EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Superconducting nanostructures have focused scientific interest, due to the ability to enhance and tune the desired characteristics. In thin film structures, the critical temperature for superconducting transition, Tc, can differ significantly from the corresponding value of their bulk counterparts. The size dependence of Tc in Al is very pronounced, leading to higher Tc values as the film thickness decreases [1]. This phenomenon affects both the density of states and the electron–phonon interaction. To this end, Al thin films of various thicknesses were grown on GaAs (001) and sapphire (0001) substrates by Molecular Beam Epitaxy (MBE). Transmission Electron Microscopy (TEM) and high-Resolution TEM (HRTEM) was employed, to evaluate the morphological, structural and interfacial characteristics of the Al thin films. The degree of the substrate surface coverage, the Al grain morphologies and the type of intergranular boundaries between adjacent grains were elucidated. TEM and HRTEM imaging revealed uniform compact thin Al films grown on (0001) sapphire, while formation of Al islands was observed on the (001) GaAs. Atomically flat crystalline interfaces with the substrates and a thin AlOx amorphous layer on the surface of the Al films were present everywhere. The Al epilayers consisted of twin related large fcc non-defected single crystals, apart from few (111) stacking faults (SFs). Conversely, Al islands on GaAs were single crystalline with either an elongated platelet-like, or a truncated pyramidal shape. Furthermore, the strain state of Al was analyzed by Moiré fringe analysis, Bragg Filtering, and Geometric Phase Analysis (GPA).

[1] K. Y. Arutyunov et. al., Physics of the Solid State, Vol. 61, No. 9, 1559 (2019)

Supported by the project “EINSTEIN” (Τ4ΔΡΩ-00031), funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (EPAnEK 2014-2020) under the federal target program “Bilateral R&D Cooperation Greece-Russia”, and co-financed by Greece and the European Union (European Regional Development Fund).