Surfaces, Interfaces, Films, and Nanosystems

Aswin kumar Anbalagan, Rebecca Cummings, Chenyu Zhou +14 more

Despite constituting a smaller fraction of the qubit's electromagnetic mode, surfaces and interfaces can exert significant influence as sources of high-loss tangents, which brings forward the need to reveal properties of these extended defects and identify routes to their control. Here, we examine the structure and composition of the metal-substrate interfacial layer that exists in Ta/sapphire-based superconducting films. Synchrotron-based X-ray reflectivity measurements of Ta films, commonly used in these qubits, reveal an unexplored interface layer at the metal-substrate interface. Scanning transmission electron microscopy and core-level electron energy loss spectroscopy identified an intermixing layer (≈0.65 ± 0.05 nm) at the metal-substrate interface containing Al, O, and Ta atoms. Density functional theory modeling reveals that the structure and properties of the Ta/sapphire heterojunctions are determined by the oxygen content on the sapphire surface prior to Ta deposition for two atomic terminations of sapphire. Using a multimodal approach, we gained deeper insights into the interface layer between the metal and substrate, which suggests that the orientation of deposited Ta films depend on the surface termination of sapphire. The observed elemental intermixing at the metal-substrate interface influences the thermodynamic stability and electronic behavior of the film, which may also affect qubit performance.

James Keaveney

Masanori Ohya, Igor Vasil'evich Volovich

Hyunbum Jang, Malcolm J. Grimson

Monte Carlo simulations have been performed to investigate the equilibrium phase behaviour of a transverse Ising model in a thin-film geometry under the action of competing surface fields. This model is a classical isotropic vector spin model with a ferromagnetic Ising Hamiltonian and a transverse field applied perpendicular to the Ising z-axis. For finite values of the transverse field strength, , the model displays an interface localization transition with a spontaneous magnetization of the film below a critical temperature Tc. However, in the limit , no spontaneous magnetization of the film is observed. In the two-phase regime, below Tc, a degeneracy exists in the magnetization profiles across the film between states of positive and negative total magnetization. The interface localization transition temperature lies below the bulk critical temperature of the film for competing surface fields.

Anthony McFadden, Trevyn F. Q. Larson, Stephen Gill +5 more

Quantum coherence in superconducting circuits has increased steadily over the last decades as a result of a growing understanding of the various loss mechanisms. Recently, tantalum (Ta) emerged as a promising material to address microscopic sources of loss found on niobium (Nb) or aluminum (Al) surfaces. However, the effects of film and interface microstructure on low-temperature microwave loss are still not well understood. Here we present a systematic study of the structural and electrical properties of Ta and Nb films sputtered on c-plane sapphire at varying growth temperatures. As growth temperature is increased, our results show that the onset of epitaxial growth of alpha-phase Ta correlates with lower Ta surface roughness, higher critical temperature, and higher residual resistivity ratio, but surprisingly also correlates with a significant increase in loss at microwave frequency. Notably, this high level of loss is not observed in Nb films prepared in the same way and having very similar structure. By experimentally controlling the surface on which the Ta film is nucleated, we determine that the source of loss is only present in samples having an epitaxial Ta/sapphire interface and show that it is apparently mitigated by either growing a thin, epitaxial Nb inter-layer between the Ta film and the substrate or by intentionally treating, and effectively damaging, the sapphire surface with an in-situ argon plasma before Ta growth. In addition to elucidating this interfacial microwave loss, this work provides adequate process details to aid reproducible growth of low-loss Ta films across fabrication facilities.

T. S. Monteiro, Adam Rançon, Janne Ruostekoski