| 1. |
- Bultmark, Fredrik, 1973-
(författare)
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Distorted Space and Multipoles in Electronic Structure Calculations
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns methods for electronic structure calculations and some applications of the methods. The augmented planewave (APW) basis set and it’s relatives LAPW (linearised APW) and APW+lo (local orbitals) have been widely used for electronic structure calculations. Here a modification of the APW basis set based on a transformation of the basis functions from a curvilinear coordinate system. Applications to a few test systems show that the modified basis set may speed up electronic structure calculations of sparse systems. The local density approximation (LDA) is used in density functional theory. Although it is the simplest possible approximation possible for the unknown exchange-correlation energy functional, it has proven to give quite accurate results for a wide range of systems. LDA fails in systems where the non-local effects are important. By including non-local effects by adding an orbital dependent term to the energy functional, through for example the LDA+U method, the calculated properties of many materials are closer to experimental observations. In the thesis the most general formulation of the LDA+U method is presented and a new way of interpreting the results of a calculations by formulating the orbital dependent part of the energy functional in terms of multipole momentum tensors. Applications to some early actinide systems leads to a reformulations of Hund’s rules for polarisations associated with the spin and orbital magnetic moment and a suggestion for similar rules, Katt’s rules, valid in the strong spin orbit coupling regime.
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| 2. |
- Castleton, Christopher, 1969-, et al.
(författare)
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Managing the supercell approximation for charged defects in semiconductors : Finite-size scaling, charge correction factors, the band-gap problem, and the ab initio dielectric constant
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 73:035215, s. 11-
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Tidskriftsartikel (refereegranskat)abstract
- The errors arising in ab initio density functional theory studies of semiconductor point defects using the supercell approximation are analyzed. It is demonstrated that (a) the leading finite size errors are inverse linear and inverse cubic in the supercell size and (b) finite size scaling over a series of supercells gives reliable isolated charged defect formation energies to around +-0.05 eV. The scaled results are used to test three correction methods. The Makov-Payne method is insufficient, but combined with the scaling parameters yields an ab initio dielectric constant of 11.6+-4.1 for InP. Gamma point corrections for defect level dispersion are completely incorrect, even for shallow levels, but realigning the total potential in real-space between defect and bulk cells actually corrects the electrostatic defect-defect interaction errors as well. Isolated defect energies to +-0.1 eV are then obtained using a 64 atom supercell, though this does not improve for larger cells. Finally, finite size scaling of known dopant levels shows how to treat the band gap problem: in < or = 200 atom supercells with no corrections, continuing to consider levels into the theoretical conductin band (extended gap) comes closest to experiment. However, for larger cells or when supercell approximation errors are removed, a scissors scheme stretching the theoretical band gap onto the experimental one is in fact correct.
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| 3. |
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| 4. |
- Castleton, C. W. M., et al.
(författare)
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Density functional theory calculations of defect energies using supercells
- 2009
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Ingår i: Modelling and Simulation in Materials Science and Engineering. - : IOP Publishing. - 0965-0393 .- 1361-651X. ; 17:8, s. 084003-
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Tidskriftsartikel (refereegranskat)abstract
- Reliable calculations of defect properties may be obtained with density functional theory (DFT) using the supercell approximation. We systematically review the known sources of error and suggest how to perform calculations of defect properties in order to minimize errors. We argue that any analytical error-correction scheme relying on electrostatic considerations alone is not appropriate to derive reliable defect formation energies, certainly not for relaxed geometries. Instead we propose finite size scaling of the calculated defect formation energies, and compare the application of this with both fully converged and 'Gamma' (Gamma) point only k-point integration. We provide a recipe for practical DFT calculations which will help to obtain reliable defect formation energies and demonstrate it using examples from III-V semiconductors.
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| 5. |
- Dziekan, Thomas, 1975-
(författare)
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Electronic Transport in Strained Materials
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis the conductivity of strained materials has been investigated using density functional theory and a semiclassical transport theory based on the Boltzmann equation.In transition metals trends are reproduced without adjustable parameters. The introduction of one temperature dependent cross section allowed the reproduction of resistivity trends between 10 and 1000K.The effect of strain on transition metals in bcc and fcc structure was studied deforming the unit cell along the tetragonal deformation path. The anisotropy of the conductivity varied on wide range of the c/a-ratio. The orbitals at the Fermi level determined the principal behavior. Pairs of elements with permutated number of electrons and holes in the 4d band showed similar behavior. The concept of the tetragonal deformation was also applied on semiconductors.The deformation of Vanadium in X/V superlattices (X=Cr,~Fe,~Mo) due to Hydrogen loading depends on the properties of X. It was found that counteracting effects due to the presence of Hydrogen influence the conductivity.It is shown that a small magnetic moment of the V host reduces the hydrogen solubility. Depending on the magnitude of the tetragonal distortion of V, the hydrogen dissolution becomes favored for larger moments.Finally, extra charge filling of the bandstructure of Cr and Mo decreases the Fermi velocity and increases the density of states at the Fermi energy.
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| 6. |
- Dziekan, Thomas, et al.
(författare)
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Influence of vanadium spin-polarization on the dissolution of hydrogen in vanadium
- 2009
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Ingår i: PHYSICAL REVIEW B. - 1098-0121 .- 1550-235X. ; 79:1, s. 012402-
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Tidskriftsartikel (refereegranskat)abstract
- Transition-metal (TM)/V superlattices (TM=Cr,Fe,Mo) show certain peculiarities under hydrogen uptake. Here we investigate the influence of an induced magnetization of the V layers on the hydrogen dissolution by means of first-principles calculations. We find that below a certain value for the magnetic moment of the V host the hydrogen solubility is slightly reduced, whereas for larger moments the hydrogen dissolution becomes favored. The actual position of this transition depends on the tetragonal distortion of the V layers.
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| 7. |
- Dziekan, Thomas, et al.
(författare)
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Theoretical calculations of mobility enhancement in strained silicon
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:19, s. 195213-
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Tidskriftsartikel (refereegranskat)abstract
- The conductivity enhancement of n -doped Si due to strain has been calculated by solving the Boltzmann equation within the relaxation-time approximation. For T=0 , we determined the Fermi surface from first principles, whereas for T>0 , we modeled the Fermi surface using the effective-mass approximation. We explain the saturation of the conductivity enhancement with increasing strain in terms of the topology of the Fermi surface. We find in accordance with experiment a decreasing enhancement with increasing doping concentration.
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| 8. |
- Hoglund, A., et al.
(författare)
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Breakdown of cation vacancies into anion vacancy-antisite complexes on III-V semiconductor surfaces
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:15
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Tidskriftsartikel (refereegranskat)abstract
- An asymmetric defect complex originating from the cation vacancy on (110) III-V semiconductor surfaces which has significantly lower formation energy than the ideal cation vacancy is presented. The complex is formed by an anion from the top layer moving into the vacancy, leaving an anion antisite-anion vacancy defect complex. By calculating the migration barrier, it is found that any ideal cation vacancies will spontaneously transform to this defect complex at room temperature. For stoichiometric semiconductors the defect formation energy of the complex is close to that of the often-observed anion vacancy, giving thermodynamic equilibrium defect concentrations on the same order. The calculated scanning tunneling microscopy (STM) plot of the defect complex is also shown to be asymmetric in the [1(1)over bar0] direction, in contrast to the symmetric one of the anion vacancy. This might therefore explain the two distinct asymmetric and symmetric vacancy structures observed experimentally by STM.
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| 9. |
- Höglund, Andreas, et al.
(författare)
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Diffusion mechanism of Zn in InP and GaP from first principles
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:11, s. 113201-
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Tidskriftsartikel (refereegranskat)abstract
- The diffusion mechanism of Zn in GaP and InP has been investigated using first-principles computational methods. It is found that the kickout mechanism is the favored diffusion process under all doping conditions for InP, and under all except n-type conditions for GaP. In n-type GaP the dissociative mechanism is probable. In both p-type GaP and InP, the diffusing species is found to be Zn. The activation energy for the kickout process is 2.49 eV in GaP and 1.60 eV in InP, and therefore unintentional diffusion of Zn should be a larger concern in InP than in GaP. The dependence of the activation energy both on the doping conditions of the material and on the stoichiometry is explained, and found to be in qualitative agreement with the experimentally observed dependencies. The calculated activation energies agree reasonably with experimental data, assuming that the region from which Zn diffuses is p type. Explanations are also found as to why Zn tends to accumulate at pn junctions in InP and to why a relatively low fraction of Zn is found on substitutional sites in InP.
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| 10. |
- Höglund, Andreas, 1977-
(författare)
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Electronic Structure Calculations of Point Defects in Semiconductors
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis point defects in semiconductors are studied by electronic structure calculations. Results are presented for the stability and equilibrium concentrations of native defects in GaP, InP, InAs, and InSb, for the entire range of doping conditions and stoichiometry. The native defects are also studied on the (110) surfaces of InP, InAs, and InSb. Comparing the relative stability at the surface and in the bulk, it is concluded that the defects have a tendency to migrate to the surface. It is found that the cation vacancy is not stable, but decomposes into an anion antisite-anion vacancy complex. The surface charge accumulation in InAs is explained by complementary intrinsic doping by native defects and extrinsic doping by residual hydrogen. A technical investigation of the supercell treatment of defects is performed, testing existing correction schemes and suggesting a more reliable alternative. It is shown that the defect level of [2VCu-IIICu] in the solarcell-material CuIn1-xGaxSe2 leads to a smaller band gap of the ordered defect γ-phase, which possibly explains why the maximal efficiency for CuIn1-xGaxSe2 has been found for x=0.3 and not for x=0.6, as expected from the band gap of the α-phase. It is found that Zn diffuses via the kick-out mechanism in InP and GaP with activation energies of 1.60 eV and 2.49 eV, respectively. Explanations are found for the tendency of Zn to accumulate at pn-junctions in InP and to why a relatively low fraction of Zn is found on substitutional sites in InP. Finally, it is shown that the equilibrium solubility of dopants in semiconductors can be increased significantly by strategic alloying. This is shown to be due to the local stress in the material, and the solubility in an alloy can in fact be much higher than in either of the constituting elements. The equilibrium solubility of Zn in Ga0.9In0.1P is for example five orders of magnitude larger than in GaP or InP.
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