Single-crystal superconducting Al thin film with high crystallinity and ultra-low interface roughness in Al₂O₃/Al/sapphire heterostructure for quantum computing

Yen-Hsun Glen Lin^1*, Lawrence Boyu Young¹, Li-Shao Chiang², Hsien-Wen Wan¹, Yi-Ting Cheng¹, Chia-Hung Hsu³, Juhn-Jong Lin⁴, Che-Te Liang¹, Yen-Hsiang Lin⁵, Jueinai Kwo⁵, Minghwei Hong^1,2

¹Graduate Institute of Applied Physics and Department of Physics, National Taiwan University, Taipei, Taiwan
²Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan
³National Synchrotron Radiation Research Center, Hsinchu, Taiwan
⁴Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
⁵Department of Physics, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Yen-Hsun Glen Lin, email:yhglenlin@ntu.edu.tw

Superconducting qubit has been the major platform for quantum computing. Despite the tremendous research efforts in both academia and industry, decoherence of quantum states due to dielectric loss remains a great challenge. The dielectric loss process mainly originated from the material imperfection of the superconducting thin films and interfaces between superconducting films and adjacent layers (substrate, air).
In this work, single crystal superconducting aluminum (Al) films were grown on sapphire substrates with thicknesses down to 3 nm. The Al layer was protected by an in-situ deposited Al₂O₃ layer without sacrificing the underlying Al film. The samples exhibited single crystal Al structure and very high crystallinity with record-low full-width at half-maximum rocking curves of 0.015^o in synchrotron radiation X-ray diffraction study. The sample surfaces showed extremely low surface roughness of 0.1-0.2 nm by atomic force microscope. The high crystallinity and atomically flat surface suggested the well-controlled growth process of the Al films. Superconductivity was obtained in the Al films down to 3 nm, suggesting the feasibility of the superconducting devices using these single crystal ultra-thin superconducting films.

Keywords: Single crystal, Superconductor, Aluminum, Quantum Computing