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  • 151.
    Iwata, H.P.
    et al.
    Department of Physics and Measurement Technology, Linköping University.
    Lindefelt, U.
    Department of Physics and Measurement Technology, Linköping University.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Ab initio study of 3 C inclusions and stacking fault-stacking fault interactions in 6H-SiC2003In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 94, no 8, p. 4972-4979Article in journal (Refereed)
    Abstract [en]

    Ab initio supercell calculations of cubic inclusions in 6H-SiC are performed. The cubic inclusions can be created in perfect 6H-SiC by the propagation of successive partial dislocations having the same Burgers vector in neighboring basal planes, i.e., multiple stacking faults. We have studied the electronic structures and the total energies of 6H-SiC single crystals that contain one, two, three, and four stacking faults, based on density functional theory in local density approximation. Our total energy calculations have revealed that the second stacking fault energy in 6H-SiC is about six to seven times larger than that of an isolated stacking fault, which is actually opposite the trend recently obtained for 4H-SiC [J. Appl. Phys. 93, 1577 (2003)]. The effects of spontaneous polarization on the electronic properties of stacking disorders are examined in detail. The calculated positions of the quantum-well-like localized bands and stacking fault energies of 3C inclusions in 6H-SiC are compared with those previously determined in 4H-SiC, and the possibility of local hexagonal to cubic polytypic transformations is discussed in light of the formation energy and quantum-well action.

  • 152.
    Iwata, H.P.
    et al.
    Department of Physics and Measurement Technology, Linköping University.
    Lindefelt, U.
    Department of Physics and Measurement Technology, Linköping University.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Energies and electronic properties of isolated and interacting twin boundaries in 3C-SiC, Si, and diamond2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 11, p. 113202-Article in journal (Refereed)
    Abstract [en]

    First-principles calculations of twin boundaries in 3C-SiC, Si, and diamond are performed, based on the density-functional theory in the local density approximation. We have investigated the formation energies and electronic properties of isolated and interacting twin boundaries. It is found that in 3C-SiC, interacting twin boundaries which are separated by more than two Si-C bilayers are actually energetically more favorable, implying a relatively frequent appearance of these defects. The effect of the spontaneous polarization associated with the hexagonal symmetry around twin boundaries is also studied, and we have observed that the 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 faulted layers, while there is no such feature in Si or diamond.

  • 153.
    Iwata, H.P.
    et al.
    Department of Physics and Measurement Technology, Linköping University.
    Lindefelt, U.
    Department of Physics and Measurement Technology, Linköping University.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Stacking faults in silicon carbide2003In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 340, p. 165-70Article in journal (Refereed)
    Abstract [en]

    We review of our theoretical work on various stacking faults in SiC polytypes. Since the discovery of the electronic degradation phenomenon in 4H-SiC p-i-n diodes, stacking faults in SiC have become a subject of intensive study around the globe. At the beginning of our research project, the aim was to find the culprit for the degradation phenomenon, but in the course of this work we uncovered a wealth of information for the general properties of stacking faults in SiC. An intuitive perspective to the diverse nature of stacking faults in SiC will be given in this conference report.

  • 154. Iwata, H.P.
    et al.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P R
    Department of Physics, University of Newcastle.
    An ab initio study of intrinsic stacking faults in GaN2004In: Proceedings of the 10th International Conference on Silicon Carbide and Related Materials, Stafa-Zuerich: Trans Tech Publications Inc., 2004, p. 1617-1620Conference paper (Refereed)
    Abstract [en]

    A first-principles study of intrinsic stacking faults in GaN is reported. Our calculations are based on density functional theory in local density approximation. We have found that the electron wave functions belonging to the conduction and valence band edge states tend to be localized almost exclusively on different sides of the faulted layer. We suggest that the electrostatic field caused by the macroscopically polarized 2H-GaN parts below and above a thin 3C-like layer around the stacking fault is responsible for these possibly shallow localized states.

  • 155.
    Janke, C.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Coutinho, J.
    University of Aveiro.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Ab initio investigation of phosphorus and boron diffusion in germanium2008In: Materials Science in Semiconductor Processing, ISSN 1369-8001, E-ISSN 1873-4081, Vol. 11, no 5, p. 324-327Article in journal (Refereed)
    Abstract [en]

    P and B diffusion has been modeled in Ge using ab initio methods along with the formation energies and electrical levels of various PV defects expected to be important in the deactivation of P in heavily n-doped Ge. The calculated activation barrier for B diffusion is found to be substantially lower than the measured barrier. However, the exceptionally large pre-exponential factor in the measured diffusivity points to a Meyer-Neldel rule operating and accounting for the discrepancy. The magnitude of the theoretical diffusivity is about a factor 10 lower than observed. For P diffusion, the experimental and theoretical results are in much closer agreement. The formation energy calculations show that all PV clusters are stable with respect to their component defects, and all but P V are predicted to introduce acceptor levels into the band gap. A simple analysis of possible formation mechanisms and Coulombic contributions suggests that as in Si, P V is the most important compensating center in heavily n-doped Ge.

  • 156.
    Janke, C.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Coutinho, J.
    Department of Physics, University of Aveiro.
    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.
    Supercell and cluster density functional calculations of the thermal stability of the divacancy in germanium2006In: Materials Science in Semiconductor Processing, ISSN 1369-8001, E-ISSN 1873-4081, Vol. 9, no 4-5, p. 484-488Article in journal (Refereed)
    Abstract [en]

    Large vacancy clusters, or voids, formed during growth have been reported in Ge. The divacancy is a precursor to such clusters, and is believed to be stable up to 150 or 180 {ring operator} C. It is also believed to form in Ge irradiated at room temperature where single vacancies are mobile. Density functional theory calculations have been performed to calculate the energy barriers for migration and dissociation of the divacancy. We find the binding energy in the neutral charge state to lie in the range 1.1-1.5 eV, and this increases for more negative charge states. The migration energies were found to vary from 1.0 to 1.3 eV from the singly positive to the doubly negative state

  • 157.
    Janke, C,
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P. R.
    University of Newcastle Upon Tyne.
    Ab initio investigation of boron diffusion paths in germanium2007In: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 18, no 7, p. 775-780Article in journal (Refereed)
    Abstract [en]

    Boron is observed to diffuse very slowly in germanium, as opposed to its behaviour in silicon where it exhibits Transient Enhanced Diffusion effects in implanted samples. As a result of this slow diffusion, boron is a very stable dopant, allowing devices to be created with very well-defined doping regions. To understand this superior performance, calculations were performed on a variety of boron diffusion paths, within vacancy and interstitial mediated methods. It was found that the vacancy mediated diffusion which is associated with the fast diffusion of many other species in germanium exhibits a total barrier of 5.8 eV. Interstitial-mediated diffusion had a total barrier for migration of 3.4 eV in the neutral and singly negative charge states, 3.2 eV for the positive charge state, but a formation barrier of 4.1 eV. Thus the barrier for interstitial-mediated diffusion is dominated by the formation energy of the self-interstitial.

  • 158.
    Janke, C.
    et al.
    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.
    University of Newcastle Upon Tyne, School of Natural Science, Newcastle Upon Tyne.
    Ab initio investigation of boron diffusion paths in germanium2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 7Article in journal (Refereed)
    Abstract [en]

    Experimental data indicate that boron diffuses very differently in Ge than in Si. To examine the kinetics of boron diffusion, density functional calculations were performed on a variety of boron diffusion mechanisms, including interstitial and vacancy-mediated paths, as well as a correlated exchange mechanism. It was found that although vacancy and correlated exchange mechanisms possess high diffusion barriers comparable with experiment, the barrier for interstitial-mediated diffusion lies around 3.8 eV and is similar to those found for boron diffusion in Si. This estimate is well below the experimental activation energy. The difference is attributed to the failure of the theory to include the effect of electronic excitations.

  • 159.
    Janke, C.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Ab initio investigation of phosphorus diffusion paths in germanium2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 19Article in journal (Refereed)
    Abstract [en]

    Ab initio calculations were performed to study phosphorus diffusion in germanium through vacancy and interstitial-mediated mechanisms as well as a correlated exchange mechanism without interaction with a mediating defect. It was found that the most favorable diffusion mechanism is sensitive to the position of the Fermi level within the band gap. For material with a midgap Fermi level, the neutral or singly positive phosphorus interstitial is the dominant diffusing species, while in n -type material, it is the doubly negative phosphorus-vacancy complex. For a Fermi level position of Ev +0.5 eV, a barrier for phosphorus diffusion via the doubly negative phosphorus-vacancy defect of ∼2.5 eV was calculated, which is roughly ∼1 eV below the equivalent process in Si.

  • 160.
    Janke, C.
    et al.
    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.
    School of Natural Science, University of Newcastle upon Tyne.
    Ab initio studies of arsenic and boron related defects in silicon mesa diodes2007In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 90, no 15, p. 152103-Article in journal (Refereed)
    Abstract [en]

    E centers are known to diffuse around 400 K in Si and may then form larger donor-vacancy defects such as As2V in heavily doped n-type Si doped with As or AsBV if they diffuse into p-type regions. Ab initio methods are used to explore these possibilities. The AsV defect possesses electrical levels in agreement with experiments. The AsBV defect is found to exhibit a charge-dependent structure, has a barrier to dissociation of ~1.4 eV, and possesses an acceptor level at 0.27 or 0.47 eV above the valence band top depending on the defect structure. The As2V defect possesses only an acceptor level at 0.22 eV below the conduction band. Comparison is made with recent experiments carried out on mesa diodes.

  • 161.
    Janke, C.
    et al.
    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.
    School of Natural Science, University of Newcastle upon Tyne.
    Supercell and cluster density functional calculations of the thermal stability of the divacancy in germanium2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 19, p. 195208-Article in journal (Refereed)
    Abstract [en]

    Large vacancy clusters, or voids, formed during crystal growth have been reported in Ge. The divacancy is a precursor to such clusters, and is believed to be stable up to 150 or 180 °C. It is also believed to form in Ge irradiated at room temperature where single vacancies are mobile. Density functional theory (DFT) cluster calculations have been performed to calculate the energy barriers for migration and dissociation of the divacancy. We find that the binding energy in the neutral charge state is ~1.5 eV and increases for negatively charged states. The migration energies were found to vary from 1.0 to 1.3 eV from the singly positive to the doubly negative charge states. These results line up well with an estimate of a migration barrier of 1.0 eV for the divacancy from experimental data. Therefore, we conclude that the divacancy in germanium will anneal by migration to trapping centers.

  • 162.
    Jones, R.
    et al.
    University of Exeter.
    Briddon, Patrick
    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.
    First-principles theory of nitrogen aggregates in diamond1992In: Philosophical Magazine Letters, ISSN 0950-0839, E-ISSN 1362-3036, Vol. 66, no 2, p. 67-74Article in journal (Refereed)
    Abstract [en]

    Local-density-functional theory is used to evaluate the structure, vibrational bands and electronic properties of two point defects in diamond a pair of adjacent substitutional N atoms and a vacancy surrounded by four N atoms. These defects are important as they are possible models of the A and B centres seen in type Ia diamond. For the A centre we find a N─N bond length of 2·14 Å, an occupied electronic energy level near midgap and four infrared absorption bands with frequencies close to the experimental values. For the model of the B centre we find a N─C length of 1·49 Å an occupied triplet electronic level within the gap consistent with the recent suggestion that the N9 absorption band is due to this defect and again vibratory modes in close agreement with experiment. These results provide strong evidence as to the validity of the models for the A and B centres.

  • 163.
    Jones, R.
    et al.
    School of Physics, University of Exeter.
    Carvalho, A.
    Department of Physics, University of Aveiro.
    Coutinho, J.
    Department of Physics, University of Aveiro.
    Torres, V.J.B.
    Department of Physics, University of Aveiro.
    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.
    Theoretical investigations of the energy levels of defects in germanium2005In: Solid State Phenomena, ISSN 1012-0394, E-ISSN 1662-9779, Vol. 108-109, p. 697-702Article in journal (Refereed)
    Abstract [en]

    The donor and acceptor levels of defects in Ge as well as in Si are found using a local density functional method applied to large H-terminated defective clusters. The surfaces of the clusters are modified so that their band gaps are aligned with experimental values. It is shown that the resulting energies of the first donor and acceptor levels are within about 0.2 eV of the experimental values

  • 164.
    Jones, R
    et al.
    University of Exeter.
    Coomer, B J
    University of Exeter.
    Goss, J P
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P R
    University of Newcastle Upon Tyne.
    Intrinsic defects and the D1 to D4 optical bands detected in plastically deformed Si2000In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 222, no 1, p. 133-140Article in journal (Refereed)
    Abstract [en]

    The properties of multi-vacancy and multi-interstitial defects that possess luminescent bands around 1 eV are reviewed. Prominent among these are the hexavacancy and tri- and tetra-self-interstitial defects. It is suggested that the formation of these defects on dislocation cores could lead to the D1 to D4 photoluminescent bands linked to dislocations in Si and SiGe.

  • 165.
    Jones, R.
    et al.
    School of Physics, University of Exeter.
    Coutinho, J.
    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.
    Thermal double donors in Si and Ge2001In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 308, p. 8-12Article in journal (Refereed)
    Abstract [en]

    Three experiments appear to cast doubt on self-interstitial-free models of the family of thermal donors, based on oxygen defects of increasing size. These are (a) the rapid transformation of TDD(N) into TDD(N+1) with activation energies considerably lower than that of oxygen diffusion, (b) the lack of any appreciable spin-density on oxygen in TDD(N)+, and (c) the observation of only two oxygen related vibrational modes associated with each donor. However, we show that the oxygen-only model of the donors is compatible with experiment for a structure involving an insulating core with normal oxygen coordination, surrounded by over-coordinated oxygen atoms which are responsible for the donor activity. It is also shown that the calculated stress-energy tensors for the early donors are in good agreement with the measurements.

  • 166.
    Jones, R.
    et al.
    Department of Physics, University of Exeter.
    Eberlein, T.A.G.
    Department of Physics, University of Exeter.
    Pinho, N.
    Department of Physics, University of Exeter.
    Coomer, B.J.
    Department of Physics, University of Exeter.
    Goss, J.P.
    Department of Physics, University of Exeter.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Self-interstitial clusters in silicon2002In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 186, no 1, p. 10-18Article in journal (Refereed)
    Abstract [en]

    Although there have been many calculations of the structures of the self-interstitial in Si and small aggregates of interstititals, In, there has been little attempt to relate the defects with experimental data. Here we discuss the assignments of the self-interstitial to the AA12 electron paramagnetic resonance (EPR) centre, and the tri-interstitial and tetra-interstitial to the W-optical centre and the B3 EPR centre, respectively. Difficulties in the assignment of I2 to the P6 EPR centre are also noted.

  • 167.
    Jones, R
    et al.
    University of Exeter.
    Elsner, J
    University of Exeter.
    Haugk, M
    Gutierrez, R
    Frauenheim, Th
    Heggie, M I
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P R
    Interaction of oxygen with threading dislocations in GaN1999In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 171, no 1, p. 167-173Article in journal (Refereed)
    Abstract [en]

    A review is given of the results of first principles calculations used to investigate the structures and electronic properties of screw and edge dislocations in GaN. The atoms at the core of the full core screw dislocation possess heavily strained bonds leading to deep gap states. Removing the first shell of Ga and N atoms gives a screw dislocation with a small open core consisting of {101-0} type surfaces. Therefore open-core screw dislocations induce only shallow gap states. In the same way we found the core of the threading edge dislocation to be reconstructed without any deep states. The interaction of oxygen with the cores of open-core screw and edge dislocations is considered and it is found that the impurity has a strong tendency to be bound by Ga vacancies leading to three types of defect trapped in the strain field. We suggest that the most stable defect leads to a poisoning of growth centres on the walls of nanopipes.

  • 168.
    Jones, R
    et al.
    University of Exeter.
    Goss, J
    University of Exeter.
    Ewels, C
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Ab initio calculations of anharmonicity of the C-H stretch mode in HCN and GaAs1994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, no 12, p. 8378-8388Article in journal (Refereed)
    Abstract [en]

    The anharmonicities of the C-H stretch modes in HCN and the passivated C acceptor in GaAs are investigated using ab initio local-density-functional cluster theory. The effective-mass parameter χ for the C-H stretch mode is shown to be less than unity in HCN, and greater than unity for the GaAs case. The calculated anharmonic parameter for the first defect is found to be 106 cm-1 and is in very good agreement with experiment. For the second defect, the anharmonicity is about 50% larger in agreement with empirical estimates. The frequencies of the fundamental transitions in both systems are shown to be very sensitive to the C-H length. This limits the accuracy of theoretical investigations of these high frequency H modes. Finally, the effects of electrical anharmonicity are considered and it is shown that they reduce the intensity of the overtone in the C-H complex in GaAs by about 70%.

  • 169. Jones, R
    et al.
    Goss, J
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P R
    Resende, A
    Theory of the NiH2 complex in Si and the CuH2 complex in GaAs1995In: ICDS-18: Proceedings of the 18th International Conference on Defects in Semiconductors, 1995, p. 921-925Conference paper (Refereed)
    Abstract [en]

    Spin-polarised local density functional cluster calculations are carried out on substitutional Ni in Si and Cu in GaAs along with TM-H-2 complexes. The Jahn-Teller distortion for Ni in Si leads to a slight displacement along (100) in agreement with EPR experiments. Several models of NiH2 are investigated and it is shown that one, with H located at anti-bonding sites to two of the Si neighbours of Ni, has the lowest energy and possesses H related local vibrational modes close to those reported for Pt-H-2. A similar structure is found for CuH2 in GaAs. The electronic properties of the complexes are described in terms of the vacancy model of TM impurities.

  • 170.
    Jones, R.
    et al.
    Department of Physics, University of Exeter.
    Goss, J.P.
    Department of Physics, University of Exeter.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Breakdown of the vacancy model for impurity-vacancy defects in diamond1997In: 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, p. 775-780Conference paper (Refereed)
    Abstract [en]

    The vacancy model for impurity vacancy defects in semiconductors assumes that the ground and low energy excited states are derivable from the four sp3-hybrid orbitals on the atoms bordering the vacancy. There are many cases where this model works but we describe here a counter-example concerning the lowest excited state of the [V-N3] defect in diamond. It is shown that a shallow electron trap, localised outside the vacancy, is involved in the first excited state and responsible for the N2 and N4 optical bands associated with this defect.

  • 171.
    Jones, R.
    et al.
    University of Exeter.
    Goss, J.P.
    University of Exeter.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    N2 and N4 optical transitions in diamond: a breakdown of the vacancy model1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 56, no 4, p. 1654-1656Article in journal (Refereed)
    Abstract [en]

    The vacancy model for impurity vacancy defects in semiconductors assumes that the ground and low-energy excited states are derivable from the four sp3 hybrid orbitals on atoms bordering the vacancy. There are many cases where this model works but we describe here a counterexample concerning the lowest excited state of the [V-N3] defect in diamond. It is shown that a shallow electron trap, localized outside the vacancy, is involved in the first excited state and responsible for the N2 and N4 optical bands associated with the defect.

  • 172.
    Jones, R.
    et al.
    School of Physics, University of Exeter.
    Hahn, I.
    School of Physics, University of Exeter.
    Goss, J.P.
    School of Natural Science, University of Newcastle upon Tyne.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Structure and electronic properties of nitrogen defects in silicon2004In: Gettering and Defect Engineering in Semiconductor Technology GADEST 2003: proceedings of the 10th International Autumn Meeting, Trans Tech Publications Inc., 2004, p. 93-98Conference paper (Refereed)
    Abstract [en]

    Nitrogen-vacancy defects in Si are of interest due to their ability to suppress the formation of large vacancy cluster during growth but there are problems in their characterisation. We use local density functional theory to determine the local vibrational modes, electrical levels and stability of a number of nitrogen defects. A prominent nitrogen local vibrational mode at 663 cm-1 is attributed to a nitrogen-vacancy centre and tentative assignments of the ABC photoluminescence line and the trigonal SL6 BPR centre are made.

  • 173.
    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 C3- 601992In: 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.

  • 174.
    Jones, R.
    et al.
    University of Exeter.
    Resende, A.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    The electronic properties of transition metal hydrogen complexes in silicon1998In: E-MRS Meeting, European Materials Research Society , 1998Conference paper (Refereed)
    Abstract [en]

    The electrical levels of various combinations of transition metal-H defects in Si are calculated using spin-polarised local density functional cluster theory with an empirical correction. The shifts of these levels with H can be understood through a displacement and splitting of the gap manifold of states due to the impurity. Passive defects are identified.

  • 175.
    Jones, R.
    et al.
    Department of Physics, University of Exeter.
    Resende, A.
    Department 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.
    The electronic properties of transition metal hydrogen complexes in silicon1999In: Materials Science & Engineering: B. Solid-state Materials for Advanced Technology, ISSN 0921-5107, E-ISSN 1873-4944, Vol. 58, no 1-2, p. 113-117Article in journal (Refereed)
    Abstract [en]

    The electrical levels of various combinations of transition metal-Hn defects in Si are calculated using spin-polarised local density functional cluster theory with an empirical correction. The shifts of these levels with H can be understood through a displacement and splitting of the gap t2 manifold of states due to the impurity. Passive defects are identified

  • 176. Jones, R.
    et al.
    Stich, P.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Heggie, M.I.
    Theory of dislocations in GaAs1994In: Proceedings of the 17th International Conference on Defects in Semiconductors: Gmunden, Austria, July 18 - 23, 1993 / [ed] Helmut Heinrich, Trans Tech Publications Inc., 1994, p. 1605-1610Conference paper (Refereed)
  • 177.
    Jones, R.
    et al.
    University of Exeter.
    Torres, V.J.B.
    Universidade de Aveiro.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Theory of nitrogen aggregates in diamond: the H3 and H4 defects1994In: Proceedings of the 17th International Conference on Defects in Semiconductors: Gmunden, Austria, July 18 - 23, 1993 / [ed] Helmut Heinrich, Trans Tech Publications Inc., 1994, p. 45-50Conference paper (Refereed)
  • 178.
    Jones, R
    et al.
    University of Exeter.
    Umerski, A
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Ab initio calculation of the local vibratory modes of interstitial oxygen in silicon1992In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 45, no 19, p. 11321-11323Article in journal (Refereed)
    Abstract [en]

    A local-density-functional cluster method is used to calculate the structure and vibrational modes of interstitial oxygen in silicon. We find Si-O lengths and the Si-O-Si bond angle to be 1.59 Å and 172°, respectively. The asymmetric and symmetric stretch frequencies are 1104 and 554 cm-1 and are close to observed modes at 1136 and 518 cm-1. The effective charge of the upper mode is 3.5e. Isotopic shifts of the modes are also reported. We find that the symmetric stretch mode at 554 cm-1 is independent of the O-isotopic mass in agreement with observation.

  • 179.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Multiple charge states of substitutional oxygen in gallium arsenide1992In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 69, no 1, p. 136-139Article in journal (Refereed)
    Abstract [en]

    A local-density-functional cluster method is used to calculate the structure and vibrational modes of substitutional oxygen, OAs, in gallium arsenide. We find the defect can exist in a surprisingly large number of five charge states: On- with n=-1, 0, 1, 2, and 3. The structures of the first two of these are very different from the last three. Their calculated modes are in fair agreement with observations on O in GaP and GaAs. The first pair of defects have a midgap level, and the third has an empty upper-gap level which can become occupied in photoillumination experiments. The consequent structural change causes a lowering in vibrational mode frequency as is observed.

  • 180. Jones, R
    et al.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Oxygen frustration and the interstitial carbon-oxygen complex in Si1993In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 70, no 8, p. 1186-1186Article in journal (Refereed)
  • 181.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Oxygen frustration and the interstitial carbon-oxygen complex in Si1992In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 68, no 1, p. 86-89Article in journal (Refereed)
    Abstract [en]

    The interstitial carbon-oxygen complex is one of the dominant defects produced in electron irradiated Si. In spite of much work, there is no consensus on its structure and a recent model of Trombetta and Watkins seems inconsistent with the lack of strong O-related vibrational modes expected from bond-centered O. Our ab initio cluster calculations reveal very unusual bonding of O in these complexes but yield vibrational modes and isotope shifts which are in reasonable agreement with experiment.

  • 182.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Structure and dynamics of the DX center in GaAs:Si1991In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 44, no 7, p. 3407-3408Article in journal (Refereed)
    Abstract [en]

    We have carried out local-density functional cluster calculations on the SiGa- defect in GaAs. We find that the distortion proposed by Chadi and Chang involving a large Si movement along 〈111〉 breaking an Si-As bond has the same energy as a simple breathing distortion. The calculated local vibratory modes of one Chadi-Chang structure do not agree with those recently assigned to DX from an infrared-absorption experiment. On the other hand, the calculated triplet mode due to the other structure above is in reasonable agreement with these observations. The effective charge of the second type of defect is about three times that of the first. It is proposed that both defects coexist with the Chadi-Chang one being almost infrared inactive.

  • 183.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Theory of carbon complexes in aluminum arsenide1994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 49, no 8, p. 5306-5312Article in journal (Refereed)
    Abstract [en]

    The structural and dynamic properties of carbon defects in aluminum arsenide are investigated using first-principles local-density-functional cluster theory. The method accounts satisfactorily for the structure and phonon modes of AlAs. The carbon acceptor and donor possess triplet modes in the band gap between the acoustic and optic branches as well as localized triplet modes. The local mode of the acceptor lies within 40 cm-1 of the observed mode. Four modes of a close-by acceptor pair lie within 40 cm-1 of the local mode of the isolated acceptor and this supports a previous assignment of four satellite lines seen in heavily doped material to this defect. The modes of a donor-acceptor pair are investigated but there is no evidence of their existence.

  • 184.
    Jones, R.
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Theory of carbon complexes in gallium arsenide and aluminium arsenide1994In: Proceedings of the 17th International Conference on Defects in Semiconductors: Gmunden, Austria, July 18 - 23, 1993 / [ed] Helmut Heinrich, Trans Tech Publications Inc., 1994, p. 253-258Conference paper (Refereed)
  • 185.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Theory of the structure and dynamics of the C impurity and C-H complex in GaAs1991In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 44, no 8, p. 3673-3677Article in journal (Refereed)
    Abstract [en]

    Carbon is an important impurity in metalorganic molecular-beam-epitaxy-grown GaAs. CAs is a single acceptor that can be passivated by H. We describe local-density-functional cluster calculations on the structure and dynamics of the impurities and passivated complexes. The lowest-energy structure of the passivated acceptor is a H atom located 1.1 Å from C in a bond-centered orientation. The H-Ga distance is 2.1 Å. The other three C-Ga lengths are 2.18 Å. The H stretch frequency is found to be 2605 cm-1 and is observed at 2635 cm-1. We have also calculated C-H bend modes that should be visible in Raman but not in infrared experiments. The activation energy for the reorientation of the complex is 0.67 eV. Also described are the local modes of the two substitutional C defects in addition to a C-C pair. Modes of the latter are found around 553 and 425 cm-1, respectively, and have effective charges of about 0.5.

  • 186.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Goss, J
    University of Exeter.
    Briddon, P R
    University of Newcastle Upon Tyne.
    Resende, A
    Universidade de Aveiro.
    Theory of nickel and nickel-hydrogen complexes in silicon1995In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 75, no 14, p. 2734-2737Article in journal (Refereed)
    Abstract [en]

    Spin-polarized local density functional cluster calculations are carried out on substitutional Ni and Ni-H2 complexes. We find that Ni- undergoes a Jahn-Teller distortion along 〈100〉 with Ni moving slightly along the cube axis. The distorted state gives b1, b2, and a1 levels in ascending order of energies within the gap in agreement with experiment. Several candidate NiH2 defects are investigated: the lowest energy structure consists of a substitutional Ni atom together with two H atoms at antibonding sites to two Si neighbors of Ni. This gives H related vibrational modes and a spin-polarized charge density close to those reported for Pt-H2.

  • 187.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Rasmussen, F Berg
    University of Aarhus.
    Nielsen, B Bech
    University of Aarhus.
    Identification of the dominant nitrogen defect in silicon1994In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 72, no 12, p. 1882-1885Article in journal (Refereed)
    Abstract [en]

    The structure of the dominant N pair defect in Si is determined from channeling, infrared local vibrational mode spectroscopy, and ab initio local density functional theory. Channeling experiments show that the N atoms are displaced by 1.1±0.1 Å from lattice sites along 〈100〉. Annealing experiments reveal that this N site is associated with two N-related local vibrational modes originating from the N pair. The ab initio calculations demonstrate that the pair consists of two neighboring 〈100〉 oriented N-Si split interstitials, arranged in an antiparallel configuration, and with four N-Si bonds forming a square lying on {011}.

  • 188.
    Jones, Robert
    et al.
    University of Exeter.
    Hounsome, Luke S.
    University of Exeter.
    Fujita, Naomi
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, Patrick R.
    University of Newcastle.
    Electrical and optical properties of multivacancy centres in diamond2007In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 204, no 9, p. 3059-3064Article in journal (Refereed)
    Abstract [en]

    Recent experiments have shown that vacancy clusters are common defects in all types of monocrystalline brown diamond. This is believed to be due to an enhanced stability of the bounding (I 11) or (I 10) surfaces, arising from the formation of pi-bonded chains. These chains lead to broad bands of gap states with acceptor and donor levels around E-v + 2 eV. They also are likely to be responsible for the broad absorption continuum extending from the NIR through the visible and into the UV region which is characteristic of brown diamonds. High temperature treatment removes or transforms the clusters in CVD and type IIa diamonds leaving the diamond colourless. In type la brown diamonds, heat treatments up to 2500 degrees C lead to the breakup of the clusters and the formation of specific vacancy-nitrogen defects. We show that ab initio modelling of the interaction of nitrogen with the chains open a gap and leads to a threshold absorption energy in the 1-2 eV range. Finally, the calculations shed light on the identity of the vacancy-nitrogen defects observed.

  • 189.
    Kaukonen, M.
    et al.
    Department of Physics, 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.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Structure of tin-vacancy defects in silicon2002In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 186, no 1, p. 24-29Article in journal (Refereed)
    Abstract [en]

    Tin-vacancy complexes Snn-Vm, n≤2, m≤2, are investigated by first-principles cluster and supercell methods. Their structures, spin-densities, electrical levels and formation energies are reported. It is found that the tin-vacancy interaction is short ranged and consequently the diffusion mechanism of Sn is somewhat different from that of the E-center.

  • 190.
    Kaukonen, M.
    et al.
    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.
    Department of Physics, University of Newcastle.
    Tin-vacancy complexes in silicon2001In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 64, no 24, p. 245213-Article in journal (Refereed)
    Abstract [en]

    The structure and electrical properties of of SnV2, Sn2V, and Sn2V2 complexes in Si are investigated using first-principles cluster and supercell methods. The formation of SnV2 and Sn2V2 is found to be energetically favorable, in agreement with the experimental results. All the tin-vacancy defects are found to possess deep donor and acceptor levels, although the number of the gap states decreases with increasing size of the defect. The diffusion of tin in silicon is considered and the mechanism found to be distinct from the diffusion of group V shallow donors. In contrast with these, the Sn-V interaction is found to extend only to the third nearest neighbor distance. This implies that the activation energy for Sn diffusion via vacancies should be nearly the same as self-diffusion by this mechanism. We find an activation energy of 3.5 eV which is close to some experimental findings but considerably less than given by others.

  • 191.
    Khirunenko, L.I.
    et al.
    Institute of Physics, Kiev.
    Pomozov, Yu.V.
    Institute of Physics, Kiev.
    Sosnin, M.
    Institute of Physics, Kiev.
    Markevich, V.P.
    University of Manchester.
    Murin, L.I.
    Joint Institute of Solid State and Semiconductor Physics, Minsk.
    Litvinov, V.V.
    Belarusian State University, Minsk.
    Carvalho, A.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Coutinho, J.
    University of Aveiro and I3N.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Complexes of self-interstitials with oxygen atoms in germanium2008In: Materials Science in Semiconductor Processing, ISSN 1369-8001, E-ISSN 1873-4081, Vol. 11, no 5, p. 344-347Article in journal (Refereed)
    Abstract [en]

    Interactions of germanium self-interstitials with interstitial oxygen atoms in Ge crystals have been studied by combining experimental and theoretical methods. Self-interstitials were created in oxygen-rich Ge crystals by irradiation with MeV electrons at 80 K, and I-O-related complexes were studied by means of infrared absorption spectroscopy, while the density functional theory was used to model structures, local vibrational modes and electronic properties of IO and I2O centers. It is argued that two absorption lines at 674 and 602 cm-1, which develop upon annealing of irradiated Ge:O crystals in the temperature range 180-220 K, are related to IO complexes, while another set of bands at 713 and 803 cm-1 is related to I2O. Those assignments are supported by the comparison with the calculated local vibrational modes of the defects.

  • 192.
    Larsen, A Nylandsted
    et al.
    University of Aarhus.
    Goubet, J. J.
    University of Aarhus.
    Mejlholm, P
    University of Aarhus.
    Christensen, J Sherman
    University of Aarhus.
    Fanciulli, M
    University of Aarhus.
    Gunnlaugsson, H P
    University of Aarhus.
    Weyer, G
    University of Aarhus.
    Petersen, J Wulff
    Technical University of Denmark.
    Resende, A
    University of Exeter.
    Kaukoen, K
    University of Exeter.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P R
    University of Newcastle Upon Tyne.
    Svensson, B G
    Royal Institute of Technology.
    Lindström, J L
    University of Lund.
    Dannefaer, S
    University of Winnipeg.
    Tin-vacancy acceptor levels in electron-irradiated n-type silicon2000In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 62, no 7, p. 4535-4544Article in journal (Refereed)
    Abstract [en]

    Si crystals (n-type, fz) with doping levels between 1.5×1014 and 2×1016 cm-3 containing in addition ∼1018 Sn/cm3 were irradiated with 2-MeV electrons to different doses and subsequently studied by deep level transient spectroscopy, Mössbauer spectroscopy, and positron annihilation. Two tin-vacancy (Sn-V) levels at Ec-0.214 eV and Ec-0.501 eV have been identified (Ec denotes the conduction band edge). Based on investigations of the temperature dependence of the electron-capture cross sections, the electric-field dependence of the electron emissivity, the anneal temperature, and the defect-introduction rate, it is concluded that these levels are the double and single acceptor levels, respectively, of the Sn-V pair. These conclusions are in agreement with electronic structure calculations carried out using a local spin-density functional theory, incorporating pseudopotentials to eliminate the core electrons, and applied to large H-terminated clusters. Thus, the Sn-V pair in Si has five different charge states corresponding to four levels in the band gap.

  • 193.
    Larsen, A. Nylandsted
    et al.
    Institute of Physics and Astronomy, University of Aarhus.
    Mesli, A.
    Institut d'Electronique du Solide et des Systèmes, CNRS/ULP, Strasbourg.
    Nielsen, K. Bonde
    Institute of Physics and Astronomy, University of Aarhus.
    Nielsen, H. Kortegaard
    Institute of Physics and Astronomy, University of Aarhus.
    Dobaczewski, L.
    Institute of Physics, Polish Academy of Sciences.
    Adey, J.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Palmer, D.W.
    School of Physics, University of Exeter.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    E center in silicon has a donor level in the band gap2006In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 97, no 10, p. 106402/1-106402/4Article in journal (Refereed)
    Abstract [en]

    It has been an accepted fact for more than 40 years that the E center in Si (the group-V impurity-vacancy pair)-one of the most studied defects in semiconductors-has only one energy level in the band gap: namely, the acceptor level at about 0.45 eV below the conduction band. We now demonstrate that it has a second level, situated in the lower half of the band gap at 0.27 eV above the valence band. The existence of this level, having a donor character, is disclosed by a combination of different transient-capacitance techniques and electronic-structure calculations. The finding seriously questions some diffusion-modeling approaches performed in the past.

  • 194.
    Larsson, Anna-Carin
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Hellström, Pär
    Fredriksson, Andreas
    Holmgren, Allan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Study of potassium O,O′-dibutyldithiophosphate combining DFT, 31P CP/MAS NMR and infrared spectroscopy2008In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 112, no 46, p. 11777-11783Article in journal (Refereed)
    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.

  • 195.
    Larsson, Anna-Carin
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Study on potassium iso-propylxanthate and its decomposition products: experimental 13C CP/MAS NMR combined with DFT calculations2011In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 115, no 8, p. 1396-1407Article in journal (Refereed)
  • 196.
    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.

  • 197.
    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

  • 198.
    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.

  • 199.
    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.

  • 200. 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.

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