ELECTRON TUNNELLING IN HIGH-Tc"Nb3Ge" FILMS

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ELECTRON TUNNELLING IN HIGH-Tc”Nb3Ge”

FILMS

R. Buitrago, L. Toth, A. Goldmann, M. Dayan

To cite this version:

JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-418

ELECTRON TUNNELLING IN HI6H-Tc"Nb G " FILMS c t>3 e

R. B u i t r a g o , L . E . T o t h , A.M. Goldmannxand M. D a y a n t

School of Chemical Engineering and Materials Soienae an School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.

Résumé.- La structure du paramètre d'ordre du supraconducteur Nb,Ge a été mesurée dans la face nor-male d'un bilame Al-ltt^Ge. On discute la possibilité d'utiliser les bilames de ce type pour étudier la fonction spectrale a2F dans NboGe.

Abstract.- Energy gap structure associated with superconducting Nb,Ge has been observed on the nor-mal side of Al-Nb3Ge proximity sandwiches. The possible use of tunnelling junctions of this type to study the electron-phonon spectra of Ift^Ge is pointed out.

Single particle tunnelling measurements are in principle a powerful technique for obtaining the electron-phonon spectral function a2F(w). While

procedures for obtaining a2F((o) from tunnelling data arewell known, methods for fabricating useful tunnelling junctions on A15 superconductors are not. Here we report the fabrication of proximity effect tunnelling junctions in the configuration Nb3Ge-Al-Al20~-Pb. Such junctions in the case of Nb base layers are very clean Nb-Al interfaces and very thin Al layers have been shown to be useful in the quantitative determination of o F(OJ) for niobum

IM. A theoretical analysis, which enables one to ob-tain a2F(w)of the superconducting later when the normal layer is thin, by explicitly taking into account the electronic density of states in the normal layer has also been given recently / 2 / .

Tunnelling in high-Tc intermetallic compounds such as Nb^Ge is somewhat more complicated than tunnelling into a pure metal such as Nb. The pro-blem of the short coherence lenght which is on the order of 50 A, is compounded by the great difficul-ties in preparing single-phase high-Tc films which are homogeneous across their thickness and by the fact that the layers of material adjacent to a tun-nelling barrier prepared by conventional thermal oxidation are rich in germanium /3/.

In the case of Nb,Ge it is relatively easy to deposit films by sputtering, CVD or electron beam deposition and achieve T 's greater than 20 K. The primary problem is that "Nb,Ge" is a metastable pha-t Supporpha-ted in parpha-t by pha-the Napha-tional Science Founda-tion under Grant DMR76-10370A01 and by the Central Administration of the University of Minnesota.

School of Physics, University of Minnesota, 116 ehurch St.S.W., Minneapolis, Minnesota, 55455 U.S.A.

se. We have found that NbgGe films with a 20 K transition temperature may contain up to five Nb-Ge compounds. These are the A15, NbcGe-,(H), NbijGeg(T), a quasiamorphous phase characterized by well defined diffraction peaks at d= 2.66 A and d = 1.33 A, and the amorphous phase.

A15 +tTetragonal) 6 2 5 - B =\ ~ AI5+ y ^ \{Hetaq.y AI5 +<Hexogondl \ / + Quosiomorphous) \ /

iY r

Fig.l : Phase relationships of sputter-deposited NbjGe films. T_is the substrate temperature during deposition

Figure 1 shows the phase relationships we have found as a function of substrate deposition tempe-rature and Nb/Ge ratio of the sputtering target. Domains where transition temperatures in excess of 21 K occur are also shown. Maximum Tc's occur in

regions very close to boundaries between two phases in fact, films with Tc's > 21 K usually contain a

small amount of second phase which is not an A15. As one moves away from these boundaries, Tc decrea-ses slightly and the amount of non - A15 phase in-creases. It is possible to produce films with Tc > 20 K that are (1) single phase A15 or (2)

about 1/3 A15, 2/3 non-A15. Thus knowing that T c

is high is not necessarily an indication that tun- nelling data willaccurately reflect the properties of the A15.

A second problem is illustrated in figure 2 which shows the localized chemistry at the top sur-

face of a sputter-deposited Nb3Ge film with a Tc > 21 K. A pronounced Ge-rich layer exists in the first 50-70 A of the original surface. We have shown that this Ge-rich layer develops as the film is formed.

or below it without decreasing Tc. Next, the Ge- rich surface layer is removed by a low-voltage glow-discharge or by ion-milling in the Auger sys- tem. A thin layer of A1 (50-70 A) is next deposi-

7

ted in a vacuum of 10- torr. This layer is then oxidized in air for three minutes. The A1 layer is masked with collodion at the same time. Finally, a counterelectrode of Pb is deposited under UHV

conditions.

Fig.3 : Current-voltage characteristic of a nomiI3 nal Nb3Ge-A1-A1 0 -Pb juction. The scales are 10

V/division, and218-I A/division. T = 4.2 K. Figure 3 shows the I-V characteristic at 4.2 K of one of the first Nb3Ge-Al-A1203-Pb junctions prepared this way. An energy gap structure which Fig.2 : Auger electron depth profile of an "Nb3Ge1' can be associated'with Nb3Ge is clearly visible. film with a Ge rich layer of 70

1.

We have not yet determined whether we have fabrica- Thermal oxidation usually used to produce

barriers is not necessarily appropriate when the electrodes are compounds. In general, preferential oxidation of one element occurs. For the specific case of Nb3Ge, the top surface of the electrode be- neath the oxide no longer has the same composition as the bulk of the film, assuming that it was homo- geneous in the first place, because as the oxide forms the remaining elements get concentrated direc- tly beneath the oxide layer. In the case of Nb3Ge the oxidation process results in an enhancement of the Ge-rich layer. It is Mghlyprobable that such a layer was responsible for the lower energy gap obser- ved by Rowell and Schmidt in their tunnelling stu- dies of NbgGe /4/.

In our procedure for fabricating proximity effect tunnelling junctions we first sputter deposit

ted an artificial barrier Nb3Ge-Al-A1203-Pb junc- tion or a proximity effect junction. In the former case it would be unnecessary to contend with the corrections for the aluminum density-of-states in analysing tunnelling data in order to study the strong-coupled phonon structure of Nb3Ge. The pre- sence of two gaps in the I-V characteristic sug- gests that the surface layer is not single-phase

Nb Ge. It remains to be demonstrated that enriched 3

surface layers can be removed and the surface appro- priately annealed so that a single-gap characteris- tic of a homogeneous material can be obtained. Ho- wever, this technique of junction fabrication ap- pears to be sufficiently reproducible that a sys- tematic study of the dependence of I-V characteris- tics on such treatment seems possible.

References

/ I / Wolf,E.L. and Zasadzinsky,J. Phys. Lett.

3

/ 2 / Arnold Gerald,B., Bull.Am.Soc.

23

/3/ Buitrago,R., Toth, L.and Goldman, A.M., sub- mitted to J.Appl.Phys.

/ 4 / Rowel1,J.M. and Schmi!dt, P.H. Appl.Phys.Lett. 29 (1976) 622.