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  • 1.
    Adjizian, J.J.
    et al.
    Department of Chemistry, University of Sussex, Falmer, Brighton.
    Latham, Chris
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Briddon, P.R.
    Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne.
    Heggie, M.I.
    Department of Chemistry, University of Sussex, Falmer, Brighton.
    DFT study of the chemistry of sulfur in graphite, including interactions with defects, edges and folds2013In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 62, p. 256-262Article in journal (Refereed)
    Abstract [en]

    Sulfur has several roles, desirable and undesirable, in graphitization. We perform density functional theory calculations within the local density approximation to define the structures and energetics of sulphur in graphite, including its interactions with point defects and edges, in order to understand its role in the later stages of graphitization. We find sulphur does not cross-link layers, except where there are defects. It reacts very strongly with vacancies in neighbouring layers to form a six coordinate split vacancy structure, analogous to that found in diamond. It is also highly stable at basal edge sites, where, as might be expected, the size and valency of sulfur can be easily accommodated. This suggests a role for sulphur in stabilizing graphene edges, and following from this, we show that sulfur dimers can open, i.e. unzip, folds in graphite rapidly and exothermically.

  • 2.
    Berezovsky, Vladimir
    et al.
    Pomorskii State University, Arkhangelsk.
    Men'shikov, Leonid
    Russian Research Centre, Kurchatov Institute, Moscow.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Latham, Chris
    Effect of dipole-dipole interactions between atoms in an active medium2008In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 25, no 3, p. 458-462Article in journal (Refereed)
    Abstract [en]

    On the basis of the results of numerical modeling, it is shown that dipole-dipole interactions among atoms in the active medium strongly influences the character of the associated superradiation. The main effect is to make the nuclear subsystem behave chaotically. Its strength increases with the atom density and leads to the suppression of distant collective correlations and superradiation. Near correlations between the atoms are established, causing a confinement effect: a shielding of radiation in the active medium.

  • 3.
    Berezovsky, V.V.
    et al.
    Lomonosov Pomor State University, Arkhangelsk.
    Men'shikov, L.I.
    Russian Research Centre, Kurchatov Institute, Moscow.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Latham, Chris
    Effect of the dipole-dipole interaction of particles in an active medium on the character of superradiation2008In: Plasma physics reports (Print), ISSN 1063-780X, E-ISSN 1562-6938, Vol. 34, no 7, p. 555-561Article in journal (Refereed)
    Abstract [en]

    The motion of a system of interacting nonlinear charged oscillators is investigated numerically. Because of nonlinearity, the total collective electric field gives rise to a phasing effect-correlations in the phases of the oscillators. The consequence is superradiation-the enhanced spontaneous short-term emission of the energy stored in the oscillators. It is shown that the oscillations of the oscillators become stochastic because of the dipole-dipole interaction between them and their nearest neighbors. As a result, as the density of the oscillators increases, distant collective correlations are suppressed, superradiation ceases to be generated, and radiation is shielded in the medium. The phenomena considered in the present paper can play an important role in cyclotron emission from a plasma and thus should be taken into account in emission calculations. The process whereby the energy of the transverse electron motion in electron cooling devices decreases is analyzed as an example. This process occurs as a result of the development of cyclotron maser instability and has the nature of superradiation. The onset of correlations between individual electrons moving in their Larmor circles is the initial, linear stage of instability developing in the plasma. Superradiation is the final, nonlinear instability stage.

  • 4.
    Dixon, P.
    et al.
    School of Physics, University of Exeter.
    Richardson, D.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Latham, Chris D.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Torres, V.J.B.
    Departamento de Fisica, Universidade de Aveiro.
    Brikddon, P.R.
    Department of Physics, University of Newcastle.
    Nitrogen-Hydrogen Defects in GaP1998In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 210, no 2, p. 321-326Article in journal (Refereed)
    Abstract [en]

    Models of the nitrogen-hydrogen defect in GaP, which contain one and two H atoms, are investigated using ab initio density functional cluster theory. We find that a single H atom binding to N possesses two infrared absorption frequencies close to those attributed to an NH2 defect. The modes shift with its charge state consistent with the photo-sensitivity found for the defect. A third mode observed for this centre is assumed to be an overtone of the bend mode. The isotope shifts of the calculated modes are in excellent agreement with experiment in contrast with the model which contains two H atoms

  • 5.
    Estreicher, S.K.
    et al.
    Physics Department, Texas Tech University, Lubbock.
    Latham, Chris D.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Heggie, M.I.
    Department of Computer Science, University of Exeter.
    Jones, R.
    Department of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Stable and metastable states of C60H: buckminsterfullerene monohydride1992In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 196, no 3-4, p. 311-316Article in journal (Refereed)
    Abstract [en]

    Approximate ab initio Hartree-Fock and first-principles density functional calculations of potential energy surfaces and electronic structures of C60H show that the stable state has H attached to one C atom, outside the buckyball. This C atom is displaced radially outward and is close to being sp3 hybridized. The unpaired electron of C60H is delocalized. The calculated Fermi contact density at the proton is in good agreement with recent low-temperature μSR data. A metastable configuration has atomic H at the center of the ball (H@C60). Once trapped there, H must overcome a large barrier to go through the surface of C60. Other configurations we considered include H attached to one C atom but inside the buckyball, and H bridging one of the two inequivalent CC bonds. The barrier for diffusion of H from outside to the center of C60 have also been calculated. The results are compared to recent muon spin rotation studies in solid C60 and to the states of hydrogen in other forms of carbon.

  • 6.
    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 University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Small aggregates of interstitials and models for platelets in diamond2000In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 12, no 49, p. 10257-10261Article in journal (Refereed)
    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

  • 7.
    Jones, R
    et al.
    Department of Physics, University of Exeter.
    Latham, Chris
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Heggie, M I
    Department of Computer Science, University of Exeter.
    Torres, V J B
    Departamento e Centro de Física, Instituto Nacional de Investigação Cientifica, Universidade de Aveiro.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Estreicher, S K
    Department of Physics, Texas Tech University.
    Ab initio calculations of the structure and dynamics of C60 and C60 3-1992In: Philosophical Magazine Letters, ISSN 0950-0839, E-ISSN 1362-3036, Vol. 65, no 6, p. 291-298Article in journal (Refereed)
    Abstract [en]

    A local-density-functional cluster method is used to calculate the structure and vibrational modes of C60. We find C–C lengths in good agreement with observed values. The effect of doping the molecule with three extra electrons is investigated and shown to result in a surprising shortening of the longer bonds. The second derivatives of the energy are evaluated and have enabled, for the first time, all the normal modes of the molecule to be found. We find these to be in fair agreement with the available experimental results.

  • 8.
    Latham, Chris
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Alatalo, M.
    Department of Electrical Engineering, Lappeenranta University of Technology.
    Nieminen, R.M.
    Laboratory of Physics, Helsinki University of Technology.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Passivation of copper in silicon by hydrogen2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 23Article in journal (Refereed)
    Abstract [en]

    The structures and energies of model defects consisting of copper and hydrogen in silicon are calculated using the AIMPRO local-spin-density functional method. For isolated copper atoms, the lowest energy location is at the interstitial site with Td symmetry. Substitutional copper atoms are found to adopt a configuration with D2d symmetry. We conclude that the symmetry is lowered from Td due to the Jahn-Teller effect. Interstitial hydrogen atoms are found to bind strongly to substitutional copper atoms with an energy that is more than the difference in formation energy over the interstitial site for Cu. The resulting complex has C2v symmetry in the -2 charge state where the H atom is situated about 1.54 Å away from the Cu atom in a [100] direction. In other charge states the symmetry of the defect is lowered to Cs or C1. A second hydrogen atom can bind to this complex with nearly the same energy as the first. Two structures are found for copper dihydride complexes that have nearly equal energies; one with C2 symmetry, and the other with Cs symmetry. The binding energy for a third hydrogen atom is slightly more than for the first. Calculated electronic levels for the model defects relative to one another are found to be in fair to good agreement with experimental data, except for the copper-dihydride complex. The copper trihydride complex has no deep levels in the bandgap, according to our calculations.

  • 9.
    Latham, Chris D.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Jones, R.
    School of Physics, University of Exeter.
    Wagner, J.
    Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg.
    Davidson, B.R.
    Interdisciplinary Research Centre for Semiconductor Materials, The Blackett Laboratory, Imperial College of Science, Technology and Medicine, London.
    Newman, R.C.
    Interdisciplinary Research Centre for Semiconductor Materials, The Blackett Laboratory, Imperial College of Science, Technology and Medicine, London.
    Button, C.C.
    Department of Electronic and Electrical Engineering, University of Surrey, Guildford.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Di-carbon complexes in AlAs and GaAs1998In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 210, no 2, p. 869-872Article in journal (Refereed)
    Abstract [en]

    Heat treatment of heavily carbon doped AlAs and GaAs results in a loss of CAS shallow acceptors. In Raman scattering experiments on annealed CBE grown GaAs with 12C and 13C isotopes, and MOVPE grown AlAs it is found that the loss of carriers is accompanied by the appearance of two high frequency lines. These lie near to the stretch mode of an isolated C2 molecule (1855 cm-1). This is consistent with the formation of two types of di-carbon defects in these materials where the C atoms are bonded together and one or both of which act as a donor. Using a local density functional method to investigate the structure and dynamics of several di-carbon defects, we find that the dimer at an As site is bistable and aligned approximately in a [100] direction in the neutral charge state, and in a [110] direction when positively ionised. The calculated frequencies lie within 10% of the measured values in both materials. Other defects are investigated too with a view of determining the structures giving rise to the modes

  • 10.
    Latham, Chris D.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Density-functional calculations of carbon doping in III-V compound semiconductors2001In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 63, no 15, p. 155202-Article in journal (Refereed)
    Abstract [en]

    This article reports the results of investigations based on local-density-functional theory into the relative formation energies for single substitutional carbon atoms in nine III-V compound semiconductors. The calculations are performed using a supercell formalism derived from the AIMPRO real-space cluster method. Only a very slight trend is discernible down the periodic table. When a metal atom is replaced with carbon, it is energetically least favorable in the phosphides, very marginally lower energy in the arsenides, and ≈0.5-0.7 eV lower in the antimonides. The situation is approximately reversed when a P, As, or Sb atom is substituted by a C atom: for the In compounds the energy is ≈0.4-0.8 eV higher than for the Al and Ga compounds.

  • 11.
    Latham, Chris D.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Nieminen, R.M.
    Laboratory of Physics, Helsinki University of Technology.
    Fall, C.J.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    Physics Centre, School of Natural Sciences, University of Newcastle upon Tyne.
    Calculated properties of point defects in Be-doped GaN2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, no 20, p. 205206-Article in journal (Refereed)
    Abstract [en]

    The properties of several point defects in hexagonal gallium nitride that can compensate beryllium shallow acceptors (BeGa) are calculated using the AIMPRO method based on local density functional theory. BeGa itself is predicted to have local vibrational modes (LVM's) very similar to magnesium acceptors. The highest frequency is about 663 cm-1. Consistent with other recent studies, we find that interstitial beryllium double donors and single-donor beryllium split interstitial pairs at gallium sites are very likely causes of compensation. The calculations predict that the split interstitial pairs possess three main LVM's at about 1041, 789, and 738 cm-1. Of these, the highest is very close to the experimental observation in Be-doped GaN. Although an oxygen donor at the nearest-neighboring site to a beryllium acceptor (BeGa-ON) is also a prime suspect among defects that are possibly responsible for compensation, its highest frequency is calculated to be about 699 cm-1 and hence is not related in any way to the observed center. Another mode for this defect is estimated to be about 523 cm-1 and is localized on the ON atom. These two vibrations of BeGa-ON are thus equivalent to those for the isolated substitutional centers perturbed by the presence of their impurity partners.

  • 12. Latham, Chris
    et al.
    Ganchenkova, M.
    Laboratory of Physics, Helsinki University of Technology.
    Nieminen, R.M.
    Laboratory of Physics, Helsinki University of Technology.
    Nicolaysen, S.
    Department of Physics, University of Oslo.
    Alatalo, M.
    Department of Electrical Engineering, Lappeenranta University of Technology.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Electronic structure calculations for substitutional copper and monovacancies in silicon2006In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 2006, no T126, p. 61-64Article in journal (Refereed)
    Abstract [en]

    Two different computer program packages based on the self-consistent local-spin-density approximation-aimpro and vasp-are employed in this study of substitutional copper CuSi and monovacancies VSi in silicon, including the effects of their charge state. The programs differ in the types of basis sets and pseudopotentials they use, each with their own relative merits, while being similar in overall quality. This approach aims to reduce uncertainty in the results, particularly for small or subtle effects, where the risk is greatest that the conclusions are affected by artifacts specific to a particular implementation. The electronic structures of the two defects are closely related, hence they are expected to behave in a similar manner. For both defects structural distortions resulting in lower point group symmetries than Td (the highest possible) are found. This is in good agreement with the results of previous studies of VSi. Much less is known about symmetry-lowering effects for CuSi; however, the electronic levels of CuSi have been measured accurately, while those for VSi are less accessible. Calculating them is a challenging task for theory. The strategy we adopt, based purely on comparing total energies of supercells in different charge states, with and without model defects, reproduces the three known levels for CuSi reasonably well. Satisfactory results are also obtained for VSi in so far as can be judged for this more complex case.

  • 13.
    Latham, Chris
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Nieminen, R.M.
    Laboratory of Physics, Helsinki University of Technology.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Calculated properties of nitrogen-vacancy complexes in beryllium- and magnesium-doped GaN2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 20, p. 205209-Article in journal (Refereed)
    Abstract [en]

    The properties of defect complexes consisting of a nitrogen vacancy with a substitutional beryllium or magnesium atom on neighboring lattice sites in hexagonal GaN are calculated using the AIMPRO local-density-functional theory method. Both types of defects VN-BeGa and VN-MgGa are bound with respect to their isolated constituents. They do not appear to have any electronic levels in the bandgap, and are expected to be neutral defects. Important structural differences are found. In its minimum energy configuration, the Be atom in the VN-BeGa complex lies nearly in the same plane as the three equivalent N atoms nearest to it. Thus, it has shorter Be-N bonds than the Ga-N distance in the bulk crystal, while the Mg atom in the VN-MgGa complex occupies a position closer the lattice site of the Ga atom it replaces. Hence, the VN-BeGa complex has a larger open volume than the VN-MgGa complex. This is consistent with positron annihilation experiments [Saarinen et al., J. Cryst. Growth 246, 281 (2002); Hautakangas et al., Phys. Rev. Lett. 90, 137402 (2003)]. The frequency of the highest local vibrational mode of the VN-BeGa center is calculated to be within 3-4 % of an infrared absorption line detected in Be-doped GaN

  • 14. Latham, Chris
    et al.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Louchet, F.
    LGGE, Saint Martin d'Hères.
    A pseudopotential density functional theory study of native defects and boron impurities in FeAl2006In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 18, no 39, p. 8859-8876Article in journal (Refereed)
    Abstract [en]

    The structures and energies of point defects and point defect complexes in B 2 iron aluminium FeAl are calculated using a local density functional theory based method with large supercells. Particular emphasis is given to pseudopotential quality, choice of chemical potentials used to calculate defect formation energies, and how these are affected by magnetism. Both purely native defects and those containing boron atoms are considered. It is found that the relative stabilities of isolated point defects versus defect complexes depends on whether the material contains excess iron or aluminium. The situation in material containing boron is further complicated by the existence of iron borides in more than one form. We propose that the interaction between point defects, dislocations, and antiphase boundaries, where the local atomic environment has some similarities with antisite defects, also depends on the alloy composition. It is likely that these interactions are part of the underlying mechanism responsible for the unusual mechanical properties of iron aluminides.

  • 15.
    Latham, Christopher D.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Heggie, Malcolm I.
    University of Surrey.
    Alatalo, Matti
    Lappeenranta University of Technology.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, Patrick R.
    University of Newcastle Upon Tyne.
    The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 13, article id 135403Article in journal (Refereed)
    Abstract [en]

    Models for radiation damage in graphite are reviewed and compared, leading to a re-examination of the contribution made by vacancies to annealing processes. A method based on density functional theory, using large supercells with orthorhombic and hexagonal symmetry, is employed to calculate properties and behaviour of lattice vacancies and displacement defects. It is concluded that annihilation of intimate Frenkel defects marks the onset of recovery in electrical resistivity, which occurs when the temperature exceeds about 160 K. Migration of isolated monovacancies is estimated to have an activation energy Ea ≈ 1.1 eV. Coalescence into divacancy defects occurs in several stages, with different barriers at each stage, depending on the path. The formation of pairs ultimately yields up to 8.9 eV energy, which is nearly 1.0 eV more than the formation energy for an isolated monovacancy. Processes resulting in vacancy coalescence and annihilation appear to be responsible for the main Wigner energy release peak in radiation-damaged graphite, occurring at about 475 K.

  • 16.
    Trevethan, Thomas
    et al.
    Department of Chemistry, University of Surrey.
    Dyulgerova, Polina
    Department of Chemistry, University of Surrey.
    Latham, Chris
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Heggie, Malcolm
    Department of Chemistry, University of Surrey.
    Seabourne, Che
    Institute for Materials Research, SPEME, University of Leeds.
    Scott, Andrew
    Institute for Materials Research, SPEME, University of Leeds.
    Briddon, Patrick
    School of Electrical and Electronic Engineering, University of Newcastle upon Tyne.
    Rayson, Mark
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Extended interplanar linking in graphite formed from vacancy aggregates2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, no 9, article id 95501Article in journal (Refereed)
    Abstract [en]

    The mechanical and electrical properties of graphite and related materials such as multilayer graphene depend strongly on the presence of defects in the lattice structure, particularly those which create links between adjacent planes. We present findings which suggest the existence of a new type of defect in the graphite or graphene structure which connects adjacent planes through continuous hexagonal sp² bonding alone and can form through the aggregation of individual vacancy defects. The energetics and kinetics of the formation of this type of defect are investigated with atomistic density functional theory calculations. The resultant structures are then employed to simulate high resolution transmission electron microscopy images, which are compared to recent experimental images of electron irradiation damaged graphite.

  • 17.
    Wagner, J
    et al.
    Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg.
    Newman, R C
    Imperial College of Science.
    Davidson, B R
    Imperial College of Science.
    Westwater, S P
    University of Liverpool.
    Bullough, T J
    University of Liverpool.
    Joyce, T B
    University of Liverpool.
    Latham, Chris
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Di-Carbon defects in annealed highly carbon doped GaAs1997In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 78, no 1, p. 74-77Article in journal (Refereed)
    Abstract [en]

    Formation of bonded dicarbon C-C centers is deduced from the observation of Raman lines at 1742, 1708, and 1674 cm -1 in GaAs codoped with 12C and 13C after annealing at 850 °C with concomitant loss of vibrational scattering from CAs. The frequencies agree with results of ab initio theory for a C-C split interstitial (deep donor) formed by the trapping of a mobile interstitial C (displaced CAs) atom by an undisplaced CAs acceptor. Other mechanisms of carrier loss are inferred since a weaker Raman triplet is detected at 1859, 1824, and 1788 cm -1 from a different C-C complex.

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