Tantalum STJ for Photon Counting Detectors

Superconducting Tunnel Junctions (STJ's) are currently being developed as photon detectors for a wide range of applications. Interest comes from their ability to cumulate photon counting with chromaticity (i.e. energy resolution) from the near infrared (2 $\mu$m) to the X-rays wavelengths and good quantum efficiency up to 80%. Resolving power can exceed 10 in the visible wavelength range. Our main goal is to use STJ's for astronomical observations at low light level in the near infrared. This paper put the emphasis on two main points: the improvement of the tantalum absorber epitaxy and the development of a new version of the fabrication process for making Ta/Al-AlOx-Al/Ta photon counting STJ's. The main features of this process are that pixels have aligned electrodes and vias patterned through a protecting SiO2 layer. These vias are then used to contact the top electrode layer. We use a double thin aluminum trapping layer on top of a 150 nm thick Ta absorber grown epitaxially. Photon counting experiments with Ta junction array are presented at \lambda = 0.78 $\mu$m. Digital filtering methods are used to compute the photon counting data in order to minimize the effects of noise

Magnetic Screening of NbN Multilayers Samples

6 pagesInternational audienceIn 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of Hc > Hc^Nb for magnetic shielding of bulk niobium to increase the breakdown magnetic field inside SC RF cavities [1]. We have deposited high quality "model" samples by magnetron sputtering on monocrystalline sapphire substrates. A 250 nm layer of niobium figures the bulk Nb. It was coated with a single and multi-stacks of NbN layers (25 or12 nm) separated by 15 nm MgO barriers, and characterized by X-Ray reflectivity and DC transport measurements. DC or AC measurement of HC1 is an important goal for multilayer evaluation during the sample evaluation phase. A clear increase of HC1 at low frequency is promising indication since HC1 is expected to increase with frequency (see e.g. [2] and references therein). We have measured the first penetration field (HP~HC1) on DC magnetization curves in a SQUID system. HP of NbN covered sample is increased compared to Nb alone. We have also developed a set-up that allows measuring a large range of field and temperature with a local probe method based on 3rd harmonic analysis. We have confirmed the screening behavior of a single 25 nm NbN layer placed on the top of a Nb Layer

Spectral dependency of superconducting single photon detectors

International audienceWe investigate the effect of varying both incoming optical wavelength and width of NbN nanowires on the superconducting single photondetectors (SSPD) detection efficiency. The SSPD are current biased close to critical value and temperature fixed at 4.2 K, far from transition. The experimental results are found to verify with a good accuracy predictions based on the "hot spot model," whose size scales with the absorbed photon energy. With larger optical power inducing multiphoton detection regime, the same scaling law remains valid, up to the three-photon regime. We demonstrate the validity of applying a limited number of measurements and using such a simple model to reasonably predict any SSPD behavior among a collection of nanowire device widths at different photon wavelengths. These results set the basis for designing efficient single photondetectors operating in the infrared (2-5 μm range)

Optimisation de détecteurs pour l'astronomie du rayonnement X (développement de jonctions supraconductrices pour l'isolation thermique dans les interconnexions)

L avenir des nouvelles caméras embarquées pour l astrophysique spatiale semble passer par unaccroissement du nombre de pixels et un fonctionnement à très basse température (en dessous de 0,1 K).Avec cette évolution, le nombre important de fils en sortie du détecteur refroidi représente souvent lacharge thermique prédominante sur la source froide (cryostat).Dans ce contexte, l isolation thermique entre les différents circuits de détection est un point crucial pources caméras. Une brique technologique innovante a été développée pour apporter une solution présentantune excellente conduction électrique couplée à une grande isolation thermique. Cette innovation,protégée par un brevet, permet de résoudre cet apparent paradoxe. La solution proposée consiste enl empilement d un grand nombre de couches minces de matériaux supraconducteurs dans lesinterconnexions.La résistance thermique à chaque interface est dépendante des propriétés élastiques des matériaux,de la qualité des interfaces et de la température à laquelle le système fonctionne. A très basse température,le modèle AMM, couplé aux mesures des caractéristiques des matériaux composants la multicouche,permettent une estimation théorique de la résistance thermique pour une interface. Les mesures effectuéesavec les liaisons supraconductrices à forte résistivité thermique concordent avec les estimationsthéoriques. Nous avons ainsi pu mesurer des résistances thermiques de l ordre de 3,3.105 K/W à 200 mKpour une multicouche composée d une succession (62 interfaces) de couches minces de nitrure de titaneet de niobium sur une surface de 16 mm2. Dans les conditions d utilisation prévues pour une camérarayons X de 4000 pixels microcalorimétriques, l utilisation de cette brique technologique devrait assurerune charge thermique sur la source froide (à 50 mK) très inférieure au W pour plus de 8000 pointsde contact. Ce dispositif pourra être utilisé à l avenir dans nombre de projets cryogéniques, lorsqu une excellenteisolation thermique associée à une excellente conduction électrique sera recherchée.Future of the next camera onboard space observatories implies a major enhancement in number of pixelsand a very low operative temperature (below 0.1 K). In this evolution, the large number of output wiresfrom the cool detector is often responsible of the most important thermal load onto the cold bath(cryostat).In this context, the thermal insulation between the different detection circuits is the bottleneck for thesecameras. An innovative technological component, protected by a patent, has been developed to tackle thisproblem. This device has both an excellent electrical resistivity and a very high thermal resistivity.The proposed solution is a stack of thin superconducting layers at electrical interconnections.The thermal resistance at each interface relies on the elastic properties of the materials used, the quality ofthe interfaces and temperature. The AMM model used in conjunction with the measured materialcharacteristics allows a theorical estimation of the thermal resistance per interface. The measurementsundertaken with superconducting connections with very high thermal resistivity are very well describedby this AMM model. We have measured thermal resistances as high as 3.3 105 K/W @ 200 mKfor a multilayer of 62 interfaces built with titaniun nitride and niobium alternatively on a 16 mm2 array.In the conditions foreseen for a 4000 micro-calorimeters camera operating at 50 mK in X-rays,this multilayer technique should allow a thermal load onto the cold bath that is much lower that 1 mWfor more than 8000 contacts.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Tantalum superconducting tunnel junctions for infrared photon counting

International audienceSuperconducting Tunnel Junctions (STJ) are currently being developed as photon detectors for a wide range of applications. Interest comes from their ability to cumulate photon counting with chromaticity (i.e. energy resolution) from the Near InfraRed (NIR) to the X-rays wavelengths and excellent quantum efficiency up to 80%. Resolving power can reach more than 10 in the visible. Our main goal is to use STJ for astronomical observation at low light level in the near infrared. This paper relates mainly to two points: The improvement of the Tantalum absorber epitaxy and the development of a new version of the fabrication process for making Ta/Al-AlOx-Al/Ta photon counting STJ. The main features of this process are that pixels have aligned electrodes and vias patterned through a protecting SiO2 layer. This vias are then used to contact the top electrode layer. We use a double thin aluminum trapping layer on top of a 150nm thick Ta absorber grown epitaxially

Transport and vortex pinning in micron-size superconducting Nb films

We have carried out Hall measurements on thin films of Nb in the flux-flow regime. The Hall bars were several microns in scale. Oscillations with magnetic field in the transverse and longitudinal resistances between the depinning field Bd and the upper critical field Bc2 are observed below Tc. The Hall effect may even change sign. The transverse and longitudinal resistances are interpreted in terms of current-driven motion of vortices in the presence of a few impurities. Simulations from time-dependent Ginzburg-Landau equations confirm this argument

Développement de jonctions supraconductrices à effet tunnel pour le comptage de photons en astronomie

Cette thèse présente le développement de Jonctions Supraconductrices à effet Tunnel (JSET) pour le comptage de photons en astronomie dans l'infrarouge proche. Ces dispositifs supraconducteurs constituent une alternative quasiment unique de détection tridimensionnelles (longueur d'onde, position et instant d'absorption) avec une sensibilité maximale jusqu'au quantum de lumière, et ce, dans une large bande d'énergie, des rayons X au ondes millimétriques.Au démarrage de cette thèse un procédé de fabrication avait permis d'obtenir des jonctions à base de niobium de bonne qualité et des résultats préliminaires en comptage de photons. L'objectif double était de passer à des jonctions à base de tantale, intrinsèquement plus sensibles, avec un nouveau procédé de fabrication collective plus performant.Dans un premier temps nous avons optimisé la qualité cristalline des dépôts de tantale. Les analyses présentées montrent que les films déposés par pulvérisation cathodique sont épitaxiés (100) pour des libre parcours moyen de l'ordre de 100 nm.Dans un second temps, un nouveau procédé original de fabrication collective des jonctions a été conçu et mis en place. Il autorise la fabrication de jonctions individuelles ou en réseau (9 pixels). Le développement de ce procédé et sa fiabilisation a permis d'obtenir un très fort pourcentage (>90%) de jonctions de qualité en terme de robustesse, de courant de fuite et de transparence tunnel.Enfin, nous avons mis en évidence le fonctionnement de ces dispositifs en comptage de photons dans l'infrarouge proche à 0,78 micron de longueur d'onde.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Quantum and Thermal Phase Slips in Superconducting Niobium Nitride (NbN) Ultrathin Crystalline Nanowire: Application to Single Photon Detection

International audienceWe present low-temperature electronic transport properties of superconducting nanowires obtained by nanolithography of 4-nm-thick niobium nitride (NbN) films epitaxially grown on sapphire substrate. Below 6 K, clear evidence of phase slippages is observed in the transport measurements. Upon lowering the temperature, we observe the signatures of a crossover between a thermal and a quantum behavior in the phase slip regimes. We find that phase slips are stable even at the lowest temperatures and that no hotspot is formed. The photoresponse of these nanowires is measured as a function of the light irradiation wavelength and temperature and exhibits a behavior comparable with previous results obtained on thicker films

Quantum and Thermal Phase Slips in Superconducting Niobium Nitride (NbN) Ultrathin Crystalline Nanowire: Application to Single Photon Detection

We present low-temperature electronic transport properties of superconducting nanowires obtained by nanolithography of 4-nm-thick niobium nitride (NbN) films epitaxially grown on sapphire substrate. Below 6 K, clear evidence of phase slippages is observed in the transport measurements. Upon lowering the temperature, we observe the signatures of a crossover between a thermal and a quantum behavior in the phase slip regimes. We find that phase slips are stable even at the lowest temperatures and that no hotspot is formed. The photoresponse of these nanowires is measured as a function of the light irradiation wavelength and temperature and exhibits a behavior comparable with previous results obtained on thicker films

Mapping of the Quantum Effyciency of a Superconducting Single Electron Detector

International audienc