The material class of hafnium oxide-based ferroelectrics adds an unexpected and huge momentum to the long-known phenomenon of pyroelectricity. In this thesis, a comprehensive study of pyroelectric and electrocaloric properties of this novel ferroelectric material class is conducted. hafnium oxide is a lead-free, non-toxic transition metal oxide, and abundant in the manufacturing of semiconductor devices. The compatibility to existing fabrication processes spawns the possibility of on-chip infrared sensing, energy harvesting, and refrigeration solutions, for which this dissertation aims to lay a foundation. A screening of the material system with respect to several dopants reveals an enhanced pyroelectric response at the morphotropic phase boundary between the polar orthorhombic and the non-polar tetragonal phase. Further, a strong pyroelectric effect is observed when applying an electric field to antiferroelectric-like films, which is attributed to a field-induced transition between the tetragonal and orthorhombic phases. Primary and secondary pyroelectric effects are separated using high-frequency temperature cycles, where the effect of frequency-dependent substrate clamping is exploited. The piezoelectric response is determined by comparing primary and secondary pyroelectric coefficients, which reproduces the expected wake-up behavior in hafnium oxide films. Further, the potential of hafnium oxide for thermal-electric energy conversion is explored. The electrocaloric temperature change of only 20 nm thick films is observed directly by using a specialized test structure. By comparing the magnitude of the effect to the pyroelectric response, it is concluded that defect charges have an important impact on the electrocaloric effect in hafnium oxide-based ferroelectrics. Energy harvesting with a conformal hafnium oxide film on a porous, nano-patterned substrate is performed, which enhances the power output. Further, the integration of a pyroelectric energy harvesting device in a microchip for waste heat recovery and more energy-efficient electronic devices is demonstrated. High dielectric breakdown fields of up to 4 MV/cm in combination with a sizable pyroelectric response and a comparably low dielectric permittivity illustrate the prospect of hafnium oxide-based devices for future energy conversion applications.

Seit der Entdeckung von Ferroelektrizität in Hafniumoxid stellt es aufgrund seiner Prozesskompatibilität im Bereich der Mikroelektronik sowie seiner besonderen Eigenschaften ein wachsendes Forschungsfeld dar. Im Speziellen wird die Anwendung in nicht-flüchtigen Speichern, in neuromorphen Bauelementen sowie in piezo-/pyroelektrischen Sensoren untersucht. Jedoch ist das Verhalten von ferroelektrischem Hafniumoxid im Vergleich zu Ferroelektrika mit Perovskit-Struktur nicht im Detail verstanden. Zudem spielen Prozesseinflüsse während und nach der Abscheidung eine entscheidende Rolle für die Materialeigenschaften aufgrund der metastabilen Natur der ferroektrischen Phase in diesem Materialsystem. In dieser Arbeit werden die grundlegenden physikalischen Eigenschaften von Hafniumoxid, Prozesseinflüsse auf die Mikrostruktur und Zuverlässigkeitsaspekte von nicht-flüchtigen sowie neuromorphen Bauelementen untersucht. Im Bezug auf die physikalischen Eigenschaften zeigen sich hier deutliche Belege für ferroelastische 90° Domänenwandbewegungen in Hafniumoxid-basierten Dünnschichten, welche in einem ähnlichen Verhalten wie ein Antiferroelektrikum resultieren. Weiterhin wird über die Entdeckung von einer mittels elektrischem Feld induzierten Kristallisation in diesem Materialsystem berichtet. Für die Charakterisierung der Mikrostruktur wird als neue Methode Transmissions-Kikuchi-Diffraktion eingeführt, welche eine detaillierte Untersuchung der lokalen kristallographischen Phase, Orientierung und Gefügestruktur ermöglicht. Hierbei zeigen sich deutliche Vorzugsorientierungen in Abhängigkeit des Substrates, der Dotierstoffkonzentration sowie der Glühtemperatur. Auf Basis dieser Ergebnisse lassen sich die beobachteten Zuverlässigkeitsverhalten in Bauelementen erklären und mittels Defektkontrolle weiter optimieren. Schließlich wird das Verhalten in neuromorphen Bauelementen untersucht und Leitlinien für Prozess- und Bauelementoptimierung gegeben.:Abstract i Abstract ii List of Figures vi List of Tables x Acronyms xi Symbols xiv 1 Introduction 1 2 Theoretical background 3 2.1 Behavior of ferroelectric materials 3 2.1.1 Phase transitions at the Curie temperature 4 2.1.2 Domains, domain walls, and microstructure 5 2.2 Ferroelectricity in HfO2 6 2.2.1 Thermodynamics and kinetics 8 2.2.2 Antiferroelectric-like behavior, wake-up effect, and fatigue 11 2.2.3 Piezo- and pyroelectric effects 13 2.3 Ferroelectric FETs 13 2.3.1 Endurance, retention and variability 14 2.3.2 Neuromorphic devices 15 3 Methodology 17 3.1 Electrical analysis 17 3.1.1 Capacitors 17 3.1.2 FeFETs 19 3.2 Structural and chemical analysis 20 3.2.1 Grazing-incident X-ray diffraction (GIXRD) 20 3.2.2 Transmission electron microscopy (TEM) 20 3.2.3 Time-of-flight secondary ion mass spectrometry (ToF-SIMS) 21 3.3 Transmission Kikuchi diffraction 21 3.4 Sample preparation 23 4 The physics of ferroelectric HfO2 25 4.1 Ferroelastic switching 25 4.2 Electric field-induced crystallization 30 5 Microstructure engineering 33 5.1 Microstructure and ferroelectric domains in HfO2 33 5.2 Doping influences 34 5.2.1 Zr doping (similar ionic radius) 35 5.2.2 Si doping (smaller ionic radius) 43 5.2.3 La doping (larger ionic radius) 50 5.2.4 Co-doping 50 5.3 Annealing influences 53 5.4 Interlayer influences 58 5.5 Interface layer influences 62 5.5.1 Structural differences in the HfO2 layer 63 5.5.2 Interactions of the interface and HfO2 layer 67 5.5.3 Substrate-driven changes in the Si-doping profile 73 5.6 Phenomenological wake-up behaviors and process guidelines 77 6 HfO2-based ferroelectric FETs 81 6.1 Endurance, retention and variability 81 6.1.1 Analytic model of HfO2-based FeFETs 84 6.1.2 Endurance improvements by interface fluorination 94 6.2 Neuromorphic devices and circuits 98 6.2.1 Current peroclation paths in FeFETs 100 6.2.2 Material and stack influences on synaptic devices 105 6.2.3 Reliability aspects of synaptic devices 106 7 Conclusion and outlook 109 Appendix 142 Density-functional-theory calculations 142 Supplementary Figures 143 Publications 145 Acknowledgment 156 Declaration 158 / The discovery of ferroelectricity in hafnium oxide spurred a growing research field due to hafnium oxides compatibility with processes in microelectronics as well as its unique properties. Notably, its application in non-volatile memories, neuromorphic devices as well as piezo- and pyroelectric sensors is investigated. However, the behavior of ferroelectric hafnium oxide is not understood into depth compared to common perovskite structure ferroelectrics. Due the the metastable nature of the ferroelectric phase, process conditions have a strong influence during and after its deposition. In this work, the physical properties of hafnium oxide, process influences on the microstructure as well as reliability aspects in non-volatile and neuromorphic devices are investigated. With respect to the physical properties, strong evidence is provided that the antiferroelectric-like behavior in hafnium oxide based thin films is governed by ferroelastic 90° domain wall movement. Furthermore, the discovery of an electric field-induced crystallization process in this material system is reported. For the analysis of the microstructure, the novel method of transmission Kikuchi diffraction is introduced, allowing an investigation of the local crystallographic phase, orientation and grain structure. Here, strong crystallographic textures are observed in dependence of the substrate, doping concentration and annealing temperature. Based on these results, the observed reliability behavior in the electronic devices is explainable and engineering of the present defect landscape enables further optimization. Finally, the behavior in neuromorphic devices is explored as well as process and design guidelines for the desired behavior are provided.:Abstract i Abstract ii List of Figures vi List of Tables x Acronyms xi Symbols xiv 1 Introduction 1 2 Theoretical background 3 2.1 Behavior of ferroelectric materials 3 2.1.1 Phase transitions at the Curie temperature 4 2.1.2 Domains, domain walls, and microstructure 5 2.2 Ferroelectricity in HfO2 6 2.2.1 Thermodynamics and kinetics 8 2.2.2 Antiferroelectric-like behavior, wake-up effect, and fatigue 11 2.2.3 Piezo- and pyroelectric effects 13 2.3 Ferroelectric FETs 13 2.3.1 Endurance, retention and variability 14 2.3.2 Neuromorphic devices 15 3 Methodology 17 3.1 Electrical analysis 17 3.1.1 Capacitors 17 3.1.2 FeFETs 19 3.2 Structural and chemical analysis 20 3.2.1 Grazing-incident X-ray diffraction (GIXRD) 20 3.2.2 Transmission electron microscopy (TEM) 20 3.2.3 Time-of-flight secondary ion mass spectrometry (ToF-SIMS) 21 3.3 Transmission Kikuchi diffraction 21 3.4 Sample preparation 23 4 The physics of ferroelectric HfO2 25 4.1 Ferroelastic switching 25 4.2 Electric field-induced crystallization 30 5 Microstructure engineering 33 5.1 Microstructure and ferroelectric domains in HfO2 33 5.2 Doping influences 34 5.2.1 Zr doping (similar ionic radius) 35 5.2.2 Si doping (smaller ionic radius) 43 5.2.3 La doping (larger ionic radius) 50 5.2.4 Co-doping 50 5.3 Annealing influences 53 5.4 Interlayer influences 58 5.5 Interface layer influences 62 5.5.1 Structural differences in the HfO2 layer 63 5.5.2 Interactions of the interface and HfO2 layer 67 5.5.3 Substrate-driven changes in the Si-doping profile 73 5.6 Phenomenological wake-up behaviors and process guidelines 77 6 HfO2-based ferroelectric FETs 81 6.1 Endurance, retention and variability 81 6.1.1 Analytic model of HfO2-based FeFETs 84 6.1.2 Endurance improvements by interface fluorination 94 6.2 Neuromorphic devices and circuits 98 6.2.1 Current peroclation paths in FeFETs 100 6.2.2 Material and stack influences on synaptic devices 105 6.2.3 Reliability aspects of synaptic devices 106 7 Conclusion and outlook 109 Appendix 142 Density-functional-theory calculations 142 Supplementary Figures 143 Publications 145 Acknowledgment 156 Declaration 158

In der vorliegenden Arbeit wird die grundsätzliche Eignung von yttriumstabilisiertem Hafniumoxidschichten als neues Dielektrikum für Speicherkondensatoren in dynamischen Halbleiterspeichern (DRAM) untersucht. Bei diesem Werkstoff handelt es sich um einen high-k Isolator der neuen Generation mit großem anwendungstechnischem Potential zur Substitution der seit vier Jahrzehnten eingesetzten siliciumbasierten Materialien. Daraus abgeleitet ergibt sich die Aufgabenstellung einer umfassenden Charakterisierung der praxisrelevanten Eigenschaften der Oxidschicht, umfassend in dem Sinne, dass aus dem Ergebnis eine wissenschaftlich fundierte Beurteilung zu den Aussichten einer Überführung in die Produktion abgeleitet werden kann. Es wird aufgezeigt, dass der Wechsel zu high-k Isolatoren erhebliche technische Neuerungen voraussetzt und weitere Entwicklungsarbeit nötig ist. Zusammenfassend kann erstmals die Eignung der ALD-Technik zur Herstellung dünnster yttriumstabilisierter Hafniumoxidschichten und deren Verwendung als Isolatorwerkstoff in zukünftigen mikroelektronischen Speicherkondensatoren anhand einer umfangreichen und anwendungstechnisch fokussierten Mikrostrukturcharakterisierung nachgewiesen werden. / This thesis investigates the basic suitability of yttrium stabilized hafnium oxide as a new dielectric for storage capacitors in dynamic random access memory (DRAM) semiconductor devices. This material is a so-called high- insulator with high dielectric constant. It is a good candidate to replace the silicon-based materials that are used for four decades now. Therefore it is necessary to extensively investigate selected properties of the oxide material. Extensively in terms of significant results that enable or object the applicability for the production process. It shows that the shift to high-insulators requires significant technological innovations and that further development work is necessary. The suitability of the ALD technique for depositing thin films of yttrium oxide and hafnium oxide is identified. The suitability of yttrium stabilized hafnium oxide layers as a dielectric material in future microelectronic storage capacitors can be given for the first time.

Die überraschende Entdeckung von ferroelektrischem Hafniumoxid durch Böscke et al. im Jahre 2011 eröffnet zahlreich technologische Möglichkeiten wie zum Beispiel voll CMOS kompatible ferroelektrische RAM Speicherzellen. Als kristallographische Ursache für dieses Verhalten erwies sich die Raumgruppe Pca21. In theoretischen Untersuchungen mit Hilfe der Dichtefunktionaltheorie erwies sich diese Phase jedoch als thermodynamisch instabil. Ziel dieser Dissertation ist daher zu klären, wie diese Phase stabilisiert werden kann. Dazu werden Faktoren wie Stöchiometrie, Temperatur, Druck, Spannung, Grenzflächenenergie sowie Defekte und Dotierung mit Hilfe der Dichtefunktionaltheorie untersucht. Die errechneten Ergebnisse werden mit Hilfe von Modellen interpretiert, welche im laufe dieser Dissertation erarbeitet werden. Es zeigt sich, dass neben dem energetischen Zustand auch der Herstellungsprozess des Materials eine bedeutende Rolle in der Stabilisierung der ferroelektrischen Phase von Hafniumoxid spielt. Abschließend wird versucht Verbindung zum Experiment herzustellen, in dem experimentell zugängliche Stellschrauben aufgezeigt werden, welche die ferroelektrischen Eingenschaften von Hafniumoxid verbessern können und sich aus den erarbeiteten Ergebnissen ableiten. / The surprising discovery of ferroelectric hafnium oxide by Böscke et al. in 2011 enables various technological possibilities like CMOS compatible ferroelectric RAM devices. The space group Pca21 was identified as the crystallographic cause of this behavior. However, this phase was proved to be thermodynamically unstable by several theoretical studies using density functional theory. Therefore, the goal of this dissertation is to investigate physical effects contributing to the stabilization of the ferroelectric phase by means of density functional theory. These effects include stoichiometry, temperature, stress, strain, interface energy, as well as defects and dopants. The computational results will be interpreted with models, which will be developed within this dissertation. It will become apparent, that in addition to the energetic state, the production process of a sample plays an important role in the stabilization of the ferroelectric phase of hafnium oxide. In the conclusion, this work will attempt to find a connection to the experiment, by identifying experimentally accessible parameters within the computational results which can be used to optimize the ferroelectric properties of ferroelectric materials.

In der vorliegenden Arbeit wird die grundsätzliche Eignung von yttriumstabilisiertem Hafniumoxidschichten als neues Dielektrikum für Speicherkondensatoren in dynamischen Halbleiterspeichern (DRAM) untersucht. Bei diesem Werkstoff handelt es sich um einen high-k Isolator der neuen Generation mit großem anwendungstechnischem Potential zur Substitution der seit vier Jahrzehnten eingesetzten siliciumbasierten Materialien. Daraus abgeleitet ergibt sich die Aufgabenstellung einer umfassenden Charakterisierung der praxisrelevanten Eigenschaften der Oxidschicht, umfassend in dem Sinne, dass aus dem Ergebnis eine wissenschaftlich fundierte Beurteilung zu den Aussichten einer Überführung in die Produktion abgeleitet werden kann. Es wird aufgezeigt, dass der Wechsel zu high-k Isolatoren erhebliche technische Neuerungen voraussetzt und weitere Entwicklungsarbeit nötig ist. Zusammenfassend kann erstmals die Eignung der ALD-Technik zur Herstellung dünnster yttriumstabilisierter Hafniumoxidschichten und deren Verwendung als Isolatorwerkstoff in zukünftigen mikroelektronischen Speicherkondensatoren anhand einer umfangreichen und anwendungstechnisch fokussierten Mikrostrukturcharakterisierung nachgewiesen werden.:Kurzbeschreibung Abstract Abkürzungen und Symbole 1. Einleitung 1.1. Motivation 1.2. DRAM-Technik 1.3. Ziel der Arbeit 2. Grundlagen 2.1. Isolatorschichten mit hoher dielektrischer Konstante 2.2. Hafniumbasierte Isolatorschichten 2.3. Weitere elektrische Kenngrößen 2.3.1. Ladungsträgertransport in Isolatoren 2.3.2. Elektrische Zuverlässigkeit 2.4. Atomlagenabscheidung 2.4.1. Grundlagen der Atomlagenabscheidung 2.4.2. Abscheidung in Strukturen mit hohem Aspektverhältnis 2.5. Eigenspannungen 3. Experimentelle Methodik 3.1. Substrate und Schichtabscheidung 3.2. Wärmebehandlung 3.2.1. Muffelofen mit Quarzglasrohr 3.2.2. Schnelle thermische Bearbeitung 3.2.3. Wärmebehandlung unter Vakuum 3.3. Präparation der Proben für die Transmissionselektronenmikroskopie . 3.4. Physikalische Analysemethoden 3.4.1. Röntgenbeugung und -reflektometrie 3.4.2. Mikroskopische Verfahren 3.4.3. Spektroskopische Verfahren 3.4.4. Substratkrümmungsmessung 3.4.5. Elektrische Messverfahren 3.4.6. Weitere Methoden 4. Ergebnisse und Diskussion 4.1. Mikrostruktur ebener Hf-Y-O-Schichten 4.1.1. Schichtwachstum 4.1.2. Rauheit und Dichte 4.1.3. Elementzusammensetzung 4.1.4. Kristallinität 4.1.5. Kristallphasen 4.1.6. Schichteigenspannungen 4.1.7. Linearer thermischer Ausdehnungskoeffizient und biaxialer Modul 4.1.8. Grenzfläche zum Substrat und der TiN-Elektrode 4.1.9. Zusammenfassung 4.2. Mikrostruktur in beschichteten Löchern mit hohem Aspektverhältnis 4.2.1. Schichtdicke als Funktion der Lochtiefe 4.2.2. Mikrostruktur und Grenzfläche 4.2.3. Modellierung der Bedeckungstiefe 4.2.4. Zusammenfassung 4.3. Einfluss der Mikrostruktur auf die elektrischen Eigenschaften 4.3.1. C-V und I-V Messungen 4.3.2. CAFM-Messungen 4.3.3. Zusammenfassung 5. Zusammenfassung und Ausblick A. Anhang A.1. Probenherstellung A.1.1. Probenbezeichnung A.1.2. Datenerfassung, Archivierung A.1.3. Probenliste A.1.4. Beschichtungsablauf für Hf-Y-Mischoxidschichten A.2. Wärmebehandlung A.3. C-V- und I-V-Messungen B. Veröffentlichungsliste C. Danksagung D. Literaturverzeichnis E. Stichwortverzeichnis / This thesis investigates the basic suitability of yttrium stabilized hafnium oxide as a new dielectric for storage capacitors in dynamic random access memory (DRAM) semiconductor devices. This material is a so-called high- insulator with high dielectric constant. It is a good candidate to replace the silicon-based materials that are used for four decades now. Therefore it is necessary to extensively investigate selected properties of the oxide material. Extensively in terms of significant results that enable or object the applicability for the production process. It shows that the shift to high-insulators requires significant technological innovations and that further development work is necessary. The suitability of the ALD technique for depositing thin films of yttrium oxide and hafnium oxide is identified. The suitability of yttrium stabilized hafnium oxide layers as a dielectric material in future microelectronic storage capacitors can be given for the first time.:Kurzbeschreibung Abstract Abkürzungen und Symbole 1. Einleitung 1.1. Motivation 1.2. DRAM-Technik 1.3. Ziel der Arbeit 2. Grundlagen 2.1. Isolatorschichten mit hoher dielektrischer Konstante 2.2. Hafniumbasierte Isolatorschichten 2.3. Weitere elektrische Kenngrößen 2.3.1. Ladungsträgertransport in Isolatoren 2.3.2. Elektrische Zuverlässigkeit 2.4. Atomlagenabscheidung 2.4.1. Grundlagen der Atomlagenabscheidung 2.4.2. Abscheidung in Strukturen mit hohem Aspektverhältnis 2.5. Eigenspannungen 3. Experimentelle Methodik 3.1. Substrate und Schichtabscheidung 3.2. Wärmebehandlung 3.2.1. Muffelofen mit Quarzglasrohr 3.2.2. Schnelle thermische Bearbeitung 3.2.3. Wärmebehandlung unter Vakuum 3.3. Präparation der Proben für die Transmissionselektronenmikroskopie . 3.4. Physikalische Analysemethoden 3.4.1. Röntgenbeugung und -reflektometrie 3.4.2. Mikroskopische Verfahren 3.4.3. Spektroskopische Verfahren 3.4.4. Substratkrümmungsmessung 3.4.5. Elektrische Messverfahren 3.4.6. Weitere Methoden 4. Ergebnisse und Diskussion 4.1. Mikrostruktur ebener Hf-Y-O-Schichten 4.1.1. Schichtwachstum 4.1.2. Rauheit und Dichte 4.1.3. Elementzusammensetzung 4.1.4. Kristallinität 4.1.5. Kristallphasen 4.1.6. Schichteigenspannungen 4.1.7. Linearer thermischer Ausdehnungskoeffizient und biaxialer Modul 4.1.8. Grenzfläche zum Substrat und der TiN-Elektrode 4.1.9. Zusammenfassung 4.2. Mikrostruktur in beschichteten Löchern mit hohem Aspektverhältnis 4.2.1. Schichtdicke als Funktion der Lochtiefe 4.2.2. Mikrostruktur und Grenzfläche 4.2.3. Modellierung der Bedeckungstiefe 4.2.4. Zusammenfassung 4.3. Einfluss der Mikrostruktur auf die elektrischen Eigenschaften 4.3.1. C-V und I-V Messungen 4.3.2. CAFM-Messungen 4.3.3. Zusammenfassung 5. Zusammenfassung und Ausblick A. Anhang A.1. Probenherstellung A.1.1. Probenbezeichnung A.1.2. Datenerfassung, Archivierung A.1.3. Probenliste A.1.4. Beschichtungsablauf für Hf-Y-Mischoxidschichten A.2. Wärmebehandlung A.3. C-V- und I-V-Messungen B. Veröffentlichungsliste C. Danksagung D. Literaturverzeichnis E. Stichwortverzeichnis

Different causes for ferroelectric properties in hafnium oxide were discussed during the last decade including various dopants, stress, electrode materials, and surface energy from different grain sizes. Recently, the focus shifted to the impact of oxygen vacancies on the phase formation process. In this progress report, the recent understanding of the influence of oxygen supplied during deposition on the structural phase formation process is reviewed and supplemented with new data for mixed HfₓZr₁₋ₓOᵧ films. Even though polar and non-polar HfₓZr₁₋ₓOᵧ thin films are well characterized, little is known about the impact of oxygen exposure during the deposition process. Here, a combination of structural and electrical characterization is applied to investigate the influence of the oxygen and zirconium content on the crystallization process during ALD deposition in comparison to other deposition techniques. Different polarization properties are assessed which correlate to the determined phase of the film. Optimized oxygen pulse times can enable the crystallization of HfₓZr₁₋ₓOᵧ in a polar orthorhombic phase rather than a non-polar monoclinic and tetragonal phase.

During the last decade ferroelectrics based on doped hafnium oxide emerged as promising candidates for realization of ultra-low-power non-volatile memories. Two spontaneous polarization states occurring in the material that can be altered by applying electrical fields rather than forcing a current through and the materials compatibility to CMOS processing are the main benefits setting the concept apart from other emerging memories. 1T1C ferroelectric random access memories (FeRAM) as well as 1T FeFET concepts are under investigation. In this article the application of hafnium based ferroelectric memories in different flavours and their ranking in the memory landscape are discussed.

Ferroelectricity in hafnium oxide thin films is known to be induced by various doping elements and in solid-solution with zirconia. While a wealth of studies is focused on their basic ferroelectric properties and memory applications, thorough studies of the related pyroelectric properties and their application potential are only rarely found. This work investigates the impact of Si doping on the phase composition and ferro- as well as pyroelectric properties of thin film capacitors. Dynamic hysteresis measurements and the field-free Sharp-Garn method were used to correlate the reported orthorhombic phase fractions with the remanent polarization and pyroelectric coefficient. Maximum values of 8.21 µC cm−2 and −46.2 µC K−1 m−2 for remanent polarization and pyroelectric coefficient were found for a Si content of 2.0 at%, respectively. Moreover, temperature-dependent measurements reveal nearly constant values for the pyroelectric coefficient and remanent polarization over the temperature range of 0 °C to 170 °C, which make the material a promising candidate for IR sensor and energy conversion applications beyond the commonly discussed use in memory applications.

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

The discovery of the ferroelectric orthorhombic phase in doped hafnia films has sparked immense research efforts. Presently, a major obstacle for hafnia's use in high-endurance memory applications like nonvolatile random-access memories is its unstable ferroelectric response during field cycling. Different mechanisms are proposed to explain this instability including field-induced phase change, electron trapping, and oxygen vacancy diffusion. However, none of these is able to fully explain the complete behavior and interdependencies of these phenomena. Up to now, no complete root cause for fatigue, wake-up, and imprint effects is presented. In this study, the first evidence for the presence of singly and doubly positively charged oxygen vacancies in hafnia–zirconia films using thermally stimulated currents and impedance spectroscopy is presented. Moreover, it is shown that interaction of these defects with electrons at the interfaces to the electrodes may cause the observed instability of the ferroelectric performance.