to retire from the university where he has worked for more than 30 years. The review will include the study of intrinsic and extrinsic defects since the defects can be introduced in various ways, either intrinsically or extrinsically.  

The journey began with the study of ferroelectric oxides applied to non-volatile ferroelectric memories 35 years ago, when one of the reliabilities for the realization of ferroelectric memories was a critical issue. The defect intensively coined at the reliability issue was an oxygen vacancy, which is a fundamental and intrinsic defect. The oxygen deficiency inevitably occurs in oxides greatly affects fatigue and imprint reliability. The fatigue had been resolved by the use of oxide electrode, either doped layered perovskite ferroelectrics. The study on the oxygen vacancy defects continued on low d-electron occupancy perovskites among transition metal oxides and rare-earth fluorite oxides in the f-electron system. The first-principles calculations suggested that oxygen vacancies tended to cluster along a specific direction, i.e., 001 direction in SrTiO3, followed by experimental validation.

Ferromagnetism evolved from heavily oxygen-deficient CeO2, known for its role as an oxygen reservoir. Oxygen vacancy engineering was also used to induce two-phase coexistence with different transition temperatures to mimic the two-phase coexistence during the first-order phase transition. From oxygen vacancy engineering, the isostructural metal-insulator transition of VO2 was predicted and validated while structural and electronic transitions were known to be coupled in the transition.

Initiated by the oxygen vacancy clustering along the specific direction, the defect study was extended to the geometrical aspect of defect distribution and location at the atomic scale. The control of extrinsic defect distribution led to considering materials dimensionality in fractional number, whereas materials dimensionality used to be defined by integral number, i.e., 0, 1, 2, 3D. Theoretical and experimental studies revealed that the geometrical control of defect distribution (La doped SrTiO3), namely geometrical doping, led to a wide span of material states from a highly symmetric charge fluid to a charge disproportionated insulating state.

Geometrical doping is added as another axis to the fundamental parameters of chemical doping, such as the amount and type of defect. The formation energy of oxygen vacancies was studied by machine learning (ML), since the oxygen vacancy is an intrinsic defect and the tendency of oxygen vacancy formation is an important concern from a material state to device performance. The formation energy of oxygen vacancies was predicted for more than 30,000 oxides available in the periodic table using an ensemble ML model, which is the last study of the defect in this journey at Sunkyunkwan University.

About the Speaker

Prof. Jaichan LEE earned his PhD in Ceramic Science and Engineering from Rutgers University, USA, in 1993. He obtained his MS in Materials Science and Engineering from KAIST, Korea, in 1985, and his BS in Metallurgical Engineering from Seoul National University, Korea, in 1983.

Prof. Lee is a Professor in the Department of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU) since 1995. From 1993 to 1994, he served as a Postdoctoral Member of Technical Staff at Bell Communications Research in the USA. Prior to his current role, he worked as a Member of Technical Staff at Samsung Advanced Institute of Technology from 1987 to 1989, and at Samsung Electro-Mechanics Co. from 1985 to 1987.

Prof. Lee’s significant contributions to the field have been recognized through various awards and honors, including the Success Award in Engineering from SKKU in 2019, and serving as President of The Korean Dielectrics Society from 2019 to 2021. He has also chaired the Ferroelectrics/Dielectrics Symposium from 2007 to 2021 and the 10th and 11th Korea-Japan Conference on Ferroelectrics from 2012 to 2016. 

For Attendees' Attention

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