We report preparation of a novel barrier structure for high Tc superconducting multilayer Josephson junctions using a simple composite target technique. Such a barrier consists of Yx Pr1-xBa2Cu3Oy with a continually varied concentration of yttrium. In this barrier no lattice mismatch and other incompatibility problems occur between adjacent layers. Thus the formation of interfacial defects and structural strain can be mostly prevented. The Josephson coupling takes place at the electrically formed interfaces rather than the structural interfaces. A particular feature of these junctions is that the effective thickness of the barrier strongly depends on the measuring temperature and the concentration gradient. The absence of the structural interface in the weak link region greatly enhances the reproducibility and the performance of these junctions.published_or_final_versio

Gao, J

Sun, JL

Tang, WH

IEEE Transactions on Applied Superconductivity

HKU Scholars Hub

Title Preparation of continually graded barriers of YPrBaCuO for HTSJosephson junctionsAuthor(s) Gao, J; Sun, JL; Tang, WHCitation IEEE Transactions on Applied Superconductivity, 2001, v. 11 n.1, p. 497-500Issued Date 2001URL http://hdl.handle.net/10722/43317Rights Creative Commons: Attribution 3.0 Hong Kong LicenseIEEE TRANSACTlONS ON APPLIED SUPERCONDUCTIVITY, VOL. I I ,  NO. I ,  MARCH 2001 497 Preparation of Continually Graded Barriers of YPrBaCuO for HTS Josephson Junctions J. Cao, J.L. Sun, and W.fl. Tang Ahsfract--We report preparation of a novel barrier structure for high T, superconducting multilayer Josephson jnnctions using a simplc composite targct technique. Such n harrier consists of Y,Prl.,Ba2CuJ0, with ii continilally varied conccntration of yttriiim. In this harrier no lattice mismatch and other incompatibility prohlems occur betwccn adjacent laycrs. Thur the formation o l  interfacial defects and structural strain can bc mostly prevented. The Josephson coupling takes place at the clectrically formcd interfaccs rather than thc structural interfaces. A particular Feeture of these junctions is that the effective thickness of the barrier strongly depends on the mcasuruig fcniperatorc nnd the concentration gradient. The absence oftlie strnctoral interface in the weak linhregion greatly enhances the reproducibility and the performance of these junctions. Index Ternrs-High Temperature Supcrcondnctors, Josephsou Junctions ne of the major problems associated with high T, Josephson junctions using an artificial barrier is the poor interface between the barrier and superconductor. It is believed that the interfaces beliveen barrier and superconducting electrodes play a cnicial role.I:l-41 In general the interface between the barrier and superconductors is damaged an0 defective, which results in poor performance and reproducibility. Studies using a high resolution transmission electron niicroscope (HRTEM) showed that the slructural, them", and chemical incompatibility at the interface could introduce various defects.[5-6] Also. the ex situ lithography process involved by either ion-milling or cheniical etching further damage the interface.[] ,7-91 To reduce the incompatibility between barrier and electrode, and to iniprove the damaged interface, various approaches like chemical etching and use o.f intermediate layers were reported.l7,9] However, the interfacial problems still remain altliough the quality oftlie interhces or the lattice niisniatch hetween barrier 0 Manuscript rewived September 16, 2000. This work has been supported by the Rcseatch (;rants Council (RGC) ofHong Kong. J. ciao is \\.ith Dtpartnieiit of Physics, The Univeoiry of Hong Kong, T'ohfiulam R a d ,  Hony Kong (tclephonc: 852-28597948. Ibx: 552-255991 51. e-mail: .iuprw@hku.hk). J.L. Sun is with Department of Physics, The llniversity of Hong Kong, Pokfulam R o d ,  Hong Kong W.H. Tang is with Deparuutent ofPhysius, The University L I ~ H L U I ~  Kong,Pokfulam Road, I-long Kong (wlephone: 852-25592 195, (ax: 852-255991 52. e-mail: ~ I i t a i ~ ~ ~ ~ h C ; u . h l i ) .  and superconductor could be improved. The reproducibility and perlbrniance 0.f high T, multilayer junctions are still not satis factory. 111 this paper, we describe the preparation of a new barrier structure with a continually graded coniposition of Y,Pr,.,Ba2Cu30, (Y,Pr,,BCOj. High Tc Josephson junctions with such a graded barrier structure have no structural interfaces and the Josephson coupling takes place at tlie electmnically formecl interhces. In atldition, the efkctive barrier thickness in our junctions, which depends on the temperature and concentration gratlienl, can be varied. Thus tlie reproducibility and uniformity ofthese junctions are greatly enhanced. Ourjunctions were fabricated on SrTi03(STO) substrates by using off-axis RF inagnelron sputtering. First, m epitaxial bi-layer consisting ofthe base YBa2Cu30, (YBCIO) electrode and the insulating layer of Pr13a2Cu30, (J'BCO) was grown in situ using off-axis sputtering. Photoresist was then spun onto the bi-layer lilms iis the stencil ancl was baked at 100 "C: for IO min. The structure was patterned by a standard lithographic process. To create a ramp edge, the samples with patterned resist were ion milled through the bi-layer and sliglitly into the substrate by using a Kaufiiiann Ar ion beam gun of 5 cni in diameter. The ion inilling was perfornied under a pressure of (5-8)X 1 O?'' inbar, and the beam current and voltage were I -  I5 inh and V = 600-800 V, respectively. 'To prevent overheating ofihe sample, the ion gun was operated in a pulse niocle, which allowed a shoit break every 10 seconds. After the ramp edge being mede, the sample was placed back into the V B C U I I ~  chamber and the barrier and top electrode were then grown. To overcome the problem caused by ihe damaged interhce, Y,Pr,.,BCO was used as the barrier with the concentration x gradually changing from 1 to 0 ancl then lkom 0 to 1 ,  as shown in Fig. 1. We first used two targets co-sputtering to make such a conrinually graded barrier of Y,Pr,.,BC!O. By controlling the sputter power of YBCO and PBCO targets, a continuous composition grdient could be fornied. The drawbacks of this method are that the gradient of concentration x is not very uniform and the deposition system is very complicated. Moreover, it is quite diflicult to control tlie process to obtain a snioothly changed composition in the g r o w  barrier. Therefore w e  developed a coinposite target technique for preparing the graded barrier. A schematic diagram of our deposition system is shown in Fig. 2. Tha composite target consists of two semi-discs, YBCO and PBCO, respectively. It w a s  made I05 I -8223/0 I $ IO .OO 0 200 I IEEE 498 simply by placing YBCO and PBCO powder in the mould, each occupying a halfvolume, and then pressing into a disc. 11 is known that the YBCO and PBCO can be sputtered under the same conditions with a similar rate. At room temperature the resistance of YBCO and PBCO are in the same order ,and tlie sputtering was operated in RF-mode. Thus the splitter power could be divided more or less equally into the two sites of the composite target, as can be seen from the shape of the sputter plasina. During the growth of barrier, the substrate was slowly rotated from the YBCO side to the PBCO side and then to the PBCO target to make a pure PBCO central layer. The reason that we insert a PBCO central layer between two graded YxPrl.,BCO layers is to enswe that no short circuits can take place even at low temperahires. After the PBCO barrier being deposited, the substrate is rotated hack to the compound YBCO/PBCO target and gradually moved from PBCO side to YBCN side, and then to the YBCO targel to grow the top electrode. By usiiig such a siinple technique, a smooth changing ofthe composition fromYBCO to PBCO is obtained. A continually graded barrier without any abrupt changes in structure and composition is therefore .formed. The SIN interfaces Ramp surface Fig. 1 Schcimlic diiigriun ofthe conlioually graded b.irrrier of Y$rl.,Ba:Cu307 with the concentration I gradually chaiiginp rrom 1 10 0 and then fiom 0 to 1. d9 is the elll.ctive harrier (hickness. Fig2. Schcmatis diayratn ot'oiir tlcposirion syskm. 111. RESULTS AND DISCISSION Fig. 3 presents the depth distribution of yttrium and praseodymium measured by x-ray photoelectron spectrometry (XPS). During the measurement, the film was sputtered through the top YBCO layer, the graded Y,Pr,.,BCO barrier, the bottom YBCO layer, and into the SrTiO3 substrate. As can be seen in Fig. 3 ,  the yttrium concentration .T was gradually changed from x-1 to x=0 to reach a pure PBCO barrier. and then x was slowly increased from 0 to I to reach the YBCO electrode. Therefore the mismatching between every adjacent atomic layer would be very small and the occurrence of interface del'ects can be greatly reduced. Hence the reproducibility and perforinance of these ju i~ t ions  can be signifcantly enhanced. On the XIJS spectnini the ytt~ium signal did not reach zero level at the center of barrier although there was a pure IJBCO centml layer in the barrier. The reason could be due to the penetration ofthe probe through the entire PBCO layer into the botlom YBCO layer as the probe depth in XPS IS typically 10 nin. In the ramp-type junctions using a conventional single PBCO barrier, the ramp surface created by ion inilliiig is typically defective. The damaged interface can result in a large boundary resistance and low transparency for the quasi-particles at the interfiice. 19,101 Also, an abrupt change of lattice coiistant and composition at the interrace can introduce strain and interfacial defects. [ 1 I] Thus the interface resistance rather than the barrier material could dominate the j~inction properties. In comparison, our junctions with a continuously graded barrier can moslly prevent the influence of the damaged ramp interface. It is known that the YyIJrl JXQcan he a superconductor at sufficiently low temperature, if the yttrium coilcentration x is larger than O S .  The results obtained on bulk material ol'Y,Prl.,BC(~indicated that the transition temperature T, decreased gradually as tlie concentration x decreased.[ 121 , 1 n W : m a) 2 Y m Q) Q 0 40 80 120 Thickness (nm) Pig.3 The depth distribution o f Y  and Pr measured by XPS. 499 0.4- 3 Q.2- ‘ Q- 0 -  -0.2- G -  5 c -0.4- I I I -0.3 -0.2 -0.1 0 0.1 0.2 v (mv) Fig. 4 I-T‘ ciirws for junction with il Y,PI.,I)CO bnrricr ill diflirent temperatures (dF 30 IUII iind dprl(.o”4 nni). The inscl shows the I-V ciiivcs with itnd wilhoul microwwe radiation at 9 Ii. Depending on the concentrationx and T. the Y,Prl.,BCO layer is partially superconductive. Therefore the S!’I interface will be formed inside the Y,Prl.,BCO layer and the damaged ramp surface is excluded from the weak link region, as shown in Fig. I .  Since the concentration .x varies very smoothly (here will be no abrupt cliange in the lattice constant and the composition. Hence no structural interrace is formed within the weak link region. To assess the influence ofthe damaged ramp surface, which now is located within the superconducting electrode, we deposited an YBCO layer on the damaged ramp surface. It was found that ihe film still showed a full transition with a high T,. We conclude that the ramp surface within the superconducting region has little effect on the performance ofthe junctions. The junctions with such a graded barrier present resistively shunted junction ( I W )  characteristics. I- I.’ curves have small excess currents (see Fig.4). Ahnost all of these junctions exhibited clear Shapiro s~eps under irradiation of a microwave field, giving unambiguous evidence of the Josephson behavior. The junctions discussed in this paper were structiired into a width ofeither 10 or 20 pin. The normal state resistance near ihe transition temperature in these j unctions is typically from 10’’ to a few Ohm. The characteristic voltages, I,R,,, measured at 10 I< are around I mv. Using such a continuously graded barrier stiircture, the yield of successful junctions, which show a supercurrent and clear Shapiro steps, is nearly 100% (Among 40 junctions made so far, 38 exhibited clear Shapiro steps above 50 IC.). In contrast, the yield of good junctions with a single bauier is typically below SO’%. The uniformity o fjunction parameters, Iuiown as an essential issue for applications, was considered too. Damaged interfaces often cause variation in the junction properties like I, and R,, efc. In most high T, .junctions with an artificial barrier, uniforniity has been inlliienced by the damaged interfaces leading to large variation. By using this barrier, the damaged interface is excluded Elom the weak link region so it can ininirnally affeci junction perforinatice. In our junction process, lc vasiatiori as low as 1-n==12o/u for I O  junctions with identical parameters was found at 60-65 K. A particular feature of such a barrier is that the egectivc barrier thickness de, which is !he spacing beriveen the two S.IN interfaces (see Fig. 1). could be varied even afier fabrication. In our junction the barrier consists of three layers, the graded Y,Prl.,BCO interlayer, PBCO layer, and another graded interlayer. The graded intermediare layers with thickness dg can be partially superc,onduc,ting depending on the temperature and the concentration.t.. For niost samples studied in this work, the thickness of the PBCO central layer is dpBc(jZ8 nin, and the thickness of the graded layer at the two sides was varied from c l S 4  nm to 4 ~ 3 8  nrn. The efl’ective barrier thickness should he the sum of dl,ljp(:) and the thickness of superconducting part within YxIJrl.,J3C0, which is a function of the ieniperature and the concentration gradient. It follows that the barrier thickness in  such j u n c h n s  would strongly depend on the ineasuring t e niperat ure. Since the dc varies as a,funclioiioften~perat~rre. it is iherefbre of great interest to study the temperature dependence of the transport properties in these junctions. The normal slate resistance R,,A ( A  is the junction arca) is plotted in Fig. 5 against temperature. We found that for small values ofd,  the RJ products were quite insensitive to the temperature. It is I<nown that I’BCO is a semiconductor, whereas YxPrl.,BCO can behaye as a normal metal. depending on the concentration 3c and the temperature. As the effective barrier thickness is also changed, the whole barrie,r presents a complicated system. The temperature insensitive &A product i s  a result of the combinaiion of Y,Prl.,BCO ant1 PRCO layers. On the other hand, with a thicker Y,Prl.,BCO layer R,J is dominated by the PBCO material, which usually shows a semiconductor behavior. The teniperahrre dependence of I, ofthese junctions was also measured and compared with that of junctions with a conventional single barrier of PBC‘O. Fig. 6 shows the normalized i, as a function of reduced temperature T/Tc for junctions with various ds (frotn 10 nm to 38 nm). Most oftlie data lie on a nearly straight line. a,pproach a [l-TIT,)] relationship. In contrast the I ,  of-junctions using a single PBClO barrier usually exhibits a (l-T/Tc)2 dependence of a SNS-type Values of d,: ~ ~ f ; n d n n m ~  1Bqnn” -A- 14nm -A- 16nm 0 10 20 30 40 50 60 70 BO T(K) Pig. 5 The temperature dependence of the product &/I for .junctions with different vaiucs ofd,. 500 1 0.8 0.6 s 3 v ‘r 0.4 0.2 0 U 0.2 0.4 0.6 0.8 1 TIT,  Pip. 6 The nornialized IC M. X’T, foi various d,. The bolid iynibols arc data measured on the junctions with a coiiventioiial single limier ofPB(’O junction. [ 1 ,  81 The eflkctive harrier thickness for junction (10-8-10) is nearly the same as that with a single barrier (0-16-0), but their I,(n dependences are far different. The power o f  two is characteristic o f a  SNS Junction. The critical current for a SNS structure can be described by [ 131 where C is a temperature dependent constant, {cL the Ginzburg-Landau coherence length, {,, the effective coherence length in the normal metal, Fo the amplitude ofcooper-pairs in electrodes and L tlie length of the normal inetal path from one superconducting electrode to the other. As {GL. and Fo(7) vary approximately as (l-T/TJ”’ and ( I -TITc)’!2for ET, 20.5 aiid varies only slowly, tlie critical current I, is proportional to (l-7‘./7‘.,]2. The fact that the IC( 7) ofjunctions with a continually graded barrier varies as ( I  -T/TJ1 implies that their transport process might difrer from that of the SNS structure. A detailed study of the behavior of these junctions will be discussed elsewhere. [ 141 IC( r )  = c I~,,(T)~”[,(T)/~(;I,(T)I~r.~~r-Llrs,(r)~ IV. CIONCLUSIONS In summary, a new barrier stnicture with a continually graded Y,Prl.,lBCO has been prepared by using a simple composite target technique. In such a barrier stnicture the structural interfaces are absent aiid the hsephson coupling takes place at the electronically ibrmed interfaces. The effective thickness of such a barrier strongly depends on the concentration gradient and leniperature. The transport process i n  these junctions sigiiificaiitly diflers from that of junctions with an ordinary single barrier layer u i d  provides a very interesting system for further study. The absence of the structural interface in the weak link region can greatly help to enhance the reproducibility aiid perforinaiice of these junctions. RLFERENCES [ I ]  J. (;ao, \+‘..4.M. Aanduk, Ci.1. (kiTiLsma. and H. 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Preparation of continually graded barriers of YPrBaCuO for HTS Josephson junctions

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Preparation of continually graded barriers of YPrBaCuO for HTS Josephson junctions

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