Wafer-scale single-domain Al films and heterostructures with superior interface quality for scalable superconducting qubits

Hsien-Wen Wan^1*, Yi-Ting Cheng¹, Chao-Kai Cheng¹, Chien-Ting Wu², Chia-Hung Hsu³, Jueinai Kwo⁴, Minghwei Hong¹

¹Graduate Institute of Applied Physics and Department of Physics, National Taiwan University, Taipei, Taiwan
²Taiwan Semiconductor Research Institute, Hsinchu, Taiwan
³National Synchrotron Radiation Research Center, Hsinchu, Taiwan
⁴Department of Physics, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Hsien-Wen Wan, email:f04222061@ntu.edu.tw

Wafer-scale, single-crystal superconducting films, characterized by low defect densities and minimal variability, enable consistent structural quality essential for large-scale manufacturing. This uniformity enhances control over device-to-device variability, improving the reproducibility of qubit devices across both individual wafers and wafer batches. These films are critical for scalable quantum processor fabrication, including Josephson junctions and microwave resonators. In this work, we report on achieving wafer-scale, single-domain superconducting aluminum (Al) films with exceptionally low twin ratios and atomically abrupt oxide/superconducting interfaces enabled by in situ oxide deposition. These features are vital for fabricating superconducting quantum circuits with improved stability.
Synchrotron X-ray diffraction normal scans confirmed the high crystallinity of our Al films, with clear Pendellösung fringes, while in-plane scans revealed a twin ratio as low as 0.005%, surpassing prior results for Al films on Si, sapphire, and GaAs substrates. Atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM) confirmed the smoothness and uniformity of both surface morphology and the oxide/Al/substrate interfaces. Furthermore, X-ray reflectivity (XRR) measurements demonstrated abrupt, uniform interfaces with well-defined interference fringes. These results indicate that our oxide/Al/substrate heterostructures exhibit superior structural quality, offering promise for reduced interface-related losses, improved coherence in superconducting qubits, and scalability in quantum device applications.
The support from the Natl. Sci. Technol. Council in Taiwan through NSTC 113-2119-M-007-008 is acknowledged.

Keywords: superconducting qubits, wafer-scale single-domain superconducting films, aluminum, abrupt interfaces, X-ray diffraction