Browsing by Author "Nieminen, Risto M."
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Item Ab initio study of Cu diffusion in alpha-cristobalite(IOP Publishing, 2012) Zelený, M.; Hegedüs, J.; Foster, Adam S.; Drabold, D.A.; Elliott, S.R.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceWe have studied the geometries, formation energies, migration barriers and diffusion of a copper interstitial with different charge states with and without an external electric field in the α-cristobalite crystalline form of SiO2 using ab initio computer simulation. The most stable state almost throughout the band gap is charge q = + 1. The height of the migration barrier depends slightly on the charge state and varies between 0.11 and 0.18 eV. However, the charge has a strong influence on the shape of the barrier, as metastable states exist in the middle of the diffusion path for Cu with q = + 1. The heights and shapes of barriers also depend on the density of SiO2, because volume expansion has a similar effect to increase the positive charge on Cu. Furthermore, diffusion coefficients have been deduced from our calculations according to transition-state theory and these calculations confirm the experimental result that oxidation of Cu is a necessary condition for diffusion. Our molecular dynamics simulations show a similar ion diffusion, and dependence on charge state. These simulations also confirm the fact that diffusion of ions can be directly simulated using ab initio molecular dynamics.Item Ab initio study of fully relaxed divacancies in GaAs(American Physical Society (APS), 1996) Pöykkö, S.; Puska, Martti J.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceWe report calculations of the electronic and atomic structures of neutral and charged divacancies in GaAs using the first-principles Car-Parrinello method. It is found that the divacancy relaxes inwards in all charge states (2-,1-,0,1+) studied. The defect-induced electron levels lie in the lower half of the fundamental band gap. The doubly negative divacancy is the most stable one for nearly all values of the electron chemical potential within the band gap. The deep-level electron density is localized at the Ga-vacancy end of the divacancy and the ionic relaxation is stronger there than at the As-vacancy end. We have also calculated the thermodynamic concentrations for several different native defects in GaAs, and the implications for self-diffusion are discussed.Item Ab initio study of gamma-Al2O3 surfaces(American Physical Society (APS), 2004) Pinto, Henry P.; Nieminen, Risto M.; Elliott, Simon D.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceStarting from the theoretical prediction of the γ−Al2O3 structure using density-functional theory in the generalized gradient approximation, we have studied the (1 1 1), (0 0 1), (1 1 0), and (1 5 0) surfaces. The surface energies and their corresponding structures are computed and compared with predictions for (0 0 0 1) α−Al2O3 and available experimental results for γ-alumina surfaces. (1 1 1) and (0 0 1) surfaces are predicted to be equally stable, but to show quite different structure and reactivity. Whereas a low coverage of highly reactive trigonal Al occurs on (1 1 1), (0 0 1) exhibits a more dense plane of both five-coordinate and tetrahedral Al. Microfaceting of a (1 1 0) surface into (1 1 1)-like planes is also observed. The implications for the structure of ultrathin dielectric films and for the surfaces of disordered transition aluminas are discussed.Item Ab initio study of oxygen point defects in GaAs, GaN, and AlN(American Physical Society (APS), 1996) Mattila, T.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe have studied oxygen point defects with the plane-wave pseudopotential method in GaAs, GaN, and AlN. The calculations demonstrate a qualitatively different behavior of oxygen impurities in these materials. OAs in GaAs acts as a deep center with an off-center displacement and negative-U behavior, in agreement with the experimental data. ON in GaN is found to be a shallow donor with a low formation energy, and is suggested to act as a partial source for the unintentional n-type conductivity commonly observed in GaN. O in AlN is also found to easily substitute for N, which is consistent with the experimentally observed large oxygen concentrations in AlN. However, ON in AlN is shown to be a deep center due to the wide band gap, in contrast with ON in GaN. Our calculations thus predict that isolated oxygen acts as a DX-type center in AlxGa1−xN alloys. Results for other oxygen point defect configurations and for the dominant native defects are also presented. © 1996 The American Physical Society.Item Ab initio study of point defects in CdF2(American Physical Society (APS), 1997) Mattila, T.; Pöykkö, S.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceThe plane-wave pseudopotential method is used to study point defects in CdF2. We present comprehensive results for the native defects as well as for dominant impurities. In addition to Fi, VCd and OF were found to be easily formed compensating acceptors. For In and Ga impurities the experimentally observed large Stokes shift could not be established, and the results rule out symmetric atomic relaxation as the mechanism leading to the bistable behavior. The limitations of the present approach utilizing density-functional theory and the local-density approximation in the case of ionic materials are addressed.Item Ab initio study of γ−Al2O3 surfaces(2004) Pinto, Henry; Nieminen, Risto M.; Eliot, S.D.; Department of Applied PhysicsStarting from the theoretical prediction of the γ−Al2O3 structure using density-functional theory in the generalized gradient approximation, we have studied the (1 1 1), (0 0 1), (1 1 0), and (1 5 0) surfaces. The surface energies and their corresponding structures are computed and compared with predictions for (0 0 0 1) α−Al2O3 and available experimental results for γ-alumina surfaces. (1 1 1) and (0 0 1) surfaces are predicted to be equally stable, but to show quite different structure and reactivity. Whereas a low coverage of highly reactive trigonal Al occurs on (1 1 1), (0 0 1) exhibits a more dense plane of both five-coordinate and tetrahedral Al. Microfaceting of a (1 1 0) surface into (1 1 1)-like planes is also observed. The implications for the structure of ultrathin dielectric films and for the surfaces of disordered transition aluminas are discussed.Item Adsorption and migration of carbon adatoms on carbon nanotubes: Density-functional ab initio and tight-binding studies(American Physical Society (APS), 2004) Krasheninnikov, A. V.; Nordlund, K.; Lehtinen, P. O.; Foster, Adam S.; Ayuela, A.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe employ density-functional plane-wave ab initio and tight-binding methods to study the adsorption and migration of carbon adatoms on single-walled carbon nanotubes. We show that the adatom adsorption and migration energies strongly depend on the nanotube diameter and chirality, which makes the model of the carbon adatom on a flat graphene sheet inappropriate. Calculated migration energies for the adatoms agree well with the activation energies obtained from experiments on annealing of irradiation damage in single-walled nanotubes and attributed to single carbon interstitials.Item Adsorption of acetic and trifluoroacetic acid on the TiO2(110) surface(AIP Publishing, 2004) Foster, Adam S.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceWe use the first-principles static and dynamic simulations to study the adsorption of acetic (CH3COOH) and trifluoroacetic (CF3COOH)acid on the TiO2(110)surface. The most favorable adsorption for both molecules is a dissociative process, which results in the two oxygens of the carboxylate ion bonding to in-plane titanium atoms in the surface. The remaining proton then bonds to a bridging oxygen site, forming a hydroxyl group. We further show that, by comparing the calculated dipoles of the molecules on the surface, it is possible to understand the difference in contrast over the acetate and trifluoroacetate molecules in the atomically resolved noncontact atomic force microscopy images.Item Agglomeration of As Antisites in As-Rich Low-Temperature GaAs: Nucleation without a Critical Nucleus Size(American Physical Society (APS), 2005) Staab, T. E. M.; Nieminen, Risto M.; Luysberg, M.; Frauenheim, Th.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceTo investigate the early stages of nucleation and growth of As precipitates in GaAs grown at low substrate temperature, we make use of a self-consistent-charge density-functional based tight-binding method. Since a pair of As antisites already shows a significant binding energy which increases when more As antisites are attached, there is no critical nucleus size. Provided that all excess As has precipitated, the clusters may grow in size since the binding energies increase with increasing agglomeration size. These findings close the gap between experimental investigation of point defects and the detection of nanometer-size precipitates in transmission electron microscopy.Item Aggregation Kinetics of Thermal Double Donors in Silicon(American Physical Society (APS), 2001) Lee, Young Joo; von Boehm, J.; Pesola, M.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceA general kinetic model based on accurate density-functional-theoretic total-energy calculations is introduced to describe the aggregation kinetics of oxygen-related thermal double donors (TDD's) in silicon. The calculated kinetics, which incorporates the reactions of associations, dissociations, and isomerizations of all relevant oxygen complexes, is in agreement with experimental annealing studies. The aggregation of TDD's takes place through parallel-consecutive reactions where both mobile oxygen dimers and fast migrating chainlike TDD's capture interstitial oxygen atoms.Item Angular distribution of positrons emitted from metal surfaces(American Physical Society (APS), 1988) Neilson, D.; Nieminen, Risto M.; Szymanski, J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceThe effect of inelastic scattering processes on the angular distribution of positrons spontaneously emitted from metal surfaces is investigated. Angular and energy spectra are calculated for positrons suffering energy loss due to electron-hole excitation in emission from materials with low (aluminum) and high (tungsten) positron work functions.Item Antisites in silicon carbide(American Physical Society (APS), 1998) Torpo, L.; Pöykkö, S.; Nieminen, Risto M.; Neurotieteen ja lääketieteellisen tekniikan laitos; Department of Neuroscience and Biomedical Engineering; Perustieteiden korkeakoulu; School of ScienceTen years ago, deep-level-transient-spectroscopy (DLTS) signals, assigned to centers labeled as H1, H2, H3, and E2, have been detected in neutron-irradiated 3C SiC. The H centers were believed to be the primary point defects and the E2 center a secondary defect, which forms after the H centers start to migrate. A conclusive identification of these signals has not been presented so far. We present computational evidence that the H centers are due to silicon antisite defects (SiC). In both cubic (3C) and hexagonal (2H) polytypes, the silicon antisite has several ionization levels in the band gap. The positions of these ionization levels in 3C SiC have been calculated accurately with the plane wave pseudopotential method using a large 128-atom site supercell, and compared with the DLTS spectrum. A very good agreement with experimental data indicates that H centers are due to the formation of SiC during neutron irradiation. The formation energies and local geometries of the antisite defects in SiC are also reported.Item Apparent activation energy during surface evolution by step formation and flow(IOP Publishing, 2006) Gosálvez, M. A.; Cheng, D.; Nieminen, Risto M.; Sato, K.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceDuring growth and etching by step flow as examples of anisotropic surface processing the apparent activation energy of the growth/etch rate depends on orientation, increasing gradually as a principal, terrace-rich surface is approached. This behaviour is traditionally explained as a change in the dominating process from step propagation to island/pit nucleation. We show that the orientation dependence of the activation energy is actually the result of a traditionally disregarded temperature dependence in the number of active step sites and is not attributable to an increasing role of step nucleation nor to a purely geometrical decrease in the number of step sites. This modifies the traditional picture of the apparent energy for a principal surface and explains how the energy can be higher than, equal to or even lower than that for vicinal orientations.Item Atomic-scale modeling of the ion-beam-induced growth of amorphous carbon(American Physical Society (APS), 2000) Kaukonen, M.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceThe results of a detailed molecular-dynamics study of the growth of amorphous carbon (a−C) are reported. Carbon atoms with kinetic energies between 10 and 150 eV are deposited on a−C surface originating from bulk a−C. Earlier simulation results of an optimal energy window at 40–70 eV are confirmed. Additionally, it is found that the growth rate is at maximum at around 40 eV. At low implantation energies (Ebeam≈10 eV), the growth of amorphous carbon takes place on the surface. At higher energies, the growth proceeds increasingly in the subsurface region by global film expansion and single atom diffusion towards the surface. Scattering events (e.g., the deposited atom does not adsorb to the surface) at intermediate energies Ebeam≈100 eV result in a densification of the growing film. Moreover, at Ebeam≈150 eV, nonpermanent diamond formation is observed.Item Atoms embedded in an electron gas: Beyond the local-density approximation(American Physical Society (APS), 1991) Puska, Martti J.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceThe chemical-binding properties of atoms belonging to the first three rows of the Periodic Table are studied within the atom-in-jellium model. The electronic structures are solved self-consistently using the density-functional theory. The extraction of the binding properties is carried out in the framework of the effective-medium theory. The emphasis is put on the systematic investigation of the trends along the 2p, 3p, and 3d series and on the effects due to different types of approximations for electron exchange and correlation. More specifically, in addition to the popular local-density approximation, the self-interaction-correction scheme and the generalized-gradient approximation are employed. The results provide insight into why the local-density approximation for solids (molecules or chemisorption systems) overestimates the cohesive (binding) energies but gives the lattice constants (bond lengths) and bulk moduli (vibration frequencies) rather well. The results obtained are also important because they give the basic parameters for the effective-medium theory, which is a versatile approximative method for calculating total energies of systems with many interacting atoms.Item Atoms embedded in an electron gas: Immersion energies(American Physical Society (APS), 1981) Puska, Martti J.; Nieminen, Risto M.; Manninen, M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceEnergies of atoms, H through Ar, embedded in a homogeneous electron gas are calculated within the density-functional scheme as a function of the electron-gas density. The energy-versus-density curves and the induced densities of states are analyzed and discussed in terms of the interaction properties of an atom with its environment. The low-density limit of the immersion energy is related to the electron-atom scattering length. The results should prove useful in detailed investigations of the recently suggested "quasiatom" or "effective-medium" approaches to chemical binding. The lowest-order estimates of the binding energies of diatomic molecules and chemisorbed atoms are obtained.Item Atoms embedded in an electron gas: Phase shifts and cross sections(American Physical Society (APS), 1983) Puska, Martti J.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceThe Fermi-level scattering phase shifts and the transport cross sections are reported for atoms embedded in a homogeneous electron gas. The applications of the results are discussed, using the electronic stopping power for slow ions and impurity resistivity as examples.Item B and N ion implantation into carbon nanotubes: Insight from atomistic simulations(American Physical Society (APS), 2005) Kotakoski, J.; Krasheninnikov, A. V.; Ma, Yuchen; Foster, Adam S.; Nordlund, K.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceBy employing atomistic computer simulations with empirical potential and density functional force models, we study B/N ion implantation onto carbon nanotubes. We simulate irradiation of single-walled nanotubes with B and N ions and show that up to 40% of the impinging ions can occupy directly the sp2 positions in the nanotube atomic network. We further estimate the optimum ion energies for direct substitution. Ab initio simulations are used to get more insight into the structure of the typical atomic configurations which appear under the impacts of the ions. As annealing should further increase the number of sp2 impurities due to dopant atom migration and annihilation with vacancies, we also study migration of impurity atoms over the tube surface. Our results indicate that irradiation-mediated doping of nanotubes is a promising way to control the nanotube electronic and even mechanical properties due to impurity-stimulated crosslinking of nanotubes.Item Band offsets at the GaInP/GaAs heterojunction(AIP Publishing, 1997) Lindell, A.; Pessa, M.; Salokatve, A.; Bernardini, F.; Nieminen, Risto M.; Paalanen, M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe have measured current–voltage curves and the temperature dependence of the zero bias conductance for a p -type Be-doped GaInP/GaAs heterojunction grown by the molecular beam epitaxy method. We have determined the valence band offset ΔEν from both measurements and find it to be 310 meV within 5% of accuracy. Similarly, we find for an n -type Si-doped sample that the conduction band offset ΔEC is 95 meV. First-principles calculations have been carried out for the atomic and electronic structures of the interfaces. For the thermodynamically favored interfaces, the valence band offset is found not to be sensitive to atomic relaxations at the interface. The calculated values are in good agreement with the experiments.Item Behavior of a magnetic impurity in graphene in the presence of a vacancy(American Physical Society (APS), 2012) Hu, F. M.; Gubernatis, J. E.; Lin, Hai-Qing; Li, Yan-Chao; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWith quantum Monte Carlo methods, we investigate the consequences of placing a magnetic adatom adjacent to a vacancy in a graphene sheet. We find that instead of the adatom properties depending on the energy of the adatom orbital, as in a single-impurity problem, they develop a dependence on the energy of the split localized state associated with the single-vacancy problem. Shifting the chemical potential through this experimentally more accessible energy scale reveals unique behavior in the spectral density, magnetic susceptibility, and the correlations of the adatom spin and charge with those of the conduction electrons. In general, the behavior of the adatom in the presence of a vacancy differs significantly from its behavior in the absence of a vacancy.
