Impact of defects on magnetic properties of spinel zinc ferrite thin films. - In: Physica status solidi, ISSN 1521-3951, Bd. 257 (2020), 7, 1900630, insges. 11 S.
The recent developments in the study of magnetic properties in the spinel zinc ferrite system are explored. Engineering of ionic valence and site distribution allows tailoring of magnetic interactions. Recent literature is reviewed, and own investigations are presented for a conclusive understanding of the mechanisms responsible for the magnetic behavior in this material system. By varying the Zn-to-Fe ratio, the deposition, as well as thermal annealing conditions, ZnFe2O4 thin films with a wide range of crystalline quality are produced. In particular, the focus is on the magnetic structure in relation to spectroscopic properties of disordered ZnFe2O4 thin films. Comparing the cation distribution in film bulk (optical transitions in the dielectric function) and near-surface region (X-ray absorption), it is found that an inhomogeneous cation distribution leads to a weaker magnetic response in films of inverse configuration, whereas defects in the normal spinel are likely to be found at the film surface. The results show that it is possible to engineer the defect distribution in the magnetic spinel ferrite film structure and tailor their magnetic properties on demand. It is demonstrated that these properties can be read out optically, which allows controlled growth of the material and applications in future magneto-optical devices.
https://doi.org/10.1002/pssb.201900630
Atomic surface structure of MOVPE-prepared GaP(111)B. - In: Applied surface science, Bd. 534 (2020), 147346
Controlling the surface formation of the group-V face of (111)-oriented III-V semiconductors is crucial for subsequent successful growth of III-V nanowires for electronic and optoelectronic applications. With a view to preparing GaP/Si(111) virtual substrates, we investigate the atomic structure of the MOVPE (metalorganic vapor phase epitaxy)-prepared GaP(111)B surface (phosphorus face). We find that upon high-temperature annealing in the H2-based MOVPE process ambience, the surface is phosphorus-depleted, as evidenced by X-ray photoemission spectroscopy (XPS). However, a combination of density functional theory calculations and scanning tunneling microscopy (STM) suggests the formation of a partially H-terminated phosphorus surface, where the STM contrast is due to electrons tunneling from non-terminated dangling bonds of the phosphorus face. Atomic force microscopy (AFM) reveals that a high proportion of the surface is covered by islands, which are confirmed as Ga-rich by Auger electron spectroscopy (AES). We conclude that the STM images of the samples after high-temperature annealing only reflect the flat regions of the partially H-terminated phosphorus face, whereas an increasing coverage with Ga-rich islands, as detected by AFM and AES, forms upon annealing and underlies the higher proportion of Ga in the XPS measurements.
https://doi.org/10.1016/j.apsusc.2020.147346
Control of magnetic properties in spinel ZnFe2O4 thin films through intrinsic defect manipulation. - In: Journal of applied physics, ISSN 1089-7550, Bd. 128 (2020), 16, 165702, insges. 7 S.
Im Titel sind "2" und "4" tiefgestellt
We present a systematic study of the magnetic properties of ZnFe2O4 thin films fabricated by pulsed laser deposition at low and high oxygen partial pressure and annealed in oxygen and argon atmosphere, respectively. The as-grown films show strong magnetization, closely related to a non-equilibrium distribution of defects, namely, Fe cations among tetrahedral and octahedral lattice sites. While the concentration of tetrahedral Fe cations declines after argon treatment at 250 ˚C, the magnetic response is enhanced by the formation of oxygen vacancies, evident by the increase in near-infrared absorption due to the Fe2+-Fe3+ exchange. After annealing at temperatures above 300 ˚C, the weakened magnetic response is related to a decline in disorder with a partial recrystallization toward a less defective spinel configuration.
https://doi.org/10.1063/5.0019712
Electrogravimetry and structural properties of thin silicon layers deposited in sulfolane and ionic liquid electrolytes. - In: ACS applied materials & interfaces, ISSN 1944-8252, Bd. 12 (2020), 51, S. 57526-57538
Potentiostatic deposition of silicon is performed in sulfolane (SL) and ionic liquid (IL) electrolytes. Electrochemical quartz crystal microbalance with damping monitoring (EQCM-D) is used as main analytical tool for the characterization of the reduction process. The apparent molar mass (Mapp) is applied for in situ estimation of the layer contamination. By means of this approach, appropriate electrolyte composition and substrate type are selected to optimize the structural properties of the layers. The application of SL electrolyte results in silicon deposition with higher efficiency compared to the IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMP][TFSI]. This has been associated with the instability of the IL in the presence of silicon tetrachloride and the enhanced incorporation of IL decomposition products into the growing silicon deposit. X-ray photoelectron spectroscopy (XPS) analysis supports the results about the layer composition, as suggested from the microgravimetric experiments. Attention has been given to the impact of practically relevant substrates (i.e., Cu, Ni, and vitreous carbon) on the reduction process. An effective deposition can be carried out on the metal electrodes in both electrolytes due to accelerated reaction kinetics for these types of substrates. However, on vitreous carbon (VC), a successful reduction of SiCl4 can only be accomplished in the IL, while the electroreduction process in SL is dominated by the decomposition of the electrolyte. For short deposition times, the scanning electron microscopy (SEM) images display rough morphologies in the nanometer range, which evolve further to structures with increased length scale of the surface roughness. The development of a rough interface during deposition, resulting in QCM damping at advanced stages of the process, is interpreted by a model accounting for the resistive force caused by the interaction of the liquid with a nonuniform layer interface. By using this approach, the individual contributions of the surface roughness and viscoelastic effects to the measured damping values are estimated.
https://doi.org/10.1021/acsami.0c14694
Structural analysis of sputtered Sc(x)Al(1-x)N layers for sensor applications. - In: Materials science and smart materials, (2020), S. 13-18
Investigation of epitaxial graphene via Raman spectroscopy: origins of phonon mode asymmetries and line width deviations. - In: Carbon, ISSN 1873-3891, Bd. 170 (2020), S. 666-676
In this work a comprehensive study is presented for the analysis of epitaxial graphene layers using Raman spectroscopy. A wide range of graphene types is covered, from defective/polycrystalline single layer graphene to multilayer graphene with low defect density. On this basis the influence of strain type, Fermi level and number of layers on the Raman spectrum of graphene is investigated. A detailed view on the 2D/G dispersion and the respective slopes of uniaxially and biaxially strained graphene is given and its implications on the asymmetry of the G peak analyzed. A linear dependency of the phonon mode asymmetry on uniaxial strain is presented in addition to the known Fermi level dependence. Additional impacts on the asymmetry are found to be arising from the defect density and transfer doping of adsorbates. The discovered transfer doping mechanism is contrary to pure phonon excitation through excitons and exhibits increasing asymmetry with increasing Fermi level. A new characteristic correlation between the 2D mode line width and the inverse I(D)/I(G) ratio is introduced that allows the determination of the strain type and layer number and explains the difference between Raman line widths of monolayer graphene on different substrates.
https://doi.org/10.1016/j.carbon.2020.07.016
Ultrafast dynamics of hot charge carriers in an oxide semiconductor probed by femtosecond spectroscopic ellipsometry. - In: New journal of physics, ISSN 1367-2630, Bd. 22 (2020), 083066, insges. 14 S.
https://doi.org/10.1088/1367-2630/aba7f3
Hybrid GA-gradient method for thin films ellipsometric data evaluation. - In: Journal of computational science, ISSN 1877-7503, Bd. 47 (2020), 101201
A global-search method which applies the concept of genetic algorithm (GA) with gradient-based optimizer is proposed for the problem of experimental data analysis from spectroscopic ellipsometry on thin films. The method is applied to evaluate the data obtained for samples with different structure complexity, starting with transparent monolayers (SiO2, HfO2) on a substrate, through absorbing film (diamond-like carbon) and multilayer structures. We demonstrate that by using this method we are able to find material parameters even for limited a priori knowledge about the sample properties, where classical methods fail.
https://doi.org/10.1016/j.jocs.2020.101201
Graphen - epitaktisches Wachstum, Charakterisierung und nicht-klassische elektrische Bauelementekonzepte. - Ilmenau : Universitätsbibliothek, 2020. - 1 Online-Ressource (163 Blätter)
Technische Universität Ilmenau, Dissertation 2020
In der vorliegenden Dissertation wurden die Schwerpunkte des Wachstums auf semi-isolierendem 6H-SiC, der Schicht- als auch der Bauelementecharakterisierung auf Basis von epitaktischem Graphen behandelt. Die Schichten wurden mittels REM, AFM, LEED, XPS und ARPES untersucht. Anhand von REM Aufnahmen wurde durch einen neuartigen Ansatz die Qualität der Schichten über die Bildentropie mit den Wachstumsparametern korreliert. Für die Bestimmung der Schichtdicke mit Hilfe von XPS wurden (a-)symmetrische Fit-Funktionen und ihr Fehler bei der Dickenbestimmung betrachtet. Der zweite Schwerpunkt befasst sich mit der Charakterisierung der hergestellten Schichten durch Raman- und FTIR-Spektroskopie. Die Einflüsse von Verspannung, Fermi-Niveau und Lagenzahl auf das Raman-Spektrum des Graphen wurden klassifiziert und quantifiziert. Uniaxiale konnte von biaxialer Dehnung anhand des Unterschieds in der G/2D-Dispersion unterschieden werden, die Asymmetrie der G-Mode wird dabei maßgeblich von uniaxialer Dehnung, der Lage des Fermi-Niveaus als auch durch Transferdotierung bei Anwesenheit von Adsorbaten beeinflusst. Zunehmende Lagenzahl verursachte eine Blauverschiebung der 2D-Mode bei zunehmender Halbwertbreite. Mittels FTIR wurden Änderung des Reststrahlenbands des SiCs in Abhängigkeit des Wachstums durch Anregung eines Oberflächenplasmon-Polaritons im Graphen untersucht. Eine Auswertemethode wurde entwickelt, um die sich im Divisionsspektrum der Reflektivitäten ausbildende Fano-Resonanz zu beschreiben. Die Intensität der resultierenden Fano-Resonanz wird dabei maßgeblich von der Verschiebung der Modell-Oszillatoren zueinander beeinflusst. Der dritte Schwerpunkt befasst sich mit der Strukturierung und Charakterisierung von vollständig aus Graphen bestehenden, Three Terminal Junctions (TTJ) und Side-Gate-Transistoren (SG-FET). Für die Vermessung kleinster Strukturbreiten anhand von REM-Aufnahmen wurden Methoden zur Schwingungskorrektur und der Breitenbestimmung nahe/unterhalb der Auflösungsgrenze des REMs hergeleitet. Es konnte gezeigt werden, dass TTJs einen Gleichrichtungseffekt mit hoher Gleichrichtungseffizienz aufweisen. Des Weiteren wurden die auftretenden Stromverstärkungseffekte untersucht. Die realisierten SG-FETs zeigen vergleichbar gute Eigenschaften wie konventionelle Top-Gate-Transistoren auf bei Minimierung parasitärer Einflüsse.
https://nbn-resolving.org/urn:nbn:de:gbv:ilm1-2020000335
Structure and dielectric function tensor of (Al,Sc)N thin films. - In: DPG-Frühjahrstagung (DPG Spring Meeting) of the Condensed Matter Section (SKM) together with the DPG Division Environmental Physics and the Working Groups Accelerator Physics; Equal Opportunities; Energy; Industry and Business; Physics, Modern IT and Artificial Intelligence, Young DPG, (2020), HL 30.29