Browsing by Subject "Density functionals"

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Density evolution in systems with slow approach to equilibrium
(2004) Nelson, Kevin Taylor; Driebe, Dean J.
This dissertation investigates the evolution of probability densities under the Frobenius-Perron operator U in chaotic iterated-map systems that are slow to reach equilibrium. It first concentrates on one-dimensional maps that are slow to reach equilibrium because they feature intermittent chaos due to the presence of a marginal fixed point. Using the method of shift states and coherent states under U, certain results are obtained concerning the spectrum of U in various functional spaces, using as the main example the cusp map f (x) = 1 - √ |1 - 2x |. Those results are applied to obtain corrections to the well-known leading 1/t form of the x-x auto correlation function C(t). The symbolic dynamics of one-dimensional maps are then investigated, with particular emphasis on the implications of the existence of intermittent chaos and with applications to topological conjugation. Next, the statistics of extreme values in one dimensional maps are investigated. Fn(x) is defined as the probability that a point chosen from an initial probability distribution and its first n- 1 iterates under a particular map are all less than x; the properties of Fn(x) are derived analytically for a wide variety of one-dimensional maps, and the on conclusions are confirmed numerically. Finally, higher-dimensional area-preserving maps are investigated. The technique of local spectral decomposition for U, in which approximate right and left eigenstates for U are constructed localized on unstable periodic points, is used to study density evolution and correlation of observables over time.
Density functional studies of magnetic semiconductors and multiferroics
(2007-12) Ciucivara, Adrian Ioan, 1976-; Kleinman, Leonard
The present work is mostly focused on theoretical study of multiferroics and magnetic semiconductors within the framework of density functional theory (DFT). We studied Bi₂NiMnO₆ which was recently synthesized as a heavily distorted double perovskite with four formula units in a monoclinic unit cell. The calculated GGA and GGA+U magnetizations per formula unit were 4.92 and 4.99 [mu]B and the calculated ferroelectric polarizations were 16.83 and 16.63 [mu]C cm⁻². We also present the results of completely relaxed electronic structure calculations for multiferroic LaMnO₃/BaTiO₃ structure which was built by joining a slab of magnetic LaMnO₃ with a slab of ferroelectric BaTiO₃. The study of Mn doped GeTe, which is a diluted magnetic semiconductor (DMS), shows the net magnetization as a function of doping and holes concentration. In addition, we present a study of ferroelectric GeTe as a function of applied pressure and the interesting properties exhibited by Si(001)/Si(110) junction which was experimentally built using hybrid orientation technology (HOT).
First principles modeling of arsenic and fluorine behavior in crystalline silicon during ultrashallow junction formation
(2006) Harrison, Scott Anthony; Hwang, Gyeong S.; Edgar, Thomas F.
The 2005 International Technology Roadmap for Semiconductors predicts ultrashallow junctions (USJs) less than 7 nm deep with unprecedented dopant activation levels will be required for silicon transistors to be manufactured in 2010. To meet these requirements, it is necessary to have a better understanding of the dopant transient enhanced diffusion (TED) and clustering behaviors that undermine the achievement of these manufacturing specifications. Arsenic (As) is a commonly used n-type dopant in USJ formation and fluorine (F) is an impurity commonly co-implanted with dopants to reduce dopant diffusion and clustering during USJ formation. In this dissertation, density functional theory within the generalized gradient approximation is used to understand the behavior of As and F in crystalline silicon during USJ formation. In the first part of this dissertation, the influence of silicon interstitials on As behavior during thermal annealing that follows dopant implantation is investigated. As a result of dopant implantation, a net excess of silicon interstitial defects exist in the silicon. First, it is shown that silicon interstitials can easily annihilate existing Asvacancy complexes in silicon with negligible recombination energy barriers. Second, experimentally observed As TED mediated by interstitials is explained by the formation of a highly mobile As-silicon interstitial pair that can exist in positive, neutral, and negative charge states. Finally, it is shown that large As-silicon interstitial complexes may form when excess interstitials are present and provide a kinetic route to As clustering that leads to As deactivation. In the second part of this dissertation, the interaction of F impurities with silicon interstitials and B dopants is investigated. First, the formation and diffusion of a highly mobile fluorine-silicon interstitial pair which has been suggested by experiment is detailed. Second, an immobile B-Sii-F structure is identified in which B has a deactivated configuration. This structure may play a role in deactivating and immobilizing B when implanted B and F profiles coincide. This research provides fundamental insight into the behavior of As dopants and F impurities during USJ formation. As the future of silicon-based devices relies on the ability to perform precise doping, these findings should be of great importance to device manufacturers.
Theoretical investigation of contact materials for emerging electronic and spintronic devices
(2007-12) Niranjan, Manish Kumar, 1977-; Kleinman, Leonard; Demkov, Alexander A.
We present a theoretical study of the electronic structure, surface energies and work functions of orthorhombic Pt monosilicide and germanides of Pt, Ni, Y and Hf within the framework of density functional theory (DFT). Calculated work functions for the (001) surfaces of PtSi, NiGe and PtGe suggest that these metals and their alloys can be used as self-aligned contacts to p-type silicon and germanium. In addition, we also study electronic structure and calculate the Schottky-barrier height at Si(001)/PtSi(001) interface and GaAs(001)/NiPtGe(001) interfaces with different GaAs(001) and NiPtGe (001) terminations. The p-type Schottky barrier height of 0.28 eV at Si/PtSi interface is found in good agreement with predictions of a simple metal induced gap states (MIGS) theory and available experiment. This low barrier suggests PtSi as a low contact resistance junction metal for silicon CMOS technology. We identify the growth conditions necessary to stabilize this orientation. The calculated p-type Schottky barrier heights (SBH) at different GaAs/NiPtGe interfaces vary by as much as 0.18 eV around the average value of 0.5 eV. We further identify and discuss factors responsible for strong Fermi level pinning resulting in small variation in the p-SBH. We also present a theoretical study of magnetic state of [beta]-MaAs and show that it is antiferromagnetic and explain the lack of observed long-range order.