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Submitted on 1 Jan 1978
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THE BEHAVIOUR OF Al/Pb PROXIMITY
SANDWICHES AS A FUNCTION OF
TEMPERATURE AND NORMAL LAYER
THICKNESS
L. van den Dries, C. van Haesendonck, Y. Bruynseraede
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplPment au no 8, Tome 39, ao2t 1978, Page 126-1245
THE BEHAVIOUR
OF
A
I / P ~PROXIMITY
SANDWICHES
AS
A
FUNCTION OF
TEMPERATURE
AND NORMAL LAYER THICKNESS
L. Van den Dries, C. Van Haesendonck and Y. Bruynseraede
Laboratoriwn voor Vaste Stof-Fysika en Magnetisme, Katholieke U n i v e r s i t e i t Leuven, Leuven (Belgium)
RQsum6.- Nous avons dtudiQ l'effet de proximitd dans les bilames Al/Pb par effet tunnel et effet Josephson dans des jonctions Nb-oxide-Al/Pb avec diffQrentes Qpaisseurs d'aluminium. Cet article dscrit la fabrication des jonctions et prssente quelques rQsultats expgrimentaux.
Abstract.- The superconducting proximity effect in the Al/Pb system was investigated by means of Josephson and quasi-particle tunneling experiments on Nb-oxide-Al/Pb structures with different Al- thicknesses. In this paper we describe the junction fabrication technique and some experimental results.
One of the problems encountered with Al/Pb systems is the irreproducibility of the interface conditions between A1 and Pb. This may complicate comparison with theory if one has to take into ac- count a different barrier reflectivity for each dif- ferent Al/Pb sandwich that is considered. To elimi- nate this inconvenience we manufactured our Nb- oxide-Al/Pb proximity junctions with different Al- thicknesses on the same substrate and within a sin- gle vacuum run, following a procedure used by NQdellec et al. 111.
The Nb film is prepared by RF sputtering on a glass substrate and the base electrode pattern is obtained by chemical etching. The tunneling area
(0.5 x 0.5 m2) is defined by a square window in a
photoresist layer on top of the Nb 121. The oxide barrier is grown by an oxygen glow discharge. The A1 layer is evaporated through a sliding mask which can be moved from the outside, so that different Al-thicknesses can be realized. After less than
2 minutes the counterelectrode is completed by eva- porating a Pb layer with fixed thickness. The pres- sure during the evaporations remains less than
3 x
lo-'
torr. The junctions are cooled down to li- quid nitrogen temperature as soon as possible to avoid deterioration of the Al/Pb interface.We present here a preliminary result for a set of 7 junctions with Al-thicknesses ranging from
230 to 530.; (Tc c 1.4 K) and an overall Pb thick- ness of 5000 (T = 7.2 K). We measured the I-V characteristics as well as the maximum dc Josephson current I in the temperature range 1.2
-
7.3 K.0 max
All junctions had a normal tunneling resistance
% 0.05 5 and showed a Fraunhofertype variation
Of I0 max with external magnetic field, which ensu-
res the absence of microshorts.
The temperature dependence of the induced energy gap
AA1
and the maximum dc Josephson current'
0 max for differnt Al-thicknesses is shown resp. in figures 1 and 2. From the regularity of these results we may conclude that we obtained very simi- lar AllPb interface conditions for all junctions on the tested substrate. The thickness dependence of
AAl
at a few temperatures is represented in figure3. Calculations indicate a fairly good agreement at low temperatures with the theoretical predictions of Bar-Sagi / 3 / if we assume a barrier reflectivity of R m 0.7. Further measurements and calculations on this base are being made at this moment and we hope to present more detailed results in the nearfuture.
Fig. 1 : Temperature dependence of the induced gap AAl for different A1 thicknesses ; Apb = 1.20 meV at T = 1.2 K
o
I
I I IJ
200 300 LOO 500 60C
dAI
( A )
-
Fig. 2 : Temperature dependence of the maximum dc Fig. 3 : Thickness dependence of the induced gap Josephson current multiplied by the normal tunne- AAl at different temperatures
ling resistance
References
/ I / NQdellec, P . , Dumoulin, L. and Guyon, E., Journal of Low Temp. Physics,
2
(1976) 663/ 2 / Folens, G., Van den Dries, L. and Bruynseraede, Y.,
Le Vide
