Defence of doctoral thesis in the field of applied physics, M.Sc. Antti Karjalainen

Title of the doctoral thesis is "Defect identification in complex oxides: positron annihilation spectroscopy of β-Ga₂O₃ and SrTiO₃"
Molekyyliketjujen verkosto vasemmalla, oikealla tuulimylly, lentokone ja laiva

The electrification of transportation and industry increases the need to control ever larger electrical powers. The properties of electrical components can be developed to a certain extent but the properties of the material used set the final limits for the possible component properties. For this reason, industry is shifting from silicon to silicon carbide in high power electrical components. However, the research laboratories in Finland and worldwide are already aiming further.

Beta-gallium oxide (β-Ga₂O₃) is an ultrawide band gap semiconductor which has been proposed as the next step of high power electronics after silicon carbide. Beta-gallium oxide can enable the control of further higher electrical powers and more economical crystal growth methods. The development of beta-gallium oxide has reached a point where structural defects the size of no more than few individual atoms start to be one of the biggest remaining factors hindering the properties of beta-gallium oxide.

This doctoral dissertation studied two research-phase semiconductor materials: beta-gallium oxide and strontium titanate (SrTiO₃). In these materials, structural defects of atomic sizes were investigated using positron annihilation spectroscopy and the latest theoretical modelling methods. The first step was to solve the peculiar direction-dependency (anisotropy) of the positron signal in beta-gallium oxide, which had been hampering positron research worldwide. A closer inspection revealed that the signal anisotropy actually carries valuable additional information that can enable defect identification in situations where it would be impossible with the conventional approaches.

By utilising the signal anisotropy, it was discovered that the disappearance of the electrical conductivity in beta-gallium oxide originates from the formation of a specific type of gallium vacancy defects. The results also suggest that hydrogen impurities have a major role in the electrical conductivity of beta-gallium oxide. The observed connections between structural defects and the electrical properties enables their elimination in the crystal growth processes, bringing the studied materials one step closer to being mature for real life applications.

Opponent is Associate Professor Michael Scarpulla, The University of Utah, USA

Custos is Professor Peter Liljeroth, Aalto University School of Science, Department of Applied Physics

Contact details of the doctoral student: [email protected]

The public defence will be organised via Zoom. Link to the event

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The dissertation is publicly displayed 10 days before the defence in the publication archive Aaltodoc of Aalto University

Electronic thesis

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