Physics of high-j _c Nb/AlO _x/Nb Josephson junctions and prospects of their applications

At critical current density of the order of 100 kA/cm ², tunnel Josephson junctions become overdamped and may be used in RSFQ circuits without external shunting, dramatically increasing circuit density. However, the physics of electron transport in such high-j _c junctions differs from the usual direct tunneling and until recently remained unclear. We have found that the observed dc I-V curves of niobium-trilayer junctions with j _c = 210 kA/cm ² can be explained quantitatively by resonant tunneling through strongly disordered barriers. According to this interpretation, random spread of critical current in high-j _c junctions may be rather small (below 1% r.m.s.) even in deep-submicron junctions, making VLSI RSFQ circuits, with density above 10 MJJ/cm ², feasible.