Stability of metallic single-electron devices with plasma oxidized, cobalt confined AlO$_{\mathrm{x}}$ tunnel barriers

Single-electron transistors (SETs) are fabricated by double-angle deposition in order to measure the electrical stability in the plasma oxidized, cobalt confined aluminum oxide (AlO$_{\mathrm{x}})$ barriers, as an estimate of the two-level fluctuator density compared with thermal oxidation. The electrical stability of metal-based SETs and superconducting devices suffers from oxide-induced high defect densities and long-term charge offset drift. These devices are typically made with thermal AlO$_{\mathrm{x}}$. Our tunnel junctions have two physical differences from those of thermal oxides: 1) plasma oxidation is shown to be more uniform and stoichiometric for AlO$_{\mathrm{x}}$ than thermal oxidation; and 2) high oxygen content is confined within the insulating regions by using a Co/AlO$_{\mathrm{x}}$/Co structure to provide a barrier against oxygen diffusion. In our prior work with large-area tunnel junctions, these AlO$_{\mathrm{x}}$ barriers sandwiched between cobalt layers exhibit better long-term resistance stability. In this work, we are developing SETs by double-angle deposition as a path toward low-capacitance and small-area Co/AlO$_{\mathrm{x}}$/Co tunnel junctions. We expect better charge offset stability on these devices than typical thermally oxidized devices with unconfined oxygen.