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YBa2Cu3O7−x thin-film Josephson junctions on 2 × 12° bicrystal (110) NdGaO3 substrates
PHYSICA ELSEVIER
Physica C 256 (1996) 149-155
Y B a 2 C u 3 0 7 _ x thin-film Josephson junctions on 2 × 12 ° bicrystal
(110) N d G a O 3 substrates Y.Y. Divin a,b,*, H. Schulz a, U. Poppe a, N. Klein a K. Urban a, P.M. Shadrin b, I.M. Kotelyanskii b, E.A. Stepantsov c a Institutffir Festkbrperforschung, Forschungszentrum Jfilich GmbH, 52425 Jiilich, Germany b Institute of Radioengineering and Electronics, Russian Academy of Sciences, Mokhovaya 11, Moscow 103907, Russian Federation c Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow 117333, Russian Federation
Received 23 August 1995; revised manuscriptreceived 20 September 1995
Abstract Grain-boundary YBa2CU3OT_x Josephson junctions with normal-state resistances from 0.1 to 10 l"~ were fabricated on (110) NdGaO 3 bicrystal substrates with a misorientation angle of 2 × 12°. Electrical imaging of the junctions in the temperature range 80 K-300 K was performed by a technique based on laser scanning microscopy. The average values of the electrical transport parameters of the fabricated junctions were shown to be comparable to those of reference junctions made on (100) SrTiO 3 bicrystals. Values of the Josephson linewidth as low as 1 GHz at 77 K have been obtained from the response of the junctions to low-intensity 9 4 G H z electromagnetic radiation. When high-intensity 94 GHz radiation was applied to the junctions, current steps appeared in the I - V curves at voltages V, = n h f / 2 e up to 6.5 mV. These results demonstrate the applicability of YBa2Cu3OT_ x grain-boundary junctions made on NdGaO 3 bicrystal subStrates for millimeter- and submillimeter-wave detection.
1. Introduction A study of the electrical transport properties of grain boundaries in high-Tc materials i s of interest both for fundamental and applied reasons. The boundaries between grains of different orientation are k n o w n to reveal lower critical current densities compared with those of the adjacent grains. For sufficiently large misorientafion a n g l e s the grain boundaries were shown to demonstrate Josephson
* Corresponding author.
behavior. In order to fabricate high-T~ grain-boundary junctions in a controllable w a y , an epitaxial growth of high-T~ thin films on bicrystal substrates was suggested [1]. For this purpose different kinds of bicrystal substrates with simple cubic structures like SrTiO 3, YSZ and MgO were used [1-3]. Recently, NdGaO 3 bicrystals have attracted attention in this field because of the much better lattice matching between NdGaO 3 and high-T~ materials. However, up to now only a few high-Tc grain-boundary junctions have been fabricated on NdGaO 3 bicrystals [4-6] and only little is k n o w n about their electrical properties. Here, we present the results of our study of integral and local electrical characteristics of
0921-4534/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00638-9
150
Y.Y. Divin et al./Physica C 256 (1996) 149-155
Y B a / C u 3 0 7 _ x grain-boundary junctions made on (110) NdGaO 3 bicrystals in connection with their potential applications in millimeter- and submillimeter-wave detection and spectroscopy.
2. Experimental techniques The mutualarrangement of the main orientations in both parts of the (110) NdGaO 3 bicrystal substrates is demonstrated in Fig. 1. The dimensions of the substrates were around 10 × 10 × 0.8 m m 3. They were fabricated by a thermal=intergrowing technique similar to that used for the fabrication of (001) NdGaO 3 bicrystals [5]. The junctions made previously on 2 × 18.4 ° (001) NdGaO 3 bicrystals showed low critical current densities, which limits their operation to the low-temperature range [5]. To increase this parameter, a lower misorientation a n g l e Of 2 × 12 ° was used in this study. Moreover, the (110) plane was chosen as the main plane for deposition of the YBa2Cu307_ x thin films because of a better matching to the parameters for the high oxygen pressure sputtering deposition [7] used in this work. A network of (110) and (112) twin boundaries was observed in the vicinity of the artificially made grain boundary of these bicrystals by optical measurements in polarized light. The deposition of the YBa2CU3OT_ x thin films was made by DC sputtering from a stoichiomeVic target at high oxygen pressure [7]. Sputtering parameters were optimized to obtain high-quality c-axis oriented thin films on the single-crystal (110) NdGaO 3 substrates. The distance between target and
Fig, 1. (110) NdGaO3 bicrystal subslrate with a misorientation angle 2or.
substrate was around 11 mm, the oxygen pressure was in the range of 3.1 to 3.2 mbar and the current was kept at values of 150-200 mA. By employing a very thin thermocouple placed inside a hole of a dummy substrate it w a s found that during sputtering the temperature of the (110) NdGaO 3 substrate just lying on the heater was 780-790°C, i.e. about 140°C lower than the heater-plate temperature. The sputtering rate was about 9 0 - 1 0 0 n m / h under these conditions. After film deposition the chamber was backfilled with oxygen to atmospheric pressure at 550°C and cooled down to room temperature within about 30 min. The film thickness was i n t h e range of 100-300 nm. Patterning of the fabricated thin films was performed by UV photolithography and wet etching in a 0.3% bromium ethanol solution. Polymethyl methacrYlate was used as a resist in this procedure and a solution of methyl isobutyl ketone in propanol was used as a developer. Sets of YBa2Cu3OT_ x thin-film bridges of 1 to 50 ixm widths were prepared across the 2 × 12 ° grain boundary of the (110) NdGaO 3 bicrystal substrates. As reference samples, YBa2Cu307_ x thin-film grain-boundary junctions were fabricated by a similar Procedure on (100) SrTiO 3 bicrystal substrate with the same misorientation angle of 2 c~ = 2 × 12 °. The deposition temperature in this case was held about 40°C lower compared to that used for NdGaO 3 bicrystal substrates. The inhomogeneity of local electrical properties of the fabricated grain-boundary junctions was char= acterized by an imaging technique based on laser scanning microscopy as was described previously [8]. The junction was placed into the cold stage of an optical microscope and the 2D spatial dependence of the laser-induced voltages on the sample were recorded at temperatures from 80 to 300 K. Two imaging modes were used. In the first mode, no bias current was applied to the junction and measure i ments were made at room temperature. In the second mode, the temperature was below the critical temperature of the YBa2Cu3OT_ x thin films and some bias current was applied. The DC parameters of the grain-boundary junctions were measured by a four-probe technique at temperature from 5 to 300 K. Samples were put into the sample holder which allows spring contacts to be made to all junctions on the same substrate, and it
151
Y.Y. Divin et al./Physica C 256 (1996) 149-155
was possible to measure all of them during one temperature run from r o o m to liquid-helium temperatures. The sample holder was sealed and helium was introduced into it as an exchange gas. All measurements were performed in transport helium and nitrogen dewars. The Josephson behavior of the fabricated junctions was studied by their interaction with millimeter-wave radiation. A 94 G H z Gunn oscillator was used as a source of radiation. Millimeter-wave radiation was directed to the cryogenic set-up through waveguides, an isolator, a modulator and a directreading attenuator. The radiation reaching the sample induced a change of the I - V curve, and the corresponding voltage response A V ( V ) at constant current bias was measured b y a conventional lock-in technique with a modulation of the radiation intensity at frequencies of about 1 kHz. A similar lock-in technique was used for the measurements of the differential resistance d V f d I of the junctions. The coupling between millimeter-wave radiation and the Josephson junction in the sample holder h a s been made sufficiently w e a k in these experiments to guarantee the operation of the Josephson detector in the small-signal l i m i t [9] and to reduce the effect of background radiation on the electrical characteristics of the junctions. For the determination of the Josephson linewidth an indirect technique was used which is based on the detector response of the Josephson junction to low=intensity microwave radiation [9,10].
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3. Imaging of electrical inhomogeneities in YBa2Cu3OT- x grain-boundary junctions The 2D spatial distributions of voltages A V ( x , y ) i n d u c e d b y the f o c u s e d l a s e r b e a m in a Y B a 2 C u 3 0 7 _ x thin-film bridge crossing an artificially made grain boundary on a N d G a O 3 bicrystal substrate are presented in Fig. 2. The 20 p.m wide thin-film bridge is placed horizontally in Fig. 2 and its edges are indicated by the arrows. The grain boundary is situated vertically in the middle o f each image and its position is indicated by an arrow and the sign GB. A grey scale p r e s e n t a t i o n of voltages A V ( x , y ) is used with white colour corresponding to m a x i m u m positive and black colour to m i n i m u m negative signals.
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Fig. 2. The 2D spatial distributions of the laser-beam-induced voltage response of a 20 Ixm wide YBa2Cu307_ x thin-film bridge made on a twinned 2× 12° (110) NdGaO3 bicrystal substrate: (a) T= 300 K and bias current I b = O, (b) T = 87 K, I b = 0.1 mA, (c) T = 84 K, I b = 0.1 mA.
152
Y.K Divin et a l . / P h y s i c a C 256 (1996) 149-155
The room-temperature zero-bias image (Fig. 2(a)) demonstrates an intensive vertical double white-black structure located along the artificially made grain boundary in the bicrystal substrate. Also, similar, but weaker structures are observed in the thin film along the twins in the substrate. One can clearly see the electrical inhomogeneity of the thin-film junction along the bicrystal boundary. In Fig. 2(b) the electrical image of the same area of the thin-film bridge is presented, but the sample was held at a temperature T of 87 K and biased at a current I of 0.1 mA. The sample temperature is below the critical temperature T~ and the critical current of the YBa2Cu307_ x thin film is higher than 0.1 mA at this temperature. The thin-film regions above the artificially made grain boundary and above the twins demonstrate responses of positive polarity. The response is proportional to the laser-beam intensity and the bias current. In spite of the difference in response symmetry across the weak parts of the film in Figs. 2(a) and (b), the general arrangements of the response-producing areas are the same in zero-bias room-temperature and finite-bias low-temperature images. The electrical image presented in Fig. 2(c) is measured at the same bias as the image in Fig. 2(b), but at a lower temperature of 84 K. The weak parts of the films near the substrate twins disappeared at this temperature due to their comparatively larger critical currents. The distribution along the grain boundary at this temperature is qualitatively the same as in the zero-bias room-temperature image (Fig. 2(a)) and the maximum intensity of the response at low temperature corresponds to the most intensive response in the zero-bias image. The degree of inhomogeneity of grain-boundary junctions made on NdGaO 3 bicrystal substrates was larger than that of reference junctions made on SrTiO 3 bicrystals. The main reason is to be found in a difference of the crystallographic structures of these two substrate materials. While SrTiO 3 is a cubic material, NdGaO 3 has an orthorhombic crystal structure. Due to orthorhombicity and a corresponding anisotropy of the thermal-expansion coefficients along the main crystallographic directions of NdGaO 3, the formation of twins in NdGaO 3 bicrystals is possible at the fabrication stage, and, additionally, the NdGaO 3 bicrystal substrates can be bent during cooling from film deposition temperature to
Table 1 Parameters of YBazCu307_, thin-film grain-boundary junctions Substrate Misofientation 1c × Rn(5 K) R n × A jc(5 K) angle [mV] (~) X cm 2) ( A / c m 2) (110) 2×12 ° NdGaO 3 (100) 2 x 12° SrTiO 3
3
3 X l O -8
1×10 5
2.5
6 x 10 -9
4 × 105
lower temperatures [4], thus introducing an inhomogeneous local strain to the grain-boundary junctions. A more refined technology of NdGaO 3 bicrystal preparation might be required to avoid the twins.
4. I - V characteristics
Due to the higher inhomogeneity, the Josephson junctions made on 2 X 12° (110) NdGaO 3 bicrystal substrates showed a higher degree of parameter spread compared to the junctions fabricated on SrTiO 3 bicrystals with the same misorientation angle. In Table 1 we present the average values of the main electrical parameters for both types of YBa2Cu307_ x grain-boundary junctions. The average values of the I c R n product are comparable for both kinds of junctions. The specific resistance R n A of the junctions made on NdGaO 3 bicrystal substrates were usually several times higher than those for the junctions made on SrTiO 3 bicrystals and, conversely, the critical current densities Jc were several times lower. The former circumstance is responsible for higher resistances R n up to 10 ~ , which can be obtained for the junctions on 2 X 12° (110) NdGaO3 bicrystals. In Fig. 3 we present the I - V curves of a YBa2Cu307_ x junction on a (110) NdGaO 3 bicrystal in the temperature range from 5 to 82 K. Especially at low temperatures, the shape of the I - V curves was close to that predicted by the RSJ model [9]. The I - V curves at higher temperatures only demonstrate an RSJ-like shape at low voltages. At higher voltages flux-flow behavior is observed. The latter may be attributed to the junctions arranged on the twin boundaries of the NdGaO 3 substrates. For comparison the I - V curves of a YBaECU307_ x junction on a SrTiO 3 bicrystal are shown in Fig. 4.
Y.Y. Divin et al./Physica C 256 (1996) 149-155
Flux-flow behavior was not observed in these junctions even at higher voltages than for junctions made on N d G a O 3 substrate. Additionally, the normal-state resistance o f junctions made on 2 × 12 ° N d G a O 3 bicrystal substrates showed a decrease of about 30% with a decrease o f temperature from 80 K to 5 K. In contrast, the normal-state resistance increases for junctions made on 2 × 12 ° SrTiO 3 substrates (see Fig. 4). To obtain the values of the critical current I c and the normal-state resistances R n we used the fitting o f the experimental I - V curves from RSJ model calculations. A quasi-linear temperature dependence of the critical current I c was found for the junctions. The IcR n product at 77 K was about 10 to 20 times smaller than that at 5 K.
15
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T=5K 48K 78 K
-8
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I
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-6
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i
i
2
4
6
8
VOLTAGE (mV) 5. M i l l i m e t e r - w a v e d e t e c t i o n grain-boundary junctions
by YBa2Cu307_
x
Due to the high values of the I c R n product and the low dielectric constant of the substrate, the YBa2Cu3OT_ x grain-boundary junctions made on N d G a O 3 bicrystal substrates are considered to be very sensitive detectors for electromagnetic radiation in a wide spectral range from the millimeter- to the
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VOLTAGE [mV] Fig. 3. I - V characteristics of a YBa2Cu307_ x grain-boundary junction made on a 2 × 12° (110) NdGaO 3 bicrystal substrate.
Fig. 4. I - V characteristics of a reference YBa2CU3OT_ x grainboundary junction made on a 2 × 1 2 ° (100) SrTiO 3 bicrystal substrate.
submillimeter-wave range. The detection of millimeter-wave electromagnetic radiation by the fabricated grain-boundary junctions was studied in the temperature range from 5 to 78 K, where, according to the results o f the DC characterization and the study o f spatial inhomogeneities, the contribution of the weak links arranged on the twin boundaries to the transport properties o f junctions is not significant. The possibility of reaching very low values of noise equivalent power (NEP ~ 10-14 W / H z J/2) in the millimeter-wave range has already been demonstrated with Josephson junctions made on N d G a O 3 bicrystal substrates [5]. In our work we concentrated on the possibility of wideband spectroscopic application [10] o f these junctions. At first, the effect o f high-intensity millimeter-wave radiation on the fabricated junctions was studied. In Fig. 5(a) the I - V characteristics o f a 3 Ixm wide junction at a temperature of 5 K are presented with and without applied high-intensity 94 GHz radiation. The normal-state resistance R n o f the junction extracted from this curves is about 7 ~ . Due to the high-intensity radiation, current steps appeared at the I - V curves at voltages Vn = n h f / 2 e. This can be seen more clearly in Fig. 5(b), where the differential resistance d V / d I
154
Y.Y. Divin et aL /Physica C 256 (1996) 149-155
of the junction is plotted as a function of voltage. The appearance of steps up to values of 6.5 mV shows the absence of Joule heating in the fabricated high-T~ junctions at voltages corresponding to frequencies up to the end of the submillimeter-wave range. This result combined with the high value of the IcR n product and the low dielectric constant of the N d G a O 3 substrates demonstrates the potential high-frequency applicability of the fabricated junctions. The response of a grain-boundary junction with a normal-state resistance of R n = 0.25 12 to low-intensity radiation at 77 K is shown in Fig. 6. The m a x i m u m of the response at low voltages corresponds to the suppression of the critical current Ic by external radiation. The characteristic odd-symmetric resonance at voltages V -.~ h f / 2 e is due to the interaction of the external monochromatic radiation with the Josephson radiation. The absence of resonances at voltages Vn = n h f / 2 e demonstrates that the response is measured at a sufficiently low intensity of the external radiation. The linewidth g f of the Josephson radiation can be calculated from the volt-
-('a)
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-0,2 2
-2
-0,4
-3
-0,6 i
0
100
200
300
400
500
VOLTAGE [~tV] Fig. 6. The I - V curve and voltage response to low-intensity 94 GHz radiation for a YBa2Cu3OT_ x grain-boundary junction made on a 2 × 12° (110) NdGaO 3 bicrystal subslrate. T = 77 K.
age separation ~ V = ( h / 2 e ) ~ f between the minim u m and the m a x i m u m of the resonant response. W e obtained a linewidth a s small as 1 GHz, which is equal to the value determined by thermal fluctuations at 77 K [9].
6. C o n c l u s i o n s
1,0 0,5 i
0,0 -0,5
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I
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-6
-4
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0
2
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I
4
6
VOLTAGE [mV] 25
i
I
'
I
I
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'
20
10 5 0
-8
-6
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I
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!
2
VOLTAGE [mV] Fig. 5. The I - V characteristics (a) and differential resistance vs. voltage (b) of a YBazCu3OT_x grain-boundary junction made on a 2X 12° (110) NdGaO3 bicrystal substrate without (1) and with (2) high-intensity 94 GHz electromagnetic radiation at T = 5 K.
Y B a 2 C u 3 0 7 _ x grain-boundary junctions made on (110) N d G a O 3 bicrystal substrates are more inhomogeneous and exhibit the Josephson effect in reduced temperature and voltage ranges compared with junctions made on (100) SrTiO 3 bicrystals with the same misorientation angle. The flux-flow behavior of the weak links in the YBa2Cu307_ x films above the twin boundaries of the N d G a O 3 bicrystal substrate might be responsible for this difference. Nevertheless, in the whole temperature range from 4 to 78 K the I - V curves and the small-signal response of YBa2Cu3OT_ x grain-boundary junctions fabricated on 2 × 12 ° (110) N d G a O 3 bicrystal substrates can be described in terms of the RSJ model. The linewidth of the millimeter-wave Josephson radiation was found to be determined by thermal fluctuations and to be as low as 1 GHz at 77 K. W h e n the junctions were irradiated with high-intensity 94 GHz radiation, current steps were observed at the 1 - V curves at voltages up to 6.5 inV. T h i s shows that, in spite of their
Y.Y. Divin et al./ Physica C 256 (1996) 149-155
electrical inhomogeneities, YBa2Cu307_ x grainboundary junctions on (110) NdGaO 3 bicrystals are very promising candidates for the detection and spectral analysis of millimeter- and submillimeter-wave radiation.
References [1] D. Dimos, P. Chaudhari, J. Mannhart and F.K. LeGoues, Phys. Rev. Lett. 61 (1988) 219. [2] Z.G. Ivanov, P.A. Nillson, D. Winkler, J.A. Alarco, T. Claeson, E.A. Stepantsov and A.Y. Tzalenchuk, Appl. Phys. Lett. 59 (1991) 3030.
155
[3] H.B. Lu, T.W. Huang, J.J. Wang, J. Lin, S.L. Tu, S.J. Yang and S.E. Hsu, IEEE Trans. Appl. Supercond. 3 (1993) 2325. [4] P.G. Quincey, Appl. Phys. Lett. 64 (1994) 517. [5] A.V. Andreev, Yu.Ya. Divin, V.N. Gubankov, I.M. Kotelyanskii, V.B. Kravchenko, S.G. Zybtsev, E.A. Stepantsov, Physica C 226 (1994) 17. [6] R. Unger, T.A. Scherer, W. Jutzi, Z.G. Ivanov and E.A. Stepantsov, Physica C 241 (1995) 316. [7] U. Poppe, N. Klein, U. DS.hne, H. Solmer, C.L. Jia, B. Kabius, K. Urban, A. Lubig, K. Schmidt, S. Hensen, S. Orbach, G. Miiller and H. Piel, J. Appl. Phys. 71 (1992) 5572. [8] Yu.Ya. Divin and P.M. Shadrin, Physica C 232 (1994) 257. [9] K.K. Likharev, Dynamics of Josephson Junctions and Circuits (Gordon and Breach, New York, 1986). [10] Yu.Ya. Divin, O.Y. Polyanskii and A.Ya. Shul'man, IEEE Trans. Magn. 19 (1983) 613.
Physica C 256 (1996) 149-155
Y B a 2 C u 3 0 7 _ x thin-film Josephson junctions on 2 × 12 ° bicrystal
(110) N d G a O 3 substrates Y.Y. Divin a,b,*, H. Schulz a, U. Poppe a, N. Klein a K. Urban a, P.M. Shadrin b, I.M. Kotelyanskii b, E.A. Stepantsov c a Institutffir Festkbrperforschung, Forschungszentrum Jfilich GmbH, 52425 Jiilich, Germany b Institute of Radioengineering and Electronics, Russian Academy of Sciences, Mokhovaya 11, Moscow 103907, Russian Federation c Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow 117333, Russian Federation
Received 23 August 1995; revised manuscriptreceived 20 September 1995
Abstract Grain-boundary YBa2CU3OT_x Josephson junctions with normal-state resistances from 0.1 to 10 l"~ were fabricated on (110) NdGaO 3 bicrystal substrates with a misorientation angle of 2 × 12°. Electrical imaging of the junctions in the temperature range 80 K-300 K was performed by a technique based on laser scanning microscopy. The average values of the electrical transport parameters of the fabricated junctions were shown to be comparable to those of reference junctions made on (100) SrTiO 3 bicrystals. Values of the Josephson linewidth as low as 1 GHz at 77 K have been obtained from the response of the junctions to low-intensity 9 4 G H z electromagnetic radiation. When high-intensity 94 GHz radiation was applied to the junctions, current steps appeared in the I - V curves at voltages V, = n h f / 2 e up to 6.5 mV. These results demonstrate the applicability of YBa2Cu3OT_ x grain-boundary junctions made on NdGaO 3 bicrystal subStrates for millimeter- and submillimeter-wave detection.
1. Introduction A study of the electrical transport properties of grain boundaries in high-Tc materials i s of interest both for fundamental and applied reasons. The boundaries between grains of different orientation are k n o w n to reveal lower critical current densities compared with those of the adjacent grains. For sufficiently large misorientafion a n g l e s the grain boundaries were shown to demonstrate Josephson
* Corresponding author.
behavior. In order to fabricate high-T~ grain-boundary junctions in a controllable w a y , an epitaxial growth of high-T~ thin films on bicrystal substrates was suggested [1]. For this purpose different kinds of bicrystal substrates with simple cubic structures like SrTiO 3, YSZ and MgO were used [1-3]. Recently, NdGaO 3 bicrystals have attracted attention in this field because of the much better lattice matching between NdGaO 3 and high-T~ materials. However, up to now only a few high-Tc grain-boundary junctions have been fabricated on NdGaO 3 bicrystals [4-6] and only little is k n o w n about their electrical properties. Here, we present the results of our study of integral and local electrical characteristics of
0921-4534/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00638-9
150
Y.Y. Divin et al./Physica C 256 (1996) 149-155
Y B a / C u 3 0 7 _ x grain-boundary junctions made on (110) NdGaO 3 bicrystals in connection with their potential applications in millimeter- and submillimeter-wave detection and spectroscopy.
2. Experimental techniques The mutualarrangement of the main orientations in both parts of the (110) NdGaO 3 bicrystal substrates is demonstrated in Fig. 1. The dimensions of the substrates were around 10 × 10 × 0.8 m m 3. They were fabricated by a thermal=intergrowing technique similar to that used for the fabrication of (001) NdGaO 3 bicrystals [5]. The junctions made previously on 2 × 18.4 ° (001) NdGaO 3 bicrystals showed low critical current densities, which limits their operation to the low-temperature range [5]. To increase this parameter, a lower misorientation a n g l e Of 2 × 12 ° was used in this study. Moreover, the (110) plane was chosen as the main plane for deposition of the YBa2Cu307_ x thin films because of a better matching to the parameters for the high oxygen pressure sputtering deposition [7] used in this work. A network of (110) and (112) twin boundaries was observed in the vicinity of the artificially made grain boundary of these bicrystals by optical measurements in polarized light. The deposition of the YBa2CU3OT_ x thin films was made by DC sputtering from a stoichiomeVic target at high oxygen pressure [7]. Sputtering parameters were optimized to obtain high-quality c-axis oriented thin films on the single-crystal (110) NdGaO 3 substrates. The distance between target and
Fig, 1. (110) NdGaO3 bicrystal subslrate with a misorientation angle 2or.
substrate was around 11 mm, the oxygen pressure was in the range of 3.1 to 3.2 mbar and the current was kept at values of 150-200 mA. By employing a very thin thermocouple placed inside a hole of a dummy substrate it w a s found that during sputtering the temperature of the (110) NdGaO 3 substrate just lying on the heater was 780-790°C, i.e. about 140°C lower than the heater-plate temperature. The sputtering rate was about 9 0 - 1 0 0 n m / h under these conditions. After film deposition the chamber was backfilled with oxygen to atmospheric pressure at 550°C and cooled down to room temperature within about 30 min. The film thickness was i n t h e range of 100-300 nm. Patterning of the fabricated thin films was performed by UV photolithography and wet etching in a 0.3% bromium ethanol solution. Polymethyl methacrYlate was used as a resist in this procedure and a solution of methyl isobutyl ketone in propanol was used as a developer. Sets of YBa2Cu3OT_ x thin-film bridges of 1 to 50 ixm widths were prepared across the 2 × 12 ° grain boundary of the (110) NdGaO 3 bicrystal substrates. As reference samples, YBa2Cu307_ x thin-film grain-boundary junctions were fabricated by a similar Procedure on (100) SrTiO 3 bicrystal substrate with the same misorientation angle of 2 c~ = 2 × 12 °. The deposition temperature in this case was held about 40°C lower compared to that used for NdGaO 3 bicrystal substrates. The inhomogeneity of local electrical properties of the fabricated grain-boundary junctions was char= acterized by an imaging technique based on laser scanning microscopy as was described previously [8]. The junction was placed into the cold stage of an optical microscope and the 2D spatial dependence of the laser-induced voltages on the sample were recorded at temperatures from 80 to 300 K. Two imaging modes were used. In the first mode, no bias current was applied to the junction and measure i ments were made at room temperature. In the second mode, the temperature was below the critical temperature of the YBa2Cu3OT_ x thin films and some bias current was applied. The DC parameters of the grain-boundary junctions were measured by a four-probe technique at temperature from 5 to 300 K. Samples were put into the sample holder which allows spring contacts to be made to all junctions on the same substrate, and it
151
Y.Y. Divin et al./Physica C 256 (1996) 149-155
was possible to measure all of them during one temperature run from r o o m to liquid-helium temperatures. The sample holder was sealed and helium was introduced into it as an exchange gas. All measurements were performed in transport helium and nitrogen dewars. The Josephson behavior of the fabricated junctions was studied by their interaction with millimeter-wave radiation. A 94 G H z Gunn oscillator was used as a source of radiation. Millimeter-wave radiation was directed to the cryogenic set-up through waveguides, an isolator, a modulator and a directreading attenuator. The radiation reaching the sample induced a change of the I - V curve, and the corresponding voltage response A V ( V ) at constant current bias was measured b y a conventional lock-in technique with a modulation of the radiation intensity at frequencies of about 1 kHz. A similar lock-in technique was used for the measurements of the differential resistance d V f d I of the junctions. The coupling between millimeter-wave radiation and the Josephson junction in the sample holder h a s been made sufficiently w e a k in these experiments to guarantee the operation of the Josephson detector in the small-signal l i m i t [9] and to reduce the effect of background radiation on the electrical characteristics of the junctions. For the determination of the Josephson linewidth an indirect technique was used which is based on the detector response of the Josephson junction to low=intensity microwave radiation [9,10].
$ GB ____>
<__-
--->
-1.25 p.V
0
-_+
+-
-+
6--
-10 gW
0
3. Imaging of electrical inhomogeneities in YBa2Cu3OT- x grain-boundary junctions The 2D spatial distributions of voltages A V ( x , y ) i n d u c e d b y the f o c u s e d l a s e r b e a m in a Y B a 2 C u 3 0 7 _ x thin-film bridge crossing an artificially made grain boundary on a N d G a O 3 bicrystal substrate are presented in Fig. 2. The 20 p.m wide thin-film bridge is placed horizontally in Fig. 2 and its edges are indicated by the arrows. The grain boundary is situated vertically in the middle o f each image and its position is indicated by an arrow and the sign GB. A grey scale p r e s e n t a t i o n of voltages A V ( x , y ) is used with white colour corresponding to m a x i m u m positive and black colour to m i n i m u m negative signals.
+1.25 ~tV
+10 rtV
<---
--+
-10 ~V
6--
0
+10 vLV
Fig. 2. The 2D spatial distributions of the laser-beam-induced voltage response of a 20 Ixm wide YBa2Cu307_ x thin-film bridge made on a twinned 2× 12° (110) NdGaO3 bicrystal substrate: (a) T= 300 K and bias current I b = O, (b) T = 87 K, I b = 0.1 mA, (c) T = 84 K, I b = 0.1 mA.
152
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The room-temperature zero-bias image (Fig. 2(a)) demonstrates an intensive vertical double white-black structure located along the artificially made grain boundary in the bicrystal substrate. Also, similar, but weaker structures are observed in the thin film along the twins in the substrate. One can clearly see the electrical inhomogeneity of the thin-film junction along the bicrystal boundary. In Fig. 2(b) the electrical image of the same area of the thin-film bridge is presented, but the sample was held at a temperature T of 87 K and biased at a current I of 0.1 mA. The sample temperature is below the critical temperature T~ and the critical current of the YBa2Cu307_ x thin film is higher than 0.1 mA at this temperature. The thin-film regions above the artificially made grain boundary and above the twins demonstrate responses of positive polarity. The response is proportional to the laser-beam intensity and the bias current. In spite of the difference in response symmetry across the weak parts of the film in Figs. 2(a) and (b), the general arrangements of the response-producing areas are the same in zero-bias room-temperature and finite-bias low-temperature images. The electrical image presented in Fig. 2(c) is measured at the same bias as the image in Fig. 2(b), but at a lower temperature of 84 K. The weak parts of the films near the substrate twins disappeared at this temperature due to their comparatively larger critical currents. The distribution along the grain boundary at this temperature is qualitatively the same as in the zero-bias room-temperature image (Fig. 2(a)) and the maximum intensity of the response at low temperature corresponds to the most intensive response in the zero-bias image. The degree of inhomogeneity of grain-boundary junctions made on NdGaO 3 bicrystal substrates was larger than that of reference junctions made on SrTiO 3 bicrystals. The main reason is to be found in a difference of the crystallographic structures of these two substrate materials. While SrTiO 3 is a cubic material, NdGaO 3 has an orthorhombic crystal structure. Due to orthorhombicity and a corresponding anisotropy of the thermal-expansion coefficients along the main crystallographic directions of NdGaO 3, the formation of twins in NdGaO 3 bicrystals is possible at the fabrication stage, and, additionally, the NdGaO 3 bicrystal substrates can be bent during cooling from film deposition temperature to
Table 1 Parameters of YBazCu307_, thin-film grain-boundary junctions Substrate Misofientation 1c × Rn(5 K) R n × A jc(5 K) angle [mV] (~) X cm 2) ( A / c m 2) (110) 2×12 ° NdGaO 3 (100) 2 x 12° SrTiO 3
3
3 X l O -8
1×10 5
2.5
6 x 10 -9
4 × 105
lower temperatures [4], thus introducing an inhomogeneous local strain to the grain-boundary junctions. A more refined technology of NdGaO 3 bicrystal preparation might be required to avoid the twins.
4. I - V characteristics
Due to the higher inhomogeneity, the Josephson junctions made on 2 X 12° (110) NdGaO 3 bicrystal substrates showed a higher degree of parameter spread compared to the junctions fabricated on SrTiO 3 bicrystals with the same misorientation angle. In Table 1 we present the average values of the main electrical parameters for both types of YBa2Cu307_ x grain-boundary junctions. The average values of the I c R n product are comparable for both kinds of junctions. The specific resistance R n A of the junctions made on NdGaO 3 bicrystal substrates were usually several times higher than those for the junctions made on SrTiO 3 bicrystals and, conversely, the critical current densities Jc were several times lower. The former circumstance is responsible for higher resistances R n up to 10 ~ , which can be obtained for the junctions on 2 X 12° (110) NdGaO3 bicrystals. In Fig. 3 we present the I - V curves of a YBa2Cu307_ x junction on a (110) NdGaO 3 bicrystal in the temperature range from 5 to 82 K. Especially at low temperatures, the shape of the I - V curves was close to that predicted by the RSJ model [9]. The I - V curves at higher temperatures only demonstrate an RSJ-like shape at low voltages. At higher voltages flux-flow behavior is observed. The latter may be attributed to the junctions arranged on the twin boundaries of the NdGaO 3 substrates. For comparison the I - V curves of a YBaECU307_ x junction on a SrTiO 3 bicrystal are shown in Fig. 4.
Y.Y. Divin et al./Physica C 256 (1996) 149-155
Flux-flow behavior was not observed in these junctions even at higher voltages than for junctions made on N d G a O 3 substrate. Additionally, the normal-state resistance o f junctions made on 2 × 12 ° N d G a O 3 bicrystal substrates showed a decrease of about 30% with a decrease o f temperature from 80 K to 5 K. In contrast, the normal-state resistance increases for junctions made on 2 × 12 ° SrTiO 3 substrates (see Fig. 4). To obtain the values of the critical current I c and the normal-state resistances R n we used the fitting o f the experimental I - V curves from RSJ model calculations. A quasi-linear temperature dependence of the critical current I c was found for the junctions. The IcR n product at 77 K was about 10 to 20 times smaller than that at 5 K.
15
153
i
,
,
10 5L
/
-5 -10 -15
T=5K 48K 78 K
-8
I
I
I
-6
-4
-2
/ 0
I
i
i
2
4
6
8
VOLTAGE (mV) 5. M i l l i m e t e r - w a v e d e t e c t i o n grain-boundary junctions
by YBa2Cu307_
x
Due to the high values of the I c R n product and the low dielectric constant of the substrate, the YBa2Cu3OT_ x grain-boundary junctions made on N d G a O 3 bicrystal substrates are considered to be very sensitive detectors for electromagnetic radiation in a wide spectral range from the millimeter- to the
-A
-2
6
0
2 •
i
4
,
6
T~)= 5 ~
~
2
9 46"4
~
j
j
Z
0
-4 -6
61 . 77:_ 2
2
-S i
82
0 -2 -4
1
-2
0
I
2
4
-6
VOLTAGE [mV] Fig. 3. I - V characteristics of a YBa2Cu307_ x grain-boundary junction made on a 2 × 12° (110) NdGaO 3 bicrystal substrate.
Fig. 4. I - V characteristics of a reference YBa2CU3OT_ x grainboundary junction made on a 2 × 1 2 ° (100) SrTiO 3 bicrystal substrate.
submillimeter-wave range. The detection of millimeter-wave electromagnetic radiation by the fabricated grain-boundary junctions was studied in the temperature range from 5 to 78 K, where, according to the results o f the DC characterization and the study o f spatial inhomogeneities, the contribution of the weak links arranged on the twin boundaries to the transport properties o f junctions is not significant. The possibility of reaching very low values of noise equivalent power (NEP ~ 10-14 W / H z J/2) in the millimeter-wave range has already been demonstrated with Josephson junctions made on N d G a O 3 bicrystal substrates [5]. In our work we concentrated on the possibility of wideband spectroscopic application [10] o f these junctions. At first, the effect o f high-intensity millimeter-wave radiation on the fabricated junctions was studied. In Fig. 5(a) the I - V characteristics o f a 3 Ixm wide junction at a temperature of 5 K are presented with and without applied high-intensity 94 GHz radiation. The normal-state resistance R n o f the junction extracted from this curves is about 7 ~ . Due to the high-intensity radiation, current steps appeared at the I - V curves at voltages Vn = n h f / 2 e. This can be seen more clearly in Fig. 5(b), where the differential resistance d V / d I
154
Y.Y. Divin et aL /Physica C 256 (1996) 149-155
of the junction is plotted as a function of voltage. The appearance of steps up to values of 6.5 mV shows the absence of Joule heating in the fabricated high-T~ junctions at voltages corresponding to frequencies up to the end of the submillimeter-wave range. This result combined with the high value of the IcR n product and the low dielectric constant of the N d G a O 3 substrates demonstrates the potential high-frequency applicability of the fabricated junctions. The response of a grain-boundary junction with a normal-state resistance of R n = 0.25 12 to low-intensity radiation at 77 K is shown in Fig. 6. The m a x i m u m of the response at low voltages corresponds to the suppression of the critical current Ic by external radiation. The characteristic odd-symmetric resonance at voltages V -.~ h f / 2 e is due to the interaction of the external monochromatic radiation with the Josephson radiation. The absence of resonances at voltages Vn = n h f / 2 e demonstrates that the response is measured at a sufficiently low intensity of the external radiation. The linewidth g f of the Josephson radiation can be calculated from the volt-
-('a)
1,5
'
'
i
'
i
i
i
i
i
i
3
0,6
2
0,4
1
0,2
0
0,0
F
-1
m
-0,2 2
-2
-0,4
-3
-0,6 i
0
100
200
300
400
500
VOLTAGE [~tV] Fig. 6. The I - V curve and voltage response to low-intensity 94 GHz radiation for a YBa2Cu3OT_ x grain-boundary junction made on a 2 × 12° (110) NdGaO 3 bicrystal subslrate. T = 77 K.
age separation ~ V = ( h / 2 e ) ~ f between the minim u m and the m a x i m u m of the resonant response. W e obtained a linewidth a s small as 1 GHz, which is equal to the value determined by thermal fluctuations at 77 K [9].
6. C o n c l u s i o n s
1,0 0,5 i
0,0 -0,5
r,.) -1,0 -1,5 -8
I
I
r
-6
-4
-2
0
2
I
I
4
6
VOLTAGE [mV] 25
i
I
'
I
I
I
'
20
10 5 0
-8
-6
-4
-2
0
I
'
I
!
2
VOLTAGE [mV] Fig. 5. The I - V characteristics (a) and differential resistance vs. voltage (b) of a YBazCu3OT_x grain-boundary junction made on a 2X 12° (110) NdGaO3 bicrystal substrate without (1) and with (2) high-intensity 94 GHz electromagnetic radiation at T = 5 K.
Y B a 2 C u 3 0 7 _ x grain-boundary junctions made on (110) N d G a O 3 bicrystal substrates are more inhomogeneous and exhibit the Josephson effect in reduced temperature and voltage ranges compared with junctions made on (100) SrTiO 3 bicrystals with the same misorientation angle. The flux-flow behavior of the weak links in the YBa2Cu307_ x films above the twin boundaries of the N d G a O 3 bicrystal substrate might be responsible for this difference. Nevertheless, in the whole temperature range from 4 to 78 K the I - V curves and the small-signal response of YBa2Cu3OT_ x grain-boundary junctions fabricated on 2 × 12 ° (110) N d G a O 3 bicrystal substrates can be described in terms of the RSJ model. The linewidth of the millimeter-wave Josephson radiation was found to be determined by thermal fluctuations and to be as low as 1 GHz at 77 K. W h e n the junctions were irradiated with high-intensity 94 GHz radiation, current steps were observed at the 1 - V curves at voltages up to 6.5 inV. T h i s shows that, in spite of their
Y.Y. Divin et al./ Physica C 256 (1996) 149-155
electrical inhomogeneities, YBa2Cu307_ x grainboundary junctions on (110) NdGaO 3 bicrystals are very promising candidates for the detection and spectral analysis of millimeter- and submillimeter-wave radiation.
References [1] D. Dimos, P. Chaudhari, J. Mannhart and F.K. LeGoues, Phys. Rev. Lett. 61 (1988) 219. [2] Z.G. Ivanov, P.A. Nillson, D. Winkler, J.A. Alarco, T. Claeson, E.A. Stepantsov and A.Y. Tzalenchuk, Appl. Phys. Lett. 59 (1991) 3030.
155
[3] H.B. Lu, T.W. Huang, J.J. Wang, J. Lin, S.L. Tu, S.J. Yang and S.E. Hsu, IEEE Trans. Appl. Supercond. 3 (1993) 2325. [4] P.G. Quincey, Appl. Phys. Lett. 64 (1994) 517. [5] A.V. Andreev, Yu.Ya. Divin, V.N. Gubankov, I.M. Kotelyanskii, V.B. Kravchenko, S.G. Zybtsev, E.A. Stepantsov, Physica C 226 (1994) 17. [6] R. Unger, T.A. Scherer, W. Jutzi, Z.G. Ivanov and E.A. Stepantsov, Physica C 241 (1995) 316. [7] U. Poppe, N. Klein, U. DS.hne, H. Solmer, C.L. Jia, B. Kabius, K. Urban, A. Lubig, K. Schmidt, S. Hensen, S. Orbach, G. Miiller and H. Piel, J. Appl. Phys. 71 (1992) 5572. [8] Yu.Ya. Divin and P.M. Shadrin, Physica C 232 (1994) 257. [9] K.K. Likharev, Dynamics of Josephson Junctions and Circuits (Gordon and Breach, New York, 1986). [10] Yu.Ya. Divin, O.Y. Polyanskii and A.Ya. Shul'man, IEEE Trans. Magn. 19 (1983) 613.