OXYGEN CONCENTRATION OF Eu1 Ba2 Cu3 O7-x IN VACUUM : AN ATOM PROBE STUDY II

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OXYGEN CONCENTRATION OF Eu1 Ba2 Cu3 O7-x IN VACUUM : AN ATOM PROBE STUDY II

P. Camus, H. Elswijk, A. Melmed

To cite this version:

P. Camus, H. Elswijk, A. Melmed. OXYGEN CONCENTRATION OF Eu1 Ba2 Cu3 O7-x IN VAC-

UUM : AN ATOM PROBE STUDY II. Journal de Physique Colloques, 1989, 50 (C8), pp.C8-477-C8-

480. �10.1051/jphyscol:1989881�. �jpa-00229979�

COLLOQUE D E PHYSIQUE

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novembre

OXYGEN CONCENTRATION OF Eu, Ba2Cu3 0,

-,

P.P. CAMUS('), H.B. ELSWIJK(')

and

National Institute of Standards and Technology, Surface Science Division, Gaithersburg, M D 20899, U.S.A.

Abstract - Atom Probe mass analysis using a wide acceptance angle instrument was used to measure the oxygen content and metallic stoichiometry of the near-surface region of the superconducting ceramic oxide EulBa2Cu307-, (x=0.1) after exposure at 85K and room temperature in vacuum. An oxygen depleted layer formed by Hz imaging must be removed before bulk concentrations are obtained. Room temperature holding in vacuum overnight

(-18h) then depletes the surface of oxygen to a depth of greater than 4 layers ( 1 . 2 ~ ~ ) . However, after holding the specimen at 85K for up to 3h either with or without an

applied field, no detectable loss of oxygen occurred. Therefore, for short time vacuum exposures at liquid nitrogen temperatures and below, no oxygen loss is expected, however, significant oxygen loss occurs for 18h vacuum exposures at room temperature.

BACKGROUND

The superconducting critical temperature, T,, is highly dependent upon the oxygen concentration in the 1,2,3 ceramic oxide materials(l,2). In the case of Y1BazC~307-x, for example, a reduction of the oxygen content from x=O to x=0.7 causes a corresponding change in the electrical behavior from superconducting to semiconducting.(l) Because the most oxygenated material is not a stable compound, oxygen out-diffusion may occur, thereby modifying the electrical properties, especially in the near surface region. This

possibility has strong implications especially for the interpretation of data obtained with surface sensitive techniques.(3,4) It must be emphasized that a decrease in the oxygen stoichiometry from x=0.1 to x=0.3 corresponds to a decrease in the oxygen concentration from 53.49% to 52.76%, or a change of only 0.73%.

Atom probe (AP) analysis of high temperature superconductors has been shown to be feasible.(5-9) If the data obtained by AP analysis on the oxygen concentration in the near- surface region are to be statistically meaningful, however, the surface concentration must be determined with an uncertainty less than the above composition change. For this accuracy, at least 4000 ions must be obtained for each surface layer. Only an AP that possesses a wide acceptance angle has the appropriate geometric configuration for this type of analysis. In an earlier study, an AP of this description was used to determine the near- surface composition of E U , B ~ ~ C U ~ O ~ - ~ . ( ~ O ) The oxygen content was found not to vary with room temperature exposures to vacuum; however, the metallic species were also found to be non-stoichiometric in the near surface layer. In the present study, the oxygen and metallic compositions were measured both before and after various time exposures to vacuum at both 85K and room temperature, after the non-stoichiometric layer was removed by extensive field evaporation.

EXPERIMENTAL

Superconducting EulBazCu307-, samples were prepared from stoichiometric mixtures by conventional methods. Superconductivity of the material was established by means of ac magnetic susceptibility measurements, showing T, to be about 92K and the value of x was estimated to be 0.1. Sharply pointed specimens suitable for field-ion microscopy (FIM) and AP chemical analysis were prepared by a mechanical fracturing process.(ll) Contamination, adsorbates, and irregularities were removed from the tip surface by chemically assisted field evaporation in Pa of Hz or a mixture H2+Nz gases at 65K and Ar gas at 85K. AP analyses were performed in =10-'Pa vacuum at 85K using a 21% pulse fraction(l2).

An AP instrument with a large acceptance angle was usedto acquire a sufficient number of ions per surface layer(l3) to determine the near-surface composition with a statistical certainty of better than f0.5 at. % for a monolayer or f0.3% for 3-4 monolayers. However, the large acceptance angle results in an increased background count from free-space ionized residual gas species which decreases the signal/noise ratio. The mass resolution of this instrument, when used in the wide-mass-scale mode, is comparable to that of a conventional electrically pulsed AP, but was insufficient to fully resolve some of the mass peaks. This

u'~lso Metallurgy Division, NIST.

C2bisiting scientist, present address : c/o Philips Research Laboratories, P .O. Box 80000, NL - 5600 JA Eindhoven, The Netherlands.

Lq~ustom Probes Unlimited, P.O. Box 3938, Gaithersburg, MD 20878 USA

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989881

restriction coupled with the reduced signal/noise ratio inhibited the identification of some peaks. Therefore, a longer, conventional AP was used to characterize many of the complex ionic species.

Fig. 1 shows a wide-angle AP mass spectrum of a specimen. The assignment of peaks used consistently throughout this study were specified earlier. If the vacuum of the AP was permitted to fully recover before collection of data, the residual gas peaks at mass-to- charge ratios (m/n) of 15, 28, 29, and 44 and the background counts were reduced to low levels, but the composition was not affected. The m/n = 32 peak is composed of 0: and C U + ~ isotopes of 31.5 and 32.5. The ratio of CU" to 0; in this peak usually varied between 0.28 and 0.33, as determined for each analysis using a graphical method, although ratios as high as 0.5 were observed. In all the experiments, the same analysis method for the mass spectra was used, so that systematical errors in peak assignments are expected to show only a minimal effect on the relative changes in local composition, while it is understood that the accuracy of the absolute compositions may not be so well characterized.

Because it was found earlier(l0) that routine specimen handling and Hz imaging at 85K produced a Ba-Eu enriched surface layer which may have affected oxygen out-diffusion, we repeated the earlier experiments. Then, much care was taken to remove this layer in high vacuum in order to reach stoichiometrically correct material with the expected high oxygen content. Both DC and pulsed field evaporation methods were employed for the surface cleaning operation with indistinguishable differences. This cleaning operation had to be continued until the evaporation voltage of the specimen was increased a few hundred volts typically indicating the removal of an estimated 40-50 atomic layers. The specimen was then analyzed to confirm the removal of the off-stoichiometric surface layer.

The subsequent specimen exposures to 10-'~a vacuum were: (1) at room temperature for up to 64h, field-free, (2) at 85K for up to 3h, field-free, and (3) at 85K with an applied DC field for up to 3h. AP analyses were then performed a second time and the oxygen

concentration and metallic stoichiometry were compared to the previous data to determine the effect of the treatment on the near-surface composition. The first treatment simulated the placement of a fresh specimen into the vacuum chamber for analysis the next day. The second treatment was for comparison with other low temperature surface analysis techniques. The last treatment was done as a control experiment to test the possible effect of an applied field on the measured oxygen concentration. Multiple regions within a specimen and multiple specimens were exposed for the described conditions for reproducibility of the phenomena.

Most analyses gave consistent results, except a few when the expected 1:2:3:7 ratio was never obtained; these data sets were not considered nor included in the results.

RESULTS and DISCUSSION

AP analyses showed that the near-surface region of each specimen, following extensive initial Hz FIM, etc., was depleted in oxygen to 52%, or x=0.5. (The number of elemental atoms were counted from which compositions were determined as simple percentages. It is customary, however, to give elemental ratios for ceramic superconductors. In this work, when ratios are given, the numbers denote atomic ratios based on the number of atoms for that element divided by one-sixth of the number of metallic ions.) After the surface cleaning operation, the expected oxygen composition of 53.8% (x=O.O) and the expected 1:2:3 metallic stoichiometry were obtained, except in a few instances when large composition fluctuations occurred. Such material was then removed by further field evaporation. If the specimen was permitted to warm to and stay at room temperature overnight, a significant decrease in the oxygen content was measured, but the metallic stoichiometry remained essentially constant. If the specimen was again permitted to warm to and remain at room temperature overnight and was analyzed without a cleaning procedure, a further (smaller) decrease in the oxygen content was measured. A subsequent cleaning operation always restored the composition to near the expected 1:2:3:7 stoichiometry. Although the exact depth of the depleted layer was not determined, it was estimated to be more than 4 layers thick, because that was the typical amount of material removed during an analysis without observing the bulk oxygen concentrations, and less than 50 layers.

It was observed that the amplitude of the m/n=32 peak increased as the oxygen content of the material increased, as reported by Kellogg and Brenner(8) and by Elswijk et a1.(10) The ratio of the C U + ~ species to the 0: species was also observed to increase as the surface oxygen content increased. This behavior indicates that increased amplitude in this peak is not due solely to the increase in the oxygen signal, but is due also to a change in the CU+/CU+~ ratio.

Cleaned-surface specimens were analyzed before and after 85K holding in vacuum for 3h field-free. No measurable changes in the elemental compositions were observed when comparing the values before and after the exposure. Composition profiles before and after the exposure show virtually no deviations, which were well within statistical errors, Fig. 2.

A specimen with a surface that had been cleaned was exposed to vacuum at 85K for 3h while maintaining the DC standing voltage. The oxygen and metallic compositions were

similar before and after the exposure, usually with a very small increase in the measured oxygen content. We believe that the small increase in oxygen is due to a field-enhanced adsorption of residual oxygen-containing gasses(l2). This indicates that the applied field used for AP analyses does not play a significant role in the measurement of oxygen out- diffusion in Eu 1,2,3 superconductors on this time scale.

Fig. 3 shows a cumulative set of results for AP analyses obtained before and after a series of vacuum treatments to a Eu 1,2,3 specimen.

Recently, data have become available eoncerning the diffusivity of oxygen in Y1Ba,Cu,O,-,.(14-16) The published data relates mainly to measurements at temperatures

above 750K although Ruckenstein and Mallick(16) indicated that the diffusion barrier may be highly temperature dependent and may drop markedly at temperatures below 773K. The present results are consistent with only the calculated diffusion distances based on the lower temperature diffusion data. It should be noted that the presence of the high vacuum (low oxygen pressure) environment was not included in the diffusion data, but might be expected to increase oxygen out-diffusion, and that grain boundary diffusion was probably dominant.

Therefore, the agreement may be fortuitous. Further AP studies should be done in which depth profiling is performed to obtain quantitative low-temperature diffusivities.

CONCLUSIONS

Oxygen concentration in the near-surface region of EulBa2Cu307-, can be measured with a wide-angle AP with a precision of f0.5 at. % for a single monolayer.

Ordinary specimen preparation and FIM characterization result in an oxygen deficient and non-stoichiometric surface region. This region, which is overly rich in Eu and Ba, acts as a chemical barrier to the subsequent out-diffusion of oxygen for as long as 80h at room temperature in vacuum. This surface layer can be removed by field evaporation to give a completely bulk stoichiometric near-surface region. Then the result of an exposure to vacuum for as long as 64h at room temperature is a measurable loss of oxygen. However, no oxygen loss occurs for vacuum exposures up to 3h at 85K.

ACKNOWLEDGEMENTS

We thank Dr. C.R. Brundle, IBM Research (San Jose, CA), for suggesting the experiment, C.K. Chiang for preparation of the samples and R.D. Shull for the magnetic susceptibility measurements. H.B. Elswijk's work was partly supported by the Netherlands Organization of Scientific Research (NWO).

References

1. R.J. Cava, B. Batlogg, A.P. Ramirez, D. Werder, C.H. Chen, E.A. Rietman and S.M.

Zahurak, Mater. Res. Soc. Symp. Proc.

s,

2. P. Strobel, J.J. Capponi, C. Chaillout, M. Marezio and J.L. Tholence, Nature 327, (1987) 306.

3. R.S. List, A.J. Arko, Z. Fisk, S.-W. Cheong, S. Conradson, J.D. Thompson, B. Pierce, D.E. Peterson, R. Barlett, N.D. Shinn, J.E. Schirber, B.W. Veal, A.P. Paulikasand and J.C. Campuzzano, Phys. Rev. B

3,

4. R.S. List, A.J. Arko, 2. Fisk, S.-W. Cheong, J.D. Thompson, J.A. O'Rourke, C.G. Olson, A.-B. Yang, Tun-Wen Pi, J.E. Schirber and N.D. Shinn, to be published in Phys. Rev.

Lett.

5. 0. Nishikawa and M. Nagai, Phys. Rev. B

21,

6. G.L. Kellogg and S.S. Brenner, Appl. Phys. Lett. S l , (1987) 1851.

7. A. Cerezo, C.R.M. Grovenor, R.M. Hoyle and G.D.W. Smith, Appl. Phys. Lett. 5 2 , (1988) 1020.

8. G. L. Kellogg and S. S. Brenner, J. de Phys. Colloque C6, @ (1988) 465.

9. G. ~aharchuk; L.v. Alvensleben, M. Oehring and P. Haasen, de Phys. Colloque C6, 49 (1988) 471.

10. H.B. Elswijk, P.P. Camus and A.J. Melmed, submitted to Appl. Phys. Let.

11. A. J. Melmed, J. de Phys. Colloque C6, Q, (1988) 67.

12. E.W. Miiller and T.T. Tsong, Field Ion Microscopy Principles and Applications, American Elsevier Publishing Company, Inc., New York (1969).

13 A.J. Melmed, M. Martinka and R. Klein, Proceedings of the 29th International Field Emission Symposium, eds. H.-0. Andrkn and H. NordBn, (Almqvist &,Wiksell International, Stockholm, Sweden, 1982) 243.

14. K.N. Tu, N.C. Yeh, S.I. Park and C.C. Tsuei, Phys. Rev. B 2 (1989) 304.

15. I. Haller, M.W. Shafer, R. Figat and D.B. Goland, Proceedings of IUPAC Symposium on the Chemistry of Oxide Superconductors, edited by C.N.R. Rao (Blackwell, Oxford, 1988), 93.

16. E. Ruckenstein and A.K. Mallick, Mat. Lett. 2 , (1988) 122.

W i d e - A n g l e AP S p e c t r u m EuBaCuO S u p e r c o n d u c t o r

Figure 1. Mass spectrum of an oxygenated Figure 2. Composition profiles for a EuiBazC% 0,

,,

,,

contains 4100 ions. The error bars indicates 1 standard deviation arising from counting statistics. Note the similar oxygen levels before and after the vacuum exposure.

Figure 3. Cumulative results for AP analyses of a Eu,Ba,Cu307-, specimen for various vacuum exposures at both room temperature and 85K. The error bars indicate one standard deviation arising from counting statistics. Note (1) the marked change in the compositions toward the expected stoichiometry following a cleaning procedure, (2) the decrease in oxygen content with room temperature exposures, and (3) the similar results after 85K exposures both with and without an applied field,

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