Ces travaux de recherche portent sur la conception et la réalisation de commutateurs RF basées sur l’intégration de matériaux innovants fonctionnels tels que le dioxyde de vanadium (VO2) et les alliages de chalcogénures de types Ge2Sb2Te5 (GST) et GeTe. Le principe de fonctionnement de ces composants repose sur le changement de résistivité que présentent ces matériaux. Le VO2 possède une transition Isolant-Métal (MIT) autour de 68°C à travers laquelle le matériau passe d’un état isolant (forte résistivité) à un état métallique (faible résistivité). La transition MIT présente l’intérêt de pouvoir être initiée sous l’effet de plusieurs types de stimuli externes (thermique, électrique et optique) avec de faibles temps de commutation. Les alliages de types GST et GeTe ont la particularité de commuter réversiblement entre un état amorphe à forte résistivité à un état cristallin à faible résistivité suite à un traitement thermique spécifique. Les commutateurs à base de GST ou de GeTe présentent l'avantage de pouvoir opérer en mode bistable car le changement de résistivité présenté par ces matériaux est de type non volatile. Les composants réalisés ont de bonnes performances électriques (isolation et pertes d’insertion) sur une large bande. Nos travaux de recherche visent à proposer une solution alternative aux solutions classiques (semi-conducteurs et MEMS-RF) pour réaliser des commutateurs RF qui peuvent être par la suite utilisés dans la conception des dispositifs reconfigurables (filtres, Antennes). / This research work focuses on the design and realization of RF switches based on the integration of new materials such as vanadium dioxide (VO2), Ge2Sb2Te5 (GST) and GeTe chalcogenides alloys. The operating principle of these devices is based on the resistivity change presented by these materials. VO2 exhibits a Metal-Insulator transition (MIT) around 68°C for which the material changes from an insulating state (high resistivity) to a metallic one (low resistivity). The MIT transition can be triggered in different ways (thermally, electrically and optically) with low switching time. GST and GeTe alloys have the particularity to be reversibly switched between a high resistive-amorphous state to low resistive-crystalline state, under a specific heat treatment. Thanks to the non-volatile resistivity change presented by these materials, GST/GeTe-based switches are able to operate in bistable mode. The fabricated devices exhibit good electrical performances (insertion loss and isolation) over a broadband. The aim of our work is to propose an alternative solution to conventional technologies (semiconductors and RF-MEMS), to design RF switches that can be used afterward in the design of reconfigurable devices (filters, antennas).

Many transition metal oxides exhibit a Metal-Insulator Transition (MIT). This is an interesting phenomenon where the material undergoes a structural phase transition, and the electronic charge is redistributed in the crystal lattice. The crystallographic, optical, electrical, and magnetic properties are drastically changed when a material undergoes a MIT. Vanadium oxides are thermochromic materials, which means that the optical properties change with temperature, closely connected to the MIT. Vanadium sesquioxide (V2O3) experiences a MIT at the transition temperature (Tt) 160-170 K. Below the transition temperature, it is a monoclinic ferromagnetic insulator, and above the transition temperature it becomes a paramagnetic rhombohedral corundum-type metal. In the insulating phase, infrared (IR) light is transmitted, whereas in the metallic phase, IR light is reflected. The aim of this Master thesis is to improve the methodology to produce V2O3 thin films, characterize them, and study how different parameters affect the structural and optical properties of the film. V2O3 thin films were synthesized by sol-gel and deposited by spin-coating on sapphire substrates. Thereafter, the resulting films underwent an annealing process in a reducing atmosphere to achieve crystalline V2O3. The obtained crystal phase was examined by X-ray diffraction. The surface structure and morphology were studied with an optical microscope and scanning electron microscope. The optical transmittance in the IR region was examined by Fourier transform infrared spectroscopy. At last, a laser test was performed on one of the samples. In conclusion, a majority of the thin films consisted of polycrystalline V2O3. The MIT was studied with the optical hysteresis loop, which showed great difference among the different samples studied. The thin film that exhibited best optical properties showed a maximum transmission of 66 % below the Tt, and a minimum transmission of 6 % above the Tt. In comparison to this one, the other samples exhibited lower transmission and a decrease in Tt. This difference was attributed to the different morphologies and crystal orientations of the samples.

No description available.

In dieser Arbeit wird das Wachstum von dünnen quasi-kristallinen Ge-Sb-Te (GST) Schichten mittels Molekularstrahlepitaxie demonstriert, die zu einer geordneten Konfiguration von intrinsischen Kristallgitterfehlstellen führen. Es wird gezeigt, wie es eine Strukturanalyse basierend auf Röntgenstrahlbeugungssimulationen, Dichtefunktionaltheorie und Transmissionselektronenmikroskopie ermöglicht, eine eindeutige Beurteilung der Kristallgitterlückenanordnung in den GST-Proben vorzunehmen. Das Verständnis für die Ordnungsprozesse der Gitterfehlstellen erlaubt eine gezielte Einstellung des Ordnungsgrades selbst, der mit der Zusammensetzung und der Kristallphase des Materials in Zusammenhang steht. Auf dieser Basis wurde ein Phasendiagramm mit verschiedenen Wachstumsfenstern für GST erstellt. Des Weiteren wird gezeigt, dass man eine hohe Ordnung der Gitterfehlstellen in GST auch durch Ausheizprozesse und anhand von Femtosekunden-gepulster Laserkristallisation von amorphem Material erhält, das zuvor auf einem als Kristallisationsgrundlage dienenden Substrat abgeschiedenen wurde. Diese Erkenntnis ist bemerkenswert, da sie zeigt, dass sich kristalline GST Schichten mit geordneten Kristallgitterlücken durch verschiedene Herstellungsprozesse realisieren lassen. Darüber hinaus wurde das Wachstum von GeTe/Sb2Te3 Übergittern durchgeführt, deren Struktur die von GST mit geordneten Gitterfehlstellen widerspiegelt. Die Möglichkeit den Grad der Gitterfehlstellenordung in GST gezielt zu manipulieren wurde mit einer Studie der Transporteigenschaften kombiniert. Die Anwendung von großflächigen Charakterisierungsmethoden wie XRD, Raman und IR-Spektroskopie, erlaubte die Bestimmung der Phase und des Fehlstellenordnungsgrades von GST und zeigte eindeutig, dass die Fehlstellenordnung für den Metall-Isolator-Übergang (MIT) verantwortlich ist. Insbesondere wird durch das Vergleichen von XRD-Messungen mit elektrischen Messungen gezeigt, dass der Übergang von isolierend zu leitend erfolgt, sobald eine Ordnung der Kristallgitterlücken einsetzt. Dieses Phänomen tritt in der kubischen Kristallphase auf, wenn Gitterfehlstellen in GST von einem ungeordneten in einen geordneten Zustand übergehen. Im zweiten Teil des Kapitels wird eine Kombination aus FIR- und Raman-Spektroskopie zur Untersuchung der Vibrationsmoden und des Ladungsträgerverhaltens in der amorphen und der kristallinen Phase angewendet, um Aktivierungsenergien für die Elektronenleitung, sowohl für die kubische, als auch für die trigonale Kristallphase von GST zu bestimmen. Hier ist es wichtig zu erwähnen, dass, in Übereinstimmung mit Ergebnissen aus anderen Untersuchungen, das Auftauchen eines MIT beim Übergang zwischen der ungeordneten und der geordneten kubischen Phase beobachtet wurde. Schlussendlich wurden verschiedene sogenannte Pump/Probe Technik, bei der man das Material mit dem Laser anregt und die Röntgenstrahlung oder Terahertz (THz)-spektroskopie als Sonde nutzt, angewandt. Dies dient um ultra-schnelle Dynamiken zu erfassen, die zum Verständnis der Umschaltmechanismen nötig sind. Die Empfindlichkeit der THz-Messungen hinsichtlich der Leitfähigkeit, sowohl in GST, als auch in GeTe/Sb2Te3 Übergittern zeigte, dass die nicht-thermische Natur der Übergitterumschaltprozesse mit Grenzflächeneffekten zusammenhängt und . Der Ablauf wird mit beeindruckender geringer Laser-Fluenz erreicht. Dieses Ergebnis stimmt mit Berichten aus der Literatur überein, in denen ein Kristall-zu Kristallwechsel von auf Übergittern basierenden Speicherzellen für effizienter gehalten wird als GST Schmelzen, was zu einen ultra-schwachen Energieverbrauch führt. / The growth by molecular beam epitaxy of Ge-Sb-Te (GST) alloys resulting in quasi-single-crystalline films with ordered configuration of intrinsic vacancies is demonstrated. It is shown how a structural characterization based on transmission electron microscopy, X-ray diffraction and density functional theory, allowed to unequivocally assess the vacancy ordering in GST samples, which was so far only predicted. The understanding of the ordering process enabled the realization of a fine tuning of the ordering degree itself, which is linked to composition and crystalline phase. A phase diagram with the different growth windows for GST is obtained. High degree of vacancy ordering in GST is also obtained through annealing and via femtosecond-pulsed laser crystallization of amorphous material deposited on a crystalline substrate, which acts as a template for the crystallization. This finding is remarkable as it demonstrates that it is possible to create a crystalline GST with ordered vacancies by using different fabrication procedures. Growth and structural characterization of GeTe/Sb2Te3 superlattices is also obtained. Their structure resembles that of ordered GST, with exception of the Sb and Ge layers stacking sequence. The possibility to tune the degree of vacancy ordering in GST has been combined with a study of its transport properties. Employing global characterization methods such as XRD, Raman and Far-Infrared spectroscopy, the phase and ordering degree of the GST was assessed, and unequivocally demonstrated that vacancy ordering in GST drives the metal-insulator transition (MIT). In particular, first it is shown that by comparing electrical measurements to XRD, the transition from insulating to metallic behavior is obtained as soon as vacancies start to order. This phenomenon occurs within the cubic phase, when GST evolves from disordered to ordered. In the second part of the chapter, a combination of Far-Infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline phases, enabling to extract activation energies for the electron conduction for both cubic and trigonal GST phases. Most important, a MIT is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase, consistently with the electrical study. Finally, pump/probe schemes based on optical-pump/X-ray absorption and Terahertz (THz) spectroscopy-probes have been employed to access ultrafast dynamics necessary for the understanding of switching mechanisms. The sensitivity of THz-probe to conductivity in both GST and GeTe/Sb2Te3 superlattices showed that the non-thermal nature of switching in superlattices is related to interface effects, and can be triggered by employing up to one order less laser fluences if compared to GST. Such result agrees with literature, in which a crystal to crystal switching of superlattice based memory cells is expected to be more efficient than GST melting, therefore enabling ultra-low energy consumption.

Electronic structure of transition metal oxides has been a subject of intense research since decades due to the wide spectrum of properties that they exhibit, like high temperature superconductivity, metal-insulator transitions (MIT), phase separation etc. Among these, colossal magnetoresistance (CMR), i.e. a sharp drop in the electrical resistance by the application of an external magnetic field, is a property of fundamental and technological importance. In the present study we investigate several of these interesting properties ranging from colossal magnetoresistance, metal-insulator transitions and phase separation phenomena on a wide range of magnetoresistive systems. All these properties originate in transition metal oxides due to a competition between the strong inter-atomic Coulomb interaction strength within the transition metal d electrons and a large hopping interaction strength between the metal d and oxygen 2p states. In this thesis we report the investigation of the electronic and magnetic structures of some magnetoresistive oxides, including various double perovskites and manganites, using various high energy spectroscopies in conjunction with various theoretical approaches. The samples for the present experimental investigation were prepared by different synthetic routes, such as solid state reaction, nitrate method, d.c arc melting and float zone method, and were characterized by x-ray diffraction, four probe resistivity, magnetic susceptibility, optical absorption and energy dispersive analysis of x-rays while some of the samples were supplied by our collaborators. Various spectroscopic techniques like x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) , bremsstrahlung isochromat spectroscopy (BIS), x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism spectroscopy (XMCD) , electron energy loss spectroscopy (EELS), spatially resolved photoelectron spectroscopy and M¨ ossbauer spectroscopy were used to probe the samples. Theoretical methods include configuration interaction cluster approach to fit the XAS and XMCD spectra while ab initio band structure calculations along with the least-square fitting procedure was used to fit some of the valence and conduction bands. Following a general introduction in Chapter 1, the details of various experimental and theoretical techniques are discussed in Chapter 2 of this thesis. Recently, a double perovskite, Sr2FeMoO6, belonging to a general family of halfmetallic ferromagnetic oxides, has shown a spectacularly large magnetoresistance even at the room temperature and at relatively small applied magnetic fields compared to the extensively investigated class of magnetoresistive manganites. Physical properties of this compound is strongly influenced by the Fe -Mo ordering. We hence synthesized Sr2FeMoO6 sample, both with high and low degree of Fe/Mo ordering. Spectroscopic investigations of these samples suggest the presence of Fe rich and Mo rich domains of the type Sr2Fe1+xMo1−xO6 in disordered Sr2FeMoO6 at times. This prompted us to prepare bulk samples of Sr2Fe1+xMo1−xO6. In Chapter 3 we address various issues related to Fe/Mo ordering like saturation magnetization, variation of TC, and CMR as well as oxidation state of Fe and Mo in Sr2FeMoO6using this new series, ”Sr2Fe1+xMo1−xO6” as it offers a better control on the Fe/Mo bonds by controlling x. On the basis of the electron spectroscopic studies in conjunction with a configuration interaction cluster calculation model coupled with the conduction band, we claim that Fe remains in 3+oxidation state throughout the series, where as Mo changes its valency to maintain the charge neutrality. An analysis of the magnetic momentas a function of x suggests that Fe at the ”wrong” crystallographic site is coupled anti-parallel to the Fe moments at the ”correct” site. Additionally, Mo depolarizes to the extend proportional to the number of Mo sites in the near-neighbor co-ordination shell. Continuing with the double perovskites in Chapter 4 we investigate the electronic and magnetic structure of Sr2FeMoO6, Ca2FeMoO6 and Ba2FeMoO6using XAS and XMCD studies. We find that the conventional XAS and XMCD calculations based on configuration interaction of a typical fragment, FeO6in this case, is insufficient to reproduce the experimental spectrum as the compounds considered here are metallic. In order to include the non local charge transfer, we coupled FeO6 octahedra to a conduction band which mimics the Mo band. Within this model we obtained a good fit to the experimental spectrum. Chapter 5 deals with another series of double perovskite (Sr1−yCay)2FeReO6which exhibits a rich phase diagram since it undergoes a metal insulator transition (MIT) with composition at low temperatures. This system becomes more interesting due to the presence of a temperature driven MIT for higher y compositions. We find that the MIT is not related to the change in valency of Fe and Re. Analysis of the near Fermi edge valence band spectra suggests opening up of a soft gap. The main reason for MIT in this system is most likely the presence of strong electron-electron correlation between multiple electrons at the Re site, which is caused by the mismatch of the Re ionic radius and change in the crystal structure across MIT. Another issue which has been extensively investigated in this thesis is phase separation in manganites presented in Chapter 6. We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional unique sensitivity to chemical compositions, to investigate high quality single crystal samples of La1/4Pr3/8Ca3/8MnO3 in the first section. This unique probe establishes the formation of distinct insulating domains embedded in the metallic host at low temperatures, significantly in the absence of any perceptible chemical inhomogeneity, with the domain-size at least an order of magnitude larger than the previous largest estimate. We also provide compelling evidence of memory effects in such domain formation and morphology, suggesting an intimate connection between these electronic domains and long-range strains, often thought to be an important ingredient in the physics of doped manganites. In second part of this chapter we discuss another system namely Eu0.5Y0.5MnO3 which undergoes a chemical phase separation forming alternate stripes of Eu rich (Y deficient) orthorhombic phase and Y rich (Eu deficient) hexagonal phases. These stripes are amazingly straight and run parallel over millimeters. One more system that we investigated is a mixture of ferromagnetic La5/8Sr3/8MnO3and insulating ferroelectric LuMnO3 taken in ratio 3:7, here too the attempt to make a single crystal resulted into a chemical phase separation forming strips of metallic La5/8Sr3/8MnO3and insulating LuMnO3 throughout the sample surface. Preliminary studies suggests that strain between the chemically and crystallographically different species may result into such interesting morphology. In Chapter 7 we study pseudo-one dimensional compounds Sr3CuIrO6 and Sr3ZnIrO6 using photo electron spectroscopy. The experimental results were fitted using band structure calculations with Full Potential Linearized Augmented Plane Wave (FP-LAPW) method.

No description available.

Transition metal oxides and chalcogenides are an ideal platform for demonstrating and investigating many interesting electronic phases of matter. These phases emerge as a result of collective many body interactions among the electrons. The omnipresent electron, depending on its interaction with other electrons and with the underlying lattice, can generate diverse phases of matter with exotic physical properties. The ultimate objective of Materials Science is to provide a complete microscopic understanding of these myriad electronic phases of matter. A proper understanding of the collective quant-tum behaviour of electrons in different system can also help in designing and tuning new electronic phases of matter that may have strong impact in the field of microelectronics, well beyond that predicted by Moore s law. Strong electron correlation effects produce a wide spectrum of ground state prop-retires like superconductivity, Metal Insulator Transition (MIT), charge-orbital ordering and many more. Similarly, different spin interactions among electrons, essentially due to various kinds of exchange coupling, give rise to varying magnetic ground state prop-retires like ferromagnetism, anti-ferromagnetism, spin glass, among others. The main objective of this thesis is to understand and rationalize diverse electronic and magnetic phases of matter in some selected strongly correlated systems. In chapter 1 we have provided an overview of various electronic and magnetic phases of matter which are relevant and necessary for understanding the chapters that follow. The first part of this chapter describes the fundamental concepts of the so called Metal Insulator Transition (MIT). A small section is dedicated to the subtle interactions among electrons and lattice that actually drive a system from a highly conducting metallic state to a strongly resistive insulating state. The second part of this chapter offers a compilation of different magnetic ground states which are discussed in detail in the last two chapters. In Chapter 2, we have explained various methodologies and experimental tech-antiques that have been used in the work reported in this thesis. In Chapter 3, we have provided a detailed understanding of the MIT in different polymorphic forms of Vanadium dioxide (VO2). Although VO2 exhibits a number of polymorphic forms, only the rutile/monoclinic VO2 phase has been studied extensively compared to other polymorphic forms. This phase shows a well-established MIT across ∼340 K, which has been extensively investigated in order to understand the relative importance of many body electron correlation effects arising primarily from on-site Coulomb interactions within the Vanadium 3d manifold, and single electron effects flounced by the dimerization of Vanadium atoms. Unlike the rutile phase of VO2, little is known about the MIT appearing across 212 K in the metastable B-phase of VO2. This phase shows dimerization of only half of the Vanadium atoms in the insulating state, in contrast to rutile/monoclinic VO2, which show complete dimerization. There is a long standing debate about the origin of the MIT in the rutile/monoclinic phase, that contrasts the role of the many-body Hubbard U term, with single particle effects of the dimerization. In light of this debate, the MIT in the B-phase offers a unique opportunity to understand and address the competition between many body and single particle effects, that has been unresolved over several decades. In this chapter we have investigated different polymorphs of VO2 to understand the underlying electronic structure and the nature of the MIT in these polymorphic forms. The MIT in VO2 B phase is very broad in nature. X-ray photoemission and optical conductivity data indicate that in case of VO2 B phase both correlation effects and dimerization is necessary to drive the MIT. We have also established that the correlation effects are more prominent for VO2 B phase compared to rutile/monoclinic phase. In Chapter 4, we have discussed the electronic structure of LaTiO3 (LTO)-SrTiO3 (STO) system. At the interface between polar LTO and non-polar (STO) oxides, an unique two dimensional electron gas (2DEG) like state appears, that exhibits a phenomenal range of unexpected transport, magnetic, and electronic properties. Thus, this interface stands as a prospective candidate for not only fundamental scientific investigation, but also application in technological and ultimately commercial frontiers. In this chapter, using variable energy Hard X-ray photoemission spectroscopy (HAXPES), we have experimentally investigated the layer resolved evolution of electronic structure across the interface in LTO-STO system. HAXPES results suggest that the interface is more coherent in nature and the coherent to incoherent feature ratio changes significantly as we probe deeper into the layer In chapter 5, we have investigated the electronic structure of the chemically exfoliated trigonal phase of MoS2. This elusive trigonal phase exists only as small patches on chemically exfoliated MoS2, and is believed to control functioning of MoS2 based devices. Its electronic structure is little understood, with total absence of any spec-troscopic data, and contradictory claims from theoretical investigations. We have ad-dressed this issue experimentally by studying the electronic structure of few layered chemically exfoliated MoS2 systems using spatially resolved X-ray photoemission spec-otoscopy and micro Raman spectroscopy in conjunction with electronic structure calculations. We have established that the ground state of this unique trigonal phase is actually a small gap (∼90 meV) semiconductor. This is in contrast with most of the claims in existing literature. In chapter 6, we have re-examined and revaluated the electronic structure of the late 3d transition metal monoxides (NiO, FeO, and CoO) using a combination of HAX-PES and state-of-the-art theoretical calculations. We have observed a strong evolution in the valence band spectra as a function of excitation energy. Theoretical results show that a combined GW+LDA+DMFT scheme is essential for explaining the observed experimental findings. Additionally, variable temperature HAXPES measurement In chapter 8, we have differentiated the surface and the bulk electronic structure in Sr2FeMoO6 and also have provided a new route to increase the Curie temperature of this material. Sr2FeMoO6 is well known for its high Curie temperature (Tc ∼410 K), half-metallic ferromagnetism, and a spectacularly large tunnelling magnetoresistance. The surface electronic structure of Sr2FeMoO6 is believed to be different from the bulk; leading to a Spin-Valve type Magnetoresistance. We have carried out variable energy HAXPES on Sr2FeMoO6 to probe electronic structure as a function of surface depth. Our experimental results indicate that surface is more Mo6+ rich. We have also demonstrated what we believe is the first direct experimental evidence of hard ferro-magnetism in the surface layer using X Ray Magnetic Circular Dichroism (XMCD) with dual detection mode. In the second part of this chapter we have designed a new route to increase the Curie temperature and have been successfully able to achieve a Curie temperature as high as 515 K.

No description available.