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  • Heikinheimo, J.; Mizohata, K.; Räisänen, J.; Ahlgren, T.; Jalkanen, P.; Lahtinen, A.; Catarino, N.; Alves, E.; Tuomisto, Filip (2019)
    Reliable and accurate knowledge of the physical properties of elementary point defects is crucial for predictive modeling of the evolution of radiation damage in materials employed in harsh conditions. We have applied positron annihilation spectroscopy to directly detect mono-vacancy defects created in tungsten through particle irradiation at cryogenic temperatures, as well as their recovery kinetics. We find that efficient self-healing of the primary damage takes place through Frenkel pair recombination already at 35 K, in line with an upper bound of 0.1 eV for the migration barrier of self-interstitials. Further self-interstitial migration is observed above 50 K with activation energies in the range of 0.12-0.42 eV through the release of the self-interstitial atoms from impurities and structural defects and following recombination with mono-vacancies. Mono-vacancy migration is activated at around 550 K with a migration barrier of E-m(V) = 1.85 +/- 0.05 eV. (C) 2019 Author(s).
  • Lu, Eryang; Makkonen, Ilja; Mizohata, Kenichiro; Li, Zhiming; Räisänen, Jyrki; Tuomisto, Filip (2020)
    Owing to their excellent radiation tolerance, some of the high-entropy alloys (HEAs) are considered as potential candidates for structural materials in extreme conditions. In order to shed light on the early-stage irradiation damage in HEAs, we performed positron annihilation spectroscopy on hydrogen implanted equiatomic FeMnNiCoCr and interstitial carbon-containing FeMnNiCoCr HEAs. We reveal primary damage as monovacancies in low dose irradiated HEAs. The enhancement of Frenkel pair recombination by C addition is observed in C-containing HEAs. In addition, the C interstitials suppress the vacancy cluster formation in high dose irradiated HEAs.
  • Lu, Eryang; Zhao, Junlei; Makkonen, Ilja; Mizohata, Kenichiro; Li, Zhiming; Hua, Mengyuan; Djurabekova, Flyura; Tuomisto, Filip (2021)
    We present evidence of homogenization of atomic diffusion properties caused by C and N interstitials in an equiatomic single-phase high entropy alloy (FeMnNiCoCr). This phenomenon is manifested by an unexpected interstitial-induced reduction and narrowing of the directly experimentally determined migration barrier distribution of mono-vacancy defects introduced by particle irradiation. Our observation by positron annihilation spectroscopy is explained by state-of-the-art theoretical calculations that predict preferential localization of C/N interstitials in regions rich in Mn and Cr, leading to a narrowing and reduction of the mono-vacancy size distribution in the random alloy. This phenomenon is likely to have a significant impact on the mechanical behavior under irradiation, as the local variations in elemental motion have a profound effect on the solute strengthening in high entropy alloys. (C) 2021 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.
  • Prozheeva, Vera; Makkonen, Ilja; Li, Haoran; Keller, Stacia; Mishra, Umesh K.; Tuomisto, Filip (2020)
    We show that N-polar GaN/(Al, Ga)N/GaN heterostructures exhibit significant N deficiency at the bottom (Al, Ga)N/GaN interface, and that these N vacancies are responsible for the trapping of holes observed in unoptimized N-polar GaN/(Al, Ga)N/GaN high electron mobility transistors. We arrive at this conclusion by performing positron annihilation experiments on GaN/(Al, Ga)N/GaN heterostructures of both N and Ga polarity, as well as state-of-the-art theoretical calculations of the positron states and positron-electron annihilation signals. We suggest that the occurrence of high interfacial N vacancy concentrations is a universal property of nitride semiconductor heterostructures at net negative polarization interfaces.
  • Vohra, Anurag; Makkonen, Ilja; Pourtois, Geoffrey; Slotte, Jonatan; Porret, Clement; Rosseel, Erik; Khanam, Afrina; Tirrito, Matteo; Douhard, Bastien; Loo, Roger; Vandervorst, Wilfried (2020)
    This paper benchmarks various epitaxial growth schemes based on n-type group-IV materials as viable source/drain candidates for Ge nMOS devices. Si:P grown at low temperature on Ge, gives an active carrier concentration as high as 3.5 x 10(20) cm(-3) and a contact resistivity down to 7.5 x 10(-9) However, Si:P growth is highly defective due to large lattice mismatch between Si and Ge. Within the material stacks assessed, one option for Ge nMOS source/drain stressors would be to stack Si:P, deposited at contact level, on top of a selectively grown n-SiyGe1-x-ySnx at source/drain level, in line with the concept of Si passivation of n-Ge surfaces to achieve low contact resistivities as reported in literature (Martens et al. 2011 Appl. Phys. Lett., 98, 013 504). The saturation in active carrier concentration with increasing P (or As)-doping is the major bottleneck in achieving low contact resistivities for as-grown Ge or SiyGe1-x-ySnx. We focus on understanding various dopant deactivation mechanisms in P-doped Ge and Ge1-xSnx alloys. First principles simulation results suggest that P deactivation in Ge and Ge1-xSnx can be explained both by P-clustering and donor-vacancy complexes. Positron annihilation spectroscopy analysis, suggests that dopant deactivation in P-doped Ge and Ge1-xSnx is primarily due to the formation of P-n-V and SnmPn-V clusters. (C) 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited.
  • Bukonte, Laura; Ahlgren, Tommy; Heinola, Kalle (2017)
    Hydrogen induced vacancy formation in metals and metal alloys has been of great interest during the past couple of decades. The main reason for this phenomenon, often referred to as the superabundant vacancy formation, is the lowering of vacancy formation energy due to the trapping of hydrogen. By means of thermodynamics, we study the equilibrium vacancy formation in fcc metals (Pd, Ni, Co, and Fe) in correlation with the H amounts. The results of this study are compared and found to be in good agreement with experiments. For the accurate description of the total energy of the metal-hydrogen system, we take into account the binding energies of each trapped impurity, the vibrational entropy of defects, and the thermodynamics of divacancy formation. We demonstrate the effect of vacancy formation energy, the hydrogen binding, and the divacancy binding energy on the total equilibrium vacancy concentration. We show that the divacancy fraction gives the major contribution to the total vacancy fraction at high H fractions and cannot be neglected when studying superabundant vacancies. Our results lead to a novel conclusion that at high hydrogen fractions, superabundant vacancy formation takes place regardless of the binding energy between vacancies and hydrogen. We also propose the reason of superabundant vacancy formation mainly in the fcc phase. The equations obtained within this work can be used for any metal-impurity system, if the impurity occupies an interstitial site in the lattice. Published by AIP Publishing.