300-GHz Operation of Divider Circuits Using High- $J_{c}\ \hbox{Nb/Nb}_{x}\hbox{Si}_{1-x}/\hbox{Nb} $ Josephson Junctions

We investigated high-current-density Josephson junctions with Nb
_x
Si
_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
_x
Si
_1-x
and a-Si barriers for J
_c
ranging from 3.5 to 85 kA/cm
²
. For junctions in the lower J
_c
range (3.5-17 kA/cm
²
), we used optical lithography, and for the 85-kA/cm
²
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/cm
²
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.