Solid State Communications, Vol. 70, No. 4, pp. 449-452, 1989. Printed in Great Britain.

0038-1098/89 $3.00 + .00 Pergamon Press plc

SYSTEMATIC EFFECTS IN THIN FILM TUNNEL JUNCTIONS OF YBa2Cu307_,~

Mark Lee, M. Naito*, A. Kapitulnik and M.R. Beasley

Department of Applied Physics, Stanford University, Stanford, CA 94305, USA

(Received 15 August 1988 by J.M. Rowell)

We report on junction fabrication, materials problems, and a more systematic study of tunneling characteristics for Pb-I-YBCO thin film sandwich-type junctions. Our results suggest that we have constructed proximity effect junctions and have observed directionaily averaged superconducting and normal properties of an oxygen deficient phase of YBCO. Among the observed properties are a linear normal density of states, a cusp feature at zero bias, and a gap opening at 60 K. The data also suggest that the normal state properties are temperature and field independent below T, but are temperature dependent above 7',.

SINCE electron tunneling into superconductors has historically yielded a wealth of insight into the micro- scopic nature of the mechanism of superconduc- tivity, there have been many attempts to tunnel into the high-T,, oxide superconductors. This has proven to be a difficult problem, and unfortunately no definitive results are presently available. For example, the tun- neling gap measurements, mostly by point contact [1] and break junction [2] have yielded many different values for the energy gap A in YBaECU307 ~ (YBCO), typically ranging from 20-60 inV. In most instances it is not obvious which aspects of tunneling behavior are related to superconductivity and which are not.

Thus, short of complete success, special strategies are necessary to extract useful physical results from the presently available junctions and to gain insights into better methods of junction construction. In this paper we discuss a more systematic investigation of tunneling into YBCO toward these ends. We have investigated the fabrication and tunneling of planar thin film tunnel junctions on YBCO subject to differ- ent surface treatments and using different barriers. Some features of the tunneling behavior appear to be independent of the details of junction preparation and are therefore worth examining in greater detail. We have measured the temperature and magnetic field dependence of the tunneling characteristics in our better junctions as a means to identify those features that are related to the superconductivity of these materials.

Our results confirm some aspects of earlier studies

* Present address: NTT Basic Research Laboratory, Tokyo, Japan.

and also extend the earlier results in important ways. In agreement with Iguchi et al. [3], our data is consis- tent with an energy gap of about 20 meV but with a gap-opening T~. around 60 K. The conductance above the gap is found to be proportional to bias voltage as reported previously [4], but we have found the conduc- tance slope to be nearly temperature and field inde- pendent below 60 K and 7 Teslas, and strongly tem- perature dependent above 60 K. Weak structure above the gap is also consistently seen but is hard to interpret unambiguously.

The YBCO films used for junction fabrication were deposited by electron beam coevaporation and post-annealed as described in [5]. SrTiO3 was used as a substrate for highly a-axis oriented films, and ZrO3 was used to form randomly oriented films. All films studied showed R = 0 above 80K. While the post- annealing method currently yields the highest quality films in terms of superconducting properties, it also yields rough and offostoichiometric surfaces [6], which is clearly a major problem in achieving good tunneling characteristics with these materials.

The simplest technique used for tunnel barrier formation was to deposit Pb strips on the as-annealed surface, thus relying on a native barrier, as has also been done by others [3, 4, 7, 8]. This typically resulted in many junctions that showed Pb gaps of varying strengths, as well as several junctions that showed no tunneling structure at all. Typical junction resistances by this method ranged from about 200 ~ to 5 k~ for junctions of area 200 x 500pm 2. In no case was a shorted or very low resistance junction observed. From this we conclude that an insulating surface barrier is naturally formed and that, judging from the junction resistances, this barrier cannot be very thick,

449

450

1.65

1.45

1.25 'o x > 1.05

0.85

0.65

THIN FILM T U N N E L JUNCTIONS

~' ~ T = O 5 K ~

~ ~ , ~ T H = O "}"'H-7T

I 5 v(~v) o' " j -i0 510 20 i I

i i I i I I | i i i I i -60 -40 -20 0 20 40 60

VOLTAGE BP~,S (mV)

Fig. 1. dI/dV of a representative junction in 0 field and in 7 Teslas at T = 0.5 K. No change in dI/dV was seen from 0.15 to 7 Teslas. Inset: I-V of a shorted junction.

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probably less than 50 A typically. Hence the remainder of the off-stoichiometric surface region, if not super- conducting, is likely a normal metal.

We then tried to alter the YBCO surface. Ion milling produced no working junctions, with or with- out deposition of an artificial A1/A1203 or SiOx barrier. A successful method of surface alteration involved passivation of the surface by a thin (50- 200 ,~) layer of Ag deposited in situ prior to annealing. It has not yet been established whether Ag remains on the surface after annealing, but there is no doubt that the surface properties are markedly different. Deposi- tion of a Pb counterelectrode directly on the surface usually yielded a shorted or low resistance junction, as compared to the native surface. We then formed an artificial barrier, usually 15-20/~ of AI or Si, which was then oxidized, to yield junctions showing strong Pb gaps. These junctions typically had resistances of 1-10k~.

A typical junction spectrum is shown in Fig. 1 along with an I-V curve of a junction in which the m1203 barrier apparently suffered from pinhole shorts, yielding SNS Josephson-type behavior. The shorted junction had 20 A oxidized A1 deposited on a 200,~, Ag passivation layer over the YBCO. The finite volt- age state resistance, ~0.01 f~, is consistent with a ~ 1 #m diameter hole through the barrier. The resist- ance below the break in the I-V curve (I < 2 ma) was "~ 10 -5 fL indicating that the Ag and whatever normal YBCO surface layer exists are capable of supporting a supercurrent. This implies that any normal YBCO surface layer is thin enough that the tunneling spec- trum shown in the main part of Fig. 1 reflects at worst a proximity effect junction.

Examining this "good" junction more carefully, we see that it is characterized by low leakage below the Pb gap, a sharp peak in G at the Pb gap, and the Pb

c: .-.E

a . = _ _J t I I L I I i I

20 30 40 50 60 70 80 90

Voltage Bias ( mV )

Fig. 2. Oscillatory deviations from linearity in G ( V ) for 3 junctions. The top two sets of data are from Ag passivated junctions with AI/AI203 barriers. The bottom data is from a natural barrier junction.

phonons evident below 10mV. Despite materials problems, the following features are always seen in both artificial barrier and natural barrier junctions: a broad conductance "hump" at about 20 mV, a linear variation of the conductance with Vat high ( > 25 mV) bias, and weak oscillatory deviations from perfect linearity in G in the 20-100mV range, shown in Fig. 2, for three different junctions. In general, the tunneling conductances were symmetric with regard to bias polarity.

We have also measured junction conductance in magnetic fields from 0.15 to 7Teslas at 0 .5K (see Fig. 1). The linear conductance even up to 7 T showed no measurable change from the zero field value at 0.5 K, while the 20 mV hump was less pronounced in low fields. The most striking feature seen in the field was the cusp-like depression in the conductance near zero bias, which was evident in any field sufficient to quench the superconductivity in the Pb counter- electrode, and in a thermally smeared form in zero field at temperatures above 7 K. The cusp was likely not due to magnetic anomalies [9] since no change in the cusp structure was seen from 0.15 to 7 T. Also, to temperatures as low as 0.5 K in 7T we did not see a Zeeman gap open as reported by Ong et al. [8].

To characterize the temperature dependence of the conductance depression due to the cusp, and to examine its possible relation to superconductivity, we tracked the ratio of the actual zero bias conductance to the value extrapolated from high bias, G0aCt/G0,ex from 9-170 K in zero field; the data is shown in Fig. 3. Although thermal smearing causes an effect of similar magnitude, the quantitative behavior observed is quite distinct from that calculated by convolving a Fermi function into the cusp. G~Ct/G~ x rises with T and satu- rates at a value slightly above unity above 60 K; ther- mal smearing of the non-analytic point at V = 0 causes G~t/G~ X to slightly exceed unity. The conduc- tance depression closes at about 60K when G~t/ Gg x = 1. This is in qualitative agreement with the

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,.oo t t

~ 0 . 9 0 y:i

0.80 <3 4.0 - -~ [~+- i . . . . . . i " " t J J 0 30 60 90 120 160

TEMPERATURE IK) l I i I i t i I , I , I , I , I ,

0 20 40 60 80 I00 120 140 160 180

TEMPERATURE (K)

Fig. 3. Normalized zero-bias conductance vs. tem- perature. The dotted line indicates calculated behavior of thermal smearing. The inset shows the temperature behaviors of the high bias average slope and its extrapolated zero-bias value.

T H I N FILM T U N N E L JUNCTIONS 451

The structure at ~ 20 mV and .~ 40 mV is consistent with that observed by Iguchi et al. [3].

A definitive interpretation of the data would be ,.5o too bold a stroke at the present time. It is instructive,

'~ however, to examine some possible implications from ,.25 _ various points of view. ~t

~o In order to characterize the cusp structure, we fit ,,o G(V) ~ V = (0 < V < 15mV) where a least squares

fit gives 0t = 0.57. This is reminiscent of the zero-bias square root singularity due to strong electron-electron correlations usually seen near a metal-insulator tran- sition [10]. In that case, MacMillan showed that the low bias density of states goes as N(eV)/N(O) = 1 + (eV/Ac) = where ct = 0.5 and Ac is the Coulomb corre- lation gap [l 1]. In our case the fit gives Ac = 94 meV. While the quantitative behavior of Ac (T) is unknown, one could reasonably expect it to close when kBT/A O(1), so for A~ = 94meV, T >> 100K. This is about an order of magnitude higher than the observed gap closing at 60 K.

Another possibility is that the cusp results from gapless behavior of a set of quasiparticle excited states with density of states N(e) ,~ e°57just above the Fermi level er = 0. Such a density of states would be qualita- tively consistent with a node in the superconducting energy gap, as is thought to exist in the case of heavy fermion superconductors [12]. Some supporting evi- dence for this kind of behavior exists in NQR studies of YBCO [13]. A related possibility is the suggestion by Deutscher and Muller [14] that the gap may be severely depressed near surfaces and interfaces. The observations of occasionally quite large clean gap structure by point contact tunneling [1] would not be inconsistent with the existence of a gap node since the point contact method probes dimensions smaller than a typical grain size and may be more directionally dependent. The observed isolated Pb gap structure could also be a consequence of low-lying states when the tunneling current is directionally diffuse, though one cannot rule out the possibility that normal metal in grain boundaries provides a parallel Pb - I -N tunnel- ing channel which would also give the Pb gap structure.

The hump in G near 20 mV may be viewed as poor gap structure due to a diffusive Pb- I -S tunneling channel averaging in all directions over an anisotropic gap. In this bias range the Pb density of states is essentially constant. Maekawa et aL have numerically integrated the directionally averaged density of states, ( N ( V ) ) , for several models of gap anisotropy using the BCS form for N(O, dp, V) [15]. They showed that the tunneling conductance gap structure becomes severely smeared for large gap anisotropy, with a peak still occurring at Am,~ at T = 0. Regardless of the exact form of N(O, c~, V), it is clear that if A varies

results of Iguchi et al. [3] who observed the 20 mV structure in their I -V curves vanish between 50-70 K.

In BCS terms, the conductance depression would be due to a suppression of the low energy density of states caused by the opening of a gap. The data in Fig. 3 is consistent with a gap opening but is quan- titatively different from BCS behavior; for example, G~Ct/G~ ~ falls roughly as a square root for T < 60 K and has a nonzero T = 0 intercept. In any event, associating the observed low bias structure with super- conductivity, we are led to the conclusion that the To, of the YBCO participating in the tunneling is ~ 60 K. Since the low bias conductance evolves smoothly to linear behavior (dI /dV ~ V) above 60K, we believe this linear conductance results from a normal state tunneling density of states linear with energy.

We also measured the temperature dependence of the average high bias slope, AG/A V and its zero-bias extrapolation G~ x. As shown in Fig. 3, we found that the presumably normal state properties AG/AV and G~ x were both nearly constant against changes in tem- perature as long as T < 60K. For T < 60K, the linear conductance appeared to be rigid with respect to changes in temperature and field (up to 7 T). Above 60 K the slope AG/A V decreased with increasing tem- perature, falling about 20% from its low temperature value by room temperature, while G~ x began to increase rapidly above 60 K, as if the normal tunneling density of states were approaching conventional metallic (i.e., G = constant) behavior at high tem- perature.

The high bias conductance is linear only on aver- age. In our best junctions we consistently observed small oscillatory deviations around the best linear fit. Examples are shown in Fig. 2 for two types of barriers. Precise measurement of the nature of these deviations was difficult, but some systematic behavior is evident.

452 THIN FILM TUNNEL JUNCTIONS Vol. 70, No. 4

continuously from Arnin to Amax ~> Amin, G will then be a broad rise whose maximum occurs about at eV = Amx. In our data, we observe the maxima to occur at about 20mV. For a 90K T,, this gives 2Amax/kT , ~ 5. In fact, as we have argued, we believe the T, of the YBCO superconducting surface region being tunneled into is about 60 K, probably due to oxygen deficiency, giving 2Amax/kT , ~ 8.

The linear high bias behavior of G(V) is seen routinely in TBCO and LSCO tunnel junctions, planar, point contact and break junction. While some barrier problems may result in such behavior [16], the omnipresence of the linear conductance, independent of junction formation, argues strongly that it is intrin- sic to the material and not a pathology. The average linear behavior of the conductance in YBCO persists out to over 500 mV bias. As we noted, G has a well defined and unusual temperature dependence that changes near T, ,~ 60K and hence seems to be affected by the superconducting transition.

Anderson and Zou have recently offered an expla- nation of the linear conductance in the framework of RVB [17]. In their model at T = 0, G(V) = C I VI + Go, where Go is proportional to the fraction of holons condensed in the superconducting state and hence should be a decreasing function of temperature. As mentioned, our data (Fig. 3) showing Go to be either constant or increasing with increasing temperature is inconsistent with this prediction.

Finally, we briefly discuss the weak structure repeatedly observed in the high bias regime. At least two possibilities come to mind. One is that they are Andreev- or Tomasch-like oscillations due to the proximity nature of the junctions [18]. The positions of the oscillation maxima fit poorly to the usual Andreev hierarchy, however. The second possibility is that the oscillations are due to strong-coupling effects, analogous to the phonon spectrum in conventional superconductors. In any event, we are convinced that they are intrinsic to the YBCO surface and not an artifact.

In conclusion, we have discussed various methods of surface preparation and sandwich tunnel junction formation in YBCO. Despite obvious materials prob- lems, an average linear conductance with weak oscil-

lations and a 20 mV conductance hump are always seen. The behavior of junction conductance with field and temperature has also been explored, leading to the observation of a conductance cusp at zero-bias that closes at 60 K, which we interpret as a gap opening. Perhaps most interestingly, we also observed a linear normal tunneling density of states that becomes tem- perature dependent only above 60 K.

Acknowledgements - We thank R.B. Laughlin for some intriguing discussions. M.L. would like to acknowledge an NSF Graduate Fellowship. This work was partly supported by the OBR under con- tract nos. N0014-83-K-0391 and N00014-89-K-0327.

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