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| Hauptseite > Publikationsdatenbank > Engineering topological superlattices and their epitaxial integration in selectively grown hybrid nanostructures via MBE |
| Hauptseite > Publikationsdatenbank > Engineering topological superlattices and their epitaxial integration in selectively grown hybrid nanostructures via MBE |
| Dissertation / PhD Thesis | FZJ-2022-03467 |
2022
RWTH Aachen University
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Please use a persistent id in citations: http://hdl.handle.net/2128/31891 doi:10.18154/RWTH-2022-06227
Abstract: The realization of advanced spintronics applications including the topological quantum computation, spin manipulation for data storage, dissipation less ballistic transport for ultra-fast quantum devices and topological switching for low energy memory applications etc. became more feasible with the experimental discovery of 3D topological insulators (TIs). The incorporation of exotic spin-momentum locked Dirac surface states (of 3D TIs) into these futuristic complex quantum devices requires not only the growth of high crystal quality epilayers but also the fabrication of pristine nanostructures, topological band engineering, ultra-smooth and defect-free surfaces, and atomically transparent epitaxial interfaces. This work deals with a systematic study of epitaxial growth of convention 3D TIs via molecular beam epitaxy(MBE) and atomic-scale structural characterization via scanning transmission electron microscope (STEM)to explore the above mentioned requirements. At first, the relation between the growth parameters and the defect density in the Van-der-Waals (VdW) based layered structures is investigated. The optimum growth parameters are extracted and the defect-free epilayers are prepared. Later, the technique of selective area epitaxy (SAE) is explored to develop a platform to achieve a scalable nano-architecture. Utilizing CMOS compatible fabrication technology, Si (111) substrates with crystalline and amorphous combinational surfaces are prepared. The precisely controlled growth parameters facilitated the realization of selectively grown topological structure. Based on statistical analysis, a generalized growth model is established that provided control over structural defects through the effective growth rate at the nanoscale and assisted in achieving high quality nanostructures. Based on conventional 3D TIs, the capabilities of VdW epitaxy are exploited further with the growth of topological-trivial hetero structures. The stoichiometric adjustment in these hetero structures is utilized as a tool to control the strength of spin-orbit coupling (SOC) and to engineer the topological band structure. Two such systems are explored including BixTey = (Bi2)m(Bi2Te3)n and GST/GBT = (GeTe)n(Sb2Te3/Bi2Te3)m. With the continuous addition of Bi2 bilayers and GeTe (materials that exhibit trivial phase) into 3D TIs, the stoichiometric modulations are achieved. Moreover, the modification of growth parameters is conducted to incorporate these stoichiometries with the pre-patterned substrates and selectively grown nanostructures of the corresponding alloys are prepared. Assisted by the atomic-scale structural characterizations, the phenomenon of VdW reconfiguration is explored to observe the transformation of layer architecture; the key mechanism in the evolution of interfacial phase change materials (IPCMs).Moreover, the systematic alterations in the atomic interaction and resulting changes in bond lengths within a pristine and hybrid VdW stacks are investigated. The focus is then shifted towards surfaces where the stability (inertness) of TI epilayers in the ambient conditions via structural and compositional investigations, is analyzed. An undeniable evidence of the aging effect in all material systems is obtained where a non-saturating oxidation process at the (0001) surfaces with a continually decreasing oxidation rate is witnessed. Using the in situ thin film deposition of Al (2 nm),the top surfaces are passivated and the aging effect is neutralized. The phenomenon of charge transfer due to band alignment at the Si (111) - TI bottom surface is investigated with a comparative growth, structural and transport analysis of TI epilayer prepared on HfO2 substrate. Finally, the interfaces between TIs and various s-wave superconductors (SCs) are explored. The challenges to achieve the induced superconductivity in TI-SC hybrid junction and highly transparent interfaces are addressed. The issue of metal diffusion into the TI epilayer and the resulting formation of Schottky-like barriers is avoided with the introduction of a thin metallic film as a diffusion barrier. Using the natural tendency of transition metals to transform into their corresponding di-chalcogenides (TMDCs) at the exposure to TI surfaces, atomically well-defined and VdW assisted epitaxial interfaces are engineered. The newly evolved interfaces assisted in achieving the induced superconductivity that was a huge limitation in realizing the complex functional devices.
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