Growth of superconducting YB$_{6\, }$thin films for the study of proximity effect in the topological Kondo insulator SmB$_{6}$

Proximity-induced superconductivity in topological insulators has been predicted to exhibit various novel phenomena including Majorana zero modes [1]. Our goal is to study this proximity effect using bilayer thin films consisting of the topological Kondo insulator samarium hexaboride (SmB$_{6})$ and superconducting yttrium hexaboride (YB$_{6})$ due to their excellent lattice match. The superconducting properties of YB$_{6\, }$ have been reported to be strongly dependent on the stoichiometry with the maximum T$_{c\, }$(\textasciitilde 7.5 K [2] and 6.1 K [3] in single crystal and thin film forms, respectively) observed in boron deficient YB$_{6}$. We have grown YB$_{6\, }$thin films by co-sputtering YB$_{6\, }$and Y targets in an ultrahigh-vacuum-compatible chamber. Towards achieving the highest T$_{c}$ (\textasciitilde 5.7 K currently), various growth parameters are explored/optimized including the sputter power, substrate temperature, and post-deposition annealing. Their composition and microstructure are characterized by energy-dispersive X-ray spectroscopy, ellipsometry, and atomic force microscopy. The superconducting properties are investigated using resistivity, magnetization, and tunneling spectroscopic measurements. [1] L. Fu {\&} C. L. Kane, Phys. Rev. Lett. \textbf{100}, 096407 (2008). [2] N. Sluchanko \textit{et al}., Phys. Rev. B. \textbf{96}, 144501 (2017). [3] S. Lee \textit{et al}., \textit{Nature}~\textbf{570,~}344--348 (2019).