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- ItemInfluence of substrate dimensionality on the growth mode of epitaxial 3D-bonded GeTe thin films: From 3D to 2D growth(Amsterdam [u.a.] : Elsevier Science, 2019) Hilmi, Isom; Lotnyk, Andriy; Gerlach, Jürgen W.; Schumacher, Philipp; Rauschenbach, BerndThe pseudo-binary line of Sb2Te3-GeTe contains alloys featuring different crystalline characteristics from two-dimensionally (2D-) bonded Sb2Te3 to three-dimensionally (3D-) bonded GeTe. Here, the growth scenario of 3D-bonded GeTe is investigated by depositing epitaxial GeTe thin films on Si(111) and Sb2Te3-buffered Si(111) substrates using pulsed laser deposition (PLD). GeTe thin films were grown in trigonal structure within a temperature window for epitaxial growth of 210–270 °C on unbuffered Si(111) substrates. An unconventional growth onset was characterized by the formation of a thin amorphous GeTe layer. Nonetheless, the as-grown film is found to be crystalline. Furthermore, by employing a 2D-bonded Sb2Te3 thin film as a seeding layer on Si(111), a 2D growth of GeTe is harnessed. The epitaxial window can substantially be extended especially towards lower temperatures down to 145 °C. Additionally, the surface quality is significantly improved. The inspection of the local structure of the epitaxial films reveals the presence of a superposition of twinned domains, which is assumed to be an intrinsic feature of such thin films. This work might open a way for an improvement of an epitaxy of a 3D-bonded material on a highly-mismatched substrate (e.g. Si (111)) by employing a 2D-bonded seeding layer (e.g. Sb2Te3).
- ItemMechanical properties and twin boundary drag in Fe-Pd ferromagnetic shape memory foils-experiments and ab initio modeling(Bristol : IOP, 2011) Claussen, I.; Mayr, S.G.We report on vibrating reed measurements combined with density functional theory-based calculations to assess the elastic and damping properties of Fe-Pd ferromagnetic shape memory alloy splats. While the austenite-martensite phase transformation is generally accompanied by lattice softening, a severe modulus defect and elevated damping behavior are characteristic of the martensitic state. We interpret the latter in terms of twin boundary motion between pinning defects via partial 'twinning' dislocations. Energy dissipation is governed by twin boundary drag, primarily due to lattice imperfections, as concluded from the temperature dependence of damping and related activation enthalpies.
- ItemMicrostructure Evaluation and Impurities in La Containing Silicon Oxynitrides(Basel : MDPI, 2021) Hakeem, Abbas Saeed; Ali, Sharafat; Höche, Thomas; Drmosh, Qasem Ahmed; Khan, Amir Azam; Jonson, BoOxynitride glasses are not yet commercialised primarily due to the impurities present in the network of these glasses. In this work, we investigated the microstructure and instinctive defects in nitrogen rich La-Si-O-N glasses. Glasses were prepared by heating a powder mixture of pure La metal, Si3N4, and SiO2 in a nitrogen atmosphere at 1650-1800 °C. The microstructure and impurities in the glasses were examined by optical microscopy, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy in conjunction with electron energy-loss spectroscopy. Analyses showed that the glasses contain a small amount of spherical metal silicide particles, mostly amorphous or poorly crystalline, and having sizes typically ranging from 1 µm and less. The amount of silicide was estimated to be less than 2 vol. %. There was no systematic relation between silicide formation and glass composition or preparation temperature. The microstructure examination revealed that the opacity of these nitrogen rich glasses is due to the elemental Si arise from the decomposition reaction of silicon nitride and silicon oxide, at a high temperature above ~1600 °C and from the metallic silicide particles formed by the reduction of silicon oxide and silicon nitride at an early stage of reaction to form a silicide intermetallic with the La metal.