No abstract

Metal-insulator-metal (MIM) tunnel junctions with the aluminum oxide tunnel barriers confined between cobalt electrodes exhibit less resistance drift over time than junctions that utilize a thick, unconfined aluminum electrode. The improved long time stability is attributed to better initial oxide quality achieved through confinement (use of a potential energy well for the oxygen) and plasma oxidation. In this work, Co/AlOx/Co and Co/Al/AlOx/Co tunnel junction aging is compared over a period of approximately 9 months using transport measurements and Wentzel-Kramers-Brillouin (WKB) based modelling. The Co/AlOx/Co (confined) tunnel junction resistance increased by (32 ± 6) % over 5400 h, while Co/Al/AlOx/Co (unconfined) tunnel junction resistance increased by (85 ± 23) % over 5200 h. Fit parameters for the tunnel barrier width and potential energy barriers were extracted using WKB transport modelling. These values change only a small amount in the confined Co/AlOx/Co tunnel junction but show a significant drift in the unconfined Co/AlOx/Co tunnel junction.

Gate-defined quantum dots benefit from the use of small grain size metals for gate materials because they aid in shrinking the device dimensions. However, it is not clear what differences arise with respect to process-induced defect densities and inhomogeneous strain. Here, we present measurements of fixed charge, Qf; interface trap density, Dit; the intrinsic film stress, σ; and the coefficient of thermal expansion, α, as a function of forming gas anneal temperature for Al, Ti/Pd, and Ti/Pt gates. We show that Dit is minimized at an anneal temperature of 350 °C for all materials, but Ti/Pd and Ti/Pt have higher Qf and Dit compared to Al. In addition, σ and α increase with anneal temperature for all three metals with α larger than the bulk value. These results indicate that there is a trade-off between minimizing defects and minimizing the impact of strain in quantum device fabrication.