Role of a capping layer on the crystalline structure of Sn thin films grown at cryogenic temperatures on InSb substrates

Abstract: Metal deposition with cryogenic cooling is widely utilized in the condensed matter community for producing ultra-thin epitaxial superconducting layers on semiconductors. However, a drawback arises when these films return to room temperature, as they tend to undergo dewetting. This issue is mitigated by capping the films with an amorphous layer. In this study, we examined the impact of different in-situ fabricated caps on the structural characteristics of Sn thin films deposited at 80 K on InSb substrates. Regardless of the type of capping, we observed that the films remained smooth upon returning to room temperature and were epitaxial on InSb in the cubic Sn ($\alpha$-Sn) phase. However, we noted a correlation between alumina capping with an electron beam evaporator and an increased presence of tetragonal Sn ($\beta$-Sn) grains. This suggests that heating from the alumina source may contribute to a partial phase transition in the Sn layer. The existence of the $\beta$-Sn phase induced superconducting behavior of the films by percolation effect. This study highlights the potential for modifying the structural properties of cryogenic Sn thin films through in-situ capping, paving the way for precise control in the production of superconducting Sn films for integration into quantum computing platforms.