Development of ramp-edge sns junctions using highly

3374

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, No. 2, JUNE 1999

Development of Ramp-Edge SNS Junctions Using Highly Stable Normal-Metal Barrier Materials Q. X. Jia, Y. Fan, C. Kwon, C. Mombourquette, D. Reagor, and R. Cantor Superconductivity Technology Center, MS K763, Los Alamos National Laboratory, Los Alamos, NM 87545 J. P. Zhou, Y. Gim', C. Jones, J. T. McDevitt, and J. B. Goodenough Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX 78712

Abstract - By using a cation-modified and corrosion-resistant compound of CPr,Gdo.6.y)Cao.4Ba,.~Lao.4Cu~0, (y = 0.4, 0.5, and 0.6) as normal-metal barrier materials, high-temperature superconducting Josephson junctions have been fabricated in a ramp-edge superconductorhormal-metalhuperconductor (SNS) configuration. We have tuned the Pr substitution level in order to achieve the optimal electrical resistivity of the barrier layer for high-performance SNS junctions. The junctions fabricated with these normal-metal barriers show well-defined RSJ-like current vs voltage characteristics at liquid-nitrogen temperature. The junction performance is mainly controlled by the N-layer instead of the interface. We have also fabricated dc superconducting quantum interference devices based on rampedge SNS technology with these normal-metal barriers. The ratio of peak-to-peak voltage modulation of the superconducting quantum interference devices to the I,R, product is more than 30%.

width of 20 pm tested over a period of more than 500 days. For robust ramp-edge SNS junctions, it is necessary to explore normal-metal materials that are not only structurally and chemically compatible with superconducting electrodes but also environmentally stable if Ag-doped YBCO is used a!< the superconducting electrode [ 111.

J

0.1

>

A

E

W

gb 0.0 a

Y

I

0

I. INTRODUCTION The ramp-edge high-temperature superconductorhormalmetal/superconductor (SNS) junction based on the proximity effect is one of the most promising technologies to fabricate reproducible and controllable high-temperature superconducting Josephson junctions and/or related devices [ 1-61. Recently, this technology has been advanced greatly for both digital applications [7,8] and the fabrication of very sensitive magnetic sensors [4-6,9]. For practical applications, it is essential that the devices hnction properly despite multiple thermal cycles and exposure to humidity. To achieve these requirements, environmentally stable superconductor electrodes and normal-metal barrier layer are needed. It has been shown that Ag-doped YBaZCu3O7 (YBCO) provides superior environmental stability compared to pure YBCO [lo]. We have also found that Ag-doped YBCO superconducting electrodes show little sign of degradation in air. Fig. 1 shows the current vs voltage characteristic of a Ag-doped YBCO electrode with a bridge Manuscript received September 14, 1998. This work was supported by the United States Department of Energy as a Los Alamos National Laboratory Directed Research and Development Project. The research in the University of Texas at Austin was supported by Offife Naval Research and Institute of Applied Technology, Now in Superconductivity Technology Center, Los Alamos National Laboratory.

10.51-8223/99$10.00

-0.2 I -300

O

-200

I

I

I

-100 0 100 current (mA)

I

1

200

300

Fig. 1 Current vs voltage characteristic of a Ag-doped YBCO electrode on a LaAlOl substrate with a bridge width of 20 pm tested (both solid-line and open circle) over a period of more than 500 days. The Ag-doped YBCO electrode shows little sign of degradation in air.

There have been many studies of normal-metal barrier materials for ramp-edge SNS junctions. The most commonly investigated normal-metal materials are those derived from YBCO compounds such as PrBaZCu3O7 [1,4-61 arid Y B ~ Z C U ~ . ~ ~ C[7,8]. O O . ~These ~ O ~ materials provide a close structural match with the superconductor YBCO. They are also chemically compatible with the electrode. However, these normal-metal materials, like the pure YBCO compound, also suffer from environmental instability. It is known that decomposition of YBCO and related compounds occurs when exposed to the atmosphere due to reactions with CO, COz, and H20 [12]. Here we report our effort to explore the environmentally stable normal-metal materials such as (Pr,Gdo.6-y)Cao.~BBai.6Lao.4CU307 (y = 0.4, 0.5, and 0.6) for ramp-edge SNS Josephson junctions. This compound possesses high corrosion resistance in water and is structurally compatible with YBCO electrodes.

0 1999 IEEE

337s

11. EXPERIMENTAL DETAILS

28 angle from the (006) diffraction peak (using the angle from LaA103 (200) diffraction as a reference) is 1.171 nm for

Thin films Of (PryGd0.6-y)Ca0.4Bal.6La0.4CU307 (y = 0.4, Pro.6C~.4Bal.6La0.4C~307. The lattice constant for a- or b0.5, and 0.6) were prepared by pulsed laser deposition (PLD) axis is 0.3836 nm, slightly smaller than the value reported for on LaA103 substrates for electrical and corrosion studies. the bulk materials [ 1 5 ] . The double-layer of The PLD used a 308 nm XeCl excimer laser producing 20 ns YBCO/(PryG&.6.y)Cao.4Ba~.6La0.4CU307 shows the same outpulses with an energy density of 2 Jlcm'. The depositions plane and in-plane diffraction patterns as that of single-layer were done at a temperature of 775 "C and an oxygen pressure (PryGdo.6.y)Cao.4Bal.6La0.4CU307 andlor YBCO. This is of 200 mTorr. These conditions were the same as used for expected because (PryGd0.6-y)Cao.4Ba~.6La0.4C~307 itself is the deposition of the Ag-doped YBCO electrodes. structurally compatible with the YBCO electrode. The structural properties such as lattice constant and film Fig. 2 shows the temperature dependence of the resistivity films were orientation of the (PryG&.6-y)Ca0.4Ba1.6La0.4C~307 ( ~ 0 . and 5 0.6) thin for the (PryGdo.6.y)Ca0.4Bal.6La0.4C~307 investigated by x-ray diffraction (XRD) from 28- and @ films. These films had a thickness of 50 nm and were grown on LaA103 at 775 "C and an oxygen pressure of 200 mTorr. scans. The electrical resistivities of the normal-metal (PryG&.6-y)Ca,,.4Ba,.6Lao4C~307 films with different Pr- As can be seen from this figure, the resistive behavior of (PryGdo.6-y)Ca0.4Ba~.6La0.4CU307 is a function of Pr content. doping concentrations were determined from four-probe resistivity vs temperature measurements. We have shown that Pr0.4Gdo.2Ca0.4Ba1 .6La0.4cu307 has a In addition to the characterization of the structural and superconducting transition at 20 K [15]. A further increase electrical properties of the (PryGdo.6.y)Ca0.4Ba~.6La0.4CU307of Pr-doping concentration in the film, on the other hand, thin films, we paid attention to the corrosion resistance of the leads to a metal-insulator transition. Compared with the most materials. The corrosion experiments were carried out by commonly used normal-metal layer material such as these materials have unique features soaking the films in aerated water at room-temperature for YBa2C~2.79C00.2107, specific periods of time. The quasi-dc four-probe resistivity such as a lower temperature coefficient of resistance around measurements were acquired as a function of real time. 60 - 80 K and a higher electrical resistivity. Using an optical microscope, we monitored the surface morphologies of the films before and after corrosion tests. 4.50 I I To fabricate ramp-edge SNS Josephson junctions using (PryGdo.6-y)Cao,4Bal,6Lao,4CU307 as a normal-metal barrier, (100) oriented LaA103 wafers (3 inch in diameter), commercially available from Lucent Technologies (AT&T), were diced into 1 x 1 cm2 chips for the substrates. A PLD technique was used to deposit the top and bottom Ag-doped YBCO electrodes, the (PryGdo.6.y)Ca0.4Bal.6La0.4CU307 (y = 0.4, 0.5, and 0.6) N-layer barrier, and an insulating layer of CeOz that was used to electrically isolate the top and the bottom superconducting electrodes. We used conventional photolithography to define the location of the device and ion milling with 250 eV Ar to etch the film and to form the ramp 0.00 edge. The angle between the edge and the substrate surface, 0 50 100 150 200 250 300 as confirmed by atomic force microscopy, was controlled in temperature (K) the range of 15"+ 3" [13]. Detailed fabrication procedures and processing considerations for ramp-edge SNS junctions Fig. 2 Resistivity vs temperature characteristics of normal metal thin films of (Pr,Gdo.~.y)Cao4Ba,.sLaa 4Cu307 on LaAIOl substrates with (a) Pr = have been published elsewhere [14]. The performance of the 0.6 and (b) Pr = 0.5. SNS junctions and dc SQUIDS was characterized from measurements of the current vs voltage characteristics and We have compared the corrosion resistance of the voltage modulation with applied magnetic fields. (PryG~,6.y)C~,4Bal &a&U307 with conventional PBCO and CO-doped YBCO normal-metal barrier materials. Fig. 3 111. RESULTS AND DISCUSSION shows the four-probe voltage or resistivity of the PBCO and films as a function of real the Pro.4G&.2Ca0.4Bal.6La0.4C~307 have a The films of (PryGd0.6-y)Ca0.4Bal.6La0.4CU307 time. For such an experiment, we used a current of 5x10-' A tetragonal structure with a very good lattice match to the and the voltage cross the film was recorded. Increase in the YBCO electrode. A slight increase in lattice constant with four-probe resistivity (or the voltage) of the film was used to increasing Pr-doping concentration is noted [ 151. We have judge the rate of corrosion. The resistivity of the PBCO found that thin films of (PryGdo.6.y)C~.4Bal,6L~.4cU~07 (y = andor CO-doped YBCO (not shown over here) increases 0.4, 0.5, and 0.6) deposited at 775 "C on LaA103 substrates with time monotonically indicating the less corrosion are single phase with the c-axis normal to the substrate resistance for these conventional normal-metal materials. On surface. The c-axis lattice constant calculated based on the the other hand, the corrosion resistance of the

3316

Pro.4Gdo.~Ca0.4Ba~.6La0.4C~307 increases substantially. The (PryGdo.~.y)C~.4Ba~,6L~,~~u3~~ with other Pr doping

concentrations shows the similar corrosion resistance characteristic as Pro.4G&.~Cao.4Ba~ .6L%.4Cu307.

Ea 0.6 o.8 M

0

2 0.4

t

the flux back and forth. The voltage modulation of the SQUID is a function of the bias current as expected. The maximum voltage modulation is achieved at a bias current slightly higher than the critical current of the SQUID. We would like to point out that the thickness of the normal-metal layer has not yet been optimized in this experiment, which can lead to a relatively low I,R, product. However, the ratio of the peak-to-peak voltage modulation (V,,) of our SQUIDs to the I,R, product is more than 30%. 10

Y

0.2

0.0

0

20

40 60 time (hours)

80

100

Fig. 3 Four-probe voltage or resistivity of the PBCO (a) and Pro.4GQ,2C~.4Bal.~La,,.4Cu307 (b) films in aerated water at roomtemperature as a function of real time.

01

magnetic flux The SNS junctions fabricated using N-layer materials of Fig. 5 Bias-current dependence of the voltage modulation of a dc SQUID ( P r y G ~ . ~ ~ y ) C ~ . ~ Bshow a l ,resistively ~ L ~ , ~ ~shunted u~~7 barrier and operated at made with a Pro.4Gdo.2C~.4Bal.~Lao.4Cu307 junction (RSJ)characteristic under dc bias at liquid nitrogen liquid nitrogen temperature. The thickness of the barrier layer is 22 nm. temperature (75 K). Fig. 4 shows typical I-V characteristics of two junctions with different normal-metal Pro.4Gdo.zCao.4Bal.6La0.4CU307 barrier thicknesses measured VI. SUMMARY at 15 K. We have identified a class of new compounds of (PTyGd0.6-y)Ca0.4Bal.aL~.4CU307 (~0.4, 0.5, and 0.6) that is

0.25 0.20

-

0.15

-

t: 0.10

-

h

W

a3

M Q

0

N-layer

- 22 nm

suitable for the normal-metal barrier in ramp-edge SNS junctions. Films of (PryG~.6-y)C~.4Bal.6L~.4C~307 a.re environmentally stable in air and structurally and chemically compatible with superconducting YBCO electrodes. 13y combining Ag-doped YBCO electrodes and these normalmetal bamer materials, we expect to be able to fabricate ve:ry stable high-temperature superconducting electronic devic:es for a wide range of applications.

r

0.05

-

0.00 0.0

REFERENCES 0.5

1.0 1.5 current (mA)

2.0

2.5

Fig. 4 Current vs voltage characteristics for two ramp-edge SNS junctions barrier with different normal-metal Pro.~Gd0.~Cao4Ba~.~Lao4Cu3O7 thicknesses.

The SQUIDs based on this ramp-edge SNS technology with (PryGdo,6-y)Cao.4Ba,.6La0.4C~307 normal-metal barriers also show well-defined voltage modulation. Fig. 5 shows the typical voltage modulation vs magnetic flux characteristic of a SQUID for different current bias levels. The curve is perfectly periodic and shows no hysteresis while sweeping

J. Ciao, W. A. M. Aarnink, G. J. Gerritsma, and H. Rogalla, “Controlled preparation of all high T, SNS-type edge junctions and DC SQUIDs,” Physica C, Vol. 171, pp. 126-130, October 1990. B. D.Hunt, M. C. Foote, and L. J. Bajuk, “All high Tc edge-geometry weak links utilizing Y-Ba-Cu-0 barrier layers,” Appl. Phys. Lett., ’dol. 59, pp. 982-984, August 199I . K. Char, M. S. Colclough, T. H. Geballe, and K. E. Myers, “High Tc superconductor-normal-superconductor Josephson junctions using CaRu03 as the metallic barrier,” Appl. Phys. Lett., Vol. 62, pp. 1.96198, January 1993. M. I. Faley, U. Poppe, K. Urban, H. Hilgenkamp, H. Hemmes, W. Arnik, J. Flokstra, and H. Rogalla, “Noise properties of direct-current SQUIDS with quasiplanar YBa2Cu307Josephson junctions,” Appl. Phys. Lett., Vol. 67, pp. 2087-2089, October 1995.

3377 Q. X. Jia, X. D. Wu, D. W. Reagor, S. R. Foltyn, C. Mombourquette, and D.E. Peterson, “Edge-geometry dc SNS SQUIDS using Ag-doped YBa2Cu30,-x electrodes,” Electron. Lett., Vol. 32, pp. 499-501, February 1996. M. Schilling, S . Krey, and R. Scharnweber, “Biomagnetic measurements with an integrated YBa2Cu307magnetometer in a handheld cryostat,” Appl. Phys. Lett., Vol. 69, pp. 2749-2751, October 1996. B. D. Hunt, M. G. Forrester, J. Talvacchio, J. D.McCambridge, and R. M. Young, “High-Tc superconductor/normal-metal/superconductor edge junctions and SQUIDSwith integrated groundplanes,” Appl. Phys. Lett.,Vol. 68, pp. 3805-3807, June 1996. W.H. Mallison, S. J. Berkowitz, A. S. Hirahara, M. J. Neal, and K. Char, “A multilayer YBa2Cu,0, Josephson junction process for digital circuit applications,” Appl. Phys. Lett., Vol. 68, pp. 3808-3810, June 1996.

[II]

[12]

[13]

[14]

Q. X. Jia, Y. Fan, C. Mombourquette, and D. W.Reagor, “Directly coupled direct current superconducting quantum interference device magnetometers based on ramp-edge Ag:YBazCu30,..lPrBa2Cu,07.,I [15] Ag:YBa,Cu,O,., junctions,” Appl. Phys. Lett., Vol. 72, pp. 3068-3070 June 1998. [lo] D. Kumar, P. R. Apte, R. Pinto, M. Sharon, and L. C. Gupta, “Aging effects and passivation studies on undoped and Ag-doped YBa2Cu,0,

laser ablated thin films,” J. Electrochem. Soc., Vol. 141, pp. 16111616, June 1994. Q. X. Jia, D. Reagor, C. Mombourquette, Y. Fan, J. Decker, and P. D’ Alessandris, “Stability of dc superconducting quantum interference devices fabricated using ramp-edge superconductorlnormal-metall superconductor technology,” Appl. Phys. Lett., Vol. 71, pp. 1721-1723, September 1997. J. M. Rosamilia, B. Miller, L. F. Schnemeyer, J. V. Waszczak, and H. M. O’Bryan, “Aqueous electrochemistry of cuprate-based high-Tc superconductors,” J. Electrochem. Soc., Vol. 134, pp. 1863-1864, July 1987. Q. X. Jia, D. W. Reagor, S. R. Foltyn, M. Hawley, C. Mombourquette, and X. D. Wu,“Superconducting YBa,Cu,O,, Based Edge Junctions with Yo,7C~.,Ba,Cu,07.,Barriers,” Physica C, Vol. 228, pp. 160-164, July 1994. Q. X. Jia, D.Reagor, H. Kung, M. Hawley, C. Mombourquette, S. R. Foltyn, X. D. Wu, and D. E. Peterson, “High-temperature superconductor edge-geometry SNS junctions with engineered normalmetal layers,” Supercond. Sci. Technol., Vol. 9, pp. 985-990, November 1996. J. P. Zhou, R. K. Lo, J. T. McDevitt, J. Talvacchio, M. G. Forrester, B. D. Hunt, Q. X. Jia, and D. Reagor, “Development of a reliable materials base for superconducting electronics,” J. Mater. Res., Vol. 12, pp. 2958-2975, November 1997.