Physikalische Chemie - Direktor: Prof. Dr. Martin Wolf
Informal Seminar
Host: Alexander Paarmann

Friday, September 30, 2022, 1:30 pm
All are invited to meet around 1:10 pm for a chat with coffee & cookies.
PC Seminar Room, G 2.06, Faradayweg 4
Mathias Schubert
University of Nebraska-Lincoln, Lincoln, USA
THz-VUV Ellipsometry and THz Electron Paramagnetic Resonance Ellipsometry Characterization of SiC and Other Wideband Gap and Ultrawideband Gap Materials
The control over electrical conductivity is critical key to enabling gallium oxide and related materials for high power electronic devices. Understanding the influence of dopants and defects onto the electrical and electronic properties is therefore of paramount importance [1]. Identifying defects and their local electronic properties remains a challenge. Here, we introduce frequency-domain Terahertz Electron Paramagnetic Resonance (EPR) ellipsometry as a new tool to study defects in gallium oxide and related materials at very high magnetic fields and very high frequencies. Traditional EPR methods exist in multiple variants and establish perhaps one of the most ubiquitous measurement techniques in science [2]. In our new concept, we determine the full polarization response of intricate defect spins as a continuous function of both field and frequency. For first investigations, we use our previously developed optical Hall effect setup [3]. We recently demonstrated this new approach analyzing the polarized spin response for the nitrogen defect in SiC [4]. Here, we investigate Fe-doped gallium oxide single crystals, and detect a large range of spin signatures which strongly vary with crystal orientation, frequency, and field. Iron is commonly used to obtain semi-insulating material where Fe2+ acts as compensating acceptor. The neutral defect Fe3+ is a high-spin system with s=5/2 and large zero-field splitting. Iron can incorporate at either Ga site but appears preferentially in octahedral configuration. Different claims exist about the nature of the spin Hamiltonian and approximate values for simplified orthorhombic models have been reported. We obtain the anisotropic g-factor as well as the zero-field Hamiltonian up to fourth order which allows to discuss the relevance of the monoclinic character of the local site symmetry. We compare our results with present knowledge from theory computation approaches. We further discuss the influence of phonons, strain, and local crystal symmetry, and we predict THz EPR ellipsometry as a new tool with potential for characterization of defects in heteroepitaxial systems.

[1] A. J. Greene et al., APL Materials 10, 029201 (2022).
[2] E. Zavoisky, Spin-magnetic resonance in paramagnetics, Fizicheskiĭ Zhurnal 9, 211 (1945).
[3] P. Kühne et al., IEEE Trans. Terahertz Sci. Technol. 8(3), 257 (2018).
[4] M. Schubert et al., Appl. Phys Lett. 120, 102101 (2022).