Study of initial growth and band diagram of e-beam evaporated Al₂O₃/ MBE Al/sapphire using in-situ x-ray photoelectron spectroscopy

Heterostructures of aluminum oxides/Al films are important for superconducting qubits. The presence of defective oxide layers in these heterostructures produces two-level systems (TLS), causing dielectric loss in superconducting circuits. Here, growth, analysis, and sample transfer were carried out in a UHV multi-chamber system.¹ We employed e-beam evaporation to in-situ deposit stoichiometric Al₂O₃ films on freshly MBE-grown Al films. Different from other methods, this approach provides better control over the oxide layer properties. We used in-situ X-ray photoelectron spectroscopy (XPS) to study the interfacial chemistry of MBE-Al/sapphire subtrate and e-beam Al₂O₃/MBE-Al.

A large band bending and the atomic form of Al were observed at the initial MBE-Al film growth. The band bending remained the same when the Al film was thicker than 2 nm, and became metallic. When Al₂O₃ was evaporated onto Al films, we observed a surface photovoltage (SPV) effect at a very thin thickness of Al₂O₃. As thickness of Al₂O₃ increased, we observed a downward band bending within the Al₂O₃ layer, shifting toward higher binding energy.

¹ K. Y. Lin et al, J. Crystal Growth 512, 223 (2019).