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Monitoring of the surface decomposition of YBaCuO and LaSrCuO by electron spectroscopy
Materials Science and Engineering, B2 (1989) 269-276
269
Monitoring of the Surface Decomposition of Y-Ba-Cu-O and La-Sr-Cu-O by Electron Spectroscopy M. G. RAMSEY and E P. NETZER
lnstitut j~ir Physikalische Chemic, Universitdt hmsbruck, A-6020 lnnsbruck (Austria) (Received August 1, 1988)
Abstract
~lhe surfaces of the high Tc superconductors L a - S r - C u - O and Y - B a - C u - O have been characterized by the laboratory-accessible techniques of ultraviolet photoelectron electron spectroscopy, electron energy loss spectroscopy and Auger electron spectroscopy. The damage to the surfaces due to the time in ultrahigh vacuum and electron beam irradiation have been followed by the various techniques which indicate a loss of oxygen. 7he decomposition of the surfaces due to heating has also been investigated. Oxygen is seen to be liberated from the samples and, as evidenced in all spectroscopy techniques, copper is completely driven from the surfaces until insulating surface layers of L a - S r - O and Ba-O are formed. 1. Introduction
The time has come for more careful characterization of the high 7~. superconductor surfaces. In this paper we present an electron spectroscopy survey of Lal.~Sr0.jsCuO4 (Tc=36K) and YBa2Cu307, surfaces both superconducting (Tc=93 K) and non-superconducting with the techniques of Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy (UPS) and electron energy loss spectroscopy (EELS). The experiments were not only confined to the freshly scraped surfaces but also to surfaces affected by time in ultrahigh vacuum, electron beam irradiation and heating. This allows less ambiguous assignment of the various features observed and the resulting electronic and chemical characterization will, hopefully, be useful to assess the surface conditions for other experimenters. The majority of the electron spectroscopy investigations reported in the literature have been synchrotron radiation studies [1-7] of the valence 1)921-511)7/89/$3.50
band and lower lying core levels and X-ray photoelectron spectroscopy (XPS) of the deeper core levels of Y - B a - C u - O [7-17]. Far fewer electron spectroscopy studies of L a - S r - C u - O have been reported [17-22] as this type was supplanted by Y - B a - C u - O with a more accessible critical temperature. Although it is improbable that electron spectroscopy will solve the riddle of the high temperature superconduction mechanism, such studies are important for characterizing the surfaces of these materials with standard techniques. This is particularly so, given the great interest in thin films, because of their applications in microelectronics and, with their greater malleability and higher current densities, in macroscopic systems. An understanding of how these materials decompose and the changes which occur at the surface, as modelled by heating, are important, given the various methods used for their production. This is also important for the single-crystal studies now being embarked on, given the experimental difficulties of working with these relatively small samples.
2. Experimental details
Two samples of nominal YBa2Cu307_ x stoichiometry and one of Lal.ssSr0.15CuO ~ were investigated. The samples were prepared according to an established procedure [23] at the National Bureau of Standards, Gaithersburg, MD, U.S.A., and kindly provided by Dr. D. L. Ederer and Dr. C. W. Clark. A.c. magnetic susceptibility measurements of L a - S r - C u - O and Y - B a - C u - O I samples showed them to be superconducting with critical temperature onsets of 36 K and 93 K respectively and transition widths of a few kelvins. Sample Y - B a - C u - O II was nonsuperconducting and was studied for the purposes of comparison. The pressed pellets were © Elsevier Sequoia/Printed in The Netherlands
270
attached to an ultrahigh vacuum manipulator via tantalum clamps to a tantalum base plate. Fresh clean surfaces characteristic of the bulk composition were obtained by repeated scraping in situ with a corundum file. The samples could be cooled to 80 K or indirectly heated to about 800 K. The investigations were performed in a VGADES 400 electron spectrometer [24] with a spherical sector electron energy analyser. A helium discharge lamp provided the He I ( h v = 2 1 . 2 eV) and He II ( h v = 4 0 . 8 eV) radiation for the photoemission experiments. The electron energy loss spectra were obtained in both N(E) and (dN/dE){N'(E)} modes with a primary energy Ep range of 10-2000 eV. The Auger spectra were excited by 3 keV incident electrons. The system was baked to only 75 °C to avoid oxygen loss from the samples, but even so a base pressure of about 5 x 10 -~I mbar was achieved.
AES of high Tc superconductors Ep=3 keY [tJ
ga
YBaEu0 I , ~
1 LaSr[uO,S
r C
La lr-
(u
~
£_
~
3. Results and discussion
Representative N'(E) Auger spectra for the freshly scraped Y - B a - C u - O and L a - S r - C u - O are displayed in Fig. 1. The features associated with the various constituents are readily identified and are marked on the spectra. Scanning across the surfaces yielded marked variations in the Auger ratios of the elements, suggesting pronounced inhomogeneities characteristic of multigrain materials. No significant differences between the composition of superconducting and that of non-superconducting Y - B a - C u - O were detectable, although the ratio of O KLL to Cu L3VV varied more across the non-superconducting sample whilst the ratio of O KLL to Ba M45N45N45 was relatively stable in both superconducting and non-superconducting Y - B a Cu-O. In a number of published studies, carbon contamination has been found even after scraping [3, 4, 14, 25]. It is therefore important to note that no carbon contamination was detectable on the surfaces of these samples after one or two scrapings. No changes in the Auger features of Y - B a - C u - O were observed on cooling below Tc. In particular the changes to the Cu L3VV reported by Sarma and coworkers [11, 15] could not be reproduced. The He I and He II UPS spectra for the freshly scraped superconducting (I) and non-supercon-
[u+Sr
J+La La
1000
500 ENERGY (eV)
(
Fig. 1. Representative Auger spectra of the freshly scraped samples. Prominent features are Cu L~VV at about 915 eV, Ba M45NasN45, at about 600 eV, La M45N45N45 at about 625 eV and O KLL at about 505 eV.
ductmg (II) Y - B a - C u - O and L a - S r - C u - O are displayed in Fig. 2. These surfaces were arrived at by repeated scrapings until no changes were observed on further scraping. It should be noted that this occurred although only one or two scrapings were required to produce contaminatefree surfaces as observed by AES. In scraping down from the surfaces as mounted, the changes in the UPS spectra of the two types of material were essentially opposite: for Y-Ba-Cu--O, scraping resulted in a decrease in the 8-13 eV binding energy structure with a concomitant increase in the feature at 2.5 eV whilst the reverse was true for L a - S r - C u - O . The spectra are consistent with those reported in the literature [1-7, 17, 19-22]. In the case of L a - S r - C u - O , however, the valence band spectra which have been reported have the
271
UPSof high Tc superconductors Hell
Hel
Y - B a - E u - ~
Hell
Hel ~
~
/t
Y-Ba-[u-O lI
\
La-S
,
l
,
15
,
,
,
l
,
,
,
,
,
J
10 5 BINDING ENERGY (eV)
,
i
l
,
EF
Fig. 2. He I and II UPS spectra of the freshly scraped samples. The region around EF is shown magnified for the superconducting Y - B a - C u - O I below and above its critical temperature. It should be noted that no changes are observed on cooling.
appearance of "damaged" spectra obtained in this work, with low intensity in the 8-13 eV region and relatively high intensity in the region of the low binding energy feature. As observed by others, there was no change in the density of states at E v on cooling below Tc. Both superconductors show a two-peak valence band structure with maxima at 2.6-2.8 eV and 4.6-5.2 eV due to the O 2p-Cu 3d derived emission and, beyond these, so-called satellites are observed. No appreciable differences were found between the superconducting and non-superconducting Y - B a - C u - O UPS spectra although EELS spectra showed significant differences [26]. Differences between the electron distribution curves for Y - B a - C u - O and
L a - S r - C u - O are observed, however. For the freshly scraped Y - B a - C u - O the peaks at 13.5 and 15.5 eV are due to emission from the Ba 5p3/2 and Ba 5pl/2 levels. The significance of the relatively low binding energy of these features is discussed below. Furthermore, freshly scraped Y-Ba-Cu-O shows a single relatively weak satellite at 9.1 eV compared with the strong satellites at 9.8 and 12.0 eV (all energies are taken from He II spectra to avoid background effects) for La-Sr-Cu-O. For both materials there is a striking anticorrelation between the intensities of the satellites and the low binding energy valence band feature at 2.6-2.8 eV. Any changes in the physical or chemical composition of the samples, as a result of the time in ultrahigh vacuum, electron beam irradiation or temperature treatment, caused intensity exchange between those satellites and the valence band structure. This is illustrated in Fig. 3 where representative freshly scraped and "damaged" (but clean) UPS spectra are shown for the two superconductors. In the case of Y-Ba-Cu-O, leaving the samples in ultrahigh vacuum results in a decrease in the 2.6 eV feature and a growth in the approximately 9 eV satellite and the appearance of a second satellite at 11.5 eV (cf. Figs. 3(a) and 3(b)). The resulting spectra are almost identical with those of the freshly filed L a - S r - C u - O except that the satellite peaks are about 0.5 eV higher in binding energy for the latter (Figs. 3(c) and 3(d)). Similar, although less pronounced, intensity exchange is observed for L a - S r - C u - O (Figs. 3(c)-3(e)). For both samples the Auger-stimulating electron beam causes changes identical, although accelerated, with those of merely leaving the samples in ultrahigh vacuum. La-Sr-Cu-O, however, exhibits a peculiar behaviour under the influence of electron beams of moderate energies (300 eV~
272 Y- B~ He \ frl
lw La -~.
aft
1'~ ....
1()
BINDING ENERGYeV
,,,,,N
5
EF
Fig. 3. Typical UPS spectra of freshly scraped and "damaged" surfaces:(a) fresh Y-Ba-Cu-O surface, He ll; (b) Y-Ba-Cu-O after 1 week in ultrahigh vacuum; (c), (d) He II and He 1 spectra of fresh La-Sr-Cu-O; (e) He 1 of La-Sr-Cu-O after 1 week in ultrahigh vacuum; (f), (g) He 1 and He II spectra of La-Sr-Cu-O showing the effects of a 300 eV electron beam. E E L S [26] spectra to that of the freshly scraped surface. It is interesting to note that Goa et al. [27], who studied the empty electronic states of L a - S r - C u - O with inverse photoemission, have also observed changes which are reversed by a higher energy electron beam. Whilst they have proposed that their results are related to chemisorbed oxygen, we argue, given the lack of oxygen in our residual atmosphere or carbon in the Auger spectra (from possible CO dissociation), that the results suggest that electrons of intermediate energies create oxygen defect states which are healed by higher energy beams. In both materials the relationship between the intensities of the satellite and the low binding energy feature supports the assignments to corre-
lation satellites and emission from hybrid O 2plike states respectively, made in studies of the valence band emission of " r - B a - C u - O as a function of photon energy [ 1,5, 6 }. In addition to the valence band emissions the Ba 5p~,, and Ba 5pl/2 levels also provide a diagnostic of the state of the Y - B a - C u - O surface accessible to laboratory UPS experiments. For Y - B a - C u - O , time in ultrahigh vacuum and electron-beam-induced changes to the valence band and satellite region are also parallelled in shifts to the Ba 5p levels. The fresh surfaces have Ba 5p3:2 and Ba 5pl/2 levels of 13.4 eV and 15.5 eV respectively. These values are in agreement with those reported by Thiry el al. [5] but are of the order of 1.0 eV lower than those reported for other B a - O systems [28-31i. Time in ultrahigh vacuum, heating or application ot an electron beam causes first broadening to higher binding energies and finally results in the peaks appearing at 14.0 eV and 16.0 eV respectively (Figs. 3(a) and 3(b)), an increase in binding energy of about 0.5 eV, and somewhat nearer the typical oxidized barium values. Jacobi el aL [2~, 30] have shown that the core levels of bulk barium all shift on oxidation to a lower binding energy by approximately the same amount. They have attributed this shift to the fact that the full O 2p level (02 is highly polarizable with the ability of shielding very effectively in the final state. The XPS spectra published for the Y - B a - C u - O Ba 4d and Ba 3d levels show the principal peaks occurring at the recognized oxidized barium valuesa however, they also indicate lower binding energy shoulders {10, 14, 16]. Steiner et al. ] 10] have noted the disappearance of these shoulders on heating and have associated them, in the manner of ,lacobi et al. [29, 30], with Ba 2- being completely coordinated with O: . Whilst we can only conjecture as to why the barium levels are even lower in binding energy for Y - B a - C u - O than for oxidized barium, it is suggested that our results which show that the 5p levels are clearly shifted to lower binding energy for the fresh surfaces rather than merely broadened to lower binding energy is significant. This suggests that the deeper core level emission results which have been reported may be due to the presence of "damage", such as the loss of oxygen, as opposed to the intrinsic existence of inequivalent B a - O configurations in Y - B a - C u - O as suggested by Werfel et al. [16J. The valence band photoemission spectra cannot be simply interpreted in terms of the oxida-
273 tion state of copper as has been the major thrust of the XPS work. The spectra bear little resemblance to those reported for CuO and Cu20 [32, 33]. On the contrary, a much closer resemblance can be found in the UPS spectra of oxygenexposed barium-dosed copper surfaces [26, 37] which are almost identical in all respects with the "damaged" Y - B a - C u - O result (Fig. 3(b)). The ultrahigh-vacuumqnduced changes observed in the UPS spectra of intensity exchange between satellites and low binding energy valence band feature and, in the case of Y-Ba-Cu-O, the shift in the Ba 5p emission energy are consistent with a change in the oxygen stoichiometry suggestive of oxygen loss from the surface region. No significant changes in the EELS spectra and only minor changes in the AES spectra occurred as a result of time in ultrahigh vacuum or higher energy electron beam irradiation. Extended application of the Auger exciting electron beam on Y-Ba-Cu-O led to a decrease in the oxygen-to-copper ratio whilst the reverse was true for the oxygen-to-barium Auger ratio. In the low energy Auger spectra there are changes in the 65-80 eV region which are interpreted as the disappearance of a 69 eV Ba-O-derived crosstransition which is known to be sensitive to the Ba-O environment [34J. Thus the oxygen environments of both barium and copper sites are affected by the electron beam and a reduction in oxygen concentration again seems to be indicated. Heating both materials resulted in the evolution of a large amount of oxygen into the vacuum system as monitored in a mass spectrometer. The copper Auger features decrease until at about 800 K there is no evidence of Auger emission associated with the annihilation of Cu El,2,3 level holes (Fig. 4). Clearly, copper is driven from the surface region of both samples. For the minor constituents, yttrium and lanthanum, the accessible Auger features unfortunately appear in the low kinetic energy region where there is severe overlap with other emissions. In Fig. 5 this region is displayed for both freshly scraped and heated samples. In .the case of Y-Ba-Cu-O it can be seen that the Y M2.3M4,sV Auger emission at about 130 eV barely alters and barium dominates the low energy region. For L a - S r - C u - O the Sr MeM4. 5N:, ,~ and Sr M3M4, ,sOl features overlap the Cu M1VV emission at about 110 eV. On heating, the emission at this energy is enhanced and is attributed to the segregation of strontium
AES of high Tc superconductors; Ep= 3 keY
Y-Ba-Cu-O Cu
1 Ba
4u u
0
IInF
0
Cu La +Sr Cu
fresh
heated
La-Sr-Cu-0 Cu r----
La
,r
---i
I II
fresh
heated
.
'
'1000
s60
o
KINETIC ENERGY (eV)
Fig. 4. Augerspectra of freshlyscraped and heated samples of Y-Ba-Cu-O heated to about 800 K and La-Sr-Cu-O heated to about 600 K.
to the surface as there is no evidence of copper as indicated by the disappearance of the strong Cu Me.~VV and higher energy Auger features. These major changes in surface composition are also evidenced in the EELS spectra. This is illustrated in Fig. 6 where the spectra of the freshly filed and heated surfaces are displayed in the N(E)and N'(E) modes, in both superconductors, strong modifications occur for loss energies less than 10 eV, As this loss region in both materials is interpreted as arising principally from Cu-O excitations, this modification is consistent with copper being driven from the surfaces. The Y-Ba-Cu-O loss spectra takes on the appearance of barium oxide with a band gap feature at about 5 eV loss energy [35]. The lack of change in
274 I
Low energyAES : heating effects
EELS: heating effects
iY-Ba-Cu-O fresh
L a - S r - Cu -0
Y-Ba -Cu -0
,.,," : _/
Cu+La r,,
fresh
fresh
,L
',
',
Ba
CuIBa La - Sr - Cu- 0 fresh
heated
heated
Sr÷La Ba
Ba'Cu
150
I00
50
~o
~6o
Q
~o
KINETIC ENERGY (eV) Fig. 5. L o w energy A u g e r features for freshly scraped and heated samples of Y - B a - C u - O heated to about 6 0 0 K and L a - S r - C u - O heated to about 800 K.
I
the higher loss energy region is consistent with the assignment that we have made [26] of these features principally to Ba 5 p ~ 5 d excitations rather than to plasmon and oxygen excitations [25]. For the higher energy losses (AE >/17 eV) of La-Sr-Cu-O, we have assigned the features of the fresh surface principally to La 5p --' 5d excitations [26]. On heating, the appearance of new features at 25 eV and 37-39 eV correspond to Sr 4p and Sr 4s excitations. This is consistent with the growth of the strontium Auger feature on heating. The changes induced by heating are quite complex as viewed with UPS. The He I UPS spectra on heating for both materials are displayed in Fig. 7. For Y-Ba-Cu-O, moderate heating (to about 450 K) (curve b) produces a spectrum similar to that of the damaged surface with a growth in the satellite region and a decrease in the low binding energy feature. In the He II spectra, not shown, the Ba 5p emissions shift to a higher binding energy. Beyond this temperature the satellites decrease in intensity. The low binding energy shoulder at about 2.6 eV disappears completely and a new shoulder at 4 eV becomes apparent. A peak also appears at 7 eV binding energy (Fig. 7, curve c). By about 600 K (Fig. 7, curve d), the
,,[
go
3b
2~
lb
LOSS ENERGY (eV) Fig. 6. Electron energy loss spectra or fresh and heated ;about 800 K) samples in the N I E - - and N ' ! E -m o d e s / E r , = 150 eV).
satellites and the low binding energy features disappear completely until the principal valence band emission at 5 eV and the new heatinginduced peak at 7 eV are evident. Prolonged heating at about 800 K produces a white surface as would be expected for BaO. No features are evident in the valence band emission region due in part to the lack of copper at the surface and charging effects of the BaO-tike surface. Scraping back showed green phase patches: as no carbon is present, this cannot be the green copper carbonate and is probably due to some copper complex or suboxide such as Cu~O. The effects on heating L a - S r - C u - O are rather similar with the disappearance of both satellites and low binding energy features. However. the heating-induced emassion feature appears al 7.5 eV rather than at 7 eV. The surface due to prolonged heating at about 800 K shows little in the
275
Y- Ba-Cu- 0
%_i lb
explains why the changes which they have observed occur at much higher temperatures than those reported here. Heating of the samples causes great changes in surface composition. Oxygen is liberated from the samples, copper is driven from the surfaces and insulating surface layers are formed. In the case of Y - B a - C u - O a BaO-like surface is produced whilst, for L a - S r - C u - O , strontium segregates to the surface and a L a - S r - O surface results. At no stage during heating, or indeed after scraping back the surfaces after heating, do the UPS spectra resemble that of Cu +. Visual inspection also showed no evidence of any red Cu~O.
La-Sr-Cu - 0
~
4. Conclusion
EF
lb
~
EF
BINDING ENERGY (eV)
Fig. 7. He I spectra on heating Y-Ba-Cu-O (curve a, fresh; curve b, about 450 K; curve c, about 550 K; curve d, about 600 K; curve e, about 800 K) and La-Sr-Cu-O (curve a, fresh; curve b, about 500 K; curve c, about 600 K: curve d, about 800 K).
way of UPS features, is yellow-white and fluoresces under electron bombardment. Few electron spectroscopy studies on heating of Y - B a - C u - O and, to our knowledge, none on L a - S r - C u - O have been reported. Steiner et al. [10] reported that prolonged heating at 400 °C results in increases in intensity of the higher binding energy Ba 4d XPS feature and a slight shift to higher binding energy in the Cu 2p3/2 emission. Sasaki et al. [36J reported disappearance of an OL site ( C u - O chains) related peak in the O Is XPS spectrum, consistent with neutron diffraction studies, and a decrease in the Cu 2p shake-up satellites at a temperature T of about 570 °C. On heating to about 740 °C the Cu 3p level shifts to a lower binding energy and the satellites completely disappear indicative of a reduction of Cu 2+ to Cu +. Their Ba 3d and Ba 4d XPS spectra show a narrowing above about 350 °C which they have suggested is due to the barium atoms located at different lattice sites. It is difficult to relate these results to those presented here as both Steiner el al. and Sasaki et al. have filed their samples after heating. This presumably
The clean, freshly scraped surfaces of YB a - C u - O and L a - S r - C u - O high 7~, superconductors have been characterized by laboratory-accessible electron spectroscopy. This has enabled the establishment of diagnostics, in the various spectra, to recognize "'good" surfaces. Changes due to time in ultrahigh vacuum and electron beam irradiation have been monitored. T h e intensity exchange between valence band and satellite features in the UPS spectra and, in the case of Y - B a - C u - O , the energy shift in the Ba 5p levels, indicates oxygen loss from the surface. T h e effects of heating on the surface composition has been followed by UPS, E E L S and AES. For both Y - B a - C u - O and L a - S r - C u - O , oxygen is liberated and copper is driven from the surfaces. T h e final result of heating, evidenced in all spectroscopy measurements and visually by colour, are insulating surface layers.
Acknowledgment This work has been supported by the Fonds zur F6rderung der Wissenschaftlichen Forschung of Austria.
References 1 R. L. Kurtz, R. L. Stockbauer, D. Mueller, A. Shih, L. E. Toth, M. Osifsky and S. A. Wolf, l'hys. Rev. B. 35 (1987) 8818. 2 E D. Johnson, S. L. Qiu, L. Tiang, M. W. Ruckman, M. Strangin, S. L. Hulbert, R. E Garrett, B. Sinkovic, N. V. Smith, R. J. Cava, C. S. Jee, D. Nichols, E. Kaczanowicz, R. E. Salomon and S. E. Crow, l'hvs. Rev. B, 35 (1987) 8811.
276 3 M. Onellion, Y. Chang, D. W. Niles, R. Joynt, G. Margarirondo, N. G. Stoffel and J. M. Tarascon, Phys. Rev. B, 35 (1987)819. 4 N. G. Stoffel, J. M. Tarascon, Y. Chang, M. Onellion, I). W. Niles and G. Margaritondo, Phys. Rev. B, 36 (19871 3986. 5 P. Thiry, G. Rossi, Y. Petroff, A. Revcolevschi and J. Jegoudez, Europhys. Lett., 5 (1988) 55. 6 T. Takahashi, F. Maeda, H. Arai, H. Katayama-Yoshida. Y. Okabe, T. Suzuki, S. Hosoya, A. Fujimori, T. Shidara, T. Koide, T. Miyahara, M. Onoda, S. Shamato and M. Sato, Phys. Rev. B, 36(19871 5686. 7 J. A. Yarmoff, D. R. Clarke, W. Drube, U. O. Karlsson, A. Taleb-Ibrahimi and F. J. Himpsel, Phys. Rev. B, 36 (1987) 3967. 8 H. lhyra, M. Hirabayashi, N. Tarada, Y. Kimiva, K. Senzaki, M. Akimoto, K. Bushida, F. Kawashima and R. Uzuka, Jpn. J. Appl. Phys., 26 (1987) IA60. 9 N. Fukushima, H. Yoshino, H. Niu, M. Hayashi, H. Sasaki, Y. Yamada and S. Murase, Jpn. J. Appl. Phys., 26 (1987) L719. 10 R Steiner, V. Kinsinger, I. Sander, B. Seigwart, S. Hfifner and C. Politis, Z. Phys. B, 67(19871 19. 11 D. D. Sarma, K. Sreedhar, E Ganguly and C. N. R. Rao, Phys. Rev. B, 36(19871 2371. 12 D. E. Ramaker, N. H. Turner, J. S. Murday, L. E. Toth, Osofsky and F. L. Hutson, Phys. Rev. B, 36 (1987) 5672. 13 B. Dauth, T. Kachel, P. Sen, K. Fischer and M. Campagna, Z. Phys. B, 68(1987)407. 14 H. Fjellvfig, R Karen, A. Kjekshus and J. K. Grepstad, Solid State Commun., 64 (1987) 917. 15 D. D. Sarma and C. N. R. Rao, Solid State Commun., 05 (1988147. 16 F. Werfel, H. Heinonen and E. Suoninen, Z. Phys. B, 70 (19881 317. 17 A. Fujimori, E. Takayama-Muromachi, Y. Uchida and B. Okai, Phys. Rev. B, 35(19871 8814. 18 B. Reihl, T. Riesterer, J. G. Bednorz and K. A. Miiller, Phys. Rev. B, 35(1987). T. Riesterer, J. G. Bednorz, K. A. Miiller and B, Reihl,
Appl. Phys. A, 44 (1987) 81. 19 H. lhara, M. Hirabayashi, N. Tcrada. Y. Kimura, K. Senzaki and M. Tokumoto, Jim. J. Appl. Phys., 26(I 9871! L4(~3. 20 N. Ni.icker, J. Fink, B. Renker, D. L:wert, C. Potitis, P. J, W. Weijs and J. C. Fuggle, Z./'hrs. B, 67(1967) !7. 21 P. Steiner, R. Courths, V. Kinsingcr, I. Sander, B. Siegwart, S. Hiifner and C. Polilis, Appl. Phys. ~t, 44 (1987) 75. 22 T. Takahashi, E Maeda, S. Hosoya and M. Sato, Jpn. J. Appl. Phys., 26 (1987) L349. 23 B. Batlogg, R. J. Cava, A. Jayaraman, R. B. van l)over, G. A. Kowouklis, S. Sunshine, D. W. Murphy, L. W. Rupp, H. S. Chen, A. White, K. T. Short. A. M, Mujsce and E. A. Rietman. l'hys. Rev. Lett., 5,~ ( 1~87, 2333. 24 F. P. Netzer and M. M. El Gomati, SurJl Sci., 124 (19831 26. 25 Y. Chang, M. Onellion, D. W. Niles, R. Joynt and G. Margaritondo, Solid State Commun.; 64 (1987) 71% 26 M. Ramsey, E E Netzer and J. A. 11. Matthew, Phys. Rev. B, in the press. 27 Y. Goa, T. ,I. Wagener, J. H. Weaver, A. J. Arko, B. Flandermeyer and D. W. Capone, Phys. Rev. B, 36 (I 987) 397t 28 M. Ayoob and M. S. Hedge, 8ur[i Sci., 147(1984) 361. 29 K. Jacobi, C. Astaldi, B. Frick and P. Geng, 8u~i Sci., 189-190 (1987) 578. 30 K. Jacobi, C. Astaldi, B. Frick and R Geng, Phys. Rev. B, in the press. 31 M.S. Hedge and M. Ayoob, SurJi ~ci., ! 73 ( 1986 ) L638. 32 C. Benndoff, H. Caus, B. Egert, H. Seidel and F. Thieme, ,I. Electron 8pectrosc. Relat. Phenom., 19(1980) 77. 33 M. R. Thuler, R. L. Benbow and Z. Hurych, Phys. Rev. B. 26(19821669. 34 G.A. Haas and A. Shih, Appl. Sutjl Sci., 31 (19881239. 35 A. R. Protheroe, A. Steinbrunn and T. E. Gallon, SurJ: S(i., 126 (1983) 534. 36 'F. A. Sasaki, Y. Baba, N. Masaki and 1. Takano, Jpn. J. Appl. Phys., 26 (1987) L1569.
269
Monitoring of the Surface Decomposition of Y-Ba-Cu-O and La-Sr-Cu-O by Electron Spectroscopy M. G. RAMSEY and E P. NETZER
lnstitut j~ir Physikalische Chemic, Universitdt hmsbruck, A-6020 lnnsbruck (Austria) (Received August 1, 1988)
Abstract
~lhe surfaces of the high Tc superconductors L a - S r - C u - O and Y - B a - C u - O have been characterized by the laboratory-accessible techniques of ultraviolet photoelectron electron spectroscopy, electron energy loss spectroscopy and Auger electron spectroscopy. The damage to the surfaces due to the time in ultrahigh vacuum and electron beam irradiation have been followed by the various techniques which indicate a loss of oxygen. 7he decomposition of the surfaces due to heating has also been investigated. Oxygen is seen to be liberated from the samples and, as evidenced in all spectroscopy techniques, copper is completely driven from the surfaces until insulating surface layers of L a - S r - O and Ba-O are formed. 1. Introduction
The time has come for more careful characterization of the high 7~. superconductor surfaces. In this paper we present an electron spectroscopy survey of Lal.~Sr0.jsCuO4 (Tc=36K) and YBa2Cu307, surfaces both superconducting (Tc=93 K) and non-superconducting with the techniques of Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy (UPS) and electron energy loss spectroscopy (EELS). The experiments were not only confined to the freshly scraped surfaces but also to surfaces affected by time in ultrahigh vacuum, electron beam irradiation and heating. This allows less ambiguous assignment of the various features observed and the resulting electronic and chemical characterization will, hopefully, be useful to assess the surface conditions for other experimenters. The majority of the electron spectroscopy investigations reported in the literature have been synchrotron radiation studies [1-7] of the valence 1)921-511)7/89/$3.50
band and lower lying core levels and X-ray photoelectron spectroscopy (XPS) of the deeper core levels of Y - B a - C u - O [7-17]. Far fewer electron spectroscopy studies of L a - S r - C u - O have been reported [17-22] as this type was supplanted by Y - B a - C u - O with a more accessible critical temperature. Although it is improbable that electron spectroscopy will solve the riddle of the high temperature superconduction mechanism, such studies are important for characterizing the surfaces of these materials with standard techniques. This is particularly so, given the great interest in thin films, because of their applications in microelectronics and, with their greater malleability and higher current densities, in macroscopic systems. An understanding of how these materials decompose and the changes which occur at the surface, as modelled by heating, are important, given the various methods used for their production. This is also important for the single-crystal studies now being embarked on, given the experimental difficulties of working with these relatively small samples.
2. Experimental details
Two samples of nominal YBa2Cu307_ x stoichiometry and one of Lal.ssSr0.15CuO ~ were investigated. The samples were prepared according to an established procedure [23] at the National Bureau of Standards, Gaithersburg, MD, U.S.A., and kindly provided by Dr. D. L. Ederer and Dr. C. W. Clark. A.c. magnetic susceptibility measurements of L a - S r - C u - O and Y - B a - C u - O I samples showed them to be superconducting with critical temperature onsets of 36 K and 93 K respectively and transition widths of a few kelvins. Sample Y - B a - C u - O II was nonsuperconducting and was studied for the purposes of comparison. The pressed pellets were © Elsevier Sequoia/Printed in The Netherlands
270
attached to an ultrahigh vacuum manipulator via tantalum clamps to a tantalum base plate. Fresh clean surfaces characteristic of the bulk composition were obtained by repeated scraping in situ with a corundum file. The samples could be cooled to 80 K or indirectly heated to about 800 K. The investigations were performed in a VGADES 400 electron spectrometer [24] with a spherical sector electron energy analyser. A helium discharge lamp provided the He I ( h v = 2 1 . 2 eV) and He II ( h v = 4 0 . 8 eV) radiation for the photoemission experiments. The electron energy loss spectra were obtained in both N(E) and (dN/dE){N'(E)} modes with a primary energy Ep range of 10-2000 eV. The Auger spectra were excited by 3 keV incident electrons. The system was baked to only 75 °C to avoid oxygen loss from the samples, but even so a base pressure of about 5 x 10 -~I mbar was achieved.
AES of high Tc superconductors Ep=3 keY [tJ
ga
YBaEu0 I , ~
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(u
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£_
~
3. Results and discussion
Representative N'(E) Auger spectra for the freshly scraped Y - B a - C u - O and L a - S r - C u - O are displayed in Fig. 1. The features associated with the various constituents are readily identified and are marked on the spectra. Scanning across the surfaces yielded marked variations in the Auger ratios of the elements, suggesting pronounced inhomogeneities characteristic of multigrain materials. No significant differences between the composition of superconducting and that of non-superconducting Y - B a - C u - O were detectable, although the ratio of O KLL to Cu L3VV varied more across the non-superconducting sample whilst the ratio of O KLL to Ba M45N45N45 was relatively stable in both superconducting and non-superconducting Y - B a Cu-O. In a number of published studies, carbon contamination has been found even after scraping [3, 4, 14, 25]. It is therefore important to note that no carbon contamination was detectable on the surfaces of these samples after one or two scrapings. No changes in the Auger features of Y - B a - C u - O were observed on cooling below Tc. In particular the changes to the Cu L3VV reported by Sarma and coworkers [11, 15] could not be reproduced. The He I and He II UPS spectra for the freshly scraped superconducting (I) and non-supercon-
[u+Sr
J+La La
1000
500 ENERGY (eV)
(
Fig. 1. Representative Auger spectra of the freshly scraped samples. Prominent features are Cu L~VV at about 915 eV, Ba M45NasN45, at about 600 eV, La M45N45N45 at about 625 eV and O KLL at about 505 eV.
ductmg (II) Y - B a - C u - O and L a - S r - C u - O are displayed in Fig. 2. These surfaces were arrived at by repeated scrapings until no changes were observed on further scraping. It should be noted that this occurred although only one or two scrapings were required to produce contaminatefree surfaces as observed by AES. In scraping down from the surfaces as mounted, the changes in the UPS spectra of the two types of material were essentially opposite: for Y-Ba-Cu--O, scraping resulted in a decrease in the 8-13 eV binding energy structure with a concomitant increase in the feature at 2.5 eV whilst the reverse was true for L a - S r - C u - O . The spectra are consistent with those reported in the literature [1-7, 17, 19-22]. In the case of L a - S r - C u - O , however, the valence band spectra which have been reported have the
271
UPSof high Tc superconductors Hell
Hel
Y - B a - E u - ~
Hell
Hel ~
~
/t
Y-Ba-[u-O lI
\
La-S
,
l
,
15
,
,
,
l
,
,
,
,
,
J
10 5 BINDING ENERGY (eV)
,
i
l
,
EF
Fig. 2. He I and II UPS spectra of the freshly scraped samples. The region around EF is shown magnified for the superconducting Y - B a - C u - O I below and above its critical temperature. It should be noted that no changes are observed on cooling.
appearance of "damaged" spectra obtained in this work, with low intensity in the 8-13 eV region and relatively high intensity in the region of the low binding energy feature. As observed by others, there was no change in the density of states at E v on cooling below Tc. Both superconductors show a two-peak valence band structure with maxima at 2.6-2.8 eV and 4.6-5.2 eV due to the O 2p-Cu 3d derived emission and, beyond these, so-called satellites are observed. No appreciable differences were found between the superconducting and non-superconducting Y - B a - C u - O UPS spectra although EELS spectra showed significant differences [26]. Differences between the electron distribution curves for Y - B a - C u - O and
L a - S r - C u - O are observed, however. For the freshly scraped Y - B a - C u - O the peaks at 13.5 and 15.5 eV are due to emission from the Ba 5p3/2 and Ba 5pl/2 levels. The significance of the relatively low binding energy of these features is discussed below. Furthermore, freshly scraped Y-Ba-Cu-O shows a single relatively weak satellite at 9.1 eV compared with the strong satellites at 9.8 and 12.0 eV (all energies are taken from He II spectra to avoid background effects) for La-Sr-Cu-O. For both materials there is a striking anticorrelation between the intensities of the satellites and the low binding energy valence band feature at 2.6-2.8 eV. Any changes in the physical or chemical composition of the samples, as a result of the time in ultrahigh vacuum, electron beam irradiation or temperature treatment, caused intensity exchange between those satellites and the valence band structure. This is illustrated in Fig. 3 where representative freshly scraped and "damaged" (but clean) UPS spectra are shown for the two superconductors. In the case of Y-Ba-Cu-O, leaving the samples in ultrahigh vacuum results in a decrease in the 2.6 eV feature and a growth in the approximately 9 eV satellite and the appearance of a second satellite at 11.5 eV (cf. Figs. 3(a) and 3(b)). The resulting spectra are almost identical with those of the freshly filed L a - S r - C u - O except that the satellite peaks are about 0.5 eV higher in binding energy for the latter (Figs. 3(c) and 3(d)). Similar, although less pronounced, intensity exchange is observed for L a - S r - C u - O (Figs. 3(c)-3(e)). For both samples the Auger-stimulating electron beam causes changes identical, although accelerated, with those of merely leaving the samples in ultrahigh vacuum. La-Sr-Cu-O, however, exhibits a peculiar behaviour under the influence of electron beams of moderate energies (300 eV~
272 Y- B~ He \ frl
lw La -~.
aft
1'~ ....
1()
BINDING ENERGYeV
,,,,,N
5
EF
Fig. 3. Typical UPS spectra of freshly scraped and "damaged" surfaces:(a) fresh Y-Ba-Cu-O surface, He ll; (b) Y-Ba-Cu-O after 1 week in ultrahigh vacuum; (c), (d) He II and He 1 spectra of fresh La-Sr-Cu-O; (e) He 1 of La-Sr-Cu-O after 1 week in ultrahigh vacuum; (f), (g) He 1 and He II spectra of La-Sr-Cu-O showing the effects of a 300 eV electron beam. E E L S [26] spectra to that of the freshly scraped surface. It is interesting to note that Goa et al. [27], who studied the empty electronic states of L a - S r - C u - O with inverse photoemission, have also observed changes which are reversed by a higher energy electron beam. Whilst they have proposed that their results are related to chemisorbed oxygen, we argue, given the lack of oxygen in our residual atmosphere or carbon in the Auger spectra (from possible CO dissociation), that the results suggest that electrons of intermediate energies create oxygen defect states which are healed by higher energy beams. In both materials the relationship between the intensities of the satellite and the low binding energy feature supports the assignments to corre-
lation satellites and emission from hybrid O 2plike states respectively, made in studies of the valence band emission of " r - B a - C u - O as a function of photon energy [ 1,5, 6 }. In addition to the valence band emissions the Ba 5p~,, and Ba 5pl/2 levels also provide a diagnostic of the state of the Y - B a - C u - O surface accessible to laboratory UPS experiments. For Y - B a - C u - O , time in ultrahigh vacuum and electron-beam-induced changes to the valence band and satellite region are also parallelled in shifts to the Ba 5p levels. The fresh surfaces have Ba 5p3:2 and Ba 5pl/2 levels of 13.4 eV and 15.5 eV respectively. These values are in agreement with those reported by Thiry el al. [5] but are of the order of 1.0 eV lower than those reported for other B a - O systems [28-31i. Time in ultrahigh vacuum, heating or application ot an electron beam causes first broadening to higher binding energies and finally results in the peaks appearing at 14.0 eV and 16.0 eV respectively (Figs. 3(a) and 3(b)), an increase in binding energy of about 0.5 eV, and somewhat nearer the typical oxidized barium values. Jacobi el aL [2~, 30] have shown that the core levels of bulk barium all shift on oxidation to a lower binding energy by approximately the same amount. They have attributed this shift to the fact that the full O 2p level (02 is highly polarizable with the ability of shielding very effectively in the final state. The XPS spectra published for the Y - B a - C u - O Ba 4d and Ba 3d levels show the principal peaks occurring at the recognized oxidized barium valuesa however, they also indicate lower binding energy shoulders {10, 14, 16]. Steiner et al. ] 10] have noted the disappearance of these shoulders on heating and have associated them, in the manner of ,lacobi et al. [29, 30], with Ba 2- being completely coordinated with O: . Whilst we can only conjecture as to why the barium levels are even lower in binding energy for Y - B a - C u - O than for oxidized barium, it is suggested that our results which show that the 5p levels are clearly shifted to lower binding energy for the fresh surfaces rather than merely broadened to lower binding energy is significant. This suggests that the deeper core level emission results which have been reported may be due to the presence of "damage", such as the loss of oxygen, as opposed to the intrinsic existence of inequivalent B a - O configurations in Y - B a - C u - O as suggested by Werfel et al. [16J. The valence band photoemission spectra cannot be simply interpreted in terms of the oxida-
273 tion state of copper as has been the major thrust of the XPS work. The spectra bear little resemblance to those reported for CuO and Cu20 [32, 33]. On the contrary, a much closer resemblance can be found in the UPS spectra of oxygenexposed barium-dosed copper surfaces [26, 37] which are almost identical in all respects with the "damaged" Y - B a - C u - O result (Fig. 3(b)). The ultrahigh-vacuumqnduced changes observed in the UPS spectra of intensity exchange between satellites and low binding energy valence band feature and, in the case of Y-Ba-Cu-O, the shift in the Ba 5p emission energy are consistent with a change in the oxygen stoichiometry suggestive of oxygen loss from the surface region. No significant changes in the EELS spectra and only minor changes in the AES spectra occurred as a result of time in ultrahigh vacuum or higher energy electron beam irradiation. Extended application of the Auger exciting electron beam on Y-Ba-Cu-O led to a decrease in the oxygen-to-copper ratio whilst the reverse was true for the oxygen-to-barium Auger ratio. In the low energy Auger spectra there are changes in the 65-80 eV region which are interpreted as the disappearance of a 69 eV Ba-O-derived crosstransition which is known to be sensitive to the Ba-O environment [34J. Thus the oxygen environments of both barium and copper sites are affected by the electron beam and a reduction in oxygen concentration again seems to be indicated. Heating both materials resulted in the evolution of a large amount of oxygen into the vacuum system as monitored in a mass spectrometer. The copper Auger features decrease until at about 800 K there is no evidence of Auger emission associated with the annihilation of Cu El,2,3 level holes (Fig. 4). Clearly, copper is driven from the surface region of both samples. For the minor constituents, yttrium and lanthanum, the accessible Auger features unfortunately appear in the low kinetic energy region where there is severe overlap with other emissions. In Fig. 5 this region is displayed for both freshly scraped and heated samples. In .the case of Y-Ba-Cu-O it can be seen that the Y M2.3M4,sV Auger emission at about 130 eV barely alters and barium dominates the low energy region. For L a - S r - C u - O the Sr MeM4. 5N:, ,~ and Sr M3M4, ,sOl features overlap the Cu M1VV emission at about 110 eV. On heating, the emission at this energy is enhanced and is attributed to the segregation of strontium
AES of high Tc superconductors; Ep= 3 keY
Y-Ba-Cu-O Cu
1 Ba
4u u
0
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0
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fresh
heated
La-Sr-Cu-0 Cu r----
La
,r
---i
I II
fresh
heated
.
'
'1000
s60
o
KINETIC ENERGY (eV)
Fig. 4. Augerspectra of freshlyscraped and heated samples of Y-Ba-Cu-O heated to about 800 K and La-Sr-Cu-O heated to about 600 K.
to the surface as there is no evidence of copper as indicated by the disappearance of the strong Cu Me.~VV and higher energy Auger features. These major changes in surface composition are also evidenced in the EELS spectra. This is illustrated in Fig. 6 where the spectra of the freshly filed and heated surfaces are displayed in the N(E)and N'(E) modes, in both superconductors, strong modifications occur for loss energies less than 10 eV, As this loss region in both materials is interpreted as arising principally from Cu-O excitations, this modification is consistent with copper being driven from the surfaces. The Y-Ba-Cu-O loss spectra takes on the appearance of barium oxide with a band gap feature at about 5 eV loss energy [35]. The lack of change in
274 I
Low energyAES : heating effects
EELS: heating effects
iY-Ba-Cu-O fresh
L a - S r - Cu -0
Y-Ba -Cu -0
,.,," : _/
Cu+La r,,
fresh
fresh
,L
',
',
Ba
CuIBa La - Sr - Cu- 0 fresh
heated
heated
Sr÷La Ba
Ba'Cu
150
I00
50
~o
~6o
Q
~o
KINETIC ENERGY (eV) Fig. 5. L o w energy A u g e r features for freshly scraped and heated samples of Y - B a - C u - O heated to about 6 0 0 K and L a - S r - C u - O heated to about 800 K.
I
the higher loss energy region is consistent with the assignment that we have made [26] of these features principally to Ba 5 p ~ 5 d excitations rather than to plasmon and oxygen excitations [25]. For the higher energy losses (AE >/17 eV) of La-Sr-Cu-O, we have assigned the features of the fresh surface principally to La 5p --' 5d excitations [26]. On heating, the appearance of new features at 25 eV and 37-39 eV correspond to Sr 4p and Sr 4s excitations. This is consistent with the growth of the strontium Auger feature on heating. The changes induced by heating are quite complex as viewed with UPS. The He I UPS spectra on heating for both materials are displayed in Fig. 7. For Y-Ba-Cu-O, moderate heating (to about 450 K) (curve b) produces a spectrum similar to that of the damaged surface with a growth in the satellite region and a decrease in the low binding energy feature. In the He II spectra, not shown, the Ba 5p emissions shift to a higher binding energy. Beyond this temperature the satellites decrease in intensity. The low binding energy shoulder at about 2.6 eV disappears completely and a new shoulder at 4 eV becomes apparent. A peak also appears at 7 eV binding energy (Fig. 7, curve c). By about 600 K (Fig. 7, curve d), the
,,[
go
3b
2~
lb
LOSS ENERGY (eV) Fig. 6. Electron energy loss spectra or fresh and heated ;about 800 K) samples in the N I E - - and N ' ! E -m o d e s / E r , = 150 eV).
satellites and the low binding energy features disappear completely until the principal valence band emission at 5 eV and the new heatinginduced peak at 7 eV are evident. Prolonged heating at about 800 K produces a white surface as would be expected for BaO. No features are evident in the valence band emission region due in part to the lack of copper at the surface and charging effects of the BaO-tike surface. Scraping back showed green phase patches: as no carbon is present, this cannot be the green copper carbonate and is probably due to some copper complex or suboxide such as Cu~O. The effects on heating L a - S r - C u - O are rather similar with the disappearance of both satellites and low binding energy features. However. the heating-induced emassion feature appears al 7.5 eV rather than at 7 eV. The surface due to prolonged heating at about 800 K shows little in the
275
Y- Ba-Cu- 0
%_i lb
explains why the changes which they have observed occur at much higher temperatures than those reported here. Heating of the samples causes great changes in surface composition. Oxygen is liberated from the samples, copper is driven from the surfaces and insulating surface layers are formed. In the case of Y - B a - C u - O a BaO-like surface is produced whilst, for L a - S r - C u - O , strontium segregates to the surface and a L a - S r - O surface results. At no stage during heating, or indeed after scraping back the surfaces after heating, do the UPS spectra resemble that of Cu +. Visual inspection also showed no evidence of any red Cu~O.
La-Sr-Cu - 0
~
4. Conclusion
EF
lb
~
EF
BINDING ENERGY (eV)
Fig. 7. He I spectra on heating Y-Ba-Cu-O (curve a, fresh; curve b, about 450 K; curve c, about 550 K; curve d, about 600 K; curve e, about 800 K) and La-Sr-Cu-O (curve a, fresh; curve b, about 500 K; curve c, about 600 K: curve d, about 800 K).
way of UPS features, is yellow-white and fluoresces under electron bombardment. Few electron spectroscopy studies on heating of Y - B a - C u - O and, to our knowledge, none on L a - S r - C u - O have been reported. Steiner et al. [10] reported that prolonged heating at 400 °C results in increases in intensity of the higher binding energy Ba 4d XPS feature and a slight shift to higher binding energy in the Cu 2p3/2 emission. Sasaki et al. [36J reported disappearance of an OL site ( C u - O chains) related peak in the O Is XPS spectrum, consistent with neutron diffraction studies, and a decrease in the Cu 2p shake-up satellites at a temperature T of about 570 °C. On heating to about 740 °C the Cu 3p level shifts to a lower binding energy and the satellites completely disappear indicative of a reduction of Cu 2+ to Cu +. Their Ba 3d and Ba 4d XPS spectra show a narrowing above about 350 °C which they have suggested is due to the barium atoms located at different lattice sites. It is difficult to relate these results to those presented here as both Steiner el al. and Sasaki et al. have filed their samples after heating. This presumably
The clean, freshly scraped surfaces of YB a - C u - O and L a - S r - C u - O high 7~, superconductors have been characterized by laboratory-accessible electron spectroscopy. This has enabled the establishment of diagnostics, in the various spectra, to recognize "'good" surfaces. Changes due to time in ultrahigh vacuum and electron beam irradiation have been monitored. T h e intensity exchange between valence band and satellite features in the UPS spectra and, in the case of Y - B a - C u - O , the energy shift in the Ba 5p levels, indicates oxygen loss from the surface. T h e effects of heating on the surface composition has been followed by UPS, E E L S and AES. For both Y - B a - C u - O and L a - S r - C u - O , oxygen is liberated and copper is driven from the surfaces. T h e final result of heating, evidenced in all spectroscopy measurements and visually by colour, are insulating surface layers.
Acknowledgment This work has been supported by the Fonds zur F6rderung der Wissenschaftlichen Forschung of Austria.
References 1 R. L. Kurtz, R. L. Stockbauer, D. Mueller, A. Shih, L. E. Toth, M. Osifsky and S. A. Wolf, l'hys. Rev. B. 35 (1987) 8818. 2 E D. Johnson, S. L. Qiu, L. Tiang, M. W. Ruckman, M. Strangin, S. L. Hulbert, R. E Garrett, B. Sinkovic, N. V. Smith, R. J. Cava, C. S. Jee, D. Nichols, E. Kaczanowicz, R. E. Salomon and S. E. Crow, l'hvs. Rev. B, 35 (1987) 8811.
276 3 M. Onellion, Y. Chang, D. W. Niles, R. Joynt, G. Margarirondo, N. G. Stoffel and J. M. Tarascon, Phys. Rev. B, 35 (1987)819. 4 N. G. Stoffel, J. M. Tarascon, Y. Chang, M. Onellion, I). W. Niles and G. Margaritondo, Phys. Rev. B, 36 (19871 3986. 5 P. Thiry, G. Rossi, Y. Petroff, A. Revcolevschi and J. Jegoudez, Europhys. Lett., 5 (1988) 55. 6 T. Takahashi, F. Maeda, H. Arai, H. Katayama-Yoshida. Y. Okabe, T. Suzuki, S. Hosoya, A. Fujimori, T. Shidara, T. Koide, T. Miyahara, M. Onoda, S. Shamato and M. Sato, Phys. Rev. B, 36(19871 5686. 7 J. A. Yarmoff, D. R. Clarke, W. Drube, U. O. Karlsson, A. Taleb-Ibrahimi and F. J. Himpsel, Phys. Rev. B, 36 (1987) 3967. 8 H. lhyra, M. Hirabayashi, N. Tarada, Y. Kimiva, K. Senzaki, M. Akimoto, K. Bushida, F. Kawashima and R. Uzuka, Jpn. J. Appl. Phys., 26 (1987) IA60. 9 N. Fukushima, H. Yoshino, H. Niu, M. Hayashi, H. Sasaki, Y. Yamada and S. Murase, Jpn. J. Appl. Phys., 26 (1987) L719. 10 R Steiner, V. Kinsinger, I. Sander, B. Seigwart, S. Hfifner and C. Politis, Z. Phys. B, 67(19871 19. 11 D. D. Sarma, K. Sreedhar, E Ganguly and C. N. R. Rao, Phys. Rev. B, 36(19871 2371. 12 D. E. Ramaker, N. H. Turner, J. S. Murday, L. E. Toth, Osofsky and F. L. Hutson, Phys. Rev. B, 36 (1987) 5672. 13 B. Dauth, T. Kachel, P. Sen, K. Fischer and M. Campagna, Z. Phys. B, 68(1987)407. 14 H. Fjellvfig, R Karen, A. Kjekshus and J. K. Grepstad, Solid State Commun., 64 (1987) 917. 15 D. D. Sarma and C. N. R. Rao, Solid State Commun., 05 (1988147. 16 F. Werfel, H. Heinonen and E. Suoninen, Z. Phys. B, 70 (19881 317. 17 A. Fujimori, E. Takayama-Muromachi, Y. Uchida and B. Okai, Phys. Rev. B, 35(19871 8814. 18 B. Reihl, T. Riesterer, J. G. Bednorz and K. A. Miiller, Phys. Rev. B, 35(1987). T. Riesterer, J. G. Bednorz, K. A. Miiller and B, Reihl,
Appl. Phys. A, 44 (1987) 81. 19 H. lhara, M. Hirabayashi, N. Tcrada. Y. Kimura, K. Senzaki and M. Tokumoto, Jim. J. Appl. Phys., 26(I 9871! L4(~3. 20 N. Ni.icker, J. Fink, B. Renker, D. L:wert, C. Potitis, P. J, W. Weijs and J. C. Fuggle, Z./'hrs. B, 67(1967) !7. 21 P. Steiner, R. Courths, V. Kinsingcr, I. Sander, B. Siegwart, S. Hiifner and C. Polilis, Appl. Phys. ~t, 44 (1987) 75. 22 T. Takahashi, E Maeda, S. Hosoya and M. Sato, Jpn. J. Appl. Phys., 26 (1987) L349. 23 B. Batlogg, R. J. Cava, A. Jayaraman, R. B. van l)over, G. A. Kowouklis, S. Sunshine, D. W. Murphy, L. W. Rupp, H. S. Chen, A. White, K. T. Short. A. M, Mujsce and E. A. Rietman. l'hys. Rev. Lett., 5,~ ( 1~87, 2333. 24 F. P. Netzer and M. M. El Gomati, SurJl Sci., 124 (19831 26. 25 Y. Chang, M. Onellion, D. W. Niles, R. Joynt and G. Margaritondo, Solid State Commun.; 64 (1987) 71% 26 M. Ramsey, E E Netzer and J. A. 11. Matthew, Phys. Rev. B, in the press. 27 Y. Goa, T. ,I. Wagener, J. H. Weaver, A. J. Arko, B. Flandermeyer and D. W. Capone, Phys. Rev. B, 36 (I 987) 397t 28 M. Ayoob and M. S. Hedge, 8ur[i Sci., 147(1984) 361. 29 K. Jacobi, C. Astaldi, B. Frick and P. Geng, 8u~i Sci., 189-190 (1987) 578. 30 K. Jacobi, C. Astaldi, B. Frick and R Geng, Phys. Rev. B, in the press. 31 M.S. Hedge and M. Ayoob, SurJi ~ci., ! 73 ( 1986 ) L638. 32 C. Benndoff, H. Caus, B. Egert, H. Seidel and F. Thieme, ,I. Electron 8pectrosc. Relat. Phenom., 19(1980) 77. 33 M. R. Thuler, R. L. Benbow and Z. Hurych, Phys. Rev. B. 26(19821669. 34 G.A. Haas and A. Shih, Appl. Sutjl Sci., 31 (19881239. 35 A. R. Protheroe, A. Steinbrunn and T. E. Gallon, SurJ: S(i., 126 (1983) 534. 36 'F. A. Sasaki, Y. Baba, N. Masaki and 1. Takano, Jpn. J. Appl. Phys., 26 (1987) L1569.