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  • 151.
    Goss, J.P.
    et al.
    University of Exeter.
    Coomer, B.J.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Fall, C.J.
    University of Exeter.
    Latham, Chris
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Small aggregates of interstitials and models for platelets in diamond2000Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 12, nr 49, s. 10257-10261Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    By examining the structure of small clusters of self-interstitials in diamond using local-density-functional techniques, we have developed models for the planar defects called platelets. We present the structures, energies and vibrational properties

  • 152.
    Goss, J.P.
    et al.
    School of Physics, University of Exeter.
    Coomer, B.J.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Shaw, T.D.
    Department of Physics, University of Newcastle.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Rayson, Mark
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Self-interstitial aggregation in diamond2001Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 63, nr 19, s. 195208-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    First-principles methods are used to investigate the self-interstitial and its aggregates in diamond. The experimental assignment of the spin-1 R2 EPR center to the single interstitial has been questioned because of the small fine-structure term observed. We calculate the spin-spin interaction tensor for the three interstitial defects I1〈001〉, I2NN, and I3 and compare with the experimental D tensors. The results give support for the assignments of the single and di-interstitials to microscopic models and allow us to conclusively identify a recently observed EPR center, O3, with I3. This identification, in turn, suggests a low-energy structure for I4 and a generic model for an extended defect called the platelet. We also determine the optical properties of I1〈001〉 as well as its piezospectroscopic or stress tensor and find these to be in agreement with experiment. Several multi-interstitial defects are found to possess different structural forms which may coexist. We propose that a different form of the charged I2 defect gives rise to the 3H optical peak. Several structures of the platelet are considered, and we find that the lowest-energy model is consistent with microscopic and infrared studies.

  • 153.
    Goss, J.P.
    et al.
    School of Physics, University of Exeter.
    Coomer, B.J.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Shaw, T.D.
    Department of Physics, University of Newcastle.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Interstitial aggregates in diamond2001Inngår i: Diamond and related materials, ISSN 0925-9635, E-ISSN 1879-0062, Vol. 10, nr 3, s. 434-438Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Theoretical modelling of magnetic resonance signals lead to convincing models for the first three self-interstitial aggregates in diamond. These in turn suggest the manner in which larger more stable aggregates including the platelet, observed in annealed type I diamonds, are formed.

  • 154.
    Goss, J.P.
    et al.
    School of Physics, University of Exeter.
    Eberlein, T.A.G.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Pinho, N.
    School of Physics, University of Exeter.
    Blumenau, A.T.
    Theoretische Physik, Universität Paderborn.
    Frauenheim, T.
    Theoretische Physik, Universität Paderborn.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Planar interstitial aggregates in Si2002Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 14, nr 48, s. 12843-12853Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Self-interstitials in silicon aggregate to form rod-like defects aligned along [110] directions and inhabiting either {111} or {113} crystallographic planes. These systems are known to be electrically and optically active. We present the results of first-principles calculations on the structure and energetics for candidate structures contained within the {113}, {111} and {001} planes and compare the results with experiment.

  • 155.
    Goss, J.P.
    et al.
    School of Physics, University of Exeter.
    Hahn, I.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Vibrational modes and electronic properties of nitrogen defects in silicon2003Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, nr 4, s. 45206-1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Nitrogen impurities form complexes with native defects such as vacancies and self-interstitials in silicon which are stable to high temperatures. These complexes can then suppress the formation of large vacancy and self-interstitial clusters. However, there is little known about their properties. We use first-principles density-functional theory to the determine the local vibrational modes, electrical levels and stability of a range of nitrogen-interstitial and vacancy complexes. Tentative assignments of the ABC photoluminescence line and the trigonal SL6 EPR center are made to substitutional-nitrogen pair and the substitutional-nitrogen-vacancy complex.

  • 156.
    Goss, J.P.
    et al.
    School of Physical Sciences, Dublin City University.
    Jones, R.
    Department of Physics, University of Exeter.
    Breuer, S.J.
    Edinburgh Parallel Computing Centre, University of Edinburgh.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A first principles study of interstitial Si in diamond1997Inngår i: Defects in Semiconductors: proceedings of the 19th International Conference on Defects in Semiconductors, Aveiro, Portugal, July 1997 / [ed] Gordon Davies, Trans Tech Publications Inc., 1997, s. 781-786Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    The results of first principles calculations using AIMPRO (a local-density-functional code) are presented to describe the stability of interstitial Si in diamond. It is found that the 〈100〉 and 〈110〉 split-interstitial configurations are only local minima in the total energy surface. Td interstitial Si reconstructs to form a substitutional Si site close to a self-interstitial. The difference in total energy between the split configuration and the substitutional-Si-self-interstitial complex is more than 6 eV. It is concluded that Si would not adopt an interstitial location in diamond as has been previously suggested from experimental evidence. Interestingly the self-interstitial remains bound to the substitutional Si with a binding energy around 1 eV.

  • 157.
    Goss, J.P.
    et al.
    School of Physics, University of Exeter, UK.
    Jones, R.
    School of Physics, University of Exeter, UK.
    Heggie, M.I.
    CPES, University of Sussex, Brighton, UK.
    Ewels, C.P.
    CPES, University of Sussex, Brighton, UK.
    Briddon, P.R.
    Department of Physics, University of Newcastle upon Tyne, Newcastle upon Tyne, UK.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    First principles studies of H in diamond2001Inngår i: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 186, nr 2, s. 263-8Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ab initio methods are used to investigate hydrogen defects in diamond. For the isolated impurity, the bond-centered site is found lowest in energy and posses both donor and acceptor levels. The neutral defect possesses a single local mode with very small infrared effective charge, but the effective charge for the negative charge state is much larger. H+ is calculated to be very mobile with a low activation barrier. Hydrogen dimers are stable as H*2 defects which are also found to be almost IR-inactive. The complex between B and H is investigated and the activation energy for the reaction B-H → B - + H+ found to be in rasonable agreement with experiment. Hydrogen is strongly bound to dislocations which, together with H*2, may form part of the hydrogen accumulation layer detected in some plasma studies.

  • 158.
    Goss, J.P.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    (CAs)2-hydrogen defects in GaAs: a first-principles study1997Inngår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 55, nr 23, s. 15576-15580Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Hydrogen-related local vibrational modes at 2643, 2651, 2688, 2725, 2729, and 2775 cm-1 are thought to arise from C-H stretch modes from defects similar to the hydrogen-passivated carbon acceptor complex CAsH. These lines appear in samples that are grown using trimethylgallium metalorganic precursors, and it has been suggested that the 2688-, 2725-, 2729-, and 2775-cm-1 bands may be due to CAs dimers decorated with one or more H atoms. We present here the structures, energies, and vibrational modes of (CAs)2, (CAs)2H, and (CAs)2H2 complexes obtained from ab initio local-density-functional cluster calculations to investigate these assignments.

  • 159.
    Gowda, Vasantha
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Larsson, Anna-Carin
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Antzutkin, Oleg
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik. Department of Physics, Warwick University, CV47AL, Coventry, UK.
    Lantto, P.
    NMR Research Group, Department of Physics, University of Oulu, P.O. Box 3000, FI-90014 Oulu, Finland.
    Telkki, V-V
    NMR Research Group, Department of Physics, University of Oulu, P.O. Box 3000, FI-90014 Oulu, Finland.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Structural investigations of rare earth dialkyl dithiocarbamate complexes: solid-state NMR, X-ray diffraction and DFT calculation studies2013Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    In this study, we made an attempt to qualitatively study the structures of few rare earth metal complexes by employing solid state NMR, X-Ray Diffraction, and preliminary DFT calculations. High resolution 13C and 15N solid state CP/MAS NMR spectra were recorded for six diamagnetic polycrystalline rare earth dialkyldithiocarbamates of the general formula [(RE2S2CNR2)3 PHEN] (where RE=La or Y, R=C2H5, C3H7, and i-C3H7) [1]. Different isotropic 13C and 15N chemical shifts for the three dialkyldithiocarbamato groups were observed. Regulacio et al. (2005) inferred that irrespective of the alkyl chains, rare earth complexes of dialkyldithiocarbamates and phenanthroline (3:1) ligands always crystallize in a monoclinic system with a space P21/c group. However, comparative analysis of solid state 13C/15N CPMAS spectra of polycrystalline yttrium and lanthanum diethyldithiocarbamate complexes shows the presence of significant differences, indicating structural variations of these complexes. Also, quite different X-Ray diffraction powder pattern was observed for the above two complexes. Finally, the computational geometry optimization of Y and La complexes, followed by the preliminary calculation of 13C and 15N chemical shifts and shielding contributions with the ADF program [2], found to be very near to the experimental results.

    Fulltekst (pdf)
    fulltext
  • 160.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Tu, Yongming
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand. Southeast University, Nanjing.
    Elfgren, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Vibrationsanalys för tillståndsbedömning av byggkonstruktioner: Tillämpningsexempel : (Main results and summary in Swedish. Detailed results in English Appendices.)2017Rapport (Annet vitenskapelig)
    Abstract [sv]

    I projektet beskrivs och tillämpasnågra metoder för tillståndsbedömningav olika typer av konstruktioner med och utan skador.Tillståndsbedömningen skermed hjälp av vibrationsanalys.Detektering av skador av olika typ och storlekdiskuteras. Randvillkorens betydelse undersöks för att få överensstämmelse mellan förutspådda och uppmätta egenmoder och egenfrekvenser.Med den programvara som utvecklats är det möjligt att detektera sprickor i enkla betongplattor med hjälp av vibrationsmätningar. För mer avancerade konstruktioner finns en potential att vidareutveckla programvaran.Mätningar och analyser presenterasför femkonstruktioner: (1) En betongplatta –Sprickor har kunnat identifieras utifrån vibrationsmätningar och en successivt uppdaterad finit element modell (FEM).(2) En bågbro av betong över Långforsen i Kalix älv–Okända randvillkor har kunnat bestämmasför t ex undergrundens styvhetmedhjälp avuppmätta vibrationer. Detekteringav specifika skadorhar ännu inte kunnat genomföras.(3) En fackverksbro av stålöver Åby älv–Enviss skadedetektering har kunnat genomföras med vibrationsmätningar. Brons sårbarhet för korrosion och lokala mekaniska skador har studerats.(4) En förspänd betongbrotill gruvan i Kiruna–Modeller av bron har arbetats framför obelastatoch brottbelastat tillståndoch jämförts med uppmätta vibrationer med gott resultat.(5) Ett betonghus med nio våningar i Luleå –En FEM-modell av huseti bruksstadiethar tagits fram och resultat från den har jämförts med uppmätta vibrationer med gott resultat.Metoder och resultatpresenteras översiktligti rapportens huvudtext. Detaljerade beskrivningar gesi appendix.I appendix presenterasäven en programvaramed färdiga moduler för de undersökta konstruktionerna. Programmet kan laddas ner och användasfritt

    Fulltekst (pdf)
    Projektsammanfattning (project summary in Swedish, 11 pages)
    Fulltekst (pdf)
    Huvudrapport (main report in Swedish, 34 pages)
    Fulltekst (pdf)
    Appendices with some more detailed results (English, 260 pages)
  • 161.
    Gulliksson, M.
    et al.
    Department of Physics and Mathematics, Mid Sweden University, SE-851 70, Sundsvall, Sweden.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Watson, G.A.
    Department of Mathematics, University of Dundee, Dundee, DD1 4HN, Scotland.
    Implicit surface fitting using directional constraints2001Inngår i: BIT Numerical Mathematics, ISSN 0006-3835, E-ISSN 1572-9125, Vol. 41, nr 1, s. 308-321Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A commonly used technique for fitting curves and surfaces to measured data is that known as orthogonal distance regression, where the sum of squares of orthogonal distances from the data points to the surface is minimized. An alternative has recently been proposed for curves and surfaces which are parametrically defined, which minimizes the sum of squares in given directions which depend on the measuring process. In addition to taking account of that process, it is claimed that this technique has the advantage of complying with traditional fixed-regressor assumptions, enabling standard inference theory to apply. Here we consider extending this idea to curves and surfaces where the only assumption made is that there is an implicit formulation. Numerical results are given to illustrate the algorithmic performance.

    Fulltekst (pdf)
    fulltext
  • 162. Gulliksson, Mårten
    et al.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Surface Fitting and Parameter Estimation with Nonlinear Least Squares1995Inngår i: Optimization Methods and Software, ISSN 1055-6788, E-ISSN 1029-4937, Vol. 5, nr 3, s. 247-269Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Algorithms for fitting a surface, explicitly or implicitly defined, to some measured points are developed, analyzed and tested. The problem class treated, includes orthogonal regression and errors-in-variables. Methods using only first order derivatives as well as second order methods are developed and the rich inherent structure of the problem is fully utilized. The main features of the algorithms, that distinguish them from earlier works, are that arbitrary nonnegative weights can be handled and that infinite weights can be used to define nonlinear equality constraints. Moreover, by using a special merit function, global convergence can be proved for the first order methods under some nondegeneracy assumptions. The local convergence for the first order algorithms is linear for nonzero residual problems and quadratic otherwise. The local convergence for the second order methods is quadratic. A mixed algorithm where only a part of the second derivatives are included is also developed and tested.

  • 163.
    Gulliksson, Mårten
    et al.
    Institution of Information Processing, University of Umeå.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Wedin, Per-Åke
    Institution of Information Processing, University of Umeå.
    Algorithms for constrained and weighted nonlinear least squares1997Inngår i: SIAM Journal on Optimization, ISSN 1052-6234, E-ISSN 1095-7189, Vol. 7, nr 1, s. 208-224Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The authors consider constrained and weighted nonlinear least squares problems in which weights can be arbitrarily large. Methods of Newton type are used for solving such problems: a Gauss-Newton method, a second-order generalized Newton-Raphson method, and an efficient hybrid approach consisting of both methods are suggested. Local convergence and global convergence under certain assumptions are proved for the Gauss-Newton method. Results of computational testing of the algorithm suggested are presented and an open problem is formulated at the end of the paper.

  • 164.
    Gutiérrez, R.
    et al.
    Technische Universität, Theoretische Physik III, D-09107 Chemnitz.
    Haugk, M.
    Technische Universität, Theoretische Physik III, D-09107 Chemnitz.
    Frauenheim, Th.
    Technische Universität, Theoretische Physik III, D-09107 Chemnitz.
    Elsner, J.
    Department of Physics, University of Exeter.
    Jones, R.
    Department of Physics, University of Exeter.
    Heggie, M.I.
    CPES, University of Sussex, Falmer, Brighton.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The formation of nanopipes caused by donor impurities in GaN: A theoretical study for the case of oxygen1999Inngår i: Philosophical Magazine Letters, ISSN 0950-0839, E-ISSN 1362-3036, Vol. 79, nr 3, s. 147-152Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Local density-functional methods are used to examine the behaviour of O and O-related defect complexes at {1010}-type surfaces in GaN. We find that O has a tendency to segregate to the (1010) surface and we identify the gallium vacancy surrounded by three oxygen impurities (VGa-(ON)3) to be a particularly stable and electrically inert complex. We suggest that these complexes impede growth at the walls of the nanopipes preventing them from growing in. Also, other donor-related defect complexes, in particular gallium vacancies surrounded by three silicon atoms as second nearest neighbours, are expected to have the same effect.

  • 165.
    Hassan, Mohsan
    et al.
    Department of mathematics, COMSATS University Islamabad, Lahore Campus 54000, Lahore, Pakistan.
    Faisal, Abrar
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Kemiteknik. Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus 54000, Lahore, Pakistan.
    Javid, Khurram
    Department of Mathematics, Northern University, 24110, Wattar-Nowshera, KPK, Pakistan.
    Khan, Salahuddin
    College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia.
    Ahmad, Ashfaq
    Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia.
    Khan, Rawaiz
    College of Dentistry, King Saud University, Riyadh 11545, Saudi Arabia.
    The Impact of Different Arrangements of Molecular Chains in Terms of Low and High Shear Rate’s Viscosities on Heat and Mass Flow of Nonnewtonian Shear thinning Fluids2022Inngår i: Combinatorial chemistry & high throughput screening, ISSN 1386-2073, E-ISSN 1875-5402, Vol. 25, nr 7, s. 1115-1126Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Background: Non-newtonian fluids, especially shear thinning fluids, have several applications in the polymer industry, food industry, and even everyday life. The viscosity of shear thinning fluids is decreased by two or three orders of magnitude due to the alignment of the molecules in order when the shear rate is increased, and it cannot be ignored in the case of polymer processing and lubrication problems.

    Objective: So, the effects of viscosities at the low and high shear rates on the heat and mass boundary layer flow of shear thinning fluid over moving belts are investigated in this study. For this purpose the generalized Carreau model of viscosity relate to shear rate is used in the momentum equation. The Carreau model contains the five parameters: low shear rate viscosity, high shear rate viscosity, viscosity curvature, consistency index, and flow behavior index. For the heat flow, the expression of the thermal conductivity model similar to the viscosity equation due to the non-Newtonian nature of the fluid is used in the energy equation.

    Methods: On the mathematical model of the problem, boundary layer approximations are applied and then simplified by applying the similarity transformations to get the solution. The solution of the simplified equations is obtained by numerical technique RK-shooting method. The results are compared with existing results for limited cases and found good agreement.

    Results: The results in the form of velocity and temperature profiles under the impact of all the viscosity’s parameters are obtained and displayed in graphical form. Moreover, the boundary layer parameters such as the thickness of the regions, momentum thickness, and displacement thickness are calculated to understand the structure of the boundary layer flow of fluid.

    Conclusion: The velocity and temperature of the fluid are decreased and increased respectively by all viscosity’s parameters of the model. So, the results of the boundary layer fluid flow under rheological parameters will not only help engineers to design superior chemical equipment but also help improve the economy and efficiency of the overall process.

  • 166.
    Hedberg, Mikael
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    CFD simulations of Luleälven: An examination of CFD as a tool forevaluating restoration possibilities of regulated dredgedrivers2018Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    Dredged rivers remain from when lumber was floated downriver for processing and shipping purposes. This has left parts of some rivers in Sweden with shallow rocky beaches and deep central furrows. Restoration efforts of dredged unregulated rivers has shown positive results for fish populations. This report compares, by use of CFD two regulated parts of Luleälven, one dredged and one natural. The report looks at habitats where the Riverine Grayling prefers to lay its eggs and evaluates whether velocity and depth conditions could give an explanation for the lower population of the Riverine Grayling in the dredged river. For the water levels tested, usable areas exist for all cases, but the dredged river has substantially smaller areas suitable as a habitat. The river geometry seems to be the large issue for Mattisudden as its shallow wide riverbanks are not conducive to habitats for the Riverine Grayling. Surprisingly the "lower" test case for the dredged river showed a larger usable area than for the "higher" water level, for the same river and higher usable area than the natural river. For this case the water level is now so low that the shallow wide riverbanks are above the water level and only the central furrow is filled with water. This shows two avenues that can be used to improve the habitats for the Riverine Grayling, lower the water level, or raise the riverbanks.

    Fulltekst (pdf)
    fulltext
  • 167.
    Hedlund, Jonas
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Grahn, Mattias
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Korelskiy, Danil
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Rayson, Mark
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, Patrick R.
    School of Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne.
    Mass transport in porous media from first principles: an experimental and theoretical study2012Inngår i: Journal of Membrane Science, ISSN 0376-7388, E-ISSN 1873-3123, Vol. 415-416, s. 271-277Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In the present work, the mass transport of helium through zeolite is experimentally determined by measuring the flow of helium through a zeolite membrane. By using a mathematical model, the mass transport through defects was accounted for to arrive at mass transport through zeolite pores. For the first time, we could thereby experimentally show that the mass transport of helium in zeolite pores is strongly controlled by the amount and location of hydrocarbons in the zeolite pores and varies several orders of magnitude. The mass transport of helium in ZSM-5 zeolite pores is first reduced gradually more than one order of magnitude when the loading of n-hexane is increased from 0 to 47% of saturation. As the loading of n-hexane is further increased to 54% of saturation, the mass transport of helium in the zeolite pores is further reduced abruptly by more than two orders of magnitude. This gradual decrease followed by an abrupt decrease of mass transport is caused by adsorption of n-hexane in the zeolite pores. In a similar yet different fashion, the mass transport of helium in the zeolite pores is reduced abruptly by almost two orders of magnitude when the loading of benzene is increased from 0 to 19% of saturation due to adsorption of benzene in the pore intersections. Effective medium approximation percolation models with parameters estimated using density functional theory employing the local density approximation, i.e. models with no adjustable parameters and the most sophisticated theory yet applied to this system, can adequately describe the experimental observations.

  • 168. Hellström, Pär
    Ab initio modeling of xanthate adsorption on ZnS surfaces2005Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Modeling surface adsorption requires systems of hundreds of atoms. To model such systems at an ab initio level successfully, we need to avoid traditional quantum chemical methods. In the present work we have shown that density functional theory is a powerful modeling tool for large chemical systems especially in combinations with pseudopotentials This is validated by an initial study of ethyl and heptyl xanthates and their sodium/potassium salts. In this study, all electron calculations using both Hartree-Fock and density functional theory methods are compared with experimental infrared results. To do this the influence of basis sets and modeling approaches on the geometrical structure and the vibrational modes are examined. This includes comparing the pseudopotential and full electron potential approaches. Results obtained from pseudopotential methods are in close agreement with both all electron calculations as well as experimental results, here used to study adsorption of heptyl xanthate ZnS surfaces. Vibrational frequencies of the adsorbed species is presented, together with calculations of the tilt angles. The investigation of the tilt angles resulted in 20.3° 20.6° and 25.2° for the 100, 110 and 111 surfaces respectively. Heptyl xanthate forms a bridging confirmation on both the 110 and 111 surfaces and a bidentate confirmation on the 100 surface. Assignments of vibrational modes of ethyl/heptyl xanthate molecule and its corresponding potassium/sodium salts are also reported.

    Fulltekst (pdf)
    FULLTEXT01
  • 169.
    Hellström, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Ab initio modelling of xanthates adsorbed on Ge and ZnS surfaces2007Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The motivation for this modelling work, in close collaboration with experimentalists, is to contribute to the understanding of xanthate adsorption on ZnS surfaces in the flotation process. Adsorption of xanthates on Ge surfaces is investigated, which also have been studied experimentally in the Agricola Research Centre. Furthermore, results for dibutyldithiophosphates—which are important in the flotation process—are reported. Modelling surface adsorption requires systems including hundreds of atoms. To model such systems at an ab initio level successfully, fast and accurate methods must be used.In the present work we use density functional theory (DFT) combined with pseudopotentials, which is a powerful modelling approach for large chemical systems. The results obtained in an initial study of ethyl and heptyl xanthates and their sodium or potassium salts using all electron Hartree-Fock and DFT calculations are used to validate the pseudopotential method used for modelling adsorption of xanthates on surfaces. Both geometrical and vibrational properties, as well as computer resources needed, are examined and compared. Results obtained using DFT and pseudopotentials are in close agreement with both experimental results and all electron potential calculations, while being obtained much faster. Hence, the pseudopotential approach is chosen to study ethyl/heptyl xanthate adsorbed on Ge and ZnS surfaces. The surfaces are modelled using periodically repeated supercells containing more than one hundred atoms. The geometrical structure and vibrational frequencies are calculated and the modes are described. The results are compared with attenuated total reflection infrared observations. The influence of additional atoms at the surface in the surrounding of the adsorbed xanthate are also investigated.

    Fulltekst (pdf)
    FULLTEXT01
  • 170.
    Hellström, Pär
    et al.
    Luleå tekniska universitet.
    Holmgren, Allan
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    An ab initio study of ethyl xanthate adsorbed on Ge(111) surfaces2007Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, nr 45, s. 16920-16926Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The interaction between adsorbed ethyl xanthate on a clean Ge(111) surface and a hydroxylated surface was studied, using first-principle density functional theory. Geometrically optimized structures are presented and discussed. Results suggest a bidentate bridging conformation on a clean Ge(111) surface, whereas a monodentate bonding is preferred when the surface is saturated by hydrogen atoms. Then the interaction with a hydroxylated surface was considered. The lowest energy configuration was obtained when xanthate binds with one sulfur to the surface and with the other to an OH group adsorbed on the surface. A marker for this configuration was found in the vibrational spectra at 3200 cm-1. Vibrational frequencies down to 250 cm-1 were calculated and assigned to vibrational modes and presented together with the calculated infrared spectra. The tilt angle of the hydrocarbon chain was also investigated.

  • 171.
    Hellström, Pär
    et al.
    Luleå tekniska universitet.
    Larsson, Anna-Carin
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Fredriksson, Andreas
    Luleå tekniska universitet.
    Holmgren, Allan
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Study of potassium O,O′-dibutyldithiophosphate combining DFT, 31P CP/MAS NMR and infrared spectroscopy2008Inngår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 112, nr 46, s. 11777-11783Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dithiophosphates are used in many different industrial applications. To explain their functions and properties in these applications, a fundamental understanding on a molecular level is needed. Potassium O,O′-Dibutyldithiophosphate and its anion have been investigated by means of a combination of DFT and 31P CP/MAS NMR and infrared spectroscopy. Several low-energy conformations were studied by DFT. Three different conformations with significantly different torsion angles of the O-C bond relative to the O-P-O plane were selected for further studies of infrared frequencies and 31P NMR chemical-shift tensors. A good agreement between theoretical and experimental results was obtained, especially when the IR spectra or 31P chemical shift tensor parameters of all three conformations were added, indicating that, because of the low energy difference between the conformations, the molecules are rapidly fluctuating between them.

  • 172.
    Hellström, Pär
    et al.
    Luleå tekniska universitet.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Fredriksson, Andreas
    Luleå tekniska universitet.
    Holmgren, Allan
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    A theoretical and experimental study of vibrational properties of alkyl xanthates2006Inngår i: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 65, nr 3-4, s. 887-895Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Geometrical structure and vibrational modes of potassium and sodium ethyl/heptyl xanthates were studied, using both theoretical and experimental methods. Both Hartree-Fock and density functional theory were used. The experimental method used was infrared absorption spectroscopy (FTIR). Our work showed that vibrational frequencies calculated with density functional theory, using the local density approximation, are in very good agreement with experiments. The results were not improved by using the more sophisticated and computationally demanding B3LYP functional.

  • 173.
    Hoffmann, L.
    et al.
    Institute of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
    Bach, J. C.
    Institute of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
    Bech Nielsen, B.
    Institute of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
    Leary, P.
    Department of Physics, University of Exeter, Exeter, United Kingdom.
    Jones, R.
    Department of Physics, University of Exeter, Exeter, United Kingdom.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Substitutional carbon in germanium1997Inngår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 55, nr 17, s. 11167-11173Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Carbon impurities implanted into single-crystalline germanium are studied with infrared absorption spectroscopy and ion channeling. After implantation of 12C+ at room temperature and subsequent annealing at 350 °C, a sharp infrared absorption line is observed at 531 cm-1. When 12C+ is substituted by 13C+, the line shifts down in frequency to 512 cm-1 and co-implantation of 12C+ and 13C+ does not give rise to additional lines. Therefore, the 531-cm-1 line represents a local vibrational mode of a defect containing a single carbon atom. Channeling measurements are carried out around the 〈100〉, 〈110〉, and 〈111〉 axes in 12C+-implanted samples annealed at 450 °C. The analysis of the data shows that 31±3 % of the carbon atoms are located at substitutional sites, while the remaining carbon atoms appear to be located randomly. The population of the substitutional site and the intensity of the 531-cm-1 mode have identical temperature dependencies. It is concluded that the 531-cm-1 mode is the three-dimensional T2 stretch mode of substitutional carbon. The effective charge of the mode is determined to be (3.4±0.5)e.mAb initio local density functional cluster theory is applied to calculate the structure and the local vibrational modes of substitutional carbon in germanium. The calculated frequencies and isotope shifts for the T2 stretch mode are in good agreement with the observations.

  • 174.
    Hoffmann, L.
    et al.
    Institute of Physics and Astronomy, University of Aarhus.
    Bach, J.C.
    Institute of Physics and Astronomy, University of Aarhus.
    Hansen, J. Lundsgaard
    Institute of Physics and Astronomy, University of Aarhus.
    Larsen, A. Nylandsted
    Institute of Physics and Astronomy, University of Aarhus.
    Nielsen, B. Bech
    Institute of Physics and Astronomy, University of Aarhus.
    Leary, P.
    Department of Physics, University of Exeter.
    Jones, R.
    Department of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Substitutional carbon in Ge and Si1-xGex1997Inngår i: Defects in semiconductors: proceedings of the 19th International Conference on Defects in Semiconductors, Aveiro, Portugal, July 1997 / [ed] Gordon Davies, Trans Tech Publications Inc., 1997, s. 97-102Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In the present work, carbon is implanted into monocrystalline Ge and into relaxed epitaxial MBE-grown Si1-xGex. The samples are studied with infrared absorption spectroscopy along with ion-channeling studies on the Ge samples. Finally, ab-initio local density functional cluster theory is applied to calculate the structure and the local vibrational modes of substitutional carbon, Cs, in Ge. After implantation of 12C+ in Ge at room temperature and subsequent annealing at 350°C, a sharp absorption line is observed at 531 cm-1. By isotope substitution, it is concluded that the 531 cm-1 line represents a local vibrational mode of a single carbon atom. From ion-channeling measurements on samples annealed at 450°C, it is found that 31±3 % of the carbon atoms are located at substitutional sites. The population of the substitutional site and the intensity of the 531 cm-1 mode have identical annealing behavior and it is concluded that the 531 cm-1 mode is the three-dimensional T2 stretch mode of Cs in Ge. The calculated frequency and isotope shift for this mode are in good agreement with the observations. In Si0.65Ge0.35, two broad absorption lines are observed at ∼551 and ∼592 cm-1 after implantation of 12C+ and subsequent annealing at 550°C. From measurements on samples implanted with 13C+ and coimplanted with 12C+ and 13C+ we conclude that these lines represent local vibrational modes of defects containing a single carbon atom. In 13C+ implanted Si1-xGex samples that contain 15 to 50 % Ge a number of modes are observed in a frequency range from ∼510 to ∼610 cm-1, i.e., in the range of Cs in Ge and in Si. From the experimental findings it is concluded that substitutional carbon in Si1-xGex binds to both Si and Ge.

  • 175.
    Hoffmann, L.
    et al.
    Institute of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark.
    Bech Nielsen, B.
    Institute of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark.
    Nylandstedt Larsen, A.
    Institute of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark.
    Leary, P.
    Department of Physics, University of Exeter, EX44QL Exeter, United Kingdom.
    Jones, R.
    Department of Physics, University of Exeter, EX44QL Exeter, United Kingdom.
    Briddon, P.R.
    Department of Physics, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, United Kingdom.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Substitutional carbon in Si1-xGex1999Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 60, nr 19, s. 13573-13581Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Local vibrational modes of carbon impurities in relaxed Si1-xGex have been studied with infrared absorption spectroscopy in the composition range 0.05≤x≤0.50. Carbon modes with frequencies in the range 512-600 cm-1 are observed in 13C+-implanted Si1-xGex after annealing at 550°C. Measurements on samples coimplanted with 12C+ and 13C+ show that these modes originate from defects containing a single carbon atom and from the variation of the mode frequencies with composition x, the modes are assigned to substitutional carbon in Si1-xGex. Based on the frequencies obtained from a simple vibrational model, the observed modes are assigned to specific combinations of the four Si and Ge neighbors to the carbon. The intensities of the modes indicate that the combination of the four neighbors deviates from a random distribution. Ab initio local-density-functional cluster theory has been applied to calculate the structure and the local mode frequencies of substitutional carbon with n Ge and 4-n Si neighbors in a Si and a Ge cluster. The calculated frequencies are ∼9% higher than those observed, but the ordering and the splitting of the mode frequencies agree with our assignments.

  • 176.
    Hoffmann, L.
    et al.
    Institute of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
    Lavrov, E. V.
    Institute of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
    Bech Nielsen, B.
    Institute of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
    Hourahine, B.
    Department of Physics, University of Exeter, Exeter, United Kingdom.
    Jones, R.
    Department of Physics, University of Exeter, Exeter, United Kingdom.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P. R.
    Department of Physics, The University of Newcastle upon Tyne, Newcastle upon Tyne, United Kingdom.
    Weakly bound carbon-hydrogen complex in silicon2000Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 61, nr 24, s. 16659-16666Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Local vibrational modes of a weakly bound carbon-hydrogen complex in silicon have been identified with infrared-absorption spectroscopy. After implantation of protons at ∼20 K and subsequent annealing at 180 K, two carbon modes at 596 and 661 cm-1, and one hydrogen mode at 1885 cm-1 are observed. The three modes originate from the same complex, which is identified as bond-centered hydrogen in the vicinity of a nearby substitutional carbon atom. Ab initio theory has been applied to calculate the structure and local modes of carbon-hydrogen complexes with hydrogen located at the first, second, and third nearest bond-center site to substitutional carbon. The results support our assignment.

  • 177.
    Holbech, J D
    et al.
    University of Aarhus.
    Nielsen, B Bech
    University of Aarhus.
    Jones, R
    University of Exeter.
    Stich, S
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    H2* defect in crystalline silicon1993Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 71, nr 6, s. 875-878Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Detailed infrared studies have been carried out on proton- and deuteron-implanted Si. A dominant trigonal defect involving a pair of inequivalent hydrogen atoms has been identified, with local modes at 2061.5, 1838.3, 1599.1, and 817.2 cm-1. The structure, the local modes, and the isotopic shifts of the H2* defect have been calculated using ab initio pseudopotential cluster theory. The structure is consistent with channeling and uniaxial stress experiments. The calculated frequencies and isotopic shifts are in close agreement with those observed.

  • 178.
    Hounsome, L.S.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Martineau, P.M.
    DTC, Maidenhead.
    Fisher, D.
    DTC, Maidenhead.
    Shaw, M.J.
    Centre, School of Natural Science, Newcastle upon Tyne.
    Briddon, P.R.
    Centre, School of Natural Science, Newcastle upon Tyne.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Origin of brown coloration in diamond2006Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, nr 12, s. 125203-1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Measurements of the absorption spectra of brown natural type IIa diamond as well as brown nitrogen-doped CVD diamond are reported. These are largely featureless and increase almost monotonically from about 1-5.5 eV. It is argued that the brown coloration is due to an extended defect and not to a point defect. First principles modeling studies demonstrate that the spectra could be attributed to vacancy disks lying on {111} planes. Such disks are unstable above about 200 vacancies and should relax to dislocation loops in natural diamond. Hydrogen is shown to passivate the optical activity of the disks.

  • 179.
    Hounsome, L.S.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Martineau, P.M.
    DTC, Maidenhead.
    Fisher, D.
    DTC, Maidenhead.
    Shaw, M.J.
    School of Natural Science, University of Newcastle upon Tyne.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Role of extended defects in brown colouration of diamond2007Inngår i: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 4, nr 8, s. 2950-2957Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The absorption spectrum of brown diamond is broad and featureless, in both natural type IIa and CVD-grown material. It is argued that such an absorption is due to an extended, rather than a point, defect. Ab-initio modelling studies have been conducted on dislocations and extended vacancy-related defects. While certain dislocations could potentially explain the origin of colour, their density is too low to account for experimentally observed absorption magnitudes. It is demonstrated that a vacancy disk lying in the {111} plane has an absorption spectrum similar to that seen in natural and CVD brown diamond. Such disks are unstable above about 200 vacancies and should relax to dislocation loops in natural diamond. Hydrogen is shown to passivate the optical activity of the disks.

  • 180.
    Hounsome, L.S.
    et al.
    School of Physics, University of Exeter, UK.
    Jones, R.
    School of Physics, University of Exeter, UK.
    Martineau, P.M.
    DTC, Maidenhead.
    Shaw, M.J.
    School of Natural Science, University of Newcastle upon Tyne, UK.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne, UK.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Blumenau, A.T.
    Max-Planck-Institut für Eisenforschung GmbH.
    Fujita, N.
    School of Physics, University of Exeter, UK.
    Optical properties of vacancy related defects in diamond2005Inngår i: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 202, nr 11, s. 2182-2187Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    ‹110› vacancy chains, multi-vacancy clusters and vacancy discs have been modeled using density functional theory within the AIMPRO and DFTB codes. While a connection can be established between the results on vacancy chains and previous EPR experiments, no connection can be made between the point defects and the optical properties of natural type IIa brown diamonds. However, a vacancy disc consisting of a {111} double plane of vacancies is stable and possesses an absorption spectrum similar to that found in brown diamonds.

  • 181.
    Hourahine, B.
    et al.
    School of Physics, The University of Exeter, Exeter, United Kingdom.
    Jones, R.
    School of Physics, The University of Exeter, Exeter, United Kingdom.
    Safonov, A. N.
    Department of Physics, University of Durham, South Road, Durham, United Kingdom.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P. R.
    Department of Physics, The University of Newcastle upon Tyne, Newcastle upon Tyne, United Kingdom.
    Estreicher, S. K.
    Department of Physics, Texas Tech University, Lubbock, Texas, USA.
    Identification of the hexavacancy in silicon with the B804 optical center2000Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 61, nr 19, s. 12594-12597Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The ring hexavacancy (V6) has been found by previous theoretical modeling to be a particularly stable defect, but it has not been identified with any observed center to date. Here, we use ab initio calculations to derive the structure and properties of two forms of V6H2 and identify these defects with the trigonal optical centers B41 and B711, which are known to contain two hydrogen atoms in equivalent and inequivalent sites, respectively. It follows from the calculations that V6 should also be optically active and we identify it with the B804 (J line) center. This allows us to place the acceptor level of V6 at Ec-0.04 eV.

  • 182.
    Hourahine, B.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Safonov, A.N.
    Department of Physics, University of Durham.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Estreicher, S.K.
    Department of Physics, Texas Tech University, Lubbock.
    Optically active hydrogen dimers in silicon1999Inngår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 273-274, s. 176-179Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    First-principles calculations are used to explore the structure and properties of several defects which are prominent luminescent centers in Si. The trigonal defects B41 and B711, which are known to contain two hydrogen atoms in equivalent and inequivalent sites, respectively, are attributed to a hexavacancy containing two H atoms in different configurations. It is suggested that the J luminescence centers arises from a stable hexavacancy without hydrogen atoms

  • 183.
    Hourahine, B
    et al.
    University of Exeter.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P R
    University of Newcastle Upon Tyne.
    Self-interstitial-hydrogen complexes in silicon1999Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 59, nr 24, s. 15729-15732Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The vibrational properties of interstitial silane (SiH4)i and silyl (SiH3)i molecules in crystalline silicon are calculated using a first-principle, cluster-based, spin-polarized local-density method. The Si-H stretch modes are found to be redshifted by ∼300 cm-1 from those of the isolated molecule, which lie around 2200 cm-1. These results refute recent suggestions that modes observed around 2200 cm-1, and previously assigned to hydrogenated vacancy defects, are due to these interstitial molecules.

  • 184.
    Hourahine, B.
    et al.
    Department of Physics, University of Exeter, Devon, England.
    Jones, R.
    Department of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, Patrick R.
    Newcastle Univ, Dept Phys, Newcastle Upon Tyne NE1 7RU, Tyne & Wear, England.
    Anomalous shift of the 1075 cm-1 oxygen-hydrogen defect in silicon1997Inngår i: Defects in semiconductors, ICDS-19: Proceedings of the 19th International Conference on Defects in Semiconductors, Aveiro, Portugal, July 1997 / [ed] Gordon Davies, Maria Helena Nazaré, Trans Tech Publications Inc., 1997, s. 277-282Konferansepaper (Fagfellevurdert)
    Abstract [en]

    First principles calculations are carried out on i) hydrogen and ii) water molecules trapped near an interstitial oxygen atom in Si. We find that it is possible for these molecules to cause an upward shift in the antisymmetric stretch mode of Oi when H is replaced by D, which could explain the anomalous shift in the 1075 cm-1 O-H related local vibrational mode. Both these molecules lead to modes in the 3500-4000 cm-1 region but those of the H2 lie close to those recently detected using Fourier transform infra-red spectroscopy.

  • 185.
    Hourahine, B.
    et al.
    Department of Physics, University of Exeter.
    Jones, r.
    Department of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Molecular hydrogen traps within silicon1999Inngår i: Materials Science & Engineering: B. Solid-state Materials for Advanced Technology, ISSN 0921-5107, E-ISSN 1873-4944, Vol. 58, nr 1-2, s. 24-25Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present the results of first principle calculations on the behaviour of molecular hydrogen within crystalline silicon, both as an isolated species, and within defects in the material. These results are compared with recent experimental infra-red and Raman data obtained for silicon treated by either hydrogen plasma or soaked in hydrogen gas. The effect of Fermi-level position on the diffusion barrier of molecular hydrogen within silicon is also discussed

  • 186.
    Hourahine, B.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Molecular hydrogen traps within silicon1998Inngår i: E-MRS Meeting, European Materials Research Society , 1998Konferansepaper (Fagfellevurdert)
    Abstract [en]

    We present the results of rst principle calculations on the behaviour of molecular hydrogen within crystalline silicon, both as an isolated species, and within defects in the material. These results are compared with recent experimental infra-red and Raman data obtained for silicon treated by either hydrogen plasma or soaked in hydrogen gas. The effect of Fermi level position on the diffusion barrier of molecular hydrogen within silicon is also discussed.

    Fulltekst (pdf)
    FULLTEXT01
  • 187.
    Hourahine, B.
    et al.
    Department of Physics, Universität Paderborn.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Frauenheim, T.
    Department of Physics, Universität Paderborn.
    Germanium-hydrogen pairs in silicon2003Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 15, nr 39, s. S2803-S2807Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Hydrogen in a dilute SiGe alloy is considered theoretically in comparison with hydrogen in Si and Ge. Structural, vibrational and electronic properties of crystalline alloys containing ~1.6 at.% Ge complexed with single hydrogen atoms are considered. The behaviour of bond-centred hydrogen is found to be weakly perturbed, when compared to hydrogen in pure Si.

  • 188.
    Hourahine, B.
    et al.
    Theoretische Physik, Universität Paderborn.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Frauenheim, T.
    Theoretische Physik, Universität Paderborn.
    Platinum and gold dihydrides in silicon2003Inngår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 340, s. 668-72Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The structural, electronic and vibrational properties of single substitutional platinum or gold atoms complexed with two hydrogen atoms in silicon are considered on the basis of local density functional theory. The calculated behaviour of these centres is compared with experimental results from infrared absorption and deep-level transient spectroscopy. In contrast with suggestions in the literature, based on hyperfine parameters of electron-paramagnetic resonance, we suggest that such complexes possess direct metal-H bonding.

  • 189.
    Hourahine, B.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Markevich, V.P.
    Centre for Electronic Materials, University of Manchester.
    Newman, R.C.
    Blacket Laboratory, Department of Physics, Imperial College, London.
    Hermansson, J.
    Department of Physics, University of Lund.
    Kleverman, M.
    Department of Physics, University of Lund.
    Lindström, J.L.
    Institute of Solid State and Semiconductor Physics, Minsk.
    Murin, L.I.
    Institute of Solid State and Semiconductor Physics, Minsk.
    Fukata, N.
    Institute of Materials Research, Tohoku University, Sendai.
    Suezawa, M.
    Institute of Materials Research, Tohoku University, Sendai.
    Evidence for H2* trapped by carbon impurities in silicon2001Inngår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 308, s. 197-201Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Local mode spectroscopy and ab initio modelling are used to investigate two trigonal defects found in carbon-rich Si into which H had been in-diffused. Isotopic shifts with D and 13C are reported along with the effect of uniaxial stress. Ab initio modelling studies suggest that the two defects are two forms of the CH2* complex where one of the two hydrogen atoms lies at an anti-bonding site attached to C or Si, respectively. The two structures are nearly degenerate and possess vibrational modes in good agreement with those observed.

  • 190.
    Hourahine, B
    et al.
    University of Exeter.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Newman, R C
    Imperial College London.
    Briddon, P R
    University of Newcastle Upon Tyne.
    Roduner, E
    Institut für Physikalische Chemie, Stuttgart.
    Hydrogen molecules in silicon located at interstitial sites and trapped in voids1998Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 57, nr 20, s. 12666-12669Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The vibrational modes of H2 molecules in Si are found using a first-principles method and compared with recent experimental investigations. The isolated molecule is found to lie at a Td interstitial site, oriented along [011] and is infrared active. The rotational barrier is at least 0.17 eV. The molecular frequency is a sensitive function of cage size and increases to lie close to the gas value for cages about 50% larger than the Td site. It is suggested that Raman-active modes around 4158 cm-1 are due to molecules within voids.

  • 191.
    Ivanovskaya, Viktoria V.
    et al.
    Institut des Matériaux Jean Rouxel (IMN), UMR 6502 CNRS, University of Nantes.
    Zobelli, Alberto
    Laboratoire de Physique des Solides, Université Paris-Sud, CNRS UMR 8502, F-91405, Orsay.
    Wagner, Philipp
    Institut des Matériaux Jean Rouxel (IMN), UMR 6502 CNRS, University of Nantes.
    Heggie, Marlcolm I.
    Department of Chemistry, University of Sussex.
    Briddon, Patrick R.
    School of Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne.
    Rayson, Mark
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Ewles, Chris P.
    Institut des Matériaux Jean Rouxel (IMN), UMR 6502 CNRS, University of Nantes.
    Low-energy termination of graphene edges via the formation of narrow nanotubes2011Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, nr 6Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nanotubes. This results in lower formation energies than any other nonfunctionalized edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution electron microscopy images show why such rolled edges may be difficult to detect. Rolled zigzag edges serve as metallic conduction channels, separated from the neighboring bulk graphene by a chain of insulating sp3-carbon atoms, and introduce van Hove singularities into the graphene density of states.

  • 192.
    Iwata, H.
    et al.
    Department of Physics and Measurement Technology, Linköping University, SE-581 83 Linköping, Sweden.
    Lindefelt, U.
    Department of Physics and Measurement Technology, Linköping University, SE-581 83 Linköping, Sweden; ABB Corporate Research, SE-721 78 Västerås, Sweden.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, Patrick R.
    Department of Physics, University of Newcastle, Newcastle upon Tyne, NE1 7RU, United Kingdom.
    Theoretical calculation of stacking fault energies in silicon carbide2002Inngår i: Silicon carbide and related materials: ICSCRM2001 : proceedings of the International Conference on Silicon Carbide and Related Materials, Tsukuba, Japan, October 28 - November 2, 2001 / [ed] S. Yoshida, Zürich-Uetikon: Trans Tech Publications Inc., 2002, Vol. 1, s. 439-442Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A first-principles calculation of stacking fault energies in 3C-, 4H-, and 6H-SiC, based on the local-density approximation within the density-functional theory, is reported. All the structurally different stacking faults -which can be introduced by glide along the (0001) basal plane are considered. The number of such stacking faults in these polytypes is one, two, and three, respectively. The stacking fault energies are also calculated using the simpler generalized axial next-nearest-neighbor Ising (ANNNI) model. Our calculations confirm that the stacking fault energy of 3C-SiC is negative, and we also find that one of the three types of stacking faults in 6H-SiC has a considerably higher stacking fault energy than the other two types.

  • 193.
    Iwata, H.
    et al.
    Department of Physics and Measurement Technology, Linköping University, SE-581 83 Linköping, Sweden.
    Lindefelt, U.
    Department of Physics and Measurement Technology, Linköping University, SE-581 83 Linköping, Sweden; Dept. G, ABB Corporate Research, SE-721 78 Västerås, Sweden.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, Patrik R.
    Department of Physics, University of Newcastle, Newcastle upon Tyne NE1 7RU, United Kingdom.
    Theoretical study of cubic polytype inclusions in 4H-SiC2002Inngår i: Silicon carbide and related materials: ICSCRM2001 : proceedings of the International Conference on Silicon Carbide and Related Materials 2001, Tsukuba, Japan, October 28 - November 2, 2001 / [ed] S. Yoshida, Zürich-Uetikon: Trans Tech Publications Inc., 2002, Vol. 1, s. 533-536Konferansepaper (Fagfellevurdert)
    Abstract [en]

    First-principles density-functional calculations of the band structure and wave functions around narrow 3C-like inclusions in 4H-SiC have been performed. 3C-like inclusions of various thicknesses, corresponding to two, three, and four stacking faults in neighbouring basal planes, have been investigated. The results for the number of bound states in the inclusion, their energies, and wave functions are well described by a simple one-dimensional quantum-well square potential. The quantum-well property of these inclusions suggests that 3C-like regions in 4H-SiC are efficient planar traps for conduction band electrons.

  • 194.
    Iwata, H.
    et al.
    Department of Physics and Measurement Technology, Linköping University.
    Lindefelt, U.
    Department of Physics and Measurement Technology, Linköping University.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    A new type of quantum wells: stacking faults in silicon carbide2003Inngår i: Microelectronics Journal, ISSN 0959-8324, Vol. 34, nr 5, s. 371-374Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report on a new type of quantum wells with the width as thin as 10 Å, which are composed of SiC only, and consequently have ideal interfaces. These quantum wells are actually stacking faults in SiC. Certain types of stacking faults in SiC polytypes create small 3C-like regions, where the stacking sequences along the c-axis become locally cubic in the hexagonal host crystals. Since the conduction band offsets between the cubic and hexagonal polytypes are very large with the conduction band minima of 3C-SiC lower than that of the other polytypes, such thin 3C inclusions can introduce locally lower conduction bands, thus acting as quantum films perpendicular to the c-axis. One mechanism for the occurrence of stacking faults in the perfect SiC single crystals is the motion of partial dislocations in the basal planes, the partial dislocations leaving behind stacking fault regions.

  • 195.
    Iwata, H.
    et al.
    Department of Physics and Measurement Technology, Linköping University.
    Lindefelt, U.
    Department of Physics and Measurement Technology, Linköping University.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Cubic polytype inclusions in 4H-SiC2003Inngår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 93, nr 3, s. 1577-1585Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Multiple stacking faults in 4H-SiC, leading to narrow 3C polytype inclusions along the hexagonal c direction, have been studied using an ab initio supercell approach with 96 atoms per supercell. The number of neighboring stacking faults considered is two, three, and four. The wave functions and the two-dimensional energy bands, located in the band gap and associated with the narrow inclusions, can be reconciled with a planar quantum-well model with quantum-well depth equal to the conduction band offset between 3C- and 4H-SiC. We show that the existence of the electronic dipole moment due to the spontaneous polarization leads to a clear asymmetry of the bound wave functions inside the quantum well, and that the perturbation associated with the change in the dipole moment caused by the 3C-like inclusion accounts for the appearance of very shallow localized states at the valence band edge. We have also calculated the stacking fault energies for successive stacking faults. It is found that the stacking fault energy for two stacking faults in adjacent basal planes is reduced by approximately a factor of 4 relative to that of one isolated stacking fault, indicating that double stacking faults in 4H-SiC could be quite common.

  • 196.
    Iwata, H.
    et al.
    Department of Physics & Measurement Technology, Linköping University.
    Lindefelt, U.
    Department of Physics Technology, Linköping University.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Effective mass of electrons in quantum-well-like stacking-fault gap states in silicon carbide2003Inngår i: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 433-436, s. 519-522Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A first-principles calculation of the effective mass of electrons in quantum-well-like gap states induced by stacking faults and cubic inclusions in 4H- and 6H-SiC is performed, based on the density functional theory in the local density approximation. Our calculated effective electron masses for perfect crystals are in very good agreement with those previously determined both theoretically and experimentally. It has been found that electrons confined in the thin 3C-like regions have clearly heavier effective masses than that in perfect 3C-SiC.

  • 197.
    Iwata, H.
    et al.
    Department of Physics and Measurement Technology, Linköping University.
    Lindefelt, U.
    Department of Physics and Measurement Technology, Linköping University.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Electronic localization around stacking faults in silicon carbide2002Inngår i: Silicon carbide and related materials: ICSCRM2001 : proceedings of the International Conference on Silicon Carbide and Related Materials, Tsukuba, Japan, October 28 - November 2, 2001 / [ed] S. Yoshida, Zürich-Uetikon: Trans Tech Publications Inc., 2002, Vol. 1, s. 529-532Konferansepaper (Fagfellevurdert)
    Abstract [en]

    First-principles band structure calculations of all the structurally different stacking faults that can be introduced by glide along the (0001) basal plane in 3C-, 4H-, and 6H-SiC are performed, based on the local-density approximation within the density-functional theory. Our calculations, using supercells containing 96 atoms, have revealed that both types of stacking faults in 4H-SiC and two of the three different SFs in 6H-SiC give rise to quasi-2D energy band states in the band gap at around 0.2 eV below the lowest conduction band, and are electrically active. The corresponding wave functions are strongly localized around the stacking fault plane. These results imply that stacking faults in these SiC polytypes are efficient planar traps for electron capture and responsible for subsequent electron-hole recombination. This can therefore have a profound influence on bipolar SiC technology

  • 198.
    Iwata, H.
    et al.
    Department of Physics, Linköping University.
    Lindefelt, U.
    Department of Physics, Linköping University.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Electronic properties of stacking faults in 15R-SiC2003Inngår i: Silicon carbide and related materials: ECSCRM 2002 : proceedings of the 4th European Conference on Silicon Carbide and Related Materials, September 2-5, 2002, Linköping, Sweden, Uetikon-Zuerich: Trans Tech Publications Inc., 2003, s. 531-534Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A first-principles calculation of stacking faults in 15R-SiC is reported. All the geometrically distinguishable stacking faults which can be introduced by the glide of partial dislocations in (0001)-basal planes are investigated: there exist as many as five different stacking faults in 15R-SiC. Electronic properties and stacking fault energies of these extended defects are studied based on the density functional theory in the local density approximation. Stacking fault energies are also calculated using the axial next nearest neighbor Ising (ANNNI) model.

  • 199.
    Iwata, H.
    et al.
    Department of Physics & Measurement Technology, Linköping University.
    Lindefelt, U.
    Department of Physics Technology, Linköping University.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Electronic structure of twin boundaries in 3C-SiC, Si and diamond2003Inngår i: Silicon carbide and related materials 2002: ECSCRM 2002 ; proceedings of the 4th European Conference on Silicon Carbide and Related Materials, September 2 - 5, 2002, Linköping, Sweden / [ed] Peder Bergman; Erik Janzén, Uetikon-Zuerich: Trans Tech Publications Inc., 2003, s. 527-30Konferansepaper (Fagfellevurdert)
    Abstract [en]

    We report on a first-principles band structure calculation of twin boundaries in 3C-SiC, Si, and diamond, based on the density functional theory in the local density approximation. It is found that the electron wave functions belonging to the conduction and valence band edge states in 3C-SiC tend to be localized almost exclusively on different sides of the boundaries, while there is no such feature in Si and diamond. We have interpreted these localization and segregation phenomena as a consequence of the electrostatic field caused by the spontaneous polarization due to the hexagonal symmetry around twin boundaries. A mechanism for the creation of twin boundaries, i.e., propagation of partial dislocations in neighboring basal planes, has been investigated using total energy calculations, and it has been realized that the double-intrinsic-stacking-fault structure in 3C-SiC, coinciding with the extrinsic stacking faults, is much energetically favored.

  • 200.
    Iwata, H.
    et al.
    Department of Physics, Linköping University.
    Lindefelt, U.
    Department of Physics, Linköping University.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Stacking fault - stacking fault interactions and cubic inclusions in 6H-SiC: an ab initio study2003Inngår i: Silicon carbide and related materials: ECSCRM 2002 : proceedings of the 4th European Conference on Silicon Carbide and Related Materials, September 2-5, 2002, Linköping, Sweden / [ed] Peder Bergman; Erik Janzén, Uetikon-Zuerich: Trans Tech Publications Inc., 2003, s. 921-924Konferansepaper (Fagfellevurdert)
    Abstract [en]

    We report on a first-principles supercell calculation of cubic inclusions in 6H-SiC. Motions of successive partial dislocations having the same Burgers vector in the basal planes can lead to a 3C-like region in the perfect 6H-SiC crystal, which corresponds to multiple stacking faults. We have calculated the electronic structures and the total energies of 6H-SiC crystals containing m stacking faults (m=l-4) in the adjacent basal planes, based on the density functional theory in the local density approximation. It has been found that 3C-like sequences in the 6H-host crystals can act as planar quantum wells, in which conduction band electrons can be confined. The total energy calculations using both the supercell method and the axial next nearest neighbor Ising model (ANNNI) have revealed that the 2nd stacking fault energy in 6H-SiC is about 6 times larger than that of an isolated stacking fault.

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