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
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:79572 |
| Date | 16 June 2022 |
| Creators | Lederer, Maximilian |
| Contributors | Eng, Lukas M., Noheda, Beatriz, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
Page generated in 0.003 seconds