Diese Arbeit befasst sich mit der Keimbildung und den Wachstumsmechanismen von GaN-Nanodrähten (NWs), die mittels Molekularstrahlepitaxie (MBE) hergestellt wurden. Die Hauptneuheiten dieser Studie sind der intensive Gebrauch von in-situ Messmethoden und der direkte Vergleich zwischen katalysatorfreien und katalysatorinduzierten NWs. In der MBE bilden sich GaN-NWs auf Silizium ohne Katalysator. Auf Saphir dagegen wachsen NWs unter den gleichen Bedingungen nur in der Anwesenheit von Ni-Partikeln. Die Nukleationsprozesse sind für beide Ansätze fundamental verschieden. In dem katalysatorinduzierten Ansatz reagiert Ga stark mit den Ni-Keimen, deren Kristallstruktur für das Nanodraht-Wachstum entscheidend sind, während in dem katalysatorfreien Ansatz bildet N eine Zwischenschicht mit Si vor der ausgeprägten GaN-Nukleation. Mittels beider Ansätze wachsen einkristalline wurtzite GaN-NWs in Ga-polarer Richtung. Allerdings sind unter denselben Wachstumsbedingungen die katalysatorinduzierten NWs länger als die katalysatorfrei gewachsenen und enthalten viele Stapelfehler. Im Vergleich sind die katalysatorfreien größtenteils defektfrei und ihre Photolumineszenz ist viel intensiver als jene der katalysatorinduzierten NWs. Alle diese Unterschiede können auf den Katalysator zurückgefürt werden. Die Ni-Partikel sammeln die an den Nanodraht-Spitzen ankommenden Ga-Atome ef?zienter ein als die unbedeckte oberste Facette im katalysatorfreien Fall. Außerdem können Stapelfehler sowohl aus der zusätzlichen Festkörperphase des Ni-Katalysators als auch aus der Verunreinigung der NWs mit Katalysatormaterial resultieren. Solch eine Kontaminierung würde schließlich nicht-strahlende Rekombinationszentren verursachen. Somit mag die Verwendung von Katalysatorkeimen zusätzliche Möglichkeiten bieten, das Wachstum von NWs zu kontrollieren. Jedoch sind sowohl die strukturellen als auch die optischen Materialeigenschaften der katalysatorfreien NWs überlegen. / This work focuses on the nucleation and growth mechanisms of GaN nanowires (NWs) by molecular beam epitaxy (MBE). The main novelties of this study are the intensive employment of in-situ techniques and the direct comparison of self-induced and catalyst-induced NWs. On silicon substrates, GaN NWs form in MBE without the use of any external catalyst seed. On sapphire, in contrast, NWs grow under identical conditions only in the presence of Ni seeds. The processes leading to NW nucleation are fundamentally different for both approaches. In the catalyst-assisted approach, Ga strongly reacts with the catalyst Ni particles whose crystal structure and phases are decisive for the NW growth, while in the catalyst-free approach, N forms an interfacial layer with Si before the intense nucleation of GaN starts. Both approaches yield monocrystalline wurtzite GaN NWs, which grow in the Ga-polar direction. However, the catalyst-assisted NWs are longer than the catalyst-free ones after growth under identical conditions, and they contain many stacking faults. By comparison the catalyst-free NWs are largely free of defects and their photoluminescence is much more intense than the one of the catalyst-assisted NWs. All of these differences can be explained as effects of the catalyst. The seed captures Ga atoms arriving at the NW tip more efficiently than the bare top facet in the catalyst-free approach. In addition, stacking faults could result from both the presence of the additional solid phase constituted by the catalyst-particles and the contamination of the NWs by the catalyst material. Finally, such contamination would generate non-radiative recombination centers. Thus, the use of catalyst seeds may offer an additional way to control the growth of NWs, but both the structural and the optical material quality of catalyst-free NWs are superior.

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This thesis reports a comprehensive study of quaternary BeMgZnO alloy and BeMgZnO/ZnO heterostructures for UV-optoelectronics electronic applications. It was shown that by tuning Be and Mg contents in the heterostructures, high carrier densities of two-dimensional electron gas (2DEG) are achievable and makes its use possible for high power RF applications. Additionally, optical bandgaps as high as 5.1 eV were achieved for single crystal wurtzite material which allows the use of the alloy for solar blind optoelectronics (Eg>4.5eV) or intersubband devices. A systematic experimental and theoretical study of lattice parameters and bandgaps of quaternary BeMgZnO alloy was performed for the whole range of compositions. Composition independent bowing parameters were determined which allows accurate predictions of experimentally measured values. The BeMgZnO thin films were grown by plasma assisted molecular beam epitaxy (P-MBE) in a wide range of compositions. The optimization of the growth conditions and its effects on the material properties were explored. The surface morphology and electrical characteristics of the films grown on (0001) sapphire were found to critically depend on the metal-to-oxygen ratio. Samples grown under slightly oxygen-rich conditions exhibited the lowest RMS surface roughness (as low as 0.5 nm). Additionally, the films grown under oxygen-rich conditions were semi-insulating (>105 Ω∙cm), while the films grown under metal-rich conditions were semiconducting (~102 Ω∙cm). Additionally, with increasing bandgap Stokes shift increases, reaching ~0.5 eV for the films with 4.6 eV absorption edge suggests the presence of band tail states introduced by potential fluctuations and alloying. From spectrally resolved PL transients, BeMgZnO films grown on a GaN/sapphire template having higher Mg/Be content ratio exhibit smaller localization depth and brighter photoluminescence at low temperatures. The optimum content ratio for better room temperature optical performance was found to be ~2.5. The BeMgZnO material system and heterostructures are promising candidates for the device fabrication. 2DEG densities of MgZnO/ZnO heterostructures were shown to improve significantly (above 1013 cm-2) by adding even a small amount of Be (1-5%). As an essential step toward device fabrication, reliable ohmic contacts to ZnO were established with remarkably low specific contact resistivities below 10-6 Ohm-cm2 for films with 1018 cm-3 carrier density.

Nesse trabalho, foi investigado um novo tipo de fotodetector de radiação infravermelha baseado em pontos quânticos de submonocamada de InAs obtidos pela técnica de epitaxia por feixe molecular (MBE, Molecular Beam Epitaxy). Suas propriedades foram comparadas com as de fotodetectores de pontos quânticos de InAs convencionais obtidos pela mesma técnica de deposição, mas no modo de crescimento Stranski-Krastanov. Medidas de corrente de escuro, de ruído, de responsividade e de absorção mostraram que, dependendo da estrutura das amostras, os dispositivos com pontos quânticos de submonocamada podem ter um excelente desempenho. / In this work, we investigated a new type of infrared photodetector based on InAs sub-monolayer quantum dots grown by molecular beam epitaxy (MBE). Their properties were compared with those of photodetectors containing conventional InAs quantum dots obtained by the same deposition technique, but in the Stranski-Krastanov growth mode. Dark current, noise, responsivity and absorption measurements have shown that, depending on the structure of the samples, the devices with sub-monolayer quantum dots can perform very well.

L'auteur n'a pas fourni de résumé en français / The epitaxial growth condition and the superconducting properties of nanostructured devices made of rhenium (superconducting below T=1.7 K) on sapphire were explored. Epitaxial growth of rhenium thin films onto a single crystal α-Al2O3(001) substrate was realised using molecular beam epitaxy. The cleanness of the substrate was verified using XPS, and the growth of rhenium was monitored using RHEED. The orientations of the two crystals are (0001)Al2O3//(0001)Re and <2110>Al2O3//<0110>Re, which was confirmed using X-ray diffraction. The in-plane misfit between the lattices is -0.43% at room temperature, which allows us to estimate the critical thickness of rhenium to be between 10 nm and 15 nm.For deposition, rhenium was heated using an electron beam. Substrates were heated during growth using either a Joule-heated W filament located behind the sample or electron bombardment. Generally deposition temperatures of 800◦C and 900◦C gave reproducible results.The effect of deposition temperature was studied on samples that had the same thickness but were deposited at different temperatures. Three thickness groups were selected: 25 nm, 50 nm and 100 nm. Every sample was dominated by the (001) epitaxial orientation. Orientations (110), (100), (101) were present, but their intensities were small and decreased with increasing deposition temperature. AFM imaging was used to observe the morphology of the films. The 25 nm thick films were decorated with grains. The diameter of the grains (∼ 50 nm) did not vary significantly on the 25 nm thick sample, however, they became more uniform with increasing deposition temperature, and the surface became smoother. On the 50 nm and 100 nm thick films spirals and holes can be observed. Diameter of spirals on the 50 nm thick film increased from 100 nm to 500 nm when the temperature of the deposition was increased from 800◦C to 900◦C. XRD rocking curves measured on all samples got narrower with increasing deposition temperatures, indicating lower mosaicity of the (001) crystals. High-resolution θ-2θ scans evidenced a disorder in the 50 nm thick film, corresponding to strain values in the range of 0.01. Deposition temperature of 1000◦C lead to the dewetting of a 50 nm thick sample, islands with atomically flat surfaces were formed.The frequently observed spirals are most likely the result of screw dislocations. The origin of the holes that accompany the spirals is a dewetting process that starts when the thickness of the film reaches ~10 nm. We quantified the temperature evolution of the film during growth taking into account emission, reflection and transmission between all surfaces. This thermal model confirmed that the temperature of the film increases as the thickness of the rhenium film grows. The dewetting was studied using Mullins’ theory of thermal grooving. A surface diffusion coefficient of 4E−12 cm2/s was obtained, which is consistent with the observed dimensions of the surface topography.Wires with widths ranging from 100 nm to 3 μm and SQUIDs were fabricated from the rhenium films. Transport measurements confirmed that the lithography process does not affect the superconducting properties of rhenium. Critical temperatures between 1.43 K and 1.96 K were measured. We could correlate the superconducting transition temperature with the topography and the crystallinity of the films. Mean free path of electrons, and the superconducting coherence length were obtained, for two of the films both mean free path and effective coherence length were over 100 nm. These two films were in the clean limit, but the fabricated wires were in the dirty limit.On one film SQUIDs of 1 um diameter with 50 nm and 20 nm wide nanobridges acting as Josephson junctions were fabricated. The SQUIDs were cooled down using a dilution refrigerator. Critical current oscillations were measured. The flux noise values obtained were as low as 2.6E−5 Φ0/Hz1/2.

Ce travail porte sur la croissance par épitaxie sous jets moléculaires (EJM) et sur les propriétés structurales et optiques de boîtes quantiques (BQs) AlyGa1-yN insérées dans une matrice AlxGa1-xN (0001). L’objectif principal est d’étudier le potentiel des BQs en tant que nouvelle voie pour la réalisation d’émetteurs ultraviolets (UV) efficaces.Tout d'abord, nous avons étudié la croissance des BQs GaN en utilisant soit une source plasma (N2, appelée PAMBE) soit une source ammoniac (NH3, appelée NH3-MBE) afin de choisir la meilleure approche pour former les BQs les plus efficaces. Il a été montré que le procédé de croissance est mieux contrôlé en utilisant l’approche PAMBE, conduisant à la croissance de BQs GaN avec des densités plus élevées, une meilleure uniformité en taille et des intensités de photoluminescence (PL) jusqu’à trois fois plus élevées. En outre, l'influence de la contrainte épitaxiale sur le processus d'auto-assemblage des BQs a été étudiée en fabriquant des BQs GaN sur différentes couche tremplins d’AlxGa1-xN (avec 0,5 ≤ x ≤ 0,7). Nous avons montré que des BQs avec des densités plus élevées et des hauteurs plus faibles sont formées en augmentant le désaccord de paramètre de maille (c.à.d en utilisant des tremplins avec xAl plus élevé). Cependant, les mesures de photoluminescence (PL) indiquent un fort décalage de l'énergie d'émission vers le rouge lorsque xAl augmente, en raison de l'augmentation de la discontinuité du champ électrique interne de 3 à 5,3 MV/cm.Ensuite, des études approfondies sur les conditions de croissance et les propriétés optiques des BQs Al0,1Ga0,9N / Al0,5Ga0,5N ont été présentées, montrant les différents défis pour fabriquer des BQs efficaces. L’optimisation de la procédure de croissance, notamment l’étape de recuit post-croissance, a montré une modification de la forme des BQs. Plus précisément, un changement d’une forme allongée (pour un recuit à 740 °C), à une forme symétrique (pour un recuit à une température proche de ou supérieure à 800°C) a été observé. En plus, une bande d’émission supplémentaire vers les plus grandes longueurs d’onde a également été observée pour les BQs formées avec un recuit à 740°C. Cette bande a été attribuée à une fluctuation de composition des BQs, induisant la formation d’une famille additionnelle de BQs avec des hauteurs plus grandes et une compostions en Al inférieure à 10 %, estimée proche de l’alliage binaire GaN. Enfin, il a été démontré qu’en faisant un recuit à plus haute température (≥ 800°C), l’émission de PL de cette famille supplémentaire de BQs (BQs riche en Ga ou (Al)GaN) diminue très fortement. De plus, cette étape de recuit impacte fortement la forme des BQs et a conduit à une amélioration de leur efficacité radiative d’un facteur 3. Ensuite, la variation de la composition en Al des BQs AlyGa1-yN (0,1 ≤ y ≤ 0,4), ainsi que la quantité de matière déposée ont permis d’évaluer la gamme de longueurs d’onde d’émission accessibles. En ajustant les conditions de croissance, l’émission des BQs a été déplacée de l’UVA vers l’UVC, atteignant une émission autour de 270 - 275 nm (pour les applications de purification de l’eau et de l’air) avec des rendements radiatifs élevés. Les mesures de photoluminescence résolue en temps (TRPL), combinées avec les mesures de PL en fonction de la température, nous ont permis de déterminer les efficacités quantiques internes (IQE) des BQs GaN / AlxGa1-xN (0001). Des valeurs d’IQE comprises entre 50 % et 66 % ont été obtenues à basse température, avec la possibilité d’atteindre un rapport d’intensité intégré de PL, entre 300 K et 9 K, allant jusqu’à 75 % pour les BQs GaN et 46 % pour les BQs AlyGa1-yN (contre 0,5 % pour des structures équivalents à base de puits quantiques).Enfin, nous avons montré la possibilité de fabriquer des DELs à base de BQs (Al,Ga)N couvrant une grande gamme de longueurs d’onde allant du bleu-violet jusqu’à l’UVB (de 415 nm à 305 nm). / This PhD deals with the epitaxial growth, structural and optical properties of AlyGa1-yN quantum dots (QDs) grown on AlxGa1-xN (0001) by molecular beam epitaxy (MBE), with the aim to study their potential as a novel route for efficient ultraviolet (UV) emitters.First, we have studied the growth of GaN QDs using either plasma MBE (PAMBE) or ammonia MBE (NH3-MBE) to find the most adapted nitrogen source for the fabrication of UV emitting QDs. It was shown that the growth process is better controlled using PAMBE, leading to the growth of GaN QDs with higher densities, better size uniformity and up to three times higher photoluminescence (PL) intensities. Also, the influence of the epitaxial strain on the QD self-assembling process was studied by fabricating GaN QDs on different AlxGa1-xN surfaces (with 0.5 ≤ x ≤ 0.7). We showed that QDs with higher densities and smaller sizes (heights) are formed by using a larger lattice-mismatch (i.e. a higher xAl composition). However, photoluminescence (PL) measurements indicated a strong redshift in the emission energy as the Al content of the AlxGa1-xN template increases due to the increase of the internal electric field discontinuity from 3 to 5.3 MV/cm.Next, in-depth investigations of the growth conditions and optical properties of Al0.1Ga0.9N QDs / Al0.5Ga0.5N were done presenting the different challenges to be solved to grow efficient QDs. Changing the growth procedure, especially the post-growth annealing step, has shown a modification of the QD shape from elongated QDs, formed with an annealing at 740°C, to symmetric QDs, formed with an annealing at a temperature around or above 800°C. An additional band emission at lower energies was also observed for QDs grown with a lower annealing temperature (740°C). This additional band emission was attributed to the formation of QDs with higher heights and a reduced Al composition less than the nominal one of 10 % (i.e. forming Ga-rich QDs). The influence of the annealing step performed at higher temperature has been shown to strongly decrease the PL emission from this additional QD family. In addition, this annealing step strongly impacted the QD shape and led to an improvement of the QD radiative efficiency by a factor 3. Then, the AlxGa1-xN barrier composition (0.5 ≤ x ≤ 0.7), the AlyGa1-yN QD composition (0.1 ≤ y ≤ 0.4) as well as the deposited amount were varied in order to assess the range of accessible emission energies. Also, the influence of varying the AlxGa1-xN barrier composition on the QD formation was studied. By varying these growth conditions, the QD wavelength emission was shifted from the UVA down to the UVC range, reaching a minimum wavelength emission of 270 - 275 nm (for water and air purification applications) with a high radiative efficiency. Time resolved photoluminescence (TRPL) combined with temperature dependent PL measurements enabled us to determine the internal quantum efficiencies (IQE) of AlyGa1-yN QDs / AlxGa1-xN (0001). IQE values between 50 % and 66 % were found at low temperature, combined with the ability to reach a PL integrated intensity ratio, between 300 K and 9 K, up to 75 % for GaN QDs and 46 % for AlyGa1-yN QDs (versus 0.5 % in a similar quantum well structure emitting in the UVC range).Finally, the demonstration of AlyGa1-yN QD-based light emitting diode prototypes, emitting in the whole UVA range, using GaN and Al0.1Ga0.9N QDs, and in the UVB range down to 305 nm with Al0.2Ga0.8N QDs active regions, was shown.

Rotationally averaged polarizabilities and intrinsic electric dipole moments of sodium clusters are measured and reported. The experimental method is a molecular beam deflection. Our precision is the highest (<5%) and the range of the cluster sizes is the broadest to date (Na₁₀ ∼ Na₃₀₀). Compared to the earlier measurements, our data covers all sizes with no gaps up to the largest cluster. The fine structure in the polarizability curve is previously unobserved. We have carefully ruled out several possible explanations. And we find an earlier existing theory could explain the facts but will lead to magic numbers which were not seen in some previous experiments. A detailed theory is needed to understand the behaviors we see. Intrinsic electric dipole moments (EDM) of sodium clusters are probed to answer the intriguing question: Do metal clusters develop electric dipole moments like molecules? Some theories have predicted the existence of EDM in ground state sodium clusters and gave their magnitudes. We put upper bounds on the EDM of sodium clusters and find that they are orders of magnitude smaller than the predictions. This provokes an interesting question: how can one define metallicity in metal clusters? Our measurements are performed at cryogenic temperature 20 Kelvin. At this temperature the clusters are believed to be in their vibronic ground states.