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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005

165

Characterization and Dynamics of [100]-Tilted Y-B-C-O Bicrystal Junctions on Nd Ga O3
Igor V. Borisenko, Iosif M. Kotelyanski, Anton V. Shadrin, Philippe V. Komissinski, and Gennady A. Ovsyannikov
Abstract--We report on the fabrication technique and electrical properties of Basal Plane Tilted YBa2 Cu3 O7 x (YBCO) superconducting bicrystal Josephson Junctions (BTJ). In order to form the grain boundary in BTJ we apply an inclination of the (001) YBCO basal planes around the direction of {100} YBCO in contrast to the common In-Plane Tilted bicrystal Josephson Junctions (ITJ) where misorientation of the crystallographic axes in the -plane of YBCO has been used. Symmetric and asymmetric junctions were fabricated on bicrystal NdGaO3 substrates 13­28 tilted from (110) NdGaO3 . DC and RF properties of the BTJ were investigates at temperatures 4.2­77 K, in magnetic fields up to 100 G, and under influence of electromagnetic radiation of 56 GHz frequency. The experimental dependences of critical current and Shapiro steps from RF current fit the Resistive Shunted model of Josephson junctions (RSJ-model). At T = 77 K the 0 5 MA cm2 BTJ reveal critical current density jC = 0 2 and characteristic voltage VC = IC RN = 0 6 0 9mV , that makes the junctions promising candidate for practical electronic devices. Index Terms--Basal plane inclination, bicrystal junction, Josephson effect, metal-oxide superconductor.

I. INTRODUCTION

A

CCORDING to the theoretical models of Josephson of the juncjunctions [1]­[5] the critical frequency tion is designated by the characteristic voltage ( --critical current, --normal resistance of the junction) and defined by through Josephson equation of the superconductors forming the an order parameter junction. Thus for (YBCO) with high critical the amplitude of superconducting temperature , and would be exorder parameter is pected to reach few millivolts at liquid nitrogen temperature do (77 K). However, experimentally obtained values of even for well developed In-Plane Tilted not exceed 300 bicrystal Josephson Junctions (ITJ), where YBCO films are

(001)-oriented on both sides of the grain boundary but and axes are rotated around [001] YBCO (Fig. 1(a)) [5], [6]. (up to 1 mV) have been Recently, significant increase in reported in Basal Plane Tilted bicrystal Josephson Junctions (BTJ) [7]­[10], where the (001) YBCO planes are symmetriby rotation around [100] cally misoriented by angles YBCO (Fig. 1(b)). However, due to an extra conductivity channel through the possible superconducting shorts with the sizes larger than the coherence length of YBCO, high values , obtained from dc measurements ( and ), may not of correspond to the dynamic characteristics of the junction like Josephson oscillations, superconducting current-phase relation, high frequency impedance and so on [5], [11]. Unfortunately, the considerable anisotropy of the electrical properties of the tilted YBCO epitaxial films may result in problems for Josephson devices based on the BTJ. In order to reduce the effect of anisotropy we suggest Asymmetric Basal Plane , Tilted bicrystal Josephson junctions (ABTJ), where (Fig. 1(b)). In this case the YBCO film on one side from the grain boundary is (001)-oriented and has no significant elecis usually small trical anisotropy in the -plane. Note that in asymmetric ITJ. In this paper we present our results on DC and RF prop(NGO) erties of BTJ and ABTJ fabricated on substrates--material with small dielectric losses at millimeter wavelengths [12]. Electrophysical and dynamic (microwave) properties of the symmetric and asymmetric junction have been studied and compared with ordinary ITJ junctions on NGO substrates with the similar values of misorientation angles. II. FABRICATION NGO bicrystal substrates for all types of junctions have been fabricated by the same technique and have bicrystal boundaries are varied of the same quality [12]. Misorientation angles from 13 to 28 . (110) NGO substrates have been used as deposition template for 150 nm thick (001) YBCO films. We applied dc sputtering of YBCO target at high oxygen pressure or laser ablation at substrate temperature 780­800 to obtain crit. ical temperatures of epitaxial YBCO films Josephson junctions namely YBCO thin film microbridges of 4 width and 10 length were patterned across the bicrystal boundary by photolithography, RF plasma etching in argon and [13]. wet chemical etching in 0.5% ethanol solution of IV-curves of the junctions were measured in the temperature , magnetic fields up to , range of and under influence of monochromatic microwave radiation of frequency.

Manuscript received October 5, 2004. This work was supported in part by INTAS-2001-0809, INTAS-2001-0249, ISTC 2369, RFBR 04-02016818, RFBR-04-02-16687, PGSS 1344.2003.2. I. V. Borisenko, I. M. Kotelyanski, and G. A. Ovsyannikov are with the Institute of Radio Engineering and Electronics Russian Academy of Sciences, Moscow 125009, Russia (e-mail: iboris@hirech.cplire.ru; imk216@ire216.msk.su; gena@hirech.cplire.ru). A. V. Shadrin is with the Institute of Radio Engineering and Electronics Russian Academy of Sciences, Moscow 125009, Russia, on leave from Moscow State University, Moscow, Russia (e-mail: shadrinant@mail.ru). P. V. Komissinski is with the Chalmers University of Technology, S 412 96, Gothenburg, Sweden, on leave from the Institute of Radio Engineering and Electronics Russian Academy of Sciences, Moscow 125009, Russia (e-mail: filipp@mc2.chalmers.se). Digital Object Identifier 10.1109/TASC.2005.849733

1051-8223/$20.00 © 2005 IEEE


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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005

Fig. 1. Types of bicrystal junctions: a) inplane tilted bicrystal junction (ITJ); b) basal plane tilted bicrystal junction (BTJ). and  are misorientation angles. = 0,  6= 0 for asymmetric junctions.

III. EXPERIMENTAL RESULTS AND DISCUSSION The morphology of the YBCO film deposited on tilted substrate depends on the inclination angle [14]. Fig. 2(a) shows an AFM 3D-image of YBCO film deposited on ABTJ substrate , . A cross section view indicates a difwith ference in the surface morphology compared with (001)-oriented films on (110) NGO substrates: the elongated step-like features can be observed, while the edges of the steps are oriented along the substrate rotation axis [001] NGO (Fig. 2(b)). For small tilted angles the thin film is very smooth (roughness less than 3 nm) and the step height is close to the YBCO lattice parameter. If the inclination angle exceeds 3­5 the step height increases rapidly and becomes much greater (7­10 nm) than the initial step size on the substrate surface (0.5­1 nm) [14]. Parameters of the three types of bicrystal junctions (ITJ, BTJ, ABTJ) obtained from DC measurements of the I-V curves at and 78 K are presented in Table I. As it follows from the Table I the values of critical current density and characteristic voltage of the BTJ at 78 K are much higher (by factor of 10) than ones of the ITJ. A. I-V Curves The I-V curves of the BTJ shown on Fig. 3 are hyperbolic-like dependences typical for the RSJ model of a Josephson junction. Two channels of current transport are presented, namely, quasiparticle current and superconducting current , where is a phase difference between two superconducting electrodes [1], [15]. Large values of excess current, , may indicate which are typically about 50% of at on the existence of an additional channel of direct (non tunnel) conductivity or other current transport mechanism [5]. Note that ABTJ and BTJ with misorientation angle 13 reveal I-V curves different from hyperbolic form and typical for viscous flow of the rise of excess current is vortices [1], [15]. At

Fig. 2. Three-dimensional AFM view for the ABTJ boundary a), crossection b) and 3-D AFM of the YBCO bridge crossing bicrystal boundary c).

(up to 10 mV) alobserved and the junctions have higher though their utilization in practical systems is difficult because of the significant deviation from RSJ model. B. Critical Current Typical dependence of BTJ is shown on Fig. 4. Its shape is rather linear and differs from the theoretically predicted one for tunnel junctions of s-superconductors (SIS) [1], where . saturation is observed at In the SIS tunnel junctions the current transport through is realized. In conhigh-energy Andreev states has been previously observed trast, linear dependence of in the junctions with direct conductivity, where within wide


BORISENKO et al.: CHARACTERIZATION AND DYNAMICS OF [100]-TILTED YBCO BICRYSTAL JUNCTIONS

167

DC P

ARAMETERS OF

TABLE I BICRYSTAL J

UNCTIONS

Fig. 4. Temperature dependences of the BTJ resistance and critical current. Almost linear I (T) dependence is clear observed.

, , J1­J3--junction number on sample.

, ,

Fig. 3. Normalized (V=V and I=I ) I-V curves for ABTJ ( = 0) for two misorientation angles = 21 (solid curve) and 28 (dashed). The reduction of excess current clearly observed with increasing angles. I-V curves for the same junctions for large voltage scale are shown on inset.

and BTJ at can be explained by the -wave in ITJ symmetry of the superconducting order parameter electrodes. In ITJ the planes are rotated around -axis of YBCO resulting in smaller along the direction perpendicular to the grain boundary. The suppression of the order parameter may as well occur in the vicinity of the grain boundary interface due to contact phenomenon [3] (Fig. 5). is decreased more than three times (Table I) when increasing the misorientation angle from 21 to 28 . Such behavior is in a good agreement with a theory [16], where large ( is the Fermi energy) typical for YBCO values of and significant Fermi surface anisotropy are taken into account. These two factors lead to dispersion in momenta of incident electrons and reflecting holes at the grain boundary and, as a result, the coherence of multiple Andreev reflection is broken and superconducting current is depressed. At the same time there is a certain critical misorientation angle according to [16], at which critical current is reduced almost to zero. Such strong dependence, however, may be decorated by boundary faceting. Note, with increasing has also been that the reduction of observed in ITJ (see Table I and [5]). Dependences of the critical current vs. magnetic field are presented in Fig. 6. The maximum of the is ob. However, dependence is different from served at the Fraunhofer-type and moreover, asymmetrical. The latter is typical for distributed Josephson junctions [1], [15]. Actually, the Josephson penetration depth of magnetic field for is , which is significantly smaller . Thus, we may consider our than the junction width junctions as distributed ones at liquid nitrogen temperature. C. Shapiro Steps In order to determine the junctions dynamical parameters we have measured the IV-curves under the influence of monochromatic radiation of millimeter wavelengths, typically , . Experimental dependences of the critical current and Shapiro steps vs. amplitude of the external electromagnetic radiation may be well fitted by the RSJ model (Fig. 6). The deviation of the normalized experimental

temperature range superconducting current transport is deter[1]. In ITJ mined by low-energy Andreev states -type of superconducting order parameter symmetry and states contributing to current presumes both transport at low temperatures. However, at high temperatures is drastically reduced. Low-energy the influence of the states do not appear in BTJ if either or crystallographic axis in YBCO is parallel to the grain boundary (no in-plane tilting) between ITJ [2]­[5]. Hence the significant difference in


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and asymmetric basal plane tilted bicrystal Josephson junctions make them very attractive for application as elements of superwas confirmed by conducting electronics. The high value of measurements at mm wave. The existence of practically isotropic (001) film on one side of the asymmetric BTJ easily allows to fabricate additional circuits and wirings while maintaining high critical parameters. Furthermore, asymmetric basal plane tilted junctions can be also fabricated using a biepitaxial technology by the proper selection of a buffer layer material for tilting of the YBCO basal planes. ACKNOWLEDGMENT The authors thank K. Y. Constantinian, M. I. Faley, A. Golubov, Yu. V. Kislinsky, and V. K. Kornev for their help with the experiment and for the fruitful discussions.
Fig. 5. Typical magnetic-field (at T = 78 K) dependences of the critical current for BTJ.

REFERENCES
[1] K. K. Likharev, "Superconducting weak links," Rev. Mod.Phys., vol. 51, no. 1, pp. 101­159, 1979. [2] R. A. Riedel and P. F. Bagwell, "Low-temperature Josephson current peak in junctions with d-wave order parameters," Phys. Rev. B, vol. 57, no. 10, pp. 6084­6089, 1998. [3] Y. S. Barash, "Quasiparticle interface states in junctions involving d-wave superconductors," Phys. Rev. B, vol. 61, no. 1, pp. 678­688, 2000. [4] Y. Tanaka and S. Kashiwaya, "Theory of the Josephson effect in d-wave superconductors," Phys. Rev. B, vol. 53, no. 18, pp. R11957­R11960, 1996. [5] H. Hilgenkamp and J. Mannhart, "Grain boundaries in high-T superconductors," Rev. Mod. Phys., vol. 74, no. 2, pp. 485­549, 2002. [6] L. R. Vale, R. H. Ono, and D. A. Rudman, "YBCO Josephson junctions on bicrystal Al O and SrTiO substrates," IEEE Trans. Appl. Supercond., vol. 7, no. 3, pp. 3193­3197, 1997. [7] D. Dimos, P. Chaudhari, and J. Manhart, "Superconducting transport properties of grain boundaries in YBCO bicrystals," Phys. Rev. B, vol. 41, no. 7, pp. 4038­4049, 1990. [8] U. Pope, Y. Y. Divin, M. I. Faley, J. S. Wu, and C. L. Jia et al., "Properties of YBCO thin films deposited on substrates and bicrystals with vicinal off cut and realization of high I R junctions," IEEE Trans. Appl. Supercond., vol. 11, no. 1, pp. 3768­3771, 2001. [9] Y. Y. Divin, I. M. Kotelyanski, P. M. Shadrin, C. L. Jia, and U. Poppe et al., "[100]-tilt YBCO grain-boundary Josephson junctions with high I R -product on NdGaO bicrystal substrates," in Abst. Sixth Eur. Conf. Applied Supercond., 2003, p. 166. [10] , "[100]-tilt YBCO grain-boundary Josephson junctions with high I R -product on NdGaO bicrystal substrates," Supercond. Sci. Technol., 2004, to be published. [11] A. D. Mashtakov, G. A. Ovsyannikov, I. V. Borisenko, K. Y. Constantinian, and I. M. Kotelyanskii et al., "High-T superconducting stepedge junctions on sapphire fabricated by nonetching technique," IEEE Trans. Appl. Supercond., vol. 9, no. 2, pp. 3001­3004, 1999. [12] Y. Y. Divin, I. M. Kotelyanski, and V. N. Gubankov, "Bicrystal Josephson junction for terahertz Hilbert-transform spectroscopy," J. Commun. Technol. Electron., vol. 48, no. 10, pp. 1137­1147, 2003. [13] G. A. Ovsyannikov, I. V. Borisenko, K. I. Constantinyan, A. D. Mashtakov, and E. A. Stepantsov, "Phase dependence of the superconducting current in YBCO Josephson junctions on a bicrystal substrate," Tech. Phys. Lett., vol. 25, pp. 913­916, 1999. [14] I. K. Bdikin, P. B. Mozhaev, G. A. Ovsyannikov, P. V. Komissinski, and I. M. Kotelyanskii, "Growth and domain structure of YBCO thin films and YBCO/CeO2 heterostructures on tilted NGO substrates," Physica C377, pp. 26­35, 2002. [15] K. K. Likharev, Dynamics of Josephson Junctions and Circuits. Moscow, Russia: Department of Physics MSU, 1978, p. 615. [16] A. Golubov and F. Tafuri, "Andreev reflection in layered structures: implications for high-Tc grain-boundary Josephson junctions," Phys. Rev. B, vol. 62, no. 22, pp. 15 200­15 203, 2000.

Fig. 6.
I (I

)=I (0) (squares) and first Shapiro step I (I )=I (0)((solid rounds) for BTJ at external frequency f = 56 GHz. Solid lines and dotted line--theoretical dependences at f =f = 0,23.

Amplitude dependences of the normalized critical current

maximum of the first Shapiro step from is about 7% at northe calculated one malized frequency . The difference shows that the effective critical current value is about 7% lower because of the inhomogeneous current distribution. Note that absence of subharmonic Shapiro steps (at voltages ), and zero minimum values of and dependences indicate pure sinusoidal superconducting current-phase relation [13]. Thus, in spite of the excess current observations we have obtained a good concordance of DC and dynamic BTJ and ABTJ junction parameters at liquid nitrogen temperature. IV. CONCLUSION at Observation of high characteristic voltages in more than 70% of the investigated symmetric