Tesi sul tema "Thin film, epitaxial"

The application of the metalorganic chemical vapour deposition technique to the production of II-VI compound semiconductor electroluminescent devices is discussed. Both low field MIS minority carrier injection devices and high field impact excitation structures are considered, and comparisons are drawn with more commiercially orientated electroluminescent displays. The epitaxial growth of ZnS and ZnSe onto (100) orientated GaAs substrates, using the reactions between dimethyl zinc and the hydrides HgS and H2Se, is described. Details are given of a novel epitaxial MISi device processing technology, in which a ZnS I-layer also acts as an etch-stop, thus enabling chemical removal of the GaAs substrate. Metal electrodes deposited directly onto the ZnS and ZnSe allow the electrical and electroluminescent characteristics of these epitaxial II-VI compound layers to be investigated in the absence of any influence from the substrate material. X-ray diffraction and reflection high energy electron dififraction confirm that the structures are epitaxial and of excellent crystallinity. It is demonstrated in an electron beam induced current study that conduction in the epitaxial MIS devices is highly uniform, and this is manifested in a uniform spatial distribution of electroluminescence. A description is given of high field impact excitation electroluminescent devices, in which the ZnS layer is doped with manganese during MOCVD growth. The spatial distribution of EL in these devices is shown to be non-uniform, and thus indicative of filamentary conduction in the ZnS:Mn, in accordance with a recently proposed dielectric breakdown model of instability. It is demonstrated that the transient characteristics of the epitaxial structures correlate with those of commercial polycrystalline devices, and are also consistent with the predictions of a dynamic model of instability. As a result of filamentary conduction, both epitaxial and polycrystalline devices are prone to degradation through localised dielectric breakdown. These breakdown events generally result in a gradual erosion of the active electrode area, although, under certain operating conditions, mobile filaments can cause rapid destruction of epitaxial structures. The columnar microstructure of sputtered devices appears to prevent such filament mobility, and it is concluded that, although filamentary conduction is a result of the carrier injection mechanism and is independent of the crystallinity, the associated damage is strongly influenced by the microstructure of the device.

This work focuses on the impact of defects, intrinsic or artificially introduced, on the functional properties of thin, epitaxial oxide films. In the first part, the origin of the ferromagnetic properties of Mn-doped and undoped zinc oxide is studied. The deposition conditions are found to have a significant impact on the structural, transport and magnetic properties of the thin films. Combining x-ray magnetic circular dichroism and magnetometry experiments, it is established that the transition metal dopants (i.e. Mn) have no influence on the ferromagnetic nature of the zinc oxide, but that localised magnetic moments on intrinsic defects are in fact responsible for the ferromagnetic behaviour. A relation between strain (related to defect concentration) and magnetisation is established. In the second part of this dissertation, artificially introduced defects are employed in order to discover the fundamental conduction mechanism behind the two-dimensionally conductive LaAlO3/SrTiO3 interface. All experiments, from varying deposition temperature, to oxygen pressure, to laser fluence or to the insertion of (doped) perovskite layers, point towards a structurally governed conduction mechanism, although the exact details are still unclear. Distinct transitions in the resistance versus temperature curves are observed at different values than the bulk phase transformation temperature. These transitions form the boundaries of different conduction modes, with tendencies towards non-Fermi-liquid behaviour observed in certain two-dimensionally conducting samples in limited temperature regimes. By optimising the (defect) structure at the interface, i.e. by introducing a single unit cell of (La0.5,Sr0.5)TiO3 or SnTiO3, it is shown that the sheet carrier density can be dramatically enhanced, up to an order of magnitude higher than unmodified LaAlO3/SrTiO3 interfaces with a value of 1e14 cm−2 at 200 K. Finally, attempts at functionalising the conductive heterointerface by doping and inserting (anti)ferromagnetic layers are made.

In this thesis, the growth of c-axis oriented Sr2RuO4 thin films using pulsed laser deposition and their electrical transport properties are systematically discussed. The deposition and optimization process involved several progressive steps. Specifically, the first focus was on the Sr2RuO4 phase optimization in films grown on lattice-matched (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. Film composition was found to be greatly influenced by changes in oxygen pressure, substrate temperature, target to substrate distance, and laser fluence. High oxygen pressure, low substrate temperature, large target to substrate distance, and high laser fluence increased the tendency to form the Ru-rich SrRuO3 phase in the film. The second focus was on improving the electrical transport properties of Sr2RuO4 from metal-insulating to fully metallic and eventually to superconducting behavior. It was observed that the full width at half maximum (FWHM) of the Sr2RuO4 (006) rocking curves in x-ray diffraction (XRD) scan was related to the quality of the electrical transport response. By fine tuning the deposition parameters to obtain low FWHM values, the electrical transport behavior of the Sr2RuO4 thin films was consistently improved from metal-insulating to fully metallic. In addition, localized superconductivity with enhanced superconducting transition temperature Tc onset was also observed among the fully metallic film. An in-depth study of the XRD results in fully metallic films indicated the existence of defects (intergrowths) along the c-axis direction, which caused localized c-axis tensile strain. The existence of structural defects within the film was likely to be responsible for the fact that only localized superconductivity was observed in the films. Furthermore, the enhanced superconducting transition temperature (Tc) relative to bulk single crystals is likely to be associated to localized strain in the film. Finally, Nb doped SrTiO3 substrates were used to achieve better quality growth of partial superconducting Sr2RuO4 thin films. Sr2RuO4 films grown on Nb doped SrTiO3 substrates had smaller FWHM values and lower level of c-axis tensile strain compared to those on LSAT substrates. Various partially superconducting films with different thicknesses and different superconducting Tc values are presented, and correlations between fabrication process, film crystalline quality as well as transport properties are discussed. This work provides better understanding of the importance of maximizing crystalline quality by delicate fine tuning of PLD deposition parameters to achieve high quality superconducting films.

Magnetoelectricity (ME) is a physical property that results from an exchange between polar (electric dipole) and spin (magnetic dipole) subsystem: i.e., a change in polarization (P) with application of magnetic field (H), or a change in magnetization (M) with applied electric field (E). Magnetoelectricity can be found both in single phase and composite materials. Compared with single phase multiferroic materials, composite multiferroics have higher ME effects. Through a strictive interaction between the piezoelectricity of the ferroelectric phase and the magnetostriction of the ferromagnetic phase, said multiferroic composites are capable of producing relatively large ME coefficients. This Dissertation focused on the deposition and characterization of two-phase composite magnetoelectric thin films. First, single phase ferroelectric thin films were studied to improve the multiferroic properties of the composite thin films. Then structural, ferroelectric, ferromagnetic, and magnetoelectric properties of composite thin films were researched. Finally, regular nano-array composite films were deposited and characterized. First, for single phase ferroelectric thin films, the phase stability was controlled by epitaxial engineering. Because ferroelectric properties are strongly related to their crystal structure, it is necessary to study the crystal structures in single phase ferroelectric thin films. Through constraint of the substrates, the phase stability of the ferroelectric thin films were able to be altered. Epitaxial thin-layers of Pb(Fe1/2Nb1/2)O3 (or PFN) grown on (001), (110), and (111) SrTiO3 substrates are tetragonal, orthorhombic, and rhombohedral respectively. The larger constraint stress induces higher piezoelectric constants in tetragonal PFN thin film. Epitaxial thin-layers of Pb(Zr0.52Ti0.48)O3 (or PZT) grown on (001), (110), and (111) SrTiO3 substrates are tetragonal, monoclinic C, and rhombohedral respectively. Enhanced ferroelectric properties were found in the low symmetry monoclinic phase. A triclinic phase in BFO was observed when it was deposited on tilted (001) STO substrates by selecting low symmetry (or interim) orientations of single crystal substrates. Then, in two phase composite magnetoelectric thin films, the morphology stability was controlled by epitaxial engineering. Because multiferroic properties are strongly related to the nano-structures of the composite thin films, it is necessary to research the nano-structures in composite thin films. Nano-belt structures were observed in both BaTiO3-CoFe2O4 and BiFeO3-CoFe2O4 systems: by changing the orientation of substrates or annealing condition, the nano-pillar structure could be changed into nano-belts structure. By doing so, the anisotropy of ferromagnetic properties changes accordingly. The multi-ferroic properties and magnetoelectric properties or (001), (110) and (111) self-assembled BiFeO3-CoFe2O4 nano-composite thin film were also measured. Finally, the regular CoFe2O4-BiFeO3 nano-array composite was deposited by pulsed laser deposition patterned using a focused ion beam. Top and cross-section views of the composite thin film showed an ordered CoFe2O4 nano-array embedded in a BiFeO3 matrix. Multiferroic and magnetoelectric properties were measured by piezoresponse force microscopy and magnetic force microscopy. Results show (i) switching of the magnetization in ferromagnetic CoFe2O4 and of the polarization in ferroelectric BiFeO3 phases under external magnetic and electric field respectively, and (ii) changes of the magnetization of CoFe2O4 by applying an electric field to the BiFeO3 phase.
Ph. D.

Fuel cells as green and sustainable energy sources are at the heart of future Hydrogen Economy. Current research is focused on creating highly active, stable and low content Pt catalysts to improve fuel cell performance up to standards suitable for commercialization. The development of bimetallic Pt structures is the most promising route for achieving this goal. Besides the lower-noble metal content, combination of Pt with other metal at the nano-scale .can result in enhancement of catalytic activity due to the combination of geometric and electronic effects. The main aim of this work is the development of a surface limited redox replacement (SLRR) protocol for the design of epitaxial Pt-films with atomic scale control of the structure. SLRR exploits underpotentially deposited (UPD) layer as sacrificial layer that is replaced by more-noble Pt through a surface-controlled limited red-ox (galvanic) reaction. Different from previously developed SLRR protocols this work explores the one-cell configuration setup as an alternative to improve the efficiency and quality of the growth. The conditions for growth have been optimized and monitored with automated control of the SLRR cycles. The successful growth of Pt films on Au has been demonstrated for SLRR growth via Pb UPD. The electrochemical characterisation showed that using Pb UPD as sacrificial layer produces epitaxial Pt films of high quality with no significant roughness evolution up to 10 layers. Scanning tunnelling microscopy (STM) examination of the morphology has shown that Pt was deposited in clusters of 5-10 nm size, homogeneously and uniformly distributed over the surface. Electrochemical quartz crystal microbalance (EQCM) showed high deposition , yield and the Pt(II):Pb replacement stoichiomentric ratio higher than expected 1 : 1, suggesting an extra reduction power present in the system. The compositional analysis of Pt layers grown by SLRR suggests incorporation of minimum 4 at% of Pb. The SLRR protocol for the homoepitaxial growth of Pt thin films using adsorbed H i.e. under potentially deposited H (H-UPD) has been developed. This work presents first application of the SLRR protocol using a nonmetal UPD system. EQCM experiments demonstrated steady displacement kinetics and a yield equal to the expected stoichiometric Pt(II):H exchange ratio (1 :2). Electrochemical and STM characterization of Pt films showed that the growth via SLRR of H-UPD results in increase of the surface roughness with the number of replacement steps. The roughness of SLRR deposited Pt films has been compared with films grown in the same solution at two constant overpotentials: with and without adsorbed H floating on the surface. The results showed clear advantages of using . the SLRR of H-UPD approach which generated films with two times lower roughness and better quality then the ones grown potentiostatically. The generality of the SLRR approach using H-UPD is validated by growth of Pt films on Pd ultrathin films on Au. The Pt films of well-defined thickness and structure grown by SLRR of Pb UPD have been used in a fundamental study of Pt dissolution during formic acid oxidation (FAO). A quantitative analysis of long term durability tests of Pt films has been conducted by potential cycling over an extended potential range. Direct proportionality between overall life and thickness of the catalyst has been observed. The characteristic stages of the activity decay were correlated with the characteristic electrochemical behaviour during FAO and the surface morphology examined by the atomic force microscopy. An average Pt dissolution rate of 1.90±O.33 ng.cm-2.cycle-1 has been determined during FAO which . was almost four times faster than the rate under the same conditions in the background solution. This study suggests that Pt dissolution mechanism during FAO is influenced in by reaction intermediates and processes on the surface.

Avec l’usage intensif de dispositifs microélectroniques modernes, il existe un besoin croissant de mémoires non volatiles. La FeRAM (mémoire ferroélectrique à accès aléatoire) est une des mémoires de nouvelle génération les plus prometteuses en raison de sa faible consommation et de sa vitesse élevé de lecture/écriture. Parmi les différents matériaux ferroélectriques, le PZT (Pb (Zr1-x,Tix)O3) présente une polarisation rémanente élevée et un faible champ coercitif qui en font un candidat de choix pour les FeRAM.Dans cette thèse, la croissance épitaxiale de couches de PZT (52/48) d’épaisseurs variables (33 à 200 nm), sur un substrat de SrTiO3 et une électrode inférieure interfaciale de SrRuO3, a été réalisé par deux méthodes pour comparaison : pulvérisation cathodique et sol – gel. Trois matériaux conducteurs différents (SrRuO3, Pt et ITO) ont été utilisés comme électrode supérieure. L’objectif a été une étude détaillée des propriétés électriques et ferroélectriques de ces structures MFM (métal-ferroélectrique-métal), avec une attention particulière sur l’influence des conditions d’élaboration et de la nature des électrodes sur le courant de fuite et la dynamique de basculement de domaines.Les capacités élaborées par pulvérisation ou sol-gel présentent des caractéristiques semblables : Au-delà d’une épaisseur minimum d’environ 100 nm, pour une structure capacitive à base de PZT de 100 × 100 μm2, elles montrent un faible courant de fuite, une permittivité relative maximale élevée (600 - 1300) et une polarisation rémanente élevée (30 - 40 μC/cm2). Les mécanismes dominants dans le courant de fuite ont été identifiés par un fit des résultats, manifestant différentes contributions en fonction du champ électrique. Des caractérisations par PFM (microscopie à force piézoélectrique) confirme l’existence de domaines ferroélectriques de directions opposées. Il est aussi montré que le champ coercitif dépend fortement de la fréquence de travail. D’autre part, les propriétés d’impression dépendent de l’électrode supérieure, de la nature du recuit et de l’épaisseur de l’électrode inférieure
With the intensive use of modern microelectronic devices in numerous areas, there is an increasing demand for non-volatile memories. FeRAM (ferroelectric random access memory) is one of the most potential next-generation memories for its ultra-low power consumption and high read/write rate. Among various ferroelectrics, PZT (Pb(Zr1-x,Tix)O3) exhibits high remnant polarization and low coercive field, which make it a promising candidate for FeRAM.In this dissertation, PZT(52/48) layers of various thicknesses (from 33 nm to 200 nm) have been epitaxially grown on SrTiO3 substrate, with a SrRuO3 interlayer as bottom electrode, using two deposition methods for comparison: sol-gel and sputtering. Three different conductive materials (SrRuO3, Pt and ITO) have been deposited as top electrode. The objective was a detailed study of the electrical and ferroelectric properties of these MFM (metal-ferroelectric-metal) capacitors, with a particular investigation of the influence of elaboration conditions and electrode material on leakage currents and domain switching dynamics.Sputtered and sol-gel-derived PZT capacitors showed similar properties: Above a minimum workable thickness of about 100 nm for a 100 × 100 μm2 PZT capacitor, they showed low leakage current, high maximum relative permittivity (600 - 1300) and high remnant polarization (30 - 40 μC/cm2). The dominant leakage current mechanisms were identified by fitting the results, showing different contributions as a function of electric field. PFM (piezoresponse force microscopy) characterizations confirmed the existence of ferroelectric domains of opposite directions. Coercive field was found to be highly dependent on work frequency. Besides, imprint properties were found to be dependent on top electrode, annealing procedure and bottom electrode thickness

Graphene has been one of the most interesting and widely investigated materials in the past decade. Because of its high mobility, high current density, inherent strength, high temperature stability and other properties, scientists consider it a promising material candidate for the future all-carbon electronics. However, graphene still exhibits a number of problems such as an unknown interface structure and no sizable band gap. Therefore, the purpose of this thesis is to probe and solve these problems to make graphene suitable for electronics. The work focuses on high-quality C-face epitaxial graphene, which is grown on the (000-1) face (C-face) of hexagonal silicon carbide using the confinement-controlled sublimation method. C-face epitaxial graphene has much higher mobility compared to Si-face graphene, resulting from its special stacking order and interface structure, the latter of which is not fully understood. Thus, the first part of the work consists of a project, which is to investigate and modify the interface and the surface of C-face graphene by silicon deposition and annealing. Results of this project show that silicon can intercalate into the graphene-SiC interface and form SiC by bonding carbon atoms on the graphene surface. Another crucial problem of graphene is the absence of a band gap, which prevents graphene from becoming an ideal candidate for traditional digital logic devices. Therefore, the second project of this work is devoted to introducing a wide band gap into the graphene electronic structure by growing from a nitrogen-seeded SiC. After successful opening of a band gap, a pre-patterning method is applied to improve graphene thickness variations, orientational epitaxy, and the gapped electronic structure.

The work presented in this thesis may be considered in two main parts; firstly a description of the design and operation of a display type photoelectron spectrometer. Secondly a series of experiments investigating the electronic properties of thin epitaxial films (1-5 atomic layers) of cobalt grown on a clean single crystal copper (001) substrate. Conventional angle resolved photoelectron spectrometers of the deflection type are only capable of observing one point in the (E,θ,φ) space at a time. This is often perfectly acceptable if one is concerned with optimal resolution in order to perform accurate band mapping experiments. However certain experiments are essentially impossible, for instance the observation of the emitted photocurrent over all θ,φ at the fermi energy. This is partly because of the time limitations imposed by the necessity to keep the sample atomically clean in the U.H.V. environment. Several previous workers have tackled this problem by designing spectrometers that observe large sections of θ,φ space simultaneously, for a given energy. The first part of this work concerns the design and implementation of a display type spectrometer which embodies some new and quite novel features. Thin epitaxial films of ferromagnetic materials grown on non-magnetic substrates have long been of interest. Partly as a prototypical surface for the investigation of surface magnetism, and partly for the investigation of the changes induced in the magnetic properties as the dimensionality is reduced or as the lattice size is changed. The second part of this thesis concerns experiments using three different spectroscopies on a system of this type, specifically Co on Cu(001). Firstly, a photoemission study using the display spectrometer is presented, observations of the spin-split bands as a function of wavevector parallel to the surface are shown. Secondly an Auger electron study of the growth mode of the epitaxial film, together with a LEED I/V study of the changing lattice strain as a function of film thickness are presented. Although none of these measurements directly probe the magnetism of the films, they provide very necessary information in order to understand their behaviour.

Magnetostriction means that the dimensions of a material depend on its magnetization. The primary goal of this dissertation was to understand the effect of magnetostriction on the magnetic anisotropy of single crystal magnetostrictive thin films, where the epitaxial pinning of the material to a substrate could inhibit its conversion to new dimensions. In order to address this goal, several Fe-based binary alloys were deposited onto various substrates by molecular beam epitaxy. The samples were characterized by an array of techniques including electron diffraction, Rutherford backscattering, vibrating sample magnetometry, ferromagnetic resonance, and x-ray absorption spectroscopies. The attempted growths of crystalline magnetostrictive thin films resulted in successful depositions of Fe 1-xGa x and Fe 1-xZn x. Depositions onto MgO(001) substrates result in an in-plane cubic magnetic anisotropy, as expected from the cubic symmetry of the Fe-based thin films, and a strong out-of-plane uniaxial anisotropy that forces the magnetization to lie in the plane of the films. Depositions onto ZnSe/GaAs(001) substrates feature an additional in-plane uniaxial anisotropy. The magnitudes and signs of the in-plane anisotropies depend on the Ga content. Furthermore, the cubic anisotropy constant of Fe 1-xGa x samples deposited onto MgO substrates switches sign at a lower Ga concentration than is seen in bulk Fe 1-xGa x. The effect on the magnetic anisotropy of depositing a magnetostrictive material as an epitaxial thin film is influenced by the material's magnetostrictive properties and the substrate upon which it is deposited. In particular, pinning a magnetoelastic material to a substrate will modify its cubic anisotropy, and depositions on substrates compliant to an anisotropic strain relaxation may result in a strong in-plane uniaxial anisotropy. 'Co authored by Steven Albert, Tino Jaeger, Hongyan Li, Paul Rugheimer, Juergen A. Schaefer, Yves U. Idzerda, Elke Arenholz, Hongyan Li, Gerrit van der Laan, Damon A. Resnick, Christopher M. Kuster.'

Germanium-on-silicon (Ge-on-Si) structure-based semiconductor devices are playing an increasingly important role in large-scale dense photonic integration, especially in silicon (Si) photonics. Si photonics has emerged as an effective solution to overcome the wiring limit imposed on integrated circuits with continued scaling by using optical interconnects instead metal interconnects. A Si-compatible laser is the last missing piece in optical interconnects on Si platforms. Recently, III-V lasers on Ge/Si substrates and Ge-on-Si lasers were demonstrated as the most promising candidates, in which Ge layers function as either the transition layers or the optical gain layers. For different applications, the requirements on Ge film quality and Ge/Si interface interdiffusion are different. Si-Ge interdiffusion during high-temperature growth or fabrication steps changes the distribution of Ge and increases atomic intermixing, which degrades device performance. However, studies on the doping impact on Ge film quality and Si-Ge interdiffusion are very limited, which were addressed in this work. We investigate Ge-on-Si film quality systematically under different types of doping conditions (phosphorus, arsenic and boron) for the first time. It’s found that the boron doping significantly impairs the Ge film quality, while arsenic and phosphorus can effectively reduce the threading dislocation density without the commonly used defect annealing. This provides a new method to fabricate high-quality Ge-on-Si films, which can avoid undesired Si-Ge interdiffusion. Si-Ge interdiffusion with different doping at Ge/Si interfaces has been investigated experimentally and theoretically. The enhancement of interdiffusion was observed in n-type (phosphorus and arsenic) doped Ge-on-Si. The phenomenon is attributed to the Fermi-level effect. A quantitative model of Si-Ge interdiffusion under high n-type doping was proposed. The model agrees well with the experimental data. This is also the first study on the quantitative modeling of Si-Ge interdiffusion with high n-type doping across the full Ge range. This work is of technical significance for the structure, doping and process design of Ge-on-Si structure-based devices including the two laser types mentioned above, Ge modulators and Ge photodetectors.
Applied Science, Faculty of
Materials Engineering, Department of
Graduate

Kyoto University (京都大学)
0048
新制・課程博士
博士(工学)
甲第14050号
工博第2962号
新制||工||1439(附属図書館)
26329
UT51-2008-F442
京都大学大学院工学研究科電子物性工学専攻
(主査)教授 鈴木 実, 教授 髙岡 義寛, 教授 藤田 静雄
学位規則第4条第1項該当

Thesis (Ph.D.)--Physics, Georgia Institute of Technology, 2008.
Committee Chair: de Heer, Walter; Committee Member: Chou, Mei-Yin; Committee Member: First, Phillip; Committee Member: Meindl, James; Committee Member: Orlando, Thomas

Utiliser l'hydrogène en tant que vecteur énergétique pour stocker l'énergie solaire et/ou remplacer le pétrole comme carburant est très attrayant, d'autant qu'il peut être produit de façon propre par photo-électrolyse de l'eau. Dans ce procédé, des paires électron-trou, générées par éclairement dans des semi-conducteurs immergés dans une solution aqueuse, réalisent les réactions d’oxydo-réduction de l’eau (production d'oxygène à la photo-anode et production d'hydrogène à la photo-cathode). Les oxydes de métaux de transition, en particulier l'hématite (α-Fe2O3) qui présente un gap quasi-idéal pour cette application, sont les matériaux de photo-anode les plus prometteurs. Des films minces d'hématite ont été déposés sur des monocristaux par épitaxie par jets moléculaires assistée par plasma d’oxygène. Ces échantillons modèles ainsi que l’utilisation de techniques de pointe, notamment utilisant le rayonnement synchrotron, rendent possible l’identification des paramètres pertinents influençant les propriétés de photo-électrolyse. Je me suis d'abord intéressé à l'impact de la structure cristallographique, de la stœchiométrie et de la morphologie de surface. Ensuite, les effets d'un dopage avec du titane ont été analysés, montrant l'existence d'un taux de dopage optimal et l'augmentation de la longueur de diffusion des porteurs de charges induisant un fort gain en photo-courant. J'ai également étudié la structure électronique et la dynamique des recombinaisons en surface d'hétérojonctions TiO2 - hématite dopée Ti, révélant une interface diffuse. Enfin, le champ électrique interne créé par un film mince ferroélectrique de BaTiO3/Nb:SrTiO3 a été considéré pour améliorer les propriétés des photo-anodes. Un premier pas vers la compréhension du lien entre polarisation ferroélectrique et photo-courant a été fait, mettant en évidence un champ électrique interne favorable pour séparer les charges
Using hydrogen as an energy carrier for solar energy storage and/or fuel alternative to oil is very appealing, especially as it can be cleanly produced by solar water splitting. In this process, electron-hole pairs, generated in illuminated semiconductors dipped in an aqueous solution, realize the water oxidoreduction reactions (oxygen production at the photoanode and hydrogen production at the photocathode). Transition metal oxides, in particular hematite (α-Fe2O3) which features a quasi ideal band-gap for this application, are the most promising photoanodes materials. Hematite thin films were deposited on single crystals by oxygen plasma assisted molecular beam epitaxy. These model samples along with the use of high-end techniques, in particular using synchrotron radiation, make possible the identification of the relevant parameters affecting the photoelectrochemical properties. I firstly focused on the impact of the crystallographic structure, the stoichiometry and the surface morphology. Then the effects of doping with titanium were investigated, demonstrating the existence of an optimal doping level and an increase of the charges diffusion length inducing a high photocurrent gain. In addition, I studied the electronic structure and the surface recombinations dynamics of TiO2 - Ti-doped hematite heterojunctions, revealing a diffuse interface. Lastly, the internal electric field created by a ferroelectric thin film of BaTiO3/Nb:SrTiO3 was considered in order to enhance the performances of photoanodes. A first step toward the comprehension of the link between ferroelectric polarization and photocurrent was achieved through the evidence of an internal electric field favourable for the separation of charges

Las capas epitaxiales de α-cuarzo, gracias a sus propiedades piezoeléctricas, pueden permitir fabricar sensores de masa de una sensibilidad jamás alcanzada que podrían dar lugar a aplicaciones novedosas en electrónica, biología y medicina. Sin embargo, para ello se debe poder controlar perfectamente su cristalización y la microestructura resultante para que sea posible nanoestructurarlas. En esta Tesis se adopta un nuevo enfoque sistemático para comprender la cristalización de capas epitaxiales de α-cuarzo sobre Silicio(100) a partir de la desvitrificación de capas de sílice mesoporosa mediante tratamientos térmicos. Se ha podido mostrar que aumentando la cantidad de agente devitrificante (Sr) se puede pasar de tener capas porosas y de muy baja rugosidad a otras cuya microestructura está caracterizada por la presencia de cristales de α-cuarzo densos. Asimismo estos estudios han permitido comprender con más detalle el mecanismo por el cual el Sr facilita la cristalización de la sílice. Además, se han podido estudiar los efectos que tienen parámetros cómo el espesor de las capas, la temperatura de tratamiento térmico, la humedad relativa o el tipo de surfactante sobre la microestructura y la homogeneidad de las capas. Aumentando el espesor es posible pasar de tener capas parcialmente cristalinas y porosas a que éstas sean completamente cristalinas y caracterizadas por la presencia de cristales densos. La temperatura del tratamiento térmico puede tener una enorme influencia en el proceso de cristalización ya que incide fuertemente en la dinámica y la reactividad del Sr contenido en las capas de sílice. Una humedad relativa suficientemente elevada da lugar a perforaciones circulares en las capas por un mecanismo de separación de fases inducido per la presencia de agua en el ambiento, siendo este fenómeno muy dependiente del tipo de surfactante utilizado en la síntesis. Las perforaciones tienen influencia en cristalización del cuarzo ya que afectan a la distribución del Sr en las capas. Todos estos estudios han permitido entender mejor el mecanismo de divitrificación asistida por el Sr y por otra parte se han obtenido claves para poder controlar la microestructura de las capas de cuarzo mediante estos diferentes parámetros. Asimismo se ha desarrollado un método de multideposición para poder aumentar el espesor de las capas desde los pocos centenares de nanómetros hasta el rango de las micras. Esto, junto con la posibilidad de controlar la microestructura ha permitido acometer con éxito la formación de nanoestructuras en capas epitaxiales de α-cuarzo sobre Silicio(100) sobre grandes áreas . Esto se ha conseguido gracias al uso combinado de las siguientes técnicas litográficas de bajo coste: (i) Litografia por transferencia laser, (ii) soft nanoimprint lithography aplicadas sobre capas de sílice dopadas con Sr obtenidas por sol-gel y (iii) el uso de máscaras nanométricas de nanopartículas de SrCO3 ensambladas. Así se han conseguido por primera vez pilares nanométricos de diferentes diámetros, que pueden llegar a ser de sólo 50 nm, y alturas de hasta 2 micras. Esta parte de la tesis demuestra que se puede conseguir controlar la forma y la micro o nano estructuración de capas delgadas epitaxiales de cuarzo preservando su cristalinidad y sus propiedades piezoeléctricas. Ello abre la puerta a la fabricación de resonadores de alta frecuencia que podrían dar lugar a sensores ultrasensibles con aplicaciones potenciales en diferentes campos.
Epitaxial films of piezoelectric α-quartz could enable the fabrication of sensors with unprecedented sensitivity for prospective applications in electronics, biology and medicine. However, a prerequisite is harnessing the crystallization of epitaxial α-quartz, tailoring suitable film microstructures for nanostructuration. In my PhD work, we bring new insights into the crystallization of epitaxial α-quartz films on Silicon(100) from the thermal devitrification of nanoporous silica and the control the film microstructures: We show that by increasing the quantity of devitrifying agent (Sr) it is possible to switch from an α-quartz microstructure consisting of porous flat film to one dominated by larger and fully dense α-quartz crystals. The mechanism of Sr-assisted devitrification was also investigated simultaneously. Then, we found that film thickness, annealing temperature, relative humidity and the nature of surfactant also play an important role in the control of the microstructure and homogeneity of the films. By increasing its, thickness it is possible to switch from a partly crystalline and porous film to fully a crystalline and dominated by dense crystals one. Annealing temperature can impact the crystallization process very deeply for it can change the dynamics and reactivity of Sr within the silica film. High relative humidity cooperates with a suitable surfactant to create perforations on the films via a water-induced phase separation phenomenon. This perforation can also influence the film crystallinity by altering the distribution of Sr inside film. All these studies on the one hand give us a better understanding of the mechanism of Sr-assisted devitrification and on the other hand can show us a versatile microstructural control of the epitaxial α-quartz film. Besides, via a multilayer deposition method, we have extended the maximum thickness of the α-quartz films from a few hundreds of nm into the µm range. Moreover, in my thesis, we report unprecedented large-scale fabrications of ordered arrays of piezoelectric epitaxial α-quartz nanostructures on silicon(100) substrates by the combination of three cost-effective lithographic techniques: (i) laser transfer lithography, (ii) soft nanoimprint lithography on Sr-doped SiO2 sol-gel thin films and (iii) self-assembled SrCO3 nanoparticles reactive nanomasks. Epitaxial α-quartz nanopillars with different diameters (down to 50 nm) and heights (up to 2000 nm) were obtained for the first time. This part of my PhD work demonstrates the control over the shape, micro- and nano-patterning of α-quartz thin films while preserving its crystallinity, texture and piezoelectricity, which opens the opportunity to fabricate new high frequency resonators and high sensitivity sensors relevant in different fields of application.

In this work, a body of knowledge is presented which pertains to the growth, characterization and exploitation of high quality, novel II-IV oxide epitaxial films and structures grown by plasma-assisted molecular beam epitaxy. The two compounds of primary interest within this research are the ternary films NixMg1-xO and ZnxMg1-xO and the investigation focuses predominantly on the realization, assessment and implementation of these two oxides as optoelectronic materials. The functioning hypothesis for this largely experimental effort has been that these cubic ternary oxides can be exploited - and possibly even juxtaposed - to realize novel wide band gap optoelectronic technologies. The results of the research conducted presented herein overwhelmingly support this hypothesis in that they confirm the possibility to grow these films with sufficient quality by this technique, as conjectured. NixMg1-xO films with varying Nickel concentrations ranging from x = 0 to x = 1 have been grown on lattice matched MgO substrates (lattice mismatch ε < 0.01) and characterized structurally, morphologically, optically and electrically. Similarly, cubic ZnxMg1-xO films with Zinc concentrations ranging from x = 0 to x ≈ 0.53, as limited by phase segregation, have also been grown and characterized. Photoconductive devices have been designed and fabricated from these films and characterized. Successfully engineered films in both categories exhibit the desired deep ultraviolet photoresponse and therefore verify the hypothesis. While the culminating work of interest here focuses on the two compounds discussed above, the investigation has also involved the characterization or exploitation of related films including hexagonal phase ZnxMg1-xO, ZnO, CdxZn1-xO and hybrid structures based on these compounds used in conjunction with GaN. These works were critical precursors to the growth of cubic oxides, however, and are closely relevant. Viewed in its entirety, this document can therefore be considered a multifaceted interrogation of several novel oxide compounds and structures, both cubic and wurtzite in structure. The conclusions of the research can be stated succinctly as a quantifiably successful effort to validate the use of these compounds and structures for wide bandgap optoelectronic technologies.
Ph.D.
Optics and Photonics
Optics and Photonics
Optics PhD

Film thickness, free carrier concentration and free carrier mobility are critical figures of merit for silicon carbide epitaxial growth. Room temperature Fourier Transform Infrared (FTIR) reflection spectroscopy can estimate these parameters non-destructively and is capable of high-resolution wafer mapping. Commercially available equipment has greatly simplified the application of this technique by coupling a high performance automated spectrometer with model-based data analysis and interpretation based on the personal computer. While powerful numerical techniques run fast and efficient on modern computers, it is essential that low-order, well-conditioned models are needed. The observed reflectance spectrum is the result of reflection and refraction of light at different interfaces due to constructive and destructive interference. The estimation of film thickness and free carrier concentration for single epitaxial layers has been improved by studying the Longitudinal Optical Phonon Plasmon (LPP) coupled modes. However, the addition of multiple layers introduces many degrees of freedom, which complicates parameter extraction. The multiple epitaxial layer stacks studied were intended for Metal Semiconductor Field Effect Transistor (MESFET?s) on both conducting and semi-insulating substrates. The thickness estimation of the n-channel in the MESFET stack on semi-insulating substrate is improved by preconditioning the curve fit for plasma frequency obtained from doping estimation from capacitance voltage profiling or by observing an LPP- peak.

The layered iridates such as Sr2IrO4 and Sr3Ir2O7, have attracted substantial attention due to their novel electronic states originating from strong spin-orbit coupling and electron-correlation. Recent studies have revealed the possibilities of novel phases such as topological insulators, Weyl semimetals, and even a potential high-Tc superconducting state with a d-wave gap. However, there are still controversial issues regarding the fundamental electronic structure of these systems: the origin of the insulating gap is disputed as arising either from an antiferromagnetic ordering, i.e. Slater scheme or electron-correlation, i.e. Mott scheme. Moreover, it is a formidable task to unveil the physics of layered iridates due to the limited number of available materials for experimental characterizations. One way to overcome this limit and extend our investigation of the layered iridates is using metastable materials. These materials which are far from their equilibrium state, often have mechanical, electronic, and magnetic properties that different from their thermodynamically stable phases. However, these materials cannot be synthesized using thermodynamic equilibrium processes. One way to synthesize these materials is by using pulsed laser deposition (PLD). PLD is able to generate nonequilibrium material phases through the use of substrate strain and deposition conditions. Using this method, we have synthesized several thermodynamically metastable iridate thin-films and have investigated their electronic and optical properties. Synthesizing and investigating metastable iridates opens a path to expand the tunability further than the ability of the bulk methods. This thesis consists of four studies on metastable layered iridate thin film systems. In the first study, three-dimensional Mott variable-range hopping transport with decreased characteristic temperatures under lattice strain or isovalent doping has been observed in Sr2IrO4 thin films. Application of lattice strain or isovalent doping exerts metastable chemical pressure in the compounds, which changes both the bandwidth and electronic hopping. The variation of the characteristic temperature under lattice strain or isovalent doping implies that the density of states near the Fermi energy is reconstructed. The increased density of states in the Sr2IrO4 thin films with strain and isovalent doping could facilitate a condition to induce unprecedented electronic properties, opening a way for electronic device applications. In the second study, the effects of tuning the bandwidth via chemical pressure (i.e., Ca and Ba doping) on the optical properties of Sr2IrO4 epitaxial thin films has been investigated. Substitution of Sr by Ca and Ba ions exerts metastable chemical pressure in the system, which changes both the bandwidth and electronic hopping. The optical conductivity results of these thin films suggest that the two-peak-like optical conductivity spectra of the layered iridates originates from the overlap between the optically-forbidden spin-orbit exciton and the inter-site optical transitions within the Jeff = ½ band, which is consistent with the results obtained from a multi-orbital Hubbard model calculation. In the third study, thermodynamically metastable Ca2IrO4 thin- films have been synthesized. Since the perovskite structure of Ca2IrO4 is not thermodynamically stable, its bulk crystals do not exist in nature. We have synthesized the layered perovskite phase Ca2IrO4 thin- films from a polycrystalline hexagonal bulk crystal using an epitaxial stabilization technique. The smaller A-site in this compound compared to Sr2IrO4 and Ba2IrO4, increases the octahedral rotation and tilting, which enhance electron-correlation. The enhanced electron-correlation is consistent with the observation of increased gap energy in this compound. This study suggest that the epitaxial stabilization of metastable-phase thin-films can be used effectively for investigating complex-oxide systems. Finally, structural, transport, and optical properties of tensile strained (Sr1-xLax)3Ir2O7 (x = 0, 0.025, 0.05) thin-films have been investigated. While high-Tc superconductivity is predicted in the system, all of the samples are insulating. The insulating behavior of the La-doped Sr3Ir2O7 thin-films is presumably due to disorder-induced localization and ineffective electron-doping of La, which brings to light the intriguing difference between epitaxial thin films and bulk single crystals of the iridates. These studies thoroughly investigate a wide array of novel electronic and optical phenomena via tuning the relative strengths of electron correlation, electronic bandwidth, and spin-orbit coupling using perturbations such as chemical doping, and the stabilization of metastable phases in the layered iridates.

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.

Oxide ultra-thin film surfaces have properties and structures that are significantly different from the terminations of the corresponding bulk crystals. For example, surface structures of epitaxial ultra-thin oxide films are highly influenced by the crystallinity and electronegativity of the metal substrates they grown on. Some enhanced properties of the novel reconstructions are related to catalysis, sensing and microelectronics, which has resulted in an increasing interest in this field. Ultra-thin TiO_x films were grown on Au(111) substrates in this work. Two well-ordered structures within monolayer coverage - honeycomb (HC) and pinwheel - were generated and investigated. Special attention has been paid to the uniform (2 x 2) Ti₂O₃ HC phase including its regular structure and imperfections such as domain boundaries (DBs) and point defects. Linear DBs with long-range repeating units have been observed; density functional theory (DFT) modelling has been used to simulate their atomic structures and calculate their formation energies. Rotational DBs/defects show up less frequently, however a six-fold symmetrical 'snowflake' DB loop stands out. Two types of point defects have been discovered and assigned to Ti vacancies and oxygen vacancies/hydroxyl groups. Their diffusion manners and pairing habits have been discussed within an experimental context. The results of growing NbO_x ultra-thin films on Au(111) are also presented in this thesis. An identical looking (2 x 2) HC structure to the Ti₂O₃ ultra-thin film has been formed; a stoichiometry of Nb2O3 is suggested. Another interesting reconstruction is a hollow triangle structure. Various sizes have been found, and sides of these equilateral triangles all show a double-line feature aligned along the { 1 ₁⁻ } directions of the Au(111) lattice. Chemical composition characterisations of NbO_x thin films are still required as is DFT modelling. Experimental techniques used in this thesis include scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). Ultra-thin oxide films were created by physical vapour deposition (PVD) in ultra-high vacuum (UHV) systems.

Ce travail de thèse porte sur les propriétés de transport thermique liées aux phonons dans un nouveau matériau nanostructuré constituée de couches minces de Ge:Mn de type "electron crystal - phonon glass". Ce matériau est élaboré par épitaxie par jets moléculaire au CEA/INAC à Grenoble sur des substrats spécifiques « Germanium-on-insulator (GOI) ». Il consiste en une matrice de germanium possédant une qualité cristalline parfaite dans laquelle sont inclues une importante concentration de nano-inclusions de Ge3Mn5 de forme quasi-sphérique. Révélé par les caractérisations de TEM, les nano-inclusions ont une distribution de diamètre variant de 5 à 50 nm. Il est par ailleurs possible de jouer sur les paramètres de croissance afin de modifier la dispersion de taille des inclusions ainsi que leur concentration. Cette possibilité nous a donc permit d'étudier l'influence des nano-inclusions sur le transport de chaleur de façon exhaustive autour de la température ambiante.Pour ce faire, une expérience de mesure de conductivité thermique « 3 omega » dédiée à ces échantillons et extrêmement sensible, a été fabriquée à l'institut Néel. Cette expérience nous a permis de mesurer les variations de conductivité thermique induites par la modification de la distribution en taille des nano-inclusions de Ge:Mn dans des couches minces d'une centaine de nanomètre d'épaisseur avec une erreur réduite d'environ 12%. Il a été montré que le transport thermique dans ces couches minces pouvait être réduit d'un facteur 20 comparé au germanium massif puisque des valeurs de conductivité thermique de l'ordre de 3 W.m-1.K-1 ont été mesurées. Ces valeurs ont été confirmées par des expériences SThM au CETHIL de Lyon. Les valeurs obtenues sont mêmes en dessous de la limite d'alliage pour le SiGe, ce qui n'est pas explicable par les théories actuelles. Ces faibles conductivités thermiques ainsi que la grande mobilité des porteurs dans le germanium font de ces matériaux à base de GeMn de bons candidats pour la thermoélectricité. Des simulations numériques ont été utilisées afin de mieux comprendre cette diminution spectaculaire de la conductivité thermique et d'interpréter les données expérimentales
This PhD project is an exhaustive study on the characterization of the thermal properties of a new type semiconducting materials based on germanium. It is a germanium matrix containing nano-inclusions with the objective of creating a perfect "electron crystal - phonon glass" material. The materials are thin films of an epitaxial germanium matrix embedded with Ge:Mn nano-inclusions, grown on a Germanium-on-insulator (GOI) substrate in CEA/INAC in Grenoble. From TEM images of the thin films it has been demonstrated that both the matrix and inclusions are monocrystalline, and the nano-inclusions have generally a spherical form with a diameter distribution ranging from 5 to 50 nm. Depending on the growth parameters in molecular beam epitaxy, i.e. the Mn concentration and the annealing temperature, the geometries, mean diameters and diameter distributions of nano-inclusions in Ge:Mn can be varied. With these unique structural features, these Ge:Mn thin films are one of the most interesting models for the study of the influence of nano-inclusions on thermal transport in a crystalline matrix.The characterization of the thermal properties of the material have been done using two advanced techniques: the 3-omega method in Institut Néel, and the Scanning Thermal Microscopy (SThM) in CETHIL (Centre d'Energétique et de Thermique de Lyon) in Lyon. A highly sensitive differential 3-omega measurement setup has been developed in the work, which permits precise (error~12%) measurements of electrical conductive thin films having low thermal conductivities. Dramatically reduced thermal conductivities have been revealed for Ge:Mn thin films containing different Mn% and having different inclusion geometries at room temperature, compared to crystalline bulk Ge. A minimum value of 3.3 Wm-1K-1 was found for Ge:Mn thin film containing 10% Mn, beating the “alloy limit” of thermal conductivity set by SiGe alloys at room temperature (6-12 Wm-1K-1). The measurement results of SThM confirmed the low thermal conductivities for all Ge:Mn/GOI samples at room temperature. Numerical simulations using different models have been performed to try to interpret the experimental results and to understand the mechanisms of the influence of the nano-inclusions on the phonon transport in semiconductor materials

La investigación en nuevos óxidos de materiales con alta conductividad mixta iónica y electrónica (MIEC) es de gran importancia con el fin de conseguir un mejor rendimiento en una amplia gama de dispositivos que tienen considerable potencial en la conversión electroquímica de energía de forma limpia y baja en carbono. En el caso de celdas de combustible de óxido sólido (SOFC), a fin de reducir la temperatura de trabajo a rangos intermedios (500-750°C), se necesitan cátodos más activos. Estudios recientes han puesto de manifiesto el gran potencial del GdBaCo2O5+δ (GBCO) como cátodo en SOFC, con una conductividad electrónica alta por encima de la temperatura de transición metal-aislante, sobresalientes propiedades de transporte de oxígeno y un alto intercambio en superficie, así como una excelente estabilidad estructural y electroquímica. Sin embargo, hasta la fecha hay poca información sobre la conductividad iónica intrínseca de este compuesto. Para permitir la medición de las propiedades intrínsecas de transporte electrónico y en particular iónico, el mayor desafío es sintetizar muestras densas, continuas o idealmente capas epitaxiales de GBCO. El objetivo de este trabajo es obtener capas epitaxiales de GBCO crecidas mediante la técnica PLD con el fin de investigar sus propiedades anisotrópicas intrínsecas para aplicaciones en SOFC. Esta investigación involucro el sinterizado de los blancos y el proceso de deposición del material. Las películas obtenidas fueron caracterizadas cristalográfica, composiciónal, morfológica y microestructuralmente. Por último, las propiedades eléctricas, de difusión de oxígeno e intercambio de oxígeno en superficie fueron estudiadas mediante medidas de impedancia electroquímica e intercambio isotópico a través de perfiles de profundidad (IEDP) usando espectrometría de masas de iones secundarios (SIMS). Después de un largo proceso de optimización, se obtuvieron películas epitaxiales de GBCO de alta calidad cristalina. La naturaleza del proceso de ablación del blanco estequiométrico de GBCO genera una desviación en la composición de las películas que básicamente consiste en un déficit de Co. Esta desviación de la composición induce la aparición de defectos de apilamiento con planos suplementarios de GdO sin afectar a la disposición general epitaxial de las películas. A pesar de los cambios observados en la orientación de las película con el eje c \\ (paralelo) a c⊥ (perpendicular) al sustrato con el aumento de la temperatura de deposición, la conductividad electrónica de las capas parece estar más correlacionada con la composición catiónica de las misma. Por lo tanto, cuanto mayor sea la desviación de la composición estequiométrica ideal, menor será la conductividad. Este efecto se ha atribuido principalmente a los defectos planares que impiden la consecución de un orden de largo alcance en la estructura. A pesar de la presencia de defectos, las conductividades en nuestras películas alcanza valores tan altos como 800 S/cm a temperaturas entre 300 y 400ºC. Lo cual hace considerar al GBCO muy prometedor para su aplicación como cátodos en SOFC de temperatura intermedia. También se demuestra que la elección apropiada sustrato permite crecer películas ya sea con orientación c⊥ o c \\ al sustrato. Esto ha permitido la exploración de la potencial anisotropía en el transporte de oxígeno y se demuestra que efectivamente, la difusión de oxígeno a bajas temperaturas es casi un orden de magnitud mayor a lo largo del eje a en comparación con el eje c. Esto se ha relacionado con la disposición de vacantes de oxígeno preferentemente en los planos GdO formando canales a lo largo del eje a, y por lo tanto proporcionando una ruta para la migración del oxígeno. Sin embargo, no se observó influencia de la anisotropía en el intercambio de oxígeno en superficie.
Research on new oxide materials with both high mixed ionic and electronic conductivity (MIEC) is of great importance in order to achieve optimum performance in a wide range of devices that have considerable potential for clean, low carbon, electrochemical energy conversion. In the case of solid oxide fuel cells (SOFCs), in order to reduce the working temperature to the intermediate range (500-750ºC), more active cathode materials are needed. Previous studies have highlighted the great potential of GdBaCo2O5+δ (GBCO), as a cathode in SOFC, with a high reported electronic conductivity above the metal-insulator transition temperature, outstanding oxygen transport properties and enhanced surface exchange, as well as excellent stability for both structural and electrochemical performance. However, to date there is an absence of information about intrinsic ionic conductivity of this GBCO compound. To enable measurement of intrinsic electronic and particularly ionic transport properties, the greatest challenge is to synthesize dense, continuous or ideally epitaxial specimens of GBCO. The aim of this work is to achieve well-defined epitaxial GBCO deposited by PLD technique in order to fundamentally investigate their intrinsic anisotropic properties for SOFC applications. This research involved PLD target synthesis and PLD deposition for these materials. The obtained films were evaluated through crystallographic, compositional, surface morphological and microstructural characterization. Finally electrical, oxygen diffusion and surface exchange properties were characterized by Electrochemical impedance and the isotopic exchange depth profile (IEDP) with secondary ion mass spectrometry (SIMS) method. After a long process of optimization, high-quality epitaxial GBCO films mainly consisting of double-perovskite regions were obtained by PLD. The nature of the ablation process from stoichiometric GBCO target generates a deviation in the composition of the films which basically consists in a Co depletion. This composition deviation induces the appearance of characteristic stacking faults with supplementary GdO planes without affecting the overall epitaxial arrangement of the films. Despite the observed changes in the film orientation from c\\ (parallel) to c⊥ (perpendicular) upon deposition temperature increase, the film electronic conductivities seem to be mainly correlated with the cation composition. So, the larger the deviation from stoichiometric ideal composition, the lower the conductivity. This effect has been mainly ascribed to the role of defects in impeding the achievement of a long-range order of the highly conducting Pmmm structure in the films. Despite the presence of defects, the conductivities in our films, which are considered very promising for their application as cathodes in intermediate temperature SOFCs, attain values as high as 800 S/cm at temperatures between 300 and 400ºC. It also demonstrates that the appropriate choice of substrate mismatch allows growing films with either pure c-axis or a-axis orientation. This has allowed exploration of the potential anisotropy in the oxygen transport and has proven that indeed the oxygen diffusion at low temperatures is almost one order of magnitude larger along the a-axis compared to the c-axis. This has been related to the arrangement of oxygen vacancies preferentially in the GdO planes forming channels along the a-axis, and therefore providing a path for oxygen migration. However, no influence of the structure anisotropy was observed in the oxygen surface exchange rates, which were of similar values regardless of the film orientation or measuring geometry.

Passivation of the crystalline silicon (c-Si) wafer surface and decreasing the number of interface defects are basic requirements for development of high efficiency a-Si:H/c-Si heterojunction solar cells. Surface passivation is generally achieved by development of detailed silicon wafer cleaning processes and the optimization of PECVD parameters for the deposition of intrinsic hydrogenated amorphous silicon layer. a-Si:H layers are grown in UHV-PECVD system. Solar cells were deposited on the p type Cz-silicon substrates in the structure of Al front contact/a-Si:H(n)/a-Si:H(i)/c-Si(p)/Al back contact. Solar cell parameters were determined under standard test conditions namely, using 1000 W/m2, AM 1.5G illumination at 25 oC. Growth of (i) a-Si:H, films on the clean wafer surface was investigated as a function of substrate temperature, RF power density, gas flow rate, hydrogen dilution ratio and deposition time and was characterized using SEM, HRTEM, AFM, SE, ATR-FTIR and I/V measurements. Structural properties of the films deposited on silicon wafer surface are directly effective on the solar cell efficiency. Morphological characterization of the grown films on the crystalline surface was found to be very complex depending on the deposition parameters and may even change during the deposition time. At 225 oC substrate temperature, at the beginning of the deposition, (i) a-Si:H films was found grown in epitaxial structure, followed by a simultaneous growth of crystalline and amorphous structure, and finally transforming to complete amorphous structure. Despite this complex structure, an efficiency of 9.2% for solar cells with total area of 72 cm2 was achieved. In this cell structure, TCO and back surface passivation do not exist. In the

Cr-N codoping approach is studied using both cluster-assembled and epitaxial TiO2 films. simultaneously injecting the two atomic species substantially modifies the TiO2 band gap, reducing it by more than 1 eV. XPS, XAS and optical spectroscopy characterizations confirm that the atoms are succesfully embedded inside the system and that the band gap is reduced. Cr-N coupling is confirmed in the case of epitaxial codoped Titania films.

Dans cette thèse, j'ai étudié la relation entre la symétrie cristalline, la symétrie des interactions magnétiques et des soliton topologiques dans des couches minces magnétiques épitaxiées. Le cas particulier de couches avec une symétrie C2v a été considéré. Ces couches ont un intérêt particulier par leurs propriétés anisotropes qui permettent une stabilisation de solitons magnétiques avec différentes symétries et nombres topologiques. J'ai utilisé des approches théoriques et expérimentaux pour étudier ce phénomène :Approche micromagnétique :La relation entre les formulations atomistes et micromagnétiques des interactions magnétiques a été étudiée en fonction de la symétrie cristalline. Ceci a permis d'expliquer la présence des interactions anisotropes et d'étudier leur effet sur la configuration des solitons magnétiques 1D et 2D.La discussion commence par le plus simple soliton 1D, la paroi des domaines, et pas par pas des nouvelles interactions et symétries sont ajoutées afin de caractériser les conditions de stabilité et les propriétés des solitons 2D, les skyrmion et anti-skyrmions.Notre méthode a permis d'étudier les solitons topologiques 2D sur une large gamme de paramètres, et de construire un diagramme de phase en fonction de l'interaction Dzyaloshinskii-Moriya (DMI) et du champ magnétique appliqué. Trois types de solitons topologiques 2D ont été identifiées (skyrmions, bulles skyrmioniques et skyrmions supercritiques) en fonction de leur taille et leur réponse à un champ magnétique externe.On a aussi montré qu'une inversion du signe de la DMI selon deux directions perpendiculaires permet la stabilisation d'anti-skyrmions. Un modèle micromagnétique a été développé pour étudier la différence de configuration et d'énergie entre skyrmions et anti-skyrmions. On montre que l'interaction dipolaire rompt la symétrie circulaire de l'anti-skyrmion et le rend plus stable que le skyrmion.Approche expérimentale :J'ai préparé différentes couches magnétiques épitaxiées de symétrie C2v. Pour chaque système, je décris les paramètres de croissance et la symétrie cristalline, suivi par les résultats des caractérisations magnétiques et finalement les résultats de microscopie magnétique.J'ai étudié la symétrie et l'intensité de la DMI dans une tricouche Au/Co/W à aimantation perpendiculaire. La DMI dans ce système induit une chiralité horaire de la modulation de spin avec une forte anisotropie de l'intensité de la DMI, venant de la symétrie C2v. Des skyrmions dans ce système devraient avoir une forme elliptique. Nous avons stabilisé des skyrmions dans des films continus et dans des nanostructures. Leur configuration magnétique a été étudiée par XMCD-PEEM et MFM, mais sans observer des propriétés anisotropes.Pour augmenter l'effet des interactions anisotropes sur la configuration des skyrmions, j'ai développé le système W/Co/Au-Pt(solution solide). Des études par microscopie ont montré la stabilisation des bandes magnétiques parallèles à l'axe facile dans le plan dans ce système. Des études par microscopie Kerr ont montré que l'origine de cette configuration en bandes parallèles est une forte anisotropie de la dynamique du mouvement des parois.Des mesures MFM en champ magnétique statique ont été effectuées afin de confiner des bulles skyrmioniques elliptiques, mais la sensibilité de ces mesures à des couches ultrafines a été insuffisante pour caractériser leurs propriétés anisotropes.Des mesures XMCD-PEEM ont permis d'observer la structure interne de parois selon l'axe planaire difficile du système. Ces mesures mettent en évidence un composant Néel de la paroi.Finalement, j'ai préparé et étudié un système W/Fe/Co/Au avec le but de stabiliser des anti-skyrmions. Cependant, le système n'a pas montré l'aimantation hors-du-plan qui est nécessaire pour stabiliser ces solitons. Ce signifie que l'anisotropie planaire de l'interface W/Fe domine l'anisotropie perpendiculaire de l'interface Co/Au
In this thesis I studied the relationship between the crystal symmetry, the symmetry of the magnetic interactions and topological solitons in epitaxial magnetic thin films. The case of thin films with C2v symmetry has been considered. These systems are particularly interesting for the anisotropic properties that allow stabilising magnetic solitons with different symmetries and topology. I used theoretical and experimental approaches to investigate this phenomenon:Micromagnetic approach:The relationship between the atomistic and the micromagnetic formulations of magnetic interactions was studied as a function of the crystal symmetry.This allowed to explain the presence of anisotropicinteractions and study their effect on the configurations of 1D and 2D magnetic solitons. The discussion starts from the simplest 1D soliton, the domain wall, and step-by-step new interactions and symmetries are added in order to characterize the stability conditions and the properties of 2D solitons, skyrmions and anti-skyrmions. Our method allowed to study 2D topological solitons over a wide range of parameters and build a phase diagram as a function of the Dzyaloshinskii-Moriya interaction (DMI) strength and magnetic field intensity. This allowed us to distinguish three kinds of 2D topological solitons (skyrmions, skyrmionic bubbles and supercritical skyrmions) as a function of their size and response to an external magnetic field. We show that an inversion of DMI strength along perpendicular directions allows the stabilisation of anti-skyrmions. A micromagnetic model is developed to study the configuration and energy differences between skyrmions and anti-skyrmions. This shows that the dipolar interaction breaks the circular symmetry of the antiskyrmion and makes it more stable than the skyrmion.Experimental approach:Epitaxial magnetic systems with C2v symmetry have been grown. For each system I describe the growth parameters and crystal symmetry, followed by the results of the magnetic characterisation and finally the results from the magnetic microscopy measurements.I have investigated the DMI symmetry and strength in an out-of-plane magnetised epitaxial Au/Co/W trilayer. The DMI in this system promotes a clockwise chirality of the spin modulation with a strong anisotropy in the DMI strength. This anisotropy arises from the C2v symmetry of the Co/W stack.Skyrmions in this system should have an elliptical shape. We stabilised skyrmions in continuous films and in nanopatterned structures. Their magnetic configurations have been displayed with different microscopic techniques, XMCD-PEEM and MFM, without identifying anisotropic properties.We designed the W/Co/Au-Pt (solid solution) system to increase the effect of the anisotropic interactions on the skyrmion configuration. Microscopy studies in naturally demagnetised areas show that stripe domains parallel to the in-plane easy axis are stable in this system. The configuration with a larger periodicity has been found even for thinner Co layer after demagnetisation with a magnetic field. Kerr microscopy studies of the DW dynamics allowed to evidence the origin of this magnetic configuration, which arises from a strong anisotropy in the DW motion.MFM measurements with the application of a static magnetic field have been performed in order to confine elliptical skyrmionic bubbles but the reduced sensitivity of this technique to thin magnetic systems did not allow to display and characterise them. XMCD-PEEM measurements allowed to display the internal structure of the DWs along the in-plane hard axis of the system. They show the presence of a Néel DW component. Finally I have grown and studied a W/Fe/Co/Au system where anti-skyrmions may in principle be stabilised. However, the system did not show the out-of-plane magnetisation which is fundamental for the stabilisation of skyrmions. This means that the W/Fe in-plane anisotropy dominates the Co/Au out-of-plane anisotropy

Els òxids conductors transparents (TCO) són essencials en dispositius tecnològics. La seva capacitat per combinar una alta conductivitat elèctrica i transparència òptica a la llum visible, els fa particularment útils en una gran varietat de dispositius: pantalles, cèl·lules solars, finestres intel·ligents, etc. L’òxid d’indi i estany (ITO) és fins ara el TCO més estès. Un gran inconvenient de l’ITO és el seu alt cost ja que l’indi, el seu component principal, és un material escàs. D’altra banda, alguns òxids metàl·lics intrínsecs de metalls de transició també són transparents. En aquests materials, la banda (3,4)d és estreta i parcialment plena. És la responsable de l’alta densitat de portadors lliures i la seva massa efectiva gran fa que la llum no es reflecteixi en el visible i que el llindar de reflexió (freqüència de plasma) estigui a la regió de l’IR proper. En aquesta tesi, hem explorat les propietats de les pel·lícules primes d’òxids metàl·lics (SrVO3 (SVO; 3d1) i SrNbO3 (SNO; 4d1)) crescudes per deposició de làser polsat (PLD). Atès que la qualitat epitaxial és fonamental per obtenir bones propietats funcionals, el primer pas va consistir en optimitzar els paràmetres de creixement de les pel·lícules SVO/SNO. Les pel·lícules han de créixer en ultra alt buit (UHV) per estabilitzar l’estat d’oxidació 4+ de V/Nb i usar temperatures de dipòsit altes (700-800° C) per permetre la mobilitat de les espècies sobre el substrat. No obstant això, l’ablació a UHV i l’expansió de la ploma del PLD, molt enèrgica, provoquen la creació de defectes puntuals en les pel·lícules. Hem resolt aquest problema utilitzant un gas inert durant el creixement, que controla l’expansió de la ploma. Finalment, hem estudiat l’impacte de la deformació epitaxial en la conductivitat elèctrica i transparència òptica. Es van obtenir pel·lícules amb una conductivitat més gran que ITO i una transparència similar. D’altra banda, l’observació que la freqüència de plasma en aquests materials estigui a l’IR proper, s’atribueix comunament a un augment de la massa efectiva dels electrons degut les correlacions e-e dins de la banda estreta 3d. Després d’una anàlisi sistemàtica de les dades de transport de SVO vam arribar a la conclusió que la teoria del líquid de Fermi no pot explicar l’augment de la massa efectiva dels portadors. En canvi, hem suggerit que l’acoblament electró-fonó i el caràcter 2D de la superfície de Fermi són molt importants. A més, hem demostrat que la imatge clàssica de banda rígida, d’un electró lliure que evoluciona en una banda 3d-t2g, és només una aproximació, com ho demostra la hibridació observada dels orbitals V 3d i O 2p. Hem observat també que la tensió epitaxial, afecta la hibridació, l’ordre orbital i últimament a la resistivitat de les capes. Hem pogut observar i explicar que a la freqüència de plasma s’exciten, en les condicions d’il·luminació adequades, plasmons de volum. Una observació i descripció poc freqüents i que, en essència, estan relacionades amb la component π de la polarització de la llum i el gradient de càrrega en la superfície del material. Finalment, hem provat la idoneïtat de SVO com elèctrode en heteroestructures fotoabsorbents “tot-òxids”. Hem observat la resposta fotovoltaica, usant capes epitaxials de LaFeO3 com absorbent, i hem posat en relleu el paper de la funció de treball de l’elèctrode en el rendiment del dispositiu. Com perspectiva, hem demostrat que els elèctrodes transparents desenvolupats (SVO i SNO), en tenir funcions de treball diferents, poden permetre ajustar i optimitzar els dispositius. Aquest treball aporta un nou coneixement fonamental de les propietats d’òxids i demostra la seva versatilitat en component fotovoltaics.
Los óxidos conductores transparentes (TCO) son esenciales en dispositivos tecnológicos. Su capacidad para combinar alta conductividad eléctrica y transparencia óptica a la luz visible los hace particularmente útiles en una gran variedad de dispositivos: pantallas, células solares, ventanas inteligentes, etc. El óxido de indio y estaño (ITO) es hasta ahora el TCO más extendido. Un gran inconveniente del ITO es su alto coste ya que el indio, su componente principal, es un material escaso. Por otro lado, algunos óxidos metálicos intrínsecos de metales de transición también son transparentes. En estos materiales, la banda (3,4)d es estrecha y parcialmente llena. Es la responsable de la alta densidad de portadores libres y su masa efectiva grande hace que la luz no se refleje en el visible y que el borde de reflexión (frecuencia de plasma) esté en la región del IR cercano. En esta tesis, hemos explorado las propiedades de las películas delgadas de óxidos metálicos (SrVO3 (SVO; 3d1) y SrNbO3 (SNO; 4d1)) crecidas por deposición de láser pulsado (PLD). Dado que la calidad epitaxial es fundamental para obtener buenas propiedades funcionales, el primer paso consistió en optimizar los parámetros de crecimiento de las películas SVO/SNO. Las películas deben crecerse en ultra alto vacío (UHV) para estabilizar el estado de oxidación 4+ de V/Nb y usar temperaturas de depósito altas (700-800°C) para permitir la movilidad de las especies sobre el sustrato. Sin embargo, la ablación en UHV y la expansión de la pluma del PLD, muy enérgica, provocan la creación de defectos puntuales en las películas. Hemos resuelto este problema utilizando un gas inerte durante el crecimiento, que controla la expansión de la pluma. Finalmente, hemos estudiado el impacto de la deformación epitaxial en la conductividad eléctrica y la transparencia óptica. Se obtuvieron películas con una conductividad mayor que ITO y una transparencia similar. Por otra parte, la observación de que la frecuencia de plasma en estos materiales esté en el IR cercano, se atribute comúnmente a un aumento de la masa efectiva de los electrones debido a las correlaciones e-e dentro de la banda estrecha 3d. Tras un análisis sistemático de los datos de transporte de SVO llegamos a la conclusión de que la teoría del líquido de Fermi no puede explicar el aumento de la masa efectiva de los portadores. En cambio, hemos sugerido que el acoplamiento electrón-fonón y el carácter 2D de la superficie de Fermi juegan un papel importante. Además, hemos demostrado que la imagen clásica de banda rígida, de un electrón libre que evoluciona en una banda 3d-t2g es solo una aproximación, como lo demuestra la hibridación observada de los orbitales V-3d y O-2p. Hemos observado también que la tensión epitaxial, afecta a la hibridación, el orden orbital y últimamente a la resistividad de las capas. Hemos podido observar y explicar que a la frecuencia de plasma se excitan, en las condiciones de iluminación adecuadas, pasmones de volumen. Una observación y descripción poco frecuentes y que, en esencia, están relacionadas con la componente π de la polarización de la luz y el gradiente de carga en la superficie del material. Finalmente, hemos probado la idoneidad de SVO como electrodo en heteroestructuras fotoabsorbentes “todo-óxido”. Hemos observado la respuesta fotovoltaica, usando capas epitaxiales de LaFeO3 como absorbente y hemos puesto de relieve el papel de la función de trabajo del electrodo en el rendimiento del dispositivo. Como perspectiva, hemos demostrado que los electrodos transparentes SVO y SNO, al tener funciones de trabajo distintas, podrán permitir ajustar y optimizar los dispositivos. Este trabajo aporta un nuevo conocimiento fundamental de las propiedades de óxidos y demuestra su versatilidad en componentes fotovoltaicos.
Transparent conducting oxides (TCOs) are key elements to many technological devices. Their ability to combine high electrical conductivity and high optical transparency to visible light, make them particularly useful in a myriad of devices such as displays, solar cells, smart windows, etc. Indium tin oxide (ITO) is so far the most widespread TCO. By Sn-doping, this wide band gap In2O3 semiconductor can reach low resistivity (only about two orders of magnitude above conventional metals) while preserving its transparency. A major drawback of ITO is its high cost as indium, its main component, is a scarce material. Moreover, due to its nature of doped-semiconductor, some physical limits impose that its properties cannot be further improved. On the other hand, some intrinsic metallic oxides composed of early transition metals also turn out to be transparent. In these materials, the partially filled narrow d band is responsible for high density of free carriers with increased effective mass, thus bringing the reflection edge down to the near-IR region. In this thesis, we were interested in exploring the properties of metallic oxide thin films grown by pulsed laser deposition (PLD), namely SrVO3 (SVO; 3d1) and SrNbO3 (SNO; 4d1). As high epitaxial quality is essential to obtain good functional properties, the first step consisted in optimizing the growth parameters for single phase and flat SVO/SNO films, displaying high crystallinity, conductivity and transparency. As anticipated, films need to be grown in ultra-high vacuum (UHV) to stabilize the 4+ oxidation state of V/Nb and using a high substrate temperature (700-800°C) to allow good mobility of the species on the substrate. However, the deposition in UHV and its subsequent highly energetic PLD plasma plume lead to a high concentration of point defects. We have solved this issue by using an inert background gas. Finally, we have studied the impact of epitaxial strain on the electrical conductivity and optical transparency window. All in all, it turned out that optimal films display larger conductivity than ITO, for a similar transparency. Conventional wisdom would suggest that a low plasma frequency would be due to the electron-electron correlations within the narrow nd1 band. In a systematic analysis of SVO transport data (temperature-dependent resistivity, etc.), we have concluded that the Fermi liquid theory alone cannot account for the carrier mass enhancement. Instead, we have suggested that the 2D-like Fermi surface and the electron-phonon coupling play a major role. In addition, we have shown that the classical rigid band picture, of one free electron evolving in a 3d-t2g band is only a rough approximation, as attested by the observed hybridization of the V 3d and O 2p orbitals. Moreover, strain affects this hybridization by modifying the orbital hierarchy and covalency which could be responsible for the observed strain-dependent resistivity and effective mass. By appropriate optical measurements, we have also discussed the nature of the plasmonic excitations at plasma frequency in SNO and SVO films. Interestingly, the possibility of exciting volume plasmons in these TCOs gives a glimpse on their potential applications in the field of plasmonics. Finally, we have tested the suitability of SVO as electrode in photoabsorbing all-oxide heterostructures. In particular, we have successfully observed a photovoltaic effect in LaFeO3-based capacitors and disclosed the important role of the electrode work function on the device performances. As outlook, we have concluded that SVO and SNO, by having distinct work functions, could allow to tune any device properties. This work demonstrates the suitability and high potential of this whole new category of TCOs as electrode material in all-oxide devices. We are convinced that it opens the way to a plethora of possible devices, photovoltaic-wise or other.
Universitat Autònoma de Barcelona. Programa de Doctorat en Física

Les propriétés élastiques sont d'une importance fondamentale pour un matériau. Avec la combinaison des techniques acoustique picoseconde et de la diffusion Brillouin, nous avons mesuré les vitesses du son et constantes élastiques dans divers films minces déposés par pulvérisation magnétron, y compris des nitrures de métaux de transition, binaires et ternaires épitaxiés (TaN, TiN, ZrN, Ti1-xZrxN), des alliages polycristallins à haute entropie ((CoCrCuFeNi)1-x(Al, Nb)x), et leurs alliages de nitrure interstitiels (CoCr(Cu,Mn)FeNiNx). D’autre part, des calculs ab-initio basés sur la théorie de la fonctionnelle de la densité (DFT) ont été effectués pour fournir des constantes élastiques monocristallines avec considération de lacunes.Il a été constaté que pour les nitrures binaires et ternaires, les lacunes jouent un rôle énorme sur les propriétés élastiques, en particulier dans le cas de TaN. Alors que dans le cas de TiN, ZrN et Ti1-xZrxN, l’influence des lacunes est moindre mais reste perceptible. Dans le cas des alliages polycristallins à plusieurs composants et des alliages interstitiels, les vitesses du son et les constantes élastiques effectives sont associées à des effets de composition chimique (Al, Nb et N), à des transitions de phase (FCC, BCC et amorphe), et des changements de textures et de porosité. Le module élastique correspond au module de nanoindentation EIT. La concordance globale entre nos mesures de propriétés élastiques et les calculs DFT a été atteinte. En outre, DFT ont également fourni des informations précieuses sur les propriétés structurelles et la stabilité des phases observées
Elastic properties are of fundamental importance for a material. With the combination of picosecond laser ultrasonic and Brillouin light scattering techniques, we measured sound velocities in various magnetron-sputtering-deposited thin films including epitaxial binary and ternary transition metal nitrides (TaN, TiN, ZrN, Ti1-xZrxN), polycrystalline high entropy alloys or multicomponent transition metal alloys ((CoCrCuFeNi)1-x(Al,Nb)x), and their nitrogen interstitial alloys (CoCr(Cu,Mn)FeNiNx). The longitudinal and shear sound velocities (VL and VT respectively) are correlated to the effective elastic constants through ⟨C33⟩= ρV2L and ⟨C44⟩= ρV2T. On the other hand, Density functional theory (DFT) based ab-initio calculations were performed to provide single crystal elastic constants w/o considering vacancies. It was found that for the epitaxial binary and ternary nitrides, vacancies play a huge role on elastic properties especially in the case of TaN. While in the case of TiN, ZrN and Ti1-xZrxN, the influence of vacancies is smaller but still noticeable. In the case of polycrystalline multicomponent alloys and interstitial alloys, the sound velocities and effective elastic constants were found to be associated to chemical alloying (Al, Nb and N) effects, phase transitions (FCC, BCC and amorphous), textures and porosity changes. The elastic modulus agrees to nanoindentation modulus EIT. The overall agreement between our elastic property measurements and DFT calculations were reached. Besides, DFT calculations also provided valuable insights into structural properties and phase stability. Additionally, we studied structural and mechanical (including elastic) properties of a bulk CoCrCuFeNi as a reference to thin film properties. With the CoCrCuFeNi films on a flexible Kapton® substrate, we performed tensile tests and studied fracture properties of thin films, as a preliminary work to further link thin film elasticity to plasticity. Keywords: Thin film, epitaxial, polycrystalline, sound velocity, elastic constant, transition metal nitride, multicomponent alloy, interstitial alloy, point defects, microstructure, porosity, texture, Brillouin light scattering, picosecond laser ultrasonic, nanoindentation, mechanical property, flexible substrate, fracture toughness, crack and buckle, ab-initio, DFT

Le but de cette these est d'etudier les proprietes structurales et magnetiques des couches epitaxiees de w/fe/w et gd/fe elaborees par depot laser pulse. La technique de depot laser pulse est presentee dans le premier chapitre. La croissance des couches minces est discutee dans le deuxieme chapitre puis l'analyse des proprietes structurales des couches elaborees est presentee dans le troisieme chapitre. Une revue des proprietes magnetiques des couches ultra-minces, telle que le moment magnetique, l'anisotropie et les excitations thermiques est presentee dans le quatrieme chapitre. La determination experimentale de ces proprietes magnetiques dans les couches de w/fe/w est discutee dans le cinquieme chapitre. Le couplage interfacial dans les couches de gd/fe est analyse dans le dernier chapitre

Al llarg de les dues darreres dècades, el polimorf d’òxid de ferro(III) ε-Fe2O3 ha passat de ser una raresa científica a estar a centre d’una intensa activitat de recerca a causa del seves propietats multiferroiques elevada coercivitat a temperatura ambient. Aquestes propietats tan poc habituals posen de relleu aspectes fonamentals relacionats amb la complexitat del seu diagrama de fases magnètic i susciten interès per les seves aplicacions potencials en tecnologies de la informació o fotocatàlisi. Tot i aquest interès creixent, la síntesi i caracterització de materials de tipus ε-(Fe1-xMx)2O3 en què el Fe3+ es substitueix per altres cations metàl·lics encara ha estat poc estudiada. Així mateix, tampoc s’acaben d’entendre els mecanismes d’estabilització de l’ε-Fe2O3 en capes primes, molt rellevants en les aplicacions. Aquesta tesi pretén fer avançar el coneixement en aquests aspectes, explorant la síntesi i caracterització de nanopartícules ε-(Fe1-xMx)2O3 dopades amb metalls de transició magnètics (M= Cr, Mn, Co, Ru) i en els mecanismes d’estabilització de les capes epitaxials d’ ε-Fe2O3, tot fent la caracterització estructural i de les seves propietats. La recerca en nanopartícules ha permès aprofundir el coneixement de les transicions de fase de l’ ε-Fe2O3 i ha posat de manifest la seva sensibilitat a la deformació i als camps magnètics. S’ha vist que el Cr3+ substitueix el Fe3+ en entorns octaèdrics regulars de l’ε-(Fe1-xCrx)2O3, i que redueix de forma dràstica l’anisotropia magnètica i la magnetització de saturació de les nanopartícules. La substitució del Fe3+ per Cr3+ s’ha pogut estudiar fins a x=0.25 sense observar-se l’aparició d’altres fases secundàries, tot i que per a x>0.10 hi ha indicis de canvis estructurals. La substitució amb Mn s’ha estudiat fins a x=0.20 sense que tampoc en aquest cas s’observés l’aparició de fases secundàries a l’augmentar el contingut de Mn. Tanmateix, per sobre de x=0.10 també s’han observat canvis estructurals. En relació amb això, una baixa substitució amb Mn fa augmentar l’anisotropia magnètica tot i que aquesta disminueixi ràpidament per a x>0.05 coincidint amb un fort increment de la magnetització. A diferència del que passa amb el Cr i el Mn, la substitució amb Co no pot anar més enllà del 3 % atòmic sense que apareguin fases secundàries. Aquests nivells tan baixos de substitució fan tanmateix augmentar a i al mateix temps comprimir b i c, molt probablement com a conseqüència d’efectes magnetoelàstics relacionats amb el moment orbital del Co2+. Aquests efectes magnetoelàstics semblen tenir una forta influència sobre la transició magnètica de l’ε-Fe2O3 a alta temperatura tot estabilitzant les subxarxes magnètiques dels ferros en entorns octaèdrics regulars i tetraèdrics. Pel que fa a la substitució amb Ru, cal dir que ha estat complicada a causa de la volatilitat d’aquest metall i que els resultats de la caracterització magnètica de diferents síntesis no ha resultat reproduïbles, fet que indica que aquestes s’han de millorar. En la recerca sobre capes primes d’ε-Fe2O3, s’ha investigat el creixement de Sc0.2Al0.4Fe1.4O3 sobre diferents substrats i s’han obtingut capes epitaxials de bona qualitat sobre LSAT (111), STO (111), Mica (001) i YSZ (001). Això ha permès estabilitzar capes primes epitaxials d’ε-Fe2O3 sobre substrats flexibles com la Mica fent servir l’Sc0.2Al0.4Fe1.4O3 com a capa tampó. La caracterització magnètica de les capes d’ε-Fe2O3 sobre mica ha posat de manifest una transició magnètica a baixa temperatura de característiques similars a la ja coneguda en nanopartícules d’ε-Fe2O3, però encara no observada en capes primes. També s’ha investigat l’estabilització d’ε-Fe2O3 sobre l’espinel·la Fe3O4 (111) com a capa tampó i sobre substrats MgAl2O4 (111). La transició a baixa temperatura de les capes d’ε-Fe2O3 crescudes directament sobre MAO(111) s’ha estudiat de forma detallada amb espectroscòpia Raman
En las dos últimas décadas, el polimorfo de óxido de hierro(III) ε-Fe2O3 ha pasado de ser una rareza científica a estar en el centro de una intensa actividad investigadora por sus propiedades multiferroicas y una elevada coercividad a temperatura ambiente. Estas propiedades ponen de relieve aspectos fundamentales relacionados con la complejidad de su diagrama de fases magnético y suscitan interés por sus aplicaciones potenciales en tecnologías de la información o fotocatálisis. A pesar de este interés creciente, la síntesis y caracterización de materiales de tipo ε-(Fe1-xMx)2O3 en que el Fe3+ se sustituye por otros cationes metálicos ha sido poco estudiada. Tampoco se acaban de entender los mecanismos que controlan la estabilización de la ε-Fe2O3 en capas delgadas, muy relevantes para las aplicaciones de interés. Esta tesis pretende hacer avanzar el conocimiento en estos aspectos, concretamente explorando la síntesis y caracterización de nanopartículas ε-(Fe1-xMx)2O3 dopadas con metales de transición magnéticos (M = Cr, Mn, Co , Ru). También se ha centrado en los mecanismos de estabilización de capas epitaxiales de ε-Fe2O3, caracterizando su estructura y propiedades. La investigación en nanopartículas ha permitido profundizar en el conocimiento de las transiciones de fase del sistema ε-Fe2O3 y ha puesto de manifiesto su sensibilidad a la deformación y los campos magnéticos. Se ha visto que el Cr3 + sustituye preferencialmente el Fe3 + en los entornos octaédricos regulares del ε-(Fe1-xCrx)2O3, y que reduce de forma drástica la anisotropía magnética y la magnetización de saturación de las nanopartículas. La sustitución del Fe3 + por Cr3 + se ha podido estudiar hasta x=0.25 sin observarse la aparición de otras fases secundarias, aunque para x>0.1 se han obtenido claros indicios de cambios estructurales. La sustitución con Mn se ha estudiado hasta x=0.20 sin que tampoco en este caso se observara la aparición de fases secundarias en aumentar el contenido de Mn. Sin embargo, por encima de x>0.1 también se han observado cambios estructurales. Para bajos niveles de sustitución el Mn aumenta la anisotropía magnética aunque esta disminuya rápidamente para x>0.05, coincidiendo con un fuerte incremento de la magnetización. A diferencia de lo que ocurre con el Cr y el Mn, la sustitución con Co no puede ir más allá del 3% atómico sin que aparezcan fases secundarias. Pero incluso estos niveles tienen el efecto de aumentar a y al mismo tiempo comprimir b y c, muy probablemente como consecuencia de fenómenos magnetoelásticos relacionados con el momento orbital del Co2 +. Estos efectos magnetoelásticos parecen tener una fuerte influencia sobre la transición magnética del sistema ε-Fe2O3 a alta temperatura, en particular estabilizando las subredes magnéticas de los hierros en entornos octaédricos regulares y tetraédricos. Respecto a la sustitución con Ru, cabe decir que ha sido complicada debido a la volatilidad de este metal y que los resultados de la caracterización magnética de diferentes síntesis no han resultado reproducibles y deben mejorarse. En cuanto a capas delgadas de ε-Fe2O3, esta Tesis ha investigado el crecimiento de Sc0.2Al0.4Fe1.4O3 sobre diferentes sustratos y se ha visto que se pueden obtener capas epitaxiales de buena calidad sobre sustratos LSAT(111), STO (111), mica (001) y YSZ (001). Esto ha permitido estabilizar capas delgadas epitaxiales de ε-Fe2O3 sobre sustratos flexibles como la Mica con Sc0.2Al0.4Fe1.4O3 como capa tampón. Con la caracterización magnética de capas ε-Fe2O3 sobre Mica se ha detectado una transición magnética a baja temperatura similares a la ya conocida para nanopartículas de ε-Fe2O3, pero desconocida para capas delgadas. También se ha investigado la estabilización de ε-Fe2O3 sobre espinela Fe3O4(111) como capa tampón y sobre MgAl2O4(111). La transición a baja se ha estudiado de forma detallada con espectroscopia Raman.
In the course of the last two decades, the ferric oxide polymorph ε-Fe2O3 has transitioned from being a scientific rarity to attracting increasing attention for its multiferroic character and huge coercive field at room temperature. Such uncommon properties bring out fundamental aspects related to its complex magnetic phase diagram and also make ε-Fe2O3 appealing for applications in information technologies or photocatalysis. In spite of this rising interest, the synthesis and characterization of ε-(Fe1-xMx)2O3 in which Fe3+ is substituted by other metallic cations remains largely unexplored. Moreover, the mechanisms governing the stabilization of ε-Fe2O3 in form of thin films, the form preferred for most applications, are still poorly understood. This Thesis aims at advancing knowledge in this particular area by exploring the synthesis and characterization of ε-(Fe1-xMx)2O3 nanoparticles substituted with magnetic transition metals (M= Cr, Mn, Co, Ru). It also focuses on the stabilization mechanisms of ε-Fe2O3 epitaxial thin films and the characterization of their structural and magnetic properties. The research on nanoparticles has brought new insights on the high and low temperature phase transitions of pure ε-Fe2O3, revealing its sensitivity to magnetic fields and strain state. It has been found that Cr3+ preferentially substitutes Fe3+ in the regular octahedral environment of ε-(Fe1-xCrx)2O3, drastically reducing the magnetic anisotropy and saturation magnetization of the nanoparticles. The Fe3+ replacement by Cr3+ was studied up x=0.25 without the appearance of other secondary phases but strong evidence of structural evolution was observed for x>0.10. The Mn substitution was studied up to x=0.20 and did not induce the appearance of additional secondary phases but above x=0.10 a structural change was also observed. In connection with this, low Mn substitution has the effect of enhancing the magnetic anisotropy which rapidly falls for x>0.05 while the magnetization is strongly increased. In contrast to Cr and Mn, the Co substitution was found to be limited to 3 at. % with secondary phases appearing above this limit. Even such small substitutions resulted in significant changes of the lattice parameters with the effect of stretching a and compressing b and c, most likely as a consequence of magnetoelastic effects related to the unquenched orbital moment of Co2+. The latter seems to strongly influence the high temperature magnetic transition of ε-Fe2O3 by magnetically stabilizing the regular octahedral and tetrahedral sublattices of Fe3+. The Ru substitution was challenging due to the volatility of this metal and the magnetic characterization of the nanoparticles presented reproducibility issues among different batches, indicating that experimental improvements are still needed. Regarding the research on ε-Fe2O3 thin films, the Thesis has investigated the growth of Sc0.2Al0.4Fe1.4O3 on different substrates finding that it yields high-quality epitaxial films on LSAT (111), STO (111), Fluorophlogopite Mica (001) and YSZ (001) substrates. This has allowed us to stabilize epitaxial ε-Fe2O3 films on flexible Mica substrates using Sc0.2Al0.4Fe1.4O3 films as a buffer layer. The magnetic characterization of the ε-Fe2O3 films on Mica revealed the existence of a low temperature magnetic transition which reminds that of ε-Fe2O3 nanoparticles, but which had not been previously reported in films. The stabilization of epitaxial ε-Fe2O3 films on Fe3O4 (111) spinel buffer layers or directly on MgAl2O4 (111) (MAO(111)) substrates was also investigated and it was found that impurities are prone to appear when Fe3O4 (111) layers are used. The low temperature magnetic transition was also found on the ε-Fe2O3 films directly grown on MAO(111) and was studied by Raman spectroscopy.
Universitat Autònoma de Barcelona. Programa de Doctorat en Ciència de Materials

Efficiency droop is a critical issue for the Group III-nitride based light-emitting diodes (LEDs) to be competitive in the general lighting application. Carrier spill-over have been suggested as an origin of the efficiency droop, and an InAlN electron-blocking layer (EBL) is suggested as a replacement of the conventional AlGaN EBL for improved performance of LED. Optimum growth condition of InAlN layer was developed, and high quality InAlN layer was grown by using metalorganic chemical vapor deposition (MOCVD). A LED structure employing an InAlN EBL was grown and its efficiency droop performance was compared with a LED with an AlGaN EBL. Characterization results suggested that the InAlN EBL delivers more effective electron blocking over AlGaN EBL. Hole-injection performance of the InAlN EBL was examined by growing and testing a series of LEDs with different InAlN EBL thickness. Analysis results by using extended quantum efficiency model shows that further improvement in the performance of LED requires better hole-injection performance of the InAlN EBL. Advanced EBL structures such as strain-engineered InAlN EBL and compositionally-graded InAlN EBLs for the delivery of higher hole-injection efficiency were also grown and tested.

Cette thèse de doctorat traite de la mise en œuvre du procédé de soulèvement épitaxial (ou ELO pour epitaxial lift-off) à partir d'un substrat d'InP permettant le détachement des couches actives et le recyclage du substrat afin de rendre économiquement compétitive la fabrication de cellules solaires multi-jonctions pour le photovoltaïque concentré. Ce procédé, qui consiste à sous-graver sélectivement une couche sacrificielle comprise entre le substrat et les couches actives, est bien connu et maîtrisé sur un substrat de GaAs avec l'utilisation d'une couche sacrificielle d'AlAs d'épaisseur voisine de 5 nm, ce qui n'est pas possible sur un substrat d'InP en raison du fort désaccord de maille cristalline existant entre l'AlAs et l'InP. Pour l'adapter à un substrat d'InP, le développement d'une couche sacrificielle spécifique basée sur un super-réseau AlAs/InAlAs a été réalisé, ce qui permet de contourner les problématiques liées au désaccord de maille et à la croissance de matériaux contraints. Après optimisation des conditions de croissance de ce super-réseau, les épaisseurs atteintes et donc les vitesses de sous-gravure obtenues en utilisant ce type de couche sacrificielle ont satisfait aux exigences du procédé ELO. Ensuite, le report et le soulèvement de structures actives de cellules solaires InGaAs en couches minces cristallines ont été développés. Les cellules solaires ainsi fabriquées ont montré des performances semblables à celles réalisées par épitaxie standard sur un substrat d'InP, voire meilleures sous concentration en raison d'effets de confinement optique. Finalement, le recyclage du substrat d'InP réalisé avec un procédé utilisant seulement deux étapes de nettoyage par voies chimiques humides, a permis de produire des surfaces d'InP de qualité suffisante pour réaliser une reprise d'épitaxie satisfaisante
This PhD thesis deals with the implementation of the epitaxial lift-off (ELO) process from an InP substrate allowing the detachment of active layers and the substrate recycling. The final target is to realize multi-junction solar cells in an economically competitive way for concentrated photovoltaic. The ELO process consists in the under-etching of a sacrificial layer inserted between the substrate and the active layers. It is well known and mastered on a GaAs substrate with the use of a sacrificial layer of AlAs with a thickness of about 5 nm. Such a layer is not usable on an InP substrate due to the high lattice mismatch between AlAs and InP. In order to adapt the ELO process to an InP substrate, this work aimed to develop a specific sacrificial layer based on an AlAs/InAlAs superlattice. Thus, it is possible to circumvent problems related to the lattice mismatch and to the strained layer growth. After optimization of growth conditions of this superlattice, using this type of sacrificial layer, we achieve a sufficient thickness and therefore a sufficient under-etching rate in order to meet the requirements of the ELO process. Then, the transfer and lift-off of thin crystalline film based InGaAs solar cells have been developed. This kind of solar cells showed performances similar to those obtained with a standard epitaxial growth on an InP substrate, or even better under concentration due to optical confinement effects. Finally, the recycling of the InP substrate carried out by a process using only two wet chemical cleaning steps made it possible to produce InP surfaces of sufficient quality to achieve a promising second epitaxial growth

This thesis deals with the synthesis and characterization of Ge-Te-Sb (GST) thin films. The films were deposited using a Pulsed Laser Deposition (PLD) method and mainly characterized with XRD, SEM, AFM and TEM. For amorphous and polycrystalline films, un-etched Si(100) was used. The amorphous films showed a similar crystallization behavior as films deposited with sputtering and evaporation techniques. When depositing GST on un-etched Si(100) substrates at elevated substrate temperatures (130-240°C), polycrystalline but highly textured films were obtained. The preferred growth orientation was either GST(111) or GST(0001) depending on if the films were cubic or hexagonal. Epitaxial films were prepared on crystalline substrates. On KCl(100), a mixed growth of hexagonal GST(0001) and cubic GST(100) was observed. The hexagonal phase dominates at low temperatures whereas the cubic phase dominates at high temperatures. The cubic phase is accompanied with a presumed GST(221) orientation when the film thickness exceeds ~70 nm. Epitaxial films were obtained with deposition rates as high as 250 nm/min. On BaF2(111), only (0001) oriented epitaxial hexagonal GST films are found, independent of substrate temperature, frequency or deposition background pressure. At high substrate temperatures there is a loss of Ge and Te which shifts the crystalline phase from Ge2Sb2Te5 towards GeSb2Te4. GST films deposited at room temperature on BaF2(111) were in an amorphous state, but after exposure to an annealing treatment they crystallize in an epitaxial cubic structure. Film deposition on pre-cleaned and buffered ammonium fluoride etched Si(111) show growth of epitaxial hexagonal GST, similar to that of the deposition on BaF2(111). When the Si-substrates were heated directly to the deposition temperature films of high crystal-line quality were obtained. An additional heat treatment of the Si-substrates prior to deposition deteriorated the crystal quality severely. The gained results show that PLD can be used as a method in order to obtain high quality epitaxial Ge-Sb-Te films from a compound target and using high deposition rates.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1998.
Includes bibliographical references (p. 135-139).
by Alexander Cherkassky.
Sc.D.