Abstract:
The Al/AlOx/Al system is widely used for the fabrication of Josephson junctions, which constitute the central element of superconducting qubits. The process parameters for growing ultra-thin AlOx barriers by thermal oxidation in-between physical vapor deposition (PVD) runs of Al films are well understood, but the resulting barriers present several drawbacks that limit performance such as thickness inhomogeneity and oxygen deficiency that leads to high defect densities in its bulk and interfaces. In this work, we present advances in the development of an alternative fabrication process for Al/Al2O3/Al structures by using thermal atomic-layer deposition (ALD) able to produce high-quality Al2O3 films with thickness close to 2 nm. We focus on reducing the low-quality interfacial oxide between Al and Al2O3 by using well-controlled wet chemical etching to eliminate the surface oxide layer that forms on the bottom PVD-deposited Al film. Additionally, the ALD deposition process for Al2O3 is also optimized by adjusting the length of the precursor pulses and purges to promote the nucleation of the Al2O3 first cycles on the Al surface. The effect of the processing conditions was observed through spectroscopic ellipsometry, electrical measurements at room temperature, and modeling of charge transport mechanisms on Al/Al2O3/Al structures. The electrical characterization revealed nonlinear J–V characteristics consistent with tunneling dominated transport, while the transport modeling enabled estimation of effective barrier thickness, interfacial layer thickness, barrier normal resistance, and critical current. The results show that interface engineering and ALD process optimization can effectively reduce the interfacial oxide contribution and improve barrier uniformity, providing useful insights for the development of Al/Al2O3/Al tunnel junctions for superconducting quantum circuit applications.