QAMSS Lecture: The ABC’s of Quantitative Defect Spectroscopies

Methodologies and Applications to Wide and Ultra-Wide Bandgap Semiconductors

(Why are we still worrying about defects and interfaces in 2026? The challenges of defects and interfaces in wide and ultrawide bandgap devices)

Electronic defect characterisation methods based on the analysis of thermally and optically stimulated carrier emission from trap states have become resurgent in the past 10–15 years due to the proliferation of wide and ultrawide bandgap (WBG and UWBG) semiconductors such as GaN, AlGaN and Ga2O3. These semiconductors, whose bandgaps range from 3.4 eV to 6 eV, are being developed for applications that include high-power radio frequency (RF) transistors, high voltage power electronics and optoelectronics for the visible to ultraviolet spectrum, that are not reachable by Si electronics. For GaN, commercial applications are already ubiquitous, from brilliant displays and high-power, compact radar, to energy-efficient power converters and systems used in harsh environments. Much of these successes have to do with the identification and mitigation of electrically active defects. As GaN has matured, the more nascent UWBG semiconductors like Ga2O3 and high Al-content AlGaN semiconductors, whose larger bandgaps push projected figures of merit toward extraordinary values, are being explored. However, UWBG semiconductors present unique challenges that get in the way of their ability to reach their projected performance. For instance, the thermodynamics of epitaxial growth of these materials can lead to large concentrations of electrically active defects contributing trap states across the large bandgaps. Additionally, the large bandgaps themselves make electronic characterisation of defects very difficult since conventional methods are not able to “see” most trap states. This presentation will therefore take two directions. The first is to provide a brief tutorial of defect spectroscopy methods appropriate for WBG and UWBG semiconductor characterisation that have been developed by our group since the late 1990’s and are now found in labs and companies the world over. The second is to show a deep dive into several key problems solved by application of deep level optical spectroscopy (DLOS) to GaN devices and also to earlier stage gallium oxide materials, for uses in the high radiation space environment. 

About the speaker

Prof. Steven A. Ringel, The Ohio State University, US.

Steven A. Ringel is a Distinguished University Professor at The Ohio State University where he also holds the Neal Smith Endowed Chair in Electrical and Computer Engineering and serves as both the senior associate vice president for research at The Ohio State University (OSU) and the (founding) executive director of OSU’s Institute for Materials and Manufacturing Research. Prof. Ringel is recognised for his pioneering research contributions in space and terrestrial compound semiconductor solar cells and for the creation and application of several defect spectroscopy methods. His seminal work on developing deep level optical spectroscopy (DLOS) and sub-variants has enabled the identification and mitigation of critical atomic defects in semiconductor materials that have contributed to the world-wide implementation of GaN devices for radio frequency (RF) and power electronics, and to the rapid advancement of next-generation ultrawide bandgap semiconductor technologies. In photovoltaics, Ringel has solved long-standing materials science challenges related to heterogeneous integration of compound semiconductors with silicon, resulting in the current certified record performance III-V/Si tandem solar cells, and has significantly helped the development of high performance, radiation-tolerant multijunction space solar cells, including putting OSU solar cells on the International Space Station. Prof. Ringel has authored/co-authored > 450 technical articles and conference presentations, edited several books and contributed many book chapters. He is a Fellow of IEEE, Fellow of AAAS, Associate Fellow of AIAA, former NSF National Young Investigator, has graduated more than 30 PhD students, and with his group has received 11 best paper/presentation awards. Ringel holds or has held leadership positions at organisations in the U.S., Singapore, Ireland and India. Currently, he is also a Distinguished Visiting Professor at IIT-Bombay and a Distinguished Visiting Fellow at the Tyndall National Institute in Ireland.