Browsing by Department "Antimatter and Nuclear Engineering"
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- Effect of Mn and Mg dopants on vacancy defect formation in ammonothermal GaN
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-10-01) Heikkinen, T.; Pavlov, J.; Ceponis, T.; Gaubas, E.; Zając, M.; Tuomisto, F.We have applied positron annihilation spectroscopy to study the formation of Ga vacancy related defects in Mg and Mn doped bulk GaN crystals grown by the ammonothermal method. We show that Mn doping has little or no effect on the formation of Ga vacancies, while Mg doping strongly suppresses their formation, in spite of both dopants leading to highly resistive material. We suggest the differences are primarily due to the hydrogen-dopant interactions. Further investigations are called for to draw a detailed picture of the atomic scale phenomena in the synthesis of ammonothermal GaN. - Effects of alloy composition and Si-doping on vacancy defect formation in (InxGa1- x)2O3 thin films
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-03-28) Prozheeva, V.; Hölldobler, R.; Von Wenckstern, H.; Grundmann, M.; Tuomisto, F.Various nominally undoped and Si-doped (InxGa1- x)2O3 thin films were grown by pulsed laser deposition in a continuous composition spread mode on c-plane α-sapphire and (100)-oriented MgO substrates. Positron annihilation spectroscopy in the Doppler broadening mode was used as the primary characterisation technique in order to investigate the effect of alloy composition and dopant atoms on the formation of vacancy-type defects. In the undoped samples, we observe a Ga2O3-like trend for low indium concentrations changing to In2O3-like behaviour along with the increase in the indium fraction. Increasing indium concentration is found to suppress defect formation in the undoped samples at [In] > 70 at. %. Si doping leads to positron saturation trapping in VIn-like defects, suggesting a vacancy concentration of at least mid-1018 cm-3 independent of the indium content. - Influence of Fermi level position on vacancy-assisted diffusion of aluminum in zinc oxide
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-12-28) Sky, T. N.; Johansen, K. M.; Venkatachalapathy, V.; Svensson, B. G.; Vines, L.; Tuomisto, F.The influence of Fermi level position and annealing ambient on the zinc vacancy VZn generation and Al diffusion is studied in monocrystalline zinc oxide (ZnO). From secondary-ion mass spectrometry and positron annihilation spectroscopy results, a quadratic dependence between the concentrations of VZn and Al is established, demonstrating the Fermi level dependence of the formation of the electrically compensating -2 charge state of VZn in conductive n-type ZnO crystals. In contrast, thermal treatment in the zinc-rich ambient is shown to efficiently reduce the VZn concentration and related complexes. Using a reaction-diffusion model, the diffusion characteristics of Al at different donor background concentrations are fully accounted for by mobile (AlZnVZn)-pairs. These pairs form via the migration and reaction of isolated VZn2- with the essentially immobile AlZn+. We obtain a migration barrier for the (AlZnVZn)- pair of 2.4±0.2 eV, in good agreement with theoretical predictions. In addition to strongly alter the shape of the Al diffusion profiles, increasing the donor background concentration also results in an enhanced effective Al diffusivity, attributed to a reduction in the VZn2-formation energy as the Fermi level position increases. - TiSrantisite
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-06-28) Karjalainen, Antti; Prozheeva, Vera; Makkonen, Ilja; Guguschev, Christo; Markurt, Toni; Bickermann, Matthias; Tuomisto, FilipWe present a systematic study of the positron lifetime as a function of measurement temperature in strontium titanate (SrTiO3) single crystals grown in different conditions and by different synthesis methods. We combine our experimental results with state-of-the-art theoretical calculations of positron annihilation parameters. We find that the essentially omnipresent 180-190 ps lifetime component is most likely the Ti Sr antisite defect, possibly coupled with one or more oxygen vacancies, supporting the importance of the Ti Sr antisite related defects in SrTiO3.