Development of low critical temperature superconducting tunnel jucntions for application as photon detectors in astronomy

This thesis describes the development of low-energy gap superconducting tunnel junctions (STJs) for use as photon detectors, with as a main goal the improvement of the energy resolution in both the optical and the x-ray energy domain. A new model for the photon detection process with STJs is presented, which includes the full energy dependence of all the quasiparticle processes occurring in the junctions. This model allows for the calculation of the time- and energy-dependent quasiparticle distribution from the moment of generation of the quasiparticles by the photon absorption process until the end of the current pulse, when all the quasiparticles have disappeared.Comment: 140 pages, Ph. D. thesi

Strong Quasiparticle Trapping In A 6x6 Array Of Vanadium-Aluminum Superconducting Tunnel Junctions

A 6x6 array of symmetrical V/Al/AlOx/Al/V Superconducting Tunnel Junctions (STJs) was fabricated. The base electrode is a high quality epitaxial film with a residual resistance ratio (RRR) of ~30. The top film is polycrystalline with an RRR of ~10. The leakage currents of the 25x25 mm^2 junctions are of the order of 0.5 pA/mm^2 at a bias voltage of 100 mV, which corresponds to a dynamical resistance of ~ 3 10^5 ohms. When the array was illuminated by 6 keV X-ray photons from a 55Fe radioactive source the single photon charge output was found to be low and strongly dependent on the temperature of the devices. This temperature dependence at X-ray energies can be explained by the existence of a very large number of quasiparticle (QP) traps in the Vanadium. QPs are confined in these traps, having a lower energy gap than the surrounding material, and are therefore not available for tunneling. The number of traps can be derived from the energy dependence of the responsivity of the devices (charge output per electron volt of photon input energy).Comment: 4 pages. presented at Low Temperature Detectors-

Optical photon detection in Al Superconducting Tunnel Junctions

We report on the successful fabrication of low leakage aluminium superconducting tunnel junctions with very homogeneous and transparent insulating barriers. The junctions were tested in an adiabatic demagnetisation refrigerator with a base temperature of 35 mK. The normal resistance of the junctions is equal to ~7 uohm cm2 with leakage currents in the bias voltage domain as low as 100 fA/um2. Optical single photon counting experiments show a very high responsivity with charge amplification factors in excess of 100. The total resolving power (including electronic noise) for 500 nm photons is equal to 13 compared to a theoretical tunnel limited value of 34. The current devices are found to be limited spectroscopically by spatial inhomogeneities in the detectors responseComment: 3 pages, 5 figure

Future optical detectors based on Al superconducting tunnel junctions

Superconducting tunnel junctions are being developed for application as photon detectors in astronomy. We present the latest results on the development of very high quality, very low critical temperature junctions, fabricated out of pure Al electrodes. The detectors are operated at 50 mK in an adiabatic demagnetisation refrigerator. The contacts to the top and base electrodes of these junctions are fabricated either out of Nb or Ta, which has strong implications on the loss time of the quasiparticles. The Nb contacted junctions show quasiparticle loss times varying between 5 and 80 usec, depending on the device size. The bias range of the Nb-contacted junctions is limited to the range 0-100 uV, because of the set-in of strong non-equilibrium quasiparticle multiplication currents at higher bias voltages. The Ta-contacted junctions, on the other hand, show quasiparticle loss times in excess of 200 usec. These long loss times lead to very strong quasiparticle multiplication, which prevents the stable biasing of the junctions even at very low bias voltages. Junction fabrication and characterisation are described, as well as the response of the detectors to monochromatic light with wavelengths varying from 250 to 1000 nm. The energy resolution of the detectors is discussed.Comment: 12 pages, 10 figure

Modelling the energy gap in transition metal/aluminium bilayers"

We present an application of the generalised proximity effect theory.Comment: 15 pages, 11 figures, presented at workshop on low temperature superconducting electronics at the University of Twente, The Netherland

Dynamics of nonequilibrium quasiparticles in a double superconducting tunnel junction detector

We study a class of superconductive radiation detectors in which the absorption of energy occurs in a long superconductive strip while the redout stage is provided by superconductive tunnel junctions positioned at the two ends of the strip. Such a device is capable both of imaging and energy resolution. In the established current scheme, well studied from the theoretical and experimental point of view, a fundamental ingredient is considered the presence of traps, or regions adjacent to the junctions made of a superconducting material of lower gap. We reconsider the problem by investigating the dynamics of the radiation induced excess quasiparticles in a simpler device, i.e. one without traps. The nonequilibrium excess quasiparticles can be seen to obey a diffusion equation whose coefficients are discontinuous functions of the position. Based on the analytical solution to this equation, we follow the dynamics of the quasiparticles in the device, predict the signal formation of the detector and discuss the potentiality offered by this configuration.Comment: 16 pages, 5 figures Submitted to Superconducting Science and Technolog

Physical routes for the synthesis of kesterite

This paper provides an overview of the physical vapor technologies used to synthesize Cu2ZnSn(S,Se)4 thin films as absorber layers for photovoltaic applications. Through the years, CZT(S,Se) thin films have been fabricated using sequential stacking or co-sputtering of precursors as well as using sequential or co-evaporation of elemental sources, leading to high-efficient solar cells. In addition, pulsed laser deposition of composite targets and monograin growth by the molten salt method were developed as alternative methods for kesterite layers deposition. This review presents the growing increase of the kesterite-based solar cell efficiencies achieved over the recent years. A historical description of the main issues limiting this efficiency and of the experimental pathways designed to prevent or limit these issues is provided and discussed as well. Afinal section is dedicated to the description of promising process steps aiming at further improvements of solar cell efficiency, such as alkali doping and bandgap grading1. R Caballero and M León acknowledge financial support via the Spanish Ministry of Science, Innovation and Universities project (WINCOST, ENE2016-80788-C5-2-R) and thank H2020 EU Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968). 2. S Canulescu and J Schou acknowledge the support from Innovation Fund Denmark. 3. D-H Kim acknowledges financial support via the DGIST R&D Program of the Ministry of Science and ICT, KOREA (18-BD-05). 4.C. Malerba acknowledges the support from the Italian Ministry of Economic development in the framework of the Operating Agreement with ENEA for the Research on the Electric System. 5.A Redinger acknowledges financial support via the FNR Attract program, Project : SUNSPOT, Nr.11244141. 6. E Saucedo thanks H2020 EU Programme under the projects STARCELL (H2020-NMBP-03-2016-720907) and INFINITE-CELL (H2020-MSCA-RISE-2017-777968), the Spanish Ministry of Science, Innovation and Universities for the IGNITE project (ENE2017-87671-C3-1-R), and the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). IREC belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the ‘Generalitat de Catalunya’ (Ref. 2017 SGR 862). 7. Taltech acknowledges financial support via the Estonian Ministry of Education and Research funding project IUT19-28 and the European Union Regional Development Fund, Project TK141. 8. B Vermang has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (Grant Agreement No 715027