Reduction of charge offset drift using plasma oxidized aluminum in SETs
Yanxue Hong, Ryan M. Stein, Michael D. Stewart, Neil M. Zimmerman, Joshua M. Pomeroy
Aluminum oxide (AlOx)-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, AlOx-based SETs exhibit time instabilities due to charge defect rearrangements and defects in AlOx often dominate the loss mechanisms in superconducting quantum computation. To characterize the charge offset stability of our AlOx-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 (ΔQ0) measured from the plasma oxidized AlOx SETs in this work is remarkably reduced (best ΔQ0 = 0.13e ± 0.01e over ≈ 7.6 days and no observation of ΔQ0 exceeding 1e), compared to the results of conventionally fabricated AlOx tunnel barriers in previous studies (best ΔQ0 = 0.43e ± 0.007e over ≈ 9 days and most ΔQ0 ≥ 1e 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.