Growth, structure and lattice dynamics of iron silicide nanostructures

In der vorliegenden Arbeit wird die Gitterdynamik von Nanostrukturen der Eisen-Silizium-Verbindungen Fe$_3$Si, $\alpha$-FeSi$_2$, und $\beta$-FeSi$_2$ untersucht. Ein umfassendes Verständnis des Einflusses einer räumlichen Beschränkung auf die Nanometerskala auf die Gitterdynamik ist Voraussetzung für die Nutzung dieser Materialien in nanoskaligen elektronischen Bauteilen sowie für die angestrebte gezielte Manipulation der Schwingungseigenschaften von Nanostrukturen. Monokristalline Nanostrukturen der untersuchten Materialien werden mittels Molekularstrahlepitaxie hergestellt und mit komplementären Methoden umfassend auf ihre strukturellen Eigenschaften charakterisiert. Die partielle Fe Phononenzustandsdichte (PDOS) der Nanostrukturen wird mittels unelastischer Kernstreuung erhalten, einer Technik, die in einzigartiger Weise für die Messung des inhärent kleinen Streuvolumens von Nanostrukturen geeignet ist. Die Kenntnis der PDOS ermöglicht die Bestimmung fundamentaler thermodynamischer und elastischer Eigenschaften, unter anderem der mittleren Kraftkonstante, der mittleren quadratischen Auslenkung, der Wärmekapazität des Gitters und der Schallgeschwindigkeit. Der Vergleich der experimentellen Ergebnisse mit ab initio Berechnungen erlaubt es eine Korrelation zwischen den strukturellen Eigenschaften der Nanostrukturen und den beobachteten Abweichungen ihrer Gitterdynamik von den dazugehörigen Volumenkristallen herzustellen. Mit diesem Ansatz werden die Gitterdynamik der Fe$_3$Si/GaAs Heterostruktur, von $\alpha$-FeSi$_2$ Nanoinseln und Nanodrähten sowie von $\beta$-FeSi$_2$ Nanostäben untersucht. Dabei werden zwei allgemeine Phänomene identifiziert: Erstens resultiert die Erhöhung des Grenzflächen-/Oberflächen-zu-Volumen-Verhältnisses in einem signifikanten Einfluss von Grenzflächen- und Oberflächen-spezifischen Schwingungsmoden auf die gesamte PDOS. Zweitens führt die kristalline Unordnung an Grenzflächen und Oberflächen zu einer verstärkten Streuung von Phononen an Defekten und einer Dämpfung der PDOS. Als Resultat zeigen die thermodynamischen und elastischen Eigenschaften eine "Aufweichung" des Gitters als Funktion der Kristallgröße, was durch eine erhöhte mittlere quadratischen Auslenkung und eine reduzierte Schallgeschwindigkeit belegt wird. Darüber hinaus offenbart die Untersuchung der Fe$_3$Si/GaAs Heterostruktur das Auftreten von Grenzflächen-spezifischen Schwingungsmoden an epitaktischen, verspannungsfreien Grenzflächen. In $\alpha$-FeSi$_2$ Nanostrukturen wird nach der Reduktion der charakteristischen Kristallgröße unter 10$\,$nm eine anormal starke Dämpfung akustischer Phononen beobachtet. Nanodrähte desselben Materials weisen eine anisotrope Dämpfung von Gitterschwingungen entlang und quer zu den Nanodrähten auf. Die Untersuchung von $\beta$-FeSi$_2$ Nanostäben offenbart das Auftreten einer neuen Schwingungsmode bei niedrigen Energien, die der Bildung einer Fe-reichen Zwischenlage zugeschrieben wird

Integración de láminas delgadas de materiales feromagnéticos medio-metálicos en silicio por medio de un contacto túnel

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de Materiales, leída el 08-07-2019The use of magnetic tunnel contacts have been proved as one of the most efficient methods for the efficient injection and detection of spin polarized currents on non-magnetic semiconductors. In this kind of systems, the spin polarized carriers of the ferromagnetic electrode are transferred by tunneling through an insulator barrier into the semiconductor. The diusive transport directly through the ferromagnet/semiconductor interface is limited by the large impedance mismatch between the two materials. The introduction of a tunnel barrier allows to improve the efficiency of the system and increase spin accumulation in the semiconductor. However, in order to achieve high spin rates the system needs to ful ll certain requirements. One of the fundamental conditions is the formation of at and abrupt interfaces. High interface roughness can induce the presence of magnetostatic elds that might induce spin precession and reduce spin polarization. Moreover, the presence of defects on the interface can act as scattering centers and reduce even more spin accumulation in the semiconductor...Se ha demostrado que el uso de contactos tunel magneticos es uno de los metodos mas eficientes para la inyeccion y deteccion de corrientes polarizadas en espín en semiconductores no magneticos. En este tipo de sistemas, los portadores de carga polarizados en espín del electrodo ferromagnetico se transfieren al semiconductor por efecto tunel a traves de una barrera aislante. La inyeccion de espín por transporte difusivo de forma directa a traves de la intercara ferromagnetico/aislante es muy poco eficiente en terminos de polarizacion debido a la gran diferencia de impedancia electrica entre los dos materiales. La introduccion de una barrera tunel permite mejorar la eficiencia del sistema y aumentar la acumulacion de espín en el semiconductor. Sin embargo, para poder obtener una inyeccion eficiente el sistema debe ademas cumplir ciertos requisitos. Una de las condiciones principales es que las heteroestructuras presenten intercaras abruptas tanto morfologicamente como desde el punto de vista qumico. Esto se debe a que una rugosidad alta puede generar campos magnetoestaticos en la intercara que induzcan la precesion de los espines y reduzcan la polarizacion en el semiconductor. Ademas, los defectos presentes en la intercara pueden funcionar como centros de dispersion y reducir a su vez la polarizacion en espín de los portadores...Fac. de Ciencias FísicasTRUEunpu

Modeling, Characterization, and Magnetic Behavior of Transition Metal Nanosystems Synthesized in Silicon Using Low Energy Ion Implantation

Magnetic nano-clusters in silicon involving iron and cobalt were synthesized using low energy (50 keV) ion implantation technique and post-implantation thermal annealing. Before the irradiation, multiple ion-solid interaction simulations were carried out to estimate optimal ion energy and fluence for each experiment. For high-fluence low-energy irradiation of heavy ions in a relatively lighter substrate, modeling the ion irradiation process using dynamic code SDTrimSP showed better agreement with the experimental results compared to the widely used static simulation code TRIM. A saturation in concentration (~ 48%) profile of the 50 keV Fe or Co implants in Si was seen at a fluence of ~ 2 × 1017 ions/cm2. Further study showed that for structures with a curved surface, particularly for nanowires, better simulation results could be extracted using a code "Iradina" as the curve geometry of the target surface can be directly defined in the input file. The compositional, structural, and magnetic properties were studied using Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy, X-ray diffraction, atom probe tomography, and vibrating sample magnetometry. Irradiation of high-current (~ 2 μA/cm2) 50 keV Fe ions into Si at a fluence of 2 × 1017 ions/cm2 showed the formation of Fe5Si3 nano structures in the near-surface region of the substrate. Post-implantation thermal annealing in vacuum at 500 οC for one hour showed a significant enhancement in structural and magnetic properties. Similar high-current irradiation of 50 keV Co with a fluence of 3.2 × 1016 ions/cm2 into Si substrate showed the formation of superparamagnetic structure even at room temperature in the as-implanted samples. The simulation results for irradiation of Co and Fe on the curved surface were validated by ion irradiation on pre-fabricated Si nano tip followed by atom probe tomography analysis

Isothermal and non-isothermal comparative study of zn-sn system using real-time rbs

>Magister Scientiae - MScInsight into the effects of isothermal and non-isothermal annealing on bi-metallic thin film is important for material synthesis and application in everyday use. The effects of isothermal annealing on bi-metallic thin films has long been studied using various heating methods from a resistively heated filaments, by transferring heat via conduction, convection and irradiation. The effect of each method have been widely reported in literature. The diffusion coefficient and activation energies of the constituent atoms can calculated for each annealing method. On the other hand, the effects of non-isothermal annealing on bi-metallic thin films has not been comprehensively studied, and there are areas of this annealing regime that need further investigation. In this study a femtosecond laser with a 1064 nm central wavelength was used to anneal bi-metallic thin films of Zinc-Tin (Zn-Sn) on a substrate

Principles and applications of CVD powder technology

Chemical vapor deposition (CVD) is an important technique for surface modification of powders through either grafting or deposition of films and coatings. The efficiency of this complex process primarily depends on appropriate contact between the reactive gas phase and the solid particles to be treated. Based on this requirement, the first part of this review focuses on the ways to ensure such contact and particularly on the formation of fluidized beds. Combination of constraints due to both fluidization and chemical vapor deposition leads to the definition of different types of reactors as an alternative to classical fluidized beds, such as spouted beds, circulating beds operating in turbulent and fast-transport regimes or vibro-fluidized beds. They operate under thermal but also plasma activation of the reactive gas and their design mainly depends on the type of powders to be treated. Modeling of both reactors and operating conditions is a valuable tool for understanding and optimizing these complex processes and materials. In the second part of the review, the state of the art on materials produced by fluidized bed chemical vapor deposition is presented. Beyond pioneering applications in the nuclear power industry, application domains, such as heterogeneous catalysis, microelectronics, photovoltaics and protection against wear, oxidation and heat are potentially concerned by processes involving chemical vapor deposition on powders. Moreover, simple and reduced cost FBCVD processes where the material to coat is immersed in the FB, allow the production of coatings for metals with different wear, oxidation and corrosion resistance. Finally, large-scale production of advanced nanomaterials is a promising area for the future extension and development of this technique