Investigation of the deposition of α-tantalum (110) films on a-plane sapphire substrate by molecular beam epitaxy for superconducting circuit
Haolin Jia, Boyi Zhou, Tao Wang, Yanfu Wu, Lina Yang, Zengqian Ding, Shuming Li, Xiao Cai, Kanglin Xiong, Jiagui Feng
Abstract
Polycrystalline α-tantalum (110) films deposited on the c-plane sapphire substrate by sputtering are used in superconducting qubits nowadays. However, these films always occasionally form other structures, such as α-tantalum (111) grains and β-tantalum grains. To improve the film quality, we investigate the growth of α-tantalum (110) films on the a-plane sapphire substrate under varying conditions by molecular beam epitaxy technology. The optimized α-tantalum (110) film is a single crystal, with a smooth surface and atomically flat metal–substrate interface. The film with thickness of 30 nm shows a Tc of 4.12 K and a high residual resistance ratio of 9.53. The quarter wavelength coplanar waveguide resonators fabricated with the 150 nm optimized α-tantalum (110) film exhibit intrinsic quality factor of over one million under single photon excitation at millikelvin temperature.