Hf₁₋ₓZrₓO₂ (x ∼ 0.5–0.7) has been the leading candidate of ferroelectric materials with a fluorite crystal structure showing highly promising compatibility with complementary metal oxide semiconductor devices. Despite the notable improvement in device performance and processing techniques, the origin of its ferroelectric crystalline phase (space group: Pca2₁) formation has not been clearly elucidated. Several recent experimental and theoretical studies evidently showed that the interface and grain boundary energies of the higher symmetry phases (orthorhombic and tetragonal) contribute to the stabilization of the metastable non-centrosymmetric orthorhombic phase or tetragonal phase. However, there was a clear quantitative discrepancy between the theoretical expectation and experiment results, suggesting that the thermodynamic model may not provide the full explanation. This work, therefore, focuses on the phase transition kinetics during the cooling step after the crystallization annealing. It was found that the large activation barrier for the transition from the tetragonal/orthorhombic to the monoclinic phase, which is the stable phase at room temperature, suppresses the phase transition, and thus, plays a critical role in the emergence of ferroelectricity.

Integration of high-K Metal-Insulator-Metal (MIM) capacitors in the Back-end-of-line (BEOL) is a topic of interest for the further development of the process at KTH Royal Institute of Technology. MIM-capacitors benefit from having constant capacitance values over a range of voltages and/or frequencies. One significant limitation in the development of better MIM-capacitors is the temperature consideration for BEOL processes. For the process at KTH Institute of Technology the temperature should not exceed 600 °C, as this would damage underlying devices. This work aims to fabricate aluminium oxide MIM-capacitors as a standard BEOL process performed at low temperature, which has been achieved via atomic layer deposition (ALD). The fabricated aluminium oxide MIM-capacitors had a good quality factor, series resistance and low dissipation. The capacitance for a 10 nm thick aluminium oxide insulator layer was 1 µF/cm2, which exceeds the set requirement. This work also aimed to make ferroelectric aluminium doped hafnium oxide MIM-capacitors using ALD. The doping ratio was varied in ALD as this had been found to affect formation of the ferroelectric crystal phase after a rapid thermal annealing step. Three wafers of 20 nm thick hafnium oxide and differing ratios were found to not be ferroelectric. The intermediate doping ratio was found to appear slightly anti-ferroelectric. A 10 nm thick doped hafnium oxide of intermediate doping was also fabricated and was found to be ferroelectric with a remnant polarisation of 1 µC/cm2. Though this polarisation is relatively small, it shows that top electrode induced strain due to lattice mismatch could be responsible for the ferroelectric properties of the capacitor. The quality of the hafnium based capacitors seemed worse in comparison to the aluminium oxide capacitors, which is suspected to be due to oxygen vacancies, resulting in a high loss tangent. While this first experiment showed promising results, the ferroelectric remnant polarisation should be increased by an order of magnitude and the electrical benchmark values should be improved before these hafnium oxide MIM-capacitors can be used in the BEOL process. / Integratie van high-K MIM-condensatoren in de Back-end-of-line (BEOL) is een onderwerp van belang voor de ontwikkeling van het proces bij de KTH. MIM-condensatoren profiteren van een constante capaciteitswaarde over een reeks spanningen en/of frequenties. Een belangrijke beperking bij de ontwikkeling van betere MIM-condensatoren is het temperatuur limiet voor BEOL-processen. Bij de KTH moet de temperatuur niet hoger zijn dan 600 °C, omdat dit de onderliggende apparaten zou beschadigen. Dit werk heeft tot doel aluminiumoxide MIM-condensatoren te fabriceren als een standaard BEOL-proces met lage temperatuur, en heeft dit inderdaad bereikt via atomaire laagafzetting (ALD). De gefabriceerde aluminiumoxide MIM-condensatoren hadden een goede kwaliteitsfactor, serieweerstand en lage dissipatie. De capaciteit voor een 10 nm dikke aluminiumoxide-isolatorlaag was 1µF/cm2, hoger dan de gestelde eisen. Dit werk was ook gericht op het maken van ferro-elektrische aluminium gedoteerde hafniumoxide MIM-condensatoren met behulp van ALD. De doteringsverhouding werd gevarieerd in ALD, aangezien bleek dat dit de vorming van de ferro-elektrische kristalfase faciliteerde na een snelle thermische gloeistap. Drie wafers van 20 nm dik hafniumoxide en verschillende verhoudingen bleken niet ferro-elektrisch te zijn. De tussenliggende doteringsverhouding bleek enigszins anti-ferro-elektrisch te zijn. Een 10 nm dik gedoteerd hafniumoxide met intermediaire dotering werd ook gefabriceerd en bleek ferro-elektrisch te zijn met een restpolarisatie van 1 µC/cm2. Hoewel deze polarisatie relatief klein is, toont het aan dat de door de topelektrode geïnduceerde spanning als gevolg van roostermismatch verantwoordelijk zou kunnen zijn voor de ferro-elektrische eigenschappen van de condensator. De kwaliteit van de op hafnium gebaseerde isolator leek slechter in vergelijking met die van aluminiumoxide, hetgeen kan worden toegeschreven aan gebrek van zuurstof in het rooster, wat in een groot verlies resulteert. De ferro-elektriciteit moet met een orde van grootte worden verhoogd en de elektrische benchmarks moeten ook verhoogd worden voordat deze hafniumoxide MIM-condensatoren kunnen worden gebruikt in het BEOLproces. Sleutelwoorden: atomaire laagafzetting (ALD), Ferro-elektrisch, Metaal-Isolator- Metaal (MIM) condensator, lage temperatuur, snelle thermische gloeiing.

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.

This dissertation focuses on projects where optical techniques were employed to characterize novel materials, developing concepts toward next generation of devices. The materials that I studied included InAsP, InMnSb and InMnAs, and BT-BCN. I have employed several advanced time resolved and magneto-optical techniques to explore unexplored properties of these structures. The first class of the materials were the ternary alloys InAsP. The electron g-factor of InAsP can be tuned, even allowing for g=0, making InAsP an ideal candidate for quantum communication devices. Furthermore, InAsP shows promises for opto-electronics and spintronics, where the development of devices requires extensive knowledge of carrier and spin dynamics. Thus, I have performed time and polarization resolved pump-probe spectroscopy on InAsP with various compositions. The carrier and spin relaxation time in these structures were observed and demonstrated tunability to the excitation wavelengths, composition and temperature. The sensitivity to these parameters provide several avenues to control carrier and spin dynamics in InAsP alloys. The second project focused on the ferromagnetic narrow gap semiconductors InMnAs and InMnSb. The incorporation of Mn can lead to ferromagnetic behavior of InMnAs and InMnSb, and enhance the g-factors, making them ideal candidates for spintronics devices. When grown using Molecular Beam Epitaxy (MBE), the Curie temperature (textit{$T_c$}) of these structures is textless 100 K, however structures grown using Metalorganic Vapor phase Epitaxy (MOVPE) have textit{$T_c$} textgreater 300 K. Magnetic circular dichroism was performed on MOVPE grown InMnAs and InMnSb. Comparison of the experimental results with the theoretical calculations provides a direct method to map the band structure, including the temperature dependence of the spin-orbit split-off band to conduction band transition and g-factors, as well as the estimated sp-d electron/hole coupling parameters. My final project was on the lead-free ferroelectric BT-BCN. Ferroelectric materials are being investigated for high speed, density, nonvolatile and energy efficient memory devices; however, commercial ferroelectric memories typically contain lead, and use a destructive reading method. Reflectometry measurements were used in order to determine the refractive index of BT-BCN with varying thicknesses, which can provide a means to nondestructively read ferroelectric memory through optical methods. / Ph. D. / This dissertation focuses on the characterization of materials that are important for the next generation computer architecture through optical techniques. These materials include the ternary alloy InAsP, the ferromagnetic semiconductors InMnAs and InMnSb, and the lead-free ferroelectric BT-BCN. InAsP is a ternary alloy composed of the technologically important InAs and InP, and by changing the alloy composition, the band gap and g-factor can be tuned. This allows for InAsP to have band gaps within the communication band, which is important for fiber optic communications as well as infrared photodetectors. As the functionality of these devices depends on the carrier dynamics, I have performed pump-probe spectroscopy in order to probe the carrier and spin relaxation times of this material system. These relaxation times were found to vary with excitation wavelengths, allowing flexibility in the application of this material system for devices. InAs and InSb are attractive materials for device applications because they offer large electron g-factor, small effective masses, and high mobilities. With the incorporation of Mn, these materials can become ferromagnetic, allowing for their use in ferromagnetic memories as well as other possible devices. The theory of ferromagnetism in semiconductors relies on the interaction between the itinerant holes and the Mn ions, however, in narrow gap semiconductors there is a large band mixing between the conduction and valence band states, and thus the interaction between the conduction band electrons and the Mn is important. In this study, my measurements revealed several interband transitions, which allowed for the calculation of the coupling constants between the electrons, holes and the Mn. My final study involved the lead-free ferroelectric BT-BCN. Ferroelectric materials are ideal for fast, low power and nonvolatile memories; however, typical implementation utilizes materials that contain lead, and a destructive reading mechanism, requiring a rewrite step. Optical, nondestructive reading methods are being explored based off of the rotation of the polarization of light as it passes through the sample. As this requires knowledge of the refractive index, I performed reflectometry measurements in order to determine the refractive indices of several BT-BCN films.

Recent analyses by polarization resolved second-harmonic (SH) microscopy have demonstrated that ferroelectric (FE) domain walls (DWs) can possess non-Ising wall characteristics and topological nature. These analyses rely on locally analyzing the properties, directionality, and magnitude of the second-order nonlinear tensor. However, when inspecting FE DWs with SH microscopy, a manifold of different effects may contribute to the observed signal difference between domains and DWs, i.e., far-field interference, Čerenkov-type phase-matching (CSHG), and changes in the aforementioned local nonlinear optical properties. They all might be present at the same time and, therefore, require careful interpretation and separation. In this work, we demonstrate how the particularly strong Čerenkov-type contrast can selectively be blocked using dark- and bright-field SH microscopy. Based on this approach, we show that other contrast mechanisms emerge that were previously overlayed by CSHG but can now be readily selected through the appropriate experimental geometry. Using the methods presented, we show that the strength of the CSHG contrast compared to the other mechanisms is approximately 22 times higher. This work lays the foundation for the in-depth analysis of FE DW topologies by SH microscopy.

Due to the voltage driven switching at low voltages combined with nonvolatility of the achieved polarization state, ferroelectric materials have a unique potential for low power nonvolatile electronic devices. The competitivity of such devices is hindered by compatibility issues of well-known ferroelectrics with established semiconductor technology. The discovery of ferroelectricity in hafnium oxide changed this situation. The natural application of nonvolatile devices is as a memory cell. Nonvolatile memory devices also built the basis for other applications like in-memory or neuromorphic computing. Three different basic ferroelectric devices can be constructed: ferroelectric capacitors, ferroelectric field effect transistors and ferroelectric tunneling junctions. In this article first the material science of the ferroelectricity in hafnium oxide will be summarized with a special focus on tailoring the switching characteristics towards different applications.The current status of nonvolatile ferroelectric memories then lays the ground for looking into applications like in-memory computing. Finally, a special focus will be given to showcase how the basic building blocks of spiking neural networks, the neuron and the synapse, can be realized and how they can be combined to realize neuromorphic computing systems. A summary, comparison with other technologies like resistive switching devices and an outlook completes the paper.

Ferroelektrische Materialien sind ein Schlüsselbereich der aktuellen Forschung und weisen zahlreiche wichtige technologische Anwendungen auf. Diese Klasse kristalliner Feststoffe zeichnet sich üblicherweise durch eine Vielzahl von para- und ferroelektrischen Phasen auf. Letztere sind dadurch charakterisiert, dass sie auch in Abwesenheit eines äußeren Feldes eine spontane elektrische Polarisation aufweisen. Diese Eigenschaft hat ihren Ursprung in der besonderen elektronischen Struktur ferroelektrischer Materialien, die sich aus einer großen Vielfalt von Gittergeometrien und mikroskopischen Ladungsdichteverteilungen ergibt. Auf atomarer Ebene sind die komplexen Eigenschaften der Ferroelektrika bis jetzt jedoch nur teilweise verstanden. Insbesondere die Verbindung zwischen mikroskopischen elektronischen Ladungsverteilungen und der daraus resultierenden makroskopischen elektrischen Polarisation wirft eine entscheidende, momentan noch offene Frage auf. Die Ladungsdynamik und ihr Zusammenspiel mit Gitteranregungen, insbesondere Softmoden, sind auf atomaren Längen- und Zeitskalen ungelöst. In dieser Arbeit wird das Potenzial der Femtosekunden-Röntgenpulverbeugung aufgezeigt, diese Frage zu adressieren. Diese Methode ermöglicht im Rahmen dieser Arbeit die Bestimmung transienter elektronischer Ladungsdichtekarten für das prototypische ferroelektrische Ammoniumsulfat direkt unterhalb seiner Curie-Temperatur nach einer optischen Anregung. Die Analyse der experimentellen Daten deckte eine bislang unbekannte niederfrequente Gitteroszillation mit einer Periode von 3 ps und nukleare Verschiebungen im Sub-Picometer-Bereich auf, die Ladungsverschiebungen auf einer 100-pm-Längenskala induzieren. Dies sind klare Merkmale, die auf die Anregung einer Softmode hinweisen. Schließlich wird zum ersten Mal die Dynamik der makroskopischen Polarisationsänderung abgeleitet, die eine oszillatorische Umkehr der Polarität aufweist und für ultraschnelle Schaltanwendungen geeignet ist. / Ferroelectrics are an area of current research, with important technological applications such as ferroelectric random access memories, infrared cameras or medical ultrasound equipment. This class of crystalline solids do not commonly only exhibit a ferroelectric phase, but rather go through an abundant variety of para- and ferroelectric phases that depend on the temperature. The ferroelectric phases present a spontaneous electric polarization even in the absence of an external field, in contrast to paraelectric phases and also exhibit a hysteresis loop in analogy to ferromagnets. This macroscopic feature has its origin in their peculiar electronic structure, which results from a rich diversity of lattice geometries and complex microscopic charge distributions. At the atomic level, however, the intricate characteristics of ferroelectrics are only partially understood. The link between microscopic charge distributions and macroscopic electric polarization poses a crucial question to be solved. The interplay of charge dynamics and lattice excitations are still unresolved on atomic length and time scales. In this thesis, femtosecond X-Ray powder diffraction is used to find solutions for these unanswered questions. This method allows for the experimental determination of time-resolved charge density maps from where the structural, charge and polarization dynamics are can be derived. These maps are determined for the photoexcited ferroelectric ammonium sulphate just below its Curie temperature. Data analysis has revealed a newly discovered low frequency lattice oscillation with a 3ps period and sub-picometer nuclear displacements that is related to periodic charge relocations on a 100pm length scale, which is a feature indicative of soft mode behavior. Finally, the dynamics of the variation of polarization are derived for the first time, showing an oscillatory reversal of polarity that holds potential for ultrafast switching applications.

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

The ferroelectricity in fluorite structure based hafnium oxide (HfO2) material expanded the horizon for realizing nonvolatile ferroelectric memory concepts. Due to the excellent HfO2 ferroelectric film properties, CMOS compatibility, and scalability; the material is foreseen as a replacement of the lead based ferroelectric materials with a big game changing potential for the emerging ferroelectric memories. In this thesis, the development of novel memory concepts based on the ferroelectric or antiferroelectric HfO2 material is reported. The ferroelectric field effect transistor (FeFET) memory concept offers a low power, high-speed, nonvolatile, and one cell memory solution ideal for embedded memory realization. As an emerging concept based on a novel ferroelectric material, the FeFET is challenged with key performance aspects intrinsic to the underlying physics of the device. A central part of this thesis is the development of FeFET through material and gate stack engineering, in turn leading to innovative novel device concepts. The conceptual innovation, process development, and electrical assessment are explored for an ferroelectric or antiferroelectric HfO2 based nonvolatile memories with focus on the underlying device physics. The impact of the ferroelectric material on the FeFET physics is explored via the screening of different HfO2 based ferroelectric materials, thicknesses, and the film doping concentration. The impact of material interfaces and substrate doping conditions are explored on the stack engineering level to achieve a low power and reliable FeFET. The material optimization leads to the concept of ferroelectric lamination, i.e. a dielectric interlayer between multi ferroelectric ones, to achieve a novel multilevel data storage in FeFET at reduced device variability. Toward a low power FeFET, the stack structure tuning and dual ferroelectric layer integration are explored through an MFM and MFIS integration in a single novel FeFET stack. The charge trapping effect during the FeFET switching captures the dynamics of the hysteresis polarization switching inside the stack with direct impact on the interfacial layer field. Even though manifesting as a clear drawback in FeFET operation, it can be utilized in Flash, leading to a novel hybrid low power and high-speed antiferroelectric based charge trap concept. Furthermore, the FeFET reliability is studied covering the role of operating temperature and the ferroelectric wakeup phenomenon observed in the FeFET. The temperature modulated operation, role of the high-temperature pyroelectric effect, and the temperature induced endurance and retention reliability are studied.:Table of Contents Abstract Table of Contents 1. Introduction 2. Fundamentals 2.1. Basics of Ferroelectricity 2.2. The FeFET Operation Principle and Gate Stack Theory 2.3. Structure and Outline of the PhD Thesis 3. The Emerging Memory Optimization Cycle: From Conceptual Design to Fabrication 3.1. The FeFET Conceptual Design and Layout Implementation 3.2. Gate First FeFET Fabrication: Material and Gate Stack Optimization 3.3. Novel Gate First based Memory Concepts: Device Integration and Stack Optimization 3.4. Device Characterization: Electrical Testing Schemes 4. The Emerging FeFET Memory: Material and Gate Stack Optimization 4.1. Material Aspect of FeFET Optimization: Role of the FE Material Properties 4.2. The Stack Aspect of FeFET Optimization: Role of the Interface Layer Properties 4.3. The Stack Aspect of FeFET Optimization: Role of the Substrate Implant Doping 4.4. Summary 5. A Novel Multilevel Cell FeFET Memory: Laminated HSO and HZO Ferroelectrics 5.1. The Laminate MFM and Stack Characteristics 5.2. The Laminate based FeFET Memory Switching 5.3. The Laminate FeFET Multilevel Coding Operation (1 bit, 2 bit, 3 bit/cell) 5.4. The Maximum Laminate FeFET MW Dependence on FE Stack Thickness 5.5. The Role of Wakeup and Charge Trapping 5.6. The Laminate MLC FeFET Area Dependence 5.7. The Laminate MLC Retention and Endurance 5.8. Impact of Pass Voltage Disturb on Laminate based NAND Array Operation 5.9. The Laminate FeFET based Synaptic Device 5.10. Summary 6. A Novel Ferroelectric MFMFIS FeFET: Toward Low Power and High-Speed NVM 6.1. The MFMFIS FeFET P-E and FET Characteristics 6.2. The MFMFIS based Memory Characteristics 6.3. The Impact of MFMFIS Stack Structure Tuning 6.4. The Maximum MFMFIS FeFET Memory Window 6.5. The Role of Device Scalability and Variability 6.6. The MFMFIS Area Tuning for Low Power Operation 6.7. The MFMFIS based FeFET Reliability 6.8. The Synaptic MFMFIS based FeFET 6.9. Summary 7. A Novel Hybrid Low Power and High-Speed Antiferroelectric Boosted Charge Trap Memory 7.1. The Hybrid Charge Trap Memory Switching Characteristics 7.2. The Role of Polarization Switching on Optimal Write Conditions 7.3. The Impact of FE/AFE Properties on the Charge Trap Maximum Memory Window 7.4. The Hybrid AFE Charge Trap Multi-level Coding and Array Operation 7.5. The Global Variability and Area Dependence of the Charge Trap Memory Window 7.6. The AFE Charge Trap Reliability 7.7. The Hybrid AFE Charge Trap based Synapse 7.8. Summary 8. The Emerging FeFET Reliability: Role of Operating Temperature and Wakeup Effect 8.1. The FeFET Temperature Reliability: A Temperature Modulated Operation 8.2. The FeFET Temperature Reliability: Role of the Pyroelectric Effect 8.3. The FeFET Temperature Reliability: Endurance and Retention 8.4. The Impact of Ferroelectric Wakeup on the FeFET Memory Reliability 8.5. Summary 9. Closure: What this Thesis has Solved? 9.1. How material selection/development influence the FeFET? 9.2. Why the FeFET Still Operates at High Write Conditions? 9.3. Why the FeFET Endurance is still a Challenge? 9.4. Can the FeFET become Multi-bit Storage Memory? 9.5. How the Scalability Determine FeFET Chances? 10. Summary 11. Bibliography List of symbols and abbreviations List of Publications Acknowledgment Erklärung

Une des méthodes utilisées pour étudier la ferroélectricité à l'échelle nanométrique dans les couches minces est la technique appelée « Piezoresponse Force Microscopy » (PFM). La PFM est un mode dérivé de l’AFM (Atomic Force Microscopy) en mode contact. Cette technique est basée sur l’effet piézoélectrique inverse : lorsqu’on applique un champ électrique sur un matériau piézoélectrique celui-ci se déforme. La pointe est posée sur la surface et mesure donc une déformation locale due à la tension appliquée. Les résultats obtenus par PFM sur des couches minces deviennent difficiles à interpréter dès lors que des charges d’origine non ferroélectriques (différentes de la charge de polarisation) entrent en jeu : charges électroniques piégées dans l’oxyde après l’injection de courant dues aux courants de fuite, charges déjà présentes dans la couche, les charges de surface, ainsi que les différents phénomènes électrochimiques due à la présence de la couche d’eau sous la pointe lors des mesures sous atmosphère ambiante. Le but de ce travail de thèse est de montrer que dans le cas de couches très minces les courants de fuite et les phénomènes électrochimiques peuvent conduire à l’interprétation de résultat PFM erroné. Des mesures PFM ont été réalisées sur des couches minces de PbZrTiO3, BaTiO3 et des nanostructures de BiFeO3 ferroélectriques. Les paramètres de mesure utilisés en PFM sont discutés avec une attention particulière sur la première résonance de contact qui permet d’amplifier le signal PFM. L’impact des phénomènes électrochimiques sur le contraste en PFM est discuté et mis en évidence d’un point de vue expérimentale. Des images PFM sur des couches minces non-ferroélectriques sont obtenues semblable à celle obtenues lors de l’utilisation d’une procédure standard sur des échantillons ferroélectriques. Ces images sont réalisées sur des couches minces d’aluminate de lanthane (LaAlO3), d'oxyde de Gadolinium (Gd2O3) et d’oxyde de Silicium (SiO2). Les motifs obtenus sur le LaAlO3 et le Gd2O3, similaires à des domaines de polarisation opposés, tiennent dans le temps sous atmosphère ambiante. Ces mesures sont comparées avec des résultats obtenus sur des couches minces de BaTiO3 préparées par MBE (Molecular Beam Epitaxy). Différentes méthodes de caractérisation électriques à l’échelle macroscopique sont présentées afin de confirmer la ferroélectricité des couches minces étudiées dans cette thèse. L'objectif est de disposer d'une procédure permettant d'affirmer qu'un échantillon dont on ne sait rien est ou n'est pas ferroélectrique. / Piezoresponse Force Microscopy (PFM) is a powerful tool for the characterization of ferroelectric materials thanks to its ability to map and control in a non destructive way domain structures in ferroelectric films. Most of the time, the ferroelectric behaviour of a film is tested by writing domains of opposite polarization with the Atomic Force Microscope (AFM) tip and/or by performing hysteresis loops with the AFM tip as a top electrode. A given sample is declared ferroelectric when domains of opposite direction have been detected; corresponding to zones of distinct contrast on the PFM image, or when an open hysteresis loop is obtained. More prudently in certain cases, the ferroelectricity is at last attested only when the contrast is stable within several hours. But as the thickness of the films studied by PFM decrease, data become difficult to interpret. In particular, charges trapped after current injection due to leakage currents and electrochemical phenomena due to the water layer under the tip may contribute in a non-negligible way to the final contrast of PFM images. In this thesis, some PFM measurements are performed on ferroelectric PbZrTiO3, BaTiO3 thin films and BiFeO3 nanostructures. Different parameters used in PFM measurements are discussed with special attention on the buckling first harmonic PFM measurements which allow the amplification of the PFM signal. The impact of electrochemical effects on the PFM contrast are discussed and are shown experimentally. Then, the standard procedure which is used in order to show the ferroelectricity of a film is applied to a non-ferroelectric sample with apparently the same results. To do so, we use a LaAlO3, Gd2O3 and SiO2 amorphous dielectric films and apply similar voltages as for artificially written ferroelectric domains. The resulting pattern is imaged by PFM and exhibit zones of distinct PFM contrasts, stable with time, similar to the one obtained with ferroelectric samples. These results are explained and is compared with results obtained on BaTiO3 thin films prepared by Molecular Beam Epitaxy which are supposed to be ferroelectric. In order to confirm the ferroelectricity of our thin films, several macroscopic electrical techniques are introduced. The aim of this study is to establish a reliable procedure which would remove any ambiguity in the characterization of the ferroelectric nature of such samples.