PROXIMITY EFFECT TUNNEL JUNCTIONS WITH BARRIERS FORMED FROM AMORPHOUS ALLOYS

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PROXIMITY EFFECT TUNNEL JUNCTIONS WITH

BARRIERS FORMED FROM AMORPHOUS ALLOYS

R. Laibowitz, C. Tsuei, P. Chaudhari, S. Raider, R. Drake, J. Viggiano

To cite this version:

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PROXIMITY EFFECT TUNNEL JUNCTIONS WITH BARRIERS FORMED FROM AMORPHOUS ALLOYS

R.B. Laibowitz, C.C. Tsuei, P. Chaudhari, S.I. Raider, R. Drake and J.M. Viggiano

IBM Thomas J. Watson Research Center, Jorktown Heights, New York 10598, USA.

Abstract.- 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 A thick). Base elec-trodes 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.

Superconductive tunneling has been observed in past work of junctions fabricated using amorphous ba-se electrodes/1/. Alloys such as Nb-Ge and Nb-Si, have been used in these studies. The materials can be formed by several techniques, e.g. coevaporation or sputtering onto substrates held at room tempera-ture or below. Using Pb or Pb alloys as the counte-relectrode and exposure to the ambient for barrier formation, high quality tunnel junctions could be fabricated. In this instance, high quality refers to low excess current below the gap. This type of junction, i.e. amorphous alloy/barrier/Pb has a li-mitation for practical applications in that the T of the amorphous alloys has been generally low, ty-pically around 3.5 K. The multilayer or proximity effect junctions studied in this work offer the pos-sibility of using the good barrier forming property of the amorphous alloys and the high T of a second superconductor. We have used pure Nb as the base, high T metal upon which amorphous films of Nb-Ge, Nb-Si and Nb-Al have been formed. In addition, lo-wer junction capacitances can in some cases be

ob-tained for barriers grown on the Nb alloys when com-pared to those grown on pure Nb.

As stated above, sample fabrication for the multilayer base electrode was carried out in aUHV sys-tem which is equipped with two e-guns. The

evapora-o

t i o n r a t e s could be feedback controlled t o a few A / s .

For example, we have fabricated a Nb/NbSi base e l e c

-o

trode in which the Nb thickness was about 1000 A

o

and the Nb-Si thickness was about 100 A. Typical evaporation rates for the Nb-Si formation were about

o

2.6 A/s for both the Nb and the Si. Pressure during

evaporation was < 3 x 10 torr. The turn-on of the Si gun was typically accomplished while the Nb was depositing. The tunnel barriers were generally for-med by exposure of this base electrode to ambient conditions ("air oxidation") and good quality junc-tions generally resulted. However, it was not pos-sible to obtain high current densities i.e. low junc-tion resistance with this barrier formajunc-tion techni-que. Generally, air oxidation limited the current density to around 10 A/cm . Higher current densities can be obtained by using plasma cleaning and in-situ counter-electrode deposition.

The UHV system used for the film fabrication also has an in-situ electron diffraction attachment (Scanning High Energy Electron Diffraction - SHEED) With this unit we can observe the structure of the top surfaces of the deposited films. Pure Nb films show a typical bcc, polycristalline structure while the addition of the alloying element renders the surface amorphous. For the Nb-Si alloys, several Nb/Si ratios have been used. However, the entire range of pure Nb to pure Si has not been studied. Such studies for these alloys would be of interest especially if intermetallic compound formation can be observed. Our past work/2/ on bulk Nb-Si alloys deposited at 77 K has shown that above a Si concen-tration of 8-10 percent, the films are always amor-phous. For the Nb-Si surfaces, Auger analysis has shown large concentrations (about 50 percent) of silicon oxides in the tunnel barrier.

Superconductive tunnel junctions formed with Pb or Pb-alloy counter electrodes often showed ve- " ry low excess currents below the gap (high quality). JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-1240

Résumé,— Nous avons fabriqué des jonctions tunnel multi-rëseaux dans lesquelles l'électrode de base est un alliage amorphe de Nb-Si ou de Nb-Al déposé sur une suface deniobium Ces jonctions ont une capacitance plus faible que celles fabriquées à partir deuiobium pur. De plus, en-dessous du gap, nous n'observons que de très faibles courants d'excès.

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Fig. la : I-V curve for a Nb/Nb-S~/OX/P~ junction measured at 1.9 K. Voltage (abscissa) 1 mV/div. Cur- rent (ordinate) 0.1 mA1div.

An example of an I-V curve of a Nb/Nb-Si/oxide/Pb junction at 1.9 K is shown in figure 1 a. At 4.2 K the excess current increases slightly. The junction as shown in the figure had a base electrode width of about I6uand a counterelectrode width of about 84

u.

The Nb-Si layer thickness is <, 100 A and the NbISi ratio is about 3 to 1. Although our prelimina- ry results indicate that the junction in figure 1 a is of high quality, the observed d.c. Josephson current was significantly lower than would be expected if the base electrode were pure M,. The cause of this low Josephson current is presently not known. The measured gap voltage, APb+ANb/Nb-Si is about 2.6 mV which is only a few tenths of a mV less than the best ANb+APb indicating that almost the full gap voltage of Nb is effective. Also shown in figure

I

a is a discontinuity which appears at the gap

0

Such a sensitivity in the <I00 A range might be expected as the coherence length in such alloys is ge-

0

nerally in the range of 50 to 100 A 15-61.

We also have examined the I-V charactelfistics of figure 1 a in a region close to the origin

for

resonance behaviour. These well-known resonances/4- 7-81 appears as a series of equally spaced current peaks at low bias voltages and are more readily ob- servable in higher current density junctions. The resonances in the junction of figure

I

a were studied in this way and the

I-V

curve is shown in figure 1 b. The 0.12 mV peak spacing corresponds to a junction capacitance of 5.6 1.~~/cm2. The capacitance

from an equivalent Nb/Nb~x/Pb junction is about

7.8 1.~~/cm2. Oxides of Nb are relatively high dielectric constant materials and it is possible that the addition of Si to the surface of the Nb to obtain silicon oxides in the tunnel barrier reduces the a- verage barrier dielectric constant and hence the capacitance.

voltage. This effect has been seen in Nb-based junc-

Fig. 1 b : I-V curve near origin at 2.2 K Voltage tions and in all Nb junctions in past workl3-4/ in

0.2 mV/div., Current 0.02 mA/div. which interfacial and proximity effect models have

been proposed.

The authors would like to acknowledge help- We have also studied junctions with surfaces

ful discussions with C. Kircher,

H.

Caswell, J. composed of amorphous NbSi2. With about 30

1

of this

Cuomo, N. Bojarczuk and the technical assistance of material on a thick Nb film, a gap of 2.4 mV is ob-

W. Kateley. tained while with 60

1

the gap is already reduced

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References

/ I / Tsuei,C.C., Johnson,W., Laibowitz,R.B. and Viggiano,J.M., Solid ~ t k t e Commun.

2

(1977) 615

/ 2 / Johnson,W.L., Tsuei,C.C., Raider,S.I. and

Laibowit2,R.B. (to be published)

/ 3 / Broom,R.F., Jaggi,R., Laibowitz,R.B., Mohr, T.O. and Walter,W., Proc. LT14 Helsinki, Finland (1975) 172

/4/ Broom,R.F., J. Appl. Phys.

67

(1976) 5432 /5/ Hake,R., Appl. Phys. Lett.

10

(1967) 189 /6/ Agyeman,K. and Tsuei,C.C. Unpublished results /7/ Matisoo,J., J. Appl. Phys.

40

(1969) 2091 /8/ Basavaiah,S. and Greiner,J.W., J. Appl. Phys.

47 (1976) 4201