Manganese-based Heusler compounds display intriguing fundamental physical properties, determined by the delicate balance of magnetic interactions that give rise to real and reciprocal-space topology, sparking the interest in their potential application in the spin-based technology of the future. In this thesis, a thorough study of thin films of two Mn-based Heusler compounds, the hexagonal MnPtGa and inverse tetragonal Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) system, was performed. The observation of Néel-type skyrmions in single-crystalline MnPtGa motivated our interest in the growth and characterization of thin films of this compound. The films were deposited by magnetron sputtering on (0001)-Al2O3 single crystalline substrates, achieving the epitaxial growth of the Ni2In-type hexagonal crystal structure (P6_3/mmc space group, no. 194). Two thermally-induced magnetic transitions were identified in MnPtGa thin films: below the ordering temperature (T_C=273 K) the system becomes ferromagnetic, followed by a spin-reorientation transition at T_sr=160 K, adopting a spin-canted magnetic structure. Resorting to single-crystal neutron diffraction (SCND), we were able to resolve the magnetic ground state of our MnPtGa thin films. The Mn magnetic moments were found to tilt 20 degrees away from the c-axis, forming a commensurate magnetic structure with a ferromagnetic component along the crystallographic c-axis and a staggered antiferromagnetic one in the basal plane. This further demonstrated the applicability of a bulk technique, such as SCND, to the study of magnetic structures in thin films. Additionally, the perpendicular magnetic anisotropy (PMA) in the system was determined by magnetometry technique. Electrical magnetotransport measurements were performed in a thickness series of MnPtGa thin films. A non-monotonous anomalous Hall conductivity (AHC) was observed, whose intrinsic Berry-curvature origin was elucidated by means of first-principle calculations. We further observed by magnetic force microscopy technique the nucleation of irregular magnetic bubbles under the application of a magnetic field. We tentatively link their appearance to the onset of an additional electron scattering mechanism contributing to the transverse resistivity. In the second part of this thesis, the inverse tetragonal Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) system was investigated. The films were grown on MgO(100) single crystalline substrates, promoting the epitaxial growth of the tetragonal structure (I-4m2 space group, no. 119). We primarily focused on the impact of the systematic substitution of iridium on the structural, magnetic and electrical (magneto)transport properties of the system. A compression of the basal lattice parameters and elongation of the c-axis, accompanied by larger crystallographic disorder, was observed as the Ir content (x) increased, altering the Mn-Mn exchange interactions and therefore the magnetic properties of the compound. Mn2RhSn have two thermally-induced magnetic transitions: first, to a collinear ferrimagnetic state below the Curie temperature (T_C=280 K), followed by a spin-reorientation transition at T_sr=80 K to a noncollinear state, determined by two inequivalent Mn sublattices. A reduction of both T_C and T_sr was observed, as well as a tendency towards a hard-axis ferromagnet and therefore larger PMA as the Ir content of the films was increased. Additionally, a reduction of the saturation magnetization suggest a change of the magnitude of the spin canting upon Ir-substitution. The electrical magnetotransport properties of the Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) thin films were acquired and analyzed in a wide temperature and magnetic field range. A strongly temperature and composition dependent non-monotonous AHC was found, suggesting two regimes in the electronic transport: (i) a nearly x-independent regime dominated by intrinsic Berry-curvature and (ii) a strongly x-dependent regime suggesting a more relevant role from extrinsic mechanisms contributing to the AHC. On the other hand, the Mn2Rh(0.95)Ir(0.05)Sn bulk system is known to host magnetic skyrmion and antiskyrmion phases. We indirectly assessed the impact of the systematic Ir-substitution on the (anti)skyrmionic phases through the analysis of the sign of the topological Hall effect in our thin films. A tendency towards the suppression of the low-T skyrmion phase stabilized by magnetic dipole-dipole interaction, and subsistence of the high-T antiskyrmion phase in Mn2Rh(1-x)Ir(x)Sn thin films was found as x > 0.2, which can be interpreted as a change of magnitude of the anisotropic DMI in this tetragonal D_2d system upon Ir-substitution. We have thus demonstrated that the magnetic and topological properties of the Mn2Rh(1-x)Ir(x)Sn system can be tailored upon chemical substitution, showing a strongly intertwined relation between composition, crystal and electronic structure, with the emergence of exotic magnetic phases, ultimately reflected in their electrical transport signatures.:Abstract iii Abbreviations iv Symbols vi Preface xii 1 Fundamentals 1 1.1 Noncollinear magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Magnetic interactions in solids . . . . . . . . . . . . . . . . . . . 2 1.1.1.1 Exchange interaction . . . . . . . . . . . . . . . . . . . 2 1.1.1.2 Dzyaloshinsky-Moriya interaction . . . . . . . . . . . . 3 1.1.1.3 Magnetic anisotropy . . . . . . . . . . . . . . . . . . . 4 1.1.1.4 Magnetic dipolar interaction . . . . . . . . . . . . . . . 5 1.1.2 Spin-reorientation transition . . . . . . . . . . . . . . . . . . . . 5 1.1.3 Magnetic skyrmions and antiskyrmions . . . . . . . . . . . . . . 6 1.1.3.1 Antiskyrmions in Heusler compounds . . . . . . . . . . 8 1.2 Magnetic Heusler compounds . . . . . . . . . . . . . . . . . . . . . . . 9 1.2.1 Cubic crystal structure . . . . . . . . . . . . . . . . . . . . . . . 10 1.2.2 Distorted crystal structures . . . . . . . . . . . . . . . . . . . . 10 1.2.2.1 Tetragonal Heusler compounds . . . . . . . . . . . . . 11 1.2.2.2 Hexagonal Heusler compounds . . . . . . . . . . . . . 11 1.3 Charge and spin transport in ferromagnets . . . . . . . . . . . . . . . . 13 1.3.1 The two-current model . . . . . . . . . . . . . . . . . . . . . . . 13 1.3.2 The Hall effect . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.3.2.1 Anomalous Hall effect . . . . . . . . . . . . . . . . . . 15 1.3.2.2 Topological Hall effect . . . . . . . . . . . . . . . . . . 17 1.4 Neutron scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.4.1 Thermal Neutrons . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.4.1.1 Scattering cross sections . . . . . . . . . . . . . . . . . 19 1.4.1.2 The four-circle diffractometer . . . . . . . . . . . . . . 23 xv 1.4.2 Magnetic neutron scattering . . . . . . . . . . . . . . . . . . . . 24 2 Experimental Techniques 29 2.1 Magnetron sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.1.1 Thin films growth modes . . . . . . . . . . . . . . . . . . . . . . 32 2.1.2 Thin films microstructure . . . . . . . . . . . . . . . . . . . . . 33 2.2 X-ray characterization of thin films . . . . . . . . . . . . . . . . . . . . 34 2.2.1 Geometry of the X-ray diffractometer . . . . . . . . . . . . . . . 35 2.2.2 Radial θ-2θ scan . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2.3 ϕ -scans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2.4 Rocking curves (ω-scans) . . . . . . . . . . . . . . . . . . . . . . 36 2.2.5 X-ray reflectivity (XRR) . . . . . . . . . . . . . . . . . . . . . . 37 2.3 Composition analysis: energy dispersive X-ray spectroscopy (EDS) . . . 38 2.4 Surface characterization: atomic and magnetic force microscopy . . . . 38 2.5 D10 thermal neutron diffractometer . . . . . . . . . . . . . . . . . . . . 39 2.6 SQUID-VSM magnetometry . . . . . . . . . . . . . . . . . . . . . . . . 40 2.7 Electrical (magneto-)transport measurements . . . . . . . . . . . . . . 41 3 Noncollinear magnetism in MnPtGa epitaxial thin films 43 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.2 MnPtGa thin films: growth and characterization . . . . . . . . . . . . . 45 3.2.1 Growth conditions . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.2.2 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.3 Magnetic properties of MnPtGa thin films . . . . . . . . . . . . . . . . 49 3.3.1 Thermal evolution of the magnetic structure . . . . . . . . . . . 49 3.3.2 Field dependent magnetization . . . . . . . . . . . . . . . . . . 50 3.3.3 Single-crystal neutron diffraction in MnPtGa thin films . . . . . 52 3.3.3.1 Ferromagnetic phase . . . . . . . . . . . . . . . . . . . 54 3.3.3.2 Noncollinear phase . . . . . . . . . . . . . . . . . . . . 55 3.4 Electronic band structure of h-MnPtGa . . . . . . . . . . . . . . . . . . 57 3.5 Electrical magnetotransport properties of MnPtGa thin films . . . . . . 59 3.5.1 Zero field longitudinal resistivity . . . . . . . . . . . . . . . . . . 60 3.5.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.5.3 Magnetic transitions under a magnetic field . . . . . . . . . . . 64 3.6 Intrinsic origin of the anomalous Hall effect . . . . . . . . . . . . . . . . 65 3.6.1 Scaling of the anomalous Hall conductivity vs. σxx . . . . . . . 68 3.7 Spin textures in MnPtGa thin films . . . . . . . . . . . . . . . . . . . . 73 3.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4 Tuning the magnetic and topological properties of Mn2Rh1−xIrxSn epitaxial thin films 83 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.2 Growth and characterization of Mn2Rh1−xIrxSn thin films . . . . . . . 86 4.2.1 Growth conditions and Ir substitution . . . . . . . . . . . . . . 86 4.2.2 Crystal structure of Mn2Rh1−xIrxSn . . . . . . . . . . . . . . . . 87 4.3 Tuning the magnetic properties of the Mn2Rh1−xIrxSn system . . . . . 91 xvi 4.3.1 Thermal magnetic transitions . . . . . . . . . . . . . . . . . . . 91 4.3.2 Increasing the magnetic anisotropy under Ir-substitution . . . . 92 4.4 Electrical (magneto-)transport properties of Mn2Rh1−xIrxSn thin films 94 4.4.1 Zero-field longitudinal resistivity and spin reorientation transition 94 4.4.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . 95 4.4.3 Hall effects: from ordinary to anomalous & topological . . . . . 96 4.4.3.1 Ordinary Hall effect . . . . . . . . . . . . . . . . . . . 97 4.4.3.2 Anomalous Hall effect . . . . . . . . . . . . . . . . . . 98 4.4.3.3 Competing mechanisms in the AHC of the Mn2Rh1−xIrxSn system . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.4.3.4 Scaling of the AHC with the magnetization . . . . . . 101 4.4.3.5 Topological Hall effect . . . . . . . . . . . . . . . . . . 102 4.5 Tuning the (Anti-)Skyrmion phases . . . . . . . . . . . . . . . . . . . . 106 4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 5 Conclusions & Outlook 111 List of Figures 117 List of Tables 120 List of Publications 124 Aknowledgements 124 Bibliography 127 Eigenständigkeitserklärung 147

ITO is widely regarded as the optimal TCO for serving as front window layer in PSK/c-Si tandem solar cells. It is known to effectively mitigate several stability issues present in perovskite solar cells while demonstrating excellent lateral conductivities and optical transparency across the entire solar spectrum. However, due to the damaging effects of traditional magnetron sputtering methods on the underlying cell precursor and the limited range of annealing temperatures viable for maintaining the stability of Perovskite Solar cells, realizing the full capability of ITO layer is constrained. This investigation focuses on developing and optimizing the front Indium Tin Oxide (ITO) layer properties for high-efficiency monolithic Perovskite/PERC tandem solarcells. The study employs two widely employed industrial techniques, Magnetron Sputtering and Screen Printing for the deposition of ITO thin-films and subsequent metallization of Ag front contacts. The sputtering process parameters, namely the carrier speed, O2 : Ar ratio, and the sputter power were varied to obtain an optimized ITO layer, which exhibited a thickness of 53nm, Rsheet of 107 ohm/□, mobility of 37 cm2/V s, and 90 % average optical transparency between 400−1200nm. A low contact resistivity of 5.4mΩ·cm2 was achieved between the ITO and metal contacts which is the lowest reported value for ITO annealed at low temperature (140 °C). Champion cells, featuring Perovskite on Ohmic substrate and 2T perovskite/PERC tandem cells, exhibited high VOC values of 1.116 V and 1.75 V on 0.97 cm2 cell aperture areas and cell efficiencies of 17.2 % and 23.85 %. Additionally, a large area (158.7 cm2) tandem cell was also fabricated which demonstrated an excellent VOC of 1.75 V . The results of this investigation demonstrates the versatility of ITO layer properties achievable at low-temperatures through Magnetron sputtering and underscores the potential of existing commercialized technologies for the fabrication of high-efficiency tandem solar cells. / ITO anses allmänt vara den optimala TCO för användning som frontfönsterskikt i PSK/c-Si tandemsolceller. Den är känd för att effektivt mildra flera stabilitetsproblem som finns i perovskitsolceller samtidigt som den uppvisar utmärkt lateral konduktivitet och optisk transparens över hela solspektrumet. På grund av de skadliga effekterna av traditionella magnetronförstoftningsmetoder på den underliggande cellprekursorn och det begränsade intervallet av glödgningstemperaturer som är användbara för att upprätthålla stabiliteten hos perovskitsolceller, begränsas dock ITO-skiktens fulla kapacitet. Denna undersökning fokuserar på att utveckla och optimera egenskaperna hos det främre Indium Tin Oxide (ITO)-skiktet för högeffektiva monolitiska Perovskite/PERC tandemsolceller. I studien används två allmänt använda industriella tekniker, magnetronförstoftning och screentryckning, fördeponering av ITO-tunnfilmer och efterföljande metallisering av Ag-frontkontakter. Parametrarna för sputteringsprocessen, nämligen bärarhastigheten, förhållandet O2 : Ar och sputterkraften varierades för att få ett optimerat ITO-lager, som uppvisade en tjocklek på 53nm, Rsheet på 107 ohm/□, rörlighet på 37 cm2/V s och 90 % genomsnittlig optisk transparens mellan 400 − 1200 nm. En låg kontaktresistivitet på 5.4mΩ.cm2 uppnåddes mellan ITO och metallkontakterna, vilket är de lägstarapporterade värdena för ITO glödgat vid låg temperatur (140 °C). Champion-cellerna, med perovskit på ohmskt substrat och 2T perovskit/PERC tandemkonfigurationer, uppvisade höga VOC-värden på 1.116 V och 1.75 V på 0.97 cm2 cellaperturområden och cellverkningsgrader på 17.2 % och 23.85 %. Dessutom tillverkades en tandemcell med stor area (158.7 cm2) som uppvisade en utmärkt VOC på 1.75 V . Resultaten av denna undersökning visar mångsidigheten hos ITO-skiktets egenskaper som kan uppnås vid låga temperaturer genom magnetronförstoftning och understryker potentialen hos befintliga kommersialiserade tekniker för tillverkning av högeffektiva tandemsolceller.

We present an active learning algorithm for identifying viable process conditions in magnetron sputtering experiments. The algorithm trains a classifier to predict which gas pressure and magnetron power combinations yield stable discharge with deposition rates exceeding a minimum threshold. A computation-based oracle that labels experiments using QCM readings facilitates a fully automated learning procedure, laying the groundwork for a self-driving lab where novel materials will be explored for next-generation solar cells. Upon evaluation, the active learning algorithm results in significantly higher sample efficiency than traditional supervised learning across a range of magnetron sputtering experiments. Moreover, a sampling sequence analysis shows that active learning enables an informed search of the process parameter space, generating patterns that approximate Paschen’s law. The work presented in this thesis serves as a first step toward a fully automated materials synthesis process, where the input region of viable synthesis parameters can be identified with minimal experimentation. The solution allows researchers to efficiently narrow the search space of optimal process conditions for targeted materials design.

The aim of this thesis is to determine the mechanical properties of copper, titanium and carbon thin films deposited on foil substrates, and identify how the properties are affected by the deposition process. This is important when such coated foils are subjected to mechanical deformation during applications. Three coating materials, Cu, Ti, and C, were evaluated on PET and Al foils. The materials were deposited by direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS). The crack initiation and propagation during tensile tests were investigated in-situ by a SEM. The coatings’ crack onset strain (COS), cohesive strength, interfacial shear strength (IFSS), and Weibull shape and scale parameters were successfully determined from the experimental data. The results showed the Cu coatings had similar cohesive strengths and IFSS, independent of the deposition process. The main difference was the COS, where thin films deposited with ion assistance displayed a higher value. The coatings also displayed different morphologies that clearly influenced the crack propagation. Larger grains hindered the crack propagation and resulted in a more ductile fragmentation, with coatings displaying short and tortuous cracks. All Ti films displayed similar fragmentation and thus mechanical properties, despite small differences in morphology. However, the surface roughness of the Al foil influences the results. Localized stresses arise in the rolling tracks from the foil production and facilitates the crack propagation, thus affecting the fragmentation of the coatings. Lastly, it could be shown that the addition of a thin Ti adhesion layer resulted in a fully adherent C film.

Ce mémoire présente les efforts effectués pour réduire l'angle de perte de couches de pentoxyde de tantale amorphes telles qu'utilisées pour les miroirs de LIGO. Afin d'améliorer le niveau de relaxation des couches, celles-ci ont été déposées par pulvérisation cathodique magnétron à des températures allant de 50 °C à 480 °C, elles ont subi un recuit thermique rapide, elles ont été implantées par des ions d'oxygène, elles ont été déposées par pulvérisation cathodique magnétron en appliquant une tension de polarisation sur le substrat lors du dépôt allant de 0 V à -450 V et elles ont été déposées par pulvérisation cathodique magnétron pulsée à haute puissance dans le cadre de différentes expériences. L'angle de perte, l'épaisseur, la rugosité, l'indice de réfraction, la composition atomique, la contrainte, l'état de relaxation et le module de Young des couches ont par la suite été trouvés à l'aide de l'ellipsométrie spectralement résolue, la spectrométrie de rétrodiffusion de Rutherford, la détection des reculs élastiques, la spectroscopie Raman, la diffraction de rayons X et la nano-indentation. Il a été trouvé que la température de dépôt améliorait légèrement le degré de relaxation des couches jusqu'à 250 °C, mais qu'elle avait peu d'impact après recuit. Aussi, lors de dépôt à température de la pièce, une forte tension de polarisation réduit l'angle de perte, mais cet effet est encore une fois perdu suite au recuit. Les autres méthodes mentionnées ci-dessus n'ont pas influencé le degré de relaxation des couches selon l'angle de perte, la spectroscopie Raman et la diffraction de rayons X. Cette recherche a été réalisée avec le support financier du CRSNG et du FRQNT (numéro de dossier 206976). / This master's thesis presents the experiments made to reduce the loss angle of tantala coatings similar to those used in LIGO. To improve the relxation level of the coatings they were deposited by magnetron sputtering at temperatures varying from 50 °C to 480 °C. They were also subjected to rapid thermal annealing, and oxygen implantation. In another experiment, the coatings were deposited by magnetron sputtering with substrate biasing varying from 0 V to -450 V at room temperature and at 250 °C. Finally, the coatings of tantala were deposited by high power impulse magnetron sputtering. The loss angle, thickness, roughness, refractive index, atomic composition, stress, the relaxation state and Young's modulus of the coatings were characterized using spectroscopic ellipsometry, Rutherford backscattering, elastic recoil detection, Raman spectroscopy, X-ray diffraction and nanoindentation. It was found that the deposition temperature improved the loss angle until it reached 250 °C. However, annealing the coatings had a superior impact and the influence of the deposition temperature was not visible after annealing. When was applied a high bias to the susbtrate at room temperature, the obtained coating was slightly more relaxed than when a low bias was applied but this effect is, once again, insignificant after annealing. The other methods of deposition mentioned did not improve the loss angle or modify the relaxation state found by Raman spectroscopy and X-ray diffraction of the tantala coatings. This research was made with the financial support of the NSERC and of the FRQNT (file number 206976).

En 2015, le Laser Interferometer Gravitational-Wave Observatory (LIGO) a observé pour la première fois des ondes gravitationnelles générées par la fusion de deux trous noirs. Cette observation résulte de 40 ans d’efforts afin de réduire au minimum les sources de bruit qui affectent l’interféromètre. À ce jour, la sensibilité de LIGO, dans son domaine de fréquence le plus sensible, est limitée par la granularité de la lumière d’une part et, d’autre part, par un phénomène de fluctuations thermiques résultat de la dissipation mécanique dans les couches minces qui constituent ses miroirs, en particulier dans le matériau ayant un haut indice de réfraction : l’oxyde de tantale. Une amélioration de la sensibilité permettrait d’observer davantage d’événement, d’autres phénomènes gravitationnels, ainsi que des détails importants permettant de mieux les comprendre. Ce mémoire présente les résultats de nos recherches afin de diminuer le bruit causé par la dissipation mécanique interne dans les couches à haut indice de réfraction. Pour ce faire, des couches d’oxyde de tantale ont été dopées soit au zirconium ou à la fois au zirconium et au titane, par co-dépôt. Des couches avec différentes quantités de dopant ont été synthétisées par pulvérisation cathodique magnétron sur des substrats de silice semblables à ceux de LIGO. Par la suite, la dissipation mécanique, l’épaisseur, la rugosité, la composition, la densité surfacique, et la microstructure ont été caractérisées par suspension nodale, ellipsométrie résolue spectralement, spectrométrie de rétrodiffusion de Rutherford et la spectroscopie Raman. Il appert que le zirconium permet d’augmenter la température de recuit avant la cristallisation, ce qui permet de diminuer plus amplement la dissipation mécanique interne, mais ne change pas la dissipation à une température de recuit donnée. Il a aussi été déterminé que la concentration de titane permettait de diminuer l’angle de perte, peu importe la concentration de zirconium. Une combinaison des deux dopages et un recuit à haute température permet ainsi de recuire par un facteur d’environ 1.5 la dissipation mécanique interne. La différence de coefficient d’expansion thermique durant les recuits à haute température induit cependant des problèmes de craquement des couches, partiellement résolus par l’application d’une couche de recouvrement en silice. / In 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) observed for the first time gravitational waves generated by the merger of two black holes. This observation was the resut of 40 years of efforts to minimize the noise source which affect the the interferometer. To this date, the sensitivity of LIGO, in its most sensitive frequency domain, is limited by the granularity of the light on one hand, on the other, by a phenomenon of thermal fluctuations resulting from the mechanical dissipation in the thin film of the miroir, in particular in the high refractive index material: tantala. An improvement of the sensitivity would allow the measurement of more events, other gravitational phenomena and some details that would result in a better understanding. This master’s thesis presents results of our research to reduce the noise caused by internal mechanical dissipation in high refractive index layers. To do so, tantala layers were doped with either zirconium and titanium by co-deposition. Layers with different amounts of dopant were synthesized by magnetron sputtering on fused silica substrate similar to those of LIGO. Subsequently, mechanical dissipation, thickness, roughness, composition, areal density and microstructure were characterized by gentle nodal suspension, spectrally resolved ellipsometry, Rutherford backscattering spectroscopy, and Raman spectroscopy. It appears that zirconium allows the annealing temperature to be increased before crystallization which further decreases internal mechanical dissipation, but does not change dissipation at a given annealing temperature. It was also determined that the concentration of titanium reduced the loss angle regardless of the zirconium concentration. A combination of the two dopant and high annealing temperatures thus enables the internal mechanical dissipation to be lower by a factor of 1.5.The difference in thermal expansion coefficient during high temperature annealing, however, induces layer cracking problems, partially resolved by the application of a silica cap.

Im Fokus der vorliegenden Arbeit lag die Untersuchung polykristalliner TiO2:Ta-Schichten, hergestellt mittels Gleichstrom-Magnetron-Sputtern durch Verwendung reduzierter keramischer Targets und anschließender thermischer Nachbehandlung im Vakuum der zunächst nichtleitfähigen amorphen Precursorschichten. Es wurden die physikalischen Zusammenhänge, welche die strukturellen, elektrischen und optischen Eigenschaften der kristallinen TiO2:Ta-Schichten beeinflussen analysiert und dabei eine empfindliche Abhängigkeit vom Sauerstofffluss während der Abscheidung festgestellt. Es zeigte sich, dass die Verringerung der kinetischen Energie der Plasmateilchen beim Magnetron-Sputtern durch die Erhöhung des Gesamtdruckes vorteilhaft ist, um das Wachstum des gegenüber Rutil besser leitfähigen Anatas in Verbindung mit dem für niedrige Widerstände notwendigen Sauerstoffdefizit zu realisieren. Bei einem Gesamtdruck von 2 Pa abgeschiedene polykristalline TiO2:Ta-Schichten haben einen spezifischen Widerstand von 1,5·10-3 Ωcm, eine hohe Ladungsträgermobilität (≈8 cm2V-1s-1) und einen geringen Extinktionskoeffizienten von 0,006. Die Abhängigkeit des elektrischen Widerstandes vom Sauerstoffdefizit in der TiO2:Ta-Schicht wurde unter dem Gesichtspunkt der Ladungsträgeraktivierung sowie der Bildung von Ti-Fehlstellen diskutiert, welche vermutlich zur Kompensation und Lokalisierung von freien Elektronen beitragen. Darüber hinaus wurde zur effizienteren Gestaltung der thermischen Nachbehandlung die konventionelle Vakuumtemperung erstmalig erfolgreich durch die Blitzlampentemperung ersetzt. / The work is focused on understanding the physical processes responsible for the modification of the structural, electrical and optical properties of polycrystalline TiO2:Ta films formed by vacuum annealing of initially not conductive amorphous films deposited by direct current magnetron sputtering. It is shown that the oxygen deficiency of amorphous and annealed TiO2:Ta films, respectively, is critical to achieve low resistivity and high optical transmittance of the crystalline films. Increasing the total pressure during magnetron sputter deposition is shown to be beneficial to achieve the desired oxygen deficient anatase growth, which is discussed in terms of energetic particle bombardment. Polycrystalline anatase TiO2:Ta films of low electrical resistivity (1,5·10-3 Ωcm), high free electron mobility (≈8 cm2V-1s-1), and low extinction (0,006) are obtained in this way at a total pressure of 2 Pa. The dependence of the polycrystalline film electrical properties on the oxygen content is discussed in terms of Ta dopant electrical activation as well as transport limiting processes taking into account the formation of Ti-vacancies. In addition, the conventional vacuum annealing has been successfully substituted by the flash lamp annealing in the millisecond range.

The work presented in this thesis deals with experimental and theoretical studies related to alumina thin films. Alumina, Al2O3, is a polymorphic material utilized in a variety of applications, e.g., in the form of thin films. However, controlling thin film growth of this material, in particular at low substrate temperatures, is not straightforward. The aim of this work is to increase the understanding of the basic mechanisms governing alumina growth and to investigate novel ways of synthesizing alumina coatings. The thesis can be divided into two main parts, where the first part deals with fundamental studies of mechanisms affecting alumina growth and the second part with more application-oriented studies of high power impulse magnetron sputter (HiPIMS) deposition of the material. In the first part, it was shown that the thermodynamically stable α phase, which normally is synthesized at substrate temperatures of around 1000 °C, can be grown using reactive sputtering at a substrate temperature of merely 500 °C by controlling the nucleation surface. This was done by predepositing a Cr2O3 nucleation layer. Moreover, it was found that an additional requirement for the formation of the α phase is that the depositions are carried out at low enough total pressure and high enough oxygen partial pressure. Based on these observations, it was concluded that energetic bombardment, plausibly originating from energetic oxygen, is necessary for the formation of α-alumina (in addition to the effect of the chromia nucleation layer). Moreover, the effects of residual water on the growth of crystalline films were investigated by varying the partial pressure of water in the ultra high vacuum (UHV) chamber. Films deposited onto chromia nucleation layers exhibited a columnar structure and consisted of crystalline α-alumina if deposited under UHV conditions. However, as water to a partial pressure of 1*10-5 Torr was introduced, the columnar α-alumina growth was disrupted. Instead, a microstructure consisting of small, equiaxed grains was formed, and the γ-alumina content was found to increase with increasing film thickness. To gain a better understanding of the atomistic processes occurring on the surface, density functional theory based computational studies of adsorption and diffusion of Al, O, AlO, and O2 on different α-alumina (0001) surfaces were also performed. The results give possible reasons for the difficulties in growing the α phase at low temperatures through the identification of several metastable adsorption sites and also show how adsorbed hydrogen might inhibit further growth of α-alumina crystallites. In addition, it was shown that the Al surface diffusion activation energies are unexpectedly low, suggesting that limited surface diffusivity is not the main obstacle for low-temperature α-alumina growth. Instead, it is suggested to be more important to find ways of reducing the amount of impurities, especially hydrogen, in the process and to facilitate α-alumina nucleation when designing new processes for low-temperature deposition of α-alumina. In the second part of the thesis, reactive HiPIMS deposition of alumina was studied. In HiPIMS, a high-density plasma is created by applying very high power to the sputtering magnetron at a low duty cycle. It was found, both from experiments and modeling, that the use of HiPIMS drastically influences the characteristics of the reactive sputtering process, causing reduced target poisoning and thereby reduced or eliminated hysteresis effects and relatively high deposition rates of stoichiometric alumina films. This is not only of importance for alumina growth, but for reactive sputter deposition in general, where hysteresis effects and loss of deposition rate pose a substantial problem. Moreover, it was found that the energetic and ionized deposition flux in the HiPIMS discharge can be used to lower the deposition temperature of α-alumina. Coatings predominantly consisting of the α phase were grown at temperatures as low as 650 °C directly onto cemented carbide substrates without the use of nucleation layers. Such coatings were also deposited onto cutting inserts and were tested in a steel turning application. The coatings were found to increase the crater wear resistance compared to a benchmark TiAlN coating, and the process consequently shows great potential for further development towards industrial applications.

In the present work Mg films prepared by RF magnetron sputtering were studied. Variable energy positron annihilation spectroscopy (VEPAS) was employed for investigation of defects in the Mg films. VEPAS characterization was combined with scanning electron microscopy and X-ray diffraction in order to determine grain size, phase composition and texture. The effect of different deposition rate and deposition temperature, annealing, various substrates and film thickness on the structure and amount of defects present in the Mg films was examined. Defect studies by VEPAS showed that positrons in studied Mg films are trapped at misfit dislocations and at vacancy-like defects in grain boundaries and their density can be reduced by the deposition at elevated temperature. 1

Grâce à ses excellentes propriétés physiques et chimiques (stable thermiquement, bon conducteur électrique et de chaleur, ductile, très dur mécaniquement, bonne inertie chimique), le matériau tantale et son nitrure TaNx sont utilisés comme revêtement de surface des outils, résistance électrique, barrière de diffusion au cuivre, croissance de nanotubes par un procédé chimique catalytique en phase vapeur. C’est ce matériau et son nitrure que nous avons étudiés lors de cette thèse.Aujourd’hui les exigences des industriels nécessitent que la pulvérisation cathodique magnétron (PCM) puisse être appliquée aux pièces de formes complexes. La principale limitation de cette méthode de dépôt est que la plupart des particules pulvérisées sont neutres. Pour contrôler l’énergie et la trajectoire des particules pulvérisées, des nouveaux procédés IPVD (Ionized Physical Vapor Deposition) ont été développés pour ioniser les atomes pulvérisés. Le procédé RF-IPVD (Radio-Frequency Ionized Physical Vapor Deposition) permet, grâce à une boucle placée entre la cible et le substrat et polarisée en RF, de créer un second plasma permettant d’ioniser la vapeur pulvérisée. Un autre procédé a été développé : nommé HIPIMS (High Power Impulse Magnetron Sputtering), ce procédé utilise une alimentation fournissant des impulsions de courte durée et de forte puissance au lieu d’une alimentation DC. Les particules pulvérisées peuvent être ionisées dans le plasma magnétron qui est très dense lors des impulsions. Nous avons réalisé des couches minces de Ta par PCM, RF-IPVD et HIPIMS, et des couches minces de TaNx par PCM et HIPIMS. Les différentes propriétés des décharges et des couches minces sont étudiées et comparées dans ce mémoire. / Thanks to their excellent physical and chimical characteristics such as good stability with temperature, good conductor of heat and electricity, ductility, hardness, chemical inertness and good corrosion resistance, tantalum and its nitride are used in a wide variety of applications such as wear and corrosion-resistant materials, thin film transistors, diffusion barrier for copper and for carbon nanotube grown by CCVD process (catalytically chemical vapor deposition). For some recent industrial demand, it is necessary to deposit on substrates with complex shape. The main disadvantage of the conventional magnetron sputtering (CMS) is that most of the sputtered particles are neutral. To controle the energy and the path of sputtered particles, new magnetron sputtering techniques have been developed for ionizing a significant fraction of sputtered material. A new sputtering process called RF-IPVD consists in ionizing the sputtered vapor by adding second plasma by a RF coil between the target and the substrate. Another method called HIPIMS (High Power Impulsed Magnetron Sputtering), uses high power impulse instead of DC power. During the impulse, the sputtered Ta atoms are ionized in the dense plasma. We have deposited Ta thin films by CMS, RF-IPVD and HIPIMS and TaNx thin films by CMS and HIPIMS. The objective of this thesis is to compare the properties of discharges and thin films deposited by these different techniques.