Atomic-layer fabrication of high-Tc tunnel junctions

Journei el

AND ~ ~ 3 ~ ' - ~ ELSEVIER

Journal of Alloys and Compounds 251 (1997) 201-205

Atomic-layer fabrication of high-T¢ tunnel junctions Ivan Bozovic*, J.N. Eckstein Vat[an Research Center. Palo Alto. CA. USA

Abstract

Using ALL-MBE. we have engineered a novel, metastable cuprate superconductor, BiSr,Ca~_,Dy,CusO,,,+,,, in which only the central Ca layer is doped by Dy. This provides, within a single unit cell. the bottom superconducting electrode, an insulating battier layer (only few A thick), and the top superconducting electrode, thus constituting an artificial intra-celi Jo~phson junction. In this way, we have fabricated the first high-T,, tunnel (SIS)junctions. They exhibit very sharp quasiparticle tunneling I-V characteristics, consistent with tunneling between 2D superconductors with d-wave pairing. Keywords: Atomic-layer fabrication: High-T,, tunnel junctions

1. Introduction

Fabrication of tunnel (SIS) junctions has always been one of the key goals in high-T~ superconducting electronics, but it proved to be elusive. The primary source of difficulties was thought to be the short coherence lengths in cuprates--~)nly 10-20 Pt in the ab direction (parallel to the of CuO~ planes) and perhaps I-2 A in the c-axis direction. This essentially implies that interlaces in an SIS trilayer structure must be perlect on an atomic scale, and this was difficult to achieve by most commonly used thin film deposition techniques. We have utilized atomic.layer.by-layer molecular beam epitaxy (ALL-MBE) to synthesize single.crystal thin lilms and heterostructures of cuprate superconductors and othe~ complex oxides (titanates, manganites) with atomically sharp interfaces. We fabricated trilayer junctions with titanate barriers from 4 to 28 ,~, thick with no pinholes. These junctions clearly showed tunneling transport, but no supercurrent, even for 4 A thick barriers. Localization in neighboring 2212 layers was observed, which explained this negative result. To circumvent this problem, we have engineered a novel, metastable superconducting compound, BiSr2Ca.7o ,Dy, CuHO,,, ,. This compound has the Bi- 1278 structure, except that the central Ca layer is doped by Dy. in this way, we have assembled the bottom superconducting electrode, an insulating barrier layer, and the top superconducting electrode, all within a single unit cell. We review this work in what follows. *Fax: (415)424-6988 (USA); e-inail: [email protected] 0925-83881971517.00 @ 1997 Elsevier Science S.A. All rights reserved P l l S0925-8388{ 96~02798-3

We have fabricated devices containing one such layer provided with further superconducting top and bottom contacts. These artificial intra-cell Josephson junctions contain highly resistive barriers, which are only few A thick. Further, they exhibit rather sharp quasiparticle tunneling I - V characteristics. So. in this way we have indeed fabricated the lirst highoT~ tunnel (SIS)junctions.

2. AI,L-MBE growth

Our technique for deposition of singleocry~tal thin films of cuprate superconductors and other complex o~ti~s, ALL-MBE (atomic layeroby=layer molecular beam epio taxy), has been described in detail elsewhere [1oo3], so it is sufficient here to just summarize Se main features. The ultrahigh-vacuum chamber contains eight thermal evaporao tion sources with computer-controlled shutters. Currently, these sources are used for Bi, St, Ca, Cu, Dy, Mn, La, and Ag. which enables synthesis of various superconducting or insulating phases from the Bi-Sr=Ca=Cu~O and La-Sr= Cu=O families, as well as L a - S r - C a - M n - O compounds that show colossal magnetoresistance |41. Silver is used to deposit electrical contacts in-situ. Atomic fluxes are monitored by a home-made system based on atomic absorption spectroscopy [5], similar to a double-beam spectrophotometer. The technique is very accurate and fast; changes of less than one percent are easily detected and corrected in real time by a feedback control. Using a pure ozone beam, vigorous oxidation is achieved under high vacuum conditions. This permits ino

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I. ttozovic. J.N. i.'cksteiu I Journal of Alloys and C, map,mmls 251 ¢ !997~ 201-205 ,

sire, real-time monitoring of the surface structure by reflection high-energy electron diffraction, RHEED 161. So far, ALL-MBE has produced single-crystal thin films of various cuprate phases with state-of-the art transport properties, as well as various heterostructures, multilayers and superlattices which involve other complex oxides (titanates, manganites) [I-31. However, for the experiments described below~studies of tunnel junctions, in particular--the film quality requirements go well beyond merely having good transport properties, in particular, the presence of any secondaryphase precipitates, inclusions, screw dislocations, etc., cannot be tolerated; in essence, atomically flat surfaces and interlaces are required. With the help of RHEED and other analytical tools available, for several compounds from the Bi-Sr-Ca-CuO family we were able to discover well defined paths through the phase space which avoid nucleation of secondary-phase defects. We emphasize that these paths are tracked by allowing for partial overlap of deposition from various sources, i.e.. we do not to deposit strictly one monolayer of an element at a time. Rather, we are trying to control the surface chemistry by supplying minute doses of atoms, with nanogram accuracy~sometimes one and at times two or more species together. Apart from monitoring of lilm growth in real time by ob~rving the RHEED patterns and intensity oscillations, post~growth characterization has been pertbrmed by a variety of techniques including x-ray diffraction, Ruthero ford backscattering (RBS), Auger electron spectroscopy (A~S), xoray fluorescence (XRF), secondary ion muss spectroscopy (SIMS), atomic three microscopy(AFM), ~canning tunneling microscopy (STM), and crossosectional transmission electron microscopy (TEM). These measure° meats confirmed fabrication of atomically smooth surfaces and inte~hces b,:tween layers of various phases under study.

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SrTiO~ slab thickness was 4, 12, 20, and 28 A, (i.e., I, 3, 5, and 7 unit cells), respectively. The base and top electrode layers consisted of 2212. For the thinnest barrier, the superconducting transition in the base- and counter-electrode can be seen in Fig. I; for thicker ones, resistance through the barrier dominates over the spreading resistance at all temperatures. The dependence of resistance on the barrier thickness is clearly exponential (see Fig. 2), at all temperatures below about 50 K, tbr over more than Ibur orders el' magnitude of resistance. It begins to saturate Ibr yet thicker barriers. This behavior is indeed suggestive o1' tunneling transport. However, no supercun'ent was observed in these devices. The differential conductance, dlldV, as a tianction of voltage, was constant at low bias voltages; above a bias of about 2kT/e, it became linearly dependent on voltage. All the above results showed t~markable reproducibility.

3. 'rrtlayer junctions with titanate harriers

S~veral titanates, including CaTiO, SrTiO, and Bi~(ri~o~, provide go(~ epitaxiai match with tile cuprate superconductors. Using ALL-MBE. we have achieved excellent heteroepitaxy between high-T~ Bi-Sr-Ca-Cu-O phases such as Bi,S%CaCu,O~ (2212) and each of these titanates. This made it possible to synthesize precise heterostructures containing cuprates and ultrathin layers of various titanates. Thus. our first attempts to fabricate highT~ SIS junctions were based on cuprate/titanate/cuprate trilayer heterostructures. So far. these attempts we~ not ~uccessful, for the reasons to be explained below. in Fig. I. we show a log-log plot of the temperature dependence of resistance for a series of devices, each containing a single barrier layer consisting of a SrTiO~ slab inserted in tire middle of a 2278 cuprate layer [21. The

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I. Bozovk'. J.N. Eckstein i Jounml of AUoys and Compounds 25I (t997) 201-205

Very little (several percent) variation in transport characteristics was seen among different devices on the same wafer, devices on different wafers, devices with SrTiOa barriers inserted in a different way (e.g. at the cleavage planes of 2212), and even devices with CaTiOa barriers. Qualitatively similar behavior was also seen with SrTiO 3, BaTiO 3, DyTiO 3, and Bi4Ti30 7. In order to comprehend the behavior of these tunnel junctioas, we have also studied few samples that consisted of repetitions of such barrier layers, e.g., a (BiSrCa3Cu40 xi 3 x SrTiO a/Cu4Ca3SrBiO~ ), supeflattice. These films showed no superconductivity. At room temperature, the resistivity was about an order of magnitude higher than in 2212; below about 50 K, it followed a nearly ideal 2-dimensional variable range hopping (2DVRH) behavior. The above observations suggest that our tunnel junctions with titanate barriers may perhaps be described as -SINILI'LINIS-; here, I' is the inserted titanate layer and L is a neighboring cuprate layer with a reduced carrier density and localized electron states [71. Empirically, we know that L layers are not superconducting; they show VRH for in-plane transport, and one- or two-hop inelastic tunneling for c-axis transport. The S-S distance here is too large (50-60 A) for direct tunneling. Because of localization, the Coulomb blockade breaks pairs and suppresses supercurrent via resonant pair tunneling 181. It is clear that in order to fabricate high-T, SIS tunnel junction, one has to avoid such localization at the S-! interfaces.

4. Synthesis of BISraCaTCuxO~o+, AI,L°MBE enables material engineering at various levels. By stacking of molecular layers oi' difl~rent corn° pounds, one can tbrm multilayers and superlatticcs. Within a molecular layer, one can add or omit atomic monolayers, and thus cast novel compounds. For example, in this manner we have synthesized thin films of Bi,Sr:CavCu~O:o, , (2278). Substantial interest in this compound has been aroused by a report 191 that it exhibited interest in this compound has been aroused by a report that it showed superconductivity at 250 K 191. The highest T~ we measured in a single-crystal 2278 thin film was around 60 K II0]. The material was metallic, but its relatively high resistivity, p~600 p,[! cm at T=300 K, indicated that it was substantially underdoped, even alter vigorous in-situ post-annealing in ozone. This was also confirmed by subsequent mutual inductance measurements which showed a very low superfluid density. in principle, good heteroepitaxy with 2212 and the reduced T~ make 2278 a good choice Ibr fabrication of high-T~ trilayer junctions. In Fig. 3, we show a crosssection TEM of a 2212/227812212 trilayer, obtained by M. Wall from Lawrence Livermore National Laboratory

203

Fig. 3. Cross-~ction TEM image of a trilayer junction. The 2212 electrodes are separated by a single 2278 molecular layer barrier, about 25 A thick. From Rel: l lll.

i l I ]. Notice the perfection of the interfaces on the atomic scale. To increase the superfluid density, we have investigated the possibility of modulation-doping by omitting entire hiP layers. Since each BiO unit donates one electron to the CuO~ state manifold, a Bi°l compound (such as Bio 1278) should have about I hole more per formula unil compared to its Bio2 counterpart (Bi°2278). IThis hole count assumes, however, that the oxygen content in the remaining layers stays the name, which indeed need not be true in reality, an discussed below.I Indeed. Biol278 is an even less stable comlu~und, and for this reason single-phase 1278 films are very difficult |o grow. However. a very thin layer of 1278 can be epitaxialo ly stabilized if it is sandwiched between layers of stable phases such as 2212 or 2201. We have therefore grown superlattice films consisting of many re~tifions of the 1278-1201-(2201),, sequence, with m= l. 2, or 3. In this case. RHEED showed a very smooth growth, and Xoray diffraction indeed confimled the desired superlattice struco ture. As an example, in Fig. 4 we show an X~ray diffraction pattern for a 2201~2201~1278~1201~2201 superlattice. The diffraction peaks can be indexed as tOOl) with c=75.73 A, close to what was predicted° Notice the pronounced finite-thickness osciflations which indicate that the film was indeed very smoo~h. In similar sttperlattice samples, 2201 ~ 1278~ 1201~2201, we measured T~.= 75 K. "l'his result demonstrated the capability of ALL°MBE method to artificially assemble novel highoT~ materials ll0,111.

204

!. Bozovie. J,N. EcL~tein / Journal ~ Alloys and Compounds 251 (1997) 201-205

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between the two 2212 electrodes, the c-axis critical currents were in general too high for practical devices. To improve the device performance, we have substituted Ca 2+ in the central layer by Dy 3+, generating a structure illustrated in Fig. 5. Again, by studying superlattices containing Dy-doped Bi- 1278, in similar 2201-1278-1201-2201 sequences, we have f,~und that even for complete one-monolayer Dy substitution this compound, Bi2Sr2DyCa7CusOi9+x, is still superconducting, with Tc=45 K. However, according to the penetration length studies from mutual inductance measurements, in this case superconductivity is confined to the outer CuO 2 planes, which are underdoped (n~-0.3 pairs per the whole formula unit). The central slab is actually insulating, as will be shown below.

6. Tunnel junction I-V characteristics $. Synthesis of BISraCa~_,Dy, CusOlg+, When we fabricated Josephson junctions by inserting a single molecular layer of 1278 to act as a barrier layer rO

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So, in this way we have fabricated, within a single unit cell, the bottom superconducting electrode, an insulating barrier layer, and the top superconducting electrode. These intra-cell Josephson junctions show a very high lcR N product. At 4.2 K, a typical !c=200 mA, while Rn=50 1"1, so that Vc = icR N= 10 inV. This makes such junctions very attractive for superconductive electronics applications. Note that this result is qualitatively different from what we have observed is all our SNS Josephson junctions. (It is also one of the highest reported so far in high-T~ Josephson junctions.) Actually, the high RN implies tunneling. In 30×30 ~m ~ mesa sWuctures, at T~4.2 K, the typical R N ~ 5 0 r ; this con~sponds to p~104 [! cm (for c-axis transport). This certainly is not a metallic conductivity. Notice further that this insulating barrier layer is only few ~ngstrom thick, so that the direct tunneling transport channel is indeed expected to be strong. Our estimate for the barrier tunnel. ing height is about 0.25-0.5 eV. Not surprisingly, these tunnel junctions are found to exhibit very sharp, gap-like quasiparticle tunneling i-V characteristics, see Fig. 6. As the temperature is raised, these gap-like structure moves to lower voltages and gets smeared as expected. In Fig. 7, we show the differential conductance (d//dV) as a function of voltage, for the same junction, at T~4.2 K. To the best of our knowledge, these are the lirst artificial high-T~ tunnel (SIS) junctions reported. However, these junctions do not show the ordinary BCSqy~ SIS behavior. A detailed quantitative analysis, which will be presented elsewhere, actually shows that these I-V curves are consistent with tunneling between 2D superconductors with d-wave pairing. In conclusion, we have utilized atomic-layer engineering to synthesize artificial (metastable) high-To compounds and fabricate trilayer junctions (two superconducting slabs separated by an insulating barrier) within a single-unit-cell

!. Bozovic, J.N. Eckstein / Journal of Alloys and Compounds 251 (11197)201-20~

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This work was supported in part by N R L and O N R via contracts N00014-93-C-2055 and N00014-94-C-2011.

References

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Acknowledgments

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[i1 J.N. Eckstein, I. Bozovic, M.E. Klausmeier-Bmwn, G.F. Vbshup and K.S. Rails, MRS Bull., 18 (8) (1992) 27. [2] J.N. Eckstein, !. Bozovic and G.F. Virshup, MRS Bull., 19 (9) (1994) 44. [31 J.N. Eckslein and I. Bozovic, Ann. Rev. Mater. Sci., 25 (1995) 6/9. 141 See e.g.S. Jin, M. MeConnack0T. Tiefel and R. Ramesh, J. App/. Phys., 76 (1994) 6929. [51 M.E. Klausmeier-Bmwn, J.N. Eckstein, !. Bozovic and G.F. Virshup, Appl. Phys. Leu., 60 (1992) 657. [6] I. Bozovic and J.N. Eckstein, MRS Bull., 20 (5) (1995) 32. [71 !. Bozovic and J.N. Eckstein, J. Supercond., 8 (1995) 537. [81 J. Halbriter, Phys. Rut,. B, 48 (1993) 9735. [91 M. Lagui!s, X. M, Xie, H. Tebbji, X.Z. Xu, V. Marken, C. Hattere, C.F. Beuran and C. Deville47.avelin,Science, 262 (1993) 1850. [10] !. Bozovic, J.N. Eckstein and G.F. Virshup, Physica ¢, 235-240 (1994) 1"/8. [I I] !. Bozovic, J.N. Eckstein, G.H. Virshup, A. Chaiken, M. Wall, R. Howell and M. Fiuss, J. Supercond., 7 (1994) 187.