Academic lectures：Defect identification in semiconductors and their alloys with positron annihilation spectroscopy
Speaker： Filip Tuomisto, Head of Helsinki Accelerator Laboratory, Department of Physics, University of Helsinki
Invitor： Future Photovoltaic Research Center
Time： 2025.09.19 15:00-16:00
Location： Room 200, East Wing, Yue-Kong Pao Library's Annex
Abstract：
Direct experimental characterization of defects in semiconductors is a challenging task. Simultaneous determination of their identity (atomic structure), quantity (density, distribution), as well as electrical, optical, and mechanical characteristics is very rarely possible. Most experimental methods for defect characterization focus on one of the various aspects: atomic structure, electrical levels in the bandgap, or optical characteristics. Vacancy defects are particularly challenging as many of the structure-sensitive methods cannot detect empty space.
Positrons provide a selective sensitive probe for vacancy-type defects in semiconductors [1, 2]. The trapping and annihilation process of positrons does not depend on the conductivity or the bandgap of the semiconductor. Hence, from the point of view of the experiment, there is no difference between narrow gap semiconductors, ultra-wide gap semiconductors, metals, and insulators. Optical properties do not affect the experiments either. The elemental sensitivity of the positron annihilation signals is very high for atoms directly neighboring the vacancy, and it it extends at a measurable level to the next-nearest-neighbors. This makes positron annihilation techniques particularly useful for analyzing vacancy-impurity interactions in elemental semiconductors, vacancy defects on various sublattices in compound semiconductors, and also the complex phenomena associated with vacancy defects in both elemental and compound semiconductor alloys. Extracting the highest level of detail requires careful design of experiments and performing state-of-the-art theoretical calculations of the expected positron electron annihilation signals.
In this talk, I will give a brief introduction to the experimental and computational methods employed in defect characterization with positron annihilation spectroscopy. This will be followed by selected examples in elemental and compound semiconductors and their alloys.
References:
[1] F. Tuomisto and I. Makkonen, Rev. Mod. Phys. 85, 1583 (2013).
[2] I. Makkonen and F. Tuomisto, J. Appl. Phys. 135, 040901 (2024).
Biography ：
Prof. Filip Tuomisto studied engineering physics and mathematics at the Helsinki University of Technology (Finland), earning his doctoral degree in 2005. He was professor of nuclear engineering at Aalto University (Finland) 2012-2019, and since 2019 he is professor of experimental materials physics and head of the Helsinki Accelerator Laboratory at University of Helsinki. His research focuses on the physics of point defects in semiconductors and metals, and on the development and applications of positron annihilation spectroscopy. He has published more than 250 papers on these topics.
