The topological insulator Bi2Te3 has been grown on Si(111)-(7 × 7) surface by molecular beam epitaxy. Reflection high energy electron diffraction, in situ scanning tunnelling microscopy, x-ray photoelectron spectroscopy and ex situ x-ray diffraction studies have been performed to analyze the quality of the growth. These analyses suggest a very good layer-by-layer epitaxial growth of Bi2Te3 on the atomically at Si surface. The magnetoresistance of the samples has been studied with magnetic field perpendicular and parallel to the sample surface, up to 9 T, over a temperature range of 2 K to 20 K. A sharp dip at low fields (0 T - 1 T) and near-linear behavior for high fields (> 4 T) have been observed in the perpendicular field magnetoresistance. The low field dip is due to weak antilocalization that agrees well with the simplified Hikami-Larkin-Nagaoka model. It has been demonstrated that both the low field dip and the high field near-linear behavior can be explained by the original Hikami-Larkin-Nagaoka formula alone in a system with strong spin-orbit coupling. From the fitting of the perpendicular field magnetoresistance the phase coherence length, the mean free path and the spin-orbit relaxation time have been estimated. The phase coherence length shows power law dependence with temperature indicating two dimensional nature of the transport. The power law also suggests electron electron interaction as the prominent dephasing mechanism. The out-of-plane spin-orbit relaxation time is determined to be small and the in-plane spin-orbit relaxation time is found to be comparable to the momentum relaxation time. The estimation of these charge and spin transport parameters is useful for topological insulator based magneto electric device applications. It also has been shown that the strong spin-orbit coupling suppresses the Zeeman contribution in perpendicular field magnetoresistance. The logarithmic divergence of perpendicular field magnetoresistance with temperature for low temperature range (2 K - 20 K) at high fields shows the presence of Coulomb interaction in the spin singlet channel. For magnetoresistance with the field parallel to the sample surface, the observed magnetoresistance has parabolic dependence for small fields (0 T - 0.6 T) and logarithmic dependence for large fields (> 3 T), which is due to the Zeeman effect. It is found that the data are inconsistent with only the Maekawa and Fukuyama theory of non interacting electrons with Zeeman contributions to the transport, but are consistent with theory if one also takes into account the electron electron interaction and the Zeeman splitting term in the electron electron interaction theory of Lee and Ramakrishnan. The Zeeman g-factor and the strength of Coulomb scattering due to electron electron interaction have been estimated from fitting of the parallel field magnetoresistance. The magnetoresistance also shows anisotropy with respect to the field directions. The angle dependent anisotropic magnetoresistance can be fitted well by the original HLN theory alone. The anisotropy can have potential application in anisotropic magnetic sensors. / text

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<p>III-Nitride materials have recently become a promising candidate for superior applications over the current technologies. However, certain issues such as lack of native substrates, and high defect density have to be overcome for further development of III-Nitride technology. This work presents research on lattice engineering of III-Nitride materials, and the structural, optical, and electrical properties of its alloys, in order to approach the ideal material for various applications. We demonstrated the non-destructive and quantitative characterization of composition modulated nanostructure in InAlN thin films with X-ray diffraction. We found the development of the nanostructure depends on growth temperature, and the composition modulation has impacts on carrier recombination dynamics. We also showed that the controlled relaxation of a very thin AlN buffer (20 ~ 30 nm) or a graded composition InGaN buffer can significantly reduce the defect density of a subsequent epitaxial layer. Finally, we synthesized an InAlGaN thin films and a multi-quantum-well structure. Significant emission enhancement in the UVB range (280 – 320 nm) was observed compared to AlGaN thin films. The nature of the enhancement was investigated experimentally and numerically, suggesting carrier confinement in the In localization centers.</p> / Dissertation

<p>GaAs1-xBix is a III-V semiconductor alloy which has generated much fundamental scientific interest. In addition, the alloy possesses numerous device-relevant beneficial characteristics. However, the synthesis of this material is very challenging and its properties are not well understood. The focus of this dissertation is to advance the understanding of its synthesis using molecular beam epitaxy (MBE) and, as a result, improve its key as-grown properties that are of great importance to device applications, such as increasing Bi concentration in the alloy and enhancing its optical emission efficiency.</p><p>In chapter 3, the discovery of a trade-off between the structural and optical characteristics of GaAs1-xBix , controlled by the degree to which the growth is kinetically-limited, is described. Chapter 4 discusses the exploitation of a growth method that utilizes the spatial distribution of MBE fluxes to facilitate numerous studies of the critical dependence of GaAs1-xBix characteristics on the V/III flux ratio. Chapter 5 describes the results of experiments utilizing vicinal substrates to modify both Bi incorporation and optical emission efficiency of synthesized GaAs1-xBix and enable new understanding of the Bi incorporation mechanism. Specifically, incorporation primarily at A steps, defined as the steps generated by misorienting the GaAs (001) substrate toward the (111)A surfaces, enhances Bi incorporation but reduces optical emission efficiency. Chapter 6 describes the identification of two new signatures in the Raman spectra of GaAs1-xBix that can be used to determine the Bi content and the hole concentration of nominally undoped GaAs1-xBix. Finally, in Chapter 7 the GaAs1-xBix growth using pulsed Ga fluxes is described. The use of pulsed-growth significantly modifies the incorporation of Bi and suggests it is a promising method for widening the GaAs1-xBix MBE growth window enabling improved synthesis control and materials properties.</p> / Dissertation

Semiconductors provide an interesting platform for studying light-matter interactions due to their unique electrically conductive behavior which can be deliberately altered in useful ways with the controlled introduction of confinement and doping, which changes the electronic band structure. This area of research has led to many important fundamental scientific discoveries that have in turn spawned a plethora of applications in areas such as photonics, microscopy, single-photon sources, and metamaterials. Silicon is the prevalent semiconductor platform for microelectronics because of its cost and electrical properties, while III-V materials are optimal for optoelectronics because of the ability to engineer a direct bandgap and create versatile heterojunctions by growing binary, ternary, or quaternary compounds.

Selektivität zu erreichen ist eines der Hauptziele der Chemie. In der Katalyse wird allgemein von einem engen Zusammenhang zwischen der Selektivität und der Katalysatorstruktur ausgegangen - allerdings erschwert die komplexe mikroskopische Struktur realer Katalysatoren ein tiefergehendes Verständnis; daher wird versucht, weitergehende Erkenntnisse an strukturell vereinfachten Materialien zu gewinnen. Für diese Arbeit wurde hierzu ein Pd/Fe3O4-Modellkatalysator verwendet. Auf diesem System wurde die Selektivität in zwei katalytische Modellreaktionen untersucht. Die Reaktantenexposition erfolgte dabei über Molekularstrahlen im Hochvakuum, und die Reaktionsraten wurden massenspektrometrisch gemessen; Adsorbate wurden IR-spektroskopisch detektiert. - Zersetzung von Methanol: Es wird gezeigt, dass Methanol auf dem Oxid Fe3O4 sehr selektiv durch Reaktion mit Oberflächensauerstoff (Mars-van-Krevelen-Mechanismus) zu Formaldehyd und Wasser dehydrogeniert wird. Auf Pd-Metall zersetzt sich Methanol im wesentlichen sehr schnell zu Kohlenstoffmonoxid und Wasserstoff (bzw. zu Kohlenstoffablagerungen in einer Nebenreaktion). Es werden Experimente gezeigt, die darauf hindeuten, dass Diffusion von oxidgebundenem Methanol/Methoxy auf die Pd-Metallpartikel signifikant zur Gesamtaktivität des Modellkatalysators beiträgt. - Umsetzung von 2-Buten mit Deuterium: Zunächst wird gezeigt, dass die Erzielung katalytischer Aktivität kritisch von der dissoziativen Adsorption des Reaktanden Deuterium abhängt, die durch Kohlenwasserstoffadsorbate stark inhibiert wird; es war allerdings möglich, diese Limitierung experimentell zu umgehen. Darüberhinaus wird gezeigt, dass die Hydrierungsreaktion durch die Anwesenheit stark zersetzter Kohlenwasserstoffablagerungen selektiv induziert werden kann, während die alternative Reaktion (H/D-Austausch/Isomerisierung) auch in Abwesenheit dieser Spezies abläuft; mögliche Erklärungsmodelle werden diskutiert. Schließlich wird die mögliche Ursache für die unter bestimmten Reaktionsbedingungen beobachteten unterschiedlichen Reaktionsraten mit cis- und trans-2-Buten als Reaktanten diskutiert. / The achievement of selectivity is one of the main objectives in chemistry. For catalysis, selectivity is generally seen to be closely linked with catalyst structure; the complex microscopic structure of real catalysts, however, obstructs to obtain a deeper understanding; for this reason, structurally simplified materials are studied. For the current work, studies have been conducted on a Pd/Fe3O4 model catayst. On this system, the selectivity in two catalytic reactions has been examined. The exposure of the reactants was effected by molecular beams in high vacuum, and the reaction rates have been measured mass spectrometrically; additionally, adsorbates were detected by IR-spectroscopy. - Decomposition of Methanol: It is shown that on the oxide Fe3O4 methanol is dehydrogenated very selectively to formaldehyde and water by reaction with surface oxygen of the oxide (Mars-van-Krevelen mechanism). On Pd metal it is mainly decomposed very quickly to carbon monoxide and hydrogen (and, in a side reaction, to carbonaceous deposits). Experiments are shown indicating that the diffusion of oxide-related methanol and methoxy to the Pd metal-particles contributes significantly to the overall activity of the model catalyst. - Conversion of 2-Butene with Deuterium: At first it is shown that the catalytic activity depends critically on the dissociative adsorption of the reactant deuterium, which is strongly inhibited by hydrocarbon adsorbates; it was, however, possible to overcome this limitation experimentally. In addition, it is shown that the hydrogenation reaction can be selectively induced in the presence of strongly dehydrogenated carbonaceous deposits, whereas the alternative reaction (H/D-exchange/isomerisation) can proceed also without the presence of those species; possible models for explanation are discussed. Finally, the possible origin of the different reaction rates with cis- and trans-2-butene that were observed only under certain reaction conditions is discussed.

Zinc nitride and magnesium nitride are examples of the relatively unexplored II3V2 group of semiconductor materials. These materials have potential applications in the electronics industry due to their excellent optical and electrical properties. This study mainly focuses on the growth and characterization of the new semiconductor materials: zinc nitride, magnesium nitride, and their alloys. The (100) oriented zinc nitride thin films were grown on both (110) sapphire substrates and (100) MgO substrates by plasma-assisted molecular beam epitaxy (MBE). The typical growth rate is in the range of 0.02-0.06 nm/s, the growth temperature is in the range of 140-180 oC, and background nitrogen pressure is around 10-5 Torr. The growth process was monitored by in-situ: reflection high energy electron diffraction (RHEED) and optical reflectivity. The RHEED and X-ray diffraction patterns of the zinc nitride indicate that the film is a single crystal material. The in-situ optical reflectivity pattern of the zinc nitride shows interference oscillations, and these oscillations are damped out as the thickness increases. The reflectivity as a function of time was accurately simulated by an optical equation. The optical constants of the thin films, the growth rate, and the thickness were derived from the simulation of the in-situ reflectance. The X-ray diffraction shows that (400) oriented zinc nitride thin films were grown on both A-plane (110) sapphire substrates and (100) MgO substrates. Optical transmittance measurements were performed on the zinc nitride thin films. The spectrum of the zinc nitride transmittance indicates that zinc nitride has a high optical absorption in the visible light region. The absorption coefficient was calculated from the transmittance spectrum, and the optical band gap of the zinc nitride thin film was found to be 1.25-1.28 eV. Ellipsometry measurements suggested that the refractive index of zinc nitride is 2.3-2.7, and the extinction coefficient is ~0.5-0.7 in the energy range 1.5-3.0 eV. The electron transport measurement shows that the single crystal zinc nitride has a mobility as high as 395 cm2 /Vs. A plasma-assisted MBE system was employed for magnesium nitride growth. The growth temperature was in the range of 300-350 oC. RHEED and laser reflectivity were employed during growth. The RHEED and X-ray diffraction patterns indicated that the epilayers are single crystal films. The optical laser reflectivity was well fitted by a modified optical equation. The optical constants and growth rate were derived from the simulation. X-ray diffraction showed that (400) oriented single crystal magnesium nitride films were grown on (100) MgO substrates. The optical transmittance spectra show that the magnesium nitride has a high absorption below 500 nm. The calculated absorption coefficient is as high as 4x10-4 cm-1 in the range of ~2.5-3.0 eV. The optical band gap was estimated to be ~2.5 eV. Ellipsometry measurements showed that the refractive index of the magnesium nitride is 2.3-2.75 and the extinction coefficient is less than 0.3 in the energy range of 1.5-3.0 eV. Zinc nitride-magnesium nitride (Zn3-3xMg3xN2) alloys were grown on (100) YSZ substrates by sputtering. The bandgap ranged from 1.2 eV to 2.1 eV for Mg content x in the 0-0.59 range. One film with a bandgap of ~1.4 eV and Mg content of 0.18 has the relatively high mobility of 47 cm2 /Vs which was expected for photovoltaics application. / Graduate