epitaxial and amorphous Al₂O₃ films on α-Ta(110)/a-plane sapphire for superconducting resonators

Yi-Ting Cheng^1,2*, Hsien-Wen Wan¹, Chao-Kai Cheng¹, Ko-Hsuan Mandy Chen², Jui-Min Chia², Chia-Hung Hsu³, Yen-Hsiang Lin², Jueinai Kwo², Minghwei Hong¹

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

* Presenter:Yi-Ting Cheng, email:ytcheng@phys.nthu.edu.tw

For building high-performance superconducting qubits and supportive circuits like a high-Q microwave resonator, we need to precisely control material properties in film crystallinity, hetero-interfaces, and capped oxide layer. Ta superconducting films are usually exposed to air, inevitably forming a native oxide layer, which may enhance energy relaxation channels such as two-level systems (TLS) for superconducting circuits. Here, the growth of deposited-oxide/α-Ta films on a-plane sapphire substrates was achieved in a multi-chamber UHV system. The research aims to investigate the microwave properties of the in-situ deposited heterostructures to understand dielectric losses in superconducting quantum circuits. α-Ta films 35 nm in thickness grew epitaxially in (110) orientation on a-plane sapphire substrates, followed by in-situ growth of epitaxial and amorphous Al₂O₃ films. The presence of Pendellösung fringes indicates a high degree of structural coherence over the entire film thickness. The Ta film has a critical temperature of 4.29 K and a high residual resistance ratio of 11 for a-Al₂O₃/Ta and 16 for epi-Al₂O₃/Ta, further confirming the high quality of the Ta films. The interface, as measured with transmission electron microscopy, differs between a-Al₂O₃/Ta and epi-Al₂O₃/Ta.

The support from the Natl. Sci. Technol. Council in Taiwan through NSTC 112-2119-M-007-009 and 112-2811-M-007-056 is acknowledged.

Keywords: Ta, in-situ growth, epitaxial oxide