Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.cplire.ru/html/oxide233/paper21h.pdf Дата изменения: Thu Sep 1 13:25:15 2005 Дата индексирования: Tue Oct 2 04:56:44 2012 Кодировка: Поисковые слова: http www.badastronomy.com phpbb index.php

Inst. Phys. ConjY Ser. No 167 Paper presented at Applied Superconductivity, Spain, 14-I 7 September 1999 0 2000 IOP Publishing Ltd

233

Current transport in Au/YBazCu30X junctions in c-axis and tilted directions of YBazCu30x thin films

P. Komissinski'92 and G.A. Ovsyannikov2
Chalmers University of Technology and Gothenburg University, Gothenburg, Sweden *Institute of Radio Engineering and Electronics RAS, Moscow, Russia ABSTRACT: We report on the fabrication technique and investigation of current transport properties of Au/YBa2Cu30X junctions. The junctions were processed in epitaxial YBa2Cu30X (YBCO) thin films grown by laser ablation. (001) LaA103 and (120) NdGa03 substrates were used to grow correspondingly (OOl)-oriented and tilted YBCO films. Current-voltage characteristics, Z-V-curves, and the dependencies of the differential resistance on the applied voltage, Rd(V), were measured and the anisotropy of the current transport in these junctions was studied. 1. INTRODUCTION

The resistance between high-temperature superconductor and normal metal, HTUN, is an important parameter from fundumental and practical point of view. The anisotropy of c-axis and a-bplane resistance (Rc and R,J has often been observed in the tunneling characteristics of hightemperature superconductors [Divin et. al. 19951. In the particular case of HTS/N junctions this anisotropy combined with the structural defects in the surface area of the superconductor can produce unexpected results in the work of circuits, containing such contacts. Recently, a number of attempts has been made to minimize the interface resistance in these contacts [Yuzi-Xu 1997, Daly 1997, Terai 1995, Sanders 1995, etc.]. The obtained results are often non-consistent with each other. This paper is devoted to the experimental investigation of the current transport in Au/YBCO junctions with a current flow along (OOl)-oriented and tilted high-temperature superconducting YBCO thin films.

2.

YBCO FILMS AND SAMPLE FABRICATION.

The junctions were prepared using the sequence of operations shown in Fig. 1. YBCO epitaxial films were grown by laser ablation (Fig. la) at temperatures 780-8OOOC and at 0.8mbar oxygen pressure. We used (001) LaA103 substrates to grow c-axis oriented YBCO and (120) NdGa03 substrates to obtain YBCO films with c-axis tilted on 18.4' from the normal to the substrate plane (tilted YBCO films). The YBCO film was in situ covered by a thin layer of normal metal, Au, with a thickness of 20nm grown at 100°C by laser ablation Junction areas and electrodes were defined by using photolithography and Ar ion milling (Fig. la). In order to provide an one-directional electrical contact plane of the YBCO film, the region of the junction was insulated on the side by a CeO;! layer with a central window having an area of lOxlOpm* (Fig. lb). The geometry of electrodes allowed for 4-point measurement of the interface resistance. (Fig. lc). YBCO films were characterized by magnetic succeptibility


234

CeO,
;o ~OOl~YBC0//[llO~N~ ye1843" w [ 2101NGO (12O)NGO d E

a

sist Au YBCO

b

area Junction
+ + ++ 6 C m ++A&

CeO2

d

30

40 20"

50

60

70

80

91

Fig. 1. A fabrication procedure of Au/YBCO junctions: a) definition of the junction area by photolitography and ion milling; b) deposition of insulating Ce02 layer; c) deposition of the contact pads; d) top view of the junction;

Fig. 2. X-ray diffraction pattern of Au/YBCO double layer structure on NdGa03 ( 120) substrate. Tilting by the angle w = 18.4" to the normal of the substrates provides (On) reflections of YBCO film.

-

measurements of the critical temperature (Tc) and the width (ATc) of the superconducting transition. Values of Tc's> 89K and AT&< OSK for c-axis YBCO films and T,`s> 85K and AT,%< 2K for tilted YBCO films were obtained. X-ray 8-28 scan was performed at the angle w= 18.4' and showed that c-axis of YBCO film on (120) NdGa03 substrates follows the tilting angle of (120) from (110) NdGa03 substrate (w= 18.4"), see Fig.2 3. EXPERIMENTAL RESULTS AND DISCUSSIONS.

The dependencies of resistance R versus temperature T, R(T), for Au/YBCO junctions and test planar YBCO microbridges (4pm wide) were measured at temperatures 4.2-300K at l-51,~A bias currents as well as their I-V curves. Parameters of c-axis and tilted Au/YBCO junctions presented in the table 1.
Table 1. Parameters of Au/YBCO junctions. Sample w,O P32J2 P32J3 P32J4 0 0 0 T,, K RdT=T,, V=O,), cl 33.2 89.3 19.5 89.5 22.9 89.9 &(T=4.2K, V=O), Q 103.0 52.0 55.3 &(T=4.2K, V=O)/RN 3.1 2.7 2.4 R$, 10-6, Rem2 33.2 19.5 22.9

R(T) dependencies for both types of investigated Au/YBCO junctions are shown in Fig. 3: (a) a solid line for w=O, the direction of the current flow coincides with the (OOl)-direction in YBCO film (a caxis junction), and (b) a dashed line for ~=18.4", the direction of the current flow deviates on ~=18.4' in relation to the (OOl)-axis of YBCO film (a tilted junction).


235
Voltage, mV

2.0

-30

-10 -20 0 10 20 30 . . . . . . . n.I..* -..l.*.....*. I... ,.....,,... L . . . . . . . . . . . . . . +()

: loo _____________---0
20 40

lo 60
80

100

120

140

160

180

2(K)

Temperature, K

Fig. 3. R(T) dependencies in the cases of tilted, ~=18.4' (dashed line), and c-axis, v=O (solid line), orientations of YBCO film to the direction of the current flow.

Fig. 4. A comparison of the Rd(V) dependencies in the case of tilted, y = 18.4" (dashed line) and c-axis, y=O (solid line), Au/YBCO junctions.

At T > Tc, the R(T) dependencies for both cases are of a metallic type, i.e., the resistance decreases with decreasing temperature, the latter is typical for YBCO film carrying current in the basal plane of YBCO. As a rule, the values of Tc for bridges and investigated tilted junctions were lower than ones of YBCO films measured immediately after deposition of the double-layer Au/YBCO structure. The degradation of superconducting properties of the films was apparently associated with a decrease in the amount of oxygen during ion etching. This effect is strong in the case of tilted YBCO films because of the high rate of the oxygen depletion in a-b basal planes of YBCO film. It can be seen from the Fig. 3 that the resistance R of tilted junctions decreases monotonously with T at T < Tc. For c-axis junctions we have R-l/T*. This different behavior indicates a different type of conductivity of the YBCO/Au interface in tilted and c-axis junctions. In the tilted junctions thecurrent transport is partially in a-b-planes which results in an ohmic type contact with metallic conductivity,. In the case of c-axis junctions an oxygen depleted semiconducting surface layer is realized. We have not observed a zero-bias conductance peak probably because of the lower junction resistances compared with [Sanders 19951. Similar R(T) behaviour and the values of characteristic interface resistance (RNS) were obtained in [Ekin 1988, Daly 19971 in the case of c-axis Au/YBCO interface. Fig. 4 presents the typical dependencies of differential resistance of Au/YBCO junctions on applied voltage. RN of the tilted junction is a constant value of 0.3-l .6Q in the current range below approximately lmA, which provides a value of RNS-~O-~- 10m6 Q-cm*. Taking into account the square of a-b-plane area, involving in the current transport, we have RNS(LZ-~)-~O-~ R-cm*. There are four orders of magnitude difference of this value with the theoretically estimated one [Kupriyanov 1995, Kupriyanov 1991, Deutscher 199 11. This difference is possibly caused by the oxygen depletion along a-6 planes during the ion milling of Au/YBCO tilted junction. According to our estimations the in-plane superconducting critical current density for proceeded tilted YBCO films is Jc-103A/cm2 at T =4.2K, i.e. a least four orders of magnitude lower than in the case of caxis YBCO films. For c-axis structures Rd(V) dependence has a maximum at zero bias current (and voltage) with the value of -10e5 L&cm*, see a solid line on Fig. 4. As it was already mentioned, an oxygen depletion effect is a possible reason for the tunnel R(T) dependence and for the high value of RNS. Fig. 5 presents Rd(v) dependencies for c-axis junction, obtained after the annealing of this junction in oxygen atmosphere at 6OOOC. The Rd(v) dependencies were measured at T=4.2K immediately after each annealing step. The value of RNS decreases rapidly with the increase of the annealing time and after 5 hours of annealing is equal to -10e6 0cm'. The subsequent annealing for 15 hours


236 did not affect the RNS value. This means that the oxygen contents in the surface layer of YBCO is close to the maximum value. 4. CONCLUSIONS.

P34J3, y=O

Voltage, mV

Fig. 5. &i(v) dependencies of c-axis Au/YBCO junction from the annealing time in oxygen atmosphere at 600°C. .

We have investigated the current transport properties of c-axis and tilted Au/YBCO junctions. A strong anisotropy of the R(T) dependencies and I-V-curves was observed, caused by the different type of conductivity in a-b-plane and in c-axis direction of YBCO. An oxygen depletion effect and its influence on the contact resistance value weas investigated. The depression of Tc and Jc in tilted YBCO film during the fabrication of Au/YBCO junction makes it questionable the applications of these films in superconducting devices. Authors are grateful1 to Z.G. Ivanov, M .Yu. Kupriyanov and P. Mozhaev for fruitful discussions of the experimental results. This work has been supported by the ESPRIT project HTS-RSFQ-23429, Swedish Materials Consortium on Superconductivity, Russian Foundation of Fundamental Research and INTAS program. 5. REFERENCES. -

Daly G M, Pond J M, Osofsky G M, Horwitz J S, Soulen R J, Chrisey D B, Auyeung R C Y 1997 IEEE Trans. Applied Superconductivity 7 pp 2153-6. Deutscher G et.al. 1991 Physica C 185189 p 216. Divin Y Y, Poppe U, Shadrin P M, Seo J W, Kabius B and Urban K 1995 Proc. 2nd Eur. Conf. on Applied Superconductivity 2 pp 1359-62. Ekin J W, Larson T M, Bergen N F, Nelson A J, Swartzlander A B, Kazmerski L L, Panson A J and Blankenship B A 1988 Appl. Phys. Lett. 52 pp 1819-21. Kupriyanov M Y and Likharev K K 1991 IEEE Trans. Magn. 27 pp 2400-3. Kupriyanov M Y 1995 Dr. of Sciences degree thesis, NPI MSU edition (Moscow), in Russian. Sanders S C, Russek S E, Clickner C C, Ekin J W 1995 IEEE Trans. Applied Superconductivity 5 pp 2404-7. Terai H, Fujimaki A, Takai Y, Hayakawa H 1995 IEEE Trans. Applied Superconductivity.5 pp 2408-11. Yuzi-Xu, Ekin J W, Russek S E, Fiske R, Clickner C C, Takeuchi I, Trajanovic 2, Venkatesan T, Rogers C T 1997 IEEE Trans. Applied Superconductivity 7 pp 2836-9.