guest ::
%0 Thesis %A Schleenvoigt, Michael %T Molecular beam epitaxy of magnetic topological insulators and their integration into superconducting hybrid devices %I RWTH Aachen University %V Dissertation %C Aachen %M RWTH-2024-12113 %P 1 Online-Ressource : Illustrationen %D 2024 %Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University 2025 %Z Dissertation, RWTH Aachen University, 2024 %X Among the few approaches towards quantum computing (QC) as the future of computing in a post-Moore’s-law time, the recent push for topological QC is one of the most auspicious. It promises fault tolerant QC by employing exotic Majorana zero mode (MZM) quasiparticle states, which build the basis of the topological QC qubit. Intrinsic protection against relaxation and dephasing distinguishes those elusive qubits from other platforms, like superconducting and spin qubits. Nanowires of 3D topological insulator (TI) – superconductor (SC) hybrid structures are predicted to exhibit MZM at each end of the wire. The one-dimensional confinement however leads to a loss of the topological phase needed to realize the MZMs. An alternative capable of avoiding this problem has gotten much traction lately, which are magnetic topological insulators (MTI), which incorporate magnetic dopants into the TI itself. This thesis focuses on the growth of magnetic TIs via molecular beam epitaxy (MBE) from established recipes for non-magnetic TIs. It will show how complex MTI-SC hybrid devices can be fabricated in situ and will illustrate the difficulties that arise when trying to combine MTIs with SC. First, the growth of 3D TI (BiSb)2Te3 via MBE is optimized to reach a low level of charge carrier density and a high mobility, signifying an appropriate host material for the magnetic dopants. By supplying Cr during thin film growth, high quality magnetically doped films are deposited. First films exhibit an anomalous Hall effect, indicating homogeneous and strong magnetism. In similar Mn-doped films, investigated in parallel, the magnetization is found to less homogeneous, ultimately leading to an focusing onto the more reliable Cr doped MTI films. By creating a trilayer film of CBST and thinning it down to <8 nm a quantization in the Hall resistance is observed, signaling a transition into a quantum anomalous Hall insulator. To investigate the interaction of the MTI and SC, Josephson junction (JJ) devices are created via a stencil lithography process established in our group. The JJ devices show no supercurrent over the junctions, but rather indications for a barrier at the MTI-SC interface due to bad interface transparency between MTI and SC. To rule out ex situ contaminations and to relate the barrier height to the magnetization, a new device layout is developed. It allows for full in situ deposition and enables Hall and JJ measurements in one device. While the low transparency issue persists, indications of induced superconductivity are found when decreasing the magnetic doping in these devices. To further investigate the interplay of topology, magnetism and superconductivity on atomic scales, a novel process is developed in the last chapter: a fully in situ process utilizing removable large-scale ultra-high vacuum lithography (LUL). With LUL, (M)TI and SC films can be grown selectively in situ and aligned to each other with nm precision. Soft cappings of Tellurium or Selenium are investigated to protect the functional surfaces. By exfoliating the stencil mask layer, samples suitable for investigations in scanning tunneling microscopes (STM) are created. In STM, atomic resolution is achieved after capping removal. Tunneling spectra of the SC gap, the TI surface and the magnetic gap are obtained after successful mask and capping removal, showing a short-ranged SC gap profile on the TI. In conclusion, this thesis establishes multiple new material systems in our institute, showcases novel UHV lithography methods for the combination of (M)TIs and SCs, paving the way for the creation of MZMs in hybrid devices, and lastly, by utilizing LUL, provides an innovative process for creating structures and devices of combinations of arbitrary quantum materials in situ that enables measurements in ways unprecedented in STM. %F PUB:(DE-HGF)11 %9 Dissertation / PhD Thesis %R 10.18154/RWTH-2024-12113 %U https://publications.rwth-aachen.de/record/999456