Electric current crowding effects in microstructured superconductors

Many applications of modern electronic devices are based on thin film geometries including sharp turns, holes and exhibiting inhomogeneities. The inevitable detour of current streamlines around such obstacles cause an inhomogeneous current density profile giving rise to current crowding. The goal of my research is to highlight the current crowding effects in micro and nanopatterned superconducting films

Magnetic flux penetration in Nb superconducting films with lithographically defined micro-indentations

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines (d-lines), caused by the abrupt bending of current streamlines around the indentations, depart from the expected parabolic trend close to the defect and depend on the shape and size of the indentation as well as on the temperature. These findings are backed up and compared with theoretical results obtained by numerical simulations and analytical calculations highlighting the key role played by demagnetization effects and the creep exponent n. In addition, we show that the presence of nearby indentations and submicrometer random roughness of the sample border can severely modify the flux front topology and dynamics. Strikingly, in contrast to what has been repeatedly predicted in the literature, we do not observe that indentations act as nucleation spots for flux avalanches, but they instead help to release the flux pressure and avoid thermomagnetic instabilities

Ultra-narrow superconducting junctions: electromigration to shed light on quantum point contacts

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and in quantum computing circuits. In this context, it is of fundamental importance to better understand the undesired and harmful appearance of thermal and quantum fluctuations of the superconducting order parameter [1]-[3] as a function of the wire width. Although superconductors in the mesoscopic regime (i.e. size comparable to ξ and/or λ) have been explored both experimentally and theoretically in depth, the superconducting nanoworld (i.e. at scales of the fermi wavelength) has received much less attention. The lack of experimental results is in part due to the difficulty of sample fabrication, at dimensions beyond the limit reached by conventional lithographic techniques. A promising direction consists of controlling the local displacement of atom by an electron wind, a process known as electromigration (EM) [4] . This effect relies on the combination of local temperature rise and substantial current crowding at nanoconstrictions. While uncontrolled, EM is responsible for the breakdown of small electronic devices, it can be used in a controllable way to further decrease locally the cross section of the nanowire towards single atomic contacts. In this work, we explore in-situ controlled EM to fabricate nano-constrictions immersed in cryogenic environment. We demonstrate that a transition from thermally assisted phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross section becomes smaller than ~ 150 nm 2 . In the regime dominated by QPS the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature [5] . We also demonstrate that the bow-tie shaped constrictions exhibit a negative magnetoresistance at low magnetic fields [5] which can be attributed to the suppression of superconductivity in the contact leads [6] . Strikingly, the detrimental effect caused by the repeated EM can be healed by simply inverting the current direction. These findings reveal the strong potential of the proposed fabrication method to explore various fascinating superconducting phenomena in atomic-size constrictions

Flux penetration in a superconducting film partially capped with a conducting layer

The influence of a conducting layer on the magnetic flux penetration in a superconducting Nb film is studied by magneto-optical imaging. The metallic layer partially covering the superconductor provides an additional velocity-dependent damping mechanism for the flux motion that helps protecting the superconducting state when thermomagnetic instabilities develop. If the flux advances with a velocity slower than w = 2/µ0σt, where σ is the cap layer conductivity and t is its thickness, the flux penetration remains unaffected, whereas for incoming flux moving faster than w, the metallic layer becomes an active screening shield. When the metallic layer is replaced by a perfect conductor, it is expected that the flux braking effect will occur for all flux velocities. We demonstrate this effect by investigating Nb samples with a thickness step. Some of the observed features, namely the deflection and the branching of the flux trajectories at the border of the thick centre, as well as the favoured flux penetration at the indentation, are reproduced by time-dependent Ginzburg-Landau simulations

BRAHMS: polarimetric bolometer arrays for the SPICA observatory camera (Conference Presentation)

International audienceIn the last decades, a very large effort has been made to measure, with high sensitivity, the intensity and spectral contents of millimetric (mm) and submillimetric (submm) light from the Universe. Today this picture is in the way to be routinely completed by polarization measurements that give access to previously hidden processes, for example the traces of primordial gravitational waves in the case of CMB (mainly mm), or the effect of magnetic field for star formation mechanisms (submm and mm optical ranges). The classical way to measure the light polarization is to split the two components by a polarizer grid and record intensities with two conjugated detection setups. This approach implies the deployment of a complex instrument system, very sensitive to external constraints (vibrations, alinement, thermal expansion…), or internal ones: determine low degrees of polarization implies a large increase in sensitivity when compared with intensity measurements. The need of detector arrays, with in pixel polarization measurement capabilities, has been well understood for years: all the complexity being reported at the focal plane level. Subsequently, the instrument integration, verification and tests procedure is considerately alleviated, specially for space applications.All silicon bolometer arrays using the same micromachining techniques than the Herschel PACS modules are well suited for this type of development. New thermometers doped for 50 mK operations permit to achieve, with a new design, sensitivities close to the aW/√Hz. It is based on all-legs bolometers (ALB), where the absorbing, insulating and thermometric functions are made by the same suspended silicon structure. This ALB structure, with in this case a spiral design, permits to separate the absorption of the two electromagnetic components of the light polarization. Each pixel consists of four bolometer divided in two pairs, each sensitive to one direction of polarization. This permits to combine the bolometer bridges in a fully differential global structure with a Wheatstone bridge arrangement. Total intensity and polarization unbalance are available directly at the detector level, thanks to a cold readout circuit integrated in the detector structure. This combination of functions is achieved by above IC manufacture techniques (IC for Integrated Circuit).All these developments take place in the prospect of the joint JAXA-ESA SPICA project, to equip a 1344 pixels polarimetric and imaging camera covering three spectral bands (100, 200 and 350 µm)

Study on the electrical properties of ultrathin in situ Boron-doped strained Si0.7Ge0.3 layers annealed by nanosecond pulsed laser

International audienceThe downscaling of CMOS transistors requires high active dopant concentrations in the source and drain terminals to minimize contact resistance. Pulsed laser annealing is an attractive option as it enables to locally reach, typically ~100 nm below the surface, high temperatures (e.g. above the melt threshold), with extremely fast temperature ramps (>109 °C/s). Structural investigations have already allowed to identify the best conditions to obtain fully strained and defect-free undoped SiGe layers by liquid phase epitaxial regrowth (LPER).In this work, we report on the electrical properties of laser annealed 30 nm-thick boron-doped strained-Si0.7Ge0.3 layers. These layers were CVD grown on p-type bulk Si (100), with three different boron concentrations probed at: 7.3x1019 (A), 1.4x1020 (B) and 2.3x1020 cm-3 (C). Electrical properties were evaluated thanks to an algorithm comparing Hall parameters calculated from boron and germanium SIMS profiles with the corresponding Hall effect measurements. Germanium redistribution occurring during laser annealing in melt conditions was taken into account through Hall scattering factor variations.For the as-grown layers, activations rates of ~100%, ~80% and ~60% were found, without any significant carrier mobility degradation. These layers were annealed in a SCREEN-LT3100 platform, equipped with a pulsed laser operating at 308 nm (XeCl laser), with a pulse duration around 160 ns and energy densities ranging from 1.20 to 2.40 J/cm2. The different laser regime transitions as well as the strain state were studied thanks to surface, structural and chemical characterizations, in addition to electrical measurements. In the three layers, the simultaneous variation of strain state and dopant activation had a definite impact on the electrical properties. Indeed, when reaching the beginning of the melt, crystal defects appeared, resulting in a partial relaxation of the compressive strain in the layers. When getting close to the complete melt of the layers, the compressive strain was recovered. The higher the dopant concentration, the earlier the strain was retrieved before reaching the full melt, certainly due to compressive strain compensation by boron atoms. Activation rates were evaluated, taking into account the relaxation phenomena by modifying the Hall scattering factor in accordance with the literature. It was found that the activation in the three layers was improved up to 100% when the energy density was increased, confirming the efficiency of the laser annealing technique.For device purposes, and as already discussed in the literature, shorter pulse laser anneals (308 nm, 25 ns) were performed on as-grown layers to circumvent the partial relaxation. Preliminary results showed a positive impact of such pulse duration reduction on the previously evidenced phenomena, although further investigations are still in progress

Study on the electrical properties of ultrathin in situ Boron-doped strained Si0.7Ge0.3 layers annealed by nanosecond pulsed laser

International audienceThe downscaling of CMOS transistors requires high active dopant concentrations in the source and drain terminals to minimize contact resistance. Pulsed laser annealing is an attractive option as it enables to locally reach, typically ~100 nm below the surface, high temperatures (e.g. above the melt threshold), with extremely fast temperature ramps (>109 °C/s). Structural investigations have already allowed to identify the best conditions to obtain fully strained and defect-free undoped SiGe layers by liquid phase epitaxial regrowth (LPER).In this work, we report on the electrical properties of laser annealed 30 nm-thick boron-doped strained-Si0.7Ge0.3 layers. These layers were CVD grown on p-type bulk Si (100), with three different boron concentrations probed at: 7.3x1019 (A), 1.4x1020 (B) and 2.3x1020 cm-3 (C). Electrical properties were evaluated thanks to an algorithm comparing Hall parameters calculated from boron and germanium SIMS profiles with the corresponding Hall effect measurements. Germanium redistribution occurring during laser annealing in melt conditions was taken into account through Hall scattering factor variations.For the as-grown layers, activations rates of ~100%, ~80% and ~60% were found, without any significant carrier mobility degradation. These layers were annealed in a SCREEN-LT3100 platform, equipped with a pulsed laser operating at 308 nm (XeCl laser), with a pulse duration around 160 ns and energy densities ranging from 1.20 to 2.40 J/cm2. The different laser regime transitions as well as the strain state were studied thanks to surface, structural and chemical characterizations, in addition to electrical measurements. In the three layers, the simultaneous variation of strain state and dopant activation had a definite impact on the electrical properties. Indeed, when reaching the beginning of the melt, crystal defects appeared, resulting in a partial relaxation of the compressive strain in the layers. When getting close to the complete melt of the layers, the compressive strain was recovered. The higher the dopant concentration, the earlier the strain was retrieved before reaching the full melt, certainly due to compressive strain compensation by boron atoms. Activation rates were evaluated, taking into account the relaxation phenomena by modifying the Hall scattering factor in accordance with the literature. It was found that the activation in the three layers was improved up to 100% when the energy density was increased, confirming the efficiency of the laser annealing technique.For device purposes, and as already discussed in the literature, shorter pulse laser anneals (308 nm, 25 ns) were performed on as-grown layers to circumvent the partial relaxation. Preliminary results showed a positive impact of such pulse duration reduction on the previously evidenced phenomena, although further investigations are still in progress

Study on the electrical properties of ultrathin in situ Boron-doped strained Si0.7Ge0.3 layers annealed by nanosecond pulsed laser

International audienceThe downscaling of CMOS transistors requires high active dopant concentrations in the source and drain terminals to minimize contact resistance. Pulsed laser annealing is an attractive option as it enables to locally reach, typically ~100 nm below the surface, high temperatures (e.g. above the melt threshold), with extremely fast temperature ramps (>109 °C/s). Structural investigations have already allowed to identify the best conditions to obtain fully strained and defect-free undoped SiGe layers by liquid phase epitaxial regrowth (LPER).In this work, we report on the electrical properties of laser annealed 30 nm-thick boron-doped strained-Si0.7Ge0.3 layers. These layers were CVD grown on p-type bulk Si (100), with three different boron concentrations probed at: 7.3x1019 (A), 1.4x1020 (B) and 2.3x1020 cm-3 (C). Electrical properties were evaluated thanks to an algorithm comparing Hall parameters calculated from boron and germanium SIMS profiles with the corresponding Hall effect measurements. Germanium redistribution occurring during laser annealing in melt conditions was taken into account through Hall scattering factor variations.For the as-grown layers, activations rates of ~100%, ~80% and ~60% were found, without any significant carrier mobility degradation. These layers were annealed in a SCREEN-LT3100 platform, equipped with a pulsed laser operating at 308 nm (XeCl laser), with a pulse duration around 160 ns and energy densities ranging from 1.20 to 2.40 J/cm2. The different laser regime transitions as well as the strain state were studied thanks to surface, structural and chemical characterizations, in addition to electrical measurements. In the three layers, the simultaneous variation of strain state and dopant activation had a definite impact on the electrical properties. Indeed, when reaching the beginning of the melt, crystal defects appeared, resulting in a partial relaxation of the compressive strain in the layers. When getting close to the complete melt of the layers, the compressive strain was recovered. The higher the dopant concentration, the earlier the strain was retrieved before reaching the full melt, certainly due to compressive strain compensation by boron atoms. Activation rates were evaluated, taking into account the relaxation phenomena by modifying the Hall scattering factor in accordance with the literature. It was found that the activation in the three layers was improved up to 100% when the energy density was increased, confirming the efficiency of the laser annealing technique.For device purposes, and as already discussed in the literature, shorter pulse laser anneals (308 nm, 25 ns) were performed on as-grown layers to circumvent the partial relaxation. Preliminary results showed a positive impact of such pulse duration reduction on the previously evidenced phenomena, although further investigations are still in progress