O.J. Luiten

Inhomogeneous response of superconducting tunnel junctions with a killed electrode for X-ray spectroscopy

Nb-based superconducting tunnel junctions are being developed as high energy resolution X-ray detectors. Unfortunately, loss of excess quasiparticles at the edges, combined with lateral diffusion, results in an inhomogeneous response. To study this degradation of energy resolution, we manufactured detectors with a Ta trap in the top or bottom electrode away from the tunneling barrier. Excess quasiparticles in this so-called killed electrode will be trapped effectively and thus removed from the tunneling process. The X-ray spectra of the active electrode can be fitted with a model based on classical diffusion of quasiparticles. On junctions with a killed bottom electrode also Low Temperature Scanning Electron Microscopy (LTSEM) measurements have been performed. The X-ray spectra and the LTSEM scans are consistent with each other and with the model. The energy resolution of the junctions presented here is limited by loss of quasiparticles at the edges.

High quality superconducting tunnel junctions on Nb and Ta single crystals for radiation detection

High quality Nb/Al-based tunnel junctions, fabricated with a superconducting interface onto thick single crystalx-ray absorbers of Nb and Ta are discussed. Current-voltage characteristics, recorded at 0.5 K, show a subgap current which is still dominated by thermally excited quasiparticles. The quality parameter Rsubgap/Rnormal reaches a value of several million, which is unequalled for nonepitaxially sputtered tunnel junctions. The fabrication process and some development steps, such as preparation of ultrasmooth crystal surfaces are described. Observations of x-ray photons absorbed in Nb and Tasingle crystals detected by the superconducting tunnel junctions are also presented.

Diffusion of quasiparticles in SIS tunnel junctions for X-ray spectroscopy

X-ray detectors based on Superconductor-Insulator-Superconductor (SIS) tunnel junctions have the potential of a very high energy resolution. We investigate, limitations on the attainable energy resolution due to the combined effect of lateral diffusion of the excess quasiparticles, generated after absorption of an X-ray photon, and loss of the quasiparticles at the edges. We present an exactly solvable model, which we apply to recent X-ray measurements on a set of tunnel junctions which only differ in size. This clearly shows the detrimental influence of loss at the edges. The model gives quantitative criteria which have to be met to reach the desired 10 eV energy resolution for 6 keV photons.

A Nb single crystal X-ray detector read out by superconductive tunnel junctions

We have fabricated a prototype imaging X-ray detector, consisting of Al/AlOx/Al/Nb superconductive tunnel junctions sputtered directly on a high purity Nb single crystal with an RRR/spl ap/15000. Such a detector in principle offers a high energy resolution, combined with a high quantum efficiency, a large detection area and imaging capabilities. The quasiparticle collection efficiency and consequently the X-ray response of the detector is still very poor due to the large thickness of the crystal (120 /spl mu/m), but agrees with proximity theory. By irradiation with a focused electron beam of a Low Temperature Scanning Electron Microscope, the spatial and temporal response of the detector has been determined. The derived diffusion coefficient is two orders of magnitude smaller than expected on the basis of the RRR.

Superconductive tunnel junction detectors as x-ray spectrometers

Superconductive tunnel junctions (STJs) are being developed as high energy resolution x-ray single photon detectors. Quasi-particle losses at the edges of such devices form a serious source of energy resolution degradation. A simple analytical relation for this source of resolution degradation has been derived from classical diffusion theory. Analyzing the x-ray spectra obtained for different series of STJs with various sizes, the edge reflectivity and the diffusion constant for our sputtered Nb films can be derived. This reflectivity can be explained by quasi- particle trapping. In addition progress is reported on the surface conditioning of single crystal, superconducting, Ta and Nb absorbers to be used for a highly efficient imaging x-ray spectrometer employing STJs as read-out.

Fabrication of tunnel junctions on thick X-ray absorbing substrates of Nb and Ta

X-ray detectors based on absorber-junction combinations can combine a large detector area with position resolution and good energy resolution. We plan to use a thick, single crystal Nb or Ta absorber with readout tunnel junctions integrated on top as our next generation X-ray detector. The thickness of the absorber should ensure detection of more than 90% of the impinging photons. Special points of attention in the fabrication of this new detector, like surface preparation, the formation of contacts, and handling of the bulk absorber during the fabrication process are discussed.

Nb-based superconducting tunnel junctions for X-ray spectroscopy: TaOx and AlOx tunnelbarriers

Abstract Results are presented of an optimization study of TaO x -tunnelbarriers in superconducting tunneljunctions for X-ray spectroscopy. The properties did not satisfy the critical demands. A comparison is made with first results on AlO x -barriers made with the same processing setup. AFM and TEM were used in studies of interface roughness.

Superconducting tunnel junctions on Nb- and Ta-single crystal substrates for X-ray spectrometry

We present the first high quality Nb/Al-based tunnel junctions, fabricated with a superconducting interface onto single crystal substrates of Nb and Ta. Current-voltage measurements down to 1.4 K show a subgap current which is dominated by thermal excitation of quasiparticles, no ohmic or superconducting leakages could be observed. The fabrication process and development steps, such as preparation of ultra-smooth crystal surfaces are described.