Pyroelectric and electrocaloric effects in hafnium oxide thin films

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

Material development of doped hafnium oxide for non-volatile ferroelectric memory application

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 158The 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 15

Radiation-tolerant ferroelectric materials for multifunctional devices

Ferroelectric materials have switchable, spontaneous polarization in addition to strong dielectric, pyroelectric and piezoelectric response. In thin films form, these materials are leveraged for numerous microelectronic devices, including mechanical logic elements, optical sensors and transducers, precision positioners, energy harvesting units, nonvolatile memory storage, and microelectromechanical systems (MEMS) sensors and actuators. Ferroelectric materials have also become attractive for use in devices for radiation-hostile environments (e.g. aerospace, medical physics, x-ray/high energy source measurement tools, nuclear monitoring systems) due to their relatively high radiation tolerance. An increased understanding of material properties responsible for radiation tolerance will allow for development of materials for the next generation of radiation-tolerant, multifunctional devices. Lead zirconate titanate (PZT), one of the most commonly used ferroelectric materials for microscale applications, is widely known for its high polarization and piezoelectric response. However, increasing demand for smaller device footprint has pushed research efforts on PZT thin films towards their limitations, creating a need for new material systems to exceed the current standards. In this thesis, two material systems are explored as radiation tolerant ferroelectric alternatives to PZT: 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) and Hf0.5Zr0.5O2 (HZO). PMN-PT films exhibit strong piezoelectric response, exceeding that of PZT, making them a strong candidate for next generation piezoelectric MEMS devices. Additionally, the large amount of chemical, polar, and structural heterogeneities in this material imply a large degree of entropy, which could result in accommodation of radiation-induced defects and enhanced radiation tolerance. HZO thin films exhibit strong polarization properties at only a few nanometers in thickness. Combined with its CMOS compatibility and the potential to fabricate complex 3D structures using atomic layer deposition, HZO has become an attractive material for (ferroelectric) non-volatile memory applications. Total ionization dose (TID) studies, using gamma-radiation doses up to 10 Mrad(Si), were performed to understand the radiation tolerance of PMN-PT and HZO thin films. Processing-structure-property relations were explored to identify the material characteristics responsible for both high functional response and high radiation tolerance. PMN-PT thin films were confirmed to exhibit equivalent or superior radiation tolerance in dielectric, polarization, and piezoelectric response than PZT thin films, largely unaffected by microstructural differences. Although the HZO thin films suffered significantly from aging, the films fabricated via plasma-enhanced atomic layer deposition exhibited superior radiation tolerance in polarization response than PZT thin films. The studies illustrate different pathways for concomitant enhanced functionality and higher radiation tolerance in ferroelectric thin films.Ph.D

High-K Dielectrics in Metal Insulator Metal (MIM) Capacitors for RF Applications

Ph.DDOCTOR OF PHILOSOPH

Plasma-assisted atomic layer deposition of metal oxides and nitrides

The atomic layer deposition (ALD) technique has recently gained considerable interest as a suitable method for the fabrication of nanoscale thin films. The virtue of this technique is that the deposition is controlled at the atomic level by self-limiting surface reactions through the alternate exposure of the substrate surface to different gaseous precursors. ALD provides ultimate control of film thickness and has the potential to achieve uniform film properties over the entire substrate surface, even in high aspect ratio structures. Over the past years it has been proven that with ALD high quality, atomically smooth, and conformal thin films of a wide variety of materials can obtained. Currently, several ALD processes are on the verge of being incorporated into the production of devices, such as CMOS transistors and DRAM memory. A recent development to broaden the applicability of ALD is the use of a plasma as an alternative reactant source. Because the activation of the reactive species already takes place in the gas phase, this so-called plasma-assisted ALD, can provide certain benefits. In this thesis work, the plasma-assisted ALD of the metal oxides (Al2O3, Ta2O5 and HfO2) and metal nitrides (TiN) were investigated. For this purpose, a versatile plasma-assisted ALD reactor based on a remote plasma configuration was constructed. To study the plasma-assisted ALD processes, in situ diagnostics were employed. New in this respect was the use of spectroscopic ellipsometry (SE) to measure film thickness in situ and optical emission spectroscopy (OES) to study the electronically excited reaction products in the plasma. Furthermore, also a quartz crystal microbalance and quadrupole mass spectrometer were employed to monitor the mass uptake per half-cycle and the reaction products created, respectively. The composition, microstructure, and electrical properties of the films were determined by ex situ techniques. During the project, the collaboration with Oxford Instruments contributed to the design of one of the first commercially available R&D tools for plasma-assisted ALD, the FlexAL. Currently, a beta version of the FlexAL tool is installed at our university and the first results on the deposition of TiN and HfO2 on 200 mm wafers were reported in this thesis work. The merits of plasma-assisted ALD have been studied and made apparent for the materials investigated. The improvement of material properties by the plasma-based process was demonstrated for the case of TiN. Good material properties in terms of impurity content and electrical resistivity were obtained for TiN, also at a deposition temperature as low as 100 °C. The feasibility of depositing good quality Al2O3, Ta2O5, and HfO2 films by plasma-assisted ALD at low substrate temperatures was also demonstrated, even down to room temperature for the case of Al2O3. The reaction mechanisms of plasma-assisted ALD have been studied for the deposition of Al2O3 and Ta2O5 from metal-organic precursors in which an O2 plasma is used as oxidant source. Two different types of metal-organic precursors, a metal-alkyl (Al(CH3)3) and metal-alkylamide (Ta[N(CH3)2]5), were used. During the O2 plasma exposure, the presence of CO, CO2 and H2O was detected in both processes. Furthermore, the depletion of the O2 source gas indicated the consumption of O radicals. These observations demonstrated that combustion-like reactions in which the surface groups are converted by O radicals into combustion products occur at the surface. Secondly, in the Al2O3 process the detection of CH4 during the O2 plasma exposure indicated that the production of H2O has an effect on the surface chemistry during the Al2O3 deposition process. The produced H2O apparently forms an alternative thermal ALD-like reaction pathway in concurrence with the combustion-like reactions. The possible existence of more reaction pathways is suggested by the presence of C2Hx and CN species during the O2 plasma exposure in the Al2O3 and Ta2O5 deposition, respectively. Additionally, the dissociation and excitation of molecules are other reactions that can take place in the plasma. This was demonstrated by the light emission coming from the plasma during Al2O3 and Ta2O5 processing, which changes in the presence of reaction products released from the surface

OXIDE-BASED MEMRISTIVE DEVICES BY BLOCK COPOLYMER SELF-ASSEMBLY

Oxide-based memristive systems represent today an emerging class of devices with a significant potential in memory, logic, and neuromorphic circuit applications. These devices have a simple capacitor structure and promise superior scalability together with favorable memory performances. This thesis presents a study of resistive switching phenomena in HfOx-based nanoscale memristive devices, with focus on material properties and development of bottom-up approaches for the fabrication of structures with dimension down to the nanoscale. One of the main issues for practical applications regarding device variability is first assessed by doping hafnium oxide films with different concentrations of aluminum atoms. Testing devices are analyzed by physico-chemical and electrical techniques in order to define the effect of oxide doping on the device properties. In the following part of the thesis, the scalability limit is explored in very high density arrays of nanodevices produced exploiting a lithographic approach based on the bottom-up self-assembly of block copolymer templates. This technique allows a tight control over the size and density of the defined features, and the possibilities offered by block copolymer patterning are here discussed. Electrical measurements of the nanodevices are performed through conductive atomic force microscopy. The device variability is examined and related to the inherent oxide non-homogeneity at the nanoscale, while a non-volatile switching of the resistance of the nanodevices is demonstrated. Further, this analysis draws the attention to a crosstalk phenomenon occurring at the nanoscale in a continuous thin film geometry. This result suggests to select different system configurations. A promising technique based on selective reactions with one copolymer block is finally discussed which allows the direct production of oxide patterns from block copolymer templates avoiding a pattern transfer process. In conclusion, the results reported in this thesis highlight the high scalability potential of oxide-based memristive devices, providing a missing piece of information for the understanding and practical development of very high density arrays

Cutting Edge Nanotechnology

The main purpose of this book is to describe important issues in various types of devices ranging from conventional transistors (opening chapters of the book) to molecular electronic devices whose fabrication and operation is discussed in the last few chapters of the book. As such, this book can serve as a guide for identifications of important areas of research in micro, nano and molecular electronics. We deeply acknowledge valuable contributions that each of the authors made in writing these excellent chapters