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

Wan-Sin Chen^1*, Yi-Ting Cheng², Y.-H. G. Lin², Chiu-Ping Cheng³, J. Kwo², M. Hong¹

¹Grad. Inst. of Appl. Phys. and Dept. of Phys., National Taiwan University, Taipei, Taiwan
²Department of Physics, , National Tsing Hua University, Hsinchu, Taiwan
³Department of Electrophysics, , National Chiayi University, Chiayi, Taiwan

* Presenter:Wan-Sin Chen, email:d06245004@ntu.edu.tw

Heterostructures composed of aluminum oxides and Al films play a pivotal role in the field of superconducting qubits. The hetero-interfaces, with defective oxide layers, give rise to two-level systems (TLS), contributing to dielectric losses in superconducting circuits. ¹ Native aluminum oxides develop when Al films are exposed to oxygen or air, making it challenging to control their thickness and stoichiometry - a significant obstacle in quantum computing applications. In this work, we utilized electron-beam evaporation to in-situ deposit Al₂O₃ films onto Al films. This approach distinguishes itself from other methods and provides better control over the properties of the oxide layer. We employed in-situ X-ray photoelectron spectroscopy (XPS) to study the interfacial chemistry between the molecular beam epitaxy (MBE) Al films, epitaxially grown on c-plane sapphire substrates, and the e-beam evaporated Al₂O₃ on the MBE epi-Al films. The entire process, from material growth and analysis to sample transfer, was carried out in a multi-chamber system operating under ultra-high vacuum (UHV). 2 The c-plane sapphire(0001) substrates were prepared by cleaning with a piranha solution and subsequent rinsing with deionized water. They were then annealed at 800°C within the MBE chamber. The Al films were deposited using an Al effusion cell, and the growth temperature was maintained nominally below 0°C. The growth of epitaxial Al films was monitored using in-situ reflection high-energy electron diffraction (RHEED).
For very thin Al films (1 nm), we observed an atomic structure and a substantial band bending of 1.5 eV. The band bending remained the same as the Al film thickness exceeded 2 nm, transitioning the film into a metallic state. Furthermore, Al was found to bond with the surface oxygen of the sapphire at the Al/sapphire interface. Upon evaporating Al₂O₃ onto a 50-nm thick Al film, a surface photovoltage (SPV) effect was observed when the Al₂O₃ thickness reached 0.5 nm. Moreover, with an increase in the thickness of Al₂O₃, a downward band bending occurred within the Al₂O₃ layer, shifting toward higher binding energy. We conducted a comparative analysis of the atomic-scale chemical bonding of Al on sapphire and e-beam-Al₂O₃ on 50-nm thick Al.
Acknowledgments
This work is supported by the National Science and Technology Council (NSTC), Taiwan through grant number NSTC 112-2119-M-007 -009 -.

Keywords: X-ray photoelectron spectroscopy, Interface study, supreconductivity Al