<p>This Licentiate thesis summarizes the work carried out by the author the years 2004 to 2008 at the University of Iceland and the Royal Institute of Technology (KTH) in Sweden. The aim of the project was to investigate the structure of sapphire (alpha-Al2O3) surfaces, both for pure scientific reasons and also for potential use as substrate for GaN-growth by molecular beam epitaxy.</p><p>More generally the thesis describes some surface science methods used for investigating the substrates; the general physical back ground, the experi- mental implementation and what information they can give. The described techniques are used for surface analysis on sapphire substrates which have been treated variously in order to optimize them for use as templates for epi- taxial growth of GaN or related III-V compounds.</p><p>The thesis is based on three published papers.</p><p>The first paper focuses on the formation a thin AlN layer on sapphire, which may act as a buffer layer for potential epitaxial growth of GaN or any related III-V materials. Two types of sapphire substrates (reconstructed and non- reconstructed) were exposed to ammonia resulting in the formation of AlN on the surface. The efficiency of the AlN formation (nitridation efficiency) for the two surfaces was then compared as a function of substrate temperature through photoelectron spectroscopy and low electron energy diffraction. The reconstructed surface showed a much higher nitridation efficiency than the non-reconstructed surface.</p><p>In the second paper, the affect of different annealing processes on the sapphire morphology, and thus its capability to act as a template for GaN growth, was studied. Atomic force microscopy, X-ray diffraction analysis together with ellipsometry measurements showed that annealing in H2 ambient and subse- quent annealing at 1300 °C in O2 for 11 hours resulted in high quality and atomically flat sapphire surface suitable for III-V epitaxial growth.</p><p>The third paper describes the effect of argon sputtering on cleaning GaN surfaces and the possibility of using indium as surfactant for establishing a clean and stoichiometric GaN surface, after such sputtering. Soft sputtering, followed by deposition of 2 ML of indium and subsequent annealing at around 500 °C resulted in a well ordered and clean GaN surface while hard sputtering introduced defects and incorporated both metallic gallium and indium in the surface.</p>

No description available.

Reproduction de : Thèse de doctorat : Physique : Lyon 1 : 2006. / Titre provenant de l'écran titre. Bibliogr. p.201-207.

Surfaces of metal oxides play a vital role in many technologically important applications. The surfaces of titanium dioxide, in particular, show quite promising properties that can be utilized in solid-state gas sensing and photocatalysis applications. In the first part of this dissertation we investigate these properties of TiO2 surfaces through a vigorous surface scientific approach. In the second part, we investigate the possibilities of modifying the TiO2 surfaces by depositing multi-component transition metal oxide monolayers so that the properties of bare TiO2 surface can be influenced in a beneficial way. For instance, via formation of new surface sites or cations that have different valance states, the chemisorption and catalytic properties can be modified. We use sophisticated experimental surface science techniques that are compatible with ultra-high vacuum technology for surface characterization. All the experimental results, except for the photocatalysis experiments, were compared to and verified by supporting DFT-based theoretical results produced by our theory collaborators. TiO2 based solid-state gas sensors have been used before for detecting trace amounts of explosives such as 2,4-dinitrololuene (DNT), a toxic decomposition product of the explosive 2,4,6-trinitrotoluene (TNT) that have very low vapor pressure. However, the adsorption, desorption and reaction mechanism were not well- understood. Here, we investigate 2,4-DNT adsorption on rutile-TiO2(110) surface in order to gain insight about these mechanisms in an atomistic level and we propose an efficient way of desorbing DNT from the surface through UV-light induced photoreactions. TiO2 exists in different polymorphs and the photocatalytic activity differs from one polymorph to another. Rutile and anatase are the most famous forms of TiO2 in photocatalysis and anatase is known to show higher activity than rutile. The photoactivity also varies depending on the surface orientation for the same polymorph. So far, a reasonable explanation as to why these differences exist was not reported. In our studies, we used high quality epitaxial rutile and anatase thin films which enabled isolating the surface effects from the bulk effects and show that it is the difference between the charge carrier diffusion lengths that causes this difference in activities. In addition to that, using different surface orientations of rutile-TiO2, we show that the anisotropic bulk charge carrier mobility may contribute to the orientation dependent photoactivity. Moreover, we show that different surface preparation methods also affect the activity of the sample and vacuum reduction results in an enhanced activity. In an effort to modify the TiO2 surfaces with monolayer/mixed monolayer oxides, we carried out experiments on (011) orientation of single crystal rutile TiO2 with few of the selected transition metal oxides namely Fe, V, Cr and Ni. We found that for specific oxidation conditions a monolayer mixed oxide is formed for all M (M= Fe, V, Cr, Ni), with one common structure with the composition MTi2O5. For small amounts of M the surface segregates into pure TiO2(011)-2×1 and into domains of MTi2O5indicating that this mixed monolayer oxide is a low energy line phase in a compositional surface phase diagram. The oxygen pressure required for the formation of this unique monolayer structure increases in the order of V2O5 mixed monolayer oxide by DFT-based simulations was verified by X-ray photoemission diffraction measurements performed at a synchrotron facility.

De nombreux oxydes de métaux de transition (TMOs) possèdent des propriétés physiques complexes (ferroélectricité, magnétisme, supraconductivité à haute Tc ou magnétorésistance colossale). Les différents degrés de liberté (le réseau, la charge, le spin ou l'ordre orbitalaire) interagissent pour donner des phases différentes, très proches en énergie, qui vont former une grande variété d'états fondamentaux accessibles. La possibilité de fabriquer des hétérostructures de TMOs a encore accru la complexité de ces systèmes, de nouveaux phénomènes apparaissant aux interfaces. Un exemple typique est le gaz d'électrons bidimensionnel (2DEG) créé à l'interface entre deux oxydes isolants, LaAlO3 et SrTiO3, qui montre une transition métal-isolant, du magnétisme ou de la supraconductivité (contrôlée par une tension de grille). Le point de départ de cette thèse a été la découverte d'un 2DEG similaire à la surface nue de SrTiO3 fracturée sous vide, rendant possible l'étude de sa structure électronique par photoémission angulaire.Dans cette thèse, l'étude de surfaces préparées, plutôt que de petites facettes fracturées, a permis l'obtention de données spectroscopiques possédant des largeurs de raie proches des valeurs intrinsèques. Il est alors possible d'étudier les effets à N corps comme la renormalisation de la self-énergie due à l'interaction électron-phonon.Ces recherches sur la structure électronique du 2DEG à la surface de SrTiO3 ont pris un tour nouveau lorsqu'une texture de spin complexe y a été mesurée par photoémission résolue en spin. Nous présentons des résultats qui contredisent ces conclusions et nous discutons des raisons pouvant expliquer ce désaccord.Une des motivations de cette thèse était de savoir si la structure électronique et les propriétés du 2DEG pouvaient être contrôlées. L'étude du 2DEG sur des surfaces (110) et (111) de SrTiO3 révèle que sa structure de bandes (ordre orbitalaire, symétrie de la surface de Fermi, masses effectives) peut être ajustée en confinant les électrons sur des surfaces de différentes orientations du même matériau.Un succès majeure est la mise en évidence de 2DEGs à la surface de nombreux autres TMOs (TiO2-anatase, CaTiO3, BaTiO3) ou d'oxydes plus simples utilisés dans les applications (ZnO). Dans tous ces oxydes, nous avons identifié les lacunes en oxygène comme étant à l'origine de la création des 2DEGs.Dans l'anatase, ou d'autres TMOs en configuration électronique initiale d0, les lacunes en oxygène produisent à la fois des électrons localisés ou itinérants (le 2DEG). Il peut être subtile de prévoir quel est le cas est le plus favorable énergétiquement comme le démontre l'étude de deux polymorphes de TiO2, anatase et rutile. Dans CaTiO3, l’octaèdre formé par les atomes d'oxygène autour du Ti est incliné. Cette rupture de symétrie provoque un mélange des orbitales d et modifie le 2DEG. Dans BaTiO3, la création d'un 2DEG entraîne la coexistence de deux phénomènes normalement incompatibles, la ferroélectricité et la métallicité, dans deux zones spatialement distinctes du même matériau. Ce travail démontre qu'un 2DEG existe aussi à la surface de ZnO qui est, contrairement aux oxydes à base de Ti, plutôt un semiconducteur conventionnel, le caractère des orbitales pour les électrons itinérants étant alors de type s et non de type d.Le principal résultat est la mise au point d'une méthode simple et versatile pour la création de 2DEGs en évaporant de l'aluminium sur des surfaces d'oxydes. Une réaction d'oxydo-réduction entre le métal et l'oxyde permet de créer un 2DEG à l'interface entre le métal oxydé et l'oxyde réduit. Dans cette thèse, les 2DEGs ont été étudiés uniquement par photoémission sous ultra-vide. Cette méthode ouvre la possibilité d'étudier ces 2DEGs dans des conditions de pression ambiante en utilisant, par exemple, des techniques de transport, un pas important vers la production de masse et à bas coûts de 2DEGs dans les oxydes pour de futures applications. / Many transition metal oxides (TMOs) show complex physics, ranging from ferroelectricity to magnetism, high-Tc superconductivity and colossal magnetoresistance. The existence of a variety of ground states often occurs as different degrees of freedom (e.g. lattice, charge, spin, orbital) interact to form different competing phases which are quite similar in energy. The capability to epitaxially grow heterostructures of TMOs increased the complexity even more as new phenomena can emerge at the interface. One typical example is the two-dimensional electron system (2DES) at the interface of two insulating oxides, namely LaAlO3/SrTiO3, which shows metal-to-insulator transitions, magnetism or gate-tunable superconductivity. The origin of this thesis was the discovery of a similar 2DES at the bare surface of SrTiO3 fractured in vacuum, making it possible to study its electronic structure by angle-resolved photoemission spectroscopy (ARPES).In this thesis, the study of well-prepared surfaces, instead of small fractured facets, results in spectroscopic data showing line widths approaching the intrinsic value. This approach allows a detailed analysis of many-body phenomena like the renormalization of the self-energy due to electron-phonon interaction.Additionally, the understanding of the electronic structure of the 2DES at the surface of SrTiO3(001) was given an additional turn by the surprising discovery of a complex spin texture measured by spin-ARPES. In this thesis data is presented which contradicts these conclusions and discusses possible reasons for the discrepancy.One major motivation of this thesis was the question if and how the electronic structure and the properties of the 2DES can be changed or controlled. In this context, the study of 2DESs at (110) and (111) surface revealed that the electronic band structure of the 2DES (orbital ordering, symmetry of the Fermi surface, effective masses) can be tuned by confining the electrons at different surface orientations of the same material, namely SrTiO3.A major achievement of this thesis is the generalization of the existence of a 2DES in SrTiO3 to many other surfaces and interfaces of TMOs (TiO2 anatase, CaTiO3, BaTiO3) and even simpler oxides already used in modern applications (ZnO). In all these oxides, we identify oxygen vacancies as the origin for the creation of the 2DESs.In anatase and other doped d0 TMOs, both localized and itinerant electrons (2DES) can exist due to oxygen vacancies. Which of the two cases is energetically favorable depends on subtle differences as demonstrated by studying two polymorphs of the same material (anatase and rutile).In CaTiO3, the oxygen octahedron around the Ti ion is slightly tilted. This symmetry breaking results in the mixing of different d-orbitals demonstrating again why and how the electronic structure of the 2DES can be altered.In BaTiO3, the creation of a 2DES results in the coexistence of the two, usually mutual exclusive, phenomena of ferroelectricity and metallicity in the same material by spatially separating the two.Moreover, this work demonstrates that the 2DES also exists in ZnO which is - compared to the Ti-based oxides - rather a conventional semiconductor as the orbital character of the itinerant electrons is of s and not d-type.The main result of this thesis is the demonstration of a simple and versatile technique for the creation of 2DESs by evaporating Al on oxide surfaces. A redox reaction between metal and oxide results in a 2DES at the interface of the oxidized metal and the reduced oxide. In this thesis the study of such interfacial 2DESs was limited to photoemission studies in ultra high vacuum. However, this technique opens up the possibility to study 2DESs in functional oxides in ambient conditions by e.g. transport techniques, and might be an important step towards cost-efficient mass production of 2DESs in oxides for future applications.

<p>This report is the result of a diploma work made at Linköping University from August 2006 till September 2007 by Erik Lundin, under the guidance of Doctor Mike Andersson and Professor Anita Lloyd Spetz. Its purpose was to find suitable materials for the construction of an oxygen sensor. The hope was not to construct such a sensor, but to investigate materials that may be suitable in creating one. In the preparatory time period of the diploma work, different papers and books were studied in order to get a proper understanding of the sensor mechanism. During this period of time, a design proposal was made and the theory behind it is presented in this thesis. The main objective in this thesis has been to investigate the response of field effect devices to oxygen and other gases that are compounds in exhaust or flue gases. Devices were created by employing the materials which were investigated. Special material combinations were proposed for field effect devices suitable for oxygen detection by Doctor Mike Andersson. One material combination showed promising results for selective detection of the oxygen concentration in exhaust gases.</p> / This diploma work has been confidential

This thesis is the result of investigations regarding the processes in InAs III-V semiconductor surfaces induced by additional charge incorporated by adsorbates. The aim of the project is to study the development of the accumulation layer on the metal/InAs(111)A/B surfaces and its electronic structure. InAs(111)A is indium-terminated and InAs(111)B is arsenic-terminated. In addition, InAs(100) is also studied. These three substrates are different; InAs(111)A has a (2x2)-termination, explained by an indium vacancy model, and the clean surface exhibits a two-dimensional electron gas (2DEG). InAs(111)B(1x1) is bulk-truncated and unreconstructed and does not host a 2DEG. InAs(100)(4x2)/c(8x2) exhibits a more covalent character of the surface bonds compared to InAs(111)A/B, and the surface is terminated by a complex reconstruction. Photoelectron spectroscopy and LEED (low energy electron diffraction) have been used as the main tools to study surfaces with sub-monolayer to monolayer amounts of adsorbates. A photoemission peak related to a two-dimensional electron gas appears close to the Fermi level. This 2DEG has in most cases InAs bulk properties, since it is located in the InAs conduction band. A systematic study of core levels and valence bands reveals that the appearance of the 2DEGs is a complex process connected to the surface order. Adsorption of lead, tin or bismuth on InAs(111)B(1x1) induces emission from a 2DEG, but only at monolayer coverage and when the surface is ordered. Cobalt reacts strongly with InAs forming InCo islands and no accumulation is observed. Examination of Cs/InAs(111)B does not reveal any 2DEG and the surface reaction is strongly related to the clean surface stabilization process. Examination of the In-terminated InAs(111)A(2x2) surface shows that In reacts strongly with cobalt and tin adatoms and with oxygen in cases of large exposure, which decreases the 2DEG intensity, while adatoms of cesium and small doses of oxygen enhance the emission from the 2DEG. InAs(100) is terminated with one kind of atom - the InAs(100)(4x2)/c(8x2) is indium terminated. Bismuth creates dimers on the surface and a 2DEG is observed. More generally, this thesis describes some of the general physical background applied to surface science and 2DEG. The first part is a general overview of the processes on the surface. The second part concentrates on the methods related to preparation of samples, and the third part on the measurement methods. The photoelectron spectroscopy part concerns the theory used in mapping electronic structure. The inserted figures are taken from different experiments, including results for InAs(111)A not previously published. / QC 20100910

This report is the result of a diploma work made at Linköping University from August 2006 till September 2007 by Erik Lundin, under the guidance of Doctor Mike Andersson and Professor Anita Lloyd Spetz. Its purpose was to find suitable materials for the construction of an oxygen sensor. The hope was not to construct such a sensor, but to investigate materials that may be suitable in creating one. In the preparatory time period of the diploma work, different papers and books were studied in order to get a proper understanding of the sensor mechanism. During this period of time, a design proposal was made and the theory behind it is presented in this thesis. The main objective in this thesis has been to investigate the response of field effect devices to oxygen and other gases that are compounds in exhaust or flue gases. Devices were created by employing the materials which were investigated. Special material combinations were proposed for field effect devices suitable for oxygen detection by Doctor Mike Andersson. One material combination showed promising results for selective detection of the oxygen concentration in exhaust gases. / This diploma work has been confidential

Photovoltaics manufactured using organic materials as a substitute for inorganic materials may provide for cheaper production of solar cells if their efficiencies can be made comparable to existing technologies. Photovoltaic devices are comprised of layered structures where the electrical, chemical, and physical properties at the multiple interfaces play a significant role in the operation of the completed device. This thesis attempts to establish a relationship between interfacial properties and overall device performance by investigation of both the organic/organic heterojunction interface, as well as the interface between the inorganic substrate and the first organic layer with useful insights towards enhancing the efficiency of organic solar cells.It has been proposed that residual chemical species may act as barriers to charge transfer at the interface between the transparent conductor (TCO) and the first organic layer, possibly causing a large contact resistance and leading to reduced device performance. Previous work has investigated the surface of the TCO but no baseline characterization of carbon-free surfaces has previously been given. In this work clean surfaces are investigated to develop a fundamental understanding of the intrinsic spectra such that further analyses of contaminated surfaces can be presented systematically and reproducibly to develop a chemical model of the TCO surface.The energy level offset at the organic/organic heterojunction has been proposed to relate to the maximum potential achievable for a solar cell under illumination, however, few experimental observations have been made where both the interface characterization and device performance are presented. Photovoltaic properties are examined in this work with titanyl phthalocyanine used as a novel donor material for enhancement of spectral absorption and optimization of the open-circuit potential. Characterization of the interface between TiOPc and C60 coupled with characterization of the interface between copper phthalocyanine and C60 shows that the higher ionization potential of TiOPc does correlate to greater open circuit potentials.Examination of photovoltaic behavior using equivalent circuit modeling relates the importance of series resistance and recombination to the homogeneity of the solar cell structure.

The magnetic and structural properties of nano magnet arrays and ferromagnetic thin films are investigated. Circular x-rays are used and extensive use is made in this Thesis of the X-ray Magnetic Circular Dichroism (XMCD) technique. By means of the XMCD magneto-optic sum rules the values of the orbital and spin moments are determined. In the case of the nano magnet arrays studied, the XMCD technique is used in a spatially resolved mode using Photo Electron Emission Microscopy (PEEM) after circular light excitation. The Extended X-ray Absorption Fine Structure (EXAFS) is studied in both the Co K- and L-edges. In situ Co L-edge X-ray XMCD spectroscopy measurements are presented, in combination with spectro-microscopy results, on Co/Pt and Co/Au based nano-dot arrays, of typical dot lateral size 250×100 nm2, on self organized Si0.5Ge0.5. The Co is only a few atomic layers thick. The dot arrays display a high degree of lateral order and the individual dots, in several cases, exhibit a stable magnetic moment at 300 K. It is found possible to characterize the spin reorientation of these dot arrays. For both systems the in- versus out-of-plane orbital moment anisotropy, is not always related with an out-of-plane magnetization and the occurrence of a spin reorientation. By performing Co K-edge EXAFS measurements the local atomic structure around the Co atoms is characterized. The feasibility of a high precision quantitative structural analysis of L-EXAFS is studied on the system Au/Co/Au/W(110). The spin reorientation transition is studied as a function of the Co thickness and Au cap thickness. The L-edge EXAFS indicates that this reorientation is correlated to a lattice expansion in the perpendicular direction. High precision angle dependent XMCD work is performed on a high temperature exchange bias system. Pinned or frozen magnetic moments are studied within an exchange biased NiFe ferromagnet at the NiFe/FeMn, ferromagnet/antiferromagnet interface by XMCD and complemented by x-ray resonant reflectivity experiments, at the Ni, Fe and Mn L-edges. The Mn L-edge XMCD MnSb and of (Ga, Mn)As layers modified by high temperature annealing is studied. For MnSb an enhanced value is obtained versus theoretical calculations. This result can be explained by means of the enhanced surface to volume ratio for the samples studied. For (Ga, Mn)As differences are found in the local environment of the Mn atoms upon annealing.