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

TL;DR

This study explores the growth of high-quality alpha-tantalum (110) films on a-plane sapphire substrates via molecular beam epitaxy, achieving single-crystal films with excellent superconducting properties for quantum circuits.

Contribution

It demonstrates the optimized growth conditions for single-crystal alpha-tantalum (110) films with superior surface quality and superconducting performance using molecular beam epitaxy.

Findings

Achieved single-crystal alpha-tantalum (110) films with atomically flat interfaces.

Obtained a superconducting transition temperature (Tc) of 4.12K.

Fabricated resonators with quality factors exceeding one million.

Abstract

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 \b{eta}-tantalum grains. 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.