Studies of contacts to YBCO

Physica B 165&166 (1990) 1625-1626 North-Holland

STUDIES OF CONTACTS TO YBCO H.W.Lean, E.J.Tarte, J.RWaldram. IRC in Superconductivity and Cavendish Laboratory, Madingley Rd, Cambridge CB3 OHE, UK. We report detailed work on the properties of silver contacts to YBCO. This includes data on the contact resistivity at various temperatures and on supercurrents observed to pass through the silver from indium. The data strongly suggests that the interfaces being produced are far from uniform. 1. INTRODUCTION There has been much previous work on contacts to high Te materials, mostly using sintered YBCO. There appears to be general agreement in the literature (eg 1,2) that a good method for producing low resistance contacts to YBCO is to deposit Silver or gold onto the surface and then to reanneal in oxygen. Wires are then attached to the noble metal pads by soldering or bonding. This approach usually produces contact resistivities in the range1 0. 7ncm 2. This is three orders of magnitude above what one expects for conventional SN interfaces (3) and two higher than has been obtained with single crystal YBCO (4). It is important to establish why these discouraging results have been so widely obtained. In this paper we present some related measurements on YBCO/silver interfaces produced by essentially the same method on sintered and thin film samples which give some indications of the nature of the imperfections in such interfaces.

attached separately to the sample; in practice the second voltage lead was attached with a second silver/indium structure formed at the same time as the first in order to keep the lead resistance as low as possible when using the SQUID voltmeter.

2. EXPERIMENTAL METHOD The experiments used either bars of sintered YBCO approximately 2x2mm cross section or sputtered thin films on YSZ substrates. In both cases the contacts were produced in the same way; silver pads about 1 lim thick were thermally evaporated and the sample then reannealed in oxygen at 600°C. Contact was then made to the sample by melting a piece of indium onto the silver pad enclosing a voltage lead. The current lead was attached to the top of the indium with silver paint. This arrangement was chosen for two reasons: (i) The more commonly used configuration of the current and voltage leads separately attached to the silver film can give results which are difficult to interpret If the contact resistance is small enough for the silver to be no longer an equipotential (4). (ii) Below Te of indium this arrangement allowed measurements to be made of supercurrents passing ri~ht across the silver layer from the YBCO to the silver. The other voltage and current leads were

FIGURE 1 Temperature dependence of Contact Resistance.

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3. RESULTS AND DISCUSSION The contact resistivity of both the sintered and thin film samples was of order 10.7 ncm 2 at 4.2K which is about what one expects from comparison with previous work. The temperature dependence of the contact resistance is typically as shown in Fig. 1, being roughly constant at low t~mperatures and then linearly increasing at higher ones although the temperatures at which these features were observed varied somewhat from sample to sample. We have found this linear dependence in a large number of samples and its cause is unclear although one possiblity is the presence of a phase of lower Te between the silver and the superconductor. At lower temperatures, below Te of the indium (3.4K), we always observe supercurrents which pass right across the silver layer from the YBCO Into the indium. A typical IV characteristic is shown in Fig. 2. In the sintered materials the le(T) curve is observed to be linear close to Te

Elsevier Science Publishers B.V. (North-Holland)

H. W. Lean, E.J. Tarte, J.R. Waldram

1626

but is a good fit to exp(rO. 5) at lower temperatures. A simple theory of the proximi~ effect (5) is found to be a good fit to this data 6). In the case of the thin film samples, the le( ) form is similar at low temperatures but often shows peaks and discontinuities close to Te and is not linear in that range. These unexplained features are probably related to the behaviour of trapped flux in the junction which, as we shall mention below, appears to be affecting the behaviour significantly.

characteristics were often observed to be assymetric with respect to current. These effects a~e poorly understood. Work is continuing to ~"mlr'ate them by reducing the size of the Junctions and the ambient B field.

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\lilA FIGURE 2 I-V characteristic below Te of Indium . A~ imP9rtant test of the uniformity of these Jun.ctlons IS the effect on Ie of the application of a B flel~ parallel to the silver film. The application of a field to the present samples (both sintered and thin film) gave an le(B) curve which was genera!ly oscillatory but with the height of sucesslve peaks and trough almost random (Fig. 3). The le(B) curve should be the Fourier transform of the supercurrent density as a function of position and in this case is consistent with many small contacts of random critical current. The fact that no envelope was observed in these curves implies that the individual contacts are smaller than 1/500th of the total sample area, and the spacing of the oscillations implies that the contacts are spread over the whole sample area. These results imply that the interfaces are strongly non-uniform with only a small proportion o~ the total area ryJaking good contact. This tallies with the observation of an unexpectedlr high contact resistance. The exact nature 0 the interface is not known at present but is under microscopic investigation. It is possible, for example, that our "small contacts" are small, thin areas where the silver is much thinnner than the nominal thickness. Most samples showed signs that the results were significantly influenced by trapped flux. The le(B) curves did not have large maxima at B..O as expected and the values of Ie at a given temper8:ture could be permanently changed by the application of a large B field or by heating the sample above Te of the indium. In addition the IV

4. CONCLUSIONS

It is clear that making contacts to YBCO by the frequently reported method produces contacts which are far from uniform when usin!;l either sintered or the present thin film matenal. In the case of the sintered samples the temperature dependence of Ie is consistent with proximity coupling across the silver layer and may therefore provide some evidence for the proximity effect. In the case of thin films the behaviour of Ip shows some less well understood features. It IS clear that in order to extract useful information about the physics of these materials it will be necessary to find a method to produce more uniform interfaces. To this end work is currently proceeding using in-situ deposited silver on thin film samples. It is also intended to start work using single crystal YBCO. ACKNOWLEDGEMENTS We would like to thank P. Freeman for the supply of samples of sintered material and R Somekh for the supply of thin films. REFERENCES (1) Van der Maas J. Nature 328 (1987) 603 (2) Mizushima K. et al. Appl. Phys. Lett. 52 (1988) 1101 (3) Lean HW. and Waldram J.R. J. Phys. Cond Matt. 1 (1989) 1285 (4) Jing TW. et. al. App. Phys. Lett. 55 (1989) 1912

(5) Clarke J. Proc. Roy. Soc. Lond. A308 (1969)

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(6) Lean HW. et. al. submitted to J. Phys. Cond Matt.