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The presented thesis explores the magnetoelectric (ME) coupling in multiferroic thin film multilayers of BaTiO3 (BTO) and BiFeO3 (BFO). Multiferroics possess more than one ferroic order parameter, in this case ferroelectricity and anti-ferromagnetism. Cross-coupling between these otherwise separate order parameters promises great advantages in the fields of multistate memory, spintronics and even medical applications. The first major challenge in this field of study is the rarity of multiferroics. Second, most known multiferroics, both intrinsic and extrinsic in nature, possess very low ME coupling coefficients. In previous studies conducted by our group, BTO-BFO multilayers deposited by pulsed laser deposition (PLD) showed a ME coupling coefficient αME enhanced by one order of magnitude, when compared to single-layers of the intrinsic multiferroic BFO. However, the mechanism of ME coupling in such heterostructures is poorly understood until now. In this thesis, we used a selection of structural, chemical, electrical and magnetic measurements to maximize the αME-coefficient and shed light on the origin of this enhanced ME effect. The comparison of BTO-BFO multilayers over single-layers revealed not only enhanced ME-coupling, but also reduced mosaicity, roughness and leakage current density in multilayers. Following a parametric sample optimization, we achieved an atomically smooth interface roughness and vast improvements in the ferroelectric properties by introducing a shadow mask in the PLD process. We measured the highest αME-value so far of 480 Vcm-1Oe-1 for a multilayer with a double-layer thickness of only 4.6 nm, two orders of magnitude larger than the coefficient of 4 Vcm-1Oe-1 measured for BFO single-layers. The αME-coefficient in these multilayers stands in an inverse correlation with the double-layer thickness ddl. The influence of oxygen pressure during growth and BTO-BFO ratio on αME was shown to be neglible in comparison to that of ddl. From the characteristic dependencies of αME on magnetic bias field, temperature and ddl, we concluded the existence of an interface-driven coupling mechanism in BTO-BFO multilayers.:1 Introduction 2 Theory of Multiferroic Magnetoelectrics 2.1 Primary Ferroic Properties 2.2 Magnetoelectric Coupling 3 Materials 3.1 The General Structure of Perovskites ABX3 3.2 Strontium Titanate SrTiO3 3.3 Barium Titanate BaTiO3 3.4 Bismuth Ferrite BiFeO3 3.5 Heterostructures Based on BiFeO3 4 Experimental Section 4.1 Thin Film Fabrication 4.2 X–Ray Diffraction 4.3 Microscopic Techniques 4.4 Chemical Analysis Techniques 4.5 Ferroelectric Characterization 4.6 Magnetic Property Measurements 4.7 Measurement of the Magnetoelectric Coupling Coefficient 5 BaTiO3–BiFeO3 Heterostructures 5.1 General Properties of Single-Layers and Multilayers of BTO and BFO 5.2 PLD–Growth of BaTiO3–BiFeO3 Multilayers 5.3 Manipulation of Multilayer Properties through Design 5.4 Effectiveness of Eclipse–PLD 5.5 Enhanced ME Effect in BaTiO3–BiFeO3 Multilayers 6 Summary and Outlook A Magnetoelectric Measurement Setup B Magnetic Background Measurements C Polarized Neutron Reflectometry Literature Own and Contributed Work Acknowledgement Erratum

This study demonstrates an experimental investigation into the efficacy of a novel surface pre-treatment technique to improve the toughness and energy dissipation of composite CFRP T-joints. This novel technique optimizes CO2 laser irradiations to remove surface contaminations and modify the surface morphology of CFRP T-joint adherents. Pull-off tests were performed on T-joints that experienced peel-ply (PP) treatment and to those that were ablated with 10% (LC) and 30% (LA) laser power respectively. A further developed alternative pattern between LA and LC surface pre-treatment was examined. Two different quasi-isotropic stacking sequences have been studied by having surface fibers aligned in 0° and 45° direction. A series of surface roughness analysis, optical microscopy, SEM, CT scan and pictorial findings have been carried out to characterize the surface morphologies and failure modes prior to and after the failure. The patterning technique promoted non-local damage mechanisms which resulted in large improvements in the toughness and energy dissipation as compared to the other pre-treatment techniques. Up to ~12 times higher energy dissipation compared to peel-ply pre-treated T-joint were achieved with patterned T-joint structures that are stacked with a 0° surface fiber direction.

This thesis deals with the deposition of epitaxial chalcogenide (Ge2Sb2Te5 (GST225), GeTe and Sb2Te3) thin films and superlattice (SL) arrangement based on GeTe-Sb2Te3 using pulsed laser deposition (PLD) technique on (111)-oriented Si substrates. The thin films are characterized using in-situ RHEED, XRD, SEM, AFM and TEM. The epitaxial trigonal GST225 films with out-of-plane c-plane orientation were grown in 2D growth mode. For the first group of the films (substrate-target distance (dts) of ~7.5 cm), the epitaxial window was observed from 200 °C to 300 °C. By varying laser frequency, deposition rate as high as 42 nm/ min can be achieved. The deposition with a slight reduction of dts to ~6 cm (second group) at moderate Ts of 220 °C results in the epitaxial films with heterogeneous vacancy structures (coexisting metastable phases. i.e. with random and ordered vacancies, and stable trigonal phase). Thermal annealing (at 220 °C) leads to a phase transformation towards a pure trigonal phase. The epitaxial Sb2Te3 films with out-of-plane (0001) oriented trigonal structure were grown at Ts from 140 to 280 °C in 2D growth mode. The optimum Ts in terms of deposition rate and film quality was determined to be 240 °C. The epitaxial growth of Sb2Te3 thin films is initiated by the self-organized formation of a Sb/Te single-atomic passivation layer on the Si surface. The growth of GeTe was initialized by the formation of an ultra-thin amorphous layer. The films were predominantly grown in the mix of 2D and 3D growth modes. The deposited films possesses trigonal structure out-of-plane (0001)-orientated on Si(111). By employing a 2D-bonded Sb2Te3 as a seeding layer on Si(111), the epitaxial window of GeTe can be extended especially towards the lower temperature regime, up to 145 °C. Additionally, the surface topography can be significantly improved, indicating that the films are grown in 2D growth mode on the buffered substrate. The epitaxial SLs can be grown starting at Ts = 140 °C. Each layer of the SLs, i.e. Sb2Te3 and GeTe layer, was grown in 2D growth mode. An intermixing of GeTe and Sb2Te3 layers occurred at a higher temperature deposition. Studies on local structure of 140 °C-deposited SL showed that the SL consists of Ge-rich Ge(x+y)Sb(2–y)Tez and Sb2Te3 units intercalated by Van der Waals gaps with the inhomogeneity of layer thickness across the SL. The obtained results demonstrate the feasibility of PLD for deposition of good quality epitaxial chalcogenide thin films and SL structure on Si(111).

High-quality lattice-matched LaNiO3/LaMnO3 superlattices with monolayer terrace structure have been grown on both (111)- and (001)-oriented SrTiO3 substrates by pulsed laser deposition. In contrast to the previously reported experiments, a magnetic exchange bias is observed that reproducibly occurs in both (111)- and (001)-oriented superlattices with the thin single layers of 5 and 7 unit cells, respectively. The exchange bias is theoretically explained by charge transfer-induced magnetic moments at Ni atoms. Furthermore, magnetization data at low temperature suggest two magnetic phases in the superlattices, with Néel temperature around 10 K. Electrical transport measurements reveal a metal-insulator transition with strong localization of electrons in the superlattices with the thin LaNiO3 layers of 4 unit cells, in which the electrical transport is dominated by two-dimensional variable range hopping.

The dielectric function of heteroepitaxial YBiO3 grown on a-Al2O3 single crystals via pulsed laser deposition is determined in the spectral range from 0.03 eV to 4.5 eV by a simultaneous modeling of the spectroscopic ellipsometry and optical transmission data of YBiO3 films of different thicknesses. The (111)-oriented YBiO3 films are nominally unstrained and crystallize in a defective fluorite-type structure with a Fm3⎯⎯m space group. From the calculated absorption spectrum, a direct electronic bandgap energy of 3.6(1) eV and the signature of an indirect electronic transition around 0.5 eV are obtained. These values provide necessary experimental feedback to previous conflicting electronic band structure calculations predicting either a topologically trivial or a non-trivial insulating ground state in YBiO3.

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CdTe and ZnTe are common semiconductors, currently used in a wide variety of applications. Heterostructures, composed of two or more layered materials, create further potential for the use of these semiconductors in the development of new technologies. In this thesis, the epitaxial growth of CdTe/ZnTe thin films and heterostructures are studied with the intention of better understanding the mechanisms by which they grow and how their overall structure and properties may be modified. Single-layer, bilayer, and multilayer structures were grown by pulsed laser deposition on sapphire substrates. The resulting crystal structure, interface, and optical properties were characterized using X-ray diffraction, UV-Vis spectroscopy, atomic force microscopy, and electron microscopy and spectroscopy techniques. It was found that the growth conditions have a direct impact on the crystal quality of these materials, that can be understood in terms of the growth dynamics and film-substrate interactions. Domain formation was also found to vary between CdTe and ZnTe depositions, revealing important information about their growth. This work presents methods of consistently producing high quality CdTe and ZnTe thin films and bilayers, and insight into how this may be applied to the growth of multilayer films. / Thesis / Master of Science (MSc)

<p>  </p> <p>Integration of functional thin films on flexible substrates has piqued interests owing to the needs of flexible devices. Selecting a suitable flexible substrate is crucial for such integration. Recently, muscovite mica has been developed as a flexible platform for functional thin film epitaxy growth. Mica can be easily peeled off due to the weak van der Waals interaction between different layers of mica, along with other advantages including cheap, high elasticity and thermal stability, biocompatible, <em>etc</em>. On the other hand, vertically aligned nanocomposites (VANs) have been attractive because of their unique anisotropic structures, which can achieve physical property anisotropy, easy tunability, out-of-plane strain engineering as well as combined multifunctionality. However, limited work on the integration of nanocomposite thin films on mica with tunable physical properties has been reported due to growth challenges. </p> <p>In this dissertation, different ceramic-metal VAN systems integrated on mica substrates towards different functionalities using pulsed laser deposition (PLD) have been demonstrated. The first chapter is on the integration of BaTiO3-Au nanocomposite system on mica. Tunable optical properties have been achieved by controlling the geometries of the Au nanostructures between nanoparticles and nanopillars by varying the growth temperature. The laser energy was also found to play a role in terms of the Au pillar dimension. The second chapter is on the integration of BaZrO3-Co VAN system on mica towards flexible spintronics. Tunable, anisotropic ferromagnetic property has been realized by controlling the aspect ratio of the Co pillars. The third chapter is on integration of BaTiO3-Fe VAN system on mica towards multiferroics. Different buffer layers have been tried out to facilitate the growth of VAN structure. Room temperature ferroelectric and anisotropic ferromagnetic properties of the films have been confirmed. The last chapter is focused on multiphase nitride-metal nanocomposite design and integration, with films showing unique optical and magnetic properties. The reliability and stability of the physical properties of the films have been verified though bending tests. The growth mechanism and criteria of ceramic-metal nanocomposite on mica have also been discussed. These demonstrations all pave a new way towards the integration and design of multifunctional nanocomposites towards flexible nanodevices.</p>

Compound semiconductors are the foundation of many electronic and optoelectronic devices. As a result semiconductor epitaxy can be viewed as the first significant step in device engineering. Accurate and reliable characterization methods are needed to measure semiconductor properties including optical, electrical, vibrational and crystal structure. In this thesis, the epitaxy of ZnTe thin films on sapphire substrate by Pulsed Laser Deposition system at different growth temperatures is studied. The texture analysis is inspected by Two Dimensional X-Ray Diffraction. The lattice constant of the films and strain studies are investigated by High Resolution X-Ray Diffraction. UV-Vis spectroscopy is applied to find absorption edge in ZnTe thin film in order to estimate optical bandgap. These common characterization methods reveal the great effect of growth temperature on crystalline and optical properties of ZnTe thin films. In addition, Raman spectroscopy is used for the first time in the Preston's group to examine vibrational modes in ZnTe thin films. This new characterization method, which is the main focus of this thesis, uncovers some new features of ZnTe thin films not accessible through other techniques. In this thesis, optimum experimental conditions, instrumentation and data analysis of Raman observations in thin films are studied in detail. The final results are in good agreement with other characterization methods and they can justify crystalline and optical observations. These results demonstrate that Raman spectroscopy is a non-destructive characterization method applicable to thin film analysis. / Thesis / Master of Applied Science (MASc)