Development of Nb-GaAs based superconductor semiconductor hybrid platform by combining in-situ dc magnetron sputtering and molecular beam epitaxy Clemens Todt, 1 Sjoerd Telkamp, 1 Filip Krizek, 1, 2, 3 Christian Reichl, 1 Mihai Gabureac, 1 Rüdiger Schott, 1 Erik Cheah, 1 Peng Zeng, 4 Thomas Weber, 5 Arnold Müller, 6 Christof Vockenhuber, 6 Mohsen Bahrami Panah, 1 and Werner Wegscheider 1 1 Solid State Physics Laboratory, ETH Zürich, CH-8093 Zürich, Switzerland 2 IBM Research Europe - Zurich, 8803 Rüschlikon, Switzerland 3 Institute of Physics, Czech Academy of Sciences, 162 00 Prague, Czech Republic 4 ETH Zürich, The Scientific Center for Optical and Electron Microscopy (ScopeM), CH 8093 Zürich, Switzerland. 5 X-ray Platform, Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5–10, 8093 Zürich, Switzerland. 6 Laboratory of Ion Beam Physics, ETH Zurich, Schafmattstrasse 20, CH-8093 Zurich, Switzerland. (Dated: April 19, 2023) We present Nb thin films deposited in-situ on GaAs by combining molecular beam epitaxy and magnetron sputtering within an ultra-high vacuum cluster. Nb films deposited at varying power, and a reference film from a commercial system, are compared. The results show clear variation between the in-situ and ex-situ deposition which we relate to differences in magnetron sputtering conditions and chamber geometry. The Nb films have critical temperatures of around 9K and critical perpendicular magnetic fields of up to Bc2 =1.4T at 4.2K. From STEM images of the GaAs-Nb interface we find the formation of an amorphous interlayer between the GaAs and the Nb for both the ex-situ and in-situ deposited material. Keywords: niobium, dc magnetron sputtering, semiconductor superconductor hybrid materials, in-situ su- perconductor growth I. INTRODUCTION Superconductor (SC) semiconductor (SE) hybrid (SSH) devices have re-emerged [1–3] fueled by the hope of finding anyons in solid state systems and their subse- quent application for fault tolerant quantum computing [4–8]. Most notably the search for the Majorana Fermion in solid state systems has attracted attention [9, 10]. This promoted numerous experiments in Andreev interaction with Quantum Hall states [11–13] and topological super- conductivity [14, 15]. The achievement of epitaxial growth of thin film Al on III-V SEs [16, 17] sparked experiments in SSH devices [18–34]. The crucial element of material synthesis is the in-situ deposition, enabling an undisturbed SC-SE com- bination, crucial for a transparent interface to electron transport [17]. The formation of sub-gap states and a electrostatic barrier, degrading the performance of the hybrid system, is associated to the surface oxide, formed when the SC is deposited ex-situ [18]. Therefore, nanowires and two dimensional electron sys- tems (2DES) based on InAs and InSb with an epitaxial Al layer have become the established material platform ex- hibiting a pronounced proximity effect [17, 18, 35]. Fur- thermore, Al is typically available in molecular beam epi- taxy (MBE) systems owing to its use in III-V semicon- ductor growth. The superconducting properties of epitaxial Al films limit the temperature and magnetic field range of Al- based SSH experiments to T c of around 1.6K at film thicknesses between 5 nm and 10 nm [19, 20, 25, 27, 28]. The reported perpendicular critical fields B c2 range from 30 mT [21] up to 164 mT [28] at dilution fridge tempera- tures. The search for an alternative to Al is the subject of a multitude of recent studies [35–41]. A wide range of elemental superconductors has been deposited onto nanowires including Pb [37, 42], In [43], Ta [36], V [44] and Sn [38, 42]. Nb is of particular interest [35, 36, 40] as it has the highest bulk critical temperature and mag- netic field of all the elemental SCs [45]. Pb appears to be the best alternative so far [37, 42] owing to its favourable lattice match to InAs [46] and relatively high Tc [37]. Exciting research proposals [47–49] call for building in- creasingly complex SSH devices and networks. In this ap- plication lithographically patterned 2DES-SC SSH repre- sent a promising approach [50]. The 2DES in InAs [32] and InSb [51] can be grown to reasonably high mobili- ties but lack far behind 2DES based on GaAs [52]. The drawback of GaAs is the Φ B > 0.7 eV Schottky barrier [53] which is expected to suppress the proximity effect [54]. Nonetheless induced superconducting gaps in bulk n-GaAs employing in-situ deposited Al have been mea- sured [55, 56]. In this context our recently developed shallow GaAs 2DESs [52] are posing an interesting unex- plored potential for SSHs. The interaction between the SE and SC is not limited to the proximity effect. A type-II superconductor can shape the magnetic field in the SE underneath via its vortices [57] forming the basis of exciting experimental proposals [58–60]. Vortex interaction mediated experi- ments have been previously attempted [2, 61] most no- arXiv:2304.08339v2 [cond-mat.mtrl-sci] 18 Apr 2023