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- ItemImproved structural and electrical properties in native Sb2Te3/GexSb2Te3+x van der Waals superlattices due to intermixing mitigation(New York : American Institute of Physics, 2017) Cecchi, Stefano; Zallo, Eugenio; Momand, Jamo; Wang, Ruining; Kooi, Bart J.; Verheijen, Marcel A.; Calarco, RaffaellaSuperlattices made of Sb2Te3/GeTe phase change materials have demonstrated outstanding performance with respect to GeSbTe alloys in memory applications. Recently, epitaxial Sb2Te3/GeTe superlattices were found to feature GexSb2Te3+x blocks as a result of intermixing between constituting layers. Here we present the epitaxy and characterization of Sb2Te3/GexSb2Te3+x van der Waals superlattices, where GexSb2Te3+x was intentionally fabricated. X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, and lateral electrical transport data are reported. The intrinsic 2D nature of both sublayers is found to mitigate the intermixing in the structures, significantly improving the interface sharpness and ultimately the superlattice structural and electrical properties.
- ItemGrowth of Fe3Si/Ge/Fe3Si trilayers on GaAs(001) using solid-phase epitaxy(New York : American Institute of Physics, 2017) Gaucher, S.; Jenichen, B.; Kalt, J.; Jahn, U.; Trampert, A.; Herfort, J.Ferromagnetic Heusler alloys can be used in combination with semiconductors to create spintronic devices. The materials have cubic crystal structures, making it possible to grow lattice-matched heterojunctions by molecular beam epitaxy. However, the development of devices is limited by the difficulty of growing epitaxial semiconductors over metallic surfaces while preventing chemical reactions, a requirement to obtain abrupt interfaces and achieve efficient spin-injection by tunneling. We used a solid-phase epitaxy approach to grow crystalline thin film stacks on GaAs(001) substrates, while preventing interfacial reactions. The crystallized Ge layer forms superlattice regions, which are caused by the migration of Fe and Si atoms into the film. X-ray diffraction and transmission electron microscopy indicate that the trilayers are fully crystalline, lattice-matched, and have ideal interface quality over extended areas.
- ItemIn Situ Transmission Electron Microscopy of Disorder–Order Transition in Epitaxially Stabilized FeGe2(Washington, DC : Soc., 2021) Terker, Markus; Nicolai, Lars; Gaucher, Samuel; Herfort, Jens; Trampert, AchimIsothermal crystallization of amorphous Ge deposited on a cubic Fe3Si/GaAs(001) substrate is performed by in situ annealing within a transmission electron microscope. It was found that the formation of epitaxially aligned tetragonal FeGe2 is associated with a disorder–order phase transition mainly consisting of a rearrangement of the Fe/vacancy sublattice from a random distribution to alternating filled and empty layers. Additionally, atomically resolved high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy demonstrated that the vertical lattice spacing of the Ge sublattice reduces across vacancy layers, indicating that strain minimization plays a role in the phase transition process. Crystallization and ordering are both found to proceed layer-by-layer and with square-root-shaped kinetics with a smaller transition rate for the latter.