Browsing by Author "Tuomisto, Filip"
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Item Amphoteric Be in GaN: Experimental Evidence for Switching between Substitutional and Interstitial Lattice Sites(2017-11-09) Tuomisto, Filip; Prozheeva, Vera; Makkonen, Ilja; Myers, Thomas H.; Bockowski, Michal; Teisseyre, Henryk; Department of Applied Physics; Antimatter and Nuclear Engineering; Department of Applied Physics; Texas State University; Polish Academy of Sciences; Institute of Physics of the Polish Academy of SciencesWe show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of BeGa in the positron annihilation signals in Be-doped GaN, while the emergence of VGa at high temperatures is a consequence of the Be impurities being driven to interstitial positions. The similarity of this behavior to that found for Na and Li in ZnO suggests that this could be a universal property of light dopants substituting for heavy cations in compound semiconductors.Item Automated Process Modelling in Safety-Critical Systems(2020-01-21) Rimppi, Alpi; Karhela, Tommi; Villberg, Antti; Perustieteiden korkeakoulu; Tuomisto, FilipProcess industry utilises various simulation tools to test and validate system performance and functionalities. Throughout testing is especially crucial for safety-critical systems, such as nuclear power plants. Nuclear energy production is heavily regulated and extensive safety analysis is requires. However, modelling complex processes by hand is a laborous task that introduces human errors. By automating the modelling workflow model quality is increased and modelling workload reduced. Functional requirements for an automated modelling tool were identified by a literature review on nuclear engineering and process simulation. Based on the requirements an automated model generation tool for Apros simulation platform was implemented. The tool uses piping and instrumentation diagrams and equipment parameter spreadsheets to construct process simulation models based on predefined transformation rules. Models generated using the tool were successfully imported into Apros. The results show that automation solutions can increase modelling consistency and ease the modelling workflow. Further development is required to expand source material support and add tools for change management.Item Calculation of positron annihilation characteristics in InGaN alloys(2013) Norrman, Vesa; Makkonen, Ilja; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of Science; Tuomisto, FilipPositron annihilation spectroscopy (PAS) is a way to get information about electronic and atomic structures of materials. By measuring lifetimes and Doppler broadening spectra of annihilating positrons it is possible to detect vacancies and characterize their chemical environments. By varying the concentration of In and Ga the band gap of InGaN alloys can be tuned to cover the whole visible spectrum of light. Hence this materials family is interesting for optoelectronic devices, such as light emitting diodes (LEDs). The quality of devices is affected by the existence of defects. In this work the electronic and atomic structure of InGaN, vacancies in InGaN, and polar InN/GaN superlattices were calculated with the local density approximation (LDA) of the density functional theyry (DFT). The wave function of the positron and PAS characteristics such as lifetime and Doppler broadening spectra were calculated for those systems. I investigated how the indium mole fraction, chemical environment of vacancies, and thickness of the InN layer affect positron annihilation characteristics. The InN/GaN superlattice was modeled by stacking 1 - 4 wurtzite unit cells of InN in the direction of the hexagonal c-axis. Because of periodic boundary conditions the layer is surrounded by GaN on both sides. There is no inversion symmetry in c-direction of InN and GaN which causes polarization in the superlattice. In calculations the positron is attracted by the InN/GaN interface. Electronic structures of superlattices were also studied with hybrid functionals which are computationally more demanding but give more accurate results.Item A case study on pair production of supersymmetric partners of electrons and muons in photon-photon interactions(2018-08-20) Mantere, Oskari; Österberg, Kenneth; Perustieteiden korkeakoulu; Tuomisto, FilipItem Cesium-päästön vähentämismahdollisuuksia ydinvoimalaitoksen vakavassa reaktorionnettomuudessa(2018-08-20) Mörsky, Sari; Harti, Mika; Insinööritieteiden korkeakoulu; Tuomisto, FilipItem CFENSS-SRS method for the uncertainty analysis of nuclear fuel and neutronics(2016-06-14) Taavitsainen, Aapo; Vanhanen, Risto; Perustieteiden korkeakoulu; Tuomisto, FilipStatistical uncertainty analysis has received much attention in the past decade. The impacts of both nuclear fuel and nuclear data uncertainties have been studied separately but not as a coupled system. The main research question of this Thesis was to confirm whether the uncertainties of fuel behaviour parameters and the nuclear data can be propagated separately. The secondary goals included comparing various statistical perturbation methods. The computations were performed close to the framework of the OECD/NEA UAM-LWR TMI-1 Pressurized Water Reactor benchmark, and more specifically, its pin cell exercise. A novel CFENSS-SRS (Coupled Fuel Behaviour and Neutronics Stochastic Sampling with Simple Random Sampling) method is presented for the combined uncertainty analysis of nuclear fuel behaviour and neutronics. The method applies the statistical uncertainty analysis to univariate nuclear fuel parameters and correlated neutron cross sections. Truncated normal distribution is used as the objective distribution for drawing samples based on the Principle of Maximum Entropy. Due to practical difficulties in employing the distribution, the distribution parameters are approximated for the nuclear fuel parameters while a normal distribution is applied for the neutron cross sections. Negative values of inherently positive parameters are re-sampled to avoid distorting the distribution to a great extent. The results support the hypothesis that the nuclear fuel parameters and the nuclear data can truly be treated as independent sources of uncertainty. Additionally, it was revealed that the details of the perturbation methodology, such as using relative covariance matrices rather than absolute ones, have a much smaller impact on the output uncertainty than neglecting some of the uncertainty data.Item Changing vacancy balance in ZnO by tuning synthesis between zinc/oxygen lean conditions(2010) Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas; Zubiaga, Asier; Tuomisto, Filip; Kuznetsov, Andrej Yu; Department of Applied PhysicsThe nature of intrinsic defects in ZnO films grown by metal organic vapor phase epitaxy was studied by positron annihilation and photoluminescence spectroscopy techniques. The supply of Zn and O during the film synthesis was varied by applying different growth temperatures (325–485 °C), affecting decomposition of the metal organic precursors. The microscopic identification of vacancy complexes was derived from a systematic variation in the defect balance in accordance with Zn/O supply trends.Item Changing vacancy balance in ZnO by tuning synthesis between zinc/oxygen lean conditions(AIP Publishing, 2010) Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas; Zubiaga, Asier; Tuomisto, Filip; Kuznetsov, Andrej Yu.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceThe nature of intrinsic defects in ZnO films grown by metal organic vapor phase epitaxy was studied by positron annihilation and photoluminescence spectroscopy techniques. The supply of Zn and O during the film synthesis was varied by applying different growth temperatures (325–485 °C), affecting decomposition of the metal organic precursors. The microscopic identification of vacancy complexes was derived from a systematic variation in the defect balance in accordance with Zn/O supply trends.Item Characterization of homoepitaxial gallium nitride grown by hydride vapor phase epitaxy(2002) Tuomisto, Filip; Saarinen, Kimmo; Teknillisen fysiikan ja matematiikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Saarinen, KimmoItem Clustering of vacancy defects in high-purity semi-insulating SiC(American Physical Society (APS), 2007) Aavikko, R.; Saarinen, K.; Tuomisto, Filip; Magnusson, B.; Son, N. T.; Janzen, E.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of SciencePositron 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.Item Compensating point defects in 4He+ -irradiated InN(American Physical Society (APS), 2007) Tuomisto, Filip; Pelli, A.; Yu, K. M.; Walukiewicz, W.; Schaff, W. J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe use positron annihilation spectroscopy to study 2 MeV 4He+ -irradiated InN grown by molecular-beam epitaxy and GaN grown by metal-organic chemical-vapor deposition. In GaN, the Ga vacancies act as important compensating centers in the irradiated material, introduced at a rate of 3600 cm exp −1. The In vacancies are introduced at a significantly lower rate of 100cm−1, making them negligible in the compensation of the irradiation-induced additional n-type conductivity in InN. On the other hand, negative non-open volume defects are introduced at a rate higher than 2000cm exp −1. These defects are tentatively attributed to interstitial nitrogen and may ultimately limit the free-electron concentration at high irradiation fluences.Item Compensating point defects in 4He+- irradiated InN(2007) Tuomisto, Filip; Pelli, Antti; Yu, Kin Man; Walukiewicz, W.; Schaff, W.J.; Department of Applied PhysicsWe use positron annihilation spectroscopy to study 2MeV 4He+-irradiated InN grown by molecular-beam epitaxy and GaN grown by metal-organic chemical-vapor deposition. In GaN, the Ga vacancies act as important compensating centers in the irradiated material, introduced at a rate of 3600cm−1. The In vacancies are introduced at a significantly lower rate of 100cm−1, making them negligible in the compensation of the irradiation-induced additional n-type conductivity in InN. On the other hand, negative non-open volume defects are introduced at a rate higher than 2000cm−1. These defects are tentatively attributed to interstitial nitrogen and may ultimately limit the free-electron concentration at high irradiation fluences.Item Compensating vacancy defects in Sn- and Mg-doped In 2O3(American Physical Society (APS), 2014) Korhonen, E.; Tuomisto, Filip; Bierwagen, O.; Speck, J. S.; Galazka, Z.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceMBE-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.Item Correlation between Zn vacancies and photoluminescence emission in ZnO films(AIP Publishing, 2006) Zubiaga, A.; Garcia, J. A.; Plazaola, F.; Tuomisto, Filip; Saarinen, K.; Zuniga Perez, J.; Munoz-Sanjose, V.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of SciencePhotoluminescence and positron annihilation spectroscopy have been used to characterize and identify vacancy-type defects produced in ZnO films grown on sapphire by metal-organic chemical-vapor deposition. The photoluminescence of the samples in the near band edge region has been studied, paying particular attention to the emission at 370.5nm (3.346eV). This emission has been correlated to the concentration of Zn vacancies in the films, which has been determined by positron annihilation spectroscopy.Item Coupling Serpent and OpenFOAM for neutronics - CFD multi-physics calculations(2015-08-25) Tuominen, Riku; Valtavirta, Ville; Perustieteiden korkeakoulu; Tuomisto, FilipThe main goal of this work was to couple the Monte Carlo neutronics code Serpent 2 with a CFD solver from the OpenFOAM toolbox. The coupling was implemented with the already available multi-physics interface of Serpent. The interface allows the passing of high fidelity density and temperature distributions from an external solver to Serpent and also the passing of fission power distribution from Serpent to the external solver. The coupled CFD-neutronics problem was solved by iteration. At each iteration Serpent solves a new power distribution based on the current temperature and density distribution. The power distribution is passed to the CFD solver to solve the corresponding temperature and density distributions which are passed back to Serpent to start a new iteration. The coupling was tested bymodelling amock-up 5x5 fuel assembly cooled with water in a steady state condition at full power. The effects of boiling were neglected and the flow was modelled as single phase. The main results of the coupled calculation were high fidelity temperature and density distributions. The effect of distribution fidelity on neutronics was studied by running Serpent simulations with varying level of axial refinement. Most obvious differences were observed in the axial power density and in the collision density in the coolant, with minor differences in e.g. the effective multiplication factor and radial capture density in the fuel. In addition performance tests related to the multiphysics interface were run and the convergence of the coupled calculation is briefly discussed. The work of this Master’s thesis can be expanded in the future for example by including the effects of boiling in the CFD solution.Item Data analysis and development of the control system for a pulsed positron beam(2009) Korhonen, Esa; Tuomisto, Filip; Reurings, Floris; Teknillisen fysiikan laitos; Teknillinen korkeakoulu; Helsinki University of Technology; Puska, MarttiIn this work a program for analyzing the spectra measured with the pulsed positron beam at the Department of Applied Physics, Helsinki University of Technology was developed. The device forms accurately timed positron pulses by modulating a monoenergic positron beam with time-varying electric fields. The pulsed positron beam can be used to measure positron lifetime in surface layers and thin films as well as probing the depth profiles of defects. Because of the pulsing the resolution function of the device is more complicated than that of an ordinary lifetime spectrometer, which makes analysis of the measurement data difficult. In addition, effects of diffusion on the results are increased when measuring lifetime in thin layers. Because of these reasons the usual method of fitting lifetime components to the measured spectrum is insufficient. The new program is based on solving the time-dependent diffusion equation, using the calculated positron density profile for calculating the lifetime spectrum and fitting the results to the measured data. The calculation takes into account diffusion, trapping, detrapping, annihilation and surface transition. The analysis software was implemented in Matlab. It uses the non-linear curve fitting routine in the Curve Fitting Toolbox. The user can select which parameters should be fitted, set limits to them and supply guessed values. The user can also select areas in the measured spectrum that will be discarded in the fitting calculation. The algorithm repeatedly calculates the lifetime spectrum with different coefficient values, iterating towards the measured spectrum. Because of the complexity of the model only one or two coefficients can he successfully solved. Since the positron density is dependant on both time and space and both have low step sizes the fitting procedure requires a high amount of computation and memory. The diffusion calculation was tested by calculating the positron density profile in silicon with varying implantation energies and observing its time development. The results indicate the program is working in this respect. During the course of this work other programs used in conjunction with the lifetime beam were modified as well. Features were added and several bugs were corrected.Item Data-driven measurement of the background with misidentified tau leptons in a search for charged Higgs bosons(2015-03-31) Pekkarinen, Erik; Lehti, Sami; Perustieteiden korkeakoulu; Tuomisto, FilipIn 2012, a neutral Higgs boson with a mass of 125 GeV was discovered by the CMS and ATLAS experiments at the CERN LHC. Models with an extended Higgs sector involving charged Higgs bosons are not excluded by the discovery. A search procedure for the charged Higgs bosons (Hpm) focusing on the Hpm -> taupm nu decay channel in the fully hadronic final state is presented. The covered mass ranges are 80-160 GeV, when the charged Higgs boson is lighter than the top quark and 180-600 GeV, when the charged Higgs boson is heavier than the top quark. The presented search is based on the data recorded by the CMS experiment in 2012 at the center-of-mass energy of 8 TeV. As charged Higgs bosons are not observed in the studied data, the goal of the analysis is the setting of 95 % model-independent upper limits for their production and decay. To observe a potential signal, it has to be extracted from the backgrounds. Of the three main backgrounds, the QCD multijet events along with the ttbar and electroweak events with genuine tau leptons are measured from data. The background consisting of ttbar and electroweak events with misidentified tau leptons has before this work been estimated from simulation. A data-driven measurement method for this background is studied. Since the QCD multijet background also lacks genuine tau leptons, the composition of these backgrounds is referred to as the background with misidentified tau leptons in this work and a method measuring this background inclusively is developed. The results given by the measurement method are in good agreement with simulated estimates and the uncertainties in the backgrounds yields are reduced by the developed method. The background purity obtained with the method is also found to increase when compared to that of the QCD multijet background measurement method. This is important especially for the LHC Run II, which requires using higher selection thresholds.Item Datankäsittelyohjelma 2D-positroni-Doppler-spektroskopialaitteistolle(2015-08-31) Kynkäänniemi, Tuomas; Korhonen, Esa; Perustieteiden korkeakoulu; Tuomisto, FilipItem Deactivation of Li by vacancy clusters in ion-implanted and flash-annealed ZnO(American Physical Society (APS), 2006) Børseth, T. Moe; Tuomisto, Filip; Christensen, J. S.; Skorupa, W.; Monakhov, E. V.; Svensson, B. G.; Kuznetsov, A. Yu.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceLi 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.Item Defect distribution in a-plane GaN on Al2O3(AIP Publishing, 2007) Tuomisto, Filip; Paskova, T.; Kröger, R.; Figge, S.; Hommel, D.; Monemar, B.; Kersting, R.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceThe authors studied the structural and point defect distributions of hydride vapor phase epitaxial GaN film grown in the [11−20] a direction on (1−102) r-plane sapphire with metal-organic vapor phase deposited a-GaN template using transmission electron microscopy, secondary ion mass spectrometry, and positron annihilation spectroscopy. Grown-in extended and point defects show constant behavior as a function of thickness, contrary to the strong nonuniform defect distribution observed in GaN grown along the [0001] direction. The observed differences are explained by orientation-dependent and kinetics related defect incorporation.
