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  • 251.
    Safonov, A N
    et al.
    King's College London.
    Lightowlers, E C
    King's College London.
    Davies, Gordon
    King's College London.
    Leary, P
    University of Exeter.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Interstitial-carbon hydrogen interaction in silicon1996Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 77, nr 23, s. 4812-4815Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The T-line luminescence system is created in Si by annealing at 400-600 °C. Shifts and splitting of the spectral features with 13C and D isotope substitution identify the presence of two C atoms and one H atom in the center. Uniaxial stress and magnetic field measurements show that the T center has monoclinic I symmetry and possesses an acceptor ( -/0) level at 0.2 eV below the conduction band. Ab initio cluster calculations lead to a structure in which an interstitial C-H defect binds with a substitutional C atom. The calculated vibrational modes are in good agreement with those observed.

  • 252.
    Budde, M
    et al.
    University of Aarhus.
    Nielsen, B Bech
    University of Aarhus.
    Jones, R
    University of Exeter.
    Goss, J
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Local modes of the H2* dimer in germanium1996Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 54, nr 8, s. 5485-5494Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Local vibrational modes of the H2* defect in crystalline germanium are identified by a combination of infrared-absorption spectroscopy, uniaxial stress measurements, and ab initio theory. Germanium crystals are implanted with protons and/or deuterons at 30 K, and subsequently annealed at room temperature. A number of local vibrational modes of hydrogen are revealed by infrared-absorption spectroscopy. In particular, modes at 765, 1499, 1774, and 1989 cm-1 originate from the same defect which has trigonal symmetry according to the uniaxial stress measurements. The 765-cm-1 mode is two dimensional, while the 1774- and 1989-cm-1 modes are one dimensional. Measurements on samples coimplanted with protons and deuterons show that the defect contains a pair of weakly coupled and inequivalent hydrogen atoms. The 765-, 1499-, 1774-, and 1989-cm-1 modes are ascribed to the H2* defect. The 765-cm-1 mode is a Ge-H bend mode with an overtone at 1499 cm-1 and the modes at 1774 and 1989 cm-1 are Ge-H stretch modes. An excellent fit to the stretch frequencies is obtained with a simple model based on two coupled Morse-potential oscillators. In addition, the model gives intensity ratios in fair agreement with those observed. The structure, the local-mode frequencies, and the isotope shifts of H2* are calculated with ab initio local-density-functional cluster theory. The theoretical frequencies are consistently 5-10 % too high, as expected from the theory which often leads to overbinding. The isotope shifts, however, are in fair agreement with observations. These results provide additional support for our assignments, and show that the 765- and 1774-cm-1 modes primarily involve the hydrogen at the antibonding site, while the 1989-cm-1 mode is related mainly to vibration of the hydrogen near the bond-center site

  • 253.
    Ewels, C P
    et al.
    University of Exeter.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Miro, J
    Technical University of Budapest.
    Deák, P
    Technical University of Budapest.
    Shallow thermal donor defects in silicon1996Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 77, nr 5, s. 865-868Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An ab initio local density functional cluster program, aimpro, is used to examine nitrogen related shallow thermal donor defects in silicon. We find the bonding of oxygen with interstitial nitrogen in Ni- O2i to be almost "normal," but the O atoms move slightly out of their bond centered sites causing the deep donor level of Ni to become shallow. The defect has properties consistent with those experimentally observed for shallow thermal donors. We also find that a CiH- O2i defect has very similar electronic properties, and suggest that shallow thermal donors do not have a unique composition.

  • 254.
    Ashwin, M.J.
    et al.
    Interdisciplinary Research Centre for Semiconductor Materials, The Blackett Laboratory, Imperial College of Science, Technology and Medicine, London.
    Pritchard, R.E.
    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.
    Joyce, T.B.
    Department of Materials Science and Engineering, Liverpool University.
    Bullough, T.J.
    Department of Materials Science and Engineering, Liverpool University.
    Wagner, J.
    Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg.
    Jeynes, C.
    Department of Electronic and Electrical Engineering, University of Surrey, Guildford.
    Breuer, S.J.
    Department of Physics, University of Exeter.
    Jones, R.
    Department of Physics, University of Exeter.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The bonding of CAs acceptors in InxGa1-xAs grown by chemical beam epitaxy using carbon tetrabromide as the source of carbon1996Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 80, nr 12, s. 6754-6760Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    InxGa1-xAs layers (0≤x≤0.37) doped with carbon (>1020 cm-3) were grown on semi-insulating GaAs substrates by chemical beam epitaxy using carbon tetrabromide (CBr4) as the dopant source. Hall measurements imply that all of the carbon was present as CAs for values x up to 0.15. The C acceptors were passivated by exposing samples to a radio frequency hydrogen plasma for periods of up to 6 h. The nearest-neighbor bonding configurations of CAs were investigated by studying the nondegenerate antisymmetric hydrogen stretch mode (A-1 symmetry) and the symmetric XH mode (A+1 symmetry) of the H-CAs pairs using IR absorption and Raman scattering, respectively. Observed modes at 2635 and 450 cm-1 had been assigned to passivated Ga4CAs clusters. New modes at 2550 and 430 cm-1 increased in strength with increasing values of x and are assigned to passivated InGa3CAs clusters. These results were compared with ab initio local density functional theory. Modes due to AlInGaCAs clusters were detected in samples containing grown in Al and In. These results demonstrate that for InGaAs, CBr4 is an efficient C doping source since both In-CAs bonds as well as Ga-CAs bonds are formed, whereas there is no evidence for the formation of In-CAs bonds in samples doped with C derived from trimethylgallium or solid sources

  • 255.
    Rasmussen, F. Berg
    et al.
    Van der Waals-Zeeman Institute, University of Amsterdam.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Jones, R.
    Department of Physics, University of Exeter.
    Ewels, C.
    Department of Physics, University of Exeter.
    Goss, J.
    Department of Physics, University of Exeter.
    Miro, J.
    Department of Atomic Physics, Technical University of Budapest.
    Deák, P.
    Department of Atomic Physics, Technical University of Budapest.
    The nitrogen-pair oxygen defect in silicon1996Ingår i: Materials Science & Engineering: B. Solid-state Materials for Advanced Technology, ISSN 0921-5107, E-ISSN 1873-4944, Vol. 36, nr 1-3, s. 91-95Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The nitrogen-pair oxygen defect in silicon has been studied by infrared absorption spectroscopy on samples implanted with various combinations of 14N, 15N, 16O and 17O. The measurements give direct evidence for the involvement of nitrogen and oxygen in the defect and show that the impurity atoms comprising the defect are only weakly coupled. Ab initio cluster calculation on several models of the nitrogen-pair oxygen defect have been performed and are compared with experiment. Based on these investigations a model consisting of a bridging oxygen atom adjacent to the nitrogen pair is suggested

  • 256.
    Goss, J P
    et al.
    University of Exeter.
    Jones, R
    University of Exeter.
    Breuer, S J
    University of Exeter.
    Briddon, P R
    University of Newcastle Upon Tyne.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The twelve-line 1.682 eV luminescence center in diamond and the vacancy-silicon complex1996Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 77, nr 14, s. 3041-3044Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ab initio cluster methods are used to investigate vacancy-impurity complexes in diamond. We assign the 1.682 eV, twelve-line optical band to a vacancy-Si complex which has a very unusual, possibly unique structure with a Si atom at the center of a split vacancy. The method also successfully accounts for the 1.945, 2.156, and 2.985 eV optical transitions in trigonal vacancy-N defects and estimates of radiative lifetimes are given.

  • 257.
    Ewels, C.P.
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Jones, R.
    University of Exeter.
    Pajot, B.
    Universite Paris.
    Briddon, P.R.
    University of Newcastle.
    Vacancy- and acceptor-H complexes in InP1996Ingår i: Semiconductor Science and Technology, ISSN 0268-1242, E-ISSN 1361-6641, Vol. 11, s. 502-507Artikel i tidskrift (Refereegranskat)
  • 258.
    Stich, P K
    et al.
    University of Exeter.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Heggie, M I
    University of Exeter.
    Ab initio investigation of the dislocation structure and activation energy for dislocation motion in silicon carbide1995Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 52, nr 7, s. 4951-4955Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The structures of straight 90° glide partial dislocations in SiC are calculated using an ab initio local density functional cluster method. Si partials containing core Si atoms are found to be strongly reconstructed with a Si-Si bond of comparable length to that in bulk silicon. The C partial possessing core C atoms is more weakly reconstructed with a bond length 16% longer than that in bulk diamond. The formation and migration energies of kinks on the partials are calculated and indicate that the C partial is the more mobile. The calculations also predict that n-type doping leads to an increase in the mobility of C partials whereas p-type doping increases the mobility of Si partials.

  • 259.
    Rasmussen, F Berg
    et al.
    University of Aarhus.
    Nielsen, B Bech
    University of Aarhus.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Comment on "Role played by N and N-N impurities in type-IV semiconductors"1995Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 51, nr 20, s. 14756-14757Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    In a recent paper [Phys. Rev. B 48, 17 806 (1993)] Cunha, Canuto, and Fazzio reported ab initio Hartree-Fock calculations on nitrogen impurities in group-IV semiconductors. In their paper it is suggested that nitrogen pairs form from substitutional atoms on adjacent lattice sites. However, the experimentally observed configuration in silicon and germanium is different from this. The aim of this Comment is to clarify the situation of the nitrogen pair in these materials.

  • 260.
    Öberg, Sven
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Stich, P K
    University of Exeter.
    Jones, R
    University of Exeter.
    Heggie, M I
    University of Exeter.
    First-principles calculations of the energy barrier to dislocation motion in Si and GaAs1995Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 51, nr 19, s. 13138-13145Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The single kink formation and migration energies Fk and Wm of 90° glide partial dislocations in Si and GaAs are calculated using an ab initio local density-functional cluster method. Kink migration occurs via a concerted exchange of an atom at a dislocation core with one of its glide plane nearest neighbors. By constraining these atoms to sit in high-energy positions and relaxing a surrounding cluster of atoms, sufficient points in configuration space can be sampled for the energy barrier for the first step in kink pair formation to be estimated. By including an estimate of the elastic energy of the interaction of kink pairs, the single kink formation energy is calculated. It is found that Fk and Wm for Si are 0.1 and 1.8 eV, respectively. For the 90° α glide partial in GaAs, these quantities are 0.07 and 0.7 eV, respectively, and 0.3 and 1.1 eV for β partials

  • 261.
    Breuer, S.J.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Interstitial hydrogen and the dissociation of C-H defects in GaAs1995Ingår i: Proceedings of the 18th International Conference on Defects in Semiconductors: ICDS-18; Sendai, Japan, July 23 - 28, 1995 / [ed] Masashi Suezawa, Trans Tech Publications Inc., 1995, s. 951-956Konferensbidrag (Refereegranskat)
  • 262. Rasmussen, F. Berg
    et al.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Jones, R.
    Ewels, C.
    Goss, J.
    Miro, J.
    Deak, P.
    The NNO defect in silicon1995Ingår i: Proceedings of the 18th International Conference on Defects in Semiconductors: ICDS-18; Sendai, Japan, July 23 - 28, 1995 / [ed] Masashi Suezawa, Trans Tech Publications Inc., 1995, s. 791-796Konferensbidrag (Refereegranskat)
  • 263.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    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 silicon1995Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 75, nr 14, s. 2734-2737Artikel i tidskrift (Refereegranskat)
    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.

  • 264. Jones, R
    et al.
    Goss, J
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P R
    Resende, A
    Theory of the NiH2 complex in Si and the CuH2 complex in GaAs1995Ingår i: ICDS-18: Proceedings of the 18th International Conference on Defects in Semiconductors, 1995, s. 921-925Konferensbidrag (Refereegranskat)
    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.

  • 265.
    Jones, R
    et al.
    University of Exeter.
    Goss, J
    University of Exeter.
    Ewels, C
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Ab initio calculations of anharmonicity of the C-H stretch mode in HCN and GaAs1994Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, nr 12, s. 8378-8388Artikel i tidskrift (Refereegranskat)
    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%.

  • 266.
    Pritchard, R E
    et al.
    Imperial College of Science Technology and Medicine.
    Newman, R C
    Imperial College of Science Technology and Medicine.
    Wagner, J
    Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg.
    Fuchs, F
    Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Bonding of H-CAs pairs in AlxGa1-xAs alloys1994Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, nr 15, s. 10628-10636Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The local structure of CAs acceptors in AlxGa1-xAs has been investigated by studying the nondegenerate localized vibrational modes of H-CAs pairs with A1 symmetry, rather than those of isolated CAs impurities. Infrared absorption and Raman scattering measurements have been made on AlxGa1-xAs: 12C epilayers that (a) had been exposed to a radio-frequency hydrogen (deuterium) plasma or (b) contained hydrogen incorporated during growth. Arguments are advanced that indicate that the H(D) atom should occupy a bond-centered site between CAs and Ga atoms rather than between CAs and Al atoms at low temperatures. An ab initio local-density-functional calculation indicates that the energy is then lowered by 0.24 eV. This analysis has led to the assignment of five antisymmetric stretch modes and five symmetric (X) modes to H-CAs pairs at sites where the carbon atom that was originally unpaired had zero, one, two, three, or four Al nearest neighbors

  • 267.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Rasmussen, F Berg
    University of Aarhus.
    Nielsen, B Bech
    University of Aarhus.
    Identification of the dominant nitrogen defect in silicon1994Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 72, nr 12, s. 1882-1885Artikel i tidskrift (Refereegranskat)
    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}.

  • 268.
    Rahbi, R.
    et al.
    Groupe de Physique des Solides, Université Denis Diderot.
    Theys, B.
    Laboratoire de Physique des Solides, CNRS.
    Pajot, B.
    Department of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Somogyi, K.
    Research Institute for Technical Physics, P.O. Box 76, Foti ùt. 56, Budapest.
    Fille, M.L.
    LPSES, CNRS, Sophia Antipolis.
    Chevallier, J.
    Laboratoire de Physique des Solides, CNRS.
    Neutralisation of group vi donors by hydrogen in gallium arsenide1994Ingår i: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 91, nr 3, s. 187-190Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The infrared absorption of GaAs:S and GaAs:Te samples partially neutralised by hydrogen show two local modes with very similar frequencies. These modes are comparable to the ones already reported in GaAs:Se. These results are interpreted by assuming that neutralisation takes place by the formation of a bond between a Ga atom first neighbour of the chalcogen and a H atom in an antibonding location. This assumption is strengthened by ab initio calculations that provide also frequencies of the right order of magnitude.

  • 269.
    Rasmussen, F Berg
    et al.
    University of Aarhus.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Nitrogen in germanium: identification of the pair defect1994Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, nr 7, s. 4378-4384Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Several nitrogen-related centers have been introduced by ion implantation of nitrogen into germanium and studied by infrared-absorption spectroscopy. Two local vibrational modes at 825.3 and 658.6 cm-1 were especially prominent. Measurements on annealed samples implanted with either pure 14N or 15N, or implanted with both isotopes showed these modes to arise from a nitrogen pair defect analogous to one previously suggested to occur in silicon. Based on this and ab initio pseudopotential cluster theory, a model of the pair is proposed which is consistent with the observed infrared absorption. This model is similar to that of the nitrogen pair in silicon and offers an explanation of the low donor efficiency of nitrogen in germanium. Several other nitrogen-related local vibrational modes are also observed in the implanted material.

  • 270.
    Nielsen, B. Bech
    et al.
    University of Aarhus.
    Holbech, J.D.
    University of Aarhus.
    Jones, R.
    University of Exeter.
    Sitch, P.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Observation and theory of the H2* defect in silicon1994Ingår i: Proceedings of the 17th International Conference on Defects in Semiconductors: Gmunden, Austria, July 18 - 23, 1993 / [ed] Helmut Heinrich, Trans Tech Publications Inc., 1994, s. 845-852Konferensbidrag (Refereegranskat)
  • 271.
    Stich, P.
    et al.
    Department of Physics, University of Exeter.
    Jones, R
    Department of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Heggie, M.I.
    Department of Computer Science, University of Exeter.
    Structures of dislocations in GaAs and their modification by impurities1994Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, s. 17717-17720Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Local density functional theory is used to show that both α and β dislocations in GaAs are reconstructed. This is done by relaxing large 158-atom H-terminated clusters of GaAs containing 90° partial dislocations. The reconstruction is strongly influenced by impurities: acceptor pairs destroy the reconstruction of β partials but strengthen it for α dislocations. Donors have opposite effects. The implication of these results for the pinning of dislocations in GaAs is discussed.

  • 272.
    Rasmussen, F. Berg
    et al.
    University of Aarhus.
    Nielsen, B. Bech
    University of Aarhus.
    Jones, R.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The nitrogen pair in crystalline silicon studied by ion channeling1994Ingår i: Proceedings of the 17th International Conference on Defects in Semiconductors: Gmunden, Austria, July 18 - 23, 1993 / [ed] Helmut Heinrich, Trans Tech Publications Inc., 1994, s. 1221-1226Konferensbidrag (Refereegranskat)
  • 273.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Theory of carbon complexes in aluminum arsenide1994Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 49, nr 8, s. 5306-5312Artikel i tidskrift (Refereegranskat)
    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.

  • 274.
    Jones, R.
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Theory of carbon complexes in gallium arsenide and aluminium arsenide1994Ingår i: Proceedings of the 17th International Conference on Defects in Semiconductors: Gmunden, Austria, July 18 - 23, 1993 / [ed] Helmut Heinrich, Trans Tech Publications Inc., 1994, s. 253-258Konferensbidrag (Refereegranskat)
  • 275. Jones, R.
    et al.
    Stich, P.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Heggie, M.I.
    Theory of dislocations in GaAs1994Ingår i: Proceedings of the 17th International Conference on Defects in Semiconductors: Gmunden, Austria, July 18 - 23, 1993 / [ed] Helmut Heinrich, Trans Tech Publications Inc., 1994, s. 1605-1610Konferensbidrag (Refereegranskat)
  • 276.
    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 tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Theory of nitrogen aggregates in diamond: the H3 and H4 defects1994Ingår i: Proceedings of the 17th International Conference on Defects in Semiconductors: Gmunden, Austria, July 18 - 23, 1993 / [ed] Helmut Heinrich, Trans Tech Publications Inc., 1994, s. 45-50Konferensbidrag (Refereegranskat)
  • 277.
    Holbech, J D
    et al.
    University of Aarhus.
    Nielsen, B Bech
    University of Aarhus.
    Jones, R
    University of Exeter.
    Stich, S
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    H2* defect in crystalline silicon1993Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 71, nr 6, s. 875-878Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 278. Jones, R
    et al.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Oxygen frustration and the interstitial carbon-oxygen complex in Si1993Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 70, nr 8, s. 1186-1186Artikel i tidskrift (Refereegranskat)
  • 279.
    Jones, R
    et al.
    University of Exeter.
    Umerski, A
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Ab initio calculation of the local vibratory modes of interstitial oxygen in silicon1992Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 45, nr 19, s. 11321-11323Artikel i tidskrift (Refereegranskat)
    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.

  • 280.
    Jones, R
    et al.
    Department of Physics, University of Exeter.
    Latham, Chris
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    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 tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Estreicher, S K
    Department of Physics, Texas Tech University.
    Ab initio calculations of the structure and dynamics of C60 and C3- 601992Ingår i: Philosophical Magazine Letters, ISSN 0950-0839, E-ISSN 1362-3036, Vol. 65, nr 6, s. 291-298Artikel i tidskrift (Refereegranskat)
    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.

  • 281.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Calculations of defect related properties in semiconductors1992Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Many of the most important and useful properties of crystalline materials, such as mechanical strength and electrical resistance, are determined by the presence of lattice defects and impurities. Well known examples are dislocations which control plastic flow and make metals ductile, and dopant atoms which control the extrinsic conduction in semiconductors and insulators. In fact the electronic device industry is based on controlled introduction of specific impurities in semiconductors and insulators and methods to avoid or eliminate destructive defects . Thus improvements of existing solid state devices and the invention of new ones are the driving forces for the large amount of experimental and theoretical investigations performed on defects and solids today. In this thesis dislocations and impurities in the most important semiconductors are analysed with mathematical methods, ranging from a method based on isotropic elasticity theory, interatomic potentials and a semi-empirical quantum-mechanical prescription, here applied on dislocations, to an ab initio quantum mechanical method by which properties of defects in crystalline solids can be obtained from first principles, that is, can be derived from the knowledge of only the atomic numbers and masses of constituent atoms, here applied on impurities. The ab initio method is based on Local Density Functional Theory, which provides a many-electron description. The semi-empirical method has been applied on dislocations in Gallium-Arsenide and Cadmium-Telluride. The ab initio method has been applied on various defects in Silicon and Gallium-Arsenide. For Silicon we have investigated interstitial Oxygen complexes, substitutional Carbon and Boron, and intersitial Carbon-Oxygen complexes. For Gallium- Arsenide de the investigation includes the Carbon impurity and the Carbon-Hydrogen complex, the Boron double acceptors, and the so called DX-centre in GaAs:Si. The interaction of hydrogen with impurities in semiconductors has also been investigated. We have also calculated the diffusion barrier of Oxygen in Silicon with excellent results. Our work shows clearly that Local Density Functional theory can provide useful information about defect structures, dynamical properties and diffusion processes in solids.

  • 282.
    Jones, R.
    et al.
    University of Exeter.
    Briddon, Patrick
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    First-principles theory of nitrogen aggregates in diamond1992Ingår i: Philosophical Magazine Letters, ISSN 0950-0839, E-ISSN 1362-3036, Vol. 66, nr 2, s. 67-74Artikel i tidskrift (Refereegranskat)
    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.

  • 283.
    Purton, J.
    et al.
    Royal Institution of Great Britain.
    Jones, R.
    Department of Physics, University of Exeter.
    Heggie, M.
    Department of Computer Science, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    LDF pseudopotential calculations of the α-quartz structure and hydrogarnet defect1992Ingår i: Physics and chemistry of minerals, ISSN 0342-1791, E-ISSN 1432-2021, Vol. 18, nr 6, s. 389-392Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ab initio LDF theory has been used to study the structure of the hydrogarnet defect in α-quartz. The predicted structure is in good agreement with the available sexperimental data. The techniques employed also yield a good model for the structure of α-quartz, giving an average Si-O bond length of 1.62 Å and average Si-O-Si angle of 142°.

  • 284.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Multiple charge states of substitutional oxygen in gallium arsenide1992Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 69, nr 1, s. 136-139Artikel i tidskrift (Refereegranskat)
    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.

  • 285.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Oxygen frustration and the interstitial carbon-oxygen complex in Si1992Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 68, nr 1, s. 86-89Artikel i tidskrift (Refereegranskat)
    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.

  • 286.
    Estreicher, S.K.
    et al.
    Physics Department, Texas Tech University, Lubbock.
    Latham, Chris D.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Heggie, M.I.
    Department of Computer Science, University of Exeter.
    Jones, R.
    Department of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Stable and metastable states of C60H: buckminsterfullerene monohydride1992Ingår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 196, nr 3-4, s. 311-316Artikel i tidskrift (Refereegranskat)
    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.

  • 287.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Structure and dynamics of the DX center in GaAs:Si1991Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 44, nr 7, s. 3407-3408Artikel i tidskrift (Refereegranskat)
    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.

  • 288.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Theory of the structure and dynamics of the C impurity and C-H complex in GaAs1991Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 44, nr 8, s. 3673-3677Artikel i tidskrift (Refereegranskat)
    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.

3456 251 - 288 av 288
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