300-GHz Operation of Divider Circuits Using High- Josephson Junctions

We investigated high-current-density Josephson junctions with Nb_xSi_1-x barriers in single-flux-quantum (SFQ) digital circuits to evaluate their performance at clock frequencies of hundreds of gigahertz. We fabricated static divider SFQ circuits with a-Nb_xSi_1-x and a-Si barriers for J_c ranging from 3.5 to 85 kA/cm². For junctions in the lower J_c range (3.5-17 kA/cm²), we used optical lithography, and for the 85-kA/cm² junctions, we used electron-beam lithography. We measured SFQ circuits with a maximum operating speed of 300 GHz by use of intrinsically shunted junctions having submicrometer dimensions and an 85-kA/cm² critical current density. These results demonstrate that silicide-barrier junctions have the potential to achieve high-speed SFQ circuits and high-density circuits because the junctions are small and do not require external shunt resistors.