Electron-beam fabrication and microwave experiments on josephson microbridges

Thin film superconducting microbridges which exhibit Josephson effects have been fabricated by electron-beam lithographic techniques. The experiments performed on these samples included the study of both the microwave-induced steps (at 35.80 GHz) in the I-V curves and the temperature dependence of the critical current near the superconducting transition temperature. The size of these rectangular bridges were measured by electron microscopy and had dimensions that were typically in the 0.1 to 0.4 μ range. Samples were fabricated using a variety of vacuum-evaporated metallurgies, e. g. Al, Sn, Pb and Nb. The fabrication technique affords the possibility of independently varying the length, width or thickness of the link. In this way, the turn-on of the Josephson effects or the transition from bulk-like superconducting properties to Josephson behavior can be studied. Thus for Nb links whose length and width were less than approximately 0.4 μ and whose thickness was about 0.1 μ, microwave-induced harmonic steps appear in the I-V curves in a range near Tc which satisfy the Josephson relation. As yet, no subharmonic steps have been observed. The sensitivity of the bridges to the rf power level appears to be quite high as will be discussed. Also as the size of these Nb samples is reduced, the behavior of the critical current near Tc also changes. An additional size effect is observed in some of the Nb bridges wherein the Tc of the link itself is reduced below that of bulk Nb. This effect will be discussed with reference to both sample size and fabrication parameters.Par des techniques lithographiques, utilisant un faisceau d'électrons, nous avons fabriqué des microponts en couches minces présentant un effet Josephson. Nous avons étudié les marches de la caractéristique I-V (induites par un rayonnement hyperfréquence à 35,80 GHz) et la variation, en fonction de la température, du courant critique près de Tc . La taille de ces ponts rectangulaires est mesurée au microscope électronique, les dimensions typiques sont 0,1 x 0,4 μ. Les échantillons sont fabriqués par différentes techniques d'évaporation utilisant les métaux Sn, Al, Pb et Nb. La technique de fabrication utilisée permet de faire varier indépendamment la longueur, la largeur ou l'épaisseur du micropont. De cette façon, on peut étudier soit l'effet Josephson, soit la transition entre les propriétés d'un supraconducteur massif et celles d'une structure Josephson. Pour des microponts en Nb dont la largeur et la longueur sont inférieures à 0,4 μ et dont l'épaisseur est d'environ 0,1 μ, des marches induites par le rayonnement hyperfréquence apparaissent dans la courbe I-V, près de Tc. Ces marches satisfont à la relation de Josephson mais jusqu'à présent, aucune marche correspondant à des sous-harmoniques de la fréquence de « gap » n'a été observée. Ces ponts semblent assez sensibles à la puissance rf. Le comportement du courant critique près de Tc change quand la taille du micropont diminue. La température critique de certains microponts en Nb est inférieure à celle du Nb massif, nous discutons cet effet en fonction de la taille de l'échantillon et des paramètres de fabrication

PROXIMITY EFFECT TUNNEL JUNCTIONS WITH BARRIERS FORMED FROM AMORPHOUS ALLOYS

Nous avons fabriqué des jonctions tunnel multi-réseaux dans lesquelles l'électrode de base est un alliage amorphe de Nb-Si ou de Nb-Al déposé sur une surface de niobium Ces jonctions ont une capacitance plus faible que celles fabriquées à partir de niobium pur. De plus, en-dessous du gap, nous n'observons que de très faibles courants d'excès.Superconducting tunnel junctions with multilayer base electrodes have been constructed in which the surface of the base electrode is composed of an amorphous alloy (≤ 100 Å thick). Base electrodes consisting of Nb with a surface layer of amorphous Nb-Si or Nb-Al have been fabricated. Tunnel barriers grown from these alloys have shown reduced capacitance when compared to barriers formed on pure Nb surfaces. In addition, low excess currents have been observed in these junctions ; however, the junction characteristics are sensitive to the thickness and composition of this amorphous layer

Competing tunneling and capacitive paths in Co-ZrO2 granular thin films

The ac electrical response is studied in thin films composed of well-defined nanometric Co particles embedded in an insulating ZrO2 matrix which tends to coat them, preventing the formation of aggregates. In the dielectric regime, ac transport originates from the competition between interparticle capacitive Cp and tunneling Rt channels, the latter being thermally assisted. This competition yields an absorption phenomenon at a characteristic frequency 1/(RtCp), which is observed in the range 1010 000 Hz. In this way, the effective ac properties mimic the universal response of disordered dielectric materials. Temperature and frequency determine the complexity and nature of the ac electrical paths, which have been successfully modeled by an Rt-Cp network