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Special Seminar

Friday, June 22

1:00 PM to 2:00 PM

106 Engineering Research Building

Speaker:  Professor Tuomisto Filip, Department of Applied Physics, Aalto University, Finland
"Irradiation Damage in Metals and Semiconductors Studied with Positrons"
Abstract:  Positron annihilation spectroscopy is an experimental method that is particularly suitable for the identification and quantification of vacancy defects in metals and semiconductors. In a crystalline solid, positrons can get trapped at a vacancy defect thanks to the missing positive ion core that strongly repels positrons. In addition, in semiconductors the vacancy charge state plays a role, and negatively charged non-open volume defects can also be detected. The trapping of positrons at these defects is observed as well-defined changes in the positron-electron annihilation radiation. The combination of positron lifetime and Doppler broadening techniques with state-of-the art theoretical calculations provides the means to deduce both the identities (including vacancy decoration by impurities) and the concentrations of the vacancies.
I will first give an introduction to the positron methods with illustrating examples from recent research. In the second part of my presentation I will review our latest results obtained in irradiated and/or ion implanted solids, ranging from elemental metals and model metal alloys to steels and novel semiconductor compounds. Finally, I will present some of the most recent developments in both theoretical and experimental positron methodologies.
Biography:  Professor Filip Tuomisto obtained his PhD in engineering physics from the Helsinki University of Technology, Finland, in 2005. He specializes in the development of experimental methods based on positron annihilation spectroscopy and in the application of these methods to studying defects in crystalline solids. At present he holds an Associate Professor position at the Department of Applied Physics of Aalto University, Finland, where he leads the positron physics research group of  ~10 people (post-docs, PhD students). He has published more than 120 refereed scientific journal articles that have together been cited more than 1000 times.