Browsing by Author "Kuznetsov, A. Yu."

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  • Deactivation of Li by vacancy clusters in ion-implanted and flash-annealed ZnO
    (2006) Børseth, T. Moe; Tuomisto, Filip; Christensen, J. S.; Skorupa, W.; Monakhov, E. V.; Svensson, B. G.; Kuznetsov, A. Yu.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Li is present in hydrothermally grown ZnO at high concentrations and is known to compensate both n- and p-type doping due to its amphoteric nature. However, Li can be manipulated by annealing and ion implantation in ZnO. Fast, 20 ms flash anneals in the 900–1400°C range result in vacancy cluster formation and, simultaneously, a low-resistive layer in the implanted part of the He- and Li-implanted ZnO. The vacancy clusters, involving 3-4 Zn vacancies, trap and deactivate Li, leaving other in-grown donors to determine the electrical properties. Such clusters are not present in sufficient concentrations after longer (1h) anneals because of a relatively low dissociation barrier ∼2.6 ± 0.3 eV, so ZnO remains compensated until Li diffuses out after 1250°C anneals.
  • Electronic structure of the phosphorus-vacancy complex in silicon: A resonant-bond model
    (2004) Ganchenkova, M. G.; Kuznetsov, A. Yu.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Using first-principles calculations, the electronic structure of the phosphorus-vacancy pair in silicon has been studied. Detailed analysis of the atomic displacement fields associated with the atomic structure optimization after the defect formation indicates a strong dependence of the character and magnitude of relaxation both on the supercell size and the E-center charge state. Our simulation results strongly suggest that the E-center structure is of the resonant-bond type with a strong localization of an electron pair at the phosphorus atom. The energy level splitting for shared electrons in a vacancy due to the appearance of the resonance distortion is discussed, as well as the nature of and the reasons for the level occupation.
  • Identification of substitutional Li in n-type ZnO and its role as an acceptor
    (2011) Johansen, K. M.; Zubiaga, A.; Makkonen, I.; Tuomisto, Filip; Neuvonen, P. T.; Knutsen, K. E.; Monakhov, E. V.; Kuznetsov, A. Yu.; Svensson, B. G.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Monocrystalline n-type zinc oxide (ZnO) samples prepared by different techniques and containing various amounts of lithium (Li) have been studied by positron annihilation spectroscopy (PAS) and secondary ion mass spectrometry. A distinct PAS signature of negatively charged Li atoms occupying a Zn-site (Li−Zn), so-called substitutional Li, is identified and thus enables a quantitative determination of the content of LiZn. In hydrothermally grown samples with a total Li concentration of ~2×10 exp 17 cm exp −3,LiZn is found to prevail strongly, with only minor influence, by other possible configurations of Li. Also in melt grown samples doped with Li to a total concentration as high as 1.5×10 exp 19 cm exp −3, a considerable fraction of the Li atoms (at least 20%) is shown to reside on the Zn-site, but despite the corresponding absolute acceptor concentration of ⩾(2–3)×10 exp 18 cm exp −3, the samples did not exhibit any detectable p-type conductivity. The presence of LiZn is demonstrated to account for the systematic difference in positron lifetime of 10–15 ps between Li-rich and Li-lean ZnO materials as found in the literature, but further work is needed to fully elucidate the role of residual hydrogen impurities and intrinsic open volume defects.
  • On the interplay of point defects and Cd in non-polar ZnCdO films
    (2013) Zubiaga, A.; Reurings, F.; Tuomisto, Filip; Plazaola, F.; Garcia, J. A.; Kuznetsov, A. Yu.; Egger, W.; Zuniga-Perez, J.; Munoz-Sanjose, V.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Non-polar ZnCdO films, grown over m- and r-sapphire with a Cd concentration ranging between 0.8% and 5%, have been studied by means of slow positron annihilation spectroscopy (PAS) combined with chemical depth profiling by secondary ion mass spectroscopy and Rutherford back-scattering. Vacancy clusters and Zn vacancies with concentrations up to 10exp17 cm−3 and 10exp18 cm−3, respectively, have been measured inside the films. Secondary ion mass spectroscopy results show that most Cd stays inside the ZnCdO film but the diffused atoms can penetrate up to 1.3 μm inside the ZnO buffer. PAS results give an insight to the structure of the meta-stable ZnCdO above the thermodynamical solubility limit of 2%. A correlation between the concentration of vacancy clusters and Cd has been measured. The concentration of Zn vacancies is one order of magnitude larger than in as-grown non-polar ZnO films and the vacancy cluster are, at least partly, created by the aggregation of smaller Zn vacancy related defects. The Zn vacancy related defects and the vacancy clusters accumulate around the Cd atoms as a way to release the strain induced by the substitutional CdZn in the ZnO crystal.
  • Stabilization of Ge-rich defect complexes originating from E centers in Si1- xGex:P
    (2010) Kilpeläinen, S.; Kuitunen, K.; Tuomisto, Filip; Slotte, J.; Radamson, H. H.; Kuznetsov, A. Yu.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Thermal evolution of vacancy complexes was studied in P-doped ([P]=10 exp 18 cm exp −3) proton irradiated Si1−xGex with Ge contents of 10%, 20%, and 30% in the range of 250–350 °C using positron annihilation spectroscopy. The radiation damage recovers in the course of anneals but the final state differs from that in as-grown samples indicating the presence of small Ge clusters in the samples, contrary to the initially random Ge distribution. The activation energy for the annealing process was estimated to be 1.4±0.3 eV and attributed to the dissociation energy of the vacancy-phosphorus-germanium (V-P-Ge) complex.
  • Vacancy defect and defect cluster energetics in ion-implanted ZnO
    (2010) Dong, Yufeng; Tuomisto, Filip; Svensson, B. G.; Kuznetsov, A. Yu.; Brillson, Leonard J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have used depth-resolved cathodoluminescence, positron annihilation, and surface photovoltage spectroscopies to determine the energy levels of Zn vacancies and vacancy clusters in bulk ZnO crystals. Doppler broadening-measured transformation of Zn vacancies to vacancy clusters with annealing shifts defect energies significantly lower in the ZnO band gap. Zn and corresponding O vacancy-related depth distributions provide a consistent explanation of depth-dependent resistivity and carrier-concentration changes induced by ion implantation.
  • Zinc vacancy and oxygen interstitial in ZnO revealed by sequential annealing and electron irradiation
    (2012) Knutsen, K. E.; Galeckas, A.; Zubiaga, A.; Tuomisto, Filip; Farlow, G. C.; Svensson, B. G.; Kuznetsov, A. Yu.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    By combining results from positron annihilation and photoluminescence spectroscopy with data from Hall effect measurements, the characteristic deep level emission centered at ∼1.75 eV and exhibiting an activation energy of thermal quenching of 11.5 meV is associated with the zinc vacancy. Further, a strong indication that oxygen interstitials act as a dominating acceptor is derived from the analysis of charge carrier losses induced by electron irradiation with variable energy below and above the threshold for Zn-atom displacement. We also demonstrate that the commonly observed green emission is related to an extrinsic acceptorlike impurity, which may be readily passivated by oxygen vacancies.