Die vorliegende Arbeit befasst sich mit den Untersuchungen zum Einfluss der Züchtungsbedingungen auf die Eigenschaften von multikristallinen (mc) Silizium-Kristallen. Im Mittelpunkt stehen Züchtungsexperimente mit einer gezielten Variation der Züchtungsaufbauten und Züchtungsgeschwindigkeiten. Die gezüchteten Kristalle wurden umfassend charakterisiert im Hinblick auf die Kohlenstoffkonzentration, die Kornstruktur, die Vesetzungsdichte, Verteilung der Ausscheidungen und Ladungsträgerlebensdauer. Zusätzlich wurde die Versetzungsanordnung in Abhängigkeit von der Wachstumsrate bzw. Abkühlrate systematisch untersucht. Als Ergebnis wurde gezeigt, dass die Züchtungsbedingungen die Kohlenstoffkonzentration, die Versetzungsdichte, die Bildung von den Ausscheidungen sowie die Ladungsträgerlebensdauer beeinflussen können, jedoch nicht die Korngröße. Es wurde ein direkter Zusammenhang zwischen Ausscheidungsgebieten und erhöhte Versetzungsdichte beobachtet. Im Rahmen der Arbeit wurde festgestellt, dass die endgültige Versetzungsstruktur sich als Resultat von Gleit- und Erholungsprozessen darstellt.

Einwandige Kohlenstoffnanoröhrchen wurden aufgrund ihrer einzigartigen elektrischen, mechanischen und thermischen Eigenschaften 1991 in den Fokus der Forschung gerückt. In dieser Dissertation wird gezeigt, dass Ramanspektroskopie eine der besten Methoden ist, um die unterschiedlichen Eigenschaften der Nanoröhrchen wie ihren elektrischen Charakter (halbleitend oder metallisch), ihren Durchmesser, die Chiralität, Defekte oder auch Dotierung zu untersuchen. Die Charakterisierung dieser Eigenschaften wird sowohl für das reine Material als auch im elektrischen Bauteil, in diesem Fall einem Feldeffekttransistor, durchgeführt. Der erste Teil der Arbeit vermittelt einen Überblick und gibt eine Einführung in Ramanspektroskopie und in die Struktur von Kohlenstoffnanoröhrchen. Es wird erklärt, welche Eigenschaften speziell mit Hilfe von Position und Intensität der Raman-Modi untersucht werden können und welche Aussagen über die Eigenschaften getroffen werden können. Im experimentellen Teil der Arbeit wurde eine Methode entwickelt, die eine rückstandslose Abscheidung von Dünnschichten aus Kohlenstoffnanoröhrchen ermöglicht. Die Quantifizierung von Defekten wurde durch die in den untersuchten Proben vorhandenen metallischen und halbleitenden Kohlenstoff-Nanoröhrchen ermöglicht. Mittels spitzenverstärkter Ramanspektroskopie wurden außerdem Defekte mit hoher Ortsauflösung (unterhalb von 10 nm) an einzelnen Nanoröhrchen charakterisiert. Der letzte Teil widmet sich den Eigenschaften in elektrische Bauteile, speziell Feldeffekttransistoren, die integrierten Kohlenstoffnanoröhrchen.:Bibliographische Beschreibung 3 Table of Contents 5 1 Introduction 7 2 Background 9 2.1 Structure of carbon nanotubes 9 2.2 Raman spectroscopy basics 10 2.3 Raman spectroscopy on graphene 14 2.4 Raman spectroscopy on carbon nanotubes 16 2.4.1 First-order Raman bands 18 2.4.2 Second-order Raman bands 20 2.5 How to analyze Raman spectra of single-wall carbon nanotubes 21 2.5.1 Diameter and chirality identification 22 2.5.2 Defect characterization 23 2.5.3 Doping and its connection to defects 25 2.5.4 Other effects that can cause frequency shifts 27 2.6 Tip-enhanced Raman spectroscopy 27 2.6.1 TERS experimental requirements 30 2.6.2 Tip and the signal enhancement 30 2.6.3 Brief summary of TERS on single-wall carbon nanotubes 31 3 Materials and Methods 33 3.1 Raman spectroscopy 33 3.2 Ion beam irradiation 34 3.3 SWCNT samples 35 3.4 SWCNT thin film preparation by vacuum filtration 36 3.5 Field effect transistor fabrication and electrical characterization 37 3.6 Tip-enhanced Raman spectroscopy 39 3.6.1 Preparation of the TERS tips 39 3.6.2 Instrumentation 39 3.6.3 SWCNT sample preparation 40 4 Preparation of carbon nanotube thin films 41 4.1 Removal of SDS 42 4.2 Removal of the density gradient medium 43 4.3 Summary 44 5 Quantifying defects in single-wall carbon nanotubes 45 5.1 Parameters of the defect creation 46 5.2 Reference measurement on ion irradiated graphite 47 5.3 Qualitative description of SWCNT defect development 48 5.3.1 Quantitative analysis of the SWCNT defects 57 5.3.2 Summary 59 6 Raman spectroscopy applied to investigate carbon nanotube transistors 61 6.1 Effect of chemical and thermal cleaning of SWCNTs 61 6.2 Effect of temperature and doping on SWCNTs in a Field-effect transistor 65 6.2.1 Investigation of temperature effect 66 6.2.2 In operando CNT-FET Raman spectroscopy measurement 67 6.3 Summary 71 7 TERS on SWCNTs 73 7.1 Preparation of TERS tips 73 7.1.1 Corrosion protection for silver TERS probes 73 7.2 Spatial resolution 76 7.3 Raman spectra of an individual nanotube at the nanoscale 77 7.4 Summary 81 8 Conclusions 83 References 85 Acknowledgement 97 Selbstständigkeitserklärung 99 Lebenslauf 101 Publication list 103

Chiral magnets such as MnSi, FeGe or Cu2OSeO3 exhibit a non-centrosymmetric lattice structure which lacks inversion symmetry. The resulting Dzyaloshinskii-Moriya interaction originating from weak spin-orbit coupling stabilizes smooth modulated magnetic textures, namely helices and skyrmions. In this thesis, we study the properties of helimagnets which are systems with a magnetic helix as ground state. First, we examine the consequences of the helical texture for spin wave excitations, so-called helimagnons. We investigate magnon-focusing effects, i.e. magnon flow in very specific directions, which result from flat bands occurring in the helimagnon band structure when the momentum component perpendicular to the helix axis is large. We show that the softness of the Goldstone mode leads to a large dissipation even at very small frequencies cut off only by magnetocrystalline anisotropies or by a magnetic field. Finally, we discuss that dipolar interactions induce non-reciprocal behavior of the spectrum at finite fields and momenta, i.e. the spectrum is not symmetric under reversing the momentum anymore. We calculate the Brillouin light scattering cross section and compare it to experimental results obtained by N. Ogawa [1]. Then, we consider reorientation processes of the helix axis due to an applied magnetic field. We compare the results to magnetic force microscopy measurements in Cu2OSeO3 performed by P. Milde et al. [2]. Afterwards, we point out that the skyrmion lattice orientation has singular points, i.e. points where the orientation is not determined, as a function of the magnetic field direction which is a consequence of the Poincaré-Hopf theorem. Afterwards, we turn to excitations in the form of the basic defects in helimagnets: disclinations and dislocations. Due to the lamellar nature of the helimagnetic texture, analogies to liquid crystals can often be used. We present an analytic parameterization of dislocations transferred from smectic liquid crystals and illustrate that dislocations carry a topological skyrmion charge. We examine dislocation motion in the presence of weak pinning due to random impurities. We derive a Thiele-Langevin equation for the dislocation position which effectively describes one dimensional motion. When reducing the system to two dimensions, this reveals ultra slow anomalous Sinai diffusion which may explain the very long time scales observed in several experiments [3,4]. Eventually, we present our work on domain walls in helimagnets. In magnetic force microscopy experiments performed by P. Schoenherr [5], we have identified three domain wall types. At small angles between the two domains, curvature walls appear. At intermediate angles, one can observe zig-zag disclination walls and at large angles, dislocation walls occur. We present analytical descriptions for curvature and dislocation walls, which we compare to micromagnetic simulation results obtained by J. Masell [5], and comment on the non-trivial topology of helimagnetic domain walls. [1] N. Ogawa, L. Köhler, M. Garst, S. Toyoda, S. Seki, and Y. Tokura, In preparation (2019). [2] P. Milde, E. Neuber, P. Ritzinger, L. Köhler, M. Garst, A. Bauer, C. Pfleiderer, H. Berger, and L. M. Eng, In preparation (2019). [3] A. Dussaux, P. Schoenherr, K. Koumpouras, J. Chico, K. Chang, L. Lorenzelli, N. Kanazawa, Y. Tokura, M. Garst, A. Bergman, C. L. Degen, and D. Meier, Nature Communications 7, 12430 (2016). [4] A. Bauer, A. Chacon, M. Wagner, M. Halder, R. Georgii, A. Rosch, C. Pfleiderer, and M. Garst, Physical Review B 95, 024429 (2017). [5] P. Schoenherr, J. Müller, L. Köhler, A. Rosch, N. Kanazawa, Y. Tokura, M. Garst, and D. Meier, Nature Physics 14, 465 (2018).:Introduction 1. Introduction to chiral magnets 1.1. Helimagnets 1.1.1. Magnetic phase diagram of chiral magnets 1.2. Skyrmions 1.2.1. Topology 1.2.2. Magnetic skyrmions 1.2.3. Skyrmion motion 1.2.4. Emergent electrodynamics 1.3. Model for chiral magnets 2. Spin waves in helimagnets 2.1. Linear spin wave theory for helimagnons 2.1.1. Fluctuations in the harmonic approximation 2.1.2. Spectrum at small momenta and fields 2.1.3. Frequency broadening from Gilbert damping 2.2. Magnon-focusing effects 2.3. Enhanced local dissipation 2.3.1. Global static susceptibility in the limit k, k' → 0 2.3.2. Local damping 2.4. Non-reciprocity 2.4.1. Non-reciprocity of the spectrum 2.4.2. Brillouin light scattering cross section 3. Orientation of magnetic order 3.1. Helix reorientation transition in MnSi 3.1.1. Effective Landau potential for the helix pitch 3.1.2. Experimental results 3.2. Helix reorientation in Cu2OSeO3 3.3. Skyrmion lattice orientation 4. Disclinations and dislocations 4.1. Liquid crystals 4.1.1. Types of liquid crystals 4.1.2. Energetics of liquid crystals 4.2. Disclinations 4.2.1. Elasticity theory for disclinations 4.3. Dislocations 4.3.1. Volterra process and Burgers vector 4.3.2. Elasticity theory for dislocations 4.3.3. Mermin-Ho relation in helimagnets 4.3.4. Topological skyrmion charge 5. Dislocation motion 5.1. Thiele approach for one helimagnetic dislocation 5.1.1. Motion in the presence of pinning 5.1.2. Corrections from elastic deformations 5.2. Dislocation diffusion 5.2.1. Sinai diffusion and toy model simulations 5.2.2. Susceptibility with Sinai diffusion 5.2.3. Dislocation string 6. Domain walls 6.1. Experimental and numerical methods 6.2. Domain wall types in helimagnets 6.3. Energetics of helimagnetic domain walls 6.3.1. Curvature wall 6.3.2. Dislocation wall 6.4. Topological domain wall structures 7. Discussion and outlook Appendix A. Details on helimagnons B. Formalism of linear-spin wave theory in helimagnets C. Deviations from the helix Bibliography List of Figures Index Danksagung

In this paper, we look at the shear-out failure of carbon fiber reinforced plastics connections in the automotive industry. Contrary to the aircraft industry, the boundary conditions of automotive applications favor this failure mode strongly. Moreover, the use of other joining technologies than that used in the aircraft industry, such as joining by forming, leads to new challenges. The different influences, typical for joining by forming, on ultimate shear-out strength were first investigated separately and then transferred and validated on connections related to praxis by an analytical model. Special attention was given to effects that resulted from oversized pre-holes, acting clamping forces, and the reduced quality of the laminates in the immediate vicinity of the joint due to the joining process.

Ferromagnetism in certain alloys with a crystalline B2-structure, such as Fe60Al40, can be switched on, and tuned, via antisite disordering of the atomic arrangement. This disorderinduced B2→A2 phase transition is accompanied by a ∼1% increase in the lattice parameter. The induced ferromagnetism can be switched off as well via atomic rearrangements causing the A2→B2 transition. In this thesis, the B2↔A2 phase transition will be manipulated by ion- as well as laser-irradiation. Ion-irradiation allows for a sensitive control of the degree of antisite disorder and thus can be applied to understand the correlation between gradual disorder and magnetic properties in the Fe60Al40 alloy. The reversibility of the laser-driven B2↔A2 transition will be shown in this work. B2-Fe60Al40 thin films have been disordered systematically by ion-irradiation and correlations between the chemical disorder (1-S), lattice parameter (a0), and the induced saturation magnetization (Ms) have been obtained. As the lattice is gradually disordered,a critical point occurs at 1-S=0.6 and a0=2.91Å, where a sharp increase of the Ms is observed. The regimes below and above the critical regime are characterized by a different, but nearly stable Ms behaving paramagnetic and ferromagnetic, respectively. Density functional theory (DFT) calculations suggest that below the critical point the system magnetically behaves as it would still be fully ordered, i.e. paramagnetic, whereas above, it is largely the increase of a0 in the disordered state that determines the Ms. Furthermore, disordered thin films possessing various open-volume defect types have been ordered via thermal annealing. The A2→B2 ordering process occurs by the vacancy diffusion mechanism and the ordering rate shows a strong dependence on the defect types, as obtained from ab-initio DFT calculations: The ordering rate is increased by mono-vacancies and decreased by triple defects and vacancy clusters. The defects can be engineered by a thermal pre-annealing and/or ion-irradiation offering a control of the subsequent ordering process. Additionally, the reversible disordering and subsequent reordering implying an on and off switching of ferromagnetism, respectively, is demonstrated by applying femtosecond laser pulse irradiation. The irradiation with a single laser pulse above the threshold fluence induces chemical disorder and hence ferromagnetism. A subsequent laser-irradiation below the threshold ŕuence causes a reordering at the surface erasing the ferromagnetism. The laser-irradiation leads to a melting and subsequent solidification of the material; if the solidification temperature is lower than the melting temperature, the liquid is supercooled. Simulations reveal the crucial role of the extent of supercooling: A single laser pulse above the threshold fluence causes a strong undercooling of the liquid phase before solidification limiting the vacancy diffusion and hence ordering. Laser pulsing below the threshold forms a limited supercooled surface region that solidifies at sufficiently high temperatures, enabling vacancy diffusion-assisted reordering.:1 Introduction and Fundamentals 1.1 Magnetism and Structure in Chemically Ordered Materials 1.1.1 Effects Induced by Chemical Disorder 1.1.2 Properties of Fe-Al Alloys 1.2 Modiőcation of B2 Materials 1.2.1 Interaction of Ions with Solids 1.2.2 Laser-Solid Interaction 1.3 Motivation 2 Experimental and Theoretical Methods 2.1 Sample Preparation 2.1.1 Magnetron Sputtering 2.1.2 Annealing Process 2.1.3 Ion-Irradiation 2.1.4 Laser-Irradiation 2.2 Structural Characterization 2.2.1 X-Ray Diffraction 2.2.2 Rutherford Backscattering Spectrometry 2.2.3 Transmission Electron Microscopy 2.3 Magnetic Characterization 2.3.1 Vibrating Sample Magnetometry 2.3.2 Spin-Resolved Photoemission Electron Microscopy 2.4 Defect Analysis by Positron Annihilation Spectroscopy 2.5 Theoretical Approaches 2.5.1 DFT Calculations on the Properties of Fe60Al40 2.5.2 Ab-initio Calculations of Positron Lifetimes in Fe60Al40 2.5.3 Simulations on the Laser-Irradiation of Fe60Al40 3 Unraveling Magneto-Structural Correlations 3.1 Characterization of Ordered B2 and Disordered A2 Films 3.1.1 Experiments 3.1.2 Structural Properties 3.1.3 Magnetic Properties 3.1.4 Summarizing Remarks 3.2 Systematic Disordering by Ion-Irradiation 3.2.1 Experiments 3.2.2 Structural Characterization 3.2.3 Analysis of Magnetic Properties 3.2.4 Correlation of Structural and Magnetic Properties 3.2.5 Comparison to Previously Reported Data 3.2.6 Theoretical Calculations 3.2.7 Discussion and Summarizing Remarks 4 Defect-Mediated Atomic Rearrangements 4.1 Experiments 4.2 Analysis of Magnetic Properties 4.3 Defect Characterization 4.4 Ab-initio Calculations of Positron Lifetimes 4.5 Discussion 5 Laser Pulse Induced Reversible Order-Disorder Transition 5.1 Experiments 5.2 Results 5.3 Simulations 5.4 Discussion 6 Conclusions / In bestimmten Legierungen mit einer kristallinen B2-Struktur, wie beispielsweise Fe60Al40, kann durch eine chemische Unordnung Ferromagnetismus erzeugt und modifiziert werden. Dieser durch Unordnung hervorgerufene B2→A2 Phasenübergang geht mit einer Vergrößerung des Gitterparameters von ungefähr 1% einher. Der erzeugte Ferromagnetismus kann durch eine atomare Neuordnung, d.h. durch den A2→B2 Phasenübergang, wieder abgeschaltet werden. In der vorliegenden Arbeit wird der B2↔A2 Phasenübergang mittels Ionen- und Laserbestrahlung hervorgerufen und kontrolliert. Ionenbestrahlung ermöglicht eine präzise Kontrolle des Unordnungsgrades und kann daher eingesetzt werden, um den Zusammenhang zwischen gradueller Unordnung und magnetischen Eigenschaften in der Fe60Al40 Legierung zu untersuchen. Die Reversibilität des laserinduzierten B2↔A2 Phasenübergangs wird in der vorliegenden Arbeit gezeigt. In B2-Fe60Al40 Dünnschichten ist mittels Ionenbestrahlung systematisch Unordnung erzeugt worden, wodurch die Zusammenhänge von atomarer Unordnung (1-S), dem Gitterparameter (a0) und der erzeugten Magnetisierung (Ms) offengelegt worden. Während der schrittweisen Unordnung des Kristallgitters tritt ein kritischer Punkt bei 1-S=0.6 und a0=2.91Å auf, an welchem Ms stark ansteigt. Die Bereiche unter- und oberhalb des kritischen Bereiches sind durch ein unterschiedliches, aber nahezu gleichbleibendes Ms charakterisiert. Das Verhalten ist para- bzw. ferromagnetisch. Berechnungen mittels Dichtefunktionaltheorie (DFT) deuten an, dass sich das System unterhalb des kritischen Punktes verhält, als wäre es noch vollständig geordnet, d.h. paramagnetisch; wohingegen Ms oberhalb des kritischen Bereiches größtenteils durch den Anstieg des Gitterparameters bestimmt wird. Darüber hinaus sind ungeordnete Dünnschichten mit verschiedenen Typen leerstellenartiger Defekte mittels Wärmebehandlung neu geordnet worden. Der A2→B2 Ordnungsprozess geschieht auf der Basis des Leerstellendiffusionsmechanismus. Die starke Abhängigkeit der Ordnungsrate von den Defekttypen ist mittels ab-initio DFT Berechnungen bestätigt worden: Die Ordnungsrate wird durch Einzel-Leerstellen erhöht und sinkt durch Dreifach-Defekte und Leerstellencluster. Die Defekte können durch eine vorherige Wärmebehandlung und/oder Ionenbestrahlung manipuliert werden, wodurch der darauffolgende Ordnungsprozess kontrolliert werden kann. Des Weiteren wird die reversible Unordnung und anschließende Neuordnung, d.h. ein Erzeugen bzw. Abschalten von Ferromagnetismus, durch die Bestrahlung mit Femtosekunden- Laserpulsen demonstriert. Die Bestrahlung mit einem einzelnen Laserpuls mit einer Fluenz über dem Schwellenwert erzeugt atomare Unordnung und damit Ferromagnetismus. Eine anschließende Laserbestrahlung unterhalb der Schwellenŕuenz bewirkt eine Neuanordnung der Atome an der Oberfläche und damit einem Auslöschen des Ferromagnetismus. Die Laserbestrahlung führt zu einem Aufschmelzen und dem anschließenden Erstarren des Materials; liegt die Erstarrungstemperatur unterhalb der Schmelztemperatur, so ist die Schmelze unterkühlt. Die ausschlaggebende Rolle des Grades der Unterkühlung wird durch Simulationen aufgezeigt: Ein einzelner Laserpuls über der Schwellenfluenz führt zu einer starken Unterkühlung der flüssigen Phase vor der Erstarrung, wodurch die Leerstellendiffusion und damit die atomare Neuordnung eingeschränkt werden. Durch Laserpulse unterhalb der Schwellenfluenz wird der Oberŕächenbereich kaum unterkühlt und erstarrt anschließend bei hinreichend hohen Temperaturen, um eine leerstellendiffusionsunterstützte Neuanordnung der Atome zu ermöglichen.:1 Introduction and Fundamentals 1.1 Magnetism and Structure in Chemically Ordered Materials 1.1.1 Effects Induced by Chemical Disorder 1.1.2 Properties of Fe-Al Alloys 1.2 Modiőcation of B2 Materials 1.2.1 Interaction of Ions with Solids 1.2.2 Laser-Solid Interaction 1.3 Motivation 2 Experimental and Theoretical Methods 2.1 Sample Preparation 2.1.1 Magnetron Sputtering 2.1.2 Annealing Process 2.1.3 Ion-Irradiation 2.1.4 Laser-Irradiation 2.2 Structural Characterization 2.2.1 X-Ray Diffraction 2.2.2 Rutherford Backscattering Spectrometry 2.2.3 Transmission Electron Microscopy 2.3 Magnetic Characterization 2.3.1 Vibrating Sample Magnetometry 2.3.2 Spin-Resolved Photoemission Electron Microscopy 2.4 Defect Analysis by Positron Annihilation Spectroscopy 2.5 Theoretical Approaches 2.5.1 DFT Calculations on the Properties of Fe60Al40 2.5.2 Ab-initio Calculations of Positron Lifetimes in Fe60Al40 2.5.3 Simulations on the Laser-Irradiation of Fe60Al40 3 Unraveling Magneto-Structural Correlations 3.1 Characterization of Ordered B2 and Disordered A2 Films 3.1.1 Experiments 3.1.2 Structural Properties 3.1.3 Magnetic Properties 3.1.4 Summarizing Remarks 3.2 Systematic Disordering by Ion-Irradiation 3.2.1 Experiments 3.2.2 Structural Characterization 3.2.3 Analysis of Magnetic Properties 3.2.4 Correlation of Structural and Magnetic Properties 3.2.5 Comparison to Previously Reported Data 3.2.6 Theoretical Calculations 3.2.7 Discussion and Summarizing Remarks 4 Defect-Mediated Atomic Rearrangements 4.1 Experiments 4.2 Analysis of Magnetic Properties 4.3 Defect Characterization 4.4 Ab-initio Calculations of Positron Lifetimes 4.5 Discussion 5 Laser Pulse Induced Reversible Order-Disorder Transition 5.1 Experiments 5.2 Results 5.3 Simulations 5.4 Discussion 6 Conclusions

The nature and role of defects is of primary importance to understand the physical properties of C and BN single walled nanotubes. Transmission electron microscopy (TEM) is a well known powerful tool to study the structure of defects in materials. However, in the case of SWNTs, the electron irradiation of the TEM may knock out atoms. This effect may alter the native structure of the tube, and has also been proposed as a potential tool for nanoengineering of nanotubular structures. Here we develop a theoretical description of the irradiation mechanism. First, the anisotropy of the emission energy threshold is obtained via density functional based calculations. Then, we numerically derive the total Mott cross section for different emission sites of carbon and boron nitride nanotubes with different chiralities. Using a dedicated STEM microscope with experimental conditions optimised on the basis of derived cross-sections, we are able to control the generation of defects in nanotubular systems. Either point or line defects can be obtained with a spatial resolution of a few nanometers. The structure, energetics and electronics of point and line defects in BN systems have been investigated. Stability of mono- and di- vacancy defects in hexagonal boron nitride layers is investigated, and their activation energies and reaction paths for diffusion have been derived using the nudged elastic band method (NEB) combined with density functional based techniques. We demonstrate that the appearance of extended linear defects under electron irradiation is more favorable than a random distribution of point defects and this is due to the existence of preferential sites for atom emission in the presence of pre-existing defects, rather than thermal vacancy nucleation and migration.

Hafnium oxide is widely used for resistive switching devices, and recently it has been discovered that ferroelectricity can be established in (un-)doped hafnium oxide as well. Previous studies showed that both switching mechanisms are influenced by oxygen vacancies. For resistive switching, typically amorphous oxide layers with an asymmetric electrode configuration are used to create a gradient of oxygen vacancies. On the other hand, ferroelectric switching is performed by having symmetric electrodes and requires crystalline structures. The coexistence of both effects has recently been demonstrated. In this work, a detailed analysis of the reversible interplay of both switching mechanisms within a single capacitor cell is investigated. First, ferroelectric switching cycles were applied in order to drive the sample into the fatigued stage characterized by increased concentration of oxygen vacancies in the oxide layer. Afterwards, a forming step that is typical for the resistive switching devices was utilized to achieve a soft breakdown. In the next step, twofold alternation between the high and low resistance state is applied to demonstrate the resistive switching behavior of the device. Having the sample in the high resistance state with a ruptured filament, ferroelectric switching behavior is again shown within the same stack. Interestingly, the same endurance as before was observed without a hard breakdown of the device. Therefore, an effective sequence of ferroelectric—resistive—ferroelectric switching is realized. Additionally, the dependence of the forming, set, and reset voltage on the ferroelectric cycling stage (pristine, woken-up and fatigued) is analyzed giving insight into the physical device operation.

The discovery of ferroelectricity in hafnium oxide has revived the interest in ferroelectric memories as a viable option for low power non-volatile memories. However, due to the high coercive field of ferroelectric hafnium oxide, instabilities in the field cycling process are commonly observed and explained by the defect movement, defect generation and field induced phase transitions. In this work, the optical and transport experiments are combined with ab-initio simulations and transport modeling to validate that the defects which act as charge traps in ferroelectric active layers are oxygen vacancies. A new oxygen vacancy generation leads to a fast growth of leakage currents and a consequent degradation of the ferroelectric response in Hf₀.₅Zr₀.₅O₂ films. Two possible pathways of the Hf₀.₅Zr₀.₅O₂ ferroelectric property degradation are discussed.

Manifold research has been done to understand the detailed mechanisms behind the performance instabilities of ferroelectric capacitors based on hafnia. The wake-up together with the imprint might be the most controversially discussed phenomena so far. Among crystallographic phase change contributions and oxygen vacancy diffusion, electron trapping as the origin has been discussed recently. In this publication, we provide evidence that the imprint is indeed caused by electron trapping into deep states at oxygen vacancies. This impedes the ferroelectric switching and causes a shift of the hysteresis. Moreover, we show that the wake-up mechanism can be caused by a local imprint of the domains in the pristine state by the very same root cause. The various domain orientations together with an electron trapping can cause a constriction of the hysteresis and an internal bias field in the pristine state. Additionally, we show that this local imprint can even cause almost anti-ferroelectric like behavior in ferroelectric films.

In this report, the operation of a normally-off vertical gallium nitride (GaN) metal-oxide field effect transistor with a threshold voltage of 5 V is demonstrated. A crucial step during device fabrication is the formation of the highly n-doped source layer. The authors infer that the use of molecular beam epitaxy (MBE) is highly beneficial for suppressing diffusion of the magnesium (Mg) p-type dopants from the body layer grown by metal-organic vapor phase epitaxy into the source cap. Repassivation of the previously activated Mg acceptors by a hydrogen out-diffusion treatment is suppressed in the ultrahigh vacuum growth environment. Structural and electrical data indicate that the defect density of the GaN substrate is currently limiting device performance much more compared to other effects like varying surface morphology resulting from fluctuations in III/N stoichiometry during the MBE growth.