Os resultados apresentados neste trabalho estão sistematizados em três partes. Em uma primeira etapa, efetuamos um estudo sistemático do comportamento da estrutura eletrônica em super-redes de deltas em função do período da super-rede e em função da concentração planar de dopantes. Uma nova abordagem, que se baseia no método celular e na solução autoconsistente das equações de Schroedinger e de Poisson, foi desenvolvida e aplicada para super-redes com dopagem planar tipo n em GaAs e em silício. Em ambos os casos, foi observada a transição de um comportamento eletrônico de caráter bi- para tridimensional conforme o período da super- rede diminui. No caso de super-redes de deltas de Si em GaAs foi empreendido o cálculo da energia de corte nos espectros de fotoluminescência de excitação. Uma boa concordância com as medidas experimentais foi obtida. O estudo da estrutura eletrônica para o caso de super-rede de deltas de Sb em Si foi pioneiro. Isto tornou os resultados de nossa investigação teórica de importância fundamental para experimentais e teóricos atuando na 6rea. A concordância entre nossas previsões teóricas e dados experimentais da literatura demonstram a consistência e o poder da abordagem desenvolvida. Em uma segunda etapa, foi efetuado o estudo da distribuição espacial do campo elétrico interno em heteroestruturas contendo camadas tipo \"bulk\", compostas por GaAs e (A1Ga)As. Uma nova abordagem foi desenvolvida para a execuqi3o de cálculos dos perfis de potencial eletrostático e de campo elétrico, sem assumir a ionização total dos dopantes e a não-degenerescência do material. Nosso método transforma a equação de Poisson em uma equação integral que deve ser resolvida autoconsistentemente. Os exemplos numéricos demonstram a aplicabilidade de nossa abordagem a sistemas reais. Perfis do campo elétrico calculados são usados para interpretar os espectros de fotorefletância. Em uma terceira etapa, a teoria geral da fotorefletância de heteroestruturas semicondutoras foi desenvolvida neste trabalho para tornar a interpretação de espectros de fotorefletância precisa e de aplicação eficiente. Um novo metodo de cdculo do coeficiente de reflexgo na presenga de inomogeneidade espacial da funggo dieletrica no interior de cada camada fmeceu um novo patamar de cornpreens20 dos espectros de fotorefletiincia. Este metodo e baseado na construgiio de uma matriz de transferhcia que iraclui as inomogeneidades no interior da camada de um mod0 integral. Portanto, para descrever uma camada de heteroestrutura e preciso ter apenas uma ma& de transferencia. 0s resultados de simulag6es numericas de espectros da fotoreflethcia estilo em uma concordhcia bastante boa com aqueles obtidos atravb de medidas opticas. A eficiencia de nosso metodo o torna aplicavel a simulag6es tip0 \"on-line\". 0s resultados dos metodos anteriores sgo reproduzidos como casos limites de nossa abordagem geral. / The results presented in this work can be displayed along the following three lines. In the first we performed a systematical study of the electronic structure behavior in delta superlattices as a function of superlattice period and sheet doping concentration. A new approach, based on the cellular method and on the selfconsistent solution of Schroedinger and Poisson equations, was developed and applied to superlattices with n-type delta doping in GaAs and silicon. In both cases, a transition from bi- to three- dimensional electronic behavior with the decrease of superlattice period was observed. For Si delta-doping superlattices in GaAs we performed calculations of the energy threshold in the photoluminescence excitation spectra. A good agreement with experimentally measured values was observed. Our investigation of the electronic structure of Sb delta-doping superlattices in Si was a pioneer theoretical study. Due to thls fact, the results of our work are of great importance for experimentalists and theoreticians acting in this area. The agreement between our theoretical predictions and the available experimental data demonstrates the consistency and the power of the developed approach. Along the second line we studied electric field spatial distribution inside of heterosinctures containing bulk layers of GaAs and (A1Ga)As. A new approach was developed to calculate the electrostatic potential and electric field profiles, providing the possibility to take .into account the incomplete ionization of impurities and the degeneracy of the materials. Our method transforms the Poisson equation into an integral equation, which must be solved selfconsistently. Numerical examples show the way to apply our approach to real systems. Internal electric field proiiles, calculated by means of our method are used to interpret photoreflectance spectra. In the third line, a general theory of photoreflectance for semiconductor heterostructures was developed in this work to make the interpretation of fotoreflectance spectra more precise and straightfornard. A new method to calculate the reflection coefficient in the presence of weak spatial inhomogenities of the dielectrical function inside each layer, provided us with a new degree of comprehension of the photoreflectance spectra. This method is based on the construction of a transfer matrix which includes the inhomogenities inside the layer in an integral way. This explains why we need only one matrix to describe one layer of the heterostructure. Results of our numerical simulations are in very good agreement with data of optical measurements. The efficiency of our method makes it suitable for on-line simulations. The results of previous methods emerge from our general approach as limit cases.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / In this work we used the density functional theory to study InAs and InP nanowires and InAs/InP nanowire heterostructures. Initially we studied the structural, electronic and mechanical properties of InAs and InP nanowires as a function of the diameter and the influence of external mechanical stress on the electronic properties of these systems. Our results show that all analyzed properties change with increasing quantum confinement. Further, the application of an external stress along the nanowire axis reveals a direct to indirect band gap transition for compressive strain in very thin nanowires. We have also studied the quantum confinement effects on the effective masses of charge carriers in InAs nanowires grown in different crystallographic directions. We found the electron and hole effective masses increase with decreasing diameter independently of the growth direction. However, in the range of the studied diameters, the hole effective mass is significantly smaller to the corresponding one at the bulk system. From the study of the stability and electronic properties of the cadmium and zinc doped InAs nanowires, we show that the Cd impurity prefers to be at the core region, whereas Zn impurity is found to be equally distributed along the nanowire diameter. The analysis of the electronic properties of these systems show that these impurities introduce shallow acceptor levels in the band gap, enabling a p-type behavior of these nanowires. Finally, we determined (i) the structural, electronic and mechanical properties of axially and radially modulated InAs/InP nanowire heterostructures for a specific diameter and (ii) the structural and electronic properties of radial InAs/InP nanowire heterostructures as a function of the diameter and composition. From (i), our calculations showed the analyzed properties have an intermediate value between those for the pure InAs and InP nanowires with similar diameters. In particular, the presence of an InP shell covering the InAs nanowires enhances the InAs electron mobility, as compared to the uncapped InAs nanowires. In addition, for the radial heterostructure, the conduction and the valence band alignments favor a type-I heterojunction, while for the axial heterostructure a transition from a type-I to a type-II heterojunction could occur at this range of diameters. From (ii), we observed that for nanowire heterostrutures of similar diameters, the variation of their structural and electronic properties with the composition possesses significant deviations from the linear behavior, which are dependent of the nanostructure diameter. The conduction band offset is approximately zero and the valence band offset decrease regardless of diameter and composition of the heterostructure. / Neste trabalho realizamos um estudo teórico, baseado na teoria do funcional da densidade, em nanofios de InAs e InP e em heteroestruturas de nanofios InAs/InP. Inicialmente estudamos a variação das propriedades estruturais, eletrônicas e mecânicas com o diâmetro em nanofios de InAs e InP, e as possíveis alterações nas propriedades eletrônicas destes sistemas sob a influência de uma tensão mecânica externa. Nossos resultados mostram que todas as propriedades analisadas são alteradas com o aumento do confinamento quântico. Além disso, a aplicação de uma tensão externa ao longo do eixo de crescimento dos fios leva a uma transição de gap direto para indireto nos nanofios de menores diâmetros. A seguir, avaliamos os efeitos do confinamento quântico na massa efetiva dos portadores de carga em nanofios de InAs crescidos em diferentes direções cristalográficas. Encontramos que as massas efetivas dos elétrons e dos buracos aumentam com a redução do diâmetro, independentemente da direção de crescimento dos nanofios. Contudo, no intervalo de diâmetro estudado, a massa efetiva dos buracos nos nanofios é significativamente menor do que a massa efetiva dos buracos no cristal. Do estudo da estabilidade e das propriedades eletrônicas de nanofios de InAs dopados substitucionalmente com cádmio e zinco observamos que, independentemente do diâmetro dessas nanoestruturas, as impurezas de Cd são mais estáveis quando estão no centro do nanofio, enquanto que as impurezas de Zn se distribuem quase que uniformemente ao longo do diâmetro do fio. Do ponto de vista eletrônico, observamos que estas impurezas introduzem níveis aceitadores rasos no gap de energia desses materiais possibitando um comportamento tipo-p desses nanofios. Por fim, determinamos: (i) as propriedades estruturais, eletrônicas e mecânicas de heteroestruturas axiais e radiais de nanofios InAs/InP para um determinado diâmetro; e (ii) as propriedades estruturais e eletrônicas de heteroestruturas radiais InAs/InP como uma função do diâmetro e da composição. Em (i), nossos resultados mostram que as propriedades analisadas possuem valores intermediários entre aqueles dos nanofios de InAs e InP de mesmo diâmetro. Em particular, observamos que a presença de uma camada de InP sobre nanofios de InAs aumenta significativamente sua mobilidade eletrônica quando comparada com a de um nanofio de InAs puro. Além disso, na heteroestrutura radial, o alinhamento das bandas de condução e das bandas de valência favorece uma heteroestrutura do tipo I, enquanto que na heteroestrutura axial, uma transição de uma heteroestrutura do tipo I para uma heteroestrutura do tipo II poderá ocorrer neste intervalo de diâmetros. Em (ii), para as heteroestruturas com diâmetros similares, observamos que a variação de suas propriedades estruturais e eletrônicas com a composição possui desvios significativos do comportamento linear, sendo estes dependentes do diâmetro dessas nanoestruturas. O descasamento da banda de condução é aproximadamente nulo enquanto que o descasamento da banda de valência diminui independente do diâmetro e da composição da heteroestrutura.

Les hétérostructures hybrides organique-inorganique présentent des propriétés intéressantes, notamment pour des applications dans le domaine de l’électronique et de la spintronique. Notre intérêt s’est porté particulièrement sur la réalisation d’hétérostructures de type Métal/Monocouche organique/Semiconducteur, dont l’étape de reprise de top-contact métallique reste actuellement un verrou majeur à la réalisation de telles jonctions. L’expérience de l’équipe sur des hétérostructures de type MOS (Métal/Oxyde/Semiconducteur), ainsi que les différentes techniques de surface et de transport disponibles au laboratoire, sont appliquées ici à l’étude de ces hétérostructures hybrides. En particulier, la Microscopie à Emission d’Electrons Balistiques (BEEM) permet d’étudier localement les propriétés électroniques des hétérostructures, avec une résolution spatiale nanométrique. A partir du système Au/GaAs(001) bien connu au laboratoire, nous avons intercalé une monocouche d’alcanethiols à l’interface, pour former des hétérostructures de type Au/Alcanethiols/GaAs(001), entièrement préparées sous ultra-vide. Lors du dépôt d’or à température ambiante, les images BEEM ont révélé des interfaces hétérogènes, avec des zones où le peigne moléculaire est court-circuité ou non par le métal. Une analyse quantitative en spectroscopie BEEM des zones non court-circuitées a mis en évidence des signatures particulières, avec une première contribution associée au passage tunnel des électrons à travers le peigne moléculaire, et une seconde contribution, à plus haute énergie, révélant l’existence de nouveaux canaux de conduction associés à l’existence d’états inoccupés dans la monocouche organique. Les effets de l’épaisseur du métal déposé, de la longueur de chaîne des molécules organiques, ainsi que du groupe terminal de la chaîne organique, ont été discutés. Afin d’améliorer le dépôt du contact métallique, un dispositif expérimental original a permis de déposer l’or sur le substrat refroidi, sur lequel une couche tampon de Xénon est condensée (méthode BLAG : Buffer Layer Assisted Growth). L’analyse BEEM de ces hétérostructures a révélé ici des interfaces homogènes, sans pénétration du métal. Des signatures spectroscopiques similaires aux zones non court-circuitées précédentes ont été mises en évidence. Une étude complète de ces hétérostructures préparées par la méthode BLAG a été réalisée via des mesures de transport à l’échelle macroscopique (J(V) et C(V)), ainsi que des mesures de photoémission par rayonnement synchrotron. Ces mesures ont confirmé le caractère reproductible des jonctions formées, avec des hauteurs de barrière en accord avec celles déterminées par BEEM. / In molecular electronics and spintronics, top-contact metal electrode deposition on organic molecular monolayer (OML)/semiconductor hybrid heterostructures is still a critical issue, leading to metal penetration through the molecules and monolayer’s damage. The experimental set-ups available in the lab and the team’s experience in inorganic-inorganic heterostructures are here applied to hybrid organic-inorganic heterostructures. In particular, the Ballistic Electron Emission Microscopy (BEEM), a technique derived from Scanning Tunneling Microscopy (STM), allows to study electronic properties of such heterostructures, at a nanometer scale. Starting from the well-known Au/GaAs(001) Schottky contact, we here intercalate an alkanethiols monolayer, in order to obtain Au/Alkanethiols/GaAs(001) heterostructures, fully grown in ultra-high vacuum environment. In the case of room-temperature metal deposition, BEEM imaging reveals domains which are short-circuited or not by the metal. A quantitative analysis of non-short-circuited interfaces is realized by BEEM in spectroscopy mode. Particular fingerprints are obtained, with a first component related to electron tunnel transport through the monolayer, and a second component, at higher energy, related to first unoccupied states of the molecular layer reachable for electrons. The effects of metal thickness, molecular chain length and terminal group are discussed. In order to minimize the degree of gold penetration, an alternative top-contact deposition method is used, based on buffer-layer assisted growth (BLAG). BEEM studies on these heterostructures reveal homogeneous interfaces without metal penetration, and similar spectroscopic fingerprints. Complementary studies at macroscopic scale (J(V) and C(V) transport measurements and photoemission by synchrotron radiation) confirm the reproducible character of the junctions with barrier height values similar to the ones obtained by BEEM.

Perovskite oxides have provided a wide variety of exotic functionalities based on their unique physical and chemical properties. By combining different perovskite oxides, interesting physical phenomena have been observed at the interfaces of perovskite heterostructures. The most interesting among these phenomena is the formation of two dimensional electron gas at the interface of two perovskite materials SrTiO3 and LaAlO3 which led to a number of fascinating physical properties such as metal-insulator transition, super-conductivity, large negative magnetoresistance and so on. This has raised the interest in exploiting the interface of various hybrids structures built on the perovskite oxide backbone. On the other hand, the two dimensional (2D) van der Waals materials such as graphene, MoS2, boron nitride etc. represent a new paradigm in the 2D electron-ics. The functionalities of these individual materials have been combined to obtain new enriched functionalities by stacking different materials together forming van der Waals heterostructures. In this work, we present a detailed study of the interface in hybrid structures made of vander Waals materials (graphene and MoS2) and their hybrids with a perovskite material namely, SrTiO3 which is known as the building block of complex oxide heterostructures. In graphene-MoS2 vertical heterostructure, we have carried out a detailed set of investigations on the modulation of the Schottky barrier at the graphene-MoS2 interface with varying external electric field. By using different stacking sequences and device structures, we obtained high mobility at large current on-off ratio at room temperature along with a tunable Schottky barrier which can be varied as high as ∼ 0.4 eV by applying electric field. We also explored the interface of graphene and SrTiO3 as well as MoS2 and SrTiO3 by electrical transport and low frequency 1/f noise measurements. We observed a hysteretic feature in the transfer characteristics of dual gated graphene and MoS2 field effect transistors on SrTiO3. The dual gated geometry enabled us to measure the effective capacitance of SrTiO3 interface which showed an enhancement indicating the possible existence of negative capacitance developed by the surface dipoles at the interface of SrTiO3 and the graphene or MoS2 channel. Our 1/f noise study and the analysis of higher order statistics of noise also support the possibility of electric field-driven reorient able surface dipoles at the interface.

Tato práce se zabývá oblastí dvourozměrných materiálů, jejich přípravou a analýzou. Pravděpodobně nejznámějším zástupcem dvourozměrných materiálů je grafen. Tento 2D allotrop uhlíku, někdy nazývaný „otec 2D materiálů“, v sobě spojuje neobyčejnou kombinaci elektrických, tepelných a mechanických vlastností. Grafen získal mnoho pozornosti a byl také připraven mnoha metodami. Jedna z těchto metod však stále vyniká nad ostatními kvalitou produkovaného grafenu. Mechanická exfoliace je ve srovnání s jinými technikami velmi jednoduchá, takto připravený grafen je však nejkvalitnější. Práce je také zaměřena na optimalizaci procesu tvorby heterostruktur složených z vrstev grafenu a hBN. Dle prezentovaného postupu bylo připraveno několik van der Waalsových heterostruktur, které byly analyzovány Ramanovskou spektroskopií, mikroskopií atomových sil a nízkoenergiovou elektronovou mikroskopií. Měření pohyblivosti nosičů náboje bylo provedeno v GFET uspořádání. Získané hodnoty pohyblivosti prokázaly vynikající transportní vlastnosti exfoliovaného grafenu v porovnání s grafenem připraveným jinými metodami. V práci popsaný proces přípravy je tedy vhodný pro výrobu kvalitních heterostruktur.

La Sonde Atomique Tomographique (SAT) assistée par laser (La-APT) est un outil puissant pour étudier la distribution atomique 3D des espèces chimiques dans une grande variété de matériaux semi-conducteurs. Cependant, des biais de composition importants affectent les analyses de sondes atomiques révélant une composition non stœchiométrique. Dans cette thèse, une étude systématique de certains semi-conducteurs binaires (GaN, GaAs, ZnO) et ternaires (AlGaN, MgZnO) a été menée afin de: (i) obtenir une description cohérente des biais de composition en APT; (ii) identifier les mécanismes physiques à l'origine de ces biais; (iii) évaluer les conditions expérimentales pour lesquelles l'analyse compositionnelle est fiable. Afin d’interpréter les résultats, l’hypothèse de l’évaporation préférentielle d’espèces métalliques (Ga, Al, Zn, Mg) à haut champ et d’émission de molécules neutres non métalliques (N2, O2) à champ bas a été proposée. Un autre objectif important de cette thèse est orienté vers la physique des matériaux. L’étude de la composition et la morphologie de certains dispositifs d’intérêt technologique, tels que les systèmes à multi-puits quantiques, est très important. Dans cette perspective, la connaissance du champ de composition 3D et de la morphologie de ces systèmes est essentielle car ces caractéristiques déterminent leurs propriétés optiques et électriques. Pour ce faire, une approche par microscopie corrélative peut être adoptée. Cette approche a été appliquée avec succès à l'étude des multi-puits quantiques ZnO/MgZnO conçus pour les lasers à cascade quantique. Les propriétés structurales, compositionnelles et optiques ont été étudiées en effectuant la tomographie par électrons (ET) - micro-photoluminescence (µ-PL) corrélative sur les mêmes échantillons de sonde atomique. Les analyses complémentaires APT et ET donnent une image claire de la structure et de la composition du système étudié, révélant d'importants phénomènes de décomposition dans l'alliage MgZnO. En particulier, La SAT s’est révélé une technique unique pour une évaluation directe de la composition locale. De plus, la µ-PL apparait extrêmement utile pour obtenir des informations relatives à la composition, en lien avec les résultats de La-APT. Enfin, nous présentons une nouvelle approche in-situ corrélative dans laquelle les mesures APT et µ-PL sont exécutées simultanément. Grâce au développement d'une sonde atomique tomographique spécialement conçue, on démontre que la µ-PL peut être mesurée avec succès sur une pointe de sonde atomique Zn :/MgZnO pendant nos analyse. Ceci est extrêmement attrayant car cela permet de corréler strictement le signal de photoluminescence avec les volumes explorés à l'échelle nanométrique. En principe, émission depuis des d'émetteurs de lumière quantiques uniques (c'est-à-dire un seul QW ou QD) peut être révélée. La nouvelle approche présentée peut être étendue à un large éventail de matériaux, ouvrant de nouvelles perspectives pour les études corrélatives. / Laser-assisted Atom Probe Tomography (La-APT) is a powerful tool for investigating the 3D atomic distribution of the chemical species in a wide variety of semiconductor materials. However, important compositional biases affect atom probe analyses revealing a non-stoichiometric composition. In the thesis a systematic study of selected binary (GaN, GaAs, ZnO) and ternary (AlGaN, MgZnO) semiconductors of high technological interest was developed in order to: (i) obtain a coherent description of the compositional biases in APT; (ii) identify the physical mechanisms leading to these biases; (iii) assess the experimental conditions for which the compositional analysis is reliable. In order to interpret the results, the hypothesis of preferential evaporation of metallic species (Ga, Al, Zn, Mg) at high field and emission of neutral non-metallic molecules (N2, O2) at low field has been proposed. Another important aim of this thesis is materials physics-oriented. It is indeed of utmost importance to study both composition and morphology of some devices of technological interest, such as in multi-quantum-well systems. In this perspective, the knowledge of 3D composition field and morphology is essential because these features determine the optical and electrical properties of the systems. In order to do it, a correlative microscopy approach can be adopted. This approach was successfully applied to the study of ZnO/MgZnO multi-quantum wells designed for quantum cascade lasers. Structural, compositional and optical properties were investigated performing correlative La-APT - Electron Tomography (ET) - micro-PhotoLuminescence (µ-PL) on the same atom probe tip specimens. The complementary APT and ET analyses yield a clear picture of the structure and composition of the system investigated, revealing important decomposition phenomena in the MgZnO alloy. In particular, La-APT proved to be a unique technique for a direct assessment of local composition. Moreover, µ-PL also proved to be extremely useful in order to get information related the composition, supporting La-APT results. Finally, a new correlative in-situ approach in which La-APT and µ-PL are simultaneously performed is presented. Thanks to the development of a specially designed tomographic atom probe, it is shown that µ-PL can be successfully performed on a single Zno/MgZnO atom probe tip during La-APT. This is extremely attractive and challenging because allows to strictly correlating the variation photoluminescence signal with nano-metric scale volumes of the tip evaporated during APT. In principle, the emission of single quantum light emitters (i.e. single QW or QD) can be revealed. The new approach presented can be extended to a wide range of materials, opening new perspectives for correlative studies of single atom probe tips.

Spin resonance of a two-dimensional electron system confined in a GaN/AlGaN heterostructure grown by molecular beam epitaxy was resistively detected over a wide range of magnetic field and microwave frequency. Although the spin-orbit interaction is strong in this type of heterostructure at zero magnetic field, surprisingly the width of the detected spin resonance line was very narrow—down to 6.5 mT at 13.3 T. The spin depolarization time extracted from the resonance linewidth was estimated to be 2 ns. The electron g-factor was measured with high accuracy, resembling a value close to the free-electron value and its dependence on the magnetic field was studied.

Optoelectronics is quickly becoming a fast emerging technology field. It refers to detect or emit electromagnetic radiation, and convert it into a form that can be read by an integrated measuring device. These devices can be a part of many applications like photodiodes, solar cells, light emitting diode (LED), telecommunications, medical equipment, and more. Due to their different applications, the semiconductor optoelectronic devices can be divided by their operating wavelength and working mechanisms. In this work, I have focused on semiconductor plasmonic systems operating in the mid-infrared and on the optical detectors made of 2D materials operating in the UV-visible spectral range. Mid-infrared plasmonic devices are very attractive for chemical sensing. Our results show that ultra-doped n-type GaAs is ideal for mid-infrared plasmonics, where the plasmon wavelength is controlled by electron concentration and can be as short as 4 μm. Ultra-doped n-type GaAs is achieved using ion implantation of chalcogenides like S and Te followed by nonequillibrium thermal annealing, namely ns-range pulsed laser melting or ms-range flash lamp annealing. I have shown that the maximum electron concentration in our GaAs layer can be as high as 7×10¹⁹ cm⁻³, which is a few times higher than that obtained by alternative techniques. In addition to plasmonic applications, the ultra-doped n-type GaAs shows negative magnetoresistance, making GaAs potential material for quantum devices and spintronic applications. UV-visible optical detectors are made of 2D materials based on van der Waals heterostructures, i.e. transition metal dichalcogenides (TMDCs) e.g. MoSe₂ and transition metal chalcogenophosphates (TMCPs) with a general formula MPX₃ where M=Fe, Ni, Mn and X=S, Se, Te. The external quantum efficiency of a self-driven broadband photodetector made of a few layers of MoSe₂/FePS₃ van der Waals heterojunctions is as high as 12 % at 532 nm. Moreover, it is shown that multilayer MoSe₂ on FePS₃ forms a type-II band alignment, while monolayer MoSe₂ on FePS₃ forms a type-I heterojunction. Due to the type-I band alignment, the PL emission from the monolayer MoSe₂ is strongly enhanced.

In dieser Arbeit werden das Wachstum und die optischen Eigenschaften von selbstorganisierten GaN Nanodrähten auf TiN und nanodrahtbasierten (In,Ga)N/GaN Heterostrukturen für LED Anwendungen untersucht. Zu diesem Zweck wird das selbstorganisierte Wachstum von langen, dünnen und nicht koaleszierten GaN Nanodrähten auf TiN mittels Molekularstrahlepitaxie demonstriert. In weiteren Untersuchungen werden diese gut separierten und nicht koaleszierten GaN Nanodrähte auf TiN als Basis für die Herstellung von axialen und radialen Heterostrukturen verwendet. Trotz der definierten Morphologie der aktiven Zonen ist die Lichtausbeute der axialen (In,Ga)N Quantentöpfen eher gering. Um das Potenzial der Molekularstrahlepitaxie für das Wachstum von Kern-Hüllen-Strukturen im Allgemeinen besser zu verstehen, wird der Aspekt, dass die Seitenfacetten der Nanodrähte nur sequentiell den verschiedenen Materialstrahlen ausgesetzt werden, durch Modellierung des Wachstums von GaN Hüllen auf GaN Nanodrähten untersucht. Es wird gezeigt, dass Ga Adatomdiffusionsprozesse zwischen verschiedenen Facetten das Wachstum auf den Seitenfacetten stark beeinflussen. Neben der Untersuchung von radialsymmetrischen (In,Ga)N Hüllen wird ein neuer Wachstumsansatz vorgestellt, der die kontrollierte Abscheidung von III-Nitridhüllen auf verschiedenen Seiten des Nanodrahtes ermöglicht. Unter Ausnutzung der Richtungsabhängigkeit der Materialstrahlen in einer Molekularstrahlepitaxieanlage ermöglicht der neuartige Ansatz die sequentielle Abscheidung verschiedener Verbundstoffmaterialien auf einer bestimmten Seite der Nanodrähte, um eine einseitige Schale zu wachsen. Diese sequentielle gerichtete Abscheidungsmethode ermöglicht prinzipiell die Kombination mehrerer aktiver Zonen mit unterschiedlichen Eigenschaften auf verschiedenen lateralen Seiten ein und derselben Nano- oder Mikrostruktur. Solche Architekturen könnten beispielsweise für die Realisierung von mehrfarbigen Pixeln für Mikro-LED-Displays interessant sein. / In this thesis, the growth and the optical characteristics of self-assembled GaN nanowires on TiN and nanowire-based (In,Ga)N/GaN heterostructures for LED applications is investigated. To this end, the self-assembled growth of long, thin and uncoalesced GaN nanowires on TiN by molecular beam epitaxy is demonstrated. Subsequently, these well-separated and uncoalesced GaN nanowires on TiN are used as a basis for the fabrication of axial and radial heterostructures. Despite the well-defined morphology of the active regions, the luminous efficiency of axial (In,Ga)N quantum wells is found to be rather low. To better understand the potential of molecular beam epitaxy for the growth of core-shell structures in general, the aspect of the side facets of the nanowires being only sequentially exposed to the different material beams is studied by modeling the shell growth of GaN shells on GaN nanowires. It is shown that Ga adatom diffusion processes between different facets strongly affect the growth on the side facets. Besides the fundamental investigation of the growth of radially symmetric (In,Ga)N shells, a new growth approach which allows the controlled deposition of III-nitride shells on different sides of the nanowire is presented. Using the directionality of the material beams in an molecular beam epitaxy system, the novel approach facilitates the sequential deposition of different compound materials on a specific side of the nanowires to grow a one-sided shell. This sequential directional deposition method may in principle allow the combination of multiple active regions with different properties on different lateral sides of one and the same nano- or microstructure. Such architectures, for instance, might be interesting for the realization of multi-color pixels for micro-LED displays.

<p>GaAs/GaP heterostructure nanowires (NWs) were grown on GaAs(111)B and Si(111) substrates by gold (Au) assisted vapor-liquid-solid (VLS) growth in a molecular beam epitaxy (MBE) system. NW morphology and crystal structure were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Early results indicated substantial differences in the length and crystal structure of the GaAs/GaP heterostructures. Efforts to remove these inhomogeneities required an improved Au VLS seed deposition method as well as a better understanding of VLS growth across GaAs/GaP hetero-interfaces.</p> <p>Experiments with GaAs/GaP heterostructures yielded the observation of changes in crystal phase in GaP, including the first reported occurrence of the 4H polytype. These observations revealed the presence of transient growth behavior during the formation of the GaAs to GaP hetero-interface that was unique to the VLS technique. Further characterization required the need to move from VLS seeds formed by annealing thin Au films to Au particles formed precisely by electron beam lithography (EBL). NW growth using EBL patterned Au seeds was discovered to be inhibited by the formation of a thin silicon oxide layer, formed at low temperatures by Au-enhanced silicon oxidation. Elimination of this layer immediately prior to growth resulted in successful patterned VLS growth.</p> <p>A systematic study of the transient GaP growth behavior was then conducted using patterned arrays to grow GaAs/GaP heterostructure NWs with frequent, periodic oscillations in the group V composition. These oscillations were measured by high angle annular dark field (HAADF) to determine the instantaneous growth rate of many NWs. A phenomenological model was fit to the data and transient growth rate behavior following a GaAs to GaP hetero-interface was understood on the basis of transient droplet compositions, which arise due to the large difference in As or P alloy concentrations required to reach the critical supersaturation.</p> / Doctor of Philosophy (PhD)