Avec les besoins croissants en enregistrement magnétique à haute densité, un effort important a été apporté à la fabrication et au contrôle des nanoalliages magnétiques. En effet les alliages magnétiques possèdent des propriétés beaucoup plus intéressantes que les métaux purs, en particulier les alliages chimiquement ordonnés (par exemple une forte anisotropie magnétique). Quand ces alliages se retrouvent à l'échelle du nanomètre, leurs propriétés peuvent de plus être exaltées ou fortement modifiées. Ces dix dernières années, une grande attention s'est tournée sur les remarquables propriétés magnétiques de l'alliage ordonné FeRh, d'un point de vue tant fondamental que technologique. En effet l'alliage FeRh présente, dans une étroite gamme de composition proche de l'équiatomique, une transition magnétique d'un état antiferromagnétique (AFM) vers un état ferromagnétique (FM). Cette transition est observée, dans l'alliage massif, à une température proche de 370K, soit au-dessus de la température ambiante. Cet alliage est de ce fait un excellent candidat pour l'enregistrement magnétique assisté thermiquement ainsi que pour la microélectronique. Le travail présenté est centré sur l'élaboration et l'étude de nanostructures de FeRh de différentes morphologies. Toutes les nanostructures ont été élaborées par voie physique dans un bâti ultra-vide de pulvérisation cathodique. Une attention particulière s'est portée sur l'évolution des caractéristiques structurales, et dans certains cas magnétiques, des nanostructures, en fonction de leur taille et des paramètres d'élaboration. Deux types de nanostructures ont été étudiés : des films minces épitaxiés sur un substrat cristallin de MgO (001) et des nanoparticules élaborées dans une matrice d'alumine amorphe. L'évolution des caractéristiques morphologique, chimique et structurale a été analysée par diffraction des rayons X et par microscopie électronique en transmission et spectroscopies associées. Les propriétés magnétiques ont été étudiées macroscopiquement par magnétométrie à échantillon vibrant (VSM) et in-situ dans un microscope par holographie électronique. / With the increasing demand for ultra-high density magnetic recording, an important effort was put on fabrication and control of magnetic nanoalloys. Indeed, magnetic alloys possess much more interesting properties than magnetic pure metals in particular chemically ordered alloys (higher magnetic anisotropy). When these alloys have a nanometer size, their properties may change significantly. In the last decade, much attention has been paid to the remarkable magnetic properties of the FeRh ordered alloy, for both fundamental and technological issues. Indeed, the FeRh alloy presents, in a very narrow range of composition close to the equiatomic one, a magnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM) state. This transition takes place at a temperature close to 370K in the bulk, i.e. slightly higher than room temperature, which make with alloy particularly attractive for applications as heat-assisted magnetic recording or for microelectronics. The present work focuses on the fabrication and study of FeRh nanostructures with different morphologies. All the nanostructures were grown by dc magnetron sputtering in an ultra-high vacuum chamber. Particular attention was paid to the evolution of the structural, and eventually magnetic, characteristics of the nanostructures as the function of their size and the growth conditions. Two types of nanostructures were studied: thin films epitaxially grown on MgO (001) and nanoparticles embedded in an amorphous alumina matrix. The evolution of the morphological chemical and structural characteristics was analyzed by high-angle X-ray diffraction (XRD) and by transmission electron microscopy (TEM) and associated spectroscopies. Magnetic properties were studied by vibrating sample magnetometer and in-situ in a microscope by electronic holography.

With the low-temperature halo-carbon epitaxial growth technique developed at MSU prior to this work, use of a halo-carbon growth precursor enabled low-temperature homoepitaxial process for 4H-SiC at temperatures below 1300 °C with good quality. Investigations of the nitrogen doping dependence are reported. It has been demonstrated that the efficiency of the nitrogen incorporation may be different for different substrate orientations, with the Cace showing the higher value of doping. The Si/C ratio is known to influence the doping during the epitaxial growth due to the site-competition mechanism. The doping on the Cace showed weak dependence on the Si/C ratio. On the Siace, the doping dependence follows the site-competition trend. At high Si/C ratio, the doping trend on Siace shows strong deviation. Both of the investigated trends are suggested for use as the main process dependencies for achieving a wide range of n type doping of SiC during the low-temperature halo-carbon homoepitaxial process.

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.

Gallium Oxide (Ga2O3) has emerged over the last decade as a new up-and-coming alternative to traditional wide bandgap semiconductors. It exists as five polymorphs (α-, β-, γ-, δ-, and ε-Ga2O3), of which β-Ga2O3 is the thermodynamically stable form, and the most extensively studied phase. β-Ga2O3 has a wide bandgap of ~4.8 eV and exhibits a superior figure-of-merit for power devices compared to other wide bandgap materials, such as SiC and GaN. These make β-Ga2O3 a promising candidate in a host of electronic and optoelectronic applications. Recent advances in β-Ga2O3 single crystals growth have also made inexpensive β-Ga2O3 single crystal grown from the melt a possibility in the near future. Despite the plethora of literature on β-Ga2O3-based devices, understanding of contacts to this material --- a device component that fundamentally determines device characteristics — remained lacking. For this research, ohmic and Schottky metal contacts to Sn-doped β-Ga2O3 (-201) single crystal substrates, unintentionally doped (UID) homoepitaxial β-Ga2O3 (010) on Sn-doped β-Ga2O3 grown by molecular beam epitaxy (MBE), and UID heteroepitaxial β-Ga2O3 (-201) epitaxial layers on c-plane sapphire by metal-organic chemical vapor deposition (MOCVD) were investigated. Each of the substrates was characterized for their structural, morphological, electrical, and optical properties, the results will be presented in the following document. Nine metals (Ti, In, Ag, Sn, W, Mo, Sc, Zn, and Zr) with low to moderate work functions were studied as possible ohmic contacts to β-Ga2O3. It was found that select metals displayed either ohmic (Ti and In) or pseudo-ohmic (Ag, Sn and Zr) behavior under certain conditions. However, the morphology was often a problem as many thin film metal contacts dewetted the substrate surface. Ti with a Au capping layer with post-metallization annealing treatment was the only consistently reliable ohmic contact to β-Ga2O3. It was concluded that metal work function is not a dominant factor in forming an ohmic contact to β-Ga2O3 and that limited interfacial reactions appear to play an important role. Prior to a systematic study of Schottky contacts to β-Ga2O3, a comparison of the effects of five different wet chemical surface treatments on the β-Ga2O3 Schottky diodes was made. It was established that a treatment with an organic solvent clean followed by HCl, H2O2 and a deionized water rinse following each step yielded the best results. Schottky diodes based on (-201) β-Ga2O3 substrates and (010) β-Ga2O3 homoepitaxial layers were formed using five different Schottky metals with moderate to high work functions: W, Cu, Ni, Ir, and Pt. Schottky barrier heights (SBHs) calculated from current-voltage (I-V) and capacitance-voltage (C-V) measurements of the five selected metals were typically in the range of 1.0 – 1.3 eV and 1.6 – 2.0 eV, respectively, and showed little dependence on the metal work function. Several diodes also displayed inhomogeneous Schottky barrier behavior at room temperature. The results indicate that bulk or near-surface defects and/or unpassivated surface states may have a more dominant effect on the electrical behavior of these diodes compared to the choice of Schottky metal and its work function. Lastly, working with collaborators at Structured Materials Industries (SMI) Inc., heteroepitaxial films of Ga2O3 were grown on c-plane sapphire (001) using a variety of vapor phase epitaxy methods, including MOVPE, and halide vapor phase epitaxy (HVPE). The stable phase β-Ga2O3 was observed when grown using MOVPE technique, regardless of precursor flow rates, at temperatures ranging between 500 – 850 °C. With HVPE growth techniques, instead of the stable β-phase, we observed the growth of the metastable α- and ε-phases, often a combination of the two. Cross-sectional transmission electron microscopy (TEM) shows the better lattice matched α-phase first growing semi-coherently on the c-plane sapphire substrate, followed by domain matched epitaxy of ε-Ga2O3 on top. Secondary ion mass spectrometry (SIMS) revealed that epilayers forming the ε-phase contain higher concentrations of chlorine, which suggests that compressive stress due to Cl- impurities may play a role in the growth of ε-Ga2O3 despite it being less than thermodynamically favorable.

My dissertation research focuses on the investigation of the transport and magnetic properties of transition metal and rare earth doped oxides, particularly SnO2 and HfO2 thin films. Cr- and Fe-doped SnO2 films were deposited on Al2O3 substrates by pulsed-laser deposition. Xray- diffraction patterns (XRD) show that the films have rutile structure and grow epitaxially along the (101) plane. The diffraction peaks of Cr-doped samples exhibit a systematic shift toward higher angles with increasing Cr concentration. This indicates that Cr dissolves in SnO2. On the other hand, there is no obvious shift of the diffraction peaks of the Fe-doped samples. The magnetization curves indicate that the Cr-doped SnO2 films are paramagnetic at 300 and 5 K. The Fe-doped SnO2 samples exhibit ferromagnetic behaviour at 300 and 5 K. Zero-field-cooled and field-cooled curves indicate super paramagnetic behavior above the blocking temperature of 100 K, suggesting that it is possible that there are ferromagnetic particles in the Fe-doped films. It was found that a Sn0.98Cr0.02O2 film became ferromagnetic at room temperature after annealing in H2. We have calculated the activation energy and found it decreasing with the annealing, which is explained by the increased oxygen vacancies/defects due to the H2 treatment of the films. The ferromagnetism may be associated with the presence of oxygen vacancies although AMR was not observed in the samples. Pure HfO2 and Gd-doped HfO2 thin films have been grown on different single crystal substrates by pulsed laser deposition. XRD patterns show that the pure HfO2 thin films are of single monoclinic phase. Gd-doped HfO2 films have the same XRD patterns except that their diffraction peaks have a shift toward lower angles, which indicates that Gd dissolves in HfO2. Transmission electron microscopy images show a columnar growth of the films. Very weak ferromagnetism is observed in pure and Gd-doped HfO2 films on different substrates at 300 and 5 K, which is attributed to either impure target materials or signals from the substrates. The magnetic properties do not change significantly with post deposition annealing of the HfO2 films.

The emergence of semiconducting nanowires as the new building blocks for photovoltaic (PV) devices has drawn considerable attention because of the great potential of achieving high efficiency and low cost. In special, nanowires with a coaxial structure, namely, core-shell structures have demonstrated significant advantages over other device configurations in terms of radial charge collection and cost reduction. In this dissertation, several core-shell nanowire structures, including ZnO/ZnSe, ZnO/ZnS, and CdSe/ZnTe, have been synthesized and the photovoltaic devices processed from a ZnO/ZnS core-shell nanowire array and a single CdSe/ZnTe core-shell nanowire have been demonstrated. By combining the chemical vapor deposition and pulsed laser deposition (PLD) techniques, type-II heterojunction ZnO/ZnSe and ZnO/ZnS core-shell nanowire array were synthesized on indium-tin-oxide substrates. Their structures and optical properties have been investigated in detail, which revealed that, despite highly mismatched interfaces between the core and shell, both systems exhibited an epitaxial growth relationship. The quenching in photoluminescence but enhancement in photocurrent with faster response upon coating the core with the shell provides the evidence that the charge separation and collection in the type II core-shell nanowire is greatly improved. This demonstration brings much greater flexibility in designing next generation PV devices in terms of material selection and device operation mechanisms for achieving their maximum energy conversion efficiencies at a low cost and in an environmentally friendly manner. In order to achieve a high quality interface in the core-shell nanowire, CdSe and ZnTe, which have close lattice parameters and thermal expansion coefficients, were chosen to fabricate nanowire solar cells. ZnTe and CdSe nanowires were first synthesized by thermal evaporation and the shells were subsequently deposited by PLD. ZnTe/CdSe nanowires represented an inhomogeneous coating while the CdSe/ZnTe core-shell exhibited a conformal coating with obvious ZnTe eptilayer. The final PV device based on an individual CdSe/ZnTe nanowire demonstrated an efficiency of ~1.7%. In addition, a controllable synthesis of CdSe nanowire array on muscovite mica substrate was presented, providing the possibility to harvest hybrid energies in an all-inorganic nanowire array.

Ce travail est consacré à l’étude de la diffraction d’atomes rapides (d’énergie cinétiquede l’ordre du keV) en incidence rasante (angles d’incidence ~ 1°) le long ou proche d’unedirection principale d’une surface cristalline. Cette géométrie en incidence rasante permet de(i) récolter, en quelques secondes, la totalité du cliché de diffraction sur un détecteur sensible enposition ; (ii) préserver la cohérence de l’onde de matière en réduisant les sources de décohérencecomme l’excitation électronique et l’agitation thermique. La grande sensibilité des atomes àla forme de la densité électronique de surface (partie répulsive) et au puits attractif de Vander Waals révèle les résonances de Fano et démontre que l’atome voyage, piégé au-dessus de lasurface, de façon cohérente, sur une distance de 0.2μm. Il est également montré que la diffractiond’atomes rapides est une technique robuste pour suivre la dynamique de croissance de semiconducteurs(GaAs) par épitaxie (reconstructions, transitions de phase). Enfin, l’observationde la diffraction sur une feuille de graphène déposé sur 6H-SiC(0001) suggère l’utilisation desatomes rapides comme outil de diagnostic pour le suivi et la caractérisation en temps réel dela croissance du graphène dont les propriétés exceptionnelles dépendent beaucoup des défautsde surface. / This work was devoted to the study of fast atom diffraction (energies in the keV range)at grazing incidence angles (~ 1°) along or close to a low indexed direction of crystalline surfaces.This specific scattering geometry bears two advantages : (i) the diffraction pattern as a wholeis collected within seconds on a position sensitive detector ; (ii) the low energy associated to themotion normal to the surface quenches decoherence due to electronic excitations and stronglyreduces decoherence due to thermal vibrations. The high sensitivity of probe atoms to thesurface electron density (repulsive part) and to the Van der Waals attractive well reveals Fanoresonances where the trapped atoms preserve their coherence over distances as long as 0.2μm.As a complement to these fundamental studies, fast atom diffraction has been proved to be arobust mean to probe the dynamics of epitaxial growth of semiconductors (GaAs). Finally, workperformed on monolayer graphene grown on 6H-SiC(0001) suggest the possibility to use fastatoms to monitor graphene growth in real time, a key process to measure the level of alterationof the intrinsic graphene electronic structure.

L’objectif de cette thèse était d’étudier les mécanismes responsables de la luminescence des ions lanthanides dans des polymères de coordination. Pour cela, des séries d’alliages moléculaires à base d’acide 4 carboxyphénylboronique ont été synthétisées, en faisant varier les proportions relatives des ions lanthanides. Les produits synthétisés ont été étudiés dans la perspective de leur application dans les domaines de la lutte anti-contrefaçon et des thermomètres moléculaires. D’autre part, de nouveaux systèmes utilisant d’autres dérivés d’acide boronique ont été synthétisés. Une nouvelle structure cristalline a été obtenue à base de l’acide 2-carboxyphénylboronique. La seconde partie de cette thèse a porté sur la synthèse et la caractérisation de poudres microcristallines de type coeur-coquille. Il s’agit de la première micro-structuration de polymères de coordination réalisée à l’échelle micrométrique. Les poudres synthétisées ont été caractérisées par microscopies électroniques à balayage, à transmission et par analyses EDS. Leurs propriétés optiques ont été comparées avec celles des alliages moléculaires. Les résultats obtenus ont mis en évidence l'intérêt de la micro-structuration de ces poudres concernant les transferts d'énergie intermétalliques. Cette technique de synthèse basée sur la croissance épitaxiale, est rendue possible par les propriétés chimiques similaires des terres rares. Nos travaux ont montré que lorsque les constantes thermodynamiques et\ou cinétiques le permettent, la phase cristalline de la coquille présente la structure cristalline du cœur même lorsque le composé constituant la coquille cristallise dans une autre phase lorsqu'il est synthétisé seul. / The aim of this thesis was to study the luminescence mechanisms of lanthanide ions in coordination polymers. Series of molecular alloys (hetero-lanthanide compounds in which lanthanide ions are randomly distributed) based on 4-carboxyphenylboronic acid were synthesized, by varying the relative contents of the lanthanide ions. The synthesized products have been studied from the perspective of their application in the fields of anti-counterfeiting and molecular thermometers. On the other hand, new systems using other boronic acid derivatives have been synthesized. A new crystal structure was obtained based on 2-carboxyphenylboronic acid. The second part of this thesis focused on the synthesis and characterization of core-shell microcrystalline powders. This is the first time that micro-structuration of a coordination polymer is achieved at the micrometric scale. The synthesized powders were characterized by scanning electron microscopy, transmission electron microscopy and EDS analyses. Their optical properties have been compared with those of the corresponding molecular alloys. The obtained results highlighted the interest of the micro-structuration concerning intermetallic energy transfers. This strategy of synthesis based on epitaxial growth, is possible because of lanthanide similar chemical properties. This work demonstrates that in the case where the thermodynamic and\or kinetic constants allow it, the shell crystallizes according to the same crystal structure than the core even if its crystal structure is different when it is synthesized alone.

A wide variety of group III-Nitride-based photonic and electronic devices have opened a new era in the field of semiconductor research in the past ten years. The direct and large bandgap nature, intrinsic high carrier mobility, and the capability of forming heterostructures allow them to dominate photonic and electronic device market such as light emitters, photodiodes, or high-speed/high-power electronic devices. Avalanche photodiodes (APDs) based on group III-Nitrides materials are of interest due to potential capabilities for low dark current densities, high sensitivities and high optical gains in the ultraviolet (UV) spectral region. Wide-bandgap GaN-based APDs are excellent candidates for short-wavelength photodetectors because they have the capability for cut-off wavelengths in the UV spectral region (λ < 290 nm). These intrinsically solar-blind UV APDs will not require filters to operate in the solar-blind spectral regime of λ < 290 nm. For the growth of GaN-based heteroepitaxial layers on lattice-mismatched substrates, a high density of defects is usually introduced during the growth; thereby, causing a device failure by premature microplasma, which has been a major issue for GaN-based APDs. The extensive research on epitaxial growth and optimization of Al_x Ga _1-x N (0 ≤ x ≤ 1) grown on low dislocation density native bulk III-N substrates have brought UV APDs into realization. GaN and AlGaN UV <i> p-i-n </i> APDs demonstrated first and record-high true avalanche gain of > 10,000 and 50, respectively. The large stable optical gains are attributed to the improved crystalline quality of epitaxial layers grown on low dislocation density bulk substrates. GaN <i>p-i-n </i> rectifiers have brought much research interest due to its superior physical properties. The AIN-free full-vertical GaN<i> p-i-n </i> rectifiers on<i> n </i>- type 6H-SiC substrates by employing a conducting AIGaN:Si buffer layer provides the advantages of the reduction of sidewall damage from plasma etching and lower forward resistance due to the reduction of current crowding at the bottom<i> n </i> -type layer. The AlGaN:Si nucleation layer was proven to provide excellent electrical properties while also acting as a good buffer role for subsequent GaN growth. The reverse breakdown voltage for a relatively thin 2.5 μm-thick<i> i </i>-region was found to be over -400V.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第18982号 / 工博第4024号 / 新制||工||1620 / 31933 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 酒井 明, 教授 邑瀬 邦明 / 学位規則第4条第1項該当