Investigation of tantalum films growth for coplanar resonators with internal quality factors above ten million

Abstract: Alpha-tantalum on silicon is a promising platform for high-coherence superconducting quantum circuits. However, the growth mechanism of alpha-tantalum on silicon remains poorly understood. We present a comprehensive study on alpha-tantalum films growth on various substrate. The decisive role of a substrate material Debye temperature on phase selection mechanism in tantalum films growth is experimentally confirmed, contradicting the prior assumptions on substrate temperature influence. Crucially, we confirm that alpha-tantalum starts growing only after a 7-10 nm thick beta-tantalum underlayer. It results in ranging the critical temperature of {\alpha}-Ta films from 3.77 K to 4.39 K for the total thickness from 20 to 150 nm, respectively. Finally, we compared high-quality Al and Ta coplanar resonators on silicon, demonstrating compact tantalum resonators (4/10.5/4 um) with an internal quality factor exceeding 10 million at single-photon excitation powers.