IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, No. 2, JUNE 1999 4521

Bi2Sr2CaCu208+6 Intrinsic Josephson Junctions Fabricated by a Simple Technique without Photolithography

Y.J. Feng. W. L. Shan, M. Jin, J. Zhou, G . D. Zhou. Z. M. Ji, L. Kang, W. W. Xu, S. Z. Yang, and P. H. Wu Department of Electronic Science and Engineering, Nanjing I Jniversity, Nanjing, 2 10093, P. R. China

Y .H.Zhang Structure Research Laboratory, 1 Jniversity of Science and Technology of China,

Hefei, Anhui 230026, P. K. China

Abstruct- Due to the roughness in the surface of the crystal sample, it is hard to use photolithography in the ptterning process of the BizSrzCaCuzOs+s intrinsic Josephson junction. In this paper, we report a simple technique for fabricating the BizSrzCaCuzOx+~ intrinsic Josephson junctions. In the patterning process, metal masks are used instead of photolithography and argon ion milling is applied to form a small mesa on the Bi,Sr,CaCu,Ox+s crystal surface. Real four-probe transport measurements are made on the Bi2SrzCaCuzOs+s intrinsic junctions and typical current- voltage characteristics with multi-branch structure have been observed, from which the superconducting gap parameter can be extracted. Additionally, from the s t r a g hysteresis in the I-V characteristics, the capacitance CJ of the unit intrinsic Josephson junction can be estimated, which is in good agreement with that evaluated from the geometric parameters of the unit junction between the two copper oxide layers.

I. INlRODI 'CTION

Since the discovery of the intrinsic Josephson effect between the Cu02 tnultilayers in the highly anisotripic cuprate superconductors [I]. continuous progress has been made for the development of devices based on this effect [2- ?I. These naturally formed Josephson junctions have been investigated intensively for several reasons. They are not only a valuable object for theoretical study of the basic mechanisms of the cuprate superconductors, but also a key element to realize the Josephson tunnel junction devices. These devices are indispensable for applications such as Josephson switching devices, millimeter and sub-millimeter wave sources. detectors and misers, since the Josephson tunnel junctions have not been obtained in the high temperature cuprate superconductors. The natural arrangement of stacked Josephson junction arrays in the cuprate superconductors may lead to high frequency

Manuscript received Septeinher 14. 1908. This work was supported partly hv The National Center for Research and

Development on Superconductivity of China, iind piirtly by the interniitional collaborative research project of Te1ect)mmunications advancement Orgiinization (TAM)) ofJ;ipai~.

applications, since the phase locking in a series Josephson junction array results in narrqw band. tunable rf sources where the bandwidth is reduced by increasing the number of the participating junctions [SI.

Most of the intrinsic Josephson junctions are prepared by patterning a small mesa structure using a combination of photolithography and ion beam etching on the Bi2Sr2CaCu20x+h (BiSrCaCuO) single crystal surface [3-4 I . Due to the roughness in the surface of the crystal samples. it is hard to use photolithography in the patterning process especially to realize a real four-probe configuration on the mesa type intrinsic junctions. In this paper. we report a simple technique of making the intrinsic Josephson junctions on a BiSrCaCuO single cr?stal. To avoid using the photolithography in the patterning process. metal inasks are used to form the mesa structure and a real four-probe geometry is established. We also report the detailed investigation of the current-voltage (I-1) characteristics of the junctions.

11. EXPERIMENTAL

For the present study we employed high quality BiSrCaCuO single crystals grown by a self-flus technique [(i]. The as-grown crystals were cut into several pieces of large crystals and were annealed at 450°C at an oqgen atmosphere for one hour to enhance the anisotropy of the samples. Prior to the patterning process these crystals were glued onto ZrzO substrates and cleaved into flat platelets with dimensions of 1.Ommx 1 .OmmxO. linin. A silver contact layer was immediately deposited on the freshly cleaved crystal surface. The patterning process of the small mesa structure with a lateral dimension of less than SOpmxSOpn is shown in Fig. 1. Instead of using photolithography, a metal mask was applied in coinbination with the argon ion milling to form the small mesa on top of the BiSrCaCuO single crystal surface. A 400nm CaF2 film was then deposited as an insulating lsyer. M e r removing the mask, a silver layer was deposited on the whole surface again and etched by argon ion milling using another metal mask. forming the four electrode patterns. The mesa height between 50 nxn and 200 nm was controlled via ion milling time. The electrical measurements were carried out by

1051-8223/99$10.00 0 1999 IEEE

4528

5.0-

2.5- 6 E != 0.0- k

v c

a,

-2.5- 3

Ar’

4 4 4 4

v- I’ I- v-

m m BSCCO metal mask CaF? Ag film

EZ. 1. Schematic drawings of the process step flow fix fiibriciition of the intrinsic Josephson junctions. Fig. 1 (d) olso shows the four-probe geometry.

mounting the samples on a closed-cycle refrigemtor down to 4.2K. The current-voltage (Z-J.’) characteristics of the samples were obtained through c-axis transport measurement by the real four-probe configuration.

111. RES1 iLTS AND DISCI ISSIONS

Fig. 2 shows a typical I- characteristic of a mesa with the dimensions of 50~tnx50~mx200nm at 20K. Frotn the mesa height of 200nm we can expect a stack of about 130 junctions. As can be seen in Fig. l(d), when patterning the four electrodes, the mesa was usually etched into two parts by argon ion milling, making the number of the junctions taking part in the transport mcasurement less than that estimated from the mesa height. A branching structure peculiar to intrinsic Josephson junctions was observed in the samples as shown in Fig. 2. In this case, the critical currents of the individual junctions are not uniform and hence switching of each junction causes the corresponding voltage drop. The multiple resistive branches are caused by the switching of each of the constituent Josephson tunnel ju:ictions with large capacitance in the mesa. When increasing the temperature. the tnultiple resistive branches can still be observed up to 70K.

In order to study the transport properties of the layered structures in the BiSrCaCuO single crystal samples more carefully, the 1-r’. characteristic of another satnple (SOO3) is depicted in Fig. 3 where only the first two resistive branches have been traced out. The I-V curves are highly hysteretic. typical for Josephson tunnel junctions with large capacitances. The first branch corresponds to the switching to the resistive state of the junction having the lowest critical

I 8 d

-200.0 -100.0 0.0 100.0 200.0

Voltage (mV)

Fig. 2. Current - voltage characteristics of the intrinsic Josephson junction ( S 0 0 2 ) measured at 20K. traced out by repeatedly cycling the hiiw current. Each jump to ;in adjacent branch corresponds to an additional junction switching to the resistive state.

current IC of about 3.5 mA. The I-J/’ dependence is altnost vertical at the voltage J<c 20 mV as shown. in Fig. 3 , similar to the gap structure reported in reference [ 3 ] . Usually in the case of a Josephson tunnel junction having the electrodes of the satne superconductor, a sharp increase of the current should take place at Jlg = ZA(T)/e. where d(T) is the temperature dependent superconducting gap parameter and e is the electron charge [7]. Thus, in our case. a A value of about 10 mcV can be deduced at 20K. which is considerably smaller than the expected 25 me‘V from the tunnelling spectroscopy experjnents [W]. The strong gap suppression. which has been reported by several groups [3 , 101, can be understood as a direct consequence of the strong quasiparticle injection into the thin superconducting CuO:

~

SO03 T=20K

Fig. 3. Current-Voltage characteristics of a 50pmxSO pm mesa at 2 0 K . Only the first two resistive branches haw hsen traced out.

4529

doublelayers[ 1 11. In addition, one can observe a slightly bending back of the first two quasiparticle branches resulting in a negative dynamic resistance in the I-V curve. This is a typical feature of the intrinsic Josephson junctions and is due to a nonequibrium heating caused by the quasiparticle in-jection to the extremely thin CuOz electrodes. As suggested by Yurgens et al. [3], an additional source of quasiparticles in our case is also due to the extra junctions underneath the current lead caused by the second argon ion milling process as shown in Fig. 1 (b). These junctions do not take part in the transport measurement, so they do not contribute to the I-V curve due to the four probe measurements.

We have also measured the first resistive branch of the

I I

0.8 ’ . O r - \

0.6

0.4

9La, I Fig. 4. The temperature dependence of the superconducting energy gap

paraineter. normalized to the value at 20K. The temperature is normalized to it5 critical temperature.

above junction (S003) at different temperatures. Fig. 4 shows the temperature dependence of the gap voltage. normalized to its value at 20K The gap voltage l’g(TI shows a large downward derivation from that expected from the BCS theory, which is quite similar to the results obtained by Yurgens et al. [3].

From the strong hysteresis in the I-V characteristics, the capacitance of the unit intrinsic Josephson junction can be estimated. The ( I r is calculated using the following relation:

based on the resistively shunted junction (RSJ) model [12], where ,@. is the McCumber parameter, R, the normal resistance and 0 0 the flux quantum. McCumber derived the relation between and the &/Ic by numerical analysis. where ZR is the minimum current in the finite voltage state [13]. From the experimental result in Fig. 3 and the McCumber’s analysis, a PI. of about 24.5 at 20K can be determined. Taking the measured 1,- of 3.5 mA and R, of about I CZ, (:I is estimated to be 2.3 pF. On the other hand, the capacitance cTr between two adjacent CuOz layers can also be estimated from the geometric parameters. Taking the distance t between the two CuOz layers to be 12 A, the

= 27dc3R:(:1/00,

junction lateral dimension S to be 50x50 pm2, and the dielectric constant sr of about 20 [14], the capacitance can be evaluated to be Cl,, = &O+S/t = 3.7 pF, which is in good agreement with the experimental result.

IV. CONCLLJSION

In summary. we have successfully fabricated the intrinsic Josephson junctions from the BiSrCaCuO single crystal. By employing metal masks instead of the photolithography, small mesa has been formed on the crystal surface and a real four-probe geometry has been realized. The I-V characteristics clearly showed the expected behaviour of series arrays of the intrinsic Josephson junctions. From the value of the voltage jumps, the c-axis superconductive energy gap was estimated to be about one half of the value reported elsewhere and to be strongly temperature dependent, suggesting a strong quasiparticle injection into the thin superconducting CuO. double layers. From the hysteresis of the I-V curves, the capacitance of the unit SIS junction naturally formed in the BiSrCaCuO crystal was also estimated to be about 2.3pF. which compares well with that evaluated from the geometric parameters of the unit junction between the two copper oxide Izyers.

REFERENCES

[ 11 R.Kleiner, F.Steinmeyer, G.Kunkel. P.Miiller. “Intrinsic Josephson &ect in BrzSrzCaCu208 single crystals.” fhys. Rev. Lett.. Vol. 68, pp2394. 1992.

[2] R.Kleiner, “Intrinsic Josephson junctions in high temperature superconductors.” .I. Low Temp. fhys., vol. 106, pp953. 1997.

[ 3 ] A.Yurgens, D. Winkler, N.Z.Zavaritsky, T.Claeson, “Strong temperature dependence of the c-axis gap pnrameter of BrzSrzCaCuzOs+n intrinsic Josephsonjunctions,” PhjLS. Rev. Lett.. vol. 53, ppR8887. 1996. A.1rie. Y.Hirai. and G.Oy;i. “Fisk and tlux-tlow modes of the intrinsic Josephson junctions in Br2SrzCaCu$ )y mews.” Appl. phys. Lett.. vol. 72. pp2159, 1998. A.V.1 istinov. h.Kohlstedt, C.Hcidzn. ”Possihle phase locking of vertically stacked Josephson flux-flow oscillators.” Appl. Phjjs. Lett., Vol. 65. pp14.57, 1994.

Giaever. .K. Megerle. “Study of superconductors hy electron tunneling.”

J.Liu, et al., “Intrinsic fe;wures of BrzSrzCaCuz08+i, tunneling spectra: scaling and syinmetry ofthe energy gap.” fhys. Rev. B, vol. 49. pp6234. 1994.

[9] Ch.R&nner. and 63. Fischer, “Vacuum tunneling spectroscopy and aspmetric density of state of BrzSr&hCuzOs+i, .” f h p . Rev. 5. vol. 5 1. pp9208, 1995.

[ lo] R.Kleiner and P.Mfiller, ”Intrinsic Josephson eEects in high-T, superconductors.” Phjw. Rev. H, vol. 49. pp1327. 1994.

[ 1 11 K.Tanahe, Y.Hidaka, S.Karimoto, M.Suzuki, “Ohservation of hoth pair and quasipiirticle tunneling ia intrinsic junction stacks fahriciited on Br&C;iCuzOs,;, single crystals.” P h j ~ . Rev. B. vol. 53, pp9348. 1996.

[ 121 A.B;irone and G.Plitemo, Physrcs and Applications of rhe .Josephson E f f . t . Wiley, New York, 1982.

[ 131 D.E.McCumher. “Effect of ac impedance on dc voltage-current chiuncteristics of superconductor weak-link junctions.” .J. Appl. Phys.. vol. 39, pp31 13, 1968.

[ 141 S.Tajima, G.D.Gu, S.Miyamoto, A.Odagawa. N.Koshizuku. “Optical evidence for strong anisotropy in the normal and superconducting states in Br2Sr2CiiC1u20~+z,” fhys. Rev. B, vol. 48. pp16164. 1993.

[4]

IS]

[6] Y.H.Zhang, private cothunication. 1907. [7]

[XI rhJj.s. RW VOI. 122, ppi io i , 1961.