• Karjalainen, Antti

Abstract

In this work, point defects in beta-gallium oxide (β-Ga2O3) and strontium titanate (SrTiO3) were studied with positron annihilation spectroscopy. β-Ga2O3 is an ultrawide bandgap semiconductor whose properties have potential stepping beyond the current state-of-the-art material, SiC, in high-power applications. Understanding of SrTiO3, a simple prototype perovskite material, can aid understanding the scientifically interesting perovskite material family. This work demonstrated how positron annihilation spectroscopy can be applied to complex materials where defect-free reference material does not exist. The low symmetry of the β-Ga2O3 crystal structure gives rise to a one-dimensional delocalised positron lattice state and colossal anisotropy in the Doppler broadening signals. The Doppler signal anisotropy is larger than the difference between the β-Ga2O3 lattice signal and vacancy defects which renders the comparison of Doppler results of unspecified lattice directions essentially meaningless. The Doppler signals are especially anisotropic in peculiar type of split Ga vacancies where a neighbouring atom has relaxed towards the vacancy site and split the open volume into two. As defect-free reference β-Ga2O3 didn't exist, the use of Doppler signal anisotropy in defect identification was demonstrated. β-Ga2O3 single crystals manifest a complicated defect distribution consisting of a high concentration of multiple different types of split Ga vacancies regardless of the dopant or conductivity, while regular (non-split) Ga vacancies were not observed. Sn-doped n-type β-Ga2O3 exhibited even higher vacancy concentrations than semi-insulating material. Particle irradiation introduces regular Ga vacancies and causes strong electrical compensation. However, the interaction of Ga vacancies with hydrogen impurities may mitigate the compensation. The work built a first understanding of the complicated and unique positron signals in β-Ga2O3 enabling future work. Three different competing defects were found in SrTiO3 while no defect-free lattice signal was observed. By combining existing experimental and theoretical knowledge, the defect present in all samples in very high concentrations was identified as Ti antisite which in literature is associated with ferroelectricity. Smaller and larger Sr vacancy - O vacancy complexes were found in lower concentrations where the oxygen concentration of growth atmosphere affected the amount of adjacent O vacancies.

Topics

• perovskite
• impedance spectroscopy
• single crystal
• Oxygen
• semiconductor
• anisotropic
• Strontium
• Hydrogen
• positron annihilation lifetime spectroscopy
• one-dimensional
• Gallium
• vacancy
• point defect