No description available.

Le Bismuth, un élément V, a longtemps été négligé dans la famille des semi-conducteurs III V. Toutefois, les matériaux bismures connaissent un intérêt croissant depuis le début des années 2000, principalement en raison de l’exceptionnelle réduction de l’énergie de bande interdite couplée à la forte augmentation de l’énergie entre la bande de valence et la bande de spin-orbite introduites par l’atome de Bismuth. Parmi les alliages III-V-Bi, le GaSbBi est particulièrement intéressant pour l’émission dans la gamme de longueurs d’onde entre 2 et 5 µm. Jusqu’à présent, ce matériau n’a été que très peu étudié, principalement à cause des difficultés d’incorporation du Bismuth. En effet, l’incorporation du Bismuth dans les matériaux III-V nécessite des conditions de croissance très spécifiques et inhabituelles. Dans ce contexte, l’objectif premier de cette thèse est d’étudier l’épitaxie par jet moléculaire et les propriétés du GaSbBi.Ainsi, l’influence des différents paramètres de croissance sur l’incorporation du Bismuth a été étudiée minutieusement. Ces expériences ont permis la réalisation de couches de GaSbBi à forte teneur en Bismuth démontrant une excellente qualité cristalline. La plus importante concentration de Bismuth atteinte est de 14%, ce qui constitue encore aujourd’hui le record mondial dans GaSb. Par ailleurs, une réduction de l’énergie de bande interdite de 28 meV/%Bismuth a été observée. Des puits quantiques GaSbBi/GaSb, émettant jusqu’à 3.5 µm à température ambiante ont ensuite été épitaxiés et caractérisés. Le premier laser à base de GaSbBi a également été réalisé. Ce composant fonctionne en continu à 80 K et une émission laser pulsée a été observée proche de 2.7 µm à température ambiante. Enfin, un autre alliage bismure méconnu, le GaInSbBi, a été épitaxié. L’influence de l’Indium sur l’incorporation du Bismuth et les propriétés de puits quantiques GaInSbBi/GaSb ont été étudiées. / Bismuth, a group-V element, has long been neglected in the III-V semiconductor family. However, dilute bismides started to attract great attention since the early 2000s, due to the giant bandgap reduction and the strong increase of the spin-orbit splitting energy introduced by the incorporation of Bismuth. Among the III-V-Bi alloys, GaSbBi is particularly interesting but has only been sporadically studied, mainly due to the very challenging incorporation of Bismuth. Bismuth requires indeed very unusual growth conditions to be incorporated into III-V materials. The main objective of this thesis was to investigate the molecular beam epitaxy and the properties of GaSbBi alloys and heterostructures.A careful study of the influence of the different growth parameters on the Bismuth incorporation was first carried out. These investigations lead to the fabrication of high quality GaSbBi layers and to the incorporation of 14% Bismuth, the highest content reached in GaSb so far. A bandgap reduction of 28 meV/%Bismuth was observed. GaSbBi/GaSb multi quantum-wells structures with various thicknesses and compositions were then fabricated and exhibited photoluminescence emission up to 3.5 µm at room-temperature. The first GaSbBi-based laser diode was also fabricated, demonstrating continuous wave operation at 80 K and a room-temperature emission close to 2.7 µm under pulsed excitation. Finally, the growth of another dilute bismide alloy, GaInSbBi, was investigated. The influence of the Indium atoms on the incorporation of Bismuth was particularly studied together with the properties of GaInSbBi/GaSb multi quantum-wells structures.

In this thesis, a custom quadrupole mass filter setup was established to independently investigate the impact of the most prominent ion species that are present during ion-assisted deposition. The setup was applied to the low temperature epitaxial growth of GaN thin films on 6H-SiC substrates. Atomic nitrogen ions at higher ion kinetic energies were for the first time independently identified to be the predominant cause of deteriorating crystalline qualities during growth. Precise control of the ion beam parameters yielded the capability to vary the average GaN phase content from almost purely wurtzite to the meta-stable zinc blende GaN phase. Even in case of comparably high crystalline quality, the atomic and molecular nitrogen ions were independently determined to yield distinct thin film topographies throughout the entire observed evolution of the thin film formation.:Bibliographical Description 1 Introduction 1.1 Epitaxial Thin Film Growth 1.2 Ion-Beam Assisted Deposition 1.2.1 Influence of Energetic Particles 1.2.2 Ion-atom Arrival Ratio 1.3 Gallium Nitride 2 Methods 2.1 Setup of the Deposition System 2.1.1 Knudsen Effusion Cell 2.1.2 Reflection High-Energy Electron Diffraction 2.1.3 Auger Electron Spectroscopy 2.1.4 Ion Sources 2.2 Quadrupole Mass Filter System 2.2.1 Components 2.2.2 Working Principle of a Quadrupole Mass Filter 2.2.3 Alternative Mass Filters 2.3 X-ray Diffraction and Reflectivity 2.4 Atomic Force Microscopy 2.5 Transmission Electron Microscopy 3 Results and Discussions 3.1 Characterization of the Quadrupole Mass Filter System 3.1.1 Mass Filter Performance and Resolution 3.1.2 Ion Beam Characteristics 3.1.3 Space Charge Considerations 3.1.4 Conclusions 3.2 Influence of the I/A Ratio and Ion Kinetic Energy 3.2.1 Determination of the GaN Phase Composition 3.2.2 Film Topography and Growth Mode 3.2.3 Crystal Structure and Orientation 3.2.4 Microstructure at the Interface 3.2.5 Conclusions 3.3 Impact of the Ion Species on Growth Instabilities 3.3.1 Growth Rates and Thin Film Topography 3.3.2 Crystal Structure 3.3.3 Growth Mode and RHEED pattern evolution 3.3.4 Conclusions 4 Summary and Conclusions Bibliography Complete Publication List of the Author Acknowledgments Declaration of Authorship / In dieser Arbeit wurde ein maßgefertigter Quadrupol-Massenfilteraufbau etabliert, um die Auswirkungen der prominentesten Ionenspezies, die während der ionengestützten Abscheidung vorhanden sind, unabhängig voneinander zu untersuchen. Der Aufbau wurde für das epitaktische Niedertemperatur-Wachstum von GaN-Dünnschichten auf 6H-SiC-Substraten angewendet. Atomare Stickstoffionen bei höheren kinetischen Ionenenergien wurden zum ersten Mal in der Abwesenheit anderer Spezies als die dominierende Ursache für die Verschlechterung der kristallinen Qualität während des Wachstums identifiziert. Eine präzise Kontrolle der Ionenstrahlparameter ergab die Fähigkeit, den durchschnittlichen GaN-Phasengehalt von der fast reinen Wurtzit- bis zur metastabilen Zinkblende-GaN-Phase zu variieren. Selbst bei vergleichbar hoher kristalliner Qualität weisen die mit atomaren und molekularen Stickstoffionen hergestellten Schichten unabhängig voneinander verschiedene Topographien auf, die sich während der gesamten beobachteten Entwicklung der Dünnschichtbildung deutlich abzeichneten.:Bibliographical Description 1 Introduction 1.1 Epitaxial Thin Film Growth 1.2 Ion-Beam Assisted Deposition 1.2.1 Influence of Energetic Particles 1.2.2 Ion-atom Arrival Ratio 1.3 Gallium Nitride 2 Methods 2.1 Setup of the Deposition System 2.1.1 Knudsen Effusion Cell 2.1.2 Reflection High-Energy Electron Diffraction 2.1.3 Auger Electron Spectroscopy 2.1.4 Ion Sources 2.2 Quadrupole Mass Filter System 2.2.1 Components 2.2.2 Working Principle of a Quadrupole Mass Filter 2.2.3 Alternative Mass Filters 2.3 X-ray Diffraction and Reflectivity 2.4 Atomic Force Microscopy 2.5 Transmission Electron Microscopy 3 Results and Discussions 3.1 Characterization of the Quadrupole Mass Filter System 3.1.1 Mass Filter Performance and Resolution 3.1.2 Ion Beam Characteristics 3.1.3 Space Charge Considerations 3.1.4 Conclusions 3.2 Influence of the I/A Ratio and Ion Kinetic Energy 3.2.1 Determination of the GaN Phase Composition 3.2.2 Film Topography and Growth Mode 3.2.3 Crystal Structure and Orientation 3.2.4 Microstructure at the Interface 3.2.5 Conclusions 3.3 Impact of the Ion Species on Growth Instabilities 3.3.1 Growth Rates and Thin Film Topography 3.3.2 Crystal Structure 3.3.3 Growth Mode and RHEED pattern evolution 3.3.4 Conclusions 4 Summary and Conclusions Bibliography Complete Publication List of the Author Acknowledgments Declaration of Authorship

No description available.

Three-dimensional topological insulators (TIs) are a class of materials for which the bulk is insulating, while the surface is necessarily metallic. A band inversion that occurs in the presence of spin-orbit coupling, and conduction and valence bands with opposite parities are necessary conditions for the existence of this class of materials. The metallicity of the surface states appears as a consequence of the topology of the bulk and these states are characterized by massless Dirac dispersions and helical spin polarization that protect the surface states against backscattering. The robustness of the topological surface states further implies that they are not destroyed by non-magnetic impurities or defects. Since their initial conception, a vast amount of theoretical studies have predicted very interesting features stemming from the topological surface states. An example of that can be found when breaking the time-reversal symmetry by introducing magnetic order in the system, which can lead to exotic phenomena such as the quantum anomalous Hall effect. The properties exhibited by these systems are expected to be of high importance both in fundamental research as well as in technological applications. However, the major difficulty remains the access to purely topological surface states. The remaining bulk conductivity of the TIs such as Bi2Se3, Bi2Te3 or Sb2Te3 still hinders the experimental realization of some of the predicted phenomena. This highlights the need of high-quality bulk-insulating materials with ultra-clean surfaces and interfaces, which can only be achieved with delicate sample preparation and characterization methods. The present work is part of the effort to fabricate high-quality TI films in a controlled manner. This shall then allow more complex investigations, such as interface effects and possibilities to engineer the band structure of the TIs. The former will be explored mainly in the form of heterostructures of Bi2Te3 and magnetic insulating layers, whereas the latter will focus on the fabrication of Sb2Te3/Bi2Te3 heterostructures. Most of the important properties of the samples are measured under ultra-high vacuum conditions, ensuring reliable results. Furthermore, in situ capping with ordered Te also allows for more sophisticated ex situ experiments. In a first step, the optimization of Bi2Te3 thin films grown on Al2O3 (0001) substrates was explored. Spectroscopic and structural characterization measurements showed that it is possible to obtain consistently bulk-insulating TI films with good structural quality, despite the lattice mismatch between Bi2Te3 and Al2O3 (0001). Magnetoconductance measurements showed a prominent weak anti-localization effect, confirming the existence of two-dimensional surface states. In order to explore the consequences of breaking the time-reversal symmetry characteristic of TIs, Bi2Te3 was interfaced with several ferro- or ferrimagnetic insulating (FI) layers in heterostructures. EuO, Fe3O4, Y3Fe5O12 and Tm3Fe5O12 were chosen as possible candidates. Systematic optimization and characterization studies showed that interfaces of Bi2Te3 and EuO, as well as Fe3O4 on top of Bi2Te3, yield poor quality samples with significant chemical reactions between the layers. Nevertheless, high-quality Bi2Te3 could be grown on Fe3O4 (001), Fe3O4 (111), Y3Fe5O12 (111) and Tm3Fe5O12 (111). Clean interfaces and intact top topological surface states were confirmed by photoemission spectroscopy. Moreover, transport signatures of a gap opening in the topological surface states were found, namely a suppression of the weak anti-localization effect and the observation of the anomalous Hall effect. However, x-ray circular magnetic dichroism (XMCD) was not observed for any of the heterostructures. A key conclusion from this study is that the ferromagnetism induced by the magnetic proximity effect is too weak to be detected by XMCD. On hindsight, one can infer that the magnetic proximity effect cannot be strong since the bonding between the TI and the magnetic insulator substrate is of the van der Waals type, and not covalent like in transition metal oxides or metallic heterostructures. It is known that a charge compensation between electron- and hole-doping can be achieved when combining Bi2Te3 and Sb2Te3, which can also tune the position of the Dirac point. With this goal in mind, the fabrication of ternary (Bi(x)Sb(1−x))2Te3 compounds and Sb2Te3/Bi2Te3 heterostructures was explored in the next step. Although pure Sb2Te3 and (Bi(x)Sb(1−x))2Te3 did not yield good quality samples, the fabrication of Sb2Te3/Bi2Te3 heterostructures emerged as a promising alternative route. Photoelectron spectroscopy allowed not only to identify the crucial role of the first few Sb2Te3 top layers, which modulate the topological surface states, but also to characterize the intermixing of the TI layers at the interface. In a final study, Fe(1+y)Te thin films were grown on MgO (001) substrates employing a Te-limited growth method. This allowed to obtain nominally stoichiometric films, as evidenced by reflection high-energy electron diffraction, x-ray absorption spectroscopy, XMCD and x-ray diffraction measurements. This preliminary study opens the way for the investigation of TI/superconductor interfaces and to delve into the topological superconductivity arising from the proximity effect.

In dieser Arbeit wurden die komplexen Mechanismen für den Einbau von Mg und In in (Ga,In)N/GaN(0001)-Heterostrukturen, die mittels PA-MBE hergestellt wurden, mit morphologischen, optischen und elektrischen Charakterisierungsmethoden untersucht. Darüber hinaus wurde die Verwendung von (Ga,In)N/GaN SPSLs als HIL oder als aktiver Bereich in herkömmlichen LED-Strukturen untersucht. In-situ-Messungen zeigten, dass die Desorption von In in Gegenwart von N und Mg auf der GaN(0001)-Oberfläche zunimmt. Ferner wurden Mg-dotierte (Ga,In)N/GaN-SLs mittels PAMBE gezüchtet und mittels QMS, XRD und SIMS charakterisiert. Die (Ga,In)N/GaN-SLs zeigten eine bessere Oberflächenmorphologie als die (Ga,In)N-Schichten, die homogen mit Mg dotiert wurden. Jedoch wurde eine deutliche Abnahme des In-Gehalts in der (Ga,In)N ML festgestellt, wenn Mg gleichzeitig mit In zugeführt wurde. Gleichzeitig nahm die Mg-Konzentration in Gegenwart von In zu, was möglicherweise auf eine Wirkung als oberflächenaktive Substanz zurückzuführen ist. Für das SL, bei dem nur die (Ga,In)N-QWs mit Mg dotiert waren, wurde vom Messergebnis von SIMS eine maximale Mg-Konzentration von 2,6 × 1022 cm-3 für eine 1 ML dicke (Ga,In)N:Mg-Schicht deduziert. Zusätzlich haben andere Experimente ähnliche Ergebnisse aufgezeigt. Thermoleistung-Studien zeigten, dass das Delta-dotierte SL und die SL-Strukturen mit Mg-Dotierung in 20% der QB p-leitfähig sind. Zusätzlich wurde ein Gleichrichterverhalten der (Ga,In)N/GaN SL-Strukturen mit einem Idealitätsfaktor von weniger als 10 für die QW-dotierten SLs demonstriert. Ausgehend von der elektrischen Charakterisierung wurden drei verschiedene LED-Strukturen, die auf den vielver-sprechendsten Mg-dotierten (Ga,In)N SL-Strukturen (Delta-dotiertes SL und 20% QB-dotiertes SL) basierten, hergestellt und charakterisiert. / In this thesis, the complex mechanisms for the incorporation of Mg and In in (Ga,In)N/GaN(0001) heterostructures prepared by PA-MBE were investigated by morphological, optical, and electrical characterization methods. Furthermore, the implementation of (Ga,In)N/GaN SPSLs as a HIL or as the active region in conventional LED structures have been studied. In-situ measurements demonstrate that the desorption of In increases in the presence of both, N and Mg on the GaN(0001) surface. Further, (Ga,In)N/GaN SLs with Mg-doping grown by PAMBE and their characterization was carried out by QMS, XRD, and SIMS. A better surface morphology was obtained for the (Ga,In)N/GaN SLs in comparison to a (Ga,In)N layer homogeneously doped with Mg. Although, a notable decrease of the In content in the (Ga,In)N ML was revealed when Mg was supplied simultaneously to In. At the same time, the Mg concentration increased in the presence of In, which can possibly be attributed to a surfactant effect. For the SL that had only its (Ga,In)N QWs doped with Mg, a maximum Mg concentration of 2.6 × 1022 cm 3 for a 1 ML thick (Ga,In)N:Mg layer was deduced from SIMS measurements. Additionally, similar results have achieved later by another set of experiments. Thermopower studies revealed the p-type conductivity of the delta doped SL and of the SL structures with 20% of the QB doped by Mg. Additionally, a rectifying behavior with an ideality factor lower than 10 was demonstrated for the (Ga,In)N/GaN SL structures with QW fully doped. Based on the electrical characterization, three different LED structures were fabricated based on the most promising Mg-doped (Ga,In)N SL structures (delta doped SL, and 20% QB doped SL) and characterized.

No description available.

In der vorliegenden Arbeit wurde das Wachstum der planaren aromatischen Moleküle Perylen-3,4,9,10-tetracarbonsäure-3,4,9,10-dianhydrid (PTCDA) und Peri-Hexabenzocoronen (HBC) auf verschiedenen einkristallinen Oberflächen von Gold und Graphit untersucht. Zur Abscheidung der Moleküle und zur Herstellung dünner hochgeordnerter organischer Schichten wurde eine Molekularstrahlepitaxieanlage mit mehreren Kammern aufgebaut. Bei den Untersuchungen des Wachstums von PTCDA auf der Au(111)- und Au(100)-Fläche wurden drei Klassen von Strukturen gefunden: Eine Fischgrätenstruktur, deren Gitterparameter und molekulare Anordnung der (102)-Ebene des PTCDA-Kristalls entsprechen, eine quadratische Struktur, sowie eine Stabstruktur, die der (010)-Ebene des PTCDA-Kristalls zugeordnet werden kann. Während die Stabstruktur auf Au(100) ein inkommensurables Wachstum zeigt, konnten alle anderen PTCDA-Strukturen sowohl auf Au(111) als auch auf Au(100) als point-on-line epitaktisch klassifiziert werden. Die HBC-Schichten auf Au(111), Au(100)hex und Graphit zeigen abweichend von der Kristallstruktur eine hexagonale Symmetrie. Auf Graphit wächst HBC in einer kommensurablen Struktur. Auf den beiden Au-Oberflächen existieren mehrere Strukturen, die sich in ihrer Orientierung und ihren Gitterkonstanten unterscheiden. Neben einer dominierenden HBC-Struktur lassen sich auf den Au-Flächen weitere Strukturen beobachten, deren Auftreten durch den Bedeckungsgrad und die Substratmorphologie, d.h. die Stufenzahl und Terrassengröße des Substrates bestimmt wird. Alle diese HBC-Strukturen konnten als kommensurabel klassifiziert werden. Die Anordnung der HBC-Moleküle in Multilagen wurde für das System HBC auf Au(100)hex mit Hilfe molekularmechanischer Berechnungen modelliert. Die HBC-Moleküle sind in der zweiten Lage gegenüber denen der ersten Lage so verschoben, dass die C-Atome der Moleküle eine graphitähnliche Anordnung zeigen. Wie die STM Untersuchung der organischen Heteroschichten aus HBC und PTCDA zeigen, ist es möglich, epitaktische organische Heteroschichten auch von Molekülen herzustellen, die sich in ihren Gitterkonstanten und in der Symmetrie unterscheiden. Erstmals ließen sich derartige Schichten mittels Rastertunnelmikroskopie direkt und durch LEED auch im reziproken Raum abbilden. Aus dem in den STM Bildern sichtbaren Moirékontrast wurde die Orientierung der beiden organischen Gitter bestimmt. PTCDA wächst bezüglich des HBC-Gitters weder kommensurabel noch point-on-line koinzident, zeigt jedoch eine feste Winkelorientierung. Es handelt sich in diesem Fall um eine inkommensurable Struktur bezüglich des HBC-Gitters, die jedoch bezüglich des zugrundeliegenden Graphitgitters point-on-line Koinzidenz zeigt. Das Versagen der einfachen geometrischen Epitaxiemodelle kann in diesem Fall auf die Existenz mehrerer, energetisch nahezu gleichwertiger Adsorptionsplätze innerhalb der Einheitszelle des Substrates zurückgeführt werden.

In this thesis, the effect of surfactants (Bi /Sb) on GaAs(111) is explored, particularly in regards to modifying the surface morphology and growth kinetics. Both molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD) techniques are discussed in this context. InAs/GaAs(111) quantum dots (QDs) have been promoted as leading candidates for efficient entangled photon sources, owing to their high degree of symmetry (c_3v). Unfortunately, GaAs(111) suffers from a defect-ridden homoepitaxial buffer layer, and the InAs/GaAs(111) material system does not natively support Stranski{Krastanov InAs QD growth. Surfactants have been identified as effective tools to alter grown surface morphologies and growth modes, potentially overcoming these obstacles, but have yet to be studied in detail in this context. For MBE, it is shown that Bi acts as a surfactant when employed in GaAs(111) homoepitaxy, and eliminates defects/hillocks, yielding atomically-smooth surfaces with step-flow growth, and RMS roughness values of 0.13 nm. The effect is more pronounced as the Bi flux increases, and Bi is suggested to be increasing adatom diffusion. A novel reflection high energy electron diffraction (RHEED)-based experiment was also designed and performed to measure the desorption activation energy (U_Des) of Bi on GaAs(111), yielding U_Des = 1.74 ± 0.38 eV. GaAs(111) homoepitaxy was also investigated using MOCVD, with GaAs(111)B exhibiting RMS roughness values of 0.09 nm. Sb is shown to provoke a morphological transition from plastically-relaxed 2D to 3D growth for InAs/GaAs(111)B, showing promise in its ability to induce QDs. Finally, simulations for GaAs-based quantum well (QW) photoluminescence were conducted, and such QWs are shown to potentially produce very sharp linewidths of 3.9 meV. These results enhance understanding of Bi surfactant behaviour on GaAs(111) and can open up its use in many technological applications, paving the way for the realization of high efficiency/viable QD entangled photon sources. / Thesis / Master of Applied Science (MASc)

<p> The mechanisms operating during the metal-assisted growth of III-V nanowires (NWs) by molecular beam epitaxy on (1 1 l)B substrates were investigated through a series of experiments aimed at determining the influence of growth conditions on the morphology and crystal structure. Using GaAs as the principal material system for these studies, it is shown that a good control of these two characteristics can be achieved via a tight control of the temperature, V /III flux ratio, and Ga flux. Low and intermediate growth temperatures of 400°C and 500°C resulted in a strongly tapered morphology, with stacking faults occurring at an average rate of 0.1 nm^(-1). NWs with uniform diameter and the occurrence of crystal defects reduced by more than an order of magnitude were achieved at 600°C, a V /III flux ratio of 2.3, and a Ga impingement rate on the surface of 0.07 nm/s, and suggest the axial growth is group V limited. Increasing the flux ratio favored uniform sidewall growth, thus making the process suitable for the fabrication of core-shell structures. Further observation of steps on the sidewall surface of strongly tapered NWs suggests that radial growth of the shell proceeds in a layer-by-layer fashion, with the edge progressing in a step-flow mode toward the tip. </p> <p> From the experimental considerations, an analytical description of the growth is proposed, based on a simple material conservation model. Direct impingement of growth species on the particle, coupled to their diffusion from the sidewall and the substrate surface, are considered in the derivation of expressions for the time evolution of both axial and radial growths. Factors that take into account the nonunity probability of inclusion of group III adatoms in the axially growing crystal are introduced. Moreover, a step-mediated growth is included to describe the axial evolution of the shell. </p> / Thesis / Doctor of Philosophy (PhD)