X-ray photoelectron spectroscopy study of chemically-etched NdCeCuO surfaces

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Solid State Communications,Vol. 78, No. 4, PP. 303-306, 1991. Printed in Great Britain.

Pergamon Press plc

X-RAY PHOTOELECTRON SPECTROSCOPY STUDY OF CHEMICALLYETCHED Nd-Ce-Cu-O SURFACES R. P. Vasquez Center for Space Microelectronics Technology Jet Propulsion Laboratory ualiforn a nst tute of Technology, Pasadena, California 91109 USA A. Gupta IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598 USA A. Kussmaul Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 USA

(Received25 January 1991 by A. A. Maradudin) Acetic acid, Br2, and HCI solutions are investigated for removing insulating species from N d 1 85Ce 0 lsCuO4.6 (NCCO) thin film surfaces. X-ray photoelectron spectfoscopy'(XPS) shows that the HCI etch is most effective, yielding O l s spectra comparable to those obtained from samples cleaned in vacuum and a clear Ferm= edge in the valence band region. Reduction and oxidation reversibly induces and eliminates, respectively, Fermi level states for undoped samples, but has no clearly observable effect on the XPS spectra for doped samples. Reactivity to air is much less for NCCO compared to hole superconductors, which is attributed to the lack of reactive alkaline earth elements in NCCO.

It is well known that the high temperature superconductors react with air to form insulating surface species. The removal of such species is necessary prior to studies utilizing surface sensitive spectroscopies such as x-ray photoelectron spectroscopy (XPS), and is also desirable in applications such as the formation of low resistance electrical contacts. For the electron superconductor N d l 85Ce 0 15CUO4.8 (NCCO), the removal of nonsuperconducting surface species has been done in vacuum by cleaving, 1-3 scraping,l,3-10 or heating. 11 Scraping has been shown to damage the material, yielding a surface which exhibits no Fermi edge in the valence band region. 1,3 Cleaving yields surfaces which do exhibit Fermi edges, but has the disadvantage that cleavage occurs at the weakest points in the material, which studies of other superconductors have shown are often segregated impurity phases. In addition, cleaving is not applicable to thin film samples, which are of interest for many applications. Heating in vacuum has been applied to thin film samples, and a clear Fermi edge is observed on such samples,11 indicating a high quality surface. An alternative approach is chemical etching, which has been shown to yield high quality surfaces on YBa:>Cu307.x, 12-14 Bl~Sr~.xCa I +xCu2OR+6,15 and TI2Ba2CaCu208+x.16 ]~r21n absolute methanol or absolute ethanol, the etchant used in the earlier studies, is also expected to effectively remove nonsuperconducting species from NCCO surfaces, and HCI in the same solvents has also been suggested 15 for this purpose. A third etchant, acetic acid, has been shown toproduce lower resistance electrical contacts on NCCO surfaces. 17 NCCO surfaces treated with these three etchants are

characterized with XPS, and preliminary results are described here. Epitaxial NCCO films with c-axis orientation are grown at IBM by laser ablation on SrTiO 3 (100) substrates. Details of the film growth and characterization ai'e described elsewhere. 18 The XPS spectra are measured at JPL on a Surface Science Instruments SSX-501 spectrometer with monochromatized AI Kcx.x-rays (1486.6 eV) and a base pressure of 3 x 10-lu Torr. Chemical etching is done in the ultrahigh purity N2 atmosphere of a glove box which encloses the sample introduction area of the XPS spectrometer. The superconducting films are immersed in the etchant followed by rinsing in ethanol and blow drying with nitrogen. Air exposures subsequent to etching are done by removing films from the glove box for a known length of time. Typical O l s spectra measured from the initial and chemically-etched NCCO surfaces are shown in Fig. 1. The low binding energy peak is observed at 528.9:1:0.2 eV on all samples, and is" not significantly affected by oxidation, reduction, or the resence or absence of the Ce dopant. Unlike the ole superconductors, the presence of a low binding energy O 1s signal is therefore not sufficient to establish the presence of the superconducting phase. The high binding energy peak, corresponding to insulating species, is reduced in intensity by etching for 2 min in 10% acetic acid (HAc) in absolute methanol, as shown in Fig. l(b). The reduced level of surface insulating species is consistent with the observed lower resistance electrical contacts obtained on HAc-etched surfaces. 17 The intensity of the high binding energy O l s signal is not reduced further with increased

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1. O l S spectra measured from thin film Ndl 85Ce0 15CUO4-8 superconductor samples (a) as grown, (b) etched in 10% acetic acid in absolute methanol for 2 min, (c) etched in 1% Br2 in absolute ethanol for 20 rain, and (d) etched in 10% HCI in absolute methanol for 30 s.

etch time up to 15 min, or by replacing the solvent with water. The presence of a C 1s signal near 289 eV, corresp.onding to carboxylate groups, is consistent w=th acetate formation. The intensity of the high binding energy O l s signal decreases monotonically with increasing etch time for the 1% Br2/ethanol etchant. However, the etch rate is very slow. A spectrum obtained after 20 min etching is shown in Fig. 1(c). This etchant leaves residual Br on the surface at a level of ~4 atomic percent (a/o) and results in a clear Cu 1+ signal, probably due to CuBr, in the Cu 2p spectra. Since use of a stronger etching solution may create a safety hazard, 19 H[3r is a promising alternative for faster etching 13 and is currently being in.vestig.ated. The high binding energy O l s signal is aJmost completely eliminated by a 30 s etch in 10% HCI in absolute methanol or absolute ethanol, as shown in Fig. 1(d). The spectrum in Fig. 1(d) is comparable to spectra published in the literature measured from samples cleaned in vacuum. The HCI etchant leaves ~7-10 a/o residual surface CI and also results in a clear Cu 1+ signal, probably due to CuCI, in the Cu 2p spectra. Despite the probable formation of CuCI by the HCI etchant, the nearly complete removal of surface species resulting from air exposure makes this etchant promising for many applications, and is hereafter used to yield standardized NCCO surfaces. The HCI etch yields surfaces which are close to the ideal stoichlometry for Nd2CuO4 and for as-deposited, but unannealed, ~ICCO samples. However, NCCO which is annealed in vacuum to produce the superconducting phase has a Cu-

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deficient surface layer, with only -1/3 the ideal concentration of Cu, which is not removed even for long etch times with any of the etchants. A Cu-deficient surface layer is not observed on a sample annealed in nitrogen. The Cu-deficient layer could only be removed by ion etching, producing surface damage which is evident in a sh ft of the low binding energy O l s peak to 529.6 eV The ion-damagedlayer is removed by the HCI etch, •ielding a surface which is close to the ideal stoich=ometry and which exhibits a clear Fermi edge in the valence band region, as discussed below. The valence band regions of Nd2CuO4 which is fully oxidized (60 min at 800" C in 02) and reduced (5 rain at 800" C in N2, 3" C/min cool) are compared in Fig. 2 to the valence band re,lion from superconducting NCCO which has been ion etched and HCI etched. Reduction of Nd2CuO4 results in a broadening of the main valence band towards the Fermi level and forms an additional band of states which crosses the Fermi level. The density of states at the Fermi level does not increase when the dwell time at 800" C during the reduction is increased from 5 min to 90 min. The changes induced by oxidation and reduction of Nd~zCuO4 are reversible. The effect of Ce doping is s=milar to the effect of reducing an undoped sample, except that the density of states near the Fermi level is higher for doped samples. Oxidation and reduction of NCCO produce no clearly observable changes. The observation of a clear Fermi edge for these samples is evidence of a high quality surface. The Nd 3d and Ce 3d spectra measured in this work are similar to spectra previously published in the literature, and are not presented here. Despite the high quality and reproducibility of the data obtained for the other core level spectra and the

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2. XPS valence band spectra measured for Nd2CuO4 fully oxidized ( - - - ) and reduced ( ) and for superconducting Ndl 85Ce0 lsCuO4.8 (---). The inset shows an expand,ion ofthe region near the Fermi level.

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CHEMICALLY-ETCHED Nd-Ce-Cu-O SURFACES

valence band spectra, considerable variation is observed in the Cu 2p spectra. Cu 2P3/2 spectra most consistent with a single species have a main peak at 932.9 eV binding energy and a satellite peak at higher binding energy with a ratio of tensities Is/Im ~ 0.25 for superconducting NCCO, significantly less that the value of ~0.4 observed for the hole superconductors in previous measurements in this lab. 14-16 However, asymmetric main peaks and Is/Im values as low as 0.15 and as high as 0.35 are also observed. Similar variability is evident in the Cu 2p data reported in the literature,l,4,5,7,9 and one studylo even reports no satellite peak. The odgin of the sample-to-sample variability in the Cu 2p spectra observed in this work is not understood at this time, but may be related to variability in material quality. The effect of air exposure can be seen in Fig. 3 to be the regrowth of insulating species, evident in the increase in the intensity of the high binding energy O l s signal with increasing air exposure time. However, the reactivity of NCCO with air is much less than is observed for the hole superconductors, which contain reactive alkaline earth elements white NCCO does not. Recent measurements in this lablS, 16 show that a higher level of nonsuperconducting surface species form on Bi2Sr2;.xCal+xCu208+~ or TI2Ba2CaCu208+x in 1 rain than is observedonNCCO after 42 hrs. In summary, the effectiveness of acetic acid, Br2, and HCI solutions for removing nonsuperconducting species from NCCO thin film surfaces has been investigated. XPS measurements show that the HCI etch is most effective, yielding O l s spectra comparable to those obtained from samples cleaned in vacuum. A clear Fermi edge in the valence band region is also observed, evidence of a high quality surface. Reduction and oxidation revermbly reduces and eliminates, respectively, Fermi level states for undoped samples, but has no clearly observable effect on the XPS spectra for doped samples. Reactivity to air is much less for NCCO compared to hole superconductors, which contain reactive alkaline earth elements not present in NCCO.

A c k n o w l e d g e m e n t s - Part of this work was performed by the Center for Space Microelectronics Technology, Jet Propulsion Laboratory (JPL), California Institute of Technology, and was jointly sponsored by the National Aeronautics and Space

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3. O l s spectra measured from an NCCO film (a) etched in 10% HCI in absolute ethanol for 30 s, followed by air exposure for (b) 15 min, (c) 12 hrs, and (d) 42 hrs. Administration (Office of Aeronautics and Space Technology), the Defense Advanced Research Projects Agency, the Strategic Defense Initiative Organization (Innovative Science and Technology Office), and the JPL Director's Discretionary Fund. The work at IBM was conducted under the auspices of and with partial support from the Consortium for Superconducting Electronics, which is partially supported by the Defense Advanced Research Projects Agency (contract No. MDA972-90-C-0021 ). The work at MIT was supported by the Air Force Office of Scientific Research. The Francis Bitter National Magnet Laboratory is supported by the National Science Foundation.

References 1. Y. Fukuda, T. Suzuki, M. Na.~oshi, Y. Syono, K. Oh-ishi, and M. Tachiki, Solid State Commun. 72, 1183 (1989). 2. J.W. Allen, C. G. Olson, M. B. Maple, J.-S. Kang, L Z. Liu, J.-H. Park, R. O. Anderson, W. P. Ellis, J. T. Markert, Y. Dalichaouch, and R. Liu, Phys. Rev. Left. 64, 595 (1990). 3. T. Suzuki, M. Nagoshi, Y. Fukuda, K. Oh-ishi, Y. Syono, and M. Tachiki, Phys. Rev. B42, 4263 (1990). 4. A. Grassman, J. Strobel, M. Klauda, J. Schlotterer, and G. Saemann-lschenko, Europhys. Left. 9, 827 (1989). 5. M.K. Rajumon, D. D. Sarma, R. Vijayaraghavan, and C. N. R. Rao, Solid State Commun. 70, 875 (1989).

S. Uji, M. Shimoda, and H. Aoki, Jap. J. Appl. Phys. 28, L804 (1989). 7. T. Hanyu, H. Takai, K. Mizoguchi, K. Kume, I. Shiozaki, and S. Yamaguchi, Jap. J. Appl. Phys. 28, L1952 (1989). 8. H. Namatame, A. Fujimori, Y. Tokura, M. Nakamura, K. Yamaguchi, A. Misu, H. Matsubara, S. Suga, H. Eisaki, T. Ito, H. Takagi, and S. Uchida, Phys. Rev. B41, 7205 (1990). 9. A. Fujimori, Y. Tokura, H. Eisaki, H. Takagi, S. Uchida, and E. Takayama- Muromachi, Phys. Rev. B42, 325 (1990). 10. S. Kohiki, J. Kawai, S. Hayashi, H. Adachi, S. Hatta, K. Setsune, and K. Wasa, J. Appl. Phys. 68, 1229 (1990). 11. Y. Sakisaka, T. Muruyama, Y. Morikawa, H.

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12. 13. 14. 15.

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Kato, K. Edemato, M. Okusawa, Y. Aiura, H. Yanashima, T. Terashima, Y. Bando, K. lijima, K. Yamamoto, and K. Hirata, Solid State Commun. 74, 609 (1990); Phys. Rev. B42, 4189 (1990). R. P. Vasquez, B. D. Hunt, and M. C. Foote, Appl. Phys. Lett. 53, 2692 (1988).. R. P. Vasquez, M. C. Foote, and B. D. Hunt, J. Appl. Phys. 66, 4866 (1989). R. P. Vasquez, M. C. Foote, B. D. Hunt, and L. Bajuk, J. Vac. Sci. Technol. A (in press). R. P. Vasquez and R. M. Housley, J. Appl. Phys. 67, 7141 (1990); Physica C (in press).

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16. R. P. Vasquez and W. L. Olson, submitted to

Physica C.

17. A. Kussmaul, J. S. Moodera, P. M. Tedrow, A. Gupta, and C. C. Tsuei, IEEE Trans. Mag. (in press). 18. A. Gupta, G. Koren, C. C. Tsuei, A. Segmuller, and T. R. McGuire, Appl. Phys. Lett. 55, 1795

(1989).

19. P. T. Bowman, E. I. Ko, and P. J. Sides, J. Electrochem. Soc. 137, 1309 (1990).