Defect analysis of Germanium and TiO₂ by means of Positron annihilation spectroscopy

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School of Science | Doctoral thesis (article-based)
Degree programme
66 + app. 36
Aalto University publication series DOCTORAL THESES, 16/2023
To date, integrated circuits (IC) are fabricated with billions of nano-transistors, e.g., with FINFETs and Gate all around (GAAs) to make electronic devices faster, smaller, and more powerful. A high electrical conductivity, i.e., an activated high charge carrier concentration is a must to fulfill the criteria of the current technology. In n-type semiconductors, a very high donor concentration (~1020 cm−3) is required to attain a high charge carrier concentration. With increased dopant concentrations, which go beyond solid solubility limits in modern transistors, defects like vacancies form abundantly during the fabrication and doping process of the materials. In the presence of n-type donors in group IV semiconductors, negatively charged vacancies form pairs with the positively charged donor atoms. These defects act as compensators for donors by introducing new energy levels in the band gap. Positron annihilation spectroscopy (PAS) is a non-destructive method to characterize vacancy-like defects. In this thesis, Doppler broadening spectroscopy (DOBS) and coincidence Doppler broadening spectroscopy (CDOBS) modes of PAS have been applied to study vacancy defects in ultra-thin and thin films. DOBS is efficient in identifying vacancy-like defects, in determining charge states of the defects and defect-complexes, and evaluating the concentrations of the defects. CDOBS is an efficient mode of PAS to identify the chemical surroundings of vacancies which complements the DOBS results. We studied vacancy complexes and their impacts on the electrical passivation of n-type dopants in epitaxially grown thin-film Ge and GeSn with PAS. In P-doped epitaxially grown Ge, we observed that, the defects hindering a high enough donor concentration are monovacancy-donor complexes. To improve charge carrier concentration, we experimented with Sn mixed Ge doped with either As or P. In highly As-doped GeSn layers, we found vacancies surrounded by four As atoms are the reason behind the massive donor passivation. In P-doped Sn mixed Ge layers, the failure of Sn in hindering the formation of Vacancy-P complexes was observed. We also studied vacancies in atomic layer deposition (ALD) grown ultra-thin anatase TiO₂ layers applying PAS. Differences in positron trapping states were found. A detailed investigation showed that these differences did not result from the intermixing of the layers, but rather resulted from the differences in growth temperatures.
Supervising professor
Liljeroth, Peter, Prof., Aalto University, Department of Applied Physics, Finland
Thesis advisor
Slotte, Jonatan, Dr., Aalto University, Finland
positron annihilation spectroscopy, defect, vacancy, semiconductor, Ge, TiO₂, epitaxy, ALD
Other note
  • [Publication 1]: A. Vohra, A. Khanam, J. Slotte, I. Makkonen, G. Pourtois, R. Loo, and W. Vandervorst. Evolution of phosphorus-vacancy clusters in epitaxial germanium. Journal of Applied Physics, 125, 025701, January 2019. Full text in Acris/Aaaltodoc:
    DOI: 10.1063/1.5054996 View at publisher
  • [Publication 2]: A. Khanam, A. Vohra, J. Slotte, I. Makkonen, R. Loo, G. Pourtois, and W. Vandervorst. A demonstration of donor passivation through direct formation of V-Asi complexes in As-doped Ge1−xSnx. Journal of Applied Physics, 127, 195703, May 2020.
    Full text in Acris/Aaltodoc:
    DOI: 10.1063/5.0003999 View at publisher
  • [Publication 3]: A. Vohra, A. Khanam, J. Slotte, I. Makkonen, G. Pourtois, C. Porret, R. Loo, and W. Vandervorst. Heavily phosphorus doped germanium: Strong interaction of phosphorus with vacancies and impact of tin alloying on doping activation. Journal of Applied Physics, 125, 225703, June 2019.
    Full text in Acris/Aaltodoc:
    DOI: 10.1063/1.5107503 View at publisher
  • [Publication 4]: A. Khanam, J. Slotte, F. Tuomisto, S. Subheccha, M. Popovici, and G. S. Kar. Open volume defects in ultra-thin TiO2 layers embedded in VMCO-like samples studied with positron annihilation spectroscopy. Journal of Applied Physics, 131, 245301, June 2022.
    Full text in Acris/Aaltodoc:
    DOI: 10.1063/5.0094558 View at publisher