Recent Developments in the Cryogenic Electronics Group

We wish to report the achievement of two important milestones in our program to develop tantalum-based superconducting tunnel junction (STJ) x-ray detectors for use in synchrotron-based XAS spectrometers.

First, by improving grounding of instrumentation and by adding shielding to cables and electronics enclosures, we have greatly reduced the severe noise that corrupted the early IV data sets taken on junctions that were cooled in the cryostat at LLNL and read out with SFSU electronics.

Second, the excellent IV data, shown below, that we recently acquired for a niobium-based STJ has validated our full-wafer microfabrication process.

IV curve

We have nearly completed a set of beta-Ta STJ devices. Beta-Ta is the structural phase that is produced in sputtered tantalum films if a niobium seed layer or other means is not used to nucleate alpha-Ta. Alpha-Ta is necessary for x-ray detection purposes because it has a higher critical temperature than the aluminum trapping layer.  If our latest beta-Ta devices also have excellent IV curves similar to those obtained for our niobium devices, then it is likely that the shunt conductance that characterized our original alpha-Ta STJs was not due to film stress in the tantalum layers but was rather a result of a small admixture of beta-phase Ta in the nominally alpha-phase Ta layers. During cool down residual magnetic flux would have been trapped in the beta-phase regions and perhaps produced NIN regions in what was supposed to have been an SIS junction.

We have several procedures in mind for rapidly identifying and correcting the fabrication steps that would produce a mixture of alpha and beta Ta.