Ce travail de thèse porte sur la croissance par épitaxie par jets moléculaires assistée plasma et sur la caractérisation de nanofils (NF) de GaN et d'hétérostructures filaires de GaN/AlN. Dans un premier temps, la morphologie des NFs de GaN (densité, longueur moyenne, diamètre moyen, dispersion de longueurs) est étudiée en fonction des paramètres de croissance. Via la diffraction d'électrons rapides, la morphologie des NFs GaN est corrélée à la dynamique de nucléation de ces derniers. Des expériences de diffraction de rayons X en incidence rasante effectuées à l'ESRF permettent également de clarifier les processus de nucléation des NFs GaN. Nous démontrons ensuite l'utilisation de la diffraction de rayons X résonnante pour déterminer la polarité des NFs GaN. Nous montrons que ces derniers sont de polarité N lorsque fabriqués sur Si nu. Des tests complémentaires de gravure sélective au KOH révèlent que les NFs GaN fabriqués sur un substrat de Si recouvert d'un fin buffer d'AlN ainsi que ceux dont la fabrication est initiée après pré-déposition de Ga sur la surface du Si, sont aussi de polarité N. Concernant les hétérostructures filaires GaN-AlN, la croissance d'AlN autour et sur les nanofils de GaN est étudiée en fonction de divers paramètres de croissance. Le rapport d'aspect des coquilles d'AlN (longueur/épaisseur) est décrit par un modèle géométrique. En utilisant une combinaison de diffraction anomale multi-longueurs d'onde, de microscopie en transmission de haute résolution et des calculs théoriques, l'état de contrainte des coeurs de GaN est analysé en fonction de l'épaisseur de la coquille. Cette contrainte augmente avec l'épaisseur de la coquille tant que l'AlN croît de manière homogène autour des NFs de GaN. Dès lors que la coquille est asymétrique, le système relaxe plastiquement. Nous étudions enfin la possibilité de fabriquer des îlots de GaN dans des NFs AlN. Nous déterminons le rayon critique de NFs AlN au-dessus duquel le GaN déposé subit une transition de forme de 2D à 3D. L'analyse des propriétés optiques de ces nanostructures originales revèle la présence de nombreux états localisés. / This work focuses on the growth by plasma-assisted molecular beam epitaxy and on the characterization of GaN nanowires (NWs) and of GaN/AlN NW heterostructures. We first investigate GaN NW morphology (density, mean length, mean diameter, length dispersion) dependence on the growth parameters. Using reflection high energy electron diffraction, GaN NW morphology is correlated to their nucleation dynamics. In situ grazing-incidence X-ray diffraction experiments performed at the ESRF allow clarifying GaN NW nucleation processes on bare Si(111) and when usinga thin AlN buffer deposited on Si(111). The use of resonant X-ray diffraction for the determination of GaN NW polarity is then successfully demonstrated. GaN NWs grown on bare Si(111) are shown to be N-polar. Additional KOH selective etching tests reveal that both GaN NWs grown using a thin AlN buffer on Si(111) and when pre-depositing Ga on the Si(111) surface are N-polar, too. Regarding GaN-AlN NW heterostructures, the growth of an AlN shell around GaN NWs is studied as a function of various growth parameters. The AlN shell aspect ratio is described by a geometrical model. Using a combination of multiwavelength anomalous diffraction, high resolution transmission microscopy and theoretical calculations, GaN core strain state is investigated as a function of the AlN shell thickness. This strain is shown to increase with the shell thickness as long as AlN grows homogeneously around GaN NWs. When the shell is asymmetric, the system relaxes plastically. Eventually, we study the possibility to fabricate island-like GaN insertions in AlN NWs. We determine the critical AlN NW radius above which GaN undergoes a 2D to 3D shape-transition. Regarding optical properties of these novel structures, the presence of multiple localized states is identified.

Ce travail a pour objectif la fabrication, en épitaxie par jets moléculaires, de nanofils coeurcoquilleà base de GaAs et AlGaAs déposés sur des substrats Si(111), en vue de réaliser desréseaux de fils pour de nouvelles cellules solaires, et pour des fils photoniques permettant uneapproche bottom-up d’émetteurs de photons uniques.La première partie de ce travail est une étude systématique des paramètres clés qui contrôlent lacroissance uni-dimensionnelle de fils GaAs élaborés par un mécanisme vapeur-liquide-solideauto-catalysé, à savoir le rapport des flux As/Ga, la température du substrat, et la vitesse decroissance.La seconde partie se concentre sur la croissance et la caractérisation de fils GaAs recouvertsd’une coquille d’alliages AlGaAs (35% Al) afin de s’affranchir des recombinaisons de surface.Ces coquillesde AlGaAs sont fabriquées en conditions riche-As (rapport As/Ga > 10) afin deconsommer les gouttes de catalyseur gallium et de promouvoir une croissance radiale (le taux decroissance maximal axial/radial est égal à 6). Diverses caractérisations optiques sont réalisées àbasse température sur ces ensembles de fils : cathodoluminescence, photoluminescence etspectroscopie résolue en temps. L’intensité de luminescence et la durée de vie des porteursaugmentent fortement avec la présence de la coquille : une épaisseur de 7 nm de cette dernièreest suffisante pour optimiser la passivation des nanofils et supprimer les recombinaisons liéesaux états de surface. Une fine couche extérieure de GaAs est nécessaire pour éviter touteoxydation de la coquille d’alliage AlGaAs.De plus, grâce à des mesures de CL résolues spatialement, les longueurs de diffusion desexcitons dans ces fils ont été obtenues, allant de 0.7 μm à 1.5 μm pour des épaisseurs decoquilles comprises entre 20 et 50 nm. Des valeurs plus petites sont mesurées pour des coquillesplus épaisses, ce qui tend à montrer l’introduction de défauts dans l’alliage qui pourraientlimiter la qualité de l’interface. Le décalage en énergie de l’émission fournit des informationssur la génération de contraintes dans ces fils coeur-coquille et sur le champ piézo-électrique quien découle. / This report focuses on the fabrication of GaAs nanowires and GaAs/AlGaAs core-shellstructures by molecular beam epitaxy, deposited on Si (111) substrates in order to providearrays of wires for innovative solar cells and bottom-up photonic wires for efficient singlephoton emitters.The first part of this work is a systematic study of the key parameters which control the onedimensionalgrowth of bare GaAs NWs with a self-assisted vapor-liquid-solid growth process,namely the As-to-Ga flux ratio, the substrate temperature, and the deposition rate.The second part concentrates on the growth and characterization of GaAs wires covered with ashell of AlGaAs alloy (35 % Al) in order to get rid of the surface recombinations. These shellswere fabricated under As-rich condition with ratio As/Ga >10 in order to consume the Gadropletscompletely and to promote a radial growth. The obtained axial-to-radial growth ratio is6. The optical characterizations on ensemble were carried out at low temperature via thecathodoluminescence (CL), photoluminescence (PL), and time-resolved PL measurements. Theresults show that the lifetime of carriers and luminescence intensity increase significantly withshell coverage. About 7 nm thick shell is enough to optimize the passivation and suppress thesurface state recombination. A thin outer cap of GaAs is required in order to prevent someoxidation of the AlGaAs alloy shell.In addition, the exciton diffusion lengths of these NWs, studied via the spatially resolved CL,are in the range of 0.7 - 1.5 μm for NWs with shell thicknesses between 20 - 50 nm. Thesevalues are smaller for thicker shells due to the defect formation, leading to limit the quality ofcore-shell interface. The shift in optical emission experiments provides the information of thestrain generation of core-shell when we vary the shell thickness. The piezoelectric field wasnoticed in these samples.

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.

No description available.

abstract: III-V-bismide semiconductor alloys are a class of materials with applications in the mid and long wave infrared spectrum. The quaternary alloy InAsSbBi is attractive because it can be grown lattice-matched to commercially available GaSb substrates, and the adjustment of the Bi and Sb mole fractions enables both lattice constant and bandgap to be tuned independently. This dissertation provides a comprehensive study of the surface morphology and the structural and chemical properties of InAsSbBi alloys grown by molecular beam epitaxy. 210 nm thick InAsSbBi layers grown at temperatures from 280 °C to 430 °C on (100) on-axis, (100) offcut 1° to (011), and (100) offcut 4° to (111)A GaSb substrates are investigated using Rutherford back scattering, X-ray diffraction, transmission electron microscopy, Nomarski optical microscopy, atomic force microscopy, and photoluminescence spectroscopy. The results indicate that the layers are coherently strained and contain dilute Bi mole fractions. Large surface droplets with diameters and densities on the order of 3 µm and 106 cm-2 are observed when the growth is performed with As overpressures around 1%. Preferential orientation of the droplets occurs along the [011 ̅] step edges offcut (100) 1° to (011) substrate. The surface droplets are not observed when the As overpressure is increased to 4%. Small crystalline droplets with diameters and densities on the order of 70 nm and 1010 cm-2 are observed between the large droplets for the growth at 430°C. Analysis of one of the small droplets indicates a misoriented zinc blende structure composed of In, Sb, and Bi, with a 6.543 ± 0.038 Å lattice constant. Lateral variation in the Bi mole fraction is observed in InAsSbBi grown at high temperature (400 °C, 420 °C) on (100) on-axis and (100) offcut 4° to (111)A substrates, but is not observed for growth at 280 °C or on (100) substrates that are offcut 1° to (011). Improved crystal and optical quality is observed in the high temperature grown InAsSbBi and CuPtB type atomic ordering on the {111}B planes is observed in the low temperature grown InAsSbBi. Strain induced tilt is observed in coherently strained InAsSbBi grown on offcut substrates. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

Die vorliegende Arbeit befasst sich mit dem Einfluss einer hyperthermischen Stickstoffionenbestrahlung (Ekin < 25 eV) auf das Galliumnitrid-Schichtwachstum. Dabei wird insbesondere der Einfluss einer Oberflächenrekonstruktion, einer Strukturierung der Oberfläche, einer Zwischenschicht (Pufferschicht) und der Einfluss verschiedener Siliziumsubstratorientierungen auf das epitaktische Wachstum von dünnen Galliumnitrid-Schichten nach einer hyperthermischen Stickstoffionenbestrahlung diskutiert. Ziel war es, möglichst dünne, epitaktische und defektarme Galliumnitrid-Schichten zu erhalten. Für die Charakterisierung der Galliumnitrid-Schichten und der Siliziumsubstrate standen diverse Analysemethoden zur Verfügung. Die kristalline Oberflächenstruktur konnte während des Wachstums mittels Reflexionsbeugung hochenergetischer Elektronen beobachtet werden. Nachfolgend wurde die Oberflächentopografie, die kristalline Struktur und Textur, sowie die optischen Eigenschaften der Galliumnitrid-Schichten mittels Rasterkraftmikroskopie, Röntgenstrahl-Diffraktometrie, hochauflösender Transmissionselektronenmikroskopie und Photolumineszenzspektroskopie untersucht.

No description available.

No description available.

<p>  </p> <p>Research that seeks to understand and develop the growth of III-nitride materials by molecular beam epitaxy (MBE) is beneficial to a broad range of the device community. MBE and the III-nitrides have been used to develop transistors, diodes, electroacoustic devices, solar cells, LEDs, LDs, intersubband devices, and quantum-cascade lasers. In this work we focus on the growth of III-nitride materials specifically for applications in near-infrared intersubband (NIR ISB) optical devices, however all this work is broadly applicable. </p> <p><br></p> <p>We begin by investigating the reduced indium incorporation in non-polar m-plane InGaN films. We find that InGaN grown on m-plane GaN has an effective activation energy for thermal decomposition of 1 eV, nearly half that reported for similar c-plane films. We produce high quality m-plane In0.16Ga0.84N and utilize it in AlGaN/InGaN devices designed for near-infrared ISB absorption measurements. We continue this work by exploring the growth of low-temperature AlGaN, necessary for these devices. We find that the utilization of an indium surfactant during low-temperature AlGaN growth enhances adatom diffusion, resulting in smoother surface morphologies, sharper interfaces, and reduced defects within the material. This growth method also prevents the anomalous suppression of the AlGaN growth rate, which we link to a reduction in the formation of high-aluminum containing defects. These investigations result in the demonstration of an Al0.24Ga0.76N/In0.16Ga0.84N heterostructure with a conduction band offset large enough to enable NIR ISB transitions.</p> <p><br></p> <p>Lastly, we explore the novel material ScAlN. This material’s large bandgap, large spontaneous polarization, ferroelectricity, and ability to be lattice matched to GaN at ~18% scandium composition make it an ideal candidate for a variety of devices, including NIR ISB devices. We investigate the reported temperature dependence of ScAlN’s <em>c</em>-lattice constant and confirm this dependence is present for high growth-temperature ScxAl1-xN with 0.11 < x < 0.23. We find that this temperature dependence is no longer present below a certain composition-dependent growth temperature. This finding, coupled with observations that samples grown at lower temperatures exhibit lower defect densities, smoother surfaces, and homogeneous chemical compositions suggest that high growth temperatures lead to defect generation that may cause the observed change in lattice parameters. We demonstrate lattice-matched, 50 repeat Sc0.18Al1-xN/GaN heterostructures with ISB absorption in excess of 500 meV with FWHM as little as 45 meV. </p>

No description available.