YBCO-based ramp-edge Josephson junctions and DC SQUIDs with a cubic-YBCO barrier layer

PHYSICA

Physlca C 207 (1993) 203-207 North-Holland

YBCO-based ramp-edge Josephson junctions and DC SQUIDs with a cubic-YBCO barrier layer J.A Agostlnelh, J.M. Chwalek, C.J. Baron, G. Lubberts and C.D. DoweU Corporate Research Laboratories, Eastman Kodak Company, Rochester, NY 14650-2011. USA

Received 12 November 1992 Revised manuscript received 8 January 1993

Recently, the discovery ofa nonsuperconduetlng metastable cubic phase of YBa2Cu307 (YBCO) has been reported Because of the structural, chemical, and deposmonal compatiblhty of cubic-YBCO w~th superconducting YBCO, It is an excellent candidate for use m heteroepltaXudstructures Such structures mvolwng superconducting YBCO/cublc YBCO/superconductmg YBCO ramp-edge proximity-effect Josephson junctions were fabneated on MgO substrates Junctions having cntical current dens~tlesof ~8X 104 A/era2 and/oR, products as high as 0 8 mV at 15 K were produced Shapiro steps were seen m the presence of RF radiation DC SQUIDs were fabncated with these junctions and showed penodic cntical current modulation with apphed magnetic field to temperatmxrs in excess of 77 K

1. Introduction A variety o f approaches to achieve high-temperature Josephson j u n c t i o n devtces have recently been d e v e l o p e d T h e types mclude weak h n k microb n d g e s [ 1 ], controlled grain b o u n d a r y j u n c t i o n s [2], biepttaxial j u n c t i o n s [ 3 ], step-edge j u n c t i o n s [4 ], a n d epitaxtal multtlayers h a v m g artificial barriers [ 5 ] The last category gives perhaps the greatest potenUal control o f j u n c t i o n characteristics Artificial b a m e r layer j u n c t i o n s using the ramp-edge geometry [ 6] offer the use o f c - o n e n t e d films, giving longer coherence lengths along the direction o f the current, and m a k i n g film d e p o s i t i o n a n d device lnterconnection simple Recently, the discovery o f a nonsuperconducting metastable cubic phase o f YBaECU307 ( Y B C O ) has been r e p o r t e d by our group [7] It has been shown that the cublc-YBCO is an excellent lattice match to the superconducting o r t h o r h o m b i c phase o f Y B C O a n d that the two phases grow well on each other [ 8 ] It was suggested that the cubic material might p r o v i d e a good choice as a b a m e r layer m a t e n a l in an all-YBCO Josephson j u n c t i o n [ 7 ] D e s c n b e d hereto are the fabrication a n d characterlzatton o f all-YBCO Josephson j u n c t i o n s a n d D C S Q U I D s on M g O substrates incorporating the cubic-

YBCO material as the barrier layer A l l - Y B C O j u n c tions fabricated on LaA103 substrates usmg an uncharacterized normally conducting Y B C O barrier have been reported [ 9 ]

2. Fabrication The ramp-edge geometry was chosen for our junctions for reasons cited above Smgle crystal MgO was used as the substrate material and the YBCO films were deposited by pulsed laser deposition ( P L D ) using conditions previously reported [ 7,8 ] In o r d e r to achieve a m o r e uniform thickness distribution in the laser deposited films, we e m p l o y e d a 10 ° wedge between the surface o f the rotating target holder and the target itself During rotatton o f a target m o u n t e d in this way, the target n o r m a l sweeps out a circular pattern F o r p r o p e r matching o f the wedge angle, target-to-substrate separation, a n d p l u m e angular distribution pattern, substantially more uniform film thicknesses m a y be achieved A cross section through the r a m p junction is shown schematically m fig 1 ( a ) Device fabrication was begun with a deposition o f a 1500 A thick layer o f Y B C O over the entire substrate with the substrate

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J A Agostmelh et al / YBCO-based ramp-edge Josephson junctsons

Buffer

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Fig 1 Junction geometry (a) SchemaUcof a ramp junction cross section (b) Top-viewphotographof a junction and a SQUID

heater temperature held at 700 °C This was followed by the deposition of a 1000 A thick cublc-YBCO buffer layer at 560°C The two-layer film was cooled slowly m 150 Torr ofO2 with a 30 mln soak at 450°C Next, approximately 5000 A of MgO was deposited on the buffer layer at room temperature using e-beam evaporation m another chamber The motivation for the inclusion of the buffer layer was to provide for a lattice-matched surface beyond the actual junction region along the ramp that will be later formed, for

the subsequent growth of the b a r n e r and upper electrode layers The Inclusion of the buffer layer ehmmates the possiblhty of a defect propagating into the barrier layer and upper electrode from the interface that would exist between the lower electrode and the nonepitaxlal MgO insulator layer Photoresist was then spun onto the three-layer film followed by patterning of the resist to define the lower electrodes The sample with patterned resist was then Ar-lon milled through the entire film thickness and

J A Agosttnelh et al / YBCO-based ramp-edge Josephson juncttons

slightly into the substrate The mIlhng operation defines both the lower electrode areas a n d the r a m p edges A milling b e a m angle o f 55 ° with respect to the surface was chosen to produce a r a m p with a calculated angle o f 19 ° The r a m p angle is significantly different from the ion b e a m angle because o f the effect o f a m o v i n g shadow o f the ion b e a m across the surface as the resist height decreases during mllhng The calculation o f r a m p angle uses experimentally d e t e r m i n e d etch rates as a function o f ion b e a m angle for photoresIst, MgO, and Y B C O together with the effect o f the moving shadow to d e t e r m i n e the final r a m p configuration Cross-sectional microscopy o f the r a m p revealed an angle o f 17 °, in close agreem e n t with the p r e d i c t e d value The r a m p angle is safely below the limit above, which grain b o u n d a r i e s form in Y B C O grown over steps in perovsklte materials, as in our case [ 4,10 ] After milling, the film was stored in nitrogen prior to the d e p o s i t i o n o f subsequent layers Once loaded into the P L D chamber, the patterned film was heated to 700°C in oxygen It has been shown previously that the metastable cubic phase remains cubic following heating to 700°C [8] Next, the substrate heater t e m p e r a t u r e was lowered to 560°C and the cubic-YBCO barrier layer was grown A barrier layer thickness o f ~ 75 A as measured perpendicular to the substrate surface was d e p o s i t e d The substrate heater t e m p e r a t u r e was then increased to 700°C a n d 2000 /k o f Y B C O were d e p o s i t e d to form the u p p e r superconducting electrode layer Following the deposition, the sample was oxygen-annealed in SltU for 30 mln at 450°C a n d cooled slowly The multilayer film was then photolithographically p a t t e r n e d to define the upper electrode areas using Ion milhng Two m o r e p h o t o h t h o g r a p h l c steps followed, the first creating vms in the contact regions o f the lower electrodes to penetrate through the MgO a n d buffer layers a n d the last to define silver pads in all contact areas C o m p l e t e d devices are shown in the mlcrograph m fig 1 ( b ) R a m p j u n c t i o n s having widths as small as 5 ~tm were fabricated D C S Q U I D S having 20 ~tm j u n c t i o n widths and loop areas o f 8100 ~tm 2 w e r e also p r o d u c e d A small area o f the mask p r o v i d e d for a grating p a t t e r n in the device wafer to facilitate microscopic study o f the edge region Each line o f the grating contains a r a m p e d edge having the same structure as the j u n c t i o n regions in the devices

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The finished wafers were diced and c h i p s w e r e m o u n t e d on carriers, wire-bonded, and then tested in a closed-cycle He cryostat The cryostat had provision for the application o f a magnetic field and microwave radiation

3. Characterization The exact nature o f charge transport in the cubicY B C O material is unknown at this time The temperature dependence o f the resistivity indicates semlconductor-hke behavior [7] The room-temperature value o f the resistivity was ~ 0 04 f l cm Increasing to ~ 0 9 f~ cm at 45 K Due to the cubic structure this material is expected to have lsotroplc charge transport properties This is in m a r k e d contrast to artificial barrier j u n c t i o n s employing anisotroplc materials such as P r - B a - C u - O [6] In this case transport is expected to occur mainly along the C u - O planes [6] In our case transport m a y occur mainly in a direction perpendicular to the r a m p This difference in transport direction for the two types o f j u n c t i o n s may lead to different effective barrier thicknesses for the same deposition thickness The validity o f this hypothesis remains to be proven experimentally The results for junctions employing the cubic-YBCO as the barrier layer are discussed below Although junctions with widths as small as 5 ~tm were fabricated with some showing good results, the results for 10 ~tm wide j u n c t i o n s had a higher yield The overall yield o f tested junctions in a ~ 1 cm 2 area was ~ 80% C u r r e n t - v o l t a g e ( l - V ) curves at several temperatures for a 10 p.m wide j u n c t i o n device are shown in fig 2 Deviations from ideal RSJ behavior m a y be due in part to a relatively large barrier thickness in c o m p a r i s o n to the normal metal coherence length (~N) However, since junctions employing various thickness barriers were not measured, ~N lS not known at this time A n o t h e r possibility for this nonideal behavior is self-field effects since the junction widths are not smaller than the Josephson penetratlon depths (2j estimated to be ~ 1 btm at 15 K ) for these junctions Also, n o n u n l f o r m l t y in the barh e r m a y be partly responsible for the non-RSJ beh a v i o r The latter two effects can be seen through the incomplete suppression o f the critical current when a magnetic field is a p p l i e d Junctions with widths

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J A Agostmelh et al / YBCO-based ramp-edge Josephson junctions

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smaller than 2j may yield better results Although rough scahng of I~ wtth area was seen, the possibility of j u n c t i o n behavior a r n v m g from gram boundaries cannot be completely excluded [ 10 ] Crmcal currents (Ie) for this j u n c U o n range from ~ 4 m A at 15 K to ~ 0 26 m A at 77 K The corresponding critical current densities ( a s s u m m g a uniform j u n c t i o n current) vary from ~ 8 × 104 A / c m 2 at 15 K to ~ 5 × 10 3 A / c m 2 at 77 K The n o r m a l resistance ( R . ) defined m this case as the slope of the l - V c u r v e far from the orlgm ranges from ~ 0 2 [2 to ~ 0 4 f l over the same temperature range These values give a I~R. product ranging from ~ 0 8 mV at the low temperatures to ~ 0 1 mV at 77 K Figure 3 plots the crmcal current for the j u n c t t o n as a funcUon of temperature At low temperatures a roughly hnear dependence may be seen At temperatures close to T~ (83 K ) a fit to the equation ( 1 - T / T e ) ~ yields n = 1 6 This value Is lower than n = 2 expected from S N S j u n c t l o n theory [ 11 ] a n d is c o m m e n s u r a t e with the n o m d e a l RSJ behavior of the device F~gure 4 shows a 10 Ixm j u n c u o n at 35 K u n d e r irradiation from a 35 G H z mtcrowave source (Klyst r o n ) at several power levels Well-defined Shapiro steps can be seen at the proper voltages w~th the step heights varying with the a m o u n t of m~crowave power At very high power levels the superconducting I - V characterlst~c can be totally suppressed The DC S Q U I D s fashioned from the cubic-barrier j u n c t i o n devices were measured next The S Q U I D loops had a calculated inductance ( L ) of 210 pH

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Fig 4 I- Vcharacteristics of a 10 gm widej uncUonlrra&ated at different power levelswith 35 GHz microwavesat 35 K The critical currents at 77 K were typically ~ 0 25 m A Because of the large L i e product the crttlcal current m o d u l a t i o n depth lS expected to be low The m o d u l a t i o n depth was calculated using a model that included self-inductance effects and assumed symmetric j u n c t i o n s [12] From these calculations a m o d u l a t i o n depth of about 6% was expected The measured depth was ~ 5% m close agreement with the model Ftgure 5 shows the voltage m o d u l a t i o n m the S Q U I D at a temperature of 80 K as a function of the current through a pmr of Helmholtz coils situated outside the cryostat A periodic m o d u l a t i o n of the voltage can be seen from the data ymldmg a peak-

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The authors gratefully acknowledge Jose M Mlr, Stephen Barry, Georgm R Torok, Lloyd A Bosworth, Samuel Chen and Ralph A N~cholas for their assistance

Current (mA) Fig 5 Voltage modulation across a DC SQUID as a function of the magnet cod current at 80 K to-peak voltage o f ~ 1 ~tV at a current bias o f 0 28 m A F o r the magnetic fields shown the S Q U I D was nonhysteret~c F o r slgmficantly larger fields, some hysteretlc b e h a v i o r was observed Some o f the S Q U I D s operated to temperatures as high as 82 K

4. Summary In summary, aI1-YBCO ramp-edge Josephson junctions with a cublc-YBCO b a m e r layer have been fabricated on a Mg O substrate tn high yield All the Y B C O layers o f the heteroepltaxlal devices were deposited by P L D Patterning was accomplished through standard p h o t o h t h o g r a p h i c techniques and Ar-lon m l l h n g J u n c t i o n s having crlt~cal current densines o f ~ 5 X 1 0 3 A / c m 2 at 77 K leading to I~R. products o f ~ 0 1 m V at this t e m p e r a t u r e were produced Wafser geometry S Q U I D s fashioned from these j u n c t i o n s showed periodic voltage m o d u l a t i o n with applied field to t e m p e r a t u r e s o f 82 K

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