Research interest in complex oxides has resurged owing to progress in modern epitaxial techniques. Among such oxides, lead-titanate-based thin films such as PbTiO3 (PTO) and Pb(ZrxTi1−x)O3 (PZT) offer attractive advantages for a wide variety of applications. Moreover, integration between functional oxides with compound semiconductors has the potential to realize multi-functional devices which enjoy the properties from both groups of materials. Ferroelectric materials with a perovskite structure (ABO3) and semiconductors such as GaN with a hexagonal structure, require a careful choice of a bridge layer and suitable epitaxial technique. Molecular beam epitaxy (MBE) has been an established technique in providing epitaxial growth with high crystal perfection and precise control over material composition. Single-crystal oxides grown by molecular beam epitaxy (MBE) can in principle avoid grain boundaries and provide a sharp interface as well. In this dissertation, the MBE growth mechanism of PZT was investigated. In-situ RHEED patterns indicate that the growth of PTO and PZT occur in a two-dimensional, layer by layer mode, as confirmed by a streaky pattern. The crystal quality of PTO, PZO, and PZT thin films prepared by MBE are evaluated by X-ray diffraction (XRD), and have a full width at half maximum (FWHM) value of 4 arcmin for an 80nm thick layer. Optical properties of the PTO thin films have been characterized by variable angle spectroscopic ellipsometry (VASE), and well resolved dielectric functions are extracted. The refractive index is determined as 2.605 at 633 nm, and bandgap energy as 3.778eV. The electrical properties of the PTO and PZT are evaluated by the measurement of polarization-field hysteresis loops, give a remanent polarization of 83 μC/cm2 and a coercive field of 77 kV/cm. Lead oxide (PbO), titanium dioxide (TiO2), and zirconium dioxide (ZrO2), on GaN templates for potential PZT/GaN integration. The epitaxial growth of TiO2, PbO, and ZrO2 is realized on GaN templates for the first time by MBE. The PbO epitaxial layer was also used as a nucleation layer to enable single crystalline, perovskite PTO growth on GaN.

Ce travail porte sur la croissance épitaxiale et la caractérisation optique de boîtes quantiques d’InAs/InP(001) en faible densité en vue de la réalisation de nouveaux composants nanophotoniques émettant à 1,55 µm. Les propriétés structurales et optiques des îlots ont été corrélés pour différents paramètres de croissance d’un système d’épitaxie par jet moléculaire à sources solides. Nos résultats soulignent l’influence des reconstructions de surface d’InAs sur la forme des îlots. Des boîtes, plutôt que des bâtonnets allongés généralement observés,peuvent être directement formées dans des conditions de croissance adéquates. Une transition de forme de bâtonnets vers des boîtes est également démontrée par des traitements postcroissance sous arsenic. Les faibles densités de boîtes sont obtenues pour des faibles épaisseurs d’InAs déposées. Leur émission est facilement contrôlée à 1,55 µm par une procédure d’encapsulation spécifique appelé « double cap ». Quelques propriétés des boîtes individuelles d’InAs/InP sont ensuite évaluées. Les études de micro-photoluminescence révèlent des pics d’émission très fins et distincts autour de 1,55 µm confirmant les propriétés« quasi-atomiques » de ces boîtes uniques. Enfin, nous proposons pour la première fois une méthode à haute résolution spatiale qui permet d’étudier le transport de charges autour d’une boîte unique grâce à une technique de cathodoluminescence à basse tension d’accélération.Une mesure directe de la longueur de diffusion des porteurs avant capture dans une boîte a été obtenue. Ces résultats ouvrent de nouvelles perspectives quant à l’intégration de ces boîtes uniques dans des microcavités optiques pour la réalisation de sources de lumières quantiques à 1,55 µm. / This thesis focus on the epitaxial growth and optical characterization of diluted InAs/InP(001) quantum dots for the realisation of new nanophotonic devices emitting at 1.55μm. The structural and optical properties of the quantum islands are correlated to different growth parameters of a solid source molecular beam epitaxy system. Our results highlight the influence of InAs surface reconstructions on the island shape. Dots rather than elongated dashes usually observed can be directly formed by adequate growth conditions. Dash to dot shape transition is also demonstrated by post-growth treatments. Low dot densities are obtained for small InAs deposited thickness. Their emission wavelength is easily tuned to1.55 µm using the “double cap” procedure for the growth of the InP capping layer. Optical properties of such single InAs/InP quantum dots are then evaluated. Microphotoluminescence studies reveal sharp and well separated emission lines near 1.55 µm from single dots confirming their atom-like properties. Last, we propose for the first time a highspatial resolution method to study the carrier transport in the vicinity of a single quantum dotusing a low-voltage cathodoluminescence technique. A direct measurement of the carrier diffusion length before capture into one dot has been obtained. These results open the way to the integration of these single dots into optical micro-cavities for the realisation of quantumlight sources at 1.55 µm.

O grafeno é um material que apresenta propriedades físicas e químicas superiores aos materiais tradicionalmente utilizados na fabricação de diferentes dispositivos. Porém, para substituir tais materiais, é imprescindível o conhecimento e controle de processos de adsorção e incorporação de diferentes compostos na superfície do grafeno, crescido ou transferido, sobre diferentes substratos. A investigação da síntese e caracterização de filmes de grafeno com a finalidade de controlar as propriedades físico-químicas e elétricas é um esforço da comunidade científica atualmente. Nesse trabalho, tivemos o objetivo de investigar o processamento térmico do grafeno em vapor de água e em óxido nítrico e sua síntese pela técnica de MBE, caracterizando as estruturas resultantes através de técnicas de análise por feixes de íons, espectroscopia de fotoelétrons e espectroscopia Raman. No caso do processamento em vapor de água, as amostras foram submetidas a tratamentos térmicos em ampla faixa de temperatura (200-1000°C), sendo possível distinguir três diferentes regimes de interação do grafeno com a água. A baixas temperaturas de processamento (200-400°C), nenhuma modificação considerável é observada. Na faixa entre 400-500°C, a estrutura plana do grafeno é corrugada para acomodar os novos grupamentos funcionais formados. A partir de 600°C, os domínios cristalinos diminuem e observa-se alto nível de dopagem oxidativa. Já no processamento em NO, evidenciou- se a introdução de N na rede cristalina e etching do grafeno em altas temperaturas. Quanto ao crescimento de grafeno sobre substratos de Si3N4 e AlN(0001) por MBE, este trabalho mostrou a viabilidade de se obter filmes de nanografeno com uma taxa de crescimento de 1 monocamada por minuto, onde a qualidade cristalina do filme formado e a espessura dependem do tempo de crescimento. / Graphene is a material which presents physical and chemical properties superior than the materials traditionally used in different devices. In order to use graphene rather than those materials, it is necessary to understand and control the adsorption process and incorporation of different compounds on graphene, grown or transferred, on different substrates. The scientific community has made efforts to control the physicochemical and electrical properties of graphene films. The objective of thesis is to investigate the physicochemical modifications of graphene induced by annealing in water vapor and nitric oxide and to grow graphene by MBE. Resulting structures were characterized by ion beam analysis, x-ray spectroscopy and Raman spectroscopy. Concerning samples treated in water vapor, annealings were performed at temperatures ranging from 200 to 1000ºC. Results evidence three different regimes with respect to annealing temperature. In the low temperature range (200–400°C), no prominent modification of graphene itself is observed. At higher temperatures (400–500°C), to accommodate newly formed oxygen functionalities, the flat and continuous sp2 bonding network of graphene is disrupted, giving rise to a puckered layer. For 600°C and above, shrinking of graphene domains and a higher doping level take place. Regarding graphene processing in NO, results show N incorporation in graphene crystal network and etching of graphene at higher temperatures. Concerning graphene synthesis on Si3N4 and AlN(0001) by MBE, this work demonstrates the feasibility of growing nanographene with a growth rate of one monolayer per minute. In addition, the crystal quality of the films and their thickness depends on growing time.

Amostras de Semicondutores III-V foram crescidas usando a técnica de Epitaxia por feixes Moleculares. As propriedades elétricas das estruturas de GaAs com dopagem planar com Silício foram investigadas, e também a saturação e a difusão do Silício nestas amostras. As propriedades ópticas e elétricas das estruturas dopadas planarmente com Selênio foram analisadas, usando as técnicas de Capacitância-voltagem e a de Tunelamento resonante. As propriedades elétricas dos poços quânticos a base de InGaAs/GaAs foram investigadas, em função da posição da impureza planarmente dopada com Silício. / III-V semiconductor samples were grown using the Molecular beam epitaxy technique, the electrical properties of the GaAs structures planar doped with silicon were investigated as well as the Silicon saturation and diffusion in these samles. The optcal and electrical properties of structures planar doped with Selenium were analyzed using the Capacitance Voltage and resonant Tunneling techniques. The electrical properties of InGaAs/ GaAs based quantum wells were investigated as a function of the planar doped with Silicon impurity position.

abstract: Compound semiconductors tend to be more ionic if the cations and anions are further apart in atomic columns, such as II-VI compared to III-V compounds, due in part to the greater electronegativity difference between group-II and group-VI atoms. As the electronegativity between the atoms increases, the materials tend to have more insulator-like properties, including higher energy band gaps and lower indices of refraction. This enables significant differences in the optical and electronic properties between III-V, II-VI, and IV-VI semiconductors. Many of these binary compounds have similar lattice constants and therefore can be grown epitaxially on top of each other to create monolithic heterovalent and heterocrystalline heterostructures with optical and electronic properties unachievable in conventional isovalent heterostructures. Due to the difference in vapor pressures and ideal growth temperatures between the different materials, precise growth methods are required to optimize the structural and optical properties of the heterovalent heterostructures. The high growth temperatures of the III-V materials can damage the II-VI barrier layers, and therefore a compromise must be found for the growth of high-quality III-V and II-VI layers in the same heterostructure. In addition, precise control of the interface termination has been shown to play a significant role in the crystal quality of the different layers in the structure. For non-polar orientations, elemental fluxes of group-II and group-V atoms consistently help to lower the stacking fault and dislocation density in the II-VI/III-V heterovalent heterostructures. This dissertation examines the epitaxial growth of heterovalent and heterocrystalline heterostructures lattice-matched to GaAs, GaSb, and InSb substrates in a single-chamber growth system. The optimal growth conditions to achieve alternating layers of III-V, II-VI, and IV-VI semiconductors have been investigated using temperature ramps, migration-enhanced epitaxy, and elemental fluxes at the interface. GaSb/ZnTe distributed Bragg reflectors grown in this study significantly outperform similar isovalent GaSb-based reflectors and show great promise for mid-infrared applications. Also, carrier confinement in GaAs/ZnSe quantum wells was achieved with a low-temperature growth technique for GaAs on ZnSe. Additionally, nearly lattice-matched heterocrystalline PbTe/CdTe/InSb heterostructures with strong infrared photoluminescence were demonstrated, along with virtual (211) CdZnTe/InSb substrates with extremely low defect densities for long-wavelength optoelectronic applications. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

Les mémoires flash non volatiles - utilisées dans les ordinateurs, téléphones portables ou clés USB - peuvent être constituées de nanocristaux semiconducteurs (SC) insérées dans une matrice isolante. Elles nécessitent l'élaboration d'hétérostructures de type "oxyde/SC/oxyde/Si(00l)" et la maîtrise de chaque interface. Dans ce cadre, nous avons étudié les mécanismes de croissance initiale du Si et du Ge (SC) sur des substrats d'oxyde cristallins LaA1O3(001) à forte permittivité (high-k). Les propriétés chimiques et structurales ont été déterminées in-situ par photoémission X (XPS et XPD) et par diffraction d'électrons (RHEED et LEED) puis ex-situ par microscopies en champ proche (AFM) et en transmission (HRTEM). Le substrat LaAlO3(001) propre présente une reconstruction de surface c(2x2) attribuée à des lacunes d'O en surface. Les croissances de Si et Ge ont été réalisées par épitaxie par jet moléculaire (MBE), soit à température ambiante suivies de recuits, soit à haute température. L'épitaxie requiert des températures de dépôt supérieures à 550°C. Le mode de croissance est de type Volmer Weber caractérisé par la formation d'îlots cristallins de dimensions nanométriques et de forte densité. Ces îlots sont relaxés et présentent une interface abrupte avec le substrat. Quant aux îlots de Ge, ils ont majoritairement des orientations aléatoires avec néanmoins une relation d'épitaxie privilégiée, la même que celle du Si. / Non-volatile flash memory used in computers, mobile phones and USB-keys can be made up of nanocrystals (SC) inserted in an insulating matrix. It requires development of "Oxide/SC/oxide/Si (001)" type hetero-structures and the control of each interface. Within this framework, we studied the initial growth mechanisms of Si and Ge (SC) on LaA1O3(001) crystal oxide substrates with high permittivity (high-k). Chemical and structural properties have been studied in-situ by X-Ray photoemission (XPS and XPD) and electron diffraction (RHEED and LEED) technics and ex­ situ by atomic force microscopy (AFM) and high resolution tunneling electron microscopy (HRTEM). Clean LaA1O3(001) substrate contains a c(2x2) surface reconstruction which attributed to gaps of oxygen (O) on the surface. Si and Ge have been deposited by molecular bearn epitaxy (MBE), at room temperature followed by series of annealings at high temperatures. Epitaxy requires temperature more than 550°C for the deposition. Volmer Weber growth mode was characterized by the formation of nanometric densely packed islands. These islands are relaxed and have an abrupt interface with the substrate. Islands of Ge have mostly random orientations with nevertheless epitaxy privileged relationship, same as that of the Si.

En utilisant l'épitaxie par jets moléculaires à basse température, nous avons élaboré des couches de (Ge,Mn), contenant des nanostructures ferromagnétiques, sur deux types de substrats GaAs d'orientation (001) : des substrats GaAs « epiready » (échantillons « Ga-GeMn »), et des substrats encapsulés par de l'arsenic amorphe (échantillons « As-GeMn »). Dans les échantillons Ga-GeMn, nous obtenons la formation de nanocolonnes riches en Mn ; celles-ci sont parallèles entre elles, ou enchevêtrées, suivant la morphologie de surface initiale. Les mesures de magnétométrie révèlent deux phases magnétiques : les nanocolonnes ferromagnétiques avec une température de Curie de 150 K, et la matrice de germanium, rendue paramagnétique par la présence de Mn dilué. Les mesures de magnétotransport montrent que ces couches sont de type p, et révèlent un l'effet Hall anormal (AHE) et plusieurs contributions à la magnétorésistance : une magnétorésistance géante négative, à basse température, la magnétorésistante orbitale, parabolique, et une contribution supplémentaire à faible champ. Un calcul des propriétés de magnétotransport a été commencé en s'appuyant sur des hypothèses de la structure de bande entre les inclusions riches en Mn et la matrice semiconductrice de type p : celui-ci montre que la présence d'AHE dans les inclusions donne naissance à un AHE sur tout l'échantillon, mais aussi à un mécanisme de magnétorésistance qui rend compte de cette contribution (que nous appelons magnétorésistance Hall). Dans les échantillons As-GeMn, la diffusion de l'arsenic change le mode de croissance, avec une décomposition spinodale qui perd son caractère bidimensionnel pour devenir tridimensionnelle, avec la formation d'agrégats riches en Mn (température de Curie de l'ordre de 50 K) et d'agrégats de la phase ferromagnétique connue Ge3Mn5. La compensation entre Mn (accepteur) et As (donneur) gouverne les propriétés de transport. Dans les couches de type n, une forte anisotropie de la magnétorésistance est observée, dont nous montrons qu'elle est due à des effets de localisation faible. Une autre contribution à la magnétorésistance est observée, que nous suggérons d'attribuer à une magnétorésistance tunnel à travers la jonction Schottky qui se forme à l'interface entre les inclusions riches en Mn, qui sont métalliques, et le semiconducteur Ge de type n.

The purpose of our research is growing the coupled InGaAs quantum dots on the n-type substrate by molecular beam epitaxy in laboratory, and we choose 5,10 and 15 nanometers to be the thicknesses of GaAs spacer between the quantum dots layer. Due to the couple effect, we hope to realize the theorem of intermediate band proved by Luque and Marti. We measure the characteristic of samples by electroluminescence spectra, photoelectric current spectra, electrical absorption spectra and electro reflectance spectra in laboratory; moreover, we acquire the basic parameters of solar cell by AM1.5G for analyzing. From the basic parameters of solar cell, we know that the quantum dots can enhance the photocurrent by absorbing additional photons , however, the strain caused by quantum dots would decay the open voltage seriously, so that the efficiency always under the baseline. Each efficiency of 9-stack QDs are 4.3%(c494),5.1%(c519),5.3% (c520),and each efficiency of 9-stack Dwells are 3.9%(c524),4.2%(c525),4.7%(c526), and 10-stack QDs(5nm) is 2.9%(c514),and 12-stack QDs(10nm) is 4.48%(c538),and 12-stack QDs(15nm) is 5.89%. The break through of this paper is that the efficiency of c529¡]VOC=0.64V,JSC=11.97mA/cm2,FF=67%,£b=5.89%¡^is higher than GaAs¡]VOC =0.87 V, JSC =7.4 mA/cm2,FF=72.3%,£b=5.6%¡^,and we attribute this performance to its good quality of miniband, because the current can be enhanced a lot, and it will make up for the lose of open voltage and filling factor, so that the efficiency can be higher than GaAs baseline.

In this thesis, we study a series of Si doped InN films. These samples are grown on sapphire (0001) by molecular beam epitaxy (MBE). We have doped Si in InN films successfully. In this experiment, we control Si cell temperature to change carrier concentration of samples during InN film growth. The carrier concentration and mobility are explored by van der Pauw Hall measurement. As carrier concentration increases, mobility decreases. Carrier concentration changes with Si cell temperature from 6.16x1018 cm-3 to 1.19x1020 cm-3. Photoluminescence (PL) emission peak energy shows blue shift when carrier concentration increases, but the intensity decreases and full width at half maximum (FWHM) broadens. The PL peak of InN film with 1.19x1020 cm-3 split into two peaks 0.74 eV and 0.89 eV. In Raman spectra, Raman modes position and FWHM do not change with carrier concentration. In temperature dependence PL, the dependence of PL spectra shows decrease when carrier concentration increases. In power dependence PL, the PL emission peak energy of InN films with 6.16x1018 cm-3 and 8.50x1018 cm-3 show blue shift, while the PL peaks of InN films with 1.43x1019 cm-3 and 2.27x1019 cm-3 show no significant move. The fitting of power density vs. intensity is linear for all samples, but all slope of them are less than 1 expect for InN film with 1.43x1019 cm-3.

The design, fabrication, and characterizations of the spin-polarized AlxGa1-xN/GaN HEMT structure have been achieved for spintronic application. By band calculation within linear combination of atomic orbitals and two-band k·p methods, the theoretical spin-splitting energy and minimum-spin-splitting surface of wurtzite structure have been investigated as a function of the Fermi wavevector with various strain-relaxations. Base on these results, the design of host material of the nonballistic spin-FET has also been proposed. By optimizing the Al composition and n2DEG, the Fermi surface of two-dimensional electron gas is supposed to reach the minimum-spin-splitting surface to produce resonant spin-lifetime. Because the high quality AlxGa1-xN/GaN HEMT structure is necessary for realizing the spin-FET, the influence of the growth conditions on the polarity and structure quality of the GaN epilayer have been studied on the sample grown by plasma-assisted molecular beam epitaxy. Ga-polar AlGaN/GaN heterostructures on c-Al2O3 has been realized by growing over the Al-rich AlN nucleation layer. And the reduction of interface roughness and threading dislocation scatterings of the electrons in two-dimensional electron gas has also been achieved by growing GaN epilayer under slightly Ga-rich condition. Furthermore, the effect of different types of threading dislocation on the electron mobility of the AlxGa1-xN/GaN HEMT structure has been investigated as well. At low temperature, the electron mobility of two-dimensional electron gas in AlGaN/GaN heterostructures is majorly scattered by the edge type dislocation rather than the screw type. The designs of proposed host material for spin-FETs have been realized through growing high quality spin-polarized AlxGa1-xN/GaN HEMT structures with various Al composition (x= 0.191 ¡V 0.397) grown on c-Al2O3 by metalorganic vapor phase epitaxy. The high mobility (10682 cm2/Vs at 0.4 K), flat interface (surface roughness < 0.5 nm), and high quality HEMT provide a good environment to study the spin-splitting energy. To investigate the spin-splitting energy as functions of the Fermi wavevector, the Shubnikov-de Haas measurements were performed. A large spin-splitting energy (10.76 meV) has been fabricated in Al0.390Ga0.61N/GaN HEMT structure with kf = 8.14 ¡Ñ 108 m-1 for the host material of the Datta-Das spin-FET. And for the first time, the minimum-spin-splitting surface has been experimentally generated in Al0.390Ga0.61N/GaN HEMT structure with kf = 8.33 ¡Ñ 108 m-1 for the host material of the nonballistic spin-FET.