Submicron tunnel junctions (original) (raw)
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Mesoscopic superconducting tunnel junction devices : experimental studies of performance limitations
2005
In this work four different mesoscopic superconducting devices have been experimentally studied: an ammeter based on a hysteretic Josephson junction switching from the superconducting state to the normal state, a conventional Cooper pair pump (CPP) based on two superconducting islands separated by tunnel junctions, a novel flux assisted Cooper pair pump and a thermometer based on a tunnel junction between a superconductor and a normal metal. These devices make use of phenomena related to superconductivity and are also benefiting from some properties of small structures, e.g., of quantum mechanical tunneling and of transport of single electrons. The present research focused on investigating performance limitations of these devices and the aim was to improve their performance. A hysteretic Josephson junction switching can be used as a sensitive current threshold detector. For many purposes, a smaller critical current I_c of the detecting junction would yield a higher current sensitivity. It was found in this Thesis that the main limitation of the ammeter is that with increasing sensitivity the negative effects due to dissipation start to dominate. In this Thesis the Josephson junction was also demonstrated as a shot-noise detector, and it was concluded that this kind of a device could probably be made into an absolute on-chip detector of Fano-factors and noise in general. A charge pump is a device which can periodically transfer a discrete amount of charge Q through an electrical circuit. The measured double island CPP introduced a frequency dependent current with a value close to the expected one at low pumping frequencies. The drawback in the measurements was that the direction of the pumped current was set by the bias voltage. The results of the measurement on flux assisted pumping were indeed very promising and the studied device was able to produce sufficiently high pumped currents of around 0.1 nA with reasonable accuracy. Thermometer based on NIS (Normal metal-Insulator-Superconductor) tunnel junctions is a very promising temperature sensor for bolometric radiation sensor applications. This work showed that it can be used as a sensitive thermometer up to MHz frequencies range and that its response time is limited by the electron-phonon scattering rate. Josephson junction, phase diffusion, Cooper pair pump, NIS tunnel junction 165 951-22-7709-3
Tunnel junctions based on superconducting/magnetic multilayers
Physica C: Superconductivity, 2002
Several high quality tunnel cross junctions based on Nb/CuMn (superconducting/spin-glass) multilayers were realized to investigate microscopic properties of superconducting/magnetic layered structures in view of potential electronic applications. The Mn concentration (2.35 at.%), which is related to the magnetic coupling, as well as the Nb layer thickness ($250 A A) were kept constant. The CuMn layer thickness d CuMn was varied from 0 to $50 A A, where a non monotonic behaviour of the superconducting temperature vs. d CuMn is observed, as a manifestation of the system entering the so-called p-phase. The geometry of the Nb/CuMn base electrode was defined by standard optical lithography. The oxide barrier (Al 2 O 3 ) and counterelectrode (Nb) were defined by lift-off and fabricated without breaking the vacuum to insure a good quality of the insulating barrier. Superconducting properties were investigated in the temperature range 2-10 K.
Superconductor Science and Technology, 2007
We present a novel Ti-based direct-write technology for fabricating Ti-TiO 2-Al tunnel junctions for bolometer and thermometry applications. The goal of our research is to develop simple and efficient technology for fabricating SIS tunnel junctions between Ti and Al with TiO 2 as an insulating barrier. The key point of this technology is the deposition of a Ti film as a base electrode and deposition of an Al electrode after oxidation of the Ti. This approach allows one to realize any geometry of the tunnel junctions and of the absorber with no limitation related to the area of the junctions or the thickness of the absorber. In particular, a very thin and completely flat absorber can be created with no bending parts, which is not possible using the shadow evaporation technique or standard trilayer technology. Besides, the proposed new approach does not require one-cycle evaporation for deposition of tunnel junctions which gives us more freedom in the geometry of the counter-electrodes. The junctions are to be used for bolometer applications, such as the fabrication of microwave receivers for sensitive measurements in new generation telescopes, e.g. CLOVER and BOOMERANG projects including polarization cosmic microwave background radiation measurements, and the OLIMPO balloon telescope project which is dedicated to measuring the Sunyaev-Zeldovich effect in clusters of galaxies. As the first step, SIN tunnel junctions have been fabricated and characterized.
Superconducting tunnel junction x-ray detectors with ultra-low subgap current
2007 2nd International Workshop on Advances in Sensors and Interface, 2007
Superconducting tunnel junctions are promising as radiation detectors because of their larger energy resolution with respect to conventional Si-based detectors. We have developed a fabrication process for Nb-based STJ devices which allows to fabricate junctions with leakage currents well below 50 pA at 300 mK, a factor of 10 −6 less than the leakage current at 4.2 K. The junctions are diamond-shaped with areas between 20 × 20 and 100 × 100 µm 2 . We report the details of the fabrication process, together with an analysis of the structure of films by AFM, SEM and x-ray spectroscopy. We show the measured I − V curves at 300 mK, together with preliminary results on x-ray measurements at 300 mK with a 5 mC 55 Fe source.
Low-leakage superconducting tunnel junctions with a single-crystal Al 2 O 3 barrier
Superconductor Science and Technology, 2005
We have developed a two-step growth scheme for single-crystal Al 2 O 3 tunnel barriers. The barriers are epitaxially grown on single-crystal rhenium (Re) base electrodes that are grown epitaxially on a sapphire substrate, while polycrystalline Al is used as the top electrode. We show that by first growing an amorphous aluminium (Al) oxide layer at room temperature and crystallizing it at a high temperature in oxygen environment, a morphologically intact single-crystal Al 2 O 3 layer is obtained. Tunnel junctions fabricated from these trilayers show very low subgap leakage current. This single-crystal Al 2 O 3 junction may open a new venue for coherent quantum devices.
Subgap biasing of superconducting tunnel junctions without a magnetic field
Journal of Applied Physics, 2008
Superconducting tunnel junctions ͑STJs͒ have been successfully used as single-photon detectors but require the use of a magnetic field to operate. A recent paper has proposed the idea to use a circuit of three junctions in place of a single junction in order to achieve the necessary biasing without applying a magnetic field. The nonlinear interaction between the different junctions in the circuit causes the existence of a stable subgap state for one of the junctions, which acts as the detector junction. In this paper, we present the first measurements demonstrating the existence of such a biasing state feasible for STJ detectors. Single junction measurements with an applied magnetic field help determine the functional form of the subgap current versus voltage; then the operating point of a three-junction circuit is measured and fit to theory. The excellent match between theory and experiment demonstrates the existence of the subgap biasing state. The outlook for possible use in detector applications is discussed.
Future optical detectors based on Al superconducting tunnel junctions
SPIE Proceedings, 2004
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 µsec, depending on the device size. The bias range of the Nb-contacted junctions is limited to the range 0-100 µV, 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 µsec. 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.
Fabrication of stable and reproducible submicron tunnel junctions
Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 2012
We have performed a detailed study of the time stability and reproducibility of sub-micron Al/AlOx/Al tunnel junctions, fabricated using standard double angle shadow evaporations. We have found that by aggressively cleaning the substrate before the evaporations, thus preventing any contamination of the junction, we obtained perfectly stable oxide barriers. We also present measurements on large ensembles of junctions which prove the reproducibility of the fabrication process. The measured tunnel resistance variance in large ensembles of identically fabricated junctions is in the range of only a few percents. Finally, we have studied the effect of different thermal treatments on the junction barrier. This is especially important for multiple step fabrication processes which imply annealing the junction.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1987
All refractory, NbN/MgO/NbN (superconductor-insulator-superconductor) tunnel junctions have been fabricated by in situ sputter deposition. The influence ofMgO thickness (0.8-6.0 nm) deposited under different sputtering ambients at various deposition rates on current-voltage (I-V) characteristics of small-area ( 30 X 30 pm) tunnel junctions is studied. The NbN/MgO/NbN trilayer is deposited in situ by de reactive magnetron (NbN), and rf magnetron (MgO) sputtering, followed by thermal evaporation of a protective Au cap. Subsequent photolithography, reactive ion etching, planarization, and top contact (Pb/ Ag) deposition completes the junction structure. Normal resistance of the junctions with MgO deposited in Ar or Ar and N 2 mixture shows good exponential dependence on the MgO thickness indicating formation of a pin-hole-free uniform barrier layer. Further, a postdeposition in situ oxygen plasma treatment of the MgO layer increases the junction resistance sharply, and reduces the subgap leakage. A possible enrichment of the MgO layer stoichiometry by the oxygen plasma treatment is suggested. A sumgap as high as 5.7 mV is observed for such a junction.