Reduction of charge offset drift using plasma oxidized aluminum in SETs

luminum oxide ( AlO_x )-based single-electron transistors (SETs) fabricated in ultra-high vacuum (UHV) chambers using in situ plasma oxidation show excellent stabilities over more than a week, enabling applications as tunnel barriers, capacitor dielectrics or gate insulators in close proximity to qubit devices. Historically, AlO_x -based SETs exhibit time instabilities due to charge defect rearrangements and defects in AlO_x often dominate the loss mechanisms in superconducting quantum computation. To characterize the charge offset stability of our AlO_x -based devices, we fabricate SETs with sub-1 e charge sensitivity and utilize charge offset drift measurements (measuring voltage shifts in the SET control curve). The charge offset drift ( ΔQ_0 ) measured from the plasma oxidized AlO_x SETs in this work is remarkably reduced (best ΔQ_0=0.13 e ± 0.01 e over ≈ 7.6 days and no observation of ΔQ_0 exceeding 1 e ), compared to the results of conventionally fabricated AlO_x tunnel barriers in previous studies (best ΔQ_0=0.43 e ± 0.007 e over ≈ 9 days and most ΔQ_0≥ 1 e within one day). We attribute this improvement primarily to using plasma oxidation, which forms the tunnel barrier with fewer two-level system (TLS) defects, and secondarily to fabricating the devices entirely within a UHV system.