Interface-sensitive microwave loss in tantalum films grown on sapphire for superconducting device application

Recently, tantalum (Ta) has emerged as a promising material for the wiring layer of superconducting quantum devices, yielding higher qubit coherence compared to niobium (Nb) or aluminum (Al). However, the effects of film and interface microstructure on low-temperature microwave loss are still not well understood. We present a systematic study varying the growth temperature of sputtered Nb and Ta thin films deposited on c-plane sapphire (Al₂O₃(001)). Structural properties of the films were studied using transmission electron microscopy and electron and X-ray diffraction, while electrical properties were studied using temperature dependent resistivity and quality factor measurements of coplanar waveguide resonators. Our measurements show that the microwave performance of superconducting resonators made from epitaxial Ta(111) films grown directly on Al₂O₃(001) was markedly poor when compared to polycrystalline Ta films or epitaxial Nb(111) films also grown on Al₂O₃(001) at the same temperature. State-of-the-art levels of low microwave loss were realized by either inserting a 5nm thick epitaxial Nb nucleation layer before growth of epitaxial Ta or by intentionally damaging the sapphire substrate with an Ar plasma before growth of polycrystalline Ta[1]. Superconducting gap spectroscopy measurements performed using a frequency-domain RF susceptometer on the lossy epitaxial Ta(111) films suggest a high density of in-gap quasiparticles with a nonmonotonic distribution in energy in contrast to films with epitaxial Nb(111) interfaces which show monotonic BCS behavior. To further understand the origin of the apparent in-gap quasiparticle distribution and associated microwave loss, epitaxial Ta(100), Ta(110), and Ta(111) films grown on r-, a-, and c-plane sapphire respectively are studied using the same techniques with an emphasis on the temperature dependence of microwave loss.