Improving the Crystal Quality of Niobium on Sapphire Thin Films in 2D Transmon Fabrication via UHV Annealing*

Superconducting metallic thin films such as Nb are utilized in the fabrication of 2D superconducting qubits, and the crystalline quality of the films is believed to be among the possible factors affecting coherence. Conventional deposition methods include DC sputtering and high-power impulse magnetron sputtering (HiPIMS), resulting in textured Nb (110) thin films.  While these textured Nb (110) thin films are suitable for superconducting transmons, the coherence times of the resulting devices are believed to be limited by material defects, thus motivating efforts to realize higher quality Nb thin films. Here, we present ultra-high vacuum (UHV) annealed Nb thin films, previously deposited by HiPIMS, at base pressures of ~10^-10 Torr and temperatures up to 1000°C. The samples were deposited both on α-Al₂O₃ (006) and (110) substrates to make use of the epitaxial relationships with Nb (110). Specular and off-specular X-ray diffraction measurements suggest improvements in crystal quality following UHV annealing. Specifically, we show that the as-grown samples have a degree of epitaxy that is further enhanced by UHV annealing. Finally, the impact of the improved Nb crystal quality on superconducting properties is probed using cryogenic charge transport measurements.