Growth and Transport Properties of Tunneling Barriers in HTS devices

In this contribution we will discuss the charge transport of ramp-type HTS Josephson junctions with a Ga-doped PBCO barrier layer. It will be demonstrated that in these junctions charge transport takes place via tunneling processes. The Cooper pairs tunnel directly, at least for T ≤ Tc/2, whereas the quasiparticles tunnel indirectly via localized states. By substituting Cu-chain atoms with Ga-atoms the density of localized states appear to be reduced, resulting in an increase in IcRn-product. Another way to increase this product is a reduction in barrier thickness. Growth studies by AFM of PBCO barriers on ramps indicate that below about 10 nm barriers become increasingly less homogeneous, and below about 6 nm pin holes are very likely to occur. This sets a lower limit on the useful barrier thickness. Presently critical-current densities up to 104 A/cm2 at 40 K, and IlR,-products up to 10 mV at 4.2 K are easily obtained.