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.

The ability of uniquely functional thermoelectric materials to convert waste heat directly into electricity is critical considering the global energy economy. Profitable, energy-efficient thermoelectrics possess thermoelectric figures of merit ZT ≥ 1. We examined the effect of metal nanoparticle – oxide film interfaces on the thermal conductivity κ and Seebeck coefficient α in bilayer and multilayer thin film oxide thermoelectrics in an effort to improve the dimensionless figure of merit ZT. Since a thermoelectric's figure of merit ZT is inversely proportional to κ and directly proportional to α, reducing κ and increasing α are key strategies to optimize ZT. We aim to reduce κ by phonon scattering due to the inclusion of metal nanoparticles in the bulk of thermoelectric thin films deposited by Pulsed Laser Deposition. XRD, AFM, XPS, and TEM analyses were carried out for structural and compositional characterization. The electrical conductivities of the samples were measured by a four-point probe apparatus. The Seebeck coefficients were measured in-plane, varying the temperature from 100K to 310K. The thermal conductivities were measured at room temperature using Time Domain Thermoreflectance. / Master of Science

Transparent oxide semiconducting films have continued to receive considerable attention, from a fundamental and application-based point of view, primarily because of their useful fundamental properties. Of particular interest is zinc oxide (ZnO), an n-type semiconductor that exhibits excellent optical, electrical, catalytic and gas-sensing properties, and has many applications in various fields. In this work, thin film transistor (TFT) arrays based on ZnO have been prepared by reactive radio frequency (RF) magnetron sputtering. Prior to the TFT fabrication, ZnO layers were sputtered on to glass and silicon substrates, and the deposition parameters optimised for electrical resistivities suitable for TFT applications. The sputtering process was carried out at room temperature with no intentional heating. The aim of this work is to prepare ZnO thin films with stable semiconducting electrical properties to be used as the active channel in TFTs; and to understand the role of intrinsic point defects in device performance and stability. The effect of oxygen (O2) adsorption on TFT device characteristics is also investigated. The structural quality of the material (defect type and concentration), electrical and optical properties (transmission/absorption) of semiconductor materials are usually closely correlated. Using the Vienna ab-initio simulation package (VASP), it is predicted that O2 adsorption may influence film transport properties only within a few atomic layers beneath the adsorption site. These findings were exploited to deposit thin films that are relatively stable in atmospheric ambient with improved TFT applications. TFTs incorporating the optimised layer were fabricated and demonstrated very impressive performance metrics, with effective channel mobilities as high as 30 cm2/V-1s-1, on-off current ratios of 107 and sub-threshold slopes of 0.9 – 3.2 V/dec. These were found to be dependent on film thickness (~15 – 60 nm) and the underlying dielectric (silicon dioxide (SiO2), gadolinium oxide (Gd2O3), yttrium oxide (Y2O3) and hafnium oxide (HfO2)). In this work, prior to sputtering the ZnO layer (using a ZnO target of 99.999 % purity), the sputtering chamber was evacuated to a base pressure ~4 x 10-6 Torr. Oxygen (O2) and argon (Ar) gas (with O2/Ar ratio of varying proportions) were then pumped into the chamber and the deposition process optimised by varying the RF power between 25 and 500 W and the O2/Ar ratio between 0.010 to 0.375. A two-level factorial design technique was implemented to test specific parameter combinations (i.e. RF power and O2/Ar ratio) and then statistical analysis was utilised to map out the responses. The ZnO films were sputtered on glass and silicon substrates for transparency and resistivity measurements, and TFT fabrication respectively. For TFT device fabrication, ZnO films were deposited onto thermally-grown silicon dioxide (SiO2) or a high-k dielectric layer (HfO2, Gd2O3 and Y2O3) deposited by a metal-organic chemical deposition (MOCVD) process. Also, by using ab initio simulation as implemented in the “Vienna ab initio simulation package (VASP)”, the role of oxygen adsorption on the electrical stability of ZnO thin film is also investigated. The results indicate that O2 adsorption on ZnO layers could modify both the electronic density of states in the vicinity of the Fermi level and the band gap of the film. This study is complemented by studying the effects of low temperature annealing in air on the properties of ZnO films. It is speculated that O2 adsorption/desorption at low temperatures (150 – 350 0C) induces variations in the electrical resistance, band gap and Urbach energy of the film, consistent with the trends predicted from DFT results.

In first part, the supercritical CO2 (SCCO2) fluid technology is employed to improve the device properties of ZnO TFT. The SCCO2 fluid exhibits liquid-like property, which has excellent transport ability. Furthermore, the SCCO2 fluid has gas-like and high-pressure properties to diffuse into the nanoscale structures without damage. Hence, the SCCO2 fluid can carry the H2O molecule effectively into the ZnO films at low temperature and passivate traps by H2O molecule at low temperature. The experimental results show that the on current, sub-threshold slope, and threshold voltage of the device were improved significantly. Next, the electrical degradation behaviors and mechanisms under drain bias stress of a-IGZO TFTs were investigated. A current crowding effect and an obvious capacitance-voltage stretch-out were observed after stress. During the drain-bias stress, the oxygen would be absorbed on the back channel near the drain region of IGZO film. Therefore, the carrier transport is impeded by the additional energy barrier near drain region induced by the adsorbed oxygen, which forms a depletion layer to generate the parasitism resistance. We also investigated the RRAM device based on IGZO film, and proposed the related physical mechanism models. The IGZO RRAM will be very promising for integration with IGZO TFTs for advanced system-on-panel display applications to be a transparent embedded system. In this part, the transparent RRAM device with ITO/IGZO/ITO structure was fabricated. The proposed device presents an excellent bipolar resistive switching characteristic and good reliability. The bipolar switching mechanism of our device is dominated by the formation and rupture of the oxygen vacancies in a conduction path. The influence of electrode material on resistance switching characteristic is investigated through Pt/IGZO/TiN and Ti/IGZO/TiN structure. As the bias applied on the Ti or TiN, the Ti or TiN electrode can play the role of oxygen reservoir to absorb/discharge oxygen ions. Therefore, the device presents a bipolar resistive switching characteristic. However, as the bias applied on the Pt electrode, the device presents a unipolar resistive switching characteristic. Because the Pt electrode can¡¦t store the oxygen ion, the device should use the joule heating mode to rupture the conduction path and present the unipolar resistive switching characteristic. Finally, the resistive switching properties of IGZO film deposited at different oxygen content were investigated, since the resistance switching behaviors are related to the formation and rupture of filaments composed of oxygen vacancies in the IGZO matrix. Experiment results show that the HRS current decreases when the oxygen partial pressure gradually increases. Based on the XPS analysis, these phenomena are related to the non-lattice oxygen concentration. With increasing oxygen ratio, the filaments will rupture completely through the abundant non-lattice oxygen inducing oxidation, which leads to HRS current decrease and an increase in the memory window.

Oxide thin films and bimetallic model catalysts have been studied using metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), low energy ion scattering spectroscopy (LEISS), X – ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) under ultra high vacuum (UHV) conditions. Of particular interest in this investigation was the characterization of the surface morphology and electronic/geometric structure of the following catalysts: SiO2/Mo(112), Ag/SiO2/Mo(112), Au–Pd/Mo(110), Au–Pd/SiO2/Mo(110), and Pd– Sn/Rh(100). Specifically, different types of oxide surface defects were directly identified by MIES. The interaction of metal clusters (Ag) with defects was examined by work function measurements. On various Pd related bimetallic alloy surfaces, CO chemisorption behavior was addressed by IRAS and TPD. Observed changes in the surface chemical properties during the CO adsorption-desorption processes were explained in terms of ensemble and ligand effects. The prospects of translating this molecular-level information into fundamental understanding of ‘real world’ catalysts are discussed.

Amorphous oxide semiconductors are of potential interest in the display industry due to their high carrier mobility, transparency at visible wavelengths and excellent operational stability. In this dissertation, n-channel zinc-tin oxide thin-film transistors are fabricated based on a solution-based deposition approach, which allows low fabrication cost and high throughput. The effects of device configuration and process conditions on transistor performance are investigated, and circuit applications including inverters, amplifiers, and ring oscillators are demonstrated. Charge transport in the zinc-tin oxide field-effect transistors is also investigated. A transition from thermally-activated to band-like transport is observed with increasing carrier concentration in high mobility samples, which agrees well with the key predictions of the multiple trap and release model and also Mott’s mobility edge model. In addition, velocity distribution of charge carriers is studied with a time-resolved technique. This provides a more detailed picture of charge transport in field-effect transistors. P-channel organic semiconductor field-effect transistors are also investigated with a view to combine them with n-channel amorphous oxide transistors to create a hybrid organic-inorganic complementary technology. / text

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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 porte sur l'étude des étapes initiales du dépôt de couches atomiques de ZnO (ALD) sur une surface (100) de In0,57Ga0,43As, par l'utilisation de techniques de caractérisation in situ (rayonnement synchrotron). En raison de la grande mobilité des électrons, le semi-conducteur III-V InGaAs est un matériau potentiel pour remplacer le canal de Silicium dans les transistors à effet de champ (MOSFET). Afin de diminuer la hauteur de la barrière Schottky et la résistance de contact, une couche ultra-mince (tunnel) de ZnO peut être insérée entre le métal et le semiconducteur InGaAs. Au cours de ces dernières années, la technique ALD, compatible avec les spécifications de l'industrie et basée sur des réactions chimiques de surface auto-limitantes, est utilisée pour la fabrication de films minces conformes et homogènes avec un contrôle sub-nanométrique de l’épaisseur. Cependant, le comportement au cours de la croissance diffère fortement en fonction de la surface du substrat. Ainsi, l'étude des premières étapes ALD est particulièrement intéressante afin d’améliorer la compréhension des mécanismes de croissance en vue de la création de films ultra-minces.Pour ce faire, nous avons développé et mis à niveau un réacteur thermique ALD (MOON) dédié. Il peut être installé sur des lignes de lumière synchrotron afin d’étudier la croissance des matériaux in situ avec des techniques telles que la fluorescence X, l’absorption X, la spectroscopie des rayons X ainsi que la diffraction X en incidence rasante. De plus, des techniques optiques de caractérisation in situ peuvent être utilisées en laboratoire ou couplées en milieu synchrotron. Les expériences au synchrotron ont été réalisées sur les lignes de lumière SIRIUS (SOLEIL, Saint-Aubin (France)) et ID3 (ESRF, Grenoble (France)).Nous montrons que dans sa phase initiale, la croissance ALD de ZnO est inhibée par le substrat (100) InGaAs, ce qui conduit à un régime transitoire avant le régime de croissance ALD stable. La première phase du régime transitoire conduit à la formation d’une couche d’oxyde de Zinc, ultra-mince (~1 nm d'épaisseur), fabriquée avec un taux de croissance très faible. L'absorption X et la diffusion X en incidence rasante montrent qu’à ce stade le matériau ZnO est désordonné (non cristallisé) et présente un ordre à courte distance caractérisé par une structure wurtzite embryonnaire. Ensuite, le régime transitoire entre dans une deuxième phase (croissance 3D), le taux de croissance par cycle (GPC) augmente, atteint un maximum puis diminue jusqu'à une valeur constante (croissance ALD stable). Afin de mieux comprendre le mode de croissance 3D nous avons développé un modèle géométrique qui schématise la croissance d’îlots hémisphériques par ALD. Ce modèle permet d'obtenir des paramètres quantitatifs de croissance.En modifiant le débit d’eau (H2O) utilisée comme réactif pendant le processus ALD, il est possible de contrôler le délai (ou le nombre de cycles) avant le début de la croissance 3D. Cet effet est très probablement lié à la variation de la densité des groupes hydroxyle à la surface de l'InGaAs. Par ailleurs, nous avons caractérisé la croissance ALD de ZnO pour différentes températures du substrat InGaAs (dans et hors fenêtre ALD). Les cartes de diffusion des RX réalisées en cours de dépôt, montrent l’apparition d’une phase cristallisée à longue distance en lien avec le démarrage de la croissance 3D. À température élevée, hors de la fenêtre ALD, nous observons une texturation de la couche ZnO lorsque son épaisseur augmente. Aucune relation d’épitaxie n’est observée.Enfin, nous rendons compte de l'utilisation de couches ZnO ultraminces sur InGaAs pour les contacts électriques. La résistance de contact des échantillons de métal/ZnO/InGaAs a été mesurée à l'aide de la méthode Transfert Length Method (TLM). Nous montrons que la résistivité de contact spécifique des tampons Al/p-InGaAs est réduite par l’insertion d’une couche tunnel ZnO entre l'Al et l'InGaAs dopé p. / This work focuses on the study of the initial stages of ZnO atomic layer deposition (ALD) on atomically flat (100) In0.57Ga0.43As surface, notably by using in situ synchrotron techniques. Due to high electron mobility, III-V InGaAs semiconductor has been recognized as a promising material to replace Silicon channel in the metal-oxide-semiconductor-field-effect transistors (MOSFET). Ultrathin ZnO layer on InGaAs can be used as a passivation layer at the interface with the gate transistor dielectric, as well as tunneling layer inserted in between metal/InGaAs contact to decrease the Schottky barrier height and the contact resistance. In the recent years, ALD technique based on self-limiting surface chemical reactions has received world-wide attention for manufacturing highly conformal and homogeneous thin films with sub-nanometer thickness control at low temperatures compatible with industry specifications. However, the growth behavior strongly differs depending on the substrate surfaces. Thus for the creation of few monolayers thick films, the study of ALD in the initial stages of growth is of particular interest for improving the understanding of the growth mechanisms.For that purpose, we have developed and upgraded a thermal ALD reactor (MOON:MOCVD/ALD growth of Oxide Nanostructures) dedicated to monitor the growth of materials by in situ characterization techniques. The MOON reactor can be moved to synchrotron centers for monitoring material growth in situ by using X-ray based techniques, notably X-ray fluorescence, X-ray absorption, XRR, and grazing incidence diffraction. Also, optical in situ techniques can be used in the laboratory. In this work, we show the results of experiments obtained at two synchrotron beamlines, i.e. SIRIUS (SOLEIL, Saint-Aubin (France)) and ID3 (ESRF, Grenoble (France)).We show that ZnO growth in the initial stages is inhibited by the (100) InGaAs substrate, leading to a transient regime prior to the steady ALD is achieved. We report a detailed investigation of this transient regime and find that an ultra-thin (~1-nm-thick) 2D layer is indeed fabricated but with a growth rate so low that one may believe that nothing has been deposited on the surface. We identify the structural and chemical properties of that ultra-thin layer. Only afterward does the substrate inhibited of type 2 growth mode begins: as the cycle number increases, the growth per cycle (GPC) increases, then reaches a maximum and level down to a constant value (steady growth). For a better understanding of the 3D growth mode by reproducing the experimental growth per cycle curves we have developed a geometric model that schematizes the growth of hemispheroid islands by ALD. We show that this model allows obtaining quantitative growth parameters.When water is used as a reactant, we showed that by changing the water flow during the ALD process, it is possible to control the time delay (or cycle number) prior to 3D growth begins. It is very likely that the water flow controls the density of hydroxyl groups on the InGaAs surface. We also demonstrated ZnO ALD for different InGaAs substrate temperatures. By combining in situ X-ray absorption and grazing incidence scattering techniques, we identified a short-range-order atomic structure of the ZnO material, with an embryonic ZnO wurtzite, prior to 3D growth, then a long-range-order structure is detected both by X-ray absorption and X-ray diffraction, together with the appearance of a microstructure. At higher growth temperature, outside of the ALD window, we observed the well-known ZnO texturing when the layer thickness increases.At last, we report on the use of ultrathin ZnO layers on InGaAs in the electrical contact structure. The contact resistance of metal/ZnO/InGaAs samples was measured using Transfer Length Method (TLM). We show that specific contact resistivity of Al/p-InGaAs pads is reduced by inserting a ZnO tunnel layer in between Al and p-doped InGaAs.