Seeking Reduced Charge Offset Drift Using Confined, Plasma Oxidized AlO_x in SETs

As an essential material for quantum computing applications, aluminum oxide (AlO_x) is extensively employed as the gate dielectric in semiconducting quantum information (QI) devices and as the tunnel barrier in superconducting QI architectures. However, AlO_x is frequently reported to have a broad distribution of electrically active defects and high density of interacting two-level systems (TLSs). Here we study the charge offset stability of single-electron transistors (SETs) fabricated with plasma oxidized, cobalt confined AlO_x tunnel junctions. The performance of metal-based SETs is critically limited by the charge offset drift, which is significantly influenced by the electrical stability of the tunnel barrier. Our tunnel junctions have two physical differences from those of thermal oxides: 1) plasma oxidation has been shown to be more uniform and stoichiometric for AlO_x; and 2) high oxygen content is confined within the insulating regions by using a Co/ AlO_x /Co structure to provide a barrier against oxygen diffusion. These two differences lead to the expectation that better charge offset stability be measured on these devices as compared to typical thermally oxidized devices with unconfined oxygen.