Effect of the atmosphere annealing on the structure of Bi2Sr2CaCu2O8 thin films grown on (100)SrTiO3 substrates

Physiea C 162-164 (1989) 707-708 North-Holland

EFFECT OF THE ATMOSPHERE ANNEALING ON THE STRUCTURE OF Bi2Sr2CaCu208 THIN FILMS GROWN ON (100)SrTiO3 SUBSTRATES *. Andr~ PERRIN, Zhi Zhong LI§, Jean PADIOU and Marcel SERGENT Laboratoire de Chimie Min~rale B, U.R.A.C.N.R.S. 254, Universit~ de Rennes I, Avenue du G~n~ral Leclerc, 35042 RENNES-C~dex (France) Thin-films of 2:2:1:2 composition have been grown by d.c. sputtering and post-annealing on (100)SrTiO3 singlecrystal substrates. When air-annealed, these films present characteristics (structure, resistivity) similar to the ones grown on (100)MgO. In contrast, films annealed in pure oxygen or nitrogen exhibit the 2:2:0:1 type structure with a strong 10011orientation and a shortened c parameter (c = 24.3 ~) suggesting a Sr-Ca disorder. However, EDS microprobe analyses show that the 2:2:1:2 stoichiometry is retained in the crystals forming the film. In fact, a subsequent short annealing in air of already oxygen-annealed films leads to the appearance of the 2:2:1:2 usual structure and, conversely, when previously air-annealed films are reannealed in oxygen, their structure reversibly transforms to the 2:2:0:1 one. This structural conversion appears to be related to the substrate nature and could explain the generally reported difference in behaviour for films deposited on SrTiO3 and MgO.

I. INTRODUCTION Following the discovery of very high Tc materials in the Bi-Sr-Ca-Cu-O system 1, a number of groups soon published superconducting thin films in this system 2-8. Most of them pointed out a noticeable influence of the substrate, (100)MgO being reported largely better than (100)SrTiO3 and, of course, than materials like sapphire. As a typical example, it was reported that two films of the same run, but deposited onto MgO and SrTiO3, turned to be superconducting and semiconducting, respectively7. All these authors annealed their films in pure oxygen. In contrast, using air instead of oxygen, and by a precise control of the annealing temperature, we obtained superconducting thin films of 2:2:1:2 composition on sapphire substrates (although the superconducting transition was not fully achieved at the nitrogen boiling point and the temperature factor in the normal state was slightly negative) and succeeded in the elaboration, on (100)SrTiO3 substrates, of films comparable in quality to the ones deposited on (100)MgO8. In order to determine the influence of the atmosphere composition during post-deposition heat treatment, we have prepared additional thin-films and annealed them in air, oxygen, and nitrogen, respectively. In addition, we performed successive annealings in different atmospheres. 2. EXPERIMENTAL Thin-films were deposited by d.c. s~uttering in the same conditions as previously reported°. X-Ray diffraction patterns (XRD) were recorded using Cu Ks radiation and e-2e scans. Thin-film surface topography was observed by scanning electron microscopy (SEM). Chemical composition was determined

by energy dispersive spectroscopy (EDS)~microprobe analysis. Resistive measurements were performed in a standard four-probe configuration. 3. RESULTS Figure 1 reports the resistive behaviour of a thin film of 2:2:1:2 composition deposited on (100) SrTiO3 and air-annealed for 40 rain. at 865 oC. XRD pattern of this film exhibits almost single-phased,

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* Research under CNET (Lannion B) contract n ° 85 8B 095 and MRT contract n ° 508 819/Thomson CSF § Present address : H.H. Wills Phys. Lab., Univ. Bristol, Tyndall Avenue, G.B.- BRISTOL BS 8 ITL

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strongly oriented I00/] material : only traces of Ca0.6Sr0.4Cu1.7503 and Bi0.9Sr1.101.45 can be detected as secondary phases in the background. SEM observation shows single crystal platelets several micrometer wide. 3.1. Oxygen annealings Similar films have been annealed in an oxygen flow in the range 845-875 °C. SEM observation shows the homogeneous formation of large platelet crystals and, in fact, these films are smoother than the airannealed ones. Microprobe analysis of these crystals, as well as of larger areas, gave evidence for a composition very close to the 2:2:1:2 stoichiometry. For films annealed at the upper limit of temperatures studied (875 °C), the growth of Sr14_xCaxCu24041 needles was also observed. In contrast, XRD exhibits the well-known 2:2:0:1 structure with a strong J00lJ orientation and c = 24.3 ~ (figure 2). As this result contradicts the microprobe data, large amounts of CuO and/or CaCuO2 or Ca2CuO3 are expected : actually, only a very small amount of the latter, apparently oriented along Ii011 and ]310[ directions, is detected. S

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FIGURE 2 XRD pattern for a 2:2:1:2 film annealed in oxygen (sample 2 of figure i). Indexations refer to the 2:2:0:1 peaks. ~Bi0.9Sr0.1Oi.45 • Ca2CuO3 The resistive behaviour of such films is characterized by a slightly negative temperature factor in the normal state, followed by an incomplete superconducting transition starting near 80 K and a further increase of the resistivity at lower temperatures (Figure 1). However no anomalies are observed in the temperature range of the 2:2:0:1 superconducting transition, so the presence of the related semiconducting material Bi2(Sr,Ca,Bi)2CuO69, with c = 24.5 A cannot be ruled out. In the special case of films annealed at lower temperatures (845 °C), we have observed the growth of unusual isotropic crystals looking like dodecahedra. EDS gives again a 2:2:1:2 composition although XRD shows incompletely oriented 2:2:0:1 structure. 3.2. Annealings under nitrogen For comparison, some films have been annealed under nitrogen flow. Quite lower annealing tempe-

rature range was used, based on the melting point evolution of Bi materials with oxygen partial pressure I0. These films are formed by large flat crystals of composition 2:2:0:1. Small amounts of CuO and CaCuO2 (Ca2CuO3) are observed on SEM photographs as white and black dots, respectively. As above, XRD of films annealed at 825 °C show a strongly oriented [00l] 2:2:0:1 structure with a specially short c lattice constant (23.9 ~) and minute amounts of CaO, (h00)Sr2CuO3 and CaCuO2. At higher annealing temperatures (855 °C) the clattice constant increases up to 24.2 ~ and a small amount of the 2:2:1:2 structure starts to appear. 4. DISCUSSION The formation of the 2:2:0:1 type structure is quite unexpected in these conditions as the composition of the films is very close to 2:2:1:2. Morever, when these films are reannealed for a short time under air atmosphere, the 2:2:1:2 phase forms again and the R(T) behaviour becomes usual as shown in figure i. Conversely, when air-annealed films are reannealed in pure oxygen, a mixture (due to kinetics) of 2:2:1:2 and 2:2:0:1 phases is observed. This stabilization of either the 2:2:0:1 or the 2:2:1:2 structure for the same film composition, depending on the nature of the atmosphere during annealings is not yet well-understood. As no significant amounts of CuO or Ca cuprates were detected, a disordered distribution of the additionnal CuO(Ca) planes has to be accounted for. As pointed out, it is difficult to actually distinguih the 2:2:0:1 from the Bi2(Sr,Ca,Bi)2CuO6 structure ; however, the (negative) temperature factor displayed on figure 1 appears too small to conclude to the formation of the latter semiconducting material. In any case, the mechanism of the structural evolution would be similar. In contrast, thin-films deposited on (100)MgO substrates appear less sensitive to this annealingatmosphere effect : this observation explains why best results are usually reported when such substrates are used instead of SrTiO3. In fact a very similar behaviour can be achieved for these two types of substrates provided that annealing is performed under quite a low oxygen partial pressure. This is in good agreement with, for instance, the recently reported stabilization of lead-doped materials under partial oxygen pressure (about 1/13 atmosphere10). REFERENCES i. H. Maeda et al., Jpn. J. Appl. Phys. 2__7(1988) L209 2. M. Hong et al., J. Cryst. Growth __91(1988) 382 3. C.E. Rice et al.,Appl. Phys. Lett. 5__22(1988) 1828 4. B.T. Sullivan et al., Appl. Phys.Lett. 52 (1988) 1992 5. J.H. Hang et al., Phys. Lett. 128 (1988) 102 6. H. Adachi et al., Jpn. J. Appl. Phys. 27 (1988) L643 7. H. Raffy et al., Solid State Comm. 8__88(1988) 235 8. A. Perrin et al., Superc. Sc. Tech. 1 (1988) 201 9. S.A. Sunshine et al., Phys. Rev. B 3-8 (1988) 893 10.U. Endo et al., Jpn. J. Appl. Phys. 2__77(1988) L1476