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This dissertation presents the development of methods based on microfabricated devices for combined structure and thermoelectric characterizations of individual nanowire and thin film materials. These nanostructured materials are being investigated for improving the thermoelectric figure of merit defined as ZT=S²[sigma]T/K, where S is the Seebeck coefficient, [sigma] is the electrical conductivity, K is the thermal conductivity, and T is the absolute temperature. The objective of the work presented in this dissertation is to address the challenges in the measurements of all the three intrinsic thermoelectric properties on the same individual nanowire sample or along the in plane direction of a thin film, and in correlating the measured properties with the crystal structure of the same nanowire or thin film sample. This objective is accomplished by the development of a four-probe thermoelectric measurement procedure based on a micro-device to measure the intrinsic K, [sigma], and S of the same nanowire or thin film and eliminate the contact thermal and electrical resistances from the measured properties. Additionally the device has an etched through hole that facilitates the structural characterization of the sample using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). This measurement method is employed to characterize individual electrodeposited Bi[subscript 1-x]Te[subscript x] nanowires. A method based on annealing the nanowire sample in a forming gas is demonstrated for making electrical contact between the nanowire and the underlying electrodes. The measurement results show that the thermoelectric propertied of the nanowires are sensitive to the crystal quality and impurity doping concentration. The highest ZT found in three nanowires is about 0.3, which is still lower than that of bulk single crystals at the optimum carrier concentration. The lower ZT found in the nanowires is attributed to the high impurity or carrier concentration and defects in the nanowires. The micro-device is further modified to extend its use to characterization of the in-plane thermoelectric properties of thin films. Existing practice for thermoelectric characterization of thin films is obtaining K in the cross plane direction using techniques such as the 3[omega] method or time domain laser thermal reflectance technique whereas the [sigma] and S are usually obtained in the in-plane direction. However, transport properties of nanostructured thin films can be highly anisotropic, making this combination of measurements along different directions unsuitable for obtaining the actual ZT value. Here, the micro-device is used to measure all three thermoelectric properties in the in-plane direction, thus obtaining the in-plane ZT. A procedure based on a nano-manipulator is developed to assemble etched thin film segments on the micro-device. Measurement results of two different types of thin films are presented in this dissertation. The first type is mis-oriented, layered thin films grown by the Modulated Elemental Reactant Technique (MERT). Three different structures of such thin films are characterized, namely WSe₂, W[subscript x](WSe₂)[subscript y] and (PbSe₀.₉₉)[subscript x](WSe₂)[subscript x] superlattice films. All three structures exhibit in-plane K values much higher than their cross-plane K values, with an increased anisotropy compared to bulk single crystals for the case of the WSe₂ film. The increased anisotropy is attributed to the in-plane ordered, cross-plane disordered nature of the mis-oriented, layered structure. While the WSe₂ film is semi-insulating and the W[subscript x](WSe₂)[subscript y] films are metallic, the (PbSe₀.₉₉)[subscript x](WSe₂)[subscript x] films are semiconducting with its power factor (S²[sigma]) greatly improved upon annealing in a Se vapor environment. The second type of thin films is semiconducting InGaAlAs films with and without embedded metallic ErAs nanoparticles. These nanoparticles are used to filter out low energy electrons with the introduction of Schottky barriers so as to increase the power factor and scatter long to mid range phonons and thus suppress K. The in-plane measurements show that both the S and [sigma] increase with increasing temperature because of the electron filtering effect. The films with the nanoparticles exhibited an increase in [sigma] by three orders of magnitude and a decrease in S by only fifty percent compared to the films without, suggesting that the nanoparticles act as dopants within the film. On the other hand, the measured in-plane K shows little difference between the films with and without nanoparticles. This finding is different from those based on published cross-plane thermal conductivity results. / text

Le PbZr₁₋ₓTiₓO₃ (PZT) est une pérovskite mixte possédant de nombreuses propriétés, dont certaines sont déjà utilisées dans l’industrie, ce qui en fait un matériau encore très étudié à l’heure actuelle, malgré la toxicité du plomb et de ses oxydes. Au cours de cette thèse, nous nous sommes intéressés à la spectroscopie d’absorption IR de ce composé, tant au niveau expérimental que théorique. Nous avons donc réalisé des modélisations, via le code de calcul CRYSTAL basé sur les méthodes de Combinaison Linéaire d’Orbitales Atomiques et de la Théorie de la Fonctionnelle de la Densité (LCAO-DFT) périodique, afin d’aider à l’interprétation des spectres expérimentaux réalisés sur la ligne AILES du synchrotron SOLEIL. Dans ce but, nous avons commencé par modéliser les composés de base du PZT : le PbTiO₃ (PT) et le PbZrO₃ (PZ). Nos résultats reproduisant très bien les données de la littérature sur ces deux composés, nous avons pu faire une analyse fine de leur spectre d’absorption IR. D’autre part, leur modélisation nous a permis de déterminer des paramètres de calcul transférables (base et fonctionnelle notamment) et de les appliquer sur le PZT en utilisant la méthode de la supermaille, couplée à une analyse statistique. Les résultats obtenus sont prometteurs pour l’interprétation, car tout à fait comparables aux spectres expérimentaux. Afin de nous rapprocher au mieux du cristal réel de PT, nous nous sommes intéressés à la modélisation de couches ultraminces et de lacunes d’oxygène, dans le but de voir leur effet sur le spectre d’absorption IR du PT. / PbZr₁₋ₓTiₓO₃ (PZT) is a complex perovskite that has many properties, some of which are already used industrially. Thus, in spite of the toxicity of lead and its oxides, this material is still under extensive investigation. In this thesis, we are interested of both experimental and theoretical IR absorption spectroscopy of this compound. To do so, we used the CRYSTAL code, based on the Linear Combination of Atomic Orbitals method and periodic Density Functional Theory (LCAO-DFT) in order to facilitate the interpretation of experimental spectra, recorded on the AILES beamline of synchrotron SOLEIL. In this goal, we first studied the two building blocks of PZT: PbTiO₃ (PT) and PbZrO₃ (PZ). Our results are in very good agreement with what has already been done in the literature. We, thus, could carry out a precise interpretation of their absorbance spectra. Moreover, transferable parameters (in particular the basis set and the functional) have been determined and used to study PZT. The supercell method, coupled with a statistical analysis, provided promising results, comparable with experimental data and, thus, helpful for their interpretation. In order to make a step towards the real PT crystal, we started the simulation of ultrathin films and oxygen vacancies to investigate their effects on the IR absorption spectrum.

The Nonlinear optical (NLO) microscopy is a very powerful and noninvasive tool to analyze the material properties, such as the local symmetry, as well as to visualize ferroelectric domains and domain walls. As a result, NLO microscopy becomes a very powerful tool in characterization and quality control which are key tasks in material development and device fabrication. One such area where NLO microscopy is widely used, is thin film materials. Thin film and nanosized materials with dimensions ranging from a few micrometers thickness down to atomically thin 2D materials, offer many innovative and intriguing features for applications in electronics, optics, and many other fields. In order to provide physical stability, these thin film and 2D materials are usually supported on substrates and handles, leading to multiple effects, such as thin film resonance and reflections at the thin film-substrate interface, that influence the genuine NLO signal from the sample. These effects are not present in bulk samples; therefore, it is natural to erroneously consider that these effects are also not present in thin film materials. This work tries to identify, quantify, and disentangle the parameters that influence nonlinear microscopy in thin film materials. To achieve this, Second Harmonic Generation (SHG) microscopy and Third Harmonic Generation (THG) microscopy were applied as two archetypal NLO processes. In particular the influence of thin film interference and phase matching on the signal strength is analyzed. Furthermore, key differences between three and four photon processes, such as the role of the Gouy-phase shift and the focal position is studied. This understanding can be extended to other three and four-photon processes, such as Coherent Anti-Stokes Raman Scattering (CARS). Wedge-shaped samples were used for the experiments here, whose thickness was varied from bulk thickness down to approximately 50 nm. In both cases, it was found that the signal in the back reflection is the phase-matched co-propagating signal and not the counter-propagating signal which may naively be expected. It was also found that the signal from the surrounding material, and support does not affect the SH signal from the sample because the second-order nonlinear tensor is only available in non-centrosymmetric material. However, the signals from the surrounding do affect the TH signal from the sample because a third-order nonlinear tensor is available in every material. Furthermore, the THG signal from the thin film starts to vanish as the thickness increases, opposite to what happens in SHG. To back up the experimental findings, two numerical models were used. The first model is the numerical simulation, while the second is a semi-analytical paraxial model. This thesis lays the groundwork for performing quantitative NLO 𝜇-spectroscopy on thin films and 2D materials, as it identifies and quantifies the impact of the corresponding sample and setup parameters on the NLO signal, in order to distinguish them from genuine material properties.:1. Introduction 1 2. Theoretical background 7 2.1. Non-linear optical polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. The non-linear susceptibility tensor . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3. Phase-matching and emission efficiency . . . . . . . . . . . . . . . . . . . . . . 12 2.4. Nonlinear effects in focused Gaussian beams . . . . . . . . . . . . . . . . . . . 17 3. Methodology and principle 25 3.1. Lithium niobate (LN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.2. Wedge preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.3. Data generation & processing . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.4. Simulation models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4. SHG in thin films 37 4.1. Coherence interaction length . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.1.1. Experimental data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4.1.2. Comparison of numerical and experimental data . . . . . . . . . . . . . 44 4.2. Thin film interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.3. Influence of NA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.4. Reproducibility of experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.5. Detection depth of coherent interaction length oscillations . . . . . . . . . . . 54 4.6. Sensitivity to focus positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 5. THG in thin films 57 5.1. Coherence interaction length . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.1.1. Quantifying the coherence interaction length . . . . . . . . . . . . . . . 61 5.2. Comparison of simulated and experimental data . . . . . . . . . . . . . . . . . 63 5.3. Thin film interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6. Conclusion and outlook 67 Appendices 69 A. Additional reflective layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 B. Focus fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 List of Figures 78 List of Tables 79 Acronyms 82 Own Publications 93 / Die nichtlineare optische Mikroskopie (NLO) ist ein sehr leistungsfähiges und nichtinvasives Instrument zur Analyse der Materialeigenschaften, z. B. der lokalen Symmetrie, sowie zur Visualisierung ferroelektrischer Domänen und Domänenwände. Dadurch wird die NLO-Mikroskopie zu den wichtigsten Aufgaben bei der Materialentwicklung und der Herstellung von Bauelementen gehören. Ein solcher Bereich, in dem die NLO-Mikroskopie weit verbreitet ist, sind Dünnschichtmaterialien. Dünnschicht- und Nanomaterialien mit Abmessungen von wenigen Mikrometern Dicke bis hin zu atomar dünnen 2D-Materialien bieten viele innovative und faszinierende Eigenschaften für Anwendungen in der Elektronik, Optik und vielen anderen Bereichen. Um die physikalische Stabilität zu gewährleisten, werden diese Dünnschicht- und 2D-Materialien in der Regel auf Substraten und Handlegriffen getragen, was zu zahlreichen Effekten führt, wie z. B. Dünnschichtresonanz und Reflexionen an der Grenzfläche zwischen Dünnschicht und Substrat, die das echte Signal der Probe beeinflussen. Diese Effekte sind bei Bulk-Proben nicht vorhanden; daher ist es naheliegend, fälschlicherweise anzunehmen, dass diese Effekte auch bei Dünnschichtmaterialien nicht vorhanden sind. In dieser Arbeit wird versucht, die Parameter, die die nichtlineare Mikroskopie in Dünnschichtmaterialien beeinflussen, zu identifizieren, zu quantifizieren und zu entflechten. Zu diesem Zweck wurden die Mikroskopie der zweiten Harmonischen (SHG) und die Mikroskopie der dritten Harmonischen (THG) als zwei archetypische NLO-Prozesse untersucht. Insbesondere wird der Einfluss von Dünnschichtinterferenzen und Phasenanpassung auf die Signalstärke analysiert. Darüber hinaus werden die wichtigsten Unterschiede zwischen Drei- und Vier-Photonen-Prozessen, wie die Rolle der Gouy-Phasenverschiebung und der Fokusposition, untersucht. Dieses Verständnis kann auf andere Drei- und Vier-Photonen-Prozesse, wie z. B. die kohärente Anti-Stokes-Raman-Streuung (CARS), ausgeweitet werden. Für die Experimente wurden keilförmige Proben verwendet, deren Dicke von der Bulk-Dicke bis hinunter zu etwa 50 nm variiert werden. In beiden Fällen wurde festgestellt, dass es sich bei dem Signal in der Rückreflexion um das phasenangepasste Mitausbreitungssignal handelt und nicht um das Gegenausbreitungssignal, das man naiverweise erwarten könnte. Es wurde auch festgestellt, dass das Signal aus der Umgebung das SH-Signal der Probe nicht beeinflusst, da der nichtlineare Tensor zweiter Ordnung nur in nicht-zentrosymmetrischem Material vorhanden ist. Die Signale aus der Umgebung beeinflussen jedoch das TH-Signal der Probe, da ein nichtlinearer Tensor dritter Ordnung in jedem Material vorhanden ist. Außerdem verschwindet das THG-Signal des dünnen Films mit zunehmender Dicke, anstatt wie bei SHG zuzunehmen. Um die experimentellen Ergebnisse zu untermauern, wurden zwei numerische Modelle verwendet. Bei dem ersten Modell handelt es sich um eine numerische Simulation, bei dem zweiten um ein halbanalytisches paraxiales Modell. Diese Arbeit legt den Grundstein für die Durchführung quantitativer NLO 𝜇-Spektroskopie an dünnen Schichten und 2D-Materialien, da sie die Auswirkungen der entsprechenden Proben und Einrichtungsparameter auf das NLO-Signal identifiziert und quantifiziert, um sie von den echten Materialeigenschaften zu unterscheiden.:1. Introduction 1 2. Theoretical background 7 2.1. Non-linear optical polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. The non-linear susceptibility tensor . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3. Phase-matching and emission efficiency . . . . . . . . . . . . . . . . . . . . . . 12 2.4. Nonlinear effects in focused Gaussian beams . . . . . . . . . . . . . . . . . . . 17 3. Methodology and principle 25 3.1. Lithium niobate (LN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.2. Wedge preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.3. Data generation & processing . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.4. Simulation models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4. SHG in thin films 37 4.1. Coherence interaction length . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.1.1. Experimental data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4.1.2. Comparison of numerical and experimental data . . . . . . . . . . . . . 44 4.2. Thin film interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.3. Influence of NA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.4. Reproducibility of experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.5. Detection depth of coherent interaction length oscillations . . . . . . . . . . . 54 4.6. Sensitivity to focus positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 5. THG in thin films 57 5.1. Coherence interaction length . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.1.1. Quantifying the coherence interaction length . . . . . . . . . . . . . . . 61 5.2. Comparison of simulated and experimental data . . . . . . . . . . . . . . . . . 63 5.3. Thin film interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6. Conclusion and outlook 67 Appendices 69 A. Additional reflective layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 B. Focus fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 List of Figures 78 List of Tables 79 Acronyms 82 Own Publications 93