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

Applied Physics;Mesoscale and Nanoscale Physics