- Email: [email protected]
X-ray photoelectron spectroscopy study of chemically etched Tl2Ba2CaCu2O8+x thin film surfaces
PhysicaC 177 (1991) 223-229 North-Holland
X-ray photoelectron spectroscopy study of chemically etched Tl,Ba,CaCu,08 +x thin film surfaces R.P. Vasquez Centerfor Space Microelectronics Technology, Jet Propulsion LaboratojS California Institute of Technology, Pasadena, CA 91109, USA
W.L. Olson Superconductor Technologies, Inc., Santa Barbara, CA 93111, USA
Received 2 1 February 199 1
X-ray photoelectron spectroscopy (XPS) is used to characterize the surfaces of epitaxial c-axis oriented Tl,Ba,CaCu,O,+, (TBCCO) thin films which have been chemically etched in a solution of BrZ in absolute ethanol. Comparison of the XPS core level spectra presented here with spectra in the literature obtained from single crystals cleaved in vacuum or polycrystalline pellets fractured or scraped in vacuum shows that the chemically etched thin films exhibit lower levels of nonsuperconducting surface species. Cation disorder is found to be restricted to the Ba sites. XPS valence band spectra show a clear Fermi edge, indicating that the room temperature conductivity near the surface is metallic. To the authors’ knowledge, this is the first report of a clear Fermi edge detected on TBCCO. Exposure of the chemically etched films to air results in a significant surface degradation even for exposure times as short as 1 min.
1. Introduction Tl-Ba-Ca-Cu-0 Superconductors in the (TBCCO) system have been studied far less than YBa-Cu-0 (YBCO) or Bi-Sr-Ca-Cu-0 (BSCCO), in spite of the higher superconducting transition temperatures. This results partially from difficulty in growth of the material due to the volatility of Tl, and partially from safety concerns due to the toxicity of Tl. As a result, there have only been a few studies published utilizing X-ray photoelectron spectroscopy (XPS) [ l-71 to characterize the chemical states of the elements in TBCCO, with the Tl : Ba : Ca : Cu ratio being either 2 : 2 : 2 : 3 or 2 : 2 : 1 : 2, and some of these studies have focused primarily or exclusively on the valence band structure or on the oxidation state of Tl. Published spectra of the other core levels are clearly influenced [ 11, or even dominated [2,3], by the presence of nonsuperconducting species, evident as high binding energy peaks in the 0 1s and Ba core level spectra (e.g., see ref. [ 81, and the studies cited therein). In addition, the published
TBCCO valence band spectra [ 1,6,7]; while exhibiting some intensity at the Fermi level, do not exhibit the clear Fermi edge observed on other hightemperature superconductor surfaces. There is therefore a need for XPS measurements on TBCCO samples with higher quality surfaces. The nonsuperconducting species on TBCCO surfaces may result from the growth process, and is also likely to result from air exposure since TBCCO, like other high-temperature superconductors (and alkaline earth comapounds in general), is known to’react with water vapor and carbon dioxide to form surface hydroxides and carbonates [ 7,9] : The removal of these nonsuperconducting species is necessary toobtain reliable measurements with surface sensitive spectroscopies, and is also.desirable from a practical viewpoint, e.g., for obtaining low-resistance electrical contacts. The insulating surface species in the earlier studies [ l-71 were removed by fracturing or scraping the samples in vacuum. An alternative approach is nonaqueous chemical etching, which has been proven to effectively remove
0921-4534/9 l/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
224
P. Vasquez,
Olson /
nonsuperconducting species from surfaces of YBCO [ lo- 12 ] and BSCCO [ 13,14 1. The nonaqueous etch consists of HBr or Brz in absolute ethanol or methanol and yields surfaces close to the ideal stoichiometry with minimal etch-induced artifacts. A clear Fermi edge has been detected on chemically etched BSCCO surfaces [ 141, indicating that the room temperature conductivity near the surface is metallic as ideally expected. The etch relies on the formation of etch products (metal bromides) which are soluble in polar nonaqueous solvents. Based on the known solubilities of the bromides of Tl, Ba, Ca, and Cu, it has been suggested [ lo] that this same etchant should effectively remove nonsuperconducting species from TBCCO surfaces. In this work, chemically etched TBCCO surfaces will be shown to yield XPS spectra exhibiting lower levels of nonsuperconducting surface species than is apparent in previously published spectra [ l-3 1. Evidence will be presented that Ca can occupy Ba sites, but Ba does not occupy Ca sites, as might be expected from the difference in ionic sizes. The intensity of the Cu*+ satellite peak relative to the main peak in the Cu 2p,,, spectra is significantly larger than found in previous TBCCO studies [ 1,2 1, but is consistent with previous measurements on YBCO [lo12 ] and BSCCO [ 13,14 1. A clear Fermi edge is detected on the chemically etched surface which, to the authors’ knowledge, is the first report of a distinct Fermi edge detected on TBCCO.
2. Experimental The TBCCO films are grown at ST1 on single-crystal LaAlO, ( 100). substrates by laser ablation followed. by post-deposition thermal processing. Depositions are performed at room temperature using a Lumonics excimer laser (KrF, 248 nm) using a repetition rate of:10 Hz. The 1.2 urn thick precursor films are heated at high temperature (830-900°C) for several minutes under a controlled atmosphere of T120 and oxygen to form the TlzBazCaCuzOs+, phase. X-ray diffraction indicates the films are crystallographically oriented with their c-axis perpendicular to the substrate surface. The epitaxial relationship of the film with the substrate has been confirmed using selected area electron beam channeling pattern
ofchemically etched
techniques [ 15 ] which showed the ( 100) superconductor orientation parallel to the ( 100) orientation of the substrate. The films used in this study have a superconducting transition midpoint of 101.5 K and a 1O%90% transition width of 1.5 K as determined by AC susceptibility measurement. The critical current densities in films grown by this process are typically [ 16 ] 0.5 to 1.Ox 1O6A/cm* at 77 K. The microwave properties of comparable TBCCO films have been characterized in detail both as films [ 16,17 ] and as devices [ 18 1. These films are consistently found to have very low loss with surface resistances of less than 0.4 mR at 77 K and 10 GHz. The XPS spectra are accumulated on a Surface Science Instruments SSX-501 spectrometer with monochromatized Al K, X-rays ( 1486.6 eV) and a base pressure of 3 x 10-l’ Torr. For these experiments, the XPS spectra are accumulated using an Xray beam diameter of 600 pm and setting the pass energy of the electron energy analyzer to 25 or 10 eV, yielding a peak full width at half maximum (FWHM) of 0.67 or 0.60 eV measured for the Au 4f,,, peak from an Au film evaporated onto Si (loo), and a FWHM of 0.60 or 0.56 eV measured for the Ag 3d5,* peak from sputter-cleaned Ag foil. 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 0.1% by volume Br2 in absolute ethanol, 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.
3. Results and discussion The etch rate of TBCCO in the Br2 solution is found to be far greater than that of YBCO. Consequently, a solution which is weaker by a factor of 10 is employed in this work. A - 1 pm thick TBCCO film is completely removed in 14 min in 0.1% Br2/ ethanol for an average etch rate of N 700 A/min. This etch rate is comparable to the - 1000 A/min measured [ 10,111 for YBCO in l.O”h Br*/ethanol. Prior to etching, the apparent Tl : Ba : Ca : Cu ratio on the surface is 3 : 2 : 3 : 2 and the XPS core level
225
R.P. Vasquez,W.L. Olson/ XPS of chemicallyetched TBCCO
spectra are dominated by nonsuperconducting species. This is most apparent in the presence of intense high binding energy signals in the 0 Is, Ba 3d, and Ca2p regions, shown in figs. l(a), 2(a), and 3(a),
I
Ca
I
I
I
2p
0 1s
i 539
I 536
I
533
I 530
BINDING ENERGY (eV) 527
524
BINDING ENERGY (eV)
Fig. 1. 0 1s spectra measured from a TBCCO thin film; (a) asgrown, (b) etched I5 s in 0.1% Br2 in absolute ethanol, and (c) etched 60 s in 0.1% Br, in absolute ethanol.
‘67
704
781
770
775
BINDING ENERGY (eV)
Fig. 2. Ba 3dS12spectra measured from a TBCCO thin film; (a) as-grown, (b) etched 15 s in 0.1% Br2 in absolute ethanol, and (c) etched 60 s in 0.1% Br, in absolute ethanol.
Fig. 3. Ca 2p spectra measured from a TBCCO thin film; (a) asgrown; etched in 0.1% Br2 in absolute ethanol for (b ) 15 s, (c ) 60s,and (d) 180s. and in the presence of a Ca 1s carbonate peak near 289 eV. The high binding energy signals are greatly reduced in the 0 1s and Ba 3d regions and completely eliminated in the Ca 2p region after a 15 s etch, as shown in figs. 1(b), 2(b), and 3 (b), respectively, and the C 1s carbonate peak is also eliminated. However, the metals ratio remains 3 : 2 : 3 : 2. The metals ratio changes to 2 : 2 : 2 : 2 after 30 s etching, and close to the ideally expected 2 : 2 : 1 : 2 after 60 s. Only small changes in apparent surface stoichiometry are observed with further etching. These results indicate that the surface has nonsuperconducting phases on the film surface. Since the surface and superconducting phases may have different etch rates, it is not possible to accurately estimate the thickness and coverage of the nonsuperconducting surface phases. However, these phases are present at sufficient coverage to completely obscure the XPS signals originating from the 2212 phase, which are obtained only after extended etching. The surface phases do not result from air exposure, as shown later, and thus appear likely to result from the film growth process. respectively,
226
R.P. Vasquez, W.L. Olson /XPS of chemically etched TBCCO
The dominant 0 1s peak in fig. 1 (b) has a binding energy of 528.6 eV and a peak full width at half maximum (FWHM) of 1.3 eV. The 0 1s peak corresponding to the superconducting phase, shown in fig. 1 (c), has a FWHM of 1.6 eV, with the broadening occurring primarily at lower binding energy. The primary signal can be fitted with a dominant peak at 528.7 eV and a smaller peak at 527.8 eV with an intensity N 15% that of the main peak. Multiple 0 1s peaks may be expected from the inequivalent 0 lattice sites, though the multiple peaks are not obvious in the measured spectra. It is to be noted that a low binding energy shoulder on the main 0 1s peak is apparent in spectra measured from YBCO [ 121, in which the chain and plane oxygens are in sufficiently different sites that multiple 0 1s signals are apparent. The 0 1s binding energy for TBCCO measured here is consistent with that reported for samples scraped or fractured in vacuum [ 1,2 1, and the higher binding energy signal associated with nonsuperconducting species is less intense than in the previously published spectra [ l-3 1. The Ba 3d,,, signal measured from the TBCCO superconducting phase, shown in fig. 2(c), differs little from that measured from the nonsuperconducting surface phase after removal of the carbonates, shown in fig. 2 (b). The FWHM of the primary peak is 1.55 eV and the binding energy is 778.3 eV, in agreement with measurements on samples scraped or fractured in vacuum [ 1 ] and significantly greater than the value of 777.6-777.7 eV measured from YBCO thin films [ 11,121 or the value of 777.4 eV measured from YBCO single crystals [ 191. The smaller high binding energy component occurs at 780.0 eV and is attributable to residual nonsuperconducting species, as is also evident in the 0 1s spectrum in fig. 1 (c). A component at 779.2 eV has also been claimed to be associated with the superconductor [ 11, with the two peaks originating from cation disorder, i.e., the Ba and Ca can substitute for each other on the inequivalent alkaline earth lattice sites, as discussed later. In this work, a second component associated with the superconductor was not found to be necessary to obtain a good fit. However, since the FWHM of the peak is significantly greater than the claimed 0.9 eV peak separation [ 11, a second component would be difficult to detect, in either this work or the earlier work. The Ba 4d signal is sig-
niticantly narrower, with a FWHM of 1.15 eV, and the spectra measured before and after etching are shown in figs. 4(a) and (b), respectively. The spectrum in fig. 4 (b) can be fitted well with a single doublet corresponding to the superconducting phase and a smaller doublet corresponding to contaminant phases. There is thus no evident for multiple Ba sites in the superconductor, and the earlier result [ 1 ] may be attributed to nonuniqueness of the fit or to contaminant phases, which are less prominent in the spectra measured in this work. The BaM,N,,N,, Auger signal is observed at a kinetic energy of 599.6 eV, compared to 600.0 eV observed for YBCO [ 111. The Auger parameter, a measure of the final state relaxation energy [ 201 and defined as the sum of the Auger kinetic energy and core level binding energy, is thus very similar for Ba in TBCCO and in YBCO. The observed difference in the Ba 3d core level binding energy in the two compounds must therefore be an initial state effect, i.e., resulting from a difference in the potentials at the Ba sites rather than from a difference in the screening of the core hole. After removal of the carbonates, a single Ca 2p doublet is observed (fig. 3(b) ) with the 2p,,, component at 345.7 eV. In the superconducting phase, a second doublet with a 2p,,, component at 344.7 eV
a
I
I
I
95
92
09
BINDING ENERGY
86
8:
(eV)
Fig. 4. Ba 4d spectra measured from a TBCCO thin film; (a) asgrown, and (b) etched 15 s in 0.1% Br, in absolute ethanol.
R. P. Vasquez, W.L. Olson / XPS of chemically etched TBCCO
is also observed, as shown in fig. 3 (c) and previously reported for samples scraped or fractured in vacuum [ 11. The FWHM of each component is 1.2 eV and their relative intensity varies with depth in the film, as can be seen by comparing figs. 3(c) and 3(d). These results are very similar to well-documented observations from BSCCO made in several labs, in which two doublets are observed in both the Ca2p and Sr 3d spectra ( [ 13,141, and citations therein). In the case of BSCCO, the Ca site is between Cu-0 planes and the Sr site is between Cu-0 and Bi-0 planes, and it was found that the Ca can easily substitute for Sr, but that Sr substitution for Ca was less likely. In the case of TBCCO, the Ca site is between Cu-0 planes and the Ba site is between Cu-0 and Tl-0 planes, and the results presented here suggest that the Ca can easily substitute for Ba, but that there is no clearly detectable Ba substitution for Ca. These observations are consistent with the differences in ionic radii of Ca2+ (0.99 A), Sr2+ (1.13 A), and Ba2+ ( 1.35 A), and the expectation that a small ion can more easily substitute at the site of a larger ion than vice versa, especially with a larger difference in ionic sizes. The Cu 2p,,, spectrum of TBCCO, shown in fig. 5(a), is typical of Cu*+ compounds, with a low binding energy peak corresponding to d” final state
Cu 2P312
I
948
944
I
940 BINDING
I
I
936
932
928
ENERGY (ev)
Fig. 5. Cu 2p,,, spectra measured from chemically etched (a) TBCCO, (b) BSCCO, and (c) YBCO.
227
screening and a less intense manifold at higher binding energy corresponding to d9 final state screening. The d” peak for TBCCO occurs at 933.1 eV and has a FWHM of 3.1 eV, and the d9 peak has 45Ohof the intensity of the d’O peak. The d9/d’0 ratio observed here is significantly larger than the values of 0.25 [ 21 and 0.38 [ 1] reported for scraped surfaces of polycrystalline samples, or the value of 0.2 1 reported [ 1] for a single crystal cleaved in vacuum. It is, however, virtually identical to the value of 0.43 reported for both YBCO [ 12,191 and BSCCO ([ 141, and citations therein). In fact, as shown in fig. 5, the Cu 2p,,, spectra for all three materials appear virtually identical in every respect. The TBCCO Cu L3Md5Ma5Auger peak is observed at 9 18.8 eV, identical within experimental error to the value of 9 18.9 eV observed for YBCO [ 111 and close to the value of 9 18.2 eV reported for BSCCO [ 141. Initial and final state effects at the Cu sites in all three compounds must therefore be very similar, though the final state relaxation energy in BSCCO does appear to differ slightly. The Tl 4f,,2 occurs at 118.0 eV after etching, with a FWHM of 1.25 eV. The binding energy observed here is consistent with observations in other studies [ 1,2,4,5], and a detailed discussion of the Tl-0 bonding can be found elsewhere [ 41. Some residual Br, at a level of -2 at.%, remains on the TBCCO surface after etching and rinsing, similar to levels observed on chemically etched YBCO [ lo- 121 and BSCCO [ 13,141. One indicator of a high-quality surface is the presence of a clear Fermi edge in the valence band spectrum. One earlier study of TBCCO detected a weak (compared to BSCCO ) Fermi edge [ 11, but a clear Fermi edge is not apparent in any published valence band spectrum [ l-31. Figure 6 shows that before etching there is little intensity at the Fermi level, but after etching a clear Fermi edge is detected, indicating that the room temperature conductivity near the surface is metallic as ideally expected. In fact, the density of states at the Fermi leve1 for TBCCO appears to be comparable to that observed for BSCCO in previous measurements in this lab [ 141. To the authors’ knowledge, this is the first report of a clear Fermi edge for TBCCO, and is further evidence of the high quality of the chemically etched surface. Degradation of the surface of chemically etched
R. P. Vmquez, W.L. Olson / XPS of chemically etched TBCCO
AIR EXPOSURE
IO
a
6 BINDING
4
2
0
-2
ENERGY (eV)
Fig. 6. Comparison of TBCCO valence band spectra measured before and after etching. The region near the Fermi level is shown in the inset.
TBCCO thin films by exposure to air is illustrated in the 0 1s spectra in fig. 7. An initial rapid degradation can be seen in fig. 7(b) after only 1 min exposure to air, after which the surface continues to react with air more slowly. The spectrum in fig. 1(e) corresponds to a 12 A thick nonsuperconducting surface layer, assuming a 20 8, photoelectron attenuation length. While this reaction with air is desirable in some circumstances, e.g., for the formation of tunnel barriers, it is undesirable for surface studies or for formation of low resistance electrical contacts. These results illustrate the deleterious effects of even brief air exposure and demonstrate the importance of processing in an inert atmosphere. A 24 h air exposure followed by a 15 s 0.1% BrJethanol etch restores the surface close to the ideal stoichiometry, in contrast to the surface of the as-grown film where the surface remained Tl- and Ca-rich even after removal of air reaction products. This Tl- and Ca-rich surface layer thus does not result from air exposure, and most likely results from the film growth process.
4. Conclusions The surfaces of TBCCO thin films which have been
L 5:39
I
I
I
536
533
530
527
524
BINDING ENERGY (eV)
Fig. 7. 0 Is spectra from a TBCCO surface (a) etched 60 s in 0.1% Br, in absolute ethanol, followed by (b ) air exposure for 1 min, (c) 5 min, (d) 15 min, and (e) 1 h.
chemically etched in a solution of Brz in absolute ethanol have been characterized with XPS. A systematic shift in the average surface composition with etching from the initial 3232 to the expected 2212 was observed. Concomitant with the change in surface composition, a dramatic improvement was noted in the qualitative features of the XPS spectra with etching, yielding results comparable to those obtained from YBCO and BSCCO surfaces. The XPS core level spectra measured from chemically etched TBCCO thin film surfaces have been shown to exhibit lower levels of nonsuperconducting surface species compared to spectra in the literature obtained from TBCCO single crystals cleaved in vacuum or polycrystalline pellets fractured or scraped in vacuum. The data are consistent with Ca occupying both of the inequivalent alkaline earth lattice sites, but Ba occupying only a single site. XPS valence band spectra show, for the first time, a clear Fermi edge, indicating that the room temperature conductivity near the surface is metallic. Significant surface degradation has been observed even for air exposure times as short as 1 mitt, indicating the importance of processing in an inert atmosphere. The hydroxide and
R.P. Viquez,
W.L. Olson /XPS of chemically etched TBCCO
carbonate surface species produced upon air exposure can be removed by chemically etching the surface and the formation of these species is minimized by maintaining the sample in a nitrogen atmosphere.
Acknowledgements Part of the work described in this paper was performed by the Center for Space Microelectronics Technology, Jet Propulsion Laboratory, California Institute of Technology, and was jointly sponsored by the National Aeronautics and Space Administration, Office of Aeronautics and Space Technology, the Defense Advanced Research Projects Agency (DARPA), and the Strategic Defense Initiative Organization, Innovative Science and Technology Office. The film preparation and characterization were supported by DARPA (contract no. NOOO14-88-C 0713) at STI.
References [ 1 ] H.M. Meyer III, T.J. Wagener, J.H. Weaver and D.S. Ginley, Phys. Rev. B 39 ( 1989) 7343. [ 21 A.E. Bocquet, J.F. Dobson, PC. Healy, S. Myrha and J.G. Thompson, Phys. Stat. Sol. (b) 152 (1989) 519. [3] I.Z. Kostadinov, V.G. Hadjiev, J. Tihov, M. Mateev, M. Mikhov, 0. Petrov, V. Popov, E. Dinolova, Ts. Zheleva, G. Tyuliev and V. Kojouharov, Physica C 156 ( 1988) 427. [4] T. Suzuki, M. Nagoshi, Y. Fukuda, Y. Syono, M. Kikuchi, N. Kobayashi and M. Tachiki, Phys. Rev. B 40 ( 1989) 5184.
229
[ 5 ] T. Suzuki, M. Nagoshi, Y. Fukuda, S. Nakajima, M. Kikuchi, Y. SyonoandM. Tachiki, PhysicaC 162-164 (1989) 1387. [6] P. Steiner, S. Hufner, A. Jungmann, V. Kinsinger and I. Sander, Z. Phys. B 74 (1989) 173. [ 7 ] R.L. Kurtz, S.W. Robey, R.L. Stockbauer, D. Mueller, A. Shih, L. Toth, A.K. Singh and M. Osofsky, Vacuum 39 (1989)611. [ 81 F. Al Shamma and J.C. Fuggle, Physica C I69 ( 1990) 325. [9]K.D. Vernon-Parry, L.T. Romano, J.S. Lees and C.R.M. Grovenor, Physica C 170 (1990) 388. [lo] R.P. Vasquez, B.D. Hunt and M.C. Foote, Appl. Phys. Lett. 53 (1988) 2692. [ 111 R.P. Vasquez, M.C. Foote and B.D. Hunt, J. Appl. Phys. 66 ( 1989) 4866. [ 121 R.P. Vasquez, M.C. Foote, B.D. Hunt and L. Bajuk, J. Vat. Sci. Technol. A, in press. [ 131 R.P. Vasquez and R.M. Housley, J. Appl. Phys. 67 ( 1990) 7141. [ 141 R.P. VasquezandR.M. Housley,PhysicaC 175 (1991) 233. [15]K.H.Young,J.Z.Sun,T.W.JamesandB.J.L.Nilsson, 1990 Fall MRS Meeting, Boston, MA, to be published in Mater. Res. Sot. Symp. Proc. on High Temperature Superconductors. [ 161 R.B. Hammond, G.V. Negrete, L.C. Boume, D.D. Strother, A.H. Cardona and M.M. Eddy, Appl. Phys. Lett. 57 ( 1990) 825. [ 171 L.D. Chang, M.J. Moskowitz, R.B. Hammond, M.M. Eddy, W.L. Olson, D.D. Casavant, E.J. Smith, M. Robinson, L. Drabeck and G. Gruner, Appl. Phys. Lett. 55 ( 1989) 1357. [IS] R.B. Hammond, G.V. Negrete, MS. Schmidt, M.J. Moskowitz, M.M. Eddy, D.D. Strother and D.L. Skogland, to be published in the Proceedings of the 1990 IEEE Microwave Symposium. [ 191 D.E. Fowler, C.R. Brundle, J. Lerczak and F. Holtzberg, J. Electron Spectrosc. Related Phenom. 52 ( 1990) 323. [20] C.D. Wagner, Faraday Discuss. Chem. Sot. 60 (1975) 291.
X-ray photoelectron spectroscopy study of chemically etched Tl,Ba,CaCu,08 +x thin film surfaces R.P. Vasquez Centerfor Space Microelectronics Technology, Jet Propulsion LaboratojS California Institute of Technology, Pasadena, CA 91109, USA
W.L. Olson Superconductor Technologies, Inc., Santa Barbara, CA 93111, USA
Received 2 1 February 199 1
X-ray photoelectron spectroscopy (XPS) is used to characterize the surfaces of epitaxial c-axis oriented Tl,Ba,CaCu,O,+, (TBCCO) thin films which have been chemically etched in a solution of BrZ in absolute ethanol. Comparison of the XPS core level spectra presented here with spectra in the literature obtained from single crystals cleaved in vacuum or polycrystalline pellets fractured or scraped in vacuum shows that the chemically etched thin films exhibit lower levels of nonsuperconducting surface species. Cation disorder is found to be restricted to the Ba sites. XPS valence band spectra show a clear Fermi edge, indicating that the room temperature conductivity near the surface is metallic. To the authors’ knowledge, this is the first report of a clear Fermi edge detected on TBCCO. Exposure of the chemically etched films to air results in a significant surface degradation even for exposure times as short as 1 min.
1. Introduction Tl-Ba-Ca-Cu-0 Superconductors in the (TBCCO) system have been studied far less than YBa-Cu-0 (YBCO) or Bi-Sr-Ca-Cu-0 (BSCCO), in spite of the higher superconducting transition temperatures. This results partially from difficulty in growth of the material due to the volatility of Tl, and partially from safety concerns due to the toxicity of Tl. As a result, there have only been a few studies published utilizing X-ray photoelectron spectroscopy (XPS) [ l-71 to characterize the chemical states of the elements in TBCCO, with the Tl : Ba : Ca : Cu ratio being either 2 : 2 : 2 : 3 or 2 : 2 : 1 : 2, and some of these studies have focused primarily or exclusively on the valence band structure or on the oxidation state of Tl. Published spectra of the other core levels are clearly influenced [ 11, or even dominated [2,3], by the presence of nonsuperconducting species, evident as high binding energy peaks in the 0 1s and Ba core level spectra (e.g., see ref. [ 81, and the studies cited therein). In addition, the published
TBCCO valence band spectra [ 1,6,7]; while exhibiting some intensity at the Fermi level, do not exhibit the clear Fermi edge observed on other hightemperature superconductor surfaces. There is therefore a need for XPS measurements on TBCCO samples with higher quality surfaces. The nonsuperconducting species on TBCCO surfaces may result from the growth process, and is also likely to result from air exposure since TBCCO, like other high-temperature superconductors (and alkaline earth comapounds in general), is known to’react with water vapor and carbon dioxide to form surface hydroxides and carbonates [ 7,9] : The removal of these nonsuperconducting species is necessary toobtain reliable measurements with surface sensitive spectroscopies, and is also.desirable from a practical viewpoint, e.g., for obtaining low-resistance electrical contacts. The insulating surface species in the earlier studies [ l-71 were removed by fracturing or scraping the samples in vacuum. An alternative approach is nonaqueous chemical etching, which has been proven to effectively remove
0921-4534/9 l/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
224
P. Vasquez,
Olson /
nonsuperconducting species from surfaces of YBCO [ lo- 12 ] and BSCCO [ 13,14 1. The nonaqueous etch consists of HBr or Brz in absolute ethanol or methanol and yields surfaces close to the ideal stoichiometry with minimal etch-induced artifacts. A clear Fermi edge has been detected on chemically etched BSCCO surfaces [ 141, indicating that the room temperature conductivity near the surface is metallic as ideally expected. The etch relies on the formation of etch products (metal bromides) which are soluble in polar nonaqueous solvents. Based on the known solubilities of the bromides of Tl, Ba, Ca, and Cu, it has been suggested [ lo] that this same etchant should effectively remove nonsuperconducting species from TBCCO surfaces. In this work, chemically etched TBCCO surfaces will be shown to yield XPS spectra exhibiting lower levels of nonsuperconducting surface species than is apparent in previously published spectra [ l-3 1. Evidence will be presented that Ca can occupy Ba sites, but Ba does not occupy Ca sites, as might be expected from the difference in ionic sizes. The intensity of the Cu*+ satellite peak relative to the main peak in the Cu 2p,,, spectra is significantly larger than found in previous TBCCO studies [ 1,2 1, but is consistent with previous measurements on YBCO [lo12 ] and BSCCO [ 13,14 1. A clear Fermi edge is detected on the chemically etched surface which, to the authors’ knowledge, is the first report of a distinct Fermi edge detected on TBCCO.
2. Experimental The TBCCO films are grown at ST1 on single-crystal LaAlO, ( 100). substrates by laser ablation followed. by post-deposition thermal processing. Depositions are performed at room temperature using a Lumonics excimer laser (KrF, 248 nm) using a repetition rate of:10 Hz. The 1.2 urn thick precursor films are heated at high temperature (830-900°C) for several minutes under a controlled atmosphere of T120 and oxygen to form the TlzBazCaCuzOs+, phase. X-ray diffraction indicates the films are crystallographically oriented with their c-axis perpendicular to the substrate surface. The epitaxial relationship of the film with the substrate has been confirmed using selected area electron beam channeling pattern
ofchemically etched
techniques [ 15 ] which showed the ( 100) superconductor orientation parallel to the ( 100) orientation of the substrate. The films used in this study have a superconducting transition midpoint of 101.5 K and a 1O%90% transition width of 1.5 K as determined by AC susceptibility measurement. The critical current densities in films grown by this process are typically [ 16 ] 0.5 to 1.Ox 1O6A/cm* at 77 K. The microwave properties of comparable TBCCO films have been characterized in detail both as films [ 16,17 ] and as devices [ 18 1. These films are consistently found to have very low loss with surface resistances of less than 0.4 mR at 77 K and 10 GHz. The XPS spectra are accumulated on a Surface Science Instruments SSX-501 spectrometer with monochromatized Al K, X-rays ( 1486.6 eV) and a base pressure of 3 x 10-l’ Torr. For these experiments, the XPS spectra are accumulated using an Xray beam diameter of 600 pm and setting the pass energy of the electron energy analyzer to 25 or 10 eV, yielding a peak full width at half maximum (FWHM) of 0.67 or 0.60 eV measured for the Au 4f,,, peak from an Au film evaporated onto Si (loo), and a FWHM of 0.60 or 0.56 eV measured for the Ag 3d5,* peak from sputter-cleaned Ag foil. 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 0.1% by volume Br2 in absolute ethanol, 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.
3. Results and discussion The etch rate of TBCCO in the Br2 solution is found to be far greater than that of YBCO. Consequently, a solution which is weaker by a factor of 10 is employed in this work. A - 1 pm thick TBCCO film is completely removed in 14 min in 0.1% Br2/ ethanol for an average etch rate of N 700 A/min. This etch rate is comparable to the - 1000 A/min measured [ 10,111 for YBCO in l.O”h Br*/ethanol. Prior to etching, the apparent Tl : Ba : Ca : Cu ratio on the surface is 3 : 2 : 3 : 2 and the XPS core level
225
R.P. Vasquez,W.L. Olson/ XPS of chemicallyetched TBCCO
spectra are dominated by nonsuperconducting species. This is most apparent in the presence of intense high binding energy signals in the 0 Is, Ba 3d, and Ca2p regions, shown in figs. l(a), 2(a), and 3(a),
I
Ca
I
I
I
2p
0 1s
i 539
I 536
I
533
I 530
BINDING ENERGY (eV) 527
524
BINDING ENERGY (eV)
Fig. 1. 0 1s spectra measured from a TBCCO thin film; (a) asgrown, (b) etched I5 s in 0.1% Br2 in absolute ethanol, and (c) etched 60 s in 0.1% Br, in absolute ethanol.
‘67
704
781
770
775
BINDING ENERGY (eV)
Fig. 2. Ba 3dS12spectra measured from a TBCCO thin film; (a) as-grown, (b) etched 15 s in 0.1% Br2 in absolute ethanol, and (c) etched 60 s in 0.1% Br, in absolute ethanol.
Fig. 3. Ca 2p spectra measured from a TBCCO thin film; (a) asgrown; etched in 0.1% Br2 in absolute ethanol for (b ) 15 s, (c ) 60s,and (d) 180s. and in the presence of a Ca 1s carbonate peak near 289 eV. The high binding energy signals are greatly reduced in the 0 1s and Ba 3d regions and completely eliminated in the Ca 2p region after a 15 s etch, as shown in figs. 1(b), 2(b), and 3 (b), respectively, and the C 1s carbonate peak is also eliminated. However, the metals ratio remains 3 : 2 : 3 : 2. The metals ratio changes to 2 : 2 : 2 : 2 after 30 s etching, and close to the ideally expected 2 : 2 : 1 : 2 after 60 s. Only small changes in apparent surface stoichiometry are observed with further etching. These results indicate that the surface has nonsuperconducting phases on the film surface. Since the surface and superconducting phases may have different etch rates, it is not possible to accurately estimate the thickness and coverage of the nonsuperconducting surface phases. However, these phases are present at sufficient coverage to completely obscure the XPS signals originating from the 2212 phase, which are obtained only after extended etching. The surface phases do not result from air exposure, as shown later, and thus appear likely to result from the film growth process. respectively,
226
R.P. Vasquez, W.L. Olson /XPS of chemically etched TBCCO
The dominant 0 1s peak in fig. 1 (b) has a binding energy of 528.6 eV and a peak full width at half maximum (FWHM) of 1.3 eV. The 0 1s peak corresponding to the superconducting phase, shown in fig. 1 (c), has a FWHM of 1.6 eV, with the broadening occurring primarily at lower binding energy. The primary signal can be fitted with a dominant peak at 528.7 eV and a smaller peak at 527.8 eV with an intensity N 15% that of the main peak. Multiple 0 1s peaks may be expected from the inequivalent 0 lattice sites, though the multiple peaks are not obvious in the measured spectra. It is to be noted that a low binding energy shoulder on the main 0 1s peak is apparent in spectra measured from YBCO [ 121, in which the chain and plane oxygens are in sufficiently different sites that multiple 0 1s signals are apparent. The 0 1s binding energy for TBCCO measured here is consistent with that reported for samples scraped or fractured in vacuum [ 1,2 1, and the higher binding energy signal associated with nonsuperconducting species is less intense than in the previously published spectra [ l-3 1. The Ba 3d,,, signal measured from the TBCCO superconducting phase, shown in fig. 2(c), differs little from that measured from the nonsuperconducting surface phase after removal of the carbonates, shown in fig. 2 (b). The FWHM of the primary peak is 1.55 eV and the binding energy is 778.3 eV, in agreement with measurements on samples scraped or fractured in vacuum [ 1 ] and significantly greater than the value of 777.6-777.7 eV measured from YBCO thin films [ 11,121 or the value of 777.4 eV measured from YBCO single crystals [ 191. The smaller high binding energy component occurs at 780.0 eV and is attributable to residual nonsuperconducting species, as is also evident in the 0 1s spectrum in fig. 1 (c). A component at 779.2 eV has also been claimed to be associated with the superconductor [ 11, with the two peaks originating from cation disorder, i.e., the Ba and Ca can substitute for each other on the inequivalent alkaline earth lattice sites, as discussed later. In this work, a second component associated with the superconductor was not found to be necessary to obtain a good fit. However, since the FWHM of the peak is significantly greater than the claimed 0.9 eV peak separation [ 11, a second component would be difficult to detect, in either this work or the earlier work. The Ba 4d signal is sig-
niticantly narrower, with a FWHM of 1.15 eV, and the spectra measured before and after etching are shown in figs. 4(a) and (b), respectively. The spectrum in fig. 4 (b) can be fitted well with a single doublet corresponding to the superconducting phase and a smaller doublet corresponding to contaminant phases. There is thus no evident for multiple Ba sites in the superconductor, and the earlier result [ 1 ] may be attributed to nonuniqueness of the fit or to contaminant phases, which are less prominent in the spectra measured in this work. The BaM,N,,N,, Auger signal is observed at a kinetic energy of 599.6 eV, compared to 600.0 eV observed for YBCO [ 111. The Auger parameter, a measure of the final state relaxation energy [ 201 and defined as the sum of the Auger kinetic energy and core level binding energy, is thus very similar for Ba in TBCCO and in YBCO. The observed difference in the Ba 3d core level binding energy in the two compounds must therefore be an initial state effect, i.e., resulting from a difference in the potentials at the Ba sites rather than from a difference in the screening of the core hole. After removal of the carbonates, a single Ca 2p doublet is observed (fig. 3(b) ) with the 2p,,, component at 345.7 eV. In the superconducting phase, a second doublet with a 2p,,, component at 344.7 eV
a
I
I
I
95
92
09
BINDING ENERGY
86
8:
(eV)
Fig. 4. Ba 4d spectra measured from a TBCCO thin film; (a) asgrown, and (b) etched 15 s in 0.1% Br, in absolute ethanol.
R. P. Vasquez, W.L. Olson / XPS of chemically etched TBCCO
is also observed, as shown in fig. 3 (c) and previously reported for samples scraped or fractured in vacuum [ 11. The FWHM of each component is 1.2 eV and their relative intensity varies with depth in the film, as can be seen by comparing figs. 3(c) and 3(d). These results are very similar to well-documented observations from BSCCO made in several labs, in which two doublets are observed in both the Ca2p and Sr 3d spectra ( [ 13,141, and citations therein). In the case of BSCCO, the Ca site is between Cu-0 planes and the Sr site is between Cu-0 and Bi-0 planes, and it was found that the Ca can easily substitute for Sr, but that Sr substitution for Ca was less likely. In the case of TBCCO, the Ca site is between Cu-0 planes and the Ba site is between Cu-0 and Tl-0 planes, and the results presented here suggest that the Ca can easily substitute for Ba, but that there is no clearly detectable Ba substitution for Ca. These observations are consistent with the differences in ionic radii of Ca2+ (0.99 A), Sr2+ (1.13 A), and Ba2+ ( 1.35 A), and the expectation that a small ion can more easily substitute at the site of a larger ion than vice versa, especially with a larger difference in ionic sizes. The Cu 2p,,, spectrum of TBCCO, shown in fig. 5(a), is typical of Cu*+ compounds, with a low binding energy peak corresponding to d” final state
Cu 2P312
I
948
944
I
940 BINDING
I
I
936
932
928
ENERGY (ev)
Fig. 5. Cu 2p,,, spectra measured from chemically etched (a) TBCCO, (b) BSCCO, and (c) YBCO.
227
screening and a less intense manifold at higher binding energy corresponding to d9 final state screening. The d” peak for TBCCO occurs at 933.1 eV and has a FWHM of 3.1 eV, and the d9 peak has 45Ohof the intensity of the d’O peak. The d9/d’0 ratio observed here is significantly larger than the values of 0.25 [ 21 and 0.38 [ 1] reported for scraped surfaces of polycrystalline samples, or the value of 0.2 1 reported [ 1] for a single crystal cleaved in vacuum. It is, however, virtually identical to the value of 0.43 reported for both YBCO [ 12,191 and BSCCO ([ 141, and citations therein). In fact, as shown in fig. 5, the Cu 2p,,, spectra for all three materials appear virtually identical in every respect. The TBCCO Cu L3Md5Ma5Auger peak is observed at 9 18.8 eV, identical within experimental error to the value of 9 18.9 eV observed for YBCO [ 111 and close to the value of 9 18.2 eV reported for BSCCO [ 141. Initial and final state effects at the Cu sites in all three compounds must therefore be very similar, though the final state relaxation energy in BSCCO does appear to differ slightly. The Tl 4f,,2 occurs at 118.0 eV after etching, with a FWHM of 1.25 eV. The binding energy observed here is consistent with observations in other studies [ 1,2,4,5], and a detailed discussion of the Tl-0 bonding can be found elsewhere [ 41. Some residual Br, at a level of -2 at.%, remains on the TBCCO surface after etching and rinsing, similar to levels observed on chemically etched YBCO [ lo- 121 and BSCCO [ 13,141. One indicator of a high-quality surface is the presence of a clear Fermi edge in the valence band spectrum. One earlier study of TBCCO detected a weak (compared to BSCCO ) Fermi edge [ 11, but a clear Fermi edge is not apparent in any published valence band spectrum [ l-31. Figure 6 shows that before etching there is little intensity at the Fermi level, but after etching a clear Fermi edge is detected, indicating that the room temperature conductivity near the surface is metallic as ideally expected. In fact, the density of states at the Fermi leve1 for TBCCO appears to be comparable to that observed for BSCCO in previous measurements in this lab [ 141. To the authors’ knowledge, this is the first report of a clear Fermi edge for TBCCO, and is further evidence of the high quality of the chemically etched surface. Degradation of the surface of chemically etched
R. P. Vmquez, W.L. Olson / XPS of chemically etched TBCCO
AIR EXPOSURE
IO
a
6 BINDING
4
2
0
-2
ENERGY (eV)
Fig. 6. Comparison of TBCCO valence band spectra measured before and after etching. The region near the Fermi level is shown in the inset.
TBCCO thin films by exposure to air is illustrated in the 0 1s spectra in fig. 7. An initial rapid degradation can be seen in fig. 7(b) after only 1 min exposure to air, after which the surface continues to react with air more slowly. The spectrum in fig. 1(e) corresponds to a 12 A thick nonsuperconducting surface layer, assuming a 20 8, photoelectron attenuation length. While this reaction with air is desirable in some circumstances, e.g., for the formation of tunnel barriers, it is undesirable for surface studies or for formation of low resistance electrical contacts. These results illustrate the deleterious effects of even brief air exposure and demonstrate the importance of processing in an inert atmosphere. A 24 h air exposure followed by a 15 s 0.1% BrJethanol etch restores the surface close to the ideal stoichiometry, in contrast to the surface of the as-grown film where the surface remained Tl- and Ca-rich even after removal of air reaction products. This Tl- and Ca-rich surface layer thus does not result from air exposure, and most likely results from the film growth process.
4. Conclusions The surfaces of TBCCO thin films which have been
L 5:39
I
I
I
536
533
530
527
524
BINDING ENERGY (eV)
Fig. 7. 0 Is spectra from a TBCCO surface (a) etched 60 s in 0.1% Br, in absolute ethanol, followed by (b ) air exposure for 1 min, (c) 5 min, (d) 15 min, and (e) 1 h.
chemically etched in a solution of Brz in absolute ethanol have been characterized with XPS. A systematic shift in the average surface composition with etching from the initial 3232 to the expected 2212 was observed. Concomitant with the change in surface composition, a dramatic improvement was noted in the qualitative features of the XPS spectra with etching, yielding results comparable to those obtained from YBCO and BSCCO surfaces. The XPS core level spectra measured from chemically etched TBCCO thin film surfaces have been shown to exhibit lower levels of nonsuperconducting surface species compared to spectra in the literature obtained from TBCCO single crystals cleaved in vacuum or polycrystalline pellets fractured or scraped in vacuum. The data are consistent with Ca occupying both of the inequivalent alkaline earth lattice sites, but Ba occupying only a single site. XPS valence band spectra show, for the first time, a clear Fermi edge, indicating that the room temperature conductivity near the surface is metallic. Significant surface degradation has been observed even for air exposure times as short as 1 mitt, indicating the importance of processing in an inert atmosphere. The hydroxide and
R.P. Viquez,
W.L. Olson /XPS of chemically etched TBCCO
carbonate surface species produced upon air exposure can be removed by chemically etching the surface and the formation of these species is minimized by maintaining the sample in a nitrogen atmosphere.
Acknowledgements Part of the work described in this paper was performed by the Center for Space Microelectronics Technology, Jet Propulsion Laboratory, California Institute of Technology, and was jointly sponsored by the National Aeronautics and Space Administration, Office of Aeronautics and Space Technology, the Defense Advanced Research Projects Agency (DARPA), and the Strategic Defense Initiative Organization, Innovative Science and Technology Office. The film preparation and characterization were supported by DARPA (contract no. NOOO14-88-C 0713) at STI.
References [ 1 ] H.M. Meyer III, T.J. Wagener, J.H. Weaver and D.S. Ginley, Phys. Rev. B 39 ( 1989) 7343. [ 21 A.E. Bocquet, J.F. Dobson, PC. Healy, S. Myrha and J.G. Thompson, Phys. Stat. Sol. (b) 152 (1989) 519. [3] I.Z. Kostadinov, V.G. Hadjiev, J. Tihov, M. Mateev, M. Mikhov, 0. Petrov, V. Popov, E. Dinolova, Ts. Zheleva, G. Tyuliev and V. Kojouharov, Physica C 156 ( 1988) 427. [4] T. Suzuki, M. Nagoshi, Y. Fukuda, Y. Syono, M. Kikuchi, N. Kobayashi and M. Tachiki, Phys. Rev. B 40 ( 1989) 5184.
229
[ 5 ] T. Suzuki, M. Nagoshi, Y. Fukuda, S. Nakajima, M. Kikuchi, Y. SyonoandM. Tachiki, PhysicaC 162-164 (1989) 1387. [6] P. Steiner, S. Hufner, A. Jungmann, V. Kinsinger and I. Sander, Z. Phys. B 74 (1989) 173. [ 7 ] R.L. Kurtz, S.W. Robey, R.L. Stockbauer, D. Mueller, A. Shih, L. Toth, A.K. Singh and M. Osofsky, Vacuum 39 (1989)611. [ 81 F. Al Shamma and J.C. Fuggle, Physica C I69 ( 1990) 325. [9]K.D. Vernon-Parry, L.T. Romano, J.S. Lees and C.R.M. Grovenor, Physica C 170 (1990) 388. [lo] R.P. Vasquez, B.D. Hunt and M.C. Foote, Appl. Phys. Lett. 53 (1988) 2692. [ 111 R.P. Vasquez, M.C. Foote and B.D. Hunt, J. Appl. Phys. 66 ( 1989) 4866. [ 121 R.P. Vasquez, M.C. Foote, B.D. Hunt and L. Bajuk, J. Vat. Sci. Technol. A, in press. [ 131 R.P. Vasquez and R.M. Housley, J. Appl. Phys. 67 ( 1990) 7141. [ 141 R.P. VasquezandR.M. Housley,PhysicaC 175 (1991) 233. [15]K.H.Young,J.Z.Sun,T.W.JamesandB.J.L.Nilsson, 1990 Fall MRS Meeting, Boston, MA, to be published in Mater. Res. Sot. Symp. Proc. on High Temperature Superconductors. [ 161 R.B. Hammond, G.V. Negrete, L.C. Boume, D.D. Strother, A.H. Cardona and M.M. Eddy, Appl. Phys. Lett. 57 ( 1990) 825. [ 171 L.D. Chang, M.J. Moskowitz, R.B. Hammond, M.M. Eddy, W.L. Olson, D.D. Casavant, E.J. Smith, M. Robinson, L. Drabeck and G. Gruner, Appl. Phys. Lett. 55 ( 1989) 1357. [IS] R.B. Hammond, G.V. Negrete, MS. Schmidt, M.J. Moskowitz, M.M. Eddy, D.D. Strother and D.L. Skogland, to be published in the Proceedings of the 1990 IEEE Microwave Symposium. [ 191 D.E. Fowler, C.R. Brundle, J. Lerczak and F. Holtzberg, J. Electron Spectrosc. Related Phenom. 52 ( 1990) 323. [20] C.D. Wagner, Faraday Discuss. Chem. Sot. 60 (1975) 291.