Browsing by Author "Tuomisto, F."

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  • Acceptors in undoped GaSb; the role of vacancy defects
    (2013) Kujala, Jiri; Slotte, J.; Tuomisto, F.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The conventional lifetime setup was used to study Czochralski grown unintentionally p- and n- type ([n] ≈ 6 × 1017cm−3) GaSb bulk samples. Several approaches were used to analyze the data. However, it was not possible to successfully analyze the obtained spectrums with the conventional trapping model. From the analyzed data it was derived that the reason for p-type behavior of GaSb was not VGa. Additionally, the role of gallium vacancy was studied and it's effect to lifetime values are shortly discussed.
  • Annealing effect on donor-acceptor interface and its impact on the performance of organic photovoltaic devices based on PSiF-DBT copolymer and C60
    (2015) Marchiori, C.F.N.; Yamamoto, N.A.D.; Matos, C.F.; Kujala, Jiri; Macedo, A.G.; Tuomisto, F.; Zarbin, A.J.G.; Koehler, M.; Roman, L.S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this work, poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) was used as active layer in bilayer solar cell with C60 as electron acceptor. As cast devices already show reasonable power conversion efficiency (PCE) that increases to 4% upon annealing at 100 °C. Space charge limited measurements of the hole mobility (μ) in PSiF-DBT give μ ∼ 1.0 × 10−4 cm2/(V s) which does not depend on the temperature of the annealing treatment. Moreover, positron annihilation spectroscopy experiments revealed that PSiF-DBT films are well stacked even without the thermal treatment. The variations in the transport of holes upon annealing are then small. As a consequence, the PCE rise was mainly induced by the increase of the polymer surface roughness that leads to a more effective interface for exciton dissociation at the PSiF-DBT/fullerene heterojunction.
  • Calculation of positron annihilation characteristics of six main defects in 6H -SiC and the possibility to distinguish them experimentally
    (2016-07-06) Linez, F.; Makkonen, I.; Tuomisto, F.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have determined positron annihilation characteristics (lifetime and Doppler broadening) in six basic vacancy-type defects of 6H-SiC and two nitrogen-vacancy complexes using ab initio calculations. The positron characteristics obtained allow us to point out which positron technique in the most adapted to identify a particular defect. They show that the coincidence Doppler broadening technique is the most relevant for observing the silicon vacancy-nitrogen complexes, VSiNC, and carbon vacancy-carbon antisite ones, VCCSi. For the other studied defects, the calculated positron characteristics are found to be too close for the defects to be easily distinguished using a single positron annihilation technique. Then it is required to use complementary techniques, positron annihilation based or other.
  • Charge transition level of GePb1 centers at interfaces of SiO2/GexSi1-x/SiO2 heterostructures investigated by positron annihilation spectroscopy
    (2014-11) Madia, O.; Segercrantz, N.; Afanasjev, V.; Stesmans, A.; Souriau, L.; Slotte, J.; Tuomisto, F.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Clustering of vacancy defects in high-purity semi-insulating SiC
    (2007-02) Aavikko, R.; Saarinen, K.; Tuomisto, F.; Magnusson, B.; Son, N.T.; Janzen, E.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Positron lifetime spectroscopy was used to study native vacancy defects in semi-insulating silicon carbide. The material is shown to contain (i) vacancy clusters consisting of four to five missing atoms and (ii) Si-vacancy-related negatively charged defects. The total open volume bound to the clusters anticorrelates with the electrical resistivity in both as-grown and annealed materials. Our results suggest that Si-vacancy-related complexes electrically compensate the as-grown material, but migrate to increase the size of the clusters during annealing, leading to loss of resistivity.
  • Compensating vacancy defects in Sn- and Mg-doped In2O3
    (2014-12) Korhonen, E.; Tuomisto, F.; Bierwagen, O.; Speck, J.S.; Galazka, Z.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    MBE-grown Sn- and Mg-doped epitaxial In2O3 thin-film samples with varying doping concentrations have been measured using positron Doppler spectroscopy and compared to a bulk crystal reference. Samples were subjected to oxygen or vacuum annealing and the effect on vacancy type defects was studied. Results indicate that after oxygen annealing the samples are dominated by cation vacancies, the concentration of which changes with the amount of doping. In highly Sn-doped In2O3, however, these vacancies are not the main compensating acceptor. Vacuum annealing increases the size of vacancies in all samples, possibly by clustering them with oxygen vacancies.
  • Deactivation of Li by vacancy clusters in ion implanted and flash-annealed ZnO
    (2006-10) Moe Børseth, T.; Tuomisto, F.; Christensen, J.S.; Skorupa, W.; Monakhov, E.V.; Svensson, B.G.; Kuznetsov, A.Yu.
     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, 20ms 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±0.3eV, so ZnO remains compensated until Li diffuses out after 1250°C anneals.
  • Defect evolution and impurity migration in Na implanted ZnO
    (2011-11) Neuvonen, P.T.; Vines, L.; Venkatachalapathy, V.; Zubiaga, A.; Tuomisto, F.; Hallén, A.; Svensson, B.G.; Kuznetsov, A.Yu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Secondary ion mass spectrometry (SIMS) and positron annihilation spectroscopy (PAS) have been applied to study impurity migration and open volume defect evolution in Na+ implanted hydrothermally grown ZnO samples. In contrast to most other elements, the presence of Na tends to decrease the concentration of open volume defects upon annealing and for temperatures above 600∘C, Na exhibits trap-limited diffusion correlating with the concentration of Li. A dominating trap for the migrating Na atoms is most likely Li residing on Zn site, but a systematic analysis of the data suggests that zinc vacancies also play an important role in the trapping process.
  • Defect identification in semiconductors with positron annihilation: Experiment and theory
    (2013-11) Tuomisto, F.; Makkonen, I.
     Literature review
    Positron annihilation spectroscopy is particularly suitable for studying vacancy-type defects in semiconductors. Combining state-of-the-art experimental and theoretical methods allows for detailed identification of the defects and their chemical surroundings. Also charge states and defect levels in the band gap are accessible. In this review the main experimental and theoretical analysis techniques are described. The usage of these methods is illustrated through examples in technologically important elemental and compound semiconductors. Future challenges include the analysis of noncrystalline materials and of transient defect-related phenomena.
  • Direct observation of mono-vacancy and self-interstitial recovery in tungsten
    (2019-02-01) Heikinheimo, J.; Mizohata, K.; Räisänen, J.; Ahlgren, Tommy; Jalkanen, P.; Lahtinen, A.; Catarino, N.; Alves, E.; Tuomisto, F.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    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 EmV=1.85±0.05 eV.
  • Direct observations of the vacancy and its annealing in germanium
    (2011-06) Slotte, J.; Kilpeläinen, S.; Tuomisto, F.; Räisänen, J.; Nylansted Larsen, A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Weakly n-type doped germanium has been irradiated with protons up to a fluence of 3×1014 cm−2 at 35 K and 100 K in a unique experimental setup. Positron annihilation measurements show a defect lifetime component of 272±4 ps at 35 K in in situ positron lifetime measurements after irradiation at 100 K. This is identified as the positron lifetime in a germanium monovacancy. Annealing experiments in the temperature interval 35–300 K reveal two annealing stages. The first at 100 K is tentatively associated with the annealing of the Frenkel pair, the second at 200 K with the annealing of the monovacancy. Above 200 K it is observed that mobile neutral monovacancies form divacancies, with a positron lifetime of 315 ps.
  • Effect of Mn and Mg dopants on vacancy defect formation in ammonothermal GaN
    (2020-10-01) Heikkinen, T.; Pavlov, J.; Ceponis, T.; Gaubas, E.; Zając, M.; Tuomisto, F.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    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.
  • Effect of the misorientation of the 4H-SiC substrate on the open volume defects in GaN grown by metal organic chemical vapour deposition
    (2006) Tengborn, E.; Rummukainen, M.; Tuomisto, F.; Saarinen, K.; Rudzinski, M.; Hageman, P.R.; Larsen, P.K.; Nordlund, A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Positron annihilation spectroscopy has been used to study GaN grown by metal-organic chemical vapor deposition on misoriented 4H-SiC substrates. Two kinds of vacancy defects are observed: Ga vacancies and larger vacancy clusters in all the studied layers. In addition to vacancies, positrons annihilate at shallow traps that are likely to be dislocations. The results show that the vacancy concentration increases and the shallow positron trap concentration decreases with the increasing substrate misorientation.
  • Effects of alloy composition and Si-doping on vacancy defect formation in (InxGa1- x)2O3 thin films
    (2018-03-28) Prozheeva, V.; Hölldobler, R.; Von Wenckstern, H.; Grundmann, M.; Tuomisto, F.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    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.
  • Electrical compensation and cation vacancies in Al rich Si-doped AlGaN
    (2020-10-05) Prozheev, I.; Mehnke, F.; Wernicke, T.; Kneissl, M.; Tuomisto, F.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report positron annihilation results on vacancy defects in Si-doped Al0.90Ga0.10N alloys grown by metalorganic vapor phase epitaxy. By combining room temperature and temperature-dependent Doppler broadening measurements, we identify negatively charged in-grown cation vacancies in the concentration range from below 1 × 10 16 cm-3 to 2 × 10 18 cm-3 in samples with a high C content, strongly correlated with the Si doping level in the samples ranging from 1 × 10 17 cm-3 to 7 × 10 18 cm-3. On the other hand, we find predominantly neutral cation vacancies with concentrations above 5 × 10 18 cm-3 in samples with a low C content. The cation vacancies are important as compensating centers only in material with a high C content at high Si doping levels.
  • Electrical compensation by Ga vacancies in Ga2O3  thin films
    (2015) Korhonen, Esa; Tuomisto, F.; Gogova, D.; Wagner, G.; Baldini, M.; Galazka, Z.; Schewski, R.; Albrecht, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The authors have applied positron annihilation spectroscopy to study the vacancy defects in undoped and Si-doped Ga2O3 thin films. The results show that Ga vacancies are formed efficiently during metal-organic vapor phase epitaxy growth of Ga2O3 thin films. Their concentrations are high enough to fully account for the electrical compensation of Si doping. This is in clear contrast to another n-type transparent semiconducting oxide In2O3, where recent results show that n-type conductivity is not limited by cation vacancies but by other intrinsic defects such as Oi.
  • Electrical compensation via vacancy-donor complexes in arsenic-implanted and laser-annealed germanium
    (2016-10-31) Kalliovaara, T.; Slotte, J.; Makkonen, I.; Kujala, J.; Tuomisto, F.; Milazzo, R.; Impellizzeri, G.; Fortunato, G.; Napolitani, E.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Highly n-type Ge attained by shallow As implantation and excimer laser annealing was studied with positron annihilation spectroscopy and theoretical calculations. We conclude that a high concentration of vacancy-arsenic complexes was introduced by the doping method, while no sign of vacancies was seen in the un-implanted laser-annealed samples. The arsenic bound to the complexes contributes substantially to the passivation of the dopants.
  • Evaluation of the concentration of point defects in GaN
    (2017-12-01) Reshchikov, M. A.; Usikov, A.; Helava, H.; Makarov, Yu N.; Prozheeva, V.; Makkonen, I.; Tuomisto, F.; Leach, J. H.; Udwary, K.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Photoluminescence (PL) was used to estimate the concentration of point defects in GaN. The results are compared with data from positron annihilation spectroscopy (PAS), secondary ion mass spectrometry (SIMS), and deep level transient spectroscopy (DLTS). Defect-related PL intensity in undoped GaN grown by hydride vapor phase epitaxy increases linearly with the concentration of related defects only up to 1016 cm-3. At higher concentrations, the PL intensity associated with individual defects tends to saturate, and accordingly, does not directly correlate with the concentration of defects. For this reason, SIMS analysis, with relatively high detection limits, may not be helpful for classifying unidentified point defects in GaN. Additionally, we highlight challenges in correlating defects identified by PL with those by PAS and DLTS methods.
  • Evidence of the Zn vacancy acting as the dominant acceptor in n-type ZnO
    (2003-11) Tuomisto, F.; Ranki, V.; Saarinen, K.; Look, D.C.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have used positron annihilation spectroscopy to determine the nature and the concentrations of the open volume defects in as-grown and electron irradiated (Eel=2   MeV, fluence 6×1017   cm−2) ZnO samples. The Zn vacancies are identified at concentrations of [VZn]≃2×1015   cm−3 in the as-grown material and [VZn]≃2×1016   cm−3 in the irradiated ZnO. These concentrations are in very good agreement with the total acceptor density determined by temperature dependent Hall experiments. Thus, the Zn vacancies are dominant acceptors in both as-grown and irradiated ZnO.
  • Evolution of E-centers during the annealing of Sb-doped Si0.8Ge0.2
    (2011-03) Kilpeläinen, S.; Tuomisto, F.; Slotte, J.; Lundsgaard Hansen, J.; Nylandsted Larsen, A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Evolution of the chemical surroundings of vacancy complexes in Sb-doped ([Sb]=2×1018 and 2×1019 cm−3) Si0.8Ge0.2 was studied with positron annihilation spectroscopy in Doppler broadening mode. The study was performed by annealing the samples both isochronally and isothermally. Defect evolution was observed at the temperature range 450–650 K. Both treatments were shown to induce changes in the chemical surroundings of the E-centers via introduction of Ge near the defects. Moreover, Sb was found to hinder these changes by stabilizing the E-centers and thus preventing them from finding Ge. The stable state reached after the anneals was found to differ from that measured from an as-grown sample. This difference was deemed to be the result of Ge gathering in small clusters during the annealing thus breaking the initially random Ge distribution.