Plasma Oxidized AlOx Tunnel Barriers and Nb/Al Bilayers Examined by Electrical Transport

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Results are reported for two related projects: the examination of material stability of plasma oxidized, free energy confined aluminum oxide and the evolution of the electronic structure in Nb/Al bilayers as a function of Al thickness. Al/AlOx and Nb are critical materials for solid-state quantum computing, mostly driven by the relatively large superconducting gap of Nb (1.5 meV) and ∼ 2 nm diffusion limited oxide formed on Al with room temperature thermal oxidation. Plasma oxidation and free energy confinement of AlOx with Co electrodes is used to produce homogeneous tunnel barriers with an O/Al ratio approaching Al2O3. The weeks long time stability of resulting metal-insulator-metal tunnel junctions is found to greatly improve, as resistance measured over ≈ 8 months increases by 34.0 ± 5.4 % in the confined devices (Co/AlOx/Co) compared to an increase of 95.4 ± 7.8 % in unconfined devices (Co/Al/AlOx/Co). In the second experiment, normal metal-insulator-superconductor (NIS) tunnel junctions are used to study the interplay of superconducting properties in Nb/Al bilayers as a function of Al thickness. The performance of superconducting quantum information devices is sensitive to thedetailed nature of the superconducting state in the materials, which is drastically altered through proximity in the case of dissimilar materials. I extract the effective Nb/Al quasiparticle DOS from the conductance spectra of NIS tunnel junctions with Nb/Al superconducting electrodes. The conductance spectra evolve from a primarily single-gapped structure for thin Al (< 20 nm) to a dual gapped structure at thicker Al. I present a modified Blonders-Tinkham-Klapwijk (BTK) based model interpreting the conductance spectra as a steady-state convolution of the Al-like DOS and the Nb-like DOS in the bilayer. These results inform future device design for quantum information by providing additional grounding to current proximity effect theory.