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- 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. |
- Lindström, Anna, 1987-
(författare)
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Defects and Impurities in CdTe : An ab Initio Study
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis defects and impurities in CdTe have been studied with ab initio methods. CdTe is a II-VI semiconductor with many important applications such as γ- and X-ray detectors, solar cells and medical imaging. Even though CdTe has been studied for more than 70 years, some of its properties connected with defects and impurities, are still shrouded in mystery. Todays experimental techniques are highly developed and can provide rather detailed data, but require elaborate theoretical analysis. Here ab initio modelling comes into play and in particular density functional theory (DFT). When reviewing different theoretical studies of defects and impurities in CdTe, one finds a vast number of discrepancies between experiment and theory. Mismatches appear even between different theoretical studies. Although many problems, such as, for example, the semiconductor band gap underestimation or the spurious interaction between charged defects, are avoided by employing corrections or implementing new functionals, some of them still remain. Employing the hybrid functional HSE06, the following topics were studied in this thesis:- Te antisites: Experimental data predict the defect state to appear in the middle of the band gap, thus "pinning" the Fermi level. In contrast, our calculations show that Te antisite alone cannot be the reason for the Fermi level pinning, since it does not form a defect level in the middle of the band gap. Instead we propose that charge compensation between Te antisites in a (+2) state and Cd vacancies in a (-2) state explains the Fermi level pinning.- Cd vacancy: Electron paramagnetic resonance experiments clearly show the existence of a hole polaron for the (-1) charged vacancy. But DFT studies report a completely delocalised hole. In our studies, for the first time, this state was found in its proper geometrical configuration with a hole localisation stabilised by a Jahn-Teller distortion, thereby removing the discrepancy between experiment and theory.- Cd chalcogenides: Additionally, with particular focus on the hole localisation problem, the series of isovalent compounds (CdTe, CdSe and CdS) was studied to understand the mechanism of hole polaron formation. We explain the trend of the hole localisation in terms of Coulomb interaction, explicitly showing that the effect of electron correlation is negligible.- Cl-doped CdTe: The formation of a Cl - Cd vacancy complex explains the selfcompensation and selfpurification mechanism. We find Cl to annihilate the hole polaron.- Te antisite under deformation: In an attempt to tailor the energy position of the Te antisite defect level in the CdTe band gap, we studied CdTe under different deformations. It is shown that by a carefully chosen deformation the defect levels can be pushed closer to the valence and/or conduction band and hence the CdTe detector performance may be improved.
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| 3. |
- Tholander, Christopher, 1986-
(författare)
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A Theoretical Study of Piezoelectricity, Phase Stability, and Surface Diffusion in Disordered Multicomponent Nitrides
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Disordered multicomponent nitride thin film can be used for various applications. The focus of this Licentiate Thesis lies on the theoretical study of piezoelectric properties, phase stability and surface diffusion in multifunctional hard coating nitrides using density functional theory (DFT).Piezoelectric thin films show great promise for microelectromechanical systems (MEMS), such as surface acoustic wave resonators or energy harvesters. One of the main benefits of nitride based piezoelectric devices is the much higher thermal stability compared to the commonly used lead zirconate titanate (PZT) based materials. This makes the nitride based material more suitable for application in, e.g., jet engines.The discovery that alloying AlN with ScN can increase the piezoelectric response more than 500% due to a phase competition between the wurtzite phase in AlN and the hexagonal phase in ScN, provides a fundamental basis for constructing highly responsive piezoelectric thin films. This approach was utilized on the neighboring nitride binaries, where ScN or YN was alloyed with AlN, GaN, or InN. It established the general role of volume matching the binaries to easily achieve a structural instability in order to obtain a maximum increase of the piezoelectric response. For Sc0.5Ga0.5N this increase is more than 900%, compared to GaN. Y1-xInxN is, however, the most promising alloy with the highest resulting piezoelectric response seconded only by Sc0.5Al0.5N.Phase stability and lattice parameters (stress-strain states) of the Y1-xAlxN alloy have been calculated in combination with experimental synthesis.Hard protective coatings based on nitride thin films have been used in industrial applications for a long time. Two of the most successful coatings are TiN and the metastable Ti1-xAlxN. Although these two materials have been extensively investigated both experimentally and theoretically, at the atomic level little is known about Ti1-xAlxN diffusion properties. This is in large part due to problems with configurational disorder in the alloy, because Ti and Al atoms are placed randomly at cation positions in the lattice, considerably increasing the complexity of the problem. To deal with this issues, we have used special quasi-random structure (SQS) models, as well as studying dilute concentrations of Al.One of the most important mechanisms related to the growth of Ti1-xAlxN is surface diffusion. Because Ti1-xAlxN is a metastable material it has to be grown as a thin film with methods such as physical vapor deposition (PVD), in which surface diffusion plays a pivotal role in determining the microstructure evolution of the film.In this work, the surface energetics and mobility of Ti and Al adatoms on a disordered Ti0.5Al0.5N(001) surface are studied. Also the effects on the adatom energetics of Ti, Al, and N by the substitution of one Ti with an Al surface atom in TiN(001), TiN(011), and TiN(111) surfaces is studied. This provides an indepth atomistic understanding of how the energetics behind surface diffusion changes as TiN transitions into Ti0.5Al0.5N.The investigations revealed many interesting results. i) That Ti adatom mobilities are dramatically reduced on the TiN and Ti0.5Al0.5N(001) surfaces while Al adatoms are largely unaffected. ii) The reverse effect is found on the TiN(111) surface, Al adatom migration is reduced while Ti adatom migration is unaffected. iii) The magnetic spin polarization of Ti adatoms is shown to have an important effect on binding energies and diffusion path, e.g., the adsorption energy at bulk sites is increased by 0.14 eV.
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