No description available.

No description available.

No description available.

No description available.

No description available.

This work presents a study on stoichiometry and structure in perovskite-type oxide thin films and investigates the role of growth–induced defects on the properties of materials. It also explores the possibility to grow thin films with properties close or similar to the ideal bulk parent compound. A novel approach to the growth of thin films, atomic layer-by-layer (ALL) laser molecular beam epitaxy (MBE) using separate oxide targets is introduced to better control the assembly of each atomic layer and to improve interface perfection and stoichiometry. It also is a way to layer materials to achieve a new structure that does not exist in nature. This thesis is divided into three sections. In the first part, we use pulsed laser deposition (PLD) to grow LaAlO3 (LAO) thin films on SrTiO3 (STO) and LAO substrates in a broad range of laser energy density and oxygen pressure. Using x-ray diffraction (θ-2θ scan and reciprocal space mapping), transmission electron microscopy (TEM) and x-ray fluorescence (XRF) we studied stoichiometry and structure of LAO films as a function of growth parameters. We show deviation from bulk–like structure and composition when films are grown at oxygen pressures lower than 10-2 Torr. We conclude that the discussion of LAO/STO interfacial properties should include the effects of growth–induced defects in the LAO films when the deposition is conducted at low oxygen pressures, as is typically reported in the literature. In the second part, we describe a new approach to atomically layer the growth of perovskite oxides: (ALL) laser MBE, using separate oxide targets to grow materials as perfectly as possible starting from the first atomic layer. We use All laser MBE to grow Ruddlesden–Popper (RP) phase Lan+1NinO3n+1 with n = 1, 2, 3 and 4 and we show that this technique enables us to construct new layered materials (n=4). In the last and main section of this thesis, we use All laser MBE from separate oxide targets to build the LaNiO3 (LNO) films as near perfectly as possible by depositing one atomic layer at a time. We study the thickness dependent metal-insulator transition (MIT) in ultrathin LNO films on an LAO substrate. In LNO, the MIT occurs in thin films and superlattices that are only a few unit cells in thickness, the understanding of which remains elusive despite tremendous effort devoted to the subject. Quantum confinement and structure distortion have been evoked as the mechanism of the MIT; however, first-principle calculations show that LaNiO3 remains metallic even at one unit cell thickness. Here, we show that thicknesses of a few unit cells, growth–induced disorders such as cation stoichiometry, oxygen vacancies, and substrate-film interface quality will impact the film properties significantly. We find that a film as thin as 2 unit cells, with LaO termination, is metallic above 150 K. An oxygen K-edge feature in the x-ray absorption spectra is clearly inked to the transition to the insulating phase as well as oxygen vacancies. We conclude that dimensionality and strain are not sufficient to induce the MIT without the contribution of oxygen vacancies in LNO ultrathin films. Dimensionality, strain, crystallinity, cation stoichiometry, and oxygen vacancies are all indispensable ingredients in a true control of the electronic properties of nanoscale strongly–correlated materials. / Physics

Quaternary GalnAsN containing a small amount of nitrogen (<2%) is a potentially promising material for realizing long-wavelength emission lasers for applications in optical communication systems. Such devices should have better high-temperature characteristics than conventional InGaAsP lasers due to an increase of the conduction band offset. In this thesis, the GS-MBE growth of quaternary GalnAsN and ternary GaAsN was carried out. Active N was produced by passing high purity nitrogen gas into either an RF or an ECR plasma source. The RF plasma source was found to produce better quality nitrides. Characterization techniques such as photoluminescence, X-ray diffraction, TEM, SIMS, and Hall effect measurements were used to characterize thick layers (e.g. 1 pm) and quantum wells of these nitride materials. The concentration of N incorporated into GalnAs and GaAs is very dependent on growth conditions and plasma conditions. The incorporation of a small amount of N into compressively strained InGaAs reduces the strain and produces a red-shift of photoluminescence peak. However, compared to N-free InGaAs materials, the optical quality is dramatically degraded yielding reduced photoluminescence intensity and a broadened FWHM of the PL peak. Hall effect measurements on un-doped, Si-doped, Bedoped thick GalnAsN layers indicate the presence of a high concentration of electron and hole traps. The results of SIMS suggest that impurity H might be responsible for the deep level defects formed. However, the nature of the defects is currently unknown. From TEM observations and comparison to samples grown with a He-plasma instead of a Nplasma, spinodal decomposition and ion-induced damage in GalnAsN may produce the reduced quality of materials, but these are not the major reasons responsible for the dramatic degradation of optical quality. Thermal annealing was found to be an effective method for significantly improving the optical quality of GalnAsN with a low N concentration. Optimum annealing conditions were obtained. Hall effect measurements on annealed samples indicate that electron and hole traps are reduced but still present after anneal. / Thesis / Master of Engineering (ME)

Le couplage des propriétés ferromagnétiques et semiconductrices représente une perspective prometteuse, afin de réaliser des technologies qui exploitent le spin des électrons. Ceci permettra de stocker et traiter des bits informatiques de façon instantanée dans le même dispositif, plutôt que dans des dispositifs séparés (mémoire et processeur). La Spintronique pourrait alors révolutionner la technologie de l'information. Un candidat potentiel pour la fabrication d'hétérostructures métal ferromagnétique/semiconducteur pour des applications en Spintronique, est le système Mn-Ge. Ce système qui est compatible avec la technologie CMOS, présente une phase intéressante pour la Spintronique qui est Mn5Ge3, avec une possibilité d'épitaxie sur le Ge(111). Afin d'intégrer cette phase dans des procédés de fabrication, nous étudions la diffusion réactive à l'état solide entre un film de Mn et un substrat de Ge (comme dans le cas de la formation des siliciures dans la technologie CMOS). L'accent a été mis sur la séquence de formation de phases lors de la réaction entre un film nanométrique de Mn et le Ge, l'influence de l'interface sur cette réaction, et sur la diffusion du Mn dans le Ge. L'incorporation du carbone dans des films minces de Mn5Ge3 a montré une augmentation notable de la température de Curie : nous présentons alors l'effet du carbone sur la réaction Mn-Ge, et sa redistribution dans les couches minces MnxGey. / Coupling ferromagnetic and semi-conducting properties represents a pathway toward producing technologies that exploit the spin of electrons. That would allow store and process computer bits instantly in a same device, rather than separate devices (memory and CPU). The Spintronics could then revolutionize the information technology. A potential candidate for the fabrication of heterostructures ferromagnetic metal / semiconductor for Spintronics applications is the Mn-Ge system. This system is compatible with CMOS technology, and presents an interesting phase for Spintronics which is Mn5Ge3 phase, which is able to be grown epitaxially on Ge(111). To integrate this phase in the manufacturing process, we study the solid state reactive diffusion between a thin Mn film and Ge substrate, to form a germanide upon the Ge substrate (as in the case of the formation of silicides in CMOS technology). Emphasis was placed on the sequence of phase formation during the reaction between a 50 nm thick Mn film and Ge, the influence of the interface on the reaction, and the diffusion of Mn in Ge. Incorporation of carbon in thin Mn5Ge3 films showed a significant increase in the Curie temperature, we then present the effect of carbon on the reaction Mn-Ge and its redistribution in thin MnxGey films.

Zweidimensionale (2D) Materialien bieten eine Vielzahl von neuartigen Eigenschaften und sind aussichtsreich Kandidaten für ein breites Spektrum an Anwendungen. Da hexagonales Bornitrid (h-BN) für eine Integration in Heterostrukturen mit anderen 2D Materialien geeignet ist, erweckte dieses in letzter Zeit großes Interesse. Insbesondere van-der-Waals-Heterostrukturen, welche h-BN und Graphen verbinden, weisen viele potenzielle Vorteile auf, verbleiben in ihrer großflächigen Herstellung von kontinuierlichen Filmen allerdings problematisch. Diese Dissertation stellt eine Untersuchung betreffend des Wachstums von h-BN und vertikalen Heterostrukturen von Graphen und h-BN auf Ni-Substraten durch Molekularstrahlepitaxie (MBE) vor. Zuerst wurde das Wachstum von h-BN mittels elementarer B- und N-Quellen auf Ni als Wachstumssubstrat untersucht. Kristalline h-BN-Schichten konnten durch Raman-spektroskopie nachgewiesen werden. Wachstumsparameter für kontinuierliche und atomar dünne Schichten wurden erlangt. Das Keimbildungs- und Wachstumsverhalten so wie die strukturelle Güte von h-BN wurden mittels einer systemischen Veränderung der Wachstumstemperatur und -dauer untersucht. Die entsprechenden Beobachtungen wie der Änderungen der bevorzugten Keimbildungszentren, der Kristallgröße und der Bedeckung des h-BN wurden diskutiert. Ein Wachstum von großflächigen vertikalen h-BN/Graphen Heterostrukturen (h-BN auf Graphen) konnte mittels einem neuartigen, MBE-basierenden Verfahren demonstriert werden, welche es h-BN und Graphen jeweils erlaubt sich in der vorteilhaften Wachstumsumgebung, welche von Ni bereitgestellt wird, zu formen. In diesem Verfahren formt sich Graphen an der Schnittstelle von h-BN und Ni durch Präzipitation von zuvor in der Ni-Schicht eingebrachten C-Atomen. Schließlich konnte noch ein großflächiges Wachstum von Graphen/h-BN-Heterostrukturen (Graphen auf h-BN) durch das direkte abscheiden von C auf MBE-gewachsenen h-BN gezeigt werden. / Two-dimensional (2D) materials offer a variety of novel properties and have shown great promise to be used in a wide range of applications. Recently, hexagonal boron nitride (h-BN) has attracted significant attention due to its suitability for integration into heterostructures with other 2D materials. In particular, van der Waals heterostructures combining h-BN and graphene offer many potential advantages, but remain difficult to produce as continuous films over large areas. This thesis presents an investigation regarding the growth of h-BN and vertical heterostructures of graphene and h-BN on Ni substrates using molecular beam epitaxy (MBE). The growth of h-BN from elemental sources of B and N was investigated initially by using Ni as the growth substrate. The presence of crystalline h-BN was confirmed using Raman spectroscopy. Growth parameters resulting in continuous and atomically thin h-BN films were obtained. By systematically varying the growth temperature and time the structural quality as well as the nucleation and growth behavior of h-BN was studied. Corresponding observations such as changes in preferred nucleation site, crystallite size, and coverage of h-BN were discussed. Growth of h-BN/graphene vertical heterostructures (h-BN on graphene) over large areas was demonstrated by employing a novel MBE-based technique, which allows both h-BN and graphene to form in the favorable growth environment provided by Ni. In this technique, graphene forms at the interface of h-BN/Ni via the precipitation of C atoms previously dissolved in the thin Ni film. No evidence for the formation of BCN alloy could be found. Additionally, the suitability of ultraviolet Raman spectroscopy for characterization of h-BN/graphene heterostructures was demonstrated. Finally, growth of large-area graphene/h-BN heterostructures (graphene on h-BN) was demonstrated via the direct deposition of C on top of MBE-grown h-BN.

Das gitterfehlangepaßte Kristallwachstum führt unter bestimmten Bedingungen zu einem 3-D Wachstumsmodus, der oft Stranski-Krastanow-Wachstum genannt wird. Resultierende Strukturgrößen liegen in der Größenordnung 10 nm und die Halbleiterstrukturen besitzen daher Quanteneigenschaften. Sie stehen im Fokus grundlagenwissenschaftlichen Interesses, da künstliche Atome und Dimensionalitätseffekte an ihnen untersucht werden können. Auch von der Anwendungsseite wächst das Interesse, da niederdimensionale Strukturen hoher Kristallqualität und mit hoher gestalterischer Freiheit geschaffen werden können. In dieser Arbeit wurden Mischhalbleiter-Heterostrukturen der Dimensionalität d= 0,1 und 2 mittels Gasphasen-MBE hergestellt. Ziel war eine Korrelation der strukturellen mit den elektronischen und optischen Eigenschaften. Selbstformierende Quantendrähte und Quantenpunkte in leitfähigen Kanälen wurden in ihrem Einfluß auf den lateralen Transport untersucht. Weiterhin wird dargestellt, wie zusätzliche, durch Quantenpunkte induzierte Oberflächenzustände eine deutliche Verschiebung der Energie des Oberflächen-Ferminiveau-Pinning einer (100)-GaAs-Oberfläche verursachen. Der senkrechte Elektronentransport durch Quantenpunkte dient der Untersuchung von Dot-induzierten, tiefen elektronischen Zuständen und der Erklärung eines eindimensionalen Modells elektronischer Kopplung zwischen denselben. Zusätzlich führen uns die Ergebnisse optischer Messungen zu einem besseren Verständnis des Vorgangs der Dotformierung und der elektronischen Kopplung zwischen zufällig verteilten Quantenpunkten. / The lattice-mismatched epitaxial growth is known to induce a three-dimensional growth mode often referred to as Stranski-Krastanov growth. The resulting structures have typical sizes of 10 nm and possess quantum properties, which are of fundamental physical interest, since artificial atoms and dimensionality effects can be studied. There is a growing interest from an applicational point of view also, since low dimensional structures of a high crystal quality and of a high degree of designerabillity can be created. In this work such structures of a dimensionality d=0,1 and 2 based on compound semiconductors have been designed and prepared by molecular beam epitaxy to perform comparative studies with respect to their electronic, structural and optical properties. Self assembled quantum wires and dots in conductive channels have been examined according to their influence on lateral electrical transport. It is demonstrated how additional surface states from quantum dots cause a distinct shift in the Surface Fermi-level of a GaAs (100) surface. Vertical transport through dots is used to support a model of one-dimensional coupling between deep states induced by the dots. Additionally, optical investigations let us attain a better understanding of the process of dot formation and the electronic coupling between the randomly distributed dots.