Aluminum nitride tunnel barrier formation with low-energy nitrogen ion beams

We report the use of low-energy nitrogen ion beams to form ultra-thin (<2 nm) layers of AlNx to act as tunnel barriers in Nb/Al–AlNx/Nb Josephson junctions. We fabricated reproducible, high-quality devices with independent control of the ion energy and dose, enabling exploration of a wide parameter space. Critical current density Jc ranged from 550 to 9400 A/cm2 with subgap-to-normal resistance ratios from 50 to 12.6. The spatial variation of ion-current density was roughly correlated with Jc over a large-area on a Si substrate. The junctions were stable on annealing up to temperatures of at least 200 °C. This technique could be applied to form other metal nitrides at room temperature for device applications where a high degree of control is desired.