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- ItemBiaxially Textured Titanium Thin Films by Oblique Angle Deposition: Conditions and Growth Mechanisms(Weinheim : Wiley-VCH, 2020) Liedtke-Grüner, Susann; Grüner, Christoph; Lotnyk, Andriy; Gerlach, Juergen W.; Rauschenbach, BerndGrowing highly crystalline nanowires over large substrate areas remains an ambiguous task nowadays. Herein, a time-efficient and easy-to-handle bottom-up approach is demonstrated that enables the self-assembled growth of biaxially textured Ti thin films composed of single-crystalline nanowires in a single-deposition step. Ti thin films are deposited under highly oblique incidence angles by electron beam evaporation on amorphous substrates. Substrate temperature, angle of the incoming particle flux, and working pressure are varied to optimize the crystallinity in those films. Height-resolved structure information of individual nanowires is provided by a transmission electron microscopy (TEM) nanobeam, high-resolution TEM, and electron diffraction. Ti nanowires are polycrystalline at 77 K, whereas for ≥300 K, single-crystalline nanowires are tendentially found. The Ti crystals grow along the thermodynamically favored c-direction, but the nanowires’ tilt angle is determined by shadowing. Biaxially textured Ti thin films require a certain temperature range combined with highly oblique deposition angles, which is proved by X-ray in-plane pole figures. A general correlation between average activation energy for surface self-diffusion and melting point of metals is given to estimate the significant influence of surface self-diffusion on the evolution of obliquely deposited metal thin films.
- ItemWeak electron irradiation suppresses the anomalous magnetization of N-doped diamond crystals(Weinheim : Wiley-VCH, 2021) Setzer, Annette; Esquinazi, Pablo D.; Daikos, Olesya; Scherzer, Tom; Pöppl, Andreas; Staacke, Robert; Lühmann, Tobias; Pezzagna, Sebastien; Knolle, Wolfgang; Buga, Sergei; Abel, Bernd; Meijer, JanSeveral diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of ≲80 ppm, the so-called C or P1 centers, are irradiated with electrons at 10 MeV energy and low fluence. The results show a complete suppression of the irreversible behavior in field and temperature of the magnetization below 30 K, after a decrease in ≲40 ppm in the concentration of C centers produced by the electron irradiation. This result indicates that magnetic C centers are at the origin of the large hysteretic behavior found recently in nitrogen-doped diamond crystals. This is remarkable because of the relatively low density of C centers, stressing the extraordinary role of the C centers in triggering those phenomena in diamond at relatively high temperatures. After annealing the samples at high temperatures in vacuum, the hysteretic behavior is partially recovered.
- ItemMolecular Beam Epitaxy Growth and Characterization of Germanium-Doped Cubic AlxGa1−xN(Weinheim : Wiley-VCH, 2020) Deppe, Michael; Henksmeier, Tobias; Gerlach, Jürgen W.; Reuter, Dirk; As, Donat J.In cubic (c-)GaN Ge has emerged as a promising alternative to Si for n-type doping, offering the advantage of slightly improved electrical properties. Herein, a study on Ge doping of the ternary alloy c-AlxGa1−xN is presented. Ge-doped c-AlxGa1−xN layers are grown by plasma-assisted molecular beam epitaxy. In two sample series, both the Al mole fraction x and the doping level are varied. The incorporation of Ge is verified by time-of-flight secondary ion mass spectrometry. Ge incorporation and donor concentrations rise exponentially with increasing Ge cell temperature. A maximum donor concentration of 1.4 × 1020 cm−3 is achieved. While the incorporation of Ge is almost independent of x, incorporation of O, which acts as an unintentional donor, increases for higher x. Dislocation densities start increasing when doping levels of around 3 × 1019 cm−3 are exceeded. Also photoluminescence intensities begin to drop at these high doping levels. Optical emission of layers with x > 0.25 is found to originate from a defect level 0.9 eV below the indirect bandgap, which is not related to Ge. In the investigated range 0 ≤ x ≤ 0.6, Ge is a suitable donor in c-AlxGa1−xN up to the low 1019 cm−3 range.
- ItemAntiphase Boundaries Constitute Fast Cation Diffusion Paths in SrTiO3 Memristive Devices(Weinheim : Wiley-VCH, 2020) Heisig, Thomas; Kler, Joe; Du, Hongchu; Baeumer, Christoph; Hensling, Felix; Glöß, Maria; Moors, Marco; Locatelli, Andrea; Menteş, Tevfik Onur; Genuzio, Francesca; Mayer, Joachim; De Souza, Roger A.; Dittmann, ReginaResistive switching in transition metal oxide-based metal-insulator-metal structures relies on the reversible drift of ions under an applied electric field on the nanoscale. In such structures, the formation of conductive filaments is believed to be induced by the electric-field driven migration of oxygen anions, while the cation sublattice is often considered to be inactive. This simple mechanistic picture of the switching process is incomplete as both oxygen anions and metal cations have been previously identified as mobile species under device operation. Here, spectromicroscopic techniques combined with atomistic simulations to elucidate the diffusion and drift processes that take place in the resistive switching model material SrTiO3 are used. It is demonstrated that the conductive filament in epitaxial SrTiO3 devices is not homogenous but exhibits a complex microstructure. Specifically, the filament consists of a conductive Ti3+-rich region and insulating Sr-rich islands. Transmission electron microscopy shows that the Sr-rich islands emerge above Ruddlesden–Popper type antiphase boundaries. The role of these extended defects is clarified by molecular static and molecular dynamic simulations, which reveal that the Ruddlesden–Popper antiphase boundaries constitute diffusion fast-paths for Sr cations in the perovskites structure. © 2020 The Authors. Published by Wiley-VCH GmbH
- ItemCompositional Patterning in Carbon Implanted Titania Nanotubes(Weinheim : Wiley-VCH, 2021) Kupferer, Astrid; Holm, Alexander; Lotnyk, Andriy; Mändl, Stephan; Mayr, Stefan G.Ranging from novel solar cells to smart biosensors, titania nanotube arrays constitute a highly functional material for various applications. A promising route to modify material characteristics while preserving the amorphous nanotube structure is present when applying low-energy ion implantation. In this study, the interplay of phenomenological effects observed upon implantation of low fluences in the unique 3D structure is reported: sputtering versus readsorption and plastic flow, amorphization versus crystallization and compositional patterning. Patterning within the oxygen and carbon subsystem is revealed using transmission electron microscopy. By applying a Cahn–Hilliard approach within the framework of driven alloys, characteristic length scales are derived and it is demonstrated that compositional patterning is expected on free enthalpy grounds, as predicted by density functional theory based ab initio calculations. Hence, an attractive material with increased conductivity for advanced devices is provided. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
- ItemDefect-Induced Magnetism in Nonmagnetic Oxides: Basic Principles, Experimental Evidence, and Possible Devices with ZnO and TiO2(Weinheim : Wiley-VCH, 2020) Esquinazi, Pablo David; Hergert, Wolfram; Stiller, Markus; Botsch, Lukas; Ohldag, Hendrik; Spemann, Daniel; Hoffmann, Martin; Adeagbo, Waheed A.; Chassé, Angelika; Nayak, Sanjeev K.; Ben Hamed, HichemThe magnetic moment and the magnetic order induced by localized defects, like vacancies, interstitials, and/or nonmagnetic (NM) ions, in a NM oxide atomic lattice are discussed. When the defect concentration is of the order of or larger than ≈3 at%, magnetic order at room temperature can appear. Herein, the theoretical basic principles needed to understand and compute this new magnetic phenomenon in solids are developed in detail. In particular, the main results of density functional theory (DFT) calculations are used to estimate the magnetization and X-ray magnetic circular dichroism (XMCD) values. The main experimental evidences on this phenomenon are reviewed, especially magnetization, the element-specific XMCD, and transport properties in two selected oxides, ZnO and TiO2. Emphasis is given on the simplicity and efficiency ion irradiation methods have to trigger magnetic order in these oxides as well as a very sensitive method to characterize magnetic impurities. Two possible applications of this phenomenon are discussed, namely spin filtering at magnetic/NM interfaces in ZnO and perpendicular magnetic anisotropy triggered in TiO2 anatase microstructures. The existing literature on defect-induced magnetism in oxides is shortly reviewed, which provides further evidence on the robustness of this phenomenon in solids.