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  • 51.
    Janke, C.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    University of Newcastle Upon Tyne, School of Natural Science, Newcastle Upon Tyne.
    Ab initio investigation of boron diffusion paths in germanium2008Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, nr 7Artikel i tidskrift (Refereegranskat)
    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.

  • 52.
    Janke, C.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Coutinho, J.
    University of Aveiro.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Ab initio investigation of phosphorus and boron diffusion in germanium2008Ingår i: Materials Science in Semiconductor Processing, ISSN 1369-8001, E-ISSN 1873-4081, Vol. 11, nr 5, s. 324-327Artikel i tidskrift (Refereegranskat)
    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.

  • 53.
    Janke, C.
    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.
    Ab initio investigation of phosphorus diffusion paths in germanium2008Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, nr 19Artikel i tidskrift (Refereegranskat)
    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.

  • 54.
    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 tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Complexes of self-interstitials with oxygen atoms in germanium2008Ingår i: Materials Science in Semiconductor Processing, ISSN 1369-8001, E-ISSN 1873-4081, Vol. 11, nr 5, s. 344-347Artikel i tidskrift (Refereegranskat)
    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.

  • 55.
    Carvalho, A.
    et al.
    School of Physics, University of Exeter.
    Coutinho, J.
    Department of Physics, University of Aveiro.
    Jones, R.
    School of Physics, University of Exeter.
    Silva, E.
    Department of Physics, University of Aveiro.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Density-functional theory study of Au, Ag and Cu defects in germanium2008Ingår i: Materials Science in Semiconductor Processing, ISSN 1369-8001, E-ISSN 1873-4081, Vol. 11, nr 5, s. 340-343Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Gold, silver and copper defects in germanium are modeled using density functional theory. The structures and electrical properties of the substitutional metals are calculated in excellent agreement with experiment. Interstitial Au, Ag and Cu are found to be shallow donors, in disagreement with a previous assignment of Cui to a hole trap in the lower half of the gap. Substitutional-interstitial metal (Mi - Ms) pairs and metal-vacancy pairs (Ms - V) are also investigated.

  • 56.
    Berezovsky, Vladimir
    et al.
    Pomorskii State University, Arkhangelsk.
    Men'shikov, Leonid
    Russian Research Centre, Kurchatov Institute, Moscow.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Latham, Chris
    Effect of dipole-dipole interactions between atoms in an active medium2008Ingår i: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 25, nr 3, s. 458-462Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 57.
    Berezovsky, V.V.
    et al.
    Lomonosov Pomor State University, Arkhangelsk.
    Men'shikov, L.I.
    Russian Research Centre, Kurchatov Institute, Moscow.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Latham, Chris
    Effect of the dipole-dipole interaction of particles in an active medium on the character of superradiation2008Ingår i: Plasma physics reports (Print), ISSN 1063-780X, E-ISSN 1562-6938, Vol. 34, nr 7, s. 555-561Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 58.
    Carvalho, A.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Janke, C.
    School of Physics, University of Exeter.
    Goss, J.P.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    First-principles study of the diffusion mechanisms of the self-interstitial in germanium2008Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 20, nr 13Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The self-interstitial in germanium can assume multiple configurations depending on the temperature and charge state. Here, we employ a first-principles density functional method to investigate the diffusion mechanisms of this defect. The energy barriers associated with the transformation between different structures are determined by the climbing nudged elastic band method, as a function of the charge state. The relation between the thermodynamic properties of the self-interstitial and the temperature evolution of electron radiation damage in germanium are discussed.

  • 59.
    Coutinho, J.
    et al.
    University of Aveiro.
    Janke, C.
    University of Exeter.
    Carvalho, A.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Torres, V. J. B.
    University of Aveiro.
    Jones, R.
    University of Exeter.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Limits to n-type doping in Ge: formation of donor-vacancy complexes2008Ingår i: Diffusion and defect data, solid state data. Part A, Defect and diffusion forum, ISSN 1012-0386, E-ISSN 1662-9507, Vol. 273-276, s. 93-98Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Vacancies and interstitials in semiconductors play a fundamental role in both high temperature diffusion and low temperature radiation and implantation damage. In Ge, a serious contender material for high-speed electronics applications, vacancies have historically been believed to dominate most diffusion related phenomena such as self-diffusivity or impurity migration. This is to be contrasted with silicon, where self-interstitials also play decisive roles, despite the similarities in the chemical nature of both materials. We report on density functional calculations of the formation and properties of vacancy-donor complexes in germanium. We predict that most vacancy-donor aggregates are deep acceptors, and together with their high solubilities, we conclude that they strongly contribute for inhibiting donor activation levels in germanium.

  • 60.
    Carvalho, A.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Janke, C.
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Primary defects in n-type irradiated germanium: a first-principles investigation2008Ingår i: Solid State Phenomena, ISSN 1012-0394, E-ISSN 1662-9779, Vol. 131-133, s. 253-258Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The properties of point defects introduced by low temperature electron irradiation of germanium are investigated by first-principles modeling. Close Frenkel pairs, including the metastable fourfold coordinated defect, are modelled and their stability is discussed. It is found that damage evolution upon annealing below room temperature can be consistently explained with the formation of correlated interstitial-vacancy pairs if the charge-dependent properties of the vacancy and self-interstitial are taken into account. We propose that Frenkel pairs can trap up to two electrons and are responsible for conductivity loss in n-type Ge at low temperatures.

  • 61.
    Larsson, Anna-Carin
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Hellström, Pär
    Fredriksson, Andreas
    Holmgren, Allan
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Study of potassium O,O′-dibutyldithiophosphate combining DFT, 31P CP/MAS NMR and infrared spectroscopy2008Ingår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 112, nr 46, s. 11777-11783Artikel i tidskrift (Refereegranskat)
    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.

  • 62.
    Fujita, N.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Eberlein, T. A. G.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P. R.
    School of Natural Science, University of Newcastle upon Tyne.
    Theoretical aspects on the formation of the tri-interstitial nitrogen defect in silicon2008Ingår i: Solid State Phenomena, ISSN 1012-0394, E-ISSN 1662-9779, Vol. 131-133, s. 265-269Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper we investigate the formation of interstitial nitrogen trimers N3 which have been suggested as a fast-diffusing species in silicon recently. Out-diffusion profiles of nitro-gen show the involvement of at least two independent nitrogen related defects in the diffusion process depending on the nitrogen concentration at different depths of the sample. When the nitrogen concentration is small it is proposed that nitrogen trimers are formed in a two step process. We present the structural properties of such a defect using density functional theory and examine the energetics of the two proposed reactions leading to the formation of N3.

  • 63.
    Fujita, N.
    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.
    School of Natural Science, University of Newcastle upon Tyne.
    Theoretical investigation on the interaction of nitrogen with dislocations in single crystal CVD diamond2008Ingår i: Diamond and related materials, ISSN 0925-9635, E-ISSN 1879-0062, Vol. 17, nr 2, s. 123-126Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In recent experimental studies, evidence for an atmosphere of nitrogen around the cores of dislocations was found in irradiated single crystal CVD diamond. In this paper, we present a first-principles study on the interaction of nitrogen with dislocations in single crystal CVD diamond, where dislocations are observed as mixed-type 45° and 90° edge-type dislocations lying along <100>. We find a strong binding of nitrogen to the dislocation core for both types of dislocations and show that our results are in consistency with the experiment.

  • 64.
    Janke, C,
    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.
    Ab initio investigation of boron diffusion paths in germanium2007Ingår i: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 18, nr 7, s. 775-780Artikel i tidskrift (Refereegranskat)
    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.

  • 65.
    Janke, C.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Ab initio studies of arsenic and boron related defects in silicon mesa diodes2007Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 90, nr 15, s. 152103-Artikel i tidskrift (Refereegranskat)
    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.

  • 66. Hellström, Pär
    et al.
    Holmgren, Allan
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    An ab initio study of ethyl xanthate adsorbed on Ge(111) surfaces2007Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, nr 45, s. 16920-16926Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 67.
    Savini, Gianluca
    et al.
    University of Sussex.
    Savini, Giancarlo
    University of Sussex.
    Marocchi, Angela
    University of Bologna.
    Suarez-Martinez, Irene
    University of Sussex.
    Haffenden, Gemma
    University of Sussex.
    Heggie, Malcolm I.
    University of Sussex.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Anomaly enhancement of the dislocation velocity in SiC2007Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 401-402, s. 62-66Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Under forward bias SiC p-i-n diodes exhibit an anomaly enhancement of the partial dislocation mobility. Through first-principle calculations, we have shown that Peierls barriers and electrical activities are strongly dependent on the dislocation core structures. Further we have found that solitons or antiphase defects along the dislocation line cannot explain alone the enhancement of the dislocation velocity. We have proposed a new theoretical model that can explain the enhancement of the dislocation mobility under forward bias. This model can be applied to any semiconductor materials in order to predict the behaviour under electron-hole plasma injections.

  • 68.
    Sque, 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.
    Carbon nanotubes and their interaction with the surface of diamond2007Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, nr 11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Calculations based on density-functional theory have been used to model armchair carbon nanotubes existing in isolation, close-packed into nanoropes, and lying prone on both the hydrogen-terminated and clean (001) surfaces of diamond. In the combined nanotube:diamond systems, the electronic band structures are indicative of electron transfer from the hydrogen-terminated diamond to the physisorbed nanotubes; that is, carbon nanotubes-like C60-appear to be capable of transfer doping diamond p type. Total-energy comparisons with separated systems suggest that the nanotubes are positively bound to the diamond substrate. Diffusion techniques have been used to model movement of the (7,7) nanotube across the hydrogen-terminated diamond surface, with the results suggesting that this substrate presents a rather smooth energy surface to the nanotube; this is consistent with the fact that no covalent bonding across the interface has been found. For comparison, the same nanotube has been modeled above a completely clean diamond (001) surface, and while the nanotube remains bound to the diamond substrate, there is no charge transfer across such an interface.

  • 69. Fredriksson, Andreas
    et al.
    Hellström, Pär
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Holmgren, Allan
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Comparison between in situ total internal reflection vibrational spectroscopy of an adsorbed collector and spectra calculated by ab initio density functional theory methods2007Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, nr 26, s. 9299-9304Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this investigation, adsorbed heptyl xanthate on a Ge(111) surface was studied, as well as the free heptyl xanthate molecule, by means of first-principle density functional theory calculations. The modeled results were compared with the in situ total internal reflection vibrational spectroscopy spectra of heptyl xanthate adsorbed on a germanium internal reflection element and the transmission infrared spectrum of a heptyl xanthate ion in aqueous solution. The assignments of the vibrational frequencies were done for the adsorbed xanthate. The region between 1150 and 1000 cm(-1) seems suitable for discussing differences between an adsorbed xanthate and its corresponding ion. The modeled spectra for both the adsorbed xanthate and the xanthate ion are in good agreement with the experimentally retrieved spectra. This investigation shows the strength of using DFT calculations in the interpretation of absorption bands from molecules adsorbed at surfaces in situ.

  • 70.
    Fujita, N.
    et al.
    School of Physics, University of Exeter.
    Blumenau, A.T.
    Max-Planck-Institut für Eisenforschung GmbH.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Core reconstructions of the (100) edge dislocation in single crystal CVD diamond2007Ingår i: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 204, nr 7, s. 2211-2215Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Dislocations are common defects in both natural as well as in CVD-grown diamond. Recent advances in the growth of high quality single crystal CVD diamond have led to an increased interest in the atomistic and electronic structure of 100 dislocations. These dislocations are observed as mixed-type 45° and pure edge dislocations. Previously we investigated both types and found that the 45° is by far lower in core energy than the proposed structure of the edge type. In this work we focus on the straight 100 dislocation only and present novel core reconstructions. We find a minimum energy structure for the edge-type dislocation which has a similar core energy as the mixed-type dislocation.

  • 71.
    Savini, G.
    et al.
    Department of Chemistry, University of Sussex.
    Heggie, M.L.
    Department of Chemistry, University of Sussex.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Core structures and kink migrations of partial dislocations in 4H-SiC2007Ingår i: Faraday discussions (Online), ISSN 1359-6640, E-ISSN 1364-5498, Vol. 134, s. 353-367Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    First-principles calculations are used to investigate the Shockley partial dislocations in 4H-SiC. We show that both dislocations can sustain the asymmetric and symmetric reconstructions along the dislocation line. The latter reconstructions are always electrically active. In particular, the Si(g) 30 degrees partials can explain the optical activation energy for the dislocation glide at similar to 2.4 eV above the VB, the narrow peak at 2.87 eV and the broadband at similar to 1.8 eV found in photoluminescence spectra. Further, we propose a new model to explain the stability of the symmetric reconstructions and the enhancement of the dislocation velocity in SiC.

  • 72.
    Coutinho, Jose
    et al.
    University of Aveiro.
    Torres, Vitor J. B.
    University of Aveiro.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Carvalho, Alexandra
    University of Exeter.
    Janke, Colin
    University of Exeter.
    Jones, Robert
    University of Exeter.
    Briddon, Patrick R.
    University of Newcastle Upon Tyne.
    Early stage donor-vacancy clusters in germanium2007Ingår i: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 18, nr 7, s. 769-773Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    There is considerable experimental evidence that vacancies in Ge dominate several solid state reactions that range from self-diffusivity to metal and dopant transport. It is therefore vital that we fully understand how vacancies interact with other point defects in Ge. Here we have a look at the properties of small donor-vacancy (Sb n V m with m,n ≤ 2) complexes in Ge by ab-initio density functional modeling. Particular attention has been payed to binding energies and to the electronic activity of the complexes. We found that all aggregates may contribute to the n→ p type conversion that is typically observed under prolonged MeV irradiation conditions. In general, Sb n V m defects are double acceptors. It is also suggested that spontaneous formation of Sb3V complexes may limit the activation level of donors introduced by ion implantation.

  • 73.
    Savini, G.
    et al.
    Department of Chemistry, University of Sussex.
    Heggie, M.I.
    Department of Chemistry, University of Sussex.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Electrical activity and migration of 90° partial dislocations in SiC2007Ingår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 9, s. 1-13Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    SiC p-i-n diodes exhibit an increase in the voltage drop under forward bias which has been linked with the increased mobility of partial dislocations. Through first-principles calculations, we investigated the Si(g) and C(g) core 90° partials in 4H-SiC. We showed that both dislocations can sustain the asymmetric and symmetric reconstructions along the dislocation line. The latter reconstructions are always electrically active with a half-filled metallic band and are always more likely to migrate with substantially lower activation energies. Further we have suggested that under forward bias, the 90° partials are less mobile than the 30° partial dislocations.

  • 74.
    Jones, Robert
    et al.
    University of Exeter.
    Hounsome, Luke S.
    University of Exeter.
    Fujita, Naomi
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, Patrick R.
    University of Newcastle.
    Electrical and optical properties of multivacancy centres in diamond2007Ingår i: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 204, nr 9, s. 3059-3064Artikel i tidskrift (Refereegranskat)
    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.

  • 75.
    Fujita, N.
    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.
    School of Natural Science, University of Newcastle upon Tyne.
    First-principles study on the local vibrational modes of nitrogen-oxygen defects in silicon2007Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 401-402, s. 159-162Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper we investigate the interaction of nitrogen and oxygen by means of local density functional theory. While nitrogen-pair-oxygen defects N2-Om have been studied in detail previously, the existence and role of nitrogen-oxygen defects containing only one nitrogen atom (N- On) is still controversial. Motivated by recent infrared absorption measurements, where several new absorption lines were observed, we present first-principles studies on the ground state configuration, binding energy and local vibrational modes of NO and NO2. We suggest that the NO2 defect gives rise to the experimentally observed lines at 1002, 973 and 855 cm-1

  • 76.
    Carvalho, A.
    et al.
    School of Physics, University of Exeter.
    Barker, S.J.
    Centre for Electronic Materials and Devices, Imperial College, London.
    Jones, R.
    School of Physics, University of Exeter.
    Williams, R.S.
    Centre for Electronic Materials and Devices, Imperial College, London.
    Ashwin, M.J.
    Centre for Electronic Materials and Devices, Imperial College, London.
    Newman, R.C.
    Centre for Electronic Materials and Devices, Imperial College, London.
    Stavrinou, P.N.
    Centre for Electronic Materials and Devices, Imperial College, London.
    Parry, G.
    Centre for Electronic Materials and Devices, Imperial College, London.
    Jones, T.S.
    Centre for Electronic Materials and Devices, Imperial College, London.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Identification of the local vibrational modes of small nitrogen clusters in dilute GaAsN2007Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 401-402, s. 339-342Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ultra-high-resolution infra-red local vibrational mode (IR LVM) spectroscopy measurements together with density-functional calculations have been used to identify the signatures of close substitutional nitrogen (NAs) pairs in GaAs1 - x Nx alloys with concentrations of x < 0.025. We show that the presence of sub-peaks close to the NAs absorption band can be attributed to nitrogen pairs up to fourth neighbor position. Additionally, we suggest that the nitrogen pairs which give rise to the deepest levels below the conduction band edge are the first to be removed upon annealing

  • 77.
    Fujita, Naomi
    et al.
    University of Exeter.
    Jones, Robert
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, Patrick
    University of Newcastle Upon Tyne.
    Local vibrational modes of N2-O n defects in Cz-Silicon2007Ingår i: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 18, nr 7, s. 683-687Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper we investigate N2-O n defect complexes in Czochralski silicon (Cz-Si) by means of local density functional theory. We consider the N2O and the N2O2 defect and determine their structural, electronic and vibrational properties. The calculated local vibrational modes of the N2O defect are in good agreement with the experiment. Furthermore the calculated binding energy matches very well with the experimental estimate. Motivated by recent experimental work, where several new absorption lines in IR absorbance spectra were observed, we present first principle studies on the ground state configuration, binding energy and local vibrational modes of the N2O2 defect and make a tentative assignment to the experimentally observed lines at 1018 and 810 cm-1.

  • 78.
    Carvalho, A.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Coutinho, J.
    University of Aveiro.
    Torres, V. J. B.
    University of Aveiro.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Alsina, J. M. Campanera
    University of Sussex.
    Shaw, M.
    University of Newcastle Upon Tyne.
    Briddon, P. R.
    University of Newcastle Upon Tyne.
    Local-density-functional calculations of the vacancy-oxygen center in Ge2007Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, nr 11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We carry out a comprehensive density-functional study of the vacancy-oxygen (VO) center in germanium using large H-terminated Ge clusters. The importance of a nonlinear core correction to account for the involvement of the 3d electrons in Ge-O bonds is discussed. We calculate the electrical levels and the vibrational modes of VO0, VO-, and VO= finding close agreement with experiment. We also explore the reorientation, migration, and dissociation mechanisms of neutral and negatively charged VO and compare the calculated energy barriers with experimental data. We conclude that the defect is likely to anneal through both mechanisms.

  • 79.
    Eberlein, T.A.G.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Blumenau, A.T.
    Max-Planck-Institut für Eisenforschung GmbH.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Movement and pinning of dislocations in SiC2007Ingår i: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 4, nr 8, s. 2923-2928Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    SiC bipolar devices show a degradation under forward-biased operation due to the formation and rapid propagation of stacking faults in the active region of the device. It is believed that the observed rapid stacking fault growth is due to a recombination-enhanced dislocation glide (REDG) mechanism at the bordering partial dislocations having either Si or C core atoms. We investigated the effect of charge on the dislocation kinks and found that only silicon kinks have a deep filled band above the valence band. Trapping of holes into this band permits dislocation glide at room temperature. This mechanism is distinct from REDG as it requires only holes to be trapped at a Si partial and not in addition electrons in stacking fault states. We furthermore looked at the pinning of dislocations by nitrogen and boron and found a strong pinning of the C core by N and of the Si core by B.

  • 80.
    Fujita, N.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Nitrogen related shallow thermal donors in silicon2007Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 91, nr 5, s. 51914-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this letter, the authors investigate the electrical properties of nitrogen related shallow thermal donor (STD) candidates and their concentrations under different doping conditions by means of density functional theory. Experimentally, the existence of STDs containing one nitrogen atom and both even and odd numbers of oxygen atoms has been proposed. However, so far first principles studies have not presented a candidate for the latter. Here, they show that the NO defect possesses a shallow donor level. Adding one or two more oxygen atoms results in the donor level to become shallower. The fraction of shallow nitrogen related donors to N dimers increases in material with low concentration of nitrogen.

  • 81.
    Carvalho, A.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Torres, V. J. B.
    University of Aveiro.
    Coutinho, J.
    University of Aveiro.
    Markevich, V.
    University of Manchester.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P. R.
    University of Newcastle Upon Tyne.
    Oxygen defects in irradiated germanium2007Ingår i: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 18, nr 7, s. 781-786Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The defects present in oxygen-rich irradiated germanium have been extensively characterised from an experimental point of view. Here, we summarise recent theoretical findings obtained using the cluster method and discuss their relation with the experimental data. In order to find a microscopic interpretation of the reactions taking place in this material upon annealing up to 400°C, we performed nudged elastic band (NEB) calculations of the migration and dissociation paths of VO, as well as a modelling of other oxygen-related complexes that are expected to form in this temperature range. Energy barriers of 1.5 (1.1) eV and 1.2 (0.9) eV for the dissociation and migration of the neutral (negatively charged) VO defect are found. We compare these with the activation energies estimated from the analysis of Hall effect, deep level transient spectroscopy (DLTS) and infra-red (IR) spectroscopy annealing data reported in literature.

  • 82.
    Savini, G.
    et al.
    University of Sussex.
    El-Barbary, A.A.
    University of Sussex.
    Heggie, M.I.
    University of Sussex.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Partial dislocations under forward bias in SiC2007Ingår i: Silicon carbide and related materials 2006: ECSCRM 2006 ; proceedings of the 6th European Conference on Silicon Carbide and Related Materials, Newcastle upon Tyne, UK, September 2006 / [ed] Nick Wright; C. Mark Johnson, Stafa-Zürich: Trans Tech Publications Inc., 2007, s. 279-282Konferensbidrag (Refereegranskat)
    Abstract [en]

    First-principles calculations are used to investigate the partial dislocations in 4H-SiC. We have shown that the Peierls barriers are strongly dependent on the dislocation core structures. Our results have revealed that the asymmetric reconstruction does not possess midgap states while the symmetric reconstructions, characterized by dangling bond on like atoms along the dislocation line, are always electrically active. We suggested that under forward bias, the free energies of the symmetric reconstructions are dynamically lowered by continuous electron-hole transitions between the respective deep levels and valence/conduction bands.

  • 83.
    Hounsome, L.S.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Martineau, P.M.
    DTC, Maidenhead.
    Fisher, D.
    DTC, Maidenhead.
    Shaw, M.J.
    School of Natural Science, University of Newcastle upon Tyne.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Role of extended defects in brown colouration of diamond2007Ingår i: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 4, nr 8, s. 2950-2957Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 84.
    Carvalho, A.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Janke, C.
    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.
    Coutinho, J.
    Department of Physics and 13N, University of Aveiro, Aveiro.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Self-interstitial in germanium2007Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 99, nr 17, s. 175502-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Low-temperature radiation damage in n- and p-type Ge is strikingly different, reflecting the charge-dependent properties of vacancies and self-interstitials. We find, using density functional theory, that in Ge the interstitial is bistable, preferring a split configuration when neutral and an open cage configuration when positively charged. The split configuration is inert while the cage configuration acts as a double donor. We evaluate the migration energies of the defects and show that the theory is able to explain the principal results of low-temperature electron-irradiation experiments.

  • 85.
    Carvalho, A.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Goss, J.
    School of Natural Science, University of Newcastle upon Tyne.
    Janke, C.
    School of Physics, University of Exeter.
    Coutinho, J.
    Department of Physics, University of Aveiro.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Self-interstitials and Frenkel pairs in electron-irradiated germanium2007Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 401-402, s. 495-498Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    First principles calculations were used to study the structures and electrical levels of the self-interstitial in Ge. We considered the possibility of structural changes consequent with change in charge state and show these have important implications in the mobility and electrical activity of the defect. The theoretical model is compared to the results of low temperature electron irradiation in germanium reported in the literature.

  • 86.
    Eberlein, T.A.G.
    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.
    Shallow acceptors in GaN2007Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 91, nr 13, s. 132105-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent high resolution photoluminescence studies of high quality Mg doped GaN show the presence of two acceptors. One is due to Mg and the other labeled A1 has a shallower acceptor defect. The authors investigate likely candidates for this shallow acceptor and conclude that CN is the most likely possibility. The authors also show that the CN is passivated by H and the passivated complex is more stable than MgGa-H

  • 87.
    Coutinho, J.
    et al.
    Department of Physics, I3N, University of Aveiro, Campus Santiago.
    Janke, C.
    School of Physics, University of Exeter.
    Carvalho, A.
    School of Physics, University of Exeter.
    Torres, V.J.B.
    Department of Physics, I3N, University of Aveiro, Campus Santiago.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Jones, R.
    School of Physics, University of Exeter.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Strong compensation of n-type Ge via formation of donor-vacancy complexes2007Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 401-402, s. 179-183Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Vacancies and interstitials in semiconductors play a fundamental role in both high-temperature diffusion and low-temperature radiation and implantation damage. In Ge, a serious contender material for high-speed electronics applications, vacancies have historically been believed to dominate most diffusion related phenomena such as self-diffusivity or impurity migration. This is to be contrasted with silicon, where self-interstitials also play decisive roles, despite the similarities in the chemical nature of both materials. We report on density functional calculations of the formation and properties of vacancy-donor complexes in germanium. We predict that most vacancy-donor aggregates are deep acceptors, and together with their high solubilities, we conclude that they strongly contribute for inhibiting donor activation levels in germanium

  • 88.
    Savini, G.
    et al.
    Department of Chemistry, University of Sussex.
    Blumenau, A.T.
    Max-Planck-Institut für Eisenforschung GmbH.
    Heggie, M.I.
    Department of Chemistry, University of Sussex.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Structure and energy of partial dislocations in wurtzite-GaN2007Ingår i: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 4, nr 8, s. 2945-1949Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    First-principle calculations have shown that both the partials can be electrically active. In particular we have shown the Ga(g) core partials are a good candidate for the observed absorption peak at 2.4 eV revealed by energy loss spectroscopy measurements. The symmetric and asymmetric reconstructions have relatively close formation energies. Our results have suggested that the asymmetric reconstructions, characterized by strong bonds along the dislocation line are favourable in intrinsic materials. However, in strongly p and n-type materials or in high stress field the symmetric reconstructions can become energetically more stable. These reconstructions are always electrically active with a deep band across the forbidden gap.

  • 89.
    Janke, C.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Supercell and cluster density functional calculations of the thermal stability of the divacancy in germanium2007Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, nr 19, s. 195208-Artikel i tidskrift (Refereegranskat)
    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.

  • 90. Latham, Chris
    et al.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Louchet, F.
    LGGE, Saint Martin d'Hères.
    A pseudopotential density functional theory study of native defects and boron impurities in FeAl2006Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 18, nr 39, s. 8859-8876Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 91. Hellström, Pär
    et al.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Fredriksson, Andreas
    Holmgren, Allan
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    A theoretical and experimental study of vibrational properties of alkyl xanthates2006Ingår i: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 65, nr 3-4, s. 887-895Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 92.
    Coutinho, J.
    et al.
    Department of Physics, University of Aveiro.
    Torres,, V.J.B.
    Department of Physics, University of Aveiro.
    Carvalho, A.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Ab initio modeling of defect levels in Ge clusters and supercells2006Ingår i: Materials Science in Semiconductor Processing, ISSN 1369-8001, E-ISSN 1873-4081, Vol. 9, nr 4-5, s. 477-483Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Most density-functional studies of defects in semiconductors invariably use (i) a supercell that imitates the host crystal, as well as (ii) a local treatment of the exchange-correlation potential. The first approximation has had an enormous success in many materials, provided that the size of cell is large enough to minimize long-range interactions between the infinite lattice of defects. For instance, these may arise from strain fields or from the overlap between shallow defect states. The second approximation, when combined with the periodic boundary conditions, leads to an essentially metallic density of states in Ge, which can compromise any investigation of electronic transitions involving gap levels. Here, we report on two approaches to surmount these difficulties, namely (i) to open the gap by reducing the host to a Ge cluster of atoms whose states are confined by a surface potential and (ii) to use supercells, but choosing carefully the Brillouin zone sampling scheme, taking k-points that minimize the admixture between defect-related gap states and the host density of states. These methods are utilized in the calculation of the electronic structure of the vacancy, divacancy, and vacancy-donor pairs in Ge

  • 93.
    Ribeiro, R.M.
    et al.
    Centro de Física, Universidade do Minho, Campus de Gualtar.
    Coutinho, J.
    Departamento de Fisica, Universidade de Aveiro.
    Torres, V.J.B.
    Departamento de Fisica, Universidade de Aveiro.
    Jones, R.
    School of Physics, University of Exeter.
    Sque, S.J.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Shaw, M.J.
    School of Natural Science, University of Newcastle upon Tyne.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Ab initio study of CsI and its surface2006Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, nr 3, s. 35430-1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Cesium iodide properties, both in the bulk and on (110) and (100) surfaces, are studied using density functional theory. The bulk lattice constant, bulk modulus, and elastic constants are in good agreement with the experimental values. The electronic band structure is also calculated, as well as the density of states. On the surfaces, relaxation of the atoms was performed, and the atomic geometry and electronic structure have been studied

  • 94.
    Coutinho, J.
    et al.
    Department of Physics, University of Aveiro, Campus Santiago.
    Torres, V.J.B.
    Department of Physics, University of Aveiro, Campus Santiago.
    Jones, R.
    School of Physics, University of Exeter.
    Carvalho, A.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon,, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Calculation of deep carrier traps in a divacancy in germanium crystals2006Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 88, nr 9, s. 91919-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present an ab initio density functional study on the electronic structure and electrical properties of divacancies in Ge. Although suffering essentially different Jahn-Teller distortions when compared to the analogous defect in Si, the relative location of the electrical levels in the gap does not differ radically in both materials. We propose a V2 model that is responsible for a donor level at Ev+0.03 eV, a first acceptor state at Ev+0.3 eV, and a second acceptor level at Ec-0.4 eV. The latter is only 0.1 eV deeper than an electron trap that has been recently linked to a divacancy in proton implanted material.

  • 95.
    Eberlein, T.A.G.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Density functional theory calculation of the DI optical center in SiC2006Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, nr 14, s. 144106-1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The DI center is a prominent defect which is detected in as-grown or irradiated SiC. It is unusual in that its intensity grows with heat treatments and survives anneals of 1700 °C. It has been assigned recently to either a close-by antisite pair or to the close-by antisite pair adjacent to a carbon antisite. We show here using local density functional calculations that these defects are not stable enough to account for DI. Instead, we assign DI to an isolated Si antisite and the four forms of the close-by antisite pair in 4H-SiC to the a, b, c, and d members of the alphabet series. The assignments allow us to understand the concentration of DI following growth, the recombination enhanced destruction of these alphabet defects and the annealing behavior of the remaining members of the series.

  • 96.
    Coutinho, J.
    et al.
    Department of Physics, University of Aveiro, Campus Santiago.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Torres, S.
    Department of Physics, University of Aveiro, Campus Santiago.
    Barroso, M.
    Department of Physics, University of Aveiro, Campus Santiago.
    Jones, R.
    School of Physics, University of Exeter.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Donor-vacancy complexes in Ge: cluster and supercell calculations2006Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, nr 23, s. 235213-1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a comprehensive spin-density functional modeling study of the structural and electronic properties of donor-vacancy complexes (PV, AsV, SbV, and BiV) in Ge crystals. Special attention is paid to spurious results which are related to the choice of the boundary conditions (supercell-cluster approach), the resulting band-gap width, and the choice of the points to sample the Brillouin zone. The underestimated energy gap, resulting from the periodic conditions together with the local-density approximation to the exchange-correlation energy, leads to defect-related gap states that are strongly coupled to crystalline states within the center of the zone. This is shown to produce a strong effect even on relative energies. Our results indicate that in all E centers the donor atom occupies a nearly substitutional site, as opposed to the split-vacancy form adopted by the SnV complex in Si. The E centers can occur in four charge states, from positive to double negative, and produce occupancy levels at E(0/+)=Ev+0.1 eV, E(-/0)=Ev+0.3 eV, and E(=/-)=Ec-0.3 eV.

  • 97.
    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 tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    E center in silicon has a donor level in the band gap2006Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 97, nr 10, s. 106402/1-106402/4Artikel i tidskrift (Refereegranskat)
    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.

  • 98.
    Torres, V.J.B.
    et al.
    Department of Physics, University of Aveiro, Campus Santiago.
    Coutinho, J.
    Department of Physics, University of Aveiro, Campus Santiago.
    Jones, R.
    School of Physics, University of Exeter.
    Barroso, M.
    Department of Physics, University of Aveiro, Campus Santiago.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Early SiO2 precipitates in Si: vacancy-oxygen versus interstitial-oxygen clusters2006Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 376-377, nr 1, s. 109-112Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Oxygen precipitation in Si strongly depends on the undergoing thermal treatment. Between 350 and 450 °C thermal donor formation is activated by a 1.4-1.6 eV barrier. On the other hand, at T>500°C, SiO2 cluster formation is limited by the interstitial oxygen (Oi) migration barrier of ∼2.5eV. Volumetric arguments imply that the formation of silica precipitates during anneals of oxygen-rich Si crystals, must be accompanied by the ejection of approximately one Si self-interstitial (Sii) per SiO 2 unit that is formed. We report on ab-initio density-functional studies of small oxygen aggregates in Si, to show that the On→VOn+Sii reaction is exothermic for n≥4. The large energy barrier required to form an intermediate Sii defect prevents the formation of VOn complexes at temperatures as low as 450 °C. Our results imply that thermal donors are not thermodynamically stable clusters, and their formation is driven by kinetics. Infra-red absorption studies can discriminate VOn and On defects. We report their local vibrational modes and compare them with the available experimental data

  • 99.
    Eberlein, T.A.G.
    et al.
    School of Physics, University of Exeter.
    Blumenau, A.T.
    Max-Planck-Institut für Eisenforschung GmbH.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Jones, R.
    School of Physics, University of Exeter.
    Effect of charge on the movement of dislocations in SiC2006Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 88, nr 8, s. 82113-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    SiC bipolar devices show a degradation under forward-biased operation which has been linked with a current induced motion of one of the two glide dislocations having either Si or C core atoms. We have carried out calculations of the core structures and dynamics of partial dislocations in 3C and 2H-SiC. In this work we present results on the effect of charge on the dislocation kinks. The calculations show that silicon kinks have a deep filled band above the valence band and the trapping of holes into this band permits motion at room temperature.

  • 100.
    Bangert, U.
    et al.
    School of Physics, University of Exeter.
    Barnes, R.
    Lunar & Planetary Lab., Arizona University, Tucson.
    Hounsome, L.S.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Blumenau, A.T.
    Max-Planck-Institut für Eisenforschung GmbH.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Shaw, M.J.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Electron energy loss spectroscopic studies of brown diamonds2006Ingår i: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 86, nr 29-31, s. 4757-4779Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate both experimentally and theoretically, low-loss electron energy losses in brown type IIa monocrystalline diamonds both before and after high-temperature, high-pressure anneals which remove the brown colouration. We find additional losses within and near the band edge for brown diamond which are significantly reduced after treatment. The additional losses are not associated with dislocations. Graphitic inclusions are detected by EELS as well as TEM studies for some brown diamonds before treatment. These lead to pronounced subgap absorption. However, all brown diamonds exhibit additional losses which are due to point defects lying in the regions between dislocations. First principles theoretical modelling shows that common dislocations are not responsible for the brown colouration but a -bonded vacancy disk lying on {111} planes gives broad bands lying in the diamond band gap, possesses an optical absorption spectrum similar to that of brown diamond, and leads to additional electron energy losses in the band edge region. These and similar defects are suggested to be responsible for the brown colouration. Mechanisms are proposed for their formation and removal.

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