Au contacts fabricated at liquid-nitrogen temperatures

Physica C 403 (2004) 52–56 www.elsevier.com/locate/physc

Controllable Bi2Sr2CaCu2O8þd/Au contacts fabricated at liquid-nitrogen temperatures X.B. Zhu a, S.P. Zhao a,*, G.H. Chen a, H.J. Tao a, C.T. Lin b, S.S. Xie a, Q.S. Yang a a

Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, PR China b Max-Planck-Institut f €ur Festk€orperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany Received 25 June 2003; received in revised form 21 October 2003; accepted 18 November 2003

Abstract Properties of the superconducting surface CuO2 double layers of a cleaved Bi2 Sr2 CaCu2 O8þd (BSCCO) crystal in contact with Au metal films are studied using mesa-structured intrinsic Josephson junctions. We show that a welldefined and experimentally controllable BSCCO/Au contact can be achieved by the stabilization of the (interstitial) oxygen atoms in the surface BiO layers at liquid nitrogen temperature during fabrication. The surface CuO2 double layers are found to be in a metallic contact with Au films and have a transition temperature Tc0 of 80 K for a BSCCO crystal with bulk Tc of 91 K. Involvement of impurity atoms or molecules in the BSCCO/Au interface is found to substantially reduce the value of Tc0 .
2003 Elsevier B.V. All rights reserved. PACS: 74.50.+r; 74.72.Hs; 74.80.Dm

1. Introduction Interface properties of the high-Tc superconductors (HTSCs) with metals remain an important concern for the device applications. The short coherence length and easy change of the oxygen content in HTSCs impose special difficulties on the reproducible device fabrication. In the case of YBa2 Cu3 O7d (YBCO) [1–3], a mostly studied HTSC in the field, it is found that oxygen-

*

Corresponding author. Fax: +86-10-82640223. E-mail address: [email protected] (S.P. Zhao).

content modifications are usually inevitable in the contact area with metals during processing. Deficiency in oxygen results in a transition layer near the interface where the properties change continuously from superconducting to metallic, or to semiconducting ones [3,4]. Such an interface is not well-defined and may not be suitable for the fabrication of practical devices. Recently, studies on interfaces with different axial orientations [5] and in some other materials [4,6] have been reported. Bi2 Sr2 CaCu2 O8þd (BSCCO) is another typical HTSC. It is a highly anisotropic material with weak inter-CuO2 -double-layer couplings, and can be easily cleaved along the Bi–O planes, giving an

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2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.11.011

X.B. Zhu et al. / Physica C 403 (2004) 52–56

2. Experimental

Metal film Au CuO2/met al bilayer

Bi O SrO Surf ace CuO2 double layer

Surf ace jun ct io n

53

SrO Bi O Bi O SrO Seco nd CuO2 double layer

Fig. 1. Schematic structure and terminology used in this work near a BSCCO/metal interface. Single crystals of BSCCO are usually cleaved along the BiO double layers.

atomically flat surface (see Fig. 1). From this point of view, BSCCO may be used as a special and also an ideal material for the studies of a HTSC/metal interface. Kim et al. [7], in connection with the studies of the intrinsic Josephson effects [8,9], have found a BSCCO/Au contact being ohmic in nature and discussed the properties of the CuO2 /Au bilayers based upon the tunneling characteristics of the surface junctions (see Fig. 1). Further discussion can also be found in our previous works [10,11]. From these studies, it appears that the properties of the surface layers are so far experimentally uncontrollable, with the bilayer superconducting transition temperature Tc0 varying anywhere below 80 K for a BSCCO crystal with bulk Tc around 90 K [7,10–12]. Josephson current Ic0 of the surface junctions is usually on the order of one tenth of the bulk value Ic and shows a strange temperature dependence [7,10,11]. One possibility leading to the results unpredictable in these experiments can be the uncontrollable change of (interstitial) oxygen content during fabrication in the surface BiO layers that serve as charge reservoirs [13]. In this work, based upon the results of the mesa-structured intrinsic Josephson junctions (IJJs), we show that this is indeed the case, and a well-defined BSCCO/metal contact and therefore a CuO2 /metal bilayer and surface junction with reproducible properties can be obtained by ÔfreezingÕ the oxygen atoms at liquid nitrogen temperature during fabrication.

Our BSCCO single crystals, which were near optimally doped with Tc  91 K, were grown by the traveling solvent floating zone method [14]. Small pieces of these crystals with a typical size of 0.6 · 0.6 · 0.05 mm3 were glued onto Si substrates. The substrate holder used in this work was attached to a liquid nitrogen container in the vacuum chamber, which had a background pressure of 2 · 105 Pa. For the cleavage of the BSCCO crystal, we connected a wire from a piece of sticky tape on the crystal to the shutter. In this way, an Au film, with thickness of 50–100 nm, was evaporated [15] immediately to the fresh surface of the crystal upon cleavage. The whole fabrication process for the IJJs, 5 · 5 lm2 in sizes, was rather conventional and has been described previously [10,11]. A 3-terminal measurement configuration was used in our experiments. The I–V curves were recorded using a dc method or from an oscilloscope. The measurements for the resistance RðT Þ across the mesas were performed using a 1-lA ac current and lock-in amplifier.

3. Results and discussion Fig. 2 shows the I–V curve of a mesa from sample S2 with the metallic linear resistance as a result of the 3-terminal measurement compensated. 1 The oscilloscope image shows only the first two or three branches due to the almost identical inner junctions. Josephson current Ic of the inner junctions is about 0.6 mA, corresponding to a current density of 2500 A/cm2 . The current at zero voltage is the Josephson current Ic0 of the surface

1

Without compensation, the part of zero-voltage current in Fig. 2 is an ohmic line, with resistance, which stems from BSCCO/Au interface and Au plus the leads, ranging from 0.3 to 3 X for the present samples. We have also performed a 4terminal measurement, from which we found the BSCCO/Au contact resistance to be below 30 mX, consistent with an estimation of Ref. [12]. Therefore, the ohmic resistance on the order of 1 X is mostly from the ‘‘outside’’ leads as a result of the 3-terminal configuration.

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X.B. Zhu et al. / Physica C 403 (2004) 52–56

120

80 R ( Ω)

S5

40 S3 S2 0 0

50

100

150

T (K) Fig. 3. Resistance versus temperature across the mesas for three samples. Note that the data of three mesas on each sample crystal are plotted together. Fig. 2. The I–V curve of a mesa of sample S2 at 4.2 K.

junction, which has a value of nearly a quarter of Ic . Compared to the value of Ic0 =Ic  10% in the case of room temperature cleavage process [7,10,11], the present fabrication process, i.e., with the BSCCO crystal cleaved at liquid nitrogen temperature in vacuum followed immediately by the evaporation of the Au film, yields a much larger Ic0 . In addition, the temperature dependence of Ic0 also shows a considerable difference (see below). In Fig. 3, we show the temperature dependence of the resistance RðT Þ across the mesas of S2 together with the data of other two samples. For each sample crystal, the data of three mesas are plotted. From the figure, we can see that with decreasing temperature, R first decreases sharply at bulk Tc and then, after increasing to some extent, it shows a second transition at Tc0 , corresponding to the superconducting transition of the CuO2 /Au bilayers. 2 The Tc0 difference DTc0 for different mesas

2

The RðT Þ curves do not show an upturn below Tc0 , indicative of an ohmic contact, instead of tunneling, between the surface CuO2 layer and the Au film. The CuO2 /Au bilayer has a reduced Tc0 compared with Tc due, probably partly, to the proximity effect. See also Ref. [7].

on the same crystal is usually 1–2 K, and in some cases can be well below 1 K, as the data from S3 in the figure. Our experiments indicate that DTc0 can be controlled within 5 K for different mesas on different crystals prepared with the same fabrication conditions. These results are in sharp contrast to those of the samples prepared with the room temperature cleavage, where Tc0 usually scatters randomly from run to run, even for samples prepared under the same fabrication conditions [7,10]. The sample S3 in Fig. 3 was prepared with the same conditions as those used for S2 except that the evaporation rate r for Au film was slower. In our experiments, we find that Tc0 increases with r in certain range and then saturates at about Tc0 ¼ 80 K. In Table 1 we list four samples (S1 –S4 ) showing such a tendency. Since only r changes for these samples, there can be two reasons responsible for this behavior. One is that the Au film crystallizes differently for different r, the other is the residual gas (CO, H2 , water and so on) adsorption on the cleaved BSCCO surface immediately before or during the Au film evaporation, which can be significant for our background pressure of 2 · 105 Pa and a surface held at liquid nitrogen temperature. The smaller the value of r, the more residual gas one would expect to be

X.B. Zhu et al. / Physica C 403 (2004) 52–56

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Table 1 Transition temperatures Tc0 (midpoint) and Josephson currents Ic0 (at T ¼ 4:2 K) for five samples, each containing three mesas, prepared with Au film evaporation of different nominal deposition rate r S1 S2 S3 S4 S5

r (nm/s)

Tc0 (K)

DTc0 (K)

Ic0 (lA)

DIc0 =Ic0 (%)

6.3 4.8 0.74 <0.3 5.7

78.5/78.7/79.3 74.7/75.9/77.0 58.9/58.9/59.1 12.3/14.9/15.9 15.8/16.7/16.9

0.8 2.3 0.2 3.6 1.1

120/142/165 122/143/205 64/76/92 6/9.5/9.5 7/13/14

±15.8 ±26.5 ±18.1 ±21.0 ±30.9

adsorbed. For the test, we prepared the sample S5 with r similar to those used for S1 and S2 but the evaporation started 20 s after the BSCCO crystal cleavage. In this case, Tc0 drops to about 16 K, comparable to that of S4 with slowest r, as can be seen in Fig. 3 and Table 1. This indicates that the involvement of the residual gas at the BSCCO/Au interface is the main cause for the decrease of Tc0 . Although the identification of the residual gas remains to be done, our results indicate that its role in affecting Tc0 , possibly through the change of the doping level in the surface CuO2 planes, 3 is as important as the stabilization of oxygen atoms in the surface BiO planes. On the quantitative side, the number of the residual gas molecules that impinge upon a samplepffiffiffiffiffiffiffiffiffi surface can be found from ffi Nr ¼ 3:513  1022 Pr = Mr Ts , where Pr and Mr are the pressure and molecular weight, respectively, and Ts is the surrounding temperature [16]. As a rough estimation, we consider air as the residual gas at a pressure of 2 · 105 Pa and a surrounding temperature of 80 K. The data of the monolayer number to adsorb and the number ratio K of the impinging molecules to the evaporant atoms are tabulated for various Pr and r in Ref. [16]. In our case, we find that K corresponds to 0.46% and 10% for samples S1 and S4 , respectively, while for sample S5 , 2.5 monolayers are adsorbed in a time period of 20 s assuming a sticking coefficient s of unity. If we consider that s is usually smaller than unity for the residual gas molecules while for the

3

Note that most residual gas molecules can in principle combine with an oxygen atom to form a new compound, e.g., CO fi CO2 , H2 fi H2 O, and H2 O fi H2 O2 .

Josephson current (µA)

Evaporation was performed immediately after the crystal cleavage for S1 to S4 , while for S5 it was done 20 s later.

100

S1 S3 S2

10

0

20

40

60

80

T(K) Fig. 4. Josephson current of the surface junctions from three mesas on three samples (solid lines with symbols). Also shown are the data of the inner junctions (dashed line) and a surface junction of a sample prepared with the usual room-temperature cleavage. Note the difference of the temperature dependencies between the solid lines with and without symbols.

evaporants we have s  1, the above quoted values should be smaller by a certain factor. In Fig. 4, Ic0 versus temperature for the samples S1 , S2 and S3 (with symbols) are plotted. The data show larger values compared to that of a sample with the usual room temperature cleavage (bare solid line [10,11]), as mentioned above. Moreover, their variations with temperature have a single curvature in the entire temperature range below Tc0 and have a shape closer to the bulk Ic ðT Þ (dashed line). These results point to the well-characterized, spatially uniform surface CuO2 layers with weakened superconductivity, which in turn give rise to the Ic0 ðT Þ characteristics of the surface junctions

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X.B. Zhu et al. / Physica C 403 (2004) 52–56

that can be better understood [17]. From these results, it is reasonable to believe that earlier reports on the surface IJJs with randomly distributed and uncontrollable Tc0 and Ic0 (and together with its strange temperature dependence) [7,10–12] are basically due to the unpredictable change of oxygen content and involvement of impurities at BSCCO/metal interface, which can often be spatially non-uniform in the case of room temperature cleavage. Our present results indicate that these effects, which are frequently encountered also in the experiments with other HTSCs [1–6], can be reduced to a minimum level in the case of BSCCO/ metal systems, which can be useful for the further study of the properties (e.g., the proximity effects) of the CuO2 /metal bilayer systems. In Table 1, the scatterings of Tc0 and Ic0 for the five samples S1 –S5 are listed. Compared with the small DTc0 , DIc0 is still quite large. Further studies are required to reduce DIc0 . For the five samples listed in Table 1, we have performed the measurements three or four times in a period of 5 months. The results show that Tc0 is very stable against thermal cycling between room and liquid helium temperatures while Ic0 shows a slight reduction.

4. Summary We have demonstrated that a well-defined BSCCO/Au contact can be prepared by stabilizing the (interstitial) oxygen atoms in the BiO planes at liquid nitrogen temperature during sample fabrication. The superconducting transition temperature Tc0 of the surface CuO2 /Au bilayer is found to be sensitive to the involvement of the impurity atoms or molecules at the bilayer interface. In the case of a clean interface, Tc0 is found to be about 80 K for a BSCCO crystal with bulk Tc of 91 K. These results may be useful for the further study of the HTSC/metal bilayer systems or in the future device applications.

Acknowledgements We are grateful to Y.F. Ren, H.W. Yu and Professor F.Z. Xu for their technical help during sample preparations. This work was supported by the National Center for R&D on Superconductivity and the Ministry of Science and Technology of China (NKBRSF-G19990646).

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