Self-Assembled Monolayers as Barriers Against Aluminum Oxidation in Quantum Circuits

Decoherence in planar superconducting quantum circuits due to two-level system (TLS) defects at material interfaces, particularly at the air-interface, continues to be a significant challenge. A substantial portion of these parasitic TLS arise from oxide growth on aluminum surfaces. Despite extensive research and development efforts to address this issue, further advancements are essential for the commercialization of superconducting circuit-based devices. Various strategies, including etching, plasma cleaning, metal encapsulation, and self-assembled monolayers (SAMs) [1, 2], have been explored to mitigate TLS density at the air-interface. In this study, we present the passivation of Al-air interfaces with SAMs and demonstrate their effectiveness in suppressing oxide growth. Aging analysis confirms the long-term stability of this oxide-suppressing effect under ambient conditions. Characterization techniques such as X-ray photoelectron spectroscopy (XPS), contact angle measurement, and scanning electron microscopy (SEM) reveal the presence of the SAM and a substantial reduction in oxide formation at the Al-air interface, even in ambient environments and over extended periods. These findings suggest that SAM-based passivation is a promising approach for reducing microwave loss and enhancing the performance of aluminum-based superconducting quantum circuits.

1. M. Alghadeer, A. Banerjee, A. Hajr, H. Hussein, H. Fariborzi, S. G. Rao, Surface Passivation of Niobium Superconducting Quantum Circuits Using Self-Assembled Monolayers, ACS Applied Materials & Interfaces 15 (1), 2319-2328 (2023).
   
   M. Alghadeer, A. Banerjee, K. Lee, H. Hussein, H. Fariborzi, S. Rao, Mitigating Coherent Loss in Superconducting Circuits using Molecular Self-Assembled Monolayers, Scientific Reports (accepted) (2024).