Investigation of the deposition of α-tantalum (110) films on a-plane sapphire substrate by molecular beam epitaxy for superconducting circuit

Polycrystalline α-tantalum (110) films deposited on the c-plane sapphire substrate by sputtering are used in superconducting qubits nowadays. However, these always occasionally form other structures, such as (111) grains and β-tantalum grains. To improve film quality, we investigate growth of a-plane under varying conditions molecular beam epitaxy technology. The optimized is a single crystal, with smooth surface atomically flat metal–substrate interface. thickness 30 nm shows Tc 4.12 K high residual resistance ratio 9.53. quarter wavelength coplanar waveguide resonators fabricated 150 exhibit intrinsic quality factor over one million photon excitation at millikelvin temperature.