Orientador: Mario Antonio Bica de Moraes / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-09T10:56:06Z (GMT). No. of bitstreams: 1 Gelamo_RogerioValentim_D.pdf: 1517991 bytes, checksum: fd93a7ffa508ced1257f9474d2674aed (MD5) Previous issue date: 2007 / Resumo: Filmes finos de polisiloxanos, polisilazanos e policarbosilanos, sintetizados por Deposição Química de Vapor Assistida por Plasma (PECVD), foram irradiados com os íons He +, Ne +, Ar +e Kr +, com energia de 170 keV, e fluências de 1x10 14 , 5x10 14 , 1x10 15 , 5x10 15e 1x10 16 íons/cm 2 . A irradiação iônica promoveu modificações significativas na composição elementar, na estrutura química, e consequentemente nas propriedades físicas dos filmes. Com o uso de espectroscopias de retro-espalhamento Rutherford (RBS) e de recuo frontal (FRS), observou-se que as razões atômicas C/Si, O/Si, N/Si e H/Si sofreram modificações. Em especial, a razão H/Si foi drasticamente diminuída, devido à perda de hidrogênio causada pela irradiação. Oxigênio foi quimicamente incorporado aos filmes, devido à recombinação das ligações pendentes, formadas durante a irradiação, com o ar ambiente. Com relação à estrutura química dos filmes, extinção e formação de novos grupos e de ligações químicas foram observadas com o uso de espectroscopias infravermelha no modo reflexão-absorção (IRRAS) e de fotoelétrons excitados por raios-X (XPS). A densidade volumétrica dos filmes aumentou significativamente com a irradiação. As constantes ópticas, medidas através de espectroscopia ultravioleta-visível e elipsometria, foram também afetadas. Com o aumento da fluência dos íons, o coeficiente de absorção e o índice de refração aumentaram e o gap óptico diminuiu. Medidas de nanoindentação mostraram notáveis aumentos nas durezas dos filmes. Nas mais altas fluências utilizadas, a dureza chegou a valores comparáveis e até maiores que a dos aços ferramenta. Os filmes são convertidos de polímero para a fase cerâmica e a intensidade da conversão é dependente da fluência dos íons. Observou-se ainda que, de modo geral, as modificações são fortemente dependentes das massas dos íons, já que as modificações mais significativas são obtidas com o uso de He+ e Ne+ . A explicação referente a esse efeito é dada em função das transferências de energia eletrônica e nuclear / Abstract: Thin films of polysiloxanes, polysilazanes and polycarbosilanes, synthesized by Plasma Enhanced Chemical Vapor Deposition (PECVD), were irradiated with 170 keV He + , Ne + , Ar + and Kr + ions, at 170 keV at fluences of 1x10 14 , 5 x10 14 , 1x10 15 , 5x10 15 and 1 x10 16 ions/cm -2 . The irradiation promoted significant modifications in the atomic composition, chemical structure, and consequently in the physical properties of the films. Changes in the atomic composition were examined using Rutherford back-scattering spectroscopy (RBS) and forward recoil spectroscopy (FRS). The former was used to determine the C/Si, N/Si and O/Si atomic ratios, while the H/Si ratio was measured by the latter. As a general behavior, these ratios changed with ion irradiation and the decrease in the H/Si ratio was particularly high, as hydrogen was drastically removed by ion bombardment. Oxygen was chemically incorporated into the films due to the reactions involving dangling bonds formed during irradiation, and ambient air. Regarding the chemical structure of the films, extinction and formation of new bonding groups and chemical bonds were observed as a function of the ion fluence using infrared reflection-absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). The volume density of the films increased significantly with irradiaton. The optical constants, determined using ultraviolet-visible spectroscopy and ellipsometry, were also affected by ion irradiation. With increasing ion fluence, the absorption coefficient and refractive index increased, and the optical gap decreased. From nanoindentation measurements. remarkable increases in surface hardness were determined. For the higher fluences, the surface hardness of the films is in the range, or even higher, of that of martensitic tool steels. Thus, ion irradiation changed the relatively soft polymer film into a high density, hard, ceramic material. It was observed that the most significant modifications occur for He+ and Ne+ ions. An explanation to this finding is offered in terms of the electronic and nuclear energy transfer functions / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Orientador: Fernando Alvarez / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-14T22:51:20Z (GMT). No. of bitstreams: 1 SaezAcuna_JoseJavier_D.pdf: 8414775 bytes, checksum: 9dca5b5e710ad86a9f6375002fc43faa (MD5) Previous issue date: 2009 / Resumo: Este trabalho apresenta um estudo sobre a Emissão Eletrônica de amostras nanoestruturadas de Carbono sobre a ação de um campo elétrico controlado. Particularmente foram estudadas nanoes- truturas de Carbono tipo Nanotubos (CNTs). Com o intuito de aprimorar as propriedades elétricas destas nano~struturas, também são apresentados estudos sobre os efeitos da irradiação com feixes iônicos de 02+ sobre a superfície local dos emissores. Este processo, que por um lado é um pós- tratamento de purificação, ou remoção seletiva dos defeitos estruturais, e, ao mesmo tempo, um pós- tratamento de dopagem da superfície, ajudando assim na emissão eletrônica. Nos processos de pós-tratamento foram utilizados íons de 02+, N2+, H2+ e combinações destes, todos irradiando com um feixe iônico controlado e in situ após crescimento das nanoestruturas. Sem quebra de vácuo, estas irradiações foram analisadas por espectroscopia de elétrons fotoemitidos por raios-X incidentes, técnica conhecida como XPS. Uma vez as amostras fora do vácuo, elas foram anali- sadas por espectroscopia Raman, microscopia eletrônica de varredura (FEG-SEM) e microscopia eletrônica de transmissão de alta resolução (HR- TEM). Para a análise da Emissão Eletrônica várias complicações indesejáveis foram com sucesso supe- radas, como, por exemplo, efeitos de borda dos eletrodos que nos levam a cálculos errados da área efetiva de emissão; incertezas do número local de pontas emissoras produto do baixo controle do crescimento das nanoestruturas; como também muitas controvérsias sobre a forma correta de se analisar os dados experimentais. Em relação a este último ponto fizemos um experimento chave no discemimento do correto uso do modelo teórico para a Emissão Eletrônica a partir de sua dedução feita por Fowler e Nordheim. Demonstraremos que o campo elétrico limiar (the threshold electric field), o parâmetro usado para comparar os dados experimentais, não é um parâmetro confiável. Isto foi observado ao medirmos uma única amostra mudando apenas a configuração geométrica do eletrodo usado para efetuar as medições. Este estudo detalhado deixou em evidência fenômenos da emissão que antes não eram perceptíveis, e nos permitiu encontrar a forma correta de analisar os dados expe- rimentais. Propomos então uma norma comparativa para a Emissão Eletrôn1ca, a qual utilizamos nesta Tese. Esta norma correlaciona-se muito bem com todas as outras técnicas acima mencionadas / Abstract: This work presents a study of the Electron Field Emission of carbon nanostructured samples on the action of an Electric Field controlled. The Carbon Nanotubes (CNTs) was the nanostructures studied. In order to improve the electrical properties of these nanostructures are also presented studies on the effects of irradiation with O2+ ion beams on the surface emitter. This process, which is a post-purification treatment, or selective removal of structural defects, is also a doping-treatment of the surface helping in electronic properties. In cases of ion bean post-treatment were used O2+, N2+, H2+ ions and combinations of these, all radiating with an ion beam controlled in situ after growth of the nanostructures. Without breaking vacuum, the samples were analyzed by X-ray photoelectron spectroscopy, technique known as XPS. Once the samples were outside the vacuum, they were analyzed by Raman spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (FEG-SEM), and high-resolution transmission electron microscopy (HR- TEM). For the analysis of the Electron Emission properties several undesirable complications have been successfully overcome, such as edge effects of the electrodes that lead to miscalculations of the effective area emission, uncertainties in the local number emitter tips due to poor control of the growth of nanostructures, as well as many controversies about the correct analysis of experimental data. On this last point, we have made a key experiment to discem the proper use of the theoretical model for Electron Emission from the deduction by Fowler-Nordheim. Here we demonstrate that the Threshold Electric Field, the parameter used to compare the experimental data, is not a reliable parameter. This was found when we made our measurements on one single sample, only changing the geometric configuration of the electrode used in the measurement. This performed with meticulous care and detail has left evidence of emission phenomena that were not obvious, and allowed us to find the correct way to analyze experimental data. From here we propose a comparative standard parameter for Electron Emission properties, and that we use in this thesis correlating very well with all the other techniques mentioned above / Doutorado / Física de Plasmas e Descargas Elétricas / Doutor em Ciências

Although ferromagnets are found in all kinds of technological applications, their natural occurrence is rather unusual because only few substances are known to be intrinsically ferromagnetic at room temperature. In the past twenty years, a plethora of new artificial ferromagnetic materials has been found by introducing defects into non-magnetic host materials. In contrast to the intrinsic ferromagnetic materials, they offer an outstanding degree of material engineering freedom, provided one finds a type of defect to functionalize every possible host material to add magnetism to its intrinsic properties. Still, some controversial questions remain: What are the mechanisms behind these ferromagnetic materials? Why are their magnetization values reported in the literature so low? Are these materials really technologically relevant ferromagnets? In this work, we aim to provide a systematic investigation of the phenomenon. We propose a universal scheme for the computational discovery of new artificial functional magnetic materials, which is guided by experimental constraints and based on first principles. The obtained predictions explain very well the experimental data found in the literature. The potential of the method is further demonstrated by the experimental realization of a truly 2D ferromagnetic phase at room temperature, created in nominally non-magnetic TiO$_2$ films by ion irradiation, which follows a characteristic 2D magnetic percolation transition and exhibits a tunable magnetic anisotropy. Furthermore, the technological relevance of these artificial ferromagnetic materials, which comes to shine when one combines the engineered magnetic with some of the intrinsic properties of the host material, is demonstrated by creating a spin filter device in a ZnO host that generates highly spin-polarized currents even at room temperature.:1 Introduction 2 Computational discovery of artificial ferromagnets 2.1 Ferromagnetism in solids 2.1.1 Exchange interaction and magnetic order 2.1.2 Artificial magnetism due to defects 2.2 Predicting defect structures from collision cascades 2.3 Finding magnetic defect candidates 2.4 Magnetic percolation 2.5 Magnetic phase diagram of anatase TiO 2 artificial ferromagnet 2.5.1 Defect creation in anatase TiO 2 2.5.2 Magnetic properties of dFP defects in anatase TiO 2 2.5.3 Constructing a magnetic phase diagram 2.6 Revisiting prior experimental results 3 Artificial ferromagnetism in TiO 2 hosts 3.1 Low energy ion irradiation 3.2 SQUID magnetometry 3.3 Experimental realization of an artificial ferromagnet in TiO2 4 Artificial magnetic monolayers and surface effects 4.1 Critical behavior and 2D magnetism 4.2 Magnetic anisotropy 4.2.1 Demagnetizing field and magnetic shape anisotropy 4.2.2 Magnetocrystalline anisotropy 4.3 Artificial ferromagnetic monolayer at TiO 2 surface with perpendicular magnetic anisotropy 4.4 DFT calculations of the defective anatase TiO 2 [001] surface 5 Spin transport through artificial ferromagnet interfaces 5.1 Artificial ferromagnetism in ZnO hosts 5.2 Spin filter effect at magnetic/non-magnetic interfaces in ZnO 5.2.1 The spin filter effect 5.2.2 Lithium and hydrogen doping in ZnO 5.2.3 Magneto-transport in artificial ferromagnetic Li:ZnO microwires 5.2.4 Spin transport through magnetic/non-magnetic interfaces 5.2.5 Minority spin filter effect 6 Conclusions and Outlook Bibliography Appendix: A List of publications B Computation inputs and codes B.1 DFT electronic structure calculations - Fleur input files B.2 Magnetic Percolation simulations B.3 SQUID raw data analysis code B.4 SRIM Monte Carlo binary collision code automation / Obwohl Ferromagnete in allen möglichen technischen Anwendungen zu finden sind, ist ihr natürliches Vorkommen eher ungewöhnlich, da nur wenige Stoffe bekannt sind, die bei Raumtemperatur intrinsisch ferromagnetisch sind. In den letzten zwanzig Jahren wurde eine Fülle neuer künstlicher ferromagnetischer Materialien durch das Einbringen von Defekten in nichtmagnetische Wirtsmaterialien entdeckt. Im Gegensatz zu den intrinsischen ferromagnetischen Materialien bieten sie einen herausragenden Grad an materialtechnischer Freiheit, vorausgesetzt man findet zu jedem möglichen Wirtsmaterial einen passenden Typus von Defekten, um dessen intrinsische Eigenschaften um Magnetismus zu ergänzen. Dennoch bleiben einige kontroverse Fragen bislang unbeantwortet: Welche Mechanismen stehen hinter diesen ferromagnetischen Materialien? Warum werden ihre Magnetisierungswerte in der Literatur meist so niedrig angegeben? Sind diese Materialien wirklich technologisch relevante Ferromagneten? In dieser Arbeit wollen wir eine systematische Untersuchung des Phänomens durchführen. Wir schlagen ein universelles ab-initio Protokoll für die computergestützte Entdeckung von neuen künstlichen funktionalen magnetischen Materialien vor, das sich an experimentellen Bedingungen orientiert. Die erhaltenen Vorhersagen erklären die in der Literatur gefundenen experimentellen Daten sehr gut. Wir demonstrieren die Wirksamkeit der Methode durch die experimentelle Realisierung einer echten 2D-ferromagnetischen Phase bei Raumtemperatur, die in nominell nicht-ma'-gne'-tischen TiO$_2$-Filmen durch Ionenbestrahlung erzeugt wird. Die so entstehende ferromagnetische Phase folgt einem charakteristischen zweidimensionalen magnetischen Perkolationsprozess und weist eine steuerbare magnetische Anisotropie auf. Weiterhin wird die technologische Relevanz dieser künstlichen ferromagnetischen Materialien gezeigt, welche besonders zum Vorschein kommt, wenn man die künstlichen magnetischen mit einigen der intrinsischen Eigenschaften des Wirtsmaterials kombiniert, und zwar indem ein Spin-Filter Element auf Basis eines ZnO-Wirts gebaut wird, das selbst bei Raumtemperatur hoch spin-polarisierte Ströme erzeugt.:1 Introduction 2 Computational discovery of artificial ferromagnets 2.1 Ferromagnetism in solids 2.1.1 Exchange interaction and magnetic order 2.1.2 Artificial magnetism due to defects 2.2 Predicting defect structures from collision cascades 2.3 Finding magnetic defect candidates 2.4 Magnetic percolation 2.5 Magnetic phase diagram of anatase TiO 2 artificial ferromagnet 2.5.1 Defect creation in anatase TiO 2 2.5.2 Magnetic properties of dFP defects in anatase TiO 2 2.5.3 Constructing a magnetic phase diagram 2.6 Revisiting prior experimental results 3 Artificial ferromagnetism in TiO 2 hosts 3.1 Low energy ion irradiation 3.2 SQUID magnetometry 3.3 Experimental realization of an artificial ferromagnet in TiO2 4 Artificial magnetic monolayers and surface effects 4.1 Critical behavior and 2D magnetism 4.2 Magnetic anisotropy 4.2.1 Demagnetizing field and magnetic shape anisotropy 4.2.2 Magnetocrystalline anisotropy 4.3 Artificial ferromagnetic monolayer at TiO 2 surface with perpendicular magnetic anisotropy 4.4 DFT calculations of the defective anatase TiO 2 [001] surface 5 Spin transport through artificial ferromagnet interfaces 5.1 Artificial ferromagnetism in ZnO hosts 5.2 Spin filter effect at magnetic/non-magnetic interfaces in ZnO 5.2.1 The spin filter effect 5.2.2 Lithium and hydrogen doping in ZnO 5.2.3 Magneto-transport in artificial ferromagnetic Li:ZnO microwires 5.2.4 Spin transport through magnetic/non-magnetic interfaces 5.2.5 Minority spin filter effect 6 Conclusions and Outlook Bibliography Appendix: A List of publications B Computation inputs and codes B.1 DFT electronic structure calculations - Fleur input files B.2 Magnetic Percolation simulations B.3 SQUID raw data analysis code B.4 SRIM Monte Carlo binary collision code automation

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Since their discovery at the beginning of the 21st century, two-dimensional (2D) materials have emerged as one of the most exciting material groups offering unique properties which promise a plethora of potential applications in nanoelectronics, quantum computing, and surface science. The progress in the study of 2D materials has advanced rapidly stimulated by the ever-growing interest in their behavior and the fact that they are the ideal specimen for transmission electron microscopy (TEM), as their geometry allows to identify every single atom. Their morphology – 2D materials consist of “surface” only – at the same time makes them sensitive to beam damage, since high-energy electrons easily sputter atoms and introduce defects. While this is in general not desirable – as non-destructive imaging is aimed at – it allows to precisely quantify the damage in TEM and even pattern the 2D material with atomic resolution using the electron beam. Alternatively, patterning of 2D materials can be achieved using focused ion irradiation, which makes studying its effect on 2D materials relevant and essential. In this thesis, we theoretically study the effects of electron and ion irradiation on 2D materials, exemplarily on 2D MoS2 . Specifically, we address the combined effect of electronic excitations and direct momentum transfer by high-energy electrons (knock-on damage) in 2D MoS2 using advanced first-principles simulation techniques, such as Ehrenfest dynamics based on time-dependent density functional theory (DFT). Here, we stress the importance of the combined effect of ionization damage and knock-on damage as neither of these alone can account for experimentally-observed defect production below the displacement threshold – the minimum energy required for the displacement of an atom from the pristine system. A mechanism of defect production relying on the localization of the electronic excitation at the emerging vacancy site is presented. The localized excitation eventually leads to a significant drop in the displacement threshold. The combination of electronic excitation and knock-on damage may in addition to beam-induced chemical etching explain the observed sub-threshold damage in low voltage TEM experiments. Apart from non-destructive imaging, electrons may be used to modify the 2D material intentionally. In this light, we consider the electron-beam driven phase transformation in 2D MoS2 , where the semiconducting polymorph transforms into its metallic counterpart. The phase energetics and a possible transformation mechanism under electron irradiation are investigated using DFT based first-principles calculations. The detailed understanding of the interaction of the electron beam with the 2D material promises to improve the patterning resolution enabling circuit design on the nanoscale. Ion irradiation employed in focussed ion beams (FIB), e.g., the helium ion microscope (HIM) constitutes another tool widely used to pattern and even image 2D materials. Ion bombardment experiment usually carried out for the 2D material placed on a substrate are frequently rationalized using simulations for free-standing systems neglecting the effect of the substrate. Combining Monte Carlo with analytical potential molecular dynamics simulations, we demonstrate that the substrate plays a crucial role in damage production under ion irradiation and cannot be neglected. Especially for light ions such as He and Ne, which are usually used in the HIM, the effect of the substrate needs to be considered to account for the increased number of defects and their broadened spatial distribution which limits the patterning resolution for typical HIM energies. / Seit ihrer Entdeckung Anfang des 21. Jahrhunderts haben sich zwei-dimensionale (2D) Materialien zu einer der spannendsten Materialklassen im Forschungsfeld aus Materialwissenschaft, Physik und Chemie entwickelt. Ihre einzigartigen Eigenschaften versprechen eine Vielzahl potentieller Anwendungen in der Nanoelektronik, für Quantencomputer und in der Oberflächenwissenschaft. Beflügelt durch das wachsende Interesse an ihrem Verhalten und der Tatsache, dass sie die idealen Proben für die Transmissions-Elektronen-Mikroskopie (TEM) darstellen – ihre Geometrie erlaubt es, jedes einzelne Atom zu identifizieren – sind die Forschungen an 2D-Materialien rapide vorangeschritten. Ihre Morphologie – 2D-Materialien bestehen nur aus “Oberfläche” – bedingt zugleich ihre Sensitivität bezüglich Strahlschäden. Hochenergetische Elektronen lösen sehr leicht Atome aus dem 2D-Material und induzieren Defekte. Obwohl dies im Allgemeinen unerwünscht ist – Ziel ist eine nicht-destruktive Bildgebung – erlaubt es doch präzise Einblicke in die Schadensentstehung im TEM. Überdies können 2D-Materialien mit Hilfe des Elektronenstrahls mit atomarer Auflösung strukturiert werden. Alternativ kann die Strukturierung des 2D-Materials über fokussierte Ionenstrahlung erfolgen, weshalb es lohnenswert erscheint, auch deren Effekt auf 2D-Materialien zu untersuchen. In dieser Arbeit werden die Effekte von Elektronen- und Ionenstrahlung auf 2D-Materialien aus theoretischer Sicht exemplarisch an 2D-MoS2 untersucht. Besonderes Augenmerk liegt dabei auf dem kombinierten Effekt von elektronischer Anregung und dem direkten Impulsübertrag durch hochenergetische Elektronen (Kollisionsschaden) in 2D-MoS2 , der durch die Anwendung von Ab-Initio-Simulationstechniken wie der Ehrenfest-Molekulardynamik, basierend auf zeitabhängiger Dichtefunktionaltheorie (DFT), studiert wird. Dabei liegt die Betonung auf der Kombination beider Effekte, da weder Ionisierungs- noch Kollisionsschäden allein die experimentell beobachtete Defekterzeugung unterhalb der Displacement Threshold – der notwendigen Mindestenergie, um ein Atom aus dem reinen Material herauszulösen – erklären. Ein möglicher Mechanismus der Defekterzeugung, basierend auf der Lokalisierung der elektronischen Anregung an der entstehenden Vakanzstelle, wird vorgeschlagen. Die lokalisierte Anregung führt dabei schließlich zu einem signifikanten Absinken der Displacement Threshold. Die Kombination von elektronischer Anregung und Kollisionsschaden trägt neben strahlinduzierten chemischen Reaktionen zur Erklärung der beobachteten Schäden unterhalb der Displacement Threshold in Niederspannungs-TEM-Experimenten bei. Neben nicht-destruktiver Bildgebung können Elektronenstrahlen auch dafür benutzt werden, 2D-Materialien gezielt zu modifizieren. In diesem Sinne wird der elektronenstrahl-induzierte Phasenübergang in 2D-MoS2 , bei dem sich das Material von einem halbleitenden in einen metallischen Zustand transformiert, betrachtet. Die Phasenenergetik und ein möglicher Transformationsmechanismus werden unter Zuhilfenahme von DFT-basierten Ab-Initio-Simulationen untersucht. Das detaillierte Verständnis der Interaktion des Elektronenstrahls mit dem 2D-Material verspricht dabei die Strukturierungsauflösung zu verbessern und ermöglicht Schaltkreisdesign auf der Nanoskala. Fokussierte Ionenstrahlen, wie sie in Ionenstrahlinstrumenten – wie dem Helium-Ionen-Mikroskop (HIM) zum Einsatz kommen – stellen ein weiteres häufig verwendetes Werkzeug zur Modifikation sowie zur Bildgebung von 2D-Materialien dar. Ionenstrahlexperimente – üblicherweise mit dem auf einem Substrat platzierten 2D-Material durchgeführt – werden hingegen oft mit Simulationen für freistehende 2D-Materialien rationalisiert, wobei jegliche Einwirkung des Substrats vernachlässigt wird. Die Kombination von Monte-Carlo-Simulationen mit Molekulardynamik-Simulationen (auf der Basis analytischer Potentiale) in dieser Arbeit verdeutlicht, dass das Substrat eine wichtige Rolle in der Defekterzeugung spielt und nicht vernachlässigt werden kann. Besonders für leichte Ionen, wie He und Ne, wie sie typischerweise im HIM zum Einsatz kommen, sollte der Effekt des Substrats berücksichtigt werden. Dieses führt für typische Ionenenergien im HIM – im Vergleich zum freistehenden 2D-Material – zu einer ansteigenden Anzahl an Defekten und einer breiteren räumlichen Defektverteilung, welche die Strukturierungsauflösung begrenzt.

Irradiation of metals with energetic particles causes heavy damage effects in microstructure and mechanical properties, which is closely associated with irradiation conditions, presence of impurities, and microstructural features. It has been proposed that the radiation tolerance of a certain material can be enhanced by introducing a high density of interfaces, acting as ‘sinks’ that can frequently involve in attracting, absorbing and annihilating defects. Nanostructured materials with large volume fraction of interfaces, therefore, are assumed to be more radiation tolerant than conventional materials. This thesis focuses on the radiation damage effects in nanostructured Cu via the methods of in-situ TEM (transmission electron microscope) radiation experiments, postirradiation TEM analyses, small-mechanical tests (nanoindentation and micro-pillar compression), and computer simulations (molecular dynamics and phase-field modeling). We design and fabricate nanostructured Cu using direct current (DC) magnetron sputtering deposition technique, a typica physical vapor deposition (PVD) method and a bottom-up way to construct various nanostructured metals. High-density twin boundaries (TBs) and nanovoids (NVs) are introduced into two distinct nanostructured Cu films, including nanovoid-nanotwinned (NVNT) Cu (111) and nanovoid (NV) Cu (110). The in-situ high-energy Kr⁺⁺ (1 MeV) and ex-situ low energy He⁺ (< 200 keV) irradiations are subsequently preformed on the as-deposited Cu samples. On the one hand, the in-situ TEM observations suggest that TBs and NVs can influence the formation, distribution and stability of radiation-induced defects. Meanwhile, the preexisting microstructures also undergo structural change through void shrinkage and twin boundary migration. On the other hand, the ex-situ micro-pillar compression tests reveal that the Heirradiated NV-NT Cu contains less defect clusters but experiences more radiation-induced hardening. The underlying mechanisms of void shrinkage, twin boundary migration, and radiationinduced hardening are fully discussed based on post-irradiation analyses and computer simulations.

Résumé : L’isolation thermique est essentielle dans de nombreux types de MEMS (micro-systèmes électro-mécaniques). Elle permet de réduire la consommation d’énergie, améliorer leurs performances, ou encore isoler la zone chaude du reste du dispositif, ce qui est essentiel dans les systèmes sur puce. Il existe quelques matériaux et techniques d’isolation pour les MEMS, mais ils sont limités. En effet, soit ils ne proposent pas un niveau d’isolation suffisant, sont trop fragiles, ou imposent des contraintes trop importantes sur la conception du dispositif et sont difficiles à intégrer. Une approche intéressante pour l’isolation, démontrée dans la littérature, est de fabriquer des pores de taille nanométrique dans le silicium par gravure électrochimique. En nanostructurant le silicium ainsi, on peut diviser sa conductivité thermique par un facteur de 100 à 1000, le transformant en isolant thermique. Cette solution est idéale pour l’intégration dans les procédés de fabrication existants des MEMS, car on garde le silicium qui est déjà utilisé pour leur fabrication, mais en le nanostructurant localement, on le rend isolant là où on en a besoin. Par contre sa porosité cause des problèmes : mauvaise résistance chimique, structure instable au-delà de 400°C, et tenue mécanique réduite. La facilité d’intégration des semiconducteurs poreux est un atout majeur, nous visons donc de réduire les désavantages de ces matériaux afin de favoriser leur intégration dans des dispositifs en silicium. Nous avons identifié deux approches pour atteindre cet objectif : i) améliorer le Si poreux ou ii) développer un nouveau matériau. La première approche consiste à amorphiser le Si poreux en l’irradiant avec des ions à haute énergie (uranium, 110 MeV). Nous avons montré que l’amorphisation, même partielle, du Si poreux entraîne une diminution de sa conductivité thermique, sans endommager sa structure poreuse. Cette technique réduit sa conductivité thermique jusqu’à un facteur de trois, et peut être combinée avec une pré-oxydation afin d’atteindre une réduction d’un facteur cinq. Donc cette méthode permet de réduire la porosité du Si poreux, et d’atténuer ainsi les problèmes de fragilité mécanique causés par la porosité élevée, tout en gardant un niveau d’isolation égal. La seconde approche est de développer un nouveau matériau. Nous avons choisi le SiC poreux : le SiC massif a des propriétés physiques supérieures à celles du Si, et donc à priori le SiC poreux devrait conserver cette supériorité. La fabrication du SiC poreux a déjà été démontrée dans la littérature, mais avec peu d’études détaillées du procédé. Sa conductivité thermique et tenue mécanique n’ont pas été caractérisées, et sa tenue en température que de façon incomplète. Nous avons mené une étude systématique de la porosification du SiC en fonction de la concentration en HF et le courant. Nous avons implémenté un banc de mesure de la conductivité thermique par la méthode « 3 oméga » et l’avons utilisé pour mesurer la conductivité thermique du SiC poreux. Nous avons montré qu’elle est environ deux ordres de grandeur plus faible que celle du SiC massif. Nous avons aussi montré que le SiC poreux est résistant à tous les produits chimiques typiquement utilisés en microfabrication sur silicium. D’après nos résultats il est stable jusqu’à au moins 1000°C et nous avons obtenu des résultats qualitatifs encourageants quant à sa tenue mécanique. Nos résultats signifient donc que le SiC poreux est compatible avec la microfabrication, et peut être intégré dans les MEMS comme isolant thermique. // Abstract : Thermal insulation is essential in several types of MEMS (micro electro-mechanical systems). It can help reduce power consumption, improve performance, and can also isolate the hot area from the rest of the device, which is essential in a system-on-chip. A few materials and techniques currently exist for thermal insulation in MEMS, but these are limited. Indeed, either they don’t have provide a sufficient level of insulation, are too fragile, or restrict design of the device and are difficult to integrate. A potentially interesting technique for thermal insulation, which has been demonstrated in the literature, is to make nanometer-scale pores in silicon by electrochemical etching. By nanostructuring silicon in this way, its thermal conductivity is reduced by a factor of 100 to 1000, transforming it into a thermal insulator. This solution is ideal for integration in existing MEMS fabrication processes, as it is based on the silicon substrates which are already used for their fabrication. By locally nanostructuring these substrates, silicon is made insulating wherever necessary. However the porosity also causes problems : poor chemical resistance, an unstable structure above 400◦C, and reduced mechanical properties. The ease of integration of porous semiconductors is a major advantage, so we aim to reduce the disadvantages of these materials in order to encourage their integration in silicon-based devices. We have pursued two approaches in order to reach this goal : i) improve porous Si, or ii) develop a new material. The first approach uses irradiation with high energy ions (100 MeV uranium) to amorphise porous Si. We have shown that amorphisation, even partial, of porous Si leads to a reduction of its thermal conductivity, without damaging its porous structure. This technique can reduce the thermal conductivity of porous Si by up to a factor of three, and can be combined with a pre-oxidation to achieve a five-fold reduction of thermal conductivity. Therefore, by using this method we can use porous Si layers with lower porosity, thus reducing the problems caused by the fragility of high-porosity layers, whilst keeping an equal level of thermal insulation. The second approach is to develop a new material. We have chosen porous SiC: bulk SiC has exceptional physical properties and is superior to bulk Si, so porous SiC should be superior to porous Si. Fabrication of porous SiC has been demonstrated in the literature, but detailed studies of the process are lacking. Its thermal conductivity and mechanical properties have never been measured and its high-temperature behaviour has only been partially characterised. We have carried out a systematic study of the effects of HF concentration and current on the porosification process. We have implemented a thermal conductivity measurement setup using the “3 omega” method and used it to measure the thermal conductivity of porous SiC. We have shown that it is about two orders of magnitude lower than that of bulk SiC. We have also shown that porous SiC is chemically inert in the most commonly used solutions for microfabrication. Our results show that porous SiC is stable up to at least 1000◦C and we have obtained encouraging qualitative results regarding its mechanical properties. This means that porous SiC is compatible with microfabrication processes, and can be integrated in MEMS as a thermal insulation material.

Ferritisch/martensitische Cr-Stähle und deren oxiddispersionsverfestigte Varianten gehören zu den potenziellen Konstruktionswerkstoffen für Komponenten zukünftiger kerntechnischer Einrichtungen, wie z. B. Fusionsreaktoren und Spaltreaktoren der IV. Generation, die Strahlungsfeldern mit hohem Neutronenfluss ausgesetzt sind. Ein Hauptproblem dieser Materialgruppen ist das Auftreten des Spröd-duktil-Übergangs und dessen maßgeblich durch die Strahlenhärtung verursachte Verschiebung zu höheren Temperaturen. In der vorliegenden Arbeit wird das Bestrahlungsverhalten von binären Fe-Cr-Modelllegierungen untersucht, die ein vereinfachtes Modell für ferritisch/martensitische Cr-Stähle darstellen. Dabei werden Bestrahlungen mit Eisenionen zur Simulation der durch Neutronen hervorgerufenen Schädigung verwendet. Die auf wenige Mikrometer begrenzte Eindringtiefe der Ionen macht es erforderlich, dass für dünne Schichten geeignete Charakterisierungsmethoden eingesetzt werden. Im Rahmen dieser Arbeit sind das Nanohärtemessungen und Transmissionselektronenmikroskopie (TEM). Im Ergebnis liegen die bestrahlungsinduzierte Härteänderung der Schicht in Abhängigkeit von Chromgehalt, Bestrahlungsfluenz und –temperatur sowie, für ausgewählte Zustände, quantitative TEM-Analysen vor. Zusammen mit begleitenden Ergebnissen von Neutronenkleinwinkelstreuexperimenten an neutronenbestrahlten Proben der gleichen Werkstoffe ermöglichen sie die Identifizierung von bestrahlungsinduzierten Versetzungsringen und nm-großen α’-Ausscheidungen als Quellen der Strahlenhärtung. Im Rahmen eines vereinfachten Modells, das auf Orowan zurückgeht, werden die Hindernisstärken dieser Gitterbaufehler für das Gleiten von Versetzungen abgeschätzt. Darauf aufbauend erfolgt ausblickartig eine Erweiterung des Untersuchungsgegenstands auf komplexere Situationen hinsichtlich der Bestrahlungsbedingungen und des Werkstoffs. Durch das Einbeziehen simultaner und sequentieller Bestrahlungen mit Eisen- und Heliumionen kann gezeigt werden, dass der Effekt von Helium auf die Strahlenhärtung von der Bestrahlungsreihenfolge abhängt und dass der simultane Eintrag fusionsrelevanter Mengen von Helium zu einer Verstärkung der Strahlenhärtung führt, die auf einem synergistischen Effekt beruht. Für Cr-Stähle mit 9 % Cr und deren oxiddispersionsverfestigte Varianten wird kein grundlegend anderes Bestrahlungsverhalten beobachtet als für binäres Fe-9at%Cr. Es gibt jedoch Hinweise, dass Oxid-dispersionsverfestigung die Strahlenhärtung unter bestimmten Bedingungen reduzieren kann. Im Ergebnis der Arbeit zeigt sich, dass Ionenbestrahlungen in Kombination mit Nanohärtemessungen zu einem vertiefenden Verständnis der Strahlenhärtung in Werkstoffen auf Fe-Cr-Basis sowie zu einer effektiven Materialvorauswahl beitragen können. Voraussetzung ist, dass der Eindruckgrößeneffekt und der Substrateffekt auf geeignete Weise in Rechnung gestellt werden.

R??sum?? : L???isolation thermique est essentielle dans de nombreux types de MEMS (micro-syst??mes ??lectro-m??caniques). Elle permet de r??duire la consommation d?????nergie, am??liorer leurs performances, ou encore isoler la zone chaude du reste du dispositif, ce qui est essentiel dans les syst??mes sur puce. Il existe quelques mat??riaux et techniques d???isolation pour les MEMS, mais ils sont limit??s. En effet, soit ils ne proposent pas un niveau d???isolation suffisant, sont trop fragiles, ou imposent des contraintes trop importantes sur la conception du dispositif et sont difficiles ?? int??grer. Une approche int??ressante pour l???isolation, d??montr??e dans la litt??rature, est de fabriquer des pores de taille nanom??trique dans le silicium par gravure ??lectrochimique. En nanostructurant le silicium ainsi, on peut diviser sa conductivit?? thermique par un facteur de 100 ?? 1000, le transformant en isolant thermique. Cette solution est id??ale pour l???int??gration dans les proc??d??s de fabrication existants des MEMS, car on garde le silicium qui est d??j?? utilis?? pour leur fabrication, mais en le nanostructurant localement, on le rend isolant l?? o?? on en a besoin. Par contre sa porosit?? cause des probl??mes : mauvaise r??sistance chimique, structure instable au-del?? de 400??C, et tenue m??canique r??duite. La facilit?? d???int??gration des semiconducteurs poreux est un atout majeur, nous visons donc de r??duire les d??savantages de ces mat??riaux afin de favoriser leur int??gration dans des dispositifs en silicium. Nous avons identifi?? deux approches pour atteindre cet objectif : i) am??liorer le Si poreux ou ii) d??velopper un nouveau mat??riau. La premi??re approche consiste ?? amorphiser le Si poreux en l???irradiant avec des ions ?? haute ??nergie (uranium, 110 MeV). Nous avons montr?? que l???amorphisation, m??me partielle, du Si poreux entra??ne une diminution de sa conductivit?? thermique, sans endommager sa structure poreuse. Cette technique r??duit sa conductivit?? thermique jusqu????? un facteur de trois, et peut ??tre combin??e avec une pr??-oxydation afin d???atteindre une r??duction d???un facteur cinq. Donc cette m??thode permet de r??duire la porosit?? du Si poreux, et d???att??nuer ainsi les probl??mes de fragilit?? m??canique caus??s par la porosit?? ??lev??e, tout en gardant un niveau d???isolation ??gal. La seconde approche est de d??velopper un nouveau mat??riau. Nous avons choisi le SiC poreux : le SiC massif a des propri??t??s physiques sup??rieures ?? celles du Si, et donc ?? priori le SiC poreux devrait conserver cette sup??riorit??. La fabrication du SiC poreux a d??j?? ??t?? d??montr??e dans la litt??rature, mais avec peu d?????tudes d??taill??es du proc??d??. Sa conductivit?? thermique et tenue m??canique n???ont pas ??t?? caract??ris??es, et sa tenue en temp??rature que de fa??on incompl??te. Nous avons men?? une ??tude syst??matique de la porosification du SiC en fonction de la concentration en HF et le courant. Nous avons impl??ment?? un banc de mesure de la conductivit?? thermique par la m??thode ?? 3 om??ga ?? et l???avons utilis?? pour mesurer la conductivit?? thermique du SiC poreux. Nous avons montr?? qu???elle est environ deux ordres de grandeur plus faible que celle du SiC massif. Nous avons aussi montr?? que le SiC poreux est r??sistant ?? tous les produits chimiques typiquement utilis??s en microfabrication sur silicium. D???apr??s nos r??sultats il est stable jusqu????? au moins 1000??C et nous avons obtenu des r??sultats qualitatifs encourageants quant ?? sa tenue m??canique. Nos r??sultats signifient donc que le SiC poreux est compatible avec la microfabrication, et peut ??tre int??gr?? dans les MEMS comme isolant thermique. // Abstract : Thermal insulation is essential in several types of MEMS (micro electro-mechanical systems). It can help reduce power consumption, improve performance, and can also isolate the hot area from the rest of the device, which is essential in a system-on-chip. A few materials and techniques currently exist for thermal insulation in MEMS, but these are limited. Indeed, either they don???t have provide a sufficient level of insulation, are too fragile, or restrict design of the device and are difficult to integrate. A potentially interesting technique for thermal insulation, which has been demonstrated in the literature, is to make nanometer-scale pores in silicon by electrochemical etching. By nanostructuring silicon in this way, its thermal conductivity is reduced by a factor of 100 to 1000, transforming it into a thermal insulator. This solution is ideal for integration in existing MEMS fabrication processes, as it is based on the silicon substrates which are already used for their fabrication. By locally nanostructuring these substrates, silicon is made insulating wherever necessary. However the porosity also causes problems : poor chemical resistance, an unstable structure above 400???C, and reduced mechanical properties. The ease of integration of porous semiconductors is a major advantage, so we aim to reduce the disadvantages of these materials in order to encourage their integration in silicon-based devices. We have pursued two approaches in order to reach this goal : i) improve porous Si, or ii) develop a new material. The first approach uses irradiation with high energy ions (100 MeV uranium) to amorphise porous Si. We have shown that amorphisation, even partial, of porous Si leads to a reduction of its thermal conductivity, without damaging its porous structure. This technique can reduce the thermal conductivity of porous Si by up to a factor of three, and can be combined with a pre-oxidation to achieve a five-fold reduction of thermal conductivity. Therefore, by using this method we can use porous Si layers with lower porosity, thus reducing the problems caused by the fragility of high-porosity layers, whilst keeping an equal level of thermal insulation. The second approach is to develop a new material. We have chosen porous SiC: bulk SiC has exceptional physical properties and is superior to bulk Si, so porous SiC should be superior to porous Si. Fabrication of porous SiC has been demonstrated in the literature, but detailed studies of the process are lacking. Its thermal conductivity and mechanical properties have never been measured and its high-temperature behaviour has only been partially characterised. We have carried out a systematic study of the effects of HF concentration and current on the porosification process. We have implemented a thermal conductivity measurement setup using the ???3 omega??? method and used it to measure the thermal conductivity of porous SiC. We have shown that it is about two orders of magnitude lower than that of bulk SiC. We have also shown that porous SiC is chemically inert in the most commonly used solutions for microfabrication. Our results show that porous SiC is stable up to at least 1000???C and we have obtained encouraging qualitative results regarding its mechanical properties. This means that porous SiC is compatible with microfabrication processes, and can be integrated in MEMS as a thermal insulation material.

Self-organization is a hot topic as it has the potential to create surface patterns on the nanoscale avoiding cost-intensive top-down approaches. Although chemists have promising results in this area, ion irradiation can create self-organized surface patterns in a more controlled manner. Different regimes of pattern formation under ion irradiation were described so far by 2D models. Here, two new regimes have been studied experimentally, which require modeling in 3D: subsurface point defect kinetics as well as ion impact-induced melt pool formation. This thesis deals with self-organized pattern formation on Ge and Si surfaces under normal incidence irradiation with heavy monatomic and polyatomic ions of energies up to several tens of keV. Irradiation has been performed using liquid metal ion sources in a focused ion beam facility with mass-separation as well as by conventional broad beam ion implantation. Irradiated samples have been analyzed mainly by scanning electron microscopy. Related to the specific irradiation conditions, investigation and discussion of pattern formation has been divided into two parts: (i) formation of Ge morphologies due to point defect kinetics and (ii) formation of Ge and Si morphologies due to melt pool dynamics. Point defect kinetics dominates pattern formation on Ge under irradiation with monatomic ions at room temperature. Irradiation of Ge with Bi and Ge ions at fluences up to 10^17 cm^(-2) has been performed. Comprehensive studies show for the first time that morphologies change from flat surfaces over hole to nanoporous, sponge-like patterns with increasing ion energy. This study is consistent with former irradiations of Ge with a few ion energies. Based on my studies, a consistent, qualitative 3D model of morphology evolution has been developed, which attributes the ion energy dependency of the surface morphology to the depth dependency of point defect creation and relaxation. This model has been proven by atomistic computer experiments, which reproduce the patterns found in real irradiation experiments. At extremely high energy densities deposited by very heavy ions another mechanism dominates pattern formation. The formation of Ge and Si dot patterns by very heavy, monatomic and polyatomic Bi ion irradiation has been studied in detail for the first time. So far, this formation of pronounced dot pattern cannot be explained by any model. Comprehensive, experimental studies have shown that pattern formation on Ge is related to extremely high energy densities deposited by each polyatomic ion locally. The simultaneous impact of several atoms leads to local energy densities sufficient to cause local melting. Heating of Ge substrates under ion irradiation increases the achievable energy density in the collision cascade substantially. This prediction has been confirmed experimentally: it has been found that the threshold for nanomelting can be lowered by substrate heating, which allows pattern formation also under heavy, monatomic ion irradiation. Extensive studies of monatomic Bi irradiation of heated Ge have shown that morphologies change from sponge-like over highly regular dot patterns to smooth surfaces with increasing substrate temperature. The change from sponge-like to dot pattern is correlated to the melting of the ion collision cascade volume, with energy densities sufficient for melt pool formation at the surface. The model of pattern formation on Ge due to extremely high deposited energy densities is not specific to a single element. Therefore, Si has been studied too. Dot patterns have been found for polyatomic Bi ion irradiation of hot Si, which creates sufficiently high energy densities to allow ion impact-induced melt pool formation. This proves that pattern formation by melt pool formation is a novel, general pattern formation mechanism. Using molecular dynamics simulations of project partners, the correlation between dot patterning and ion impact-induced melt pool formation has been proven. The driving force for dot pattern formation due to high deposited energy densities has been identified and approximated in a first continuum description.