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New structural aspects of Tl2Ba2CaCu2Oy epitaxial thin films grown by MOCVD on LaAlO3
PHYSICA ELSEVIER
Physica C 275 (1997) 146-154
New structural aspects of TlaBaaCaCu2Oy epitaxial thin films grown by MOCVD on LaA10 3 X.F. Zhang a, *, Y.S. Sung a, D.J. Miller a, B.J. Hinds b, R.J. McNeely b, D.L. Studebaker b, T.J. Marks b a Materials Science Division, and the Science and Technology Center for Superconductivity, Argonne National Laboratory, Argonne, IL 60439, USA b Department of Chemistry, and the Science and Technology CenterJbr Superconductivity, Northwestern University, Evanston, IL 60208-3113, USA Received 10 June 1996
Abstract The microstructure of a Tl2Ba2CaCu2Oy (TI-2212) superconducting thin film grown by metal-organic chemical vapor deposition (MOCVD) on a pseudo-cubic (001) LaAIO 3 substrate has been examined by analytical transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Over large regions, the film is epitaxial and T1-2212 phase is found to be the major phase. The film/substrate interface is abrupt and smooth on the atomic scale but a strain-field is induced by the lattice mismatch between the film and the substrate. In addition to the intrinsic modulation structure of the T1-2212 phase, a very different modulation structure has also been found. The space group for the T1-2212 phase in this thin film was determined to be I4mm rather than I4/mmm as usually reported. The loss of the (001) mirror plane is attributed to a defective T1-2212 structure in which a considerable amount of T1 vacancies and TI disorder occurred inhomogeneously in T1-O layers. The reason for the Tl-deficiency is discussed. PACS: 61.16.Bg; 61.50.Em; 68.55.Jk Keywords: TI2Ba2CaCu2Oy superconducting thin film; Microstructure
I. Introduction Although the basic structures for the Tl-based T l 2 B a 2 C a , C u , + l O y (n = 0,1,2...) superconducting cuprates have been known for quite some time [1-3], detailed studies of the microstructure of these materials are still needed to evaluate various defect struc-
* Corresponding author. Fax: + 1 630 252-7777; e-mail: [email protected].~,ov.
tures. For example, homologous intergrown phases (different n values) [4,5]; structural modulations [2,6,7]; non-stoichiometry which is related not only to the oxygen content but also to the inter-layer substitutions [8-11]; mixtures of TI 3+ and TI 1÷ in the T I - O layers [9,12-14]; TI and O disorders [12,14,15] and other defects all contribute to the properties and behavior of these materials. Meanwhile, synthesis and characterization o f TIbased high transition temperature (Tc) superconducting thin films remains attractive due to their great
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potential applications in various electronic and passive microwave devices. Unlike thin films based on other high Tc superconducting compounds, in-situ preparation of Tl-based thin films of high quality is quite difficult due to the high volatility of T120. A two-step ex-situ thallination process has thus been developed. Polycrystalline B a - C a - C u - O precursor films of an appropriate stoichiometric ratio are first deposited onto substrates. The precursor films are then subjected to an annealing process in the presence of TI20 vapor. Thin films consisting of T1-2212 phase or T1-2223 phase have been synthesized by this method [16-18]. According to previous TEM investigations, intergrowth of homologous phases [16], intrinsic modulation [19], granularity based on small misorientations of epitaxial films [18,20] and interfacial layers [21] are present in the Tl-based superconducting thin films. However, detailed TEM characterizations and especially HREM studies on an atomic scale are still valuable in establishing the relationship among the microstructure, the physical properties and the processing conditions of the thin films. This is particularly true for the Tl-based thin films because their microstructure may be affected by many factors as mentioned above. As is shown in this paper, the processing parameters substantially influence the microstructure of the T1-2212 phase formed in the present thin film, which was found to be Tl-deficient as a consequence of annealing under particular conditions.
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ducting transition temperature for zero resistance (To,0) of 105 K and a transport critical current density (Jc) at 77 K of 1.2 × 105 A / c m 2. More details of the processing conditions and physical properties are reported elsewhere [18]. Cross-sectional and plane-view TEM samples were prepared by conventional techniques involving grinding, dimpling, and finally ion milling at liquid nitrogen temperature to final perforation. TEM characterizations were carried out with both a Philips CM30 and a high-resolution Jeol 4000EX transmission electron microscope, operated at 200 and 400 kV, respectively. Energy-dispersive X-ray spectroscopy (EDS) was performed with the Philips CM30 transmission electron microscope equipped with an ultra-thin window.
3. Results Fig. 1 illustrates some typical features observed in the epitaxially oriented film studied. The thickness of the film is about 3500 ,~,. The interface between the substrate and the film appears to be abrupt and straight. Inhomogeneous image contrast can be seen which is usually associated with very small misorientations (one part of the crystal may be tilted a degree or so) but may also be partly due to secondary phases. The selected-area electron diffraction (SAED) patterns obtained from various areas of the film confirm T1-2212 (c = 29.2 ,g,) as the major phase in the film. The c-axis of the film is perpendicular to
2. Experimental Epitaxial Tl2Ba2CaCu2Oy superconducting thin films were grown in a two-step process involving MOCVD growth of B a - C a - C u - O ( F ) thin films on pseudo-cubic (001) LaAIO 3 substrates, followed by a post-annealing in the presence of TI20 vapor. The TI-2212 phase was formed during annealing. This thallination annealing was performed by sealing the precursor film together with a presintered pellet of 2212 stoichiometry in a gold foil pouch to minimize T1 loss. Annealing was carried out at 867°C for 1 hour in a pure oxygen atmosphere. Mixtures of T1-2212/1212 formed when the annealing temperature was beyond a range of (870 + 20)°C. The best films prepared by this method exhibit a supercon-
Fig. I. Morphologyof the TI-2212thin films(denotedas TBCCO) observed along a cross-sectional direction. Interface planes between the film and the LaAIO3 substrate are indicated by white arrows. Arrowheadsindicate grains of differentimage contrast.
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the substrate surface plane in most areas according to electron diffraction. However, in a few places a larger c-axis misorientation can be seen as shown in Fig. 2, in which grain boundaries in the T1-2212 film are easily recognized. A very different orientation was observed for the grain marked by an asterisk, where the [110] axis is close to the substrate surface normal. However, such highly misoriented grains were not common. There is no evidence for a correlation between the misoriented grains in the film and the microstructure (e.g. twinning) or surface morphology of the substrate. The misorientation between grains in the film can be readily seen from the lattice-fringe images shown in Fig. 3. Fig. 3a shows a small angle tilt grain boundary across which the c-lattice fringes in two grains are tilted by 3.5 ° with respect to each other about the incident beam direction (b-axis). Fig. 3b illustrates another small angle tilt grain boundary in the same TEM sample, in this case a 2 ° tilt occurs about the a-axis direction marked in the upper grain. Therefore, the image for the bottom grain is interrupted due to the 2° tilt away from the perfect [010] zone-axis. Fig. 3 indicates that the small angle c-axis misorientation can take place about both a- and b-axes, therefore these c-axis misorientations appear unrelated to the surface morphology, i.e. miscut, of the substrate. We also find that a higher annealing temperature of 890°C causes significant misorientations in the (a-b) plane and the formation of an interfacial phase.
Fig. 3. (a) Lattice-fringe image displaying a 3.5 ° misorientation between the c-axes of two grains. (b) HREM image showing two neighboring grains in the T1-2212 film. The [010] zone-axis of the upper grain is exactly parallel to the incident electron beam direction while the grain at the bottom is rotated by 2 ° about the a-axis. A bold arrow indicates the grain boundary line.
Fig. 2. Bright-field image showing the granularity in the TI-2212 film. c-axes in some misoriented grains are marked. A grain marked by an asterisk has a very different orientation: the [110] direction is parallel to the substrate normal.
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The general orientation relationships between the film and the substrate can be determined by SAED patterns. Fig. 4a shows such an SAED pattern taken along the [010] direction of the T1-2212 with both the film and the substrate included in the selected area. Reflections arising from the LaA103 substrate are marked by black dots. Indexing the substrate based on the pseudo-cubic perovskite structure of the LaA103, the orientation relationships between the substrate and the film can be described as (001)LaAlOs//(001)2212 and [100]LaAIO3//[10012212. In Fig. 4a, satellite reflections appearing around the main reflections associated with the T1-2212 indicate an intrinsic modulation structure in the film. The [010] projected wave vector of the intrinsic modulation is q p = 0 . 1 7 a * + c * giving rise to a 5.9a X c projected super-unit cell in real space, consistent with the intrinsic modulation structure reported for bulk crystals [2,6,7]. This intrinsic modulation also possesses an incommensurate component along the b'-direction, as will be shown later in the SAED patterns from a plane-view sample so that the wave vector should be expressed as q = 0.17a* + 0.17b* + c * . The intrinsic modulation in the present T1-2212 film can be directly imaged by HREM. A prominent feature in these images is the alternately stacked darker and brighter blocks which are reminiscent of the similar image features for the well-characterized
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modulation in Bi2Sr2CaCu2Oy (Bi-2212) compounds [22,23]. It has been established that atomic disorder is the origin for the modulation of the Bi-2212 phase. Atoms, especially Bi, shift away from the standard crystallographic positions, leading to an uneven atomic distribution in the structure [24]. Those areas containing more cations scatter the incident electrons more strongly resulting in darker contrast, whereas areas containing less condensed cations display a brighter contrast [22,23]. The fact that a similar image contrast is observed for the intrinsic modulation in the T1-2212 phase implies a displacive rather than a compositional character of the modulation. Another type of modulation structure has been found in some areas of the film as shown in Fig. 4b and by the representative drawing in Fig. 4c. The difference in the position of the satellite reflections is obvious compared with that shown in Fig. 4a. The [010] projected modulation wave vector is q p = 0.104a* + 0.52c*. This type of modulation has not been reported previously for the T 1 - B a - C a - C u - O superconductors. The new modulation shown in Fig. 4b is not dominant in the film, therefore the intensity of the satellite reflections is weak, and the coexistence of two types of modulation reflections can be distinguished in SAED patterns obtaining from an area of 0.3 /xm in diameter. According to the SAED pattern in Fig. 4a, a lattice mismatch of 2.1% (T1-2212: a = 3.87 ~k;
Fig. 4. (a) SAED pattern taken along the [010] direction of the film. Some reflections from the substrate are marked by black dots. Small arrows indicate satellite reflections associating with the modulation structure of the T1-2212 phase. (b) [010] SAED pattern taken from a local area (0.2 p~m in diameter) in the T1-2212film. A new modulation is present, as also seen in (c), in which reflections arising from the basic structure of the T1-2212 phase are represented by full circles while open circles correspond to the satellite reflections having a different arrangement from those in (a).
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Fig. 5. Morphology of the film/substrate interfacial area. The periodic change of the image contrast due to the strain field along the interface is indicated.
LaAIO3: a = 3.79 ,&) along the a-axis of the film can be calculated between the substrate and the T1-2212 thin film. This interfacial lattice mismatch is accommodated by misfit dislocations. A periodic distribution of dislocation cores was formed along the interface plane as can be seen in the image in Fig. 5. Points along the interface at which the image contrast is changed due to the dislocation cores are indicated b~¢ arrowheads in Fig. 5. A periodicity of about 200 A (20 nm) is observed which is consistent with the expected value based on the lattice mismatch: L = [ a 2 / ( a I --a2)]a t = 183 ,&, with a I = 3.87 ,& and a 2 = 3.79 A the a-axis lattice parameter in the film and the substrate, respectively. Fig. 6 presents SAED patterns along the [001] and the [101] zone-axes of the T1-2212 film. The 4-fold symmetry is confirmed by Fig. 6a for the basic structure of the TI-2212 phase. The satellite reflec-
1 Fig. 6. SAED pattems taken along (a) the [001] and (b) the [101] directions of the TI-2212 phase. Satellite reflections indicated in (b) reveal the two-dimensional superlattice in the T1-2212 structure.
Fig. 7. (a) [001] CBED pattern of the TI-2212 film. 4mvm' v symmetry can be seen in the whole pattern and the bright-field disk (the central disk), m v and m'v denote the vertical mirror planes parallel to the (100), (010) and the (110), (1~0) planes, respectively.
tions associated with the intrinsic modulation can be observed only when the sample is tilted slightly (e.g. 3.5 °) away from the [001] zone-axis because the satellite reflections locate on the (00/) planes with l = odd integral numbers. Fig. 6b is such an SAED pattern, in which the two-dimensional modulation is visible. The four-fold symmetry is thus not disturbed by imposing the intrinsic modulation on the basic structure of the T1-2212 phase. This is verified by the [001] convergent-beam electron diffraction (CBED) pattern shown in Fig. 7. The 4mvm' v symmetry in the (a *-b* ) plane can be seen in the whole pattern (WP) and the bright-field (BF) disk (zero-beam disk). Here m v and m'v denote the vertical mirror planes parallel to the (100), (010) and the (110), (1~0) lattice planes, respectively. It should be noted that the places used for the CBED studies have been carefully selected. EDS was used to ensure no residual LaA103 was attached to the T1-2212 film. A single crystalline [001] SAED pattern and 4mvm' ~ symmetry of the [001] CBED pattern must be observed on the selected places so that the possible influence of local defects (e.g., c-axis misorientations) on the symmetry determination can be avoided. Although the [001] CBED pattern yields symmetry information in the (a-b) plane, information about the symmetry along the c-axis is still needed to
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Table 1 Summary of symmetries presented by the [001] CBED patterns in Figs. 7 and 8; BF, WP, DF denote the bright-field, whole pattern and dark-field, respectively; m v and m' v denote the mirror planes parallel to the (100), (010) and the (110), (1~0) lattice planes, respectively
Fig. 8. (a) (200) and (b) (110) CBED dark-field patterns of the TI-2212 film. Dark-field disks are circled by dashed lines and the center of the zero-beam disk in each pattern is marked by a cross. m_~ and m'v denote the mirror planes parallel to the (020) and (110) lattice planes, respectively.
determine the space group. Usually this information can be obtained by tilting the sample in the microscope. Based on the reflection conditions obtained in a series of SAED patterns from the T1-2212 film, we were able to deduce that the space group is either I 4 / m m m (no. 139), I4m2 (no. 119), or I4mm (no. 107). An important difference between I 4 / m m m and latter two space groups is the lack of the (001) mirror plane for the I4m2 and I4mm assuming the 4-fold axis is parallel to the [001] axis. In principle, a CBED pattern along any direction perpendicular to the c-axis may be used to examine the (001) mirror symmetry. However, the (001) reflections in reciprocal space of the T1-2212 structure are too closely spaced, leading to severely overlapped (00l) disks. The symmetry information contained in the BF and WP is thus distorted. Therefore, we utilized the [001] dark-field (DF) symmetry to determine the space group. Fig. 8a and 8b show the (200) and the (110) CBED DF patterns, respectively, obtained from the same spot on the sample from which Fig. 7 was obtained. A single mirror plane can be seen in each DF disk marked. This observation excludes the possibility of a 4 / m m m point group for which (200) and (110) DF patterns should present 2mm symmetry rather than m [25]. Combined with the WP and BF symmetries (see Table 1), we can conclude that the point group is 4mm [25] so that the space group for the T1-2212 phase structure studied is I4mm rather than I 4 / m m m as reported for the ideal T1-2212 structure [1-3]. The fact that the I4mm has been found in several randomly selected areas in the film
WP
BF
(200) DF
( l l 0 ) DF
4m v m'v
4m v m'v
mv
m'v
suggests that the symmetry of the T1-2212 phase in the film has been changed globally rather than only locally. Further details of the CBED analyses will be presented elsewhere. As mentioned above, an I4mm space group indicates the loss of the (001) mirror plane compared with I4/mmm. This loss of (001) mirror symmetry can be directly verified by [100] HREM images as shown in Fig. 9. The T1-2212 structure as well as the film/substrate orientation relationships determined by the SAED patterns can be seen. The film/substrate interface is smooth and reasonably abrupt. However, on this atomic scale a disordered interfacial layer with an abnormally bright image contrast
Fig. 9. HREM image showing a film/substrate interfacial area. Black dots correspond to projected columns of cations. Epitaxial growth of the T1-2212 film can be seen from the very first layer. A disordered interfacial layer with an abnormally bright contrast is exhibited in the right part of the figure. Arrowheads and small arrows indicate the T1-O layers showing different image contrast.
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can be seen in some interfacial areas such as the right part of Fig. 9. The thickness of this layer is about 4 - 5 A along the c-axis of the film but the origin and identity of the layer has not yet been established. It has also been observed that the presence of small steps (a few .~, in height) does not disrupt the epitaxial growth of the T1-2212 film. The layer stacking sequence of the T1-2212 structure can be derived from Fig. 9 using the known inter-layer spacing [1-3] and the image contrast of projected atomic columns. In computer simulations, columns associated with cations are projected as black dots in the images taken under our imaging conditions (Scherzer defocus - 4 0 nm and crystal thickness up to 4 nm in the Jeol 4000EX electron microscope). Black dots corresponding to TI columns generally have the larger size than those corresponding to other cations. The two T1-O layers in the ideal T1-2212 structure should appear identical because of their identical structural environments [1-3]. However, a considerable difference in image contrast is exhibited between the neighboring T1-O layers marked by arrowheads and arrows, respectively, and the spacing between each type of T1-O layer is c/2. As a result, the (001) mirror plane of the T1-2212 structure does not exist, verifying the conclusion derived from the CBED results. This change in symmetry is observed throughout the film, from the surface to the film/substrate interface. No new ordering in the T I - O layers has been distinguished by SAED patterns and HREM images along both plane-view and cross-sectional directions. It should be pointed out that the different image contrast between the neighboring T1-O layers can not be induced by varied imaging conditions (various defocus and sample thickness, etc.) due to the spatial proximity of neighboring TI-O layers. Regardless of the imaging conditions, the two neighboring TI-O layers should show identical contrast for the ideal TI-2212 phase structure [1-3]. Therefore, a substantial difference in structure between two neighboring T1-O layers must exist. A similar phenomenon can be observed in HREM images published by other groups although the authors did not address the issue [5,26]. EDS was performed to determine the composition of the T1-2212 phase in the film studied. The results suggest a considerable Tl-deficiency (as much as 30 at% deficient in Tl) and Ca-enrichment (as much as
20 at% enriched in Ca) for the TI-2212 film. Similar results have also been reported by other groups [3,8,10,17]. In spite of some Ca substitution for T1, T1 vacancies can still be expected based on the EDS analysis, leaving a defective structure for the T1-2212 phase formed in the film studied.
4. Discussion
The relative Tl-deficiency in the MOCVD-derived T1-2212 film detected by EDS is likely related to the high volatility of TI. This Tl-deficiency might be a result of the processing conditions used in synthesis of the film or it might arise as an artifact during TEM sample preparation (e.g., during ion milling) or TEM examination (e.g., beam heating). In order to exclude the possibility that the Tl-deficiency arises as an artifact, in situ heating experiments were performed. Bulk crystals of T1-2212 were heated in situ in the TEM (10 -6 torr vacuum) while the composition and crystal structure were monitored by EDS and SAED, respectively. It was found that the composition did not change until the sample temperature reached 400°C at which point a dramatic loss of T1 occurred. Consistent with this behavior, SAED patterns showed no discemible changes until the sample temperature exceeded 400°C. Thus, it appears that the thermallystimulated loss of T1 from the T1-2212 is not a factor at temperatures below about 400°C. It is highly unlikely that the sample reached temperatures approaching 400°C during either sample preparation or TEM imaging, so we speculate that the Tl-deficiency observed is a result of the synthesis conditions. In addition to the annealing temperature which was controlled to form the phase-pure T1-2212 phase rather than mixed TI-2212/T1212 in these experiments [18], the TI20 vapor pressure is also considered a key factor which influences phase formation and T1 content. Generally, a higher T120 vapor pressure stabilizes the double-Tl-layer compounds while lower T120 pressures leads to formation of single-Tl-layer phases. Decreasing the T1 vapor pressure leads to a change from T1-2212 to either T1-1212 or Tl-free phases, depending on the oxygen partial pressure [27]. However, the T1-2212 phase is stable over a range of TI partial pressures spanning nearly
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an order of magnitude [27] although the T1-2212 phase with less than 100% occupancy of the TI sites is likely when lower TIzO vapor pressure within the range is applied. In case of the existence of a remarkable Tl-deficiency in the structure, the phase is actually an intermediate state between the T1-2212 and the T1-1212 phase with fully occupied T1-O layers. Our T1-2212 films were synthesized by annealing B a - C a - C u - O ( F ) precursor films in TI20 vapor. The TI20 vapor is provided from a bulk T1-2212 pellet while the pressure is maintained by sealing the film and pellet together in a gold foil pouch. Presumably, the TI20 vapor pressure provided by the bulk pellet cannot be maintained infinitely in this type of system so that TI loss from the film could occur for long annealing times. Chrzanowski et al. have shown that the optimum thallination time in such a semiopen system is related to the mass of the bulk pellet contained in the system [17]. For the 1 g pellet used in the present experiments, the 60 minute thallination time may result in some loss of TI from the film, resulting in less than 100% occupancy of the TI sites (Tl-deficiency) as suggested by the EDS results [17]. Tl-deficiency is thus very likely to occur in our T1-2212 film produced in the similar way. A shorter thallination time may improve the Tl-occupancy but could also have an influence on the oxidation state of the TI in the T1-2212 phase, resulting in a mixture of T11+ and TI 3+ as has been observed in some bulk crystals [9,12,14]. Either the Tl-deficiency measured for this sample or the possible coexistence of TI3+/TI j÷ could lead to the change in space group of the T1-2212 film studied here as long as these defects are inhomogeneously distributed in T I - O layers. Our computer simulations show that a non-uniform distribution of vacant TI sites a n d / o r TI3+/TI l+ could lead to the unusual image contrast noted for Fig. 9. For example, a difference in contrast between the adjacent T I - O layers similar to that observed in Fig. 9 becomes distinct when a T1-O layer has about 20% of the cation sites unoccupied while the adjacent T I - O layer is fully occupied. Details of these simulations will be discussed elsewhere. Since the T! loss is inevitable during processing procedures because of the high volatility of TI, a quite common tendency of Tl-deficiency can be ex-
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pected in the TI-2212 phase. The ideal T1-2212 structure with the space group of I 4 / m m m can be maintained when this Tl-deficiency is not remarkable as in cases of [1-3]. However, considerable amount of TI vacancies could occur under some annealing conditions. As a consequence, the structure should be influenced and is able to be imaged by HREM as shown in Fig. 9. In fact, in addition to Fig. 9 from the T1-2212 thin film studied, image contrast effect which implies the modified T1-2212 structure can also be recognized in HREM images published previously by other groups [5,26]. These observations suggest the possibility that this modified T1-2212 structure is not uncommon. In our samples, although some of the vacant TI sites may be occupied by some Ca, the remaining unoccupied sites (up to 30%) could lead to an unstable structure and the formation of single-Tl-layer (T1-1212) intergrowth. However, carbon which has recently been shown to promote the formation of double-Tl-layer phases [28] may be incorporated around TI vacancies. Thus, it is possible that carbon stabilizes the double-layer phases so that, rather than forming single-Tl-layer intergrowth, a Tl-deficient double-layer phase remains stable. 5. Concluding r e m a r k s Structural characterization of the epitaxial TI-2212 thin film grown by MOCVD on (001) LaAlO 3 substrate has been performed by TEM. The lattice mismatch between the film and the substrate, granularity based on small c-axis misorientation, and two kinds of modulation structures have been observed. The space group of the T1-2212 phase in the film was determined to be I4mm which is different from the 1 4 / m m m reported. The lower symmetry is mainly due to the loss of the (00t) mirror plane which can be directly verified by HREM images. Tl-deficiency a n d / o r Tl-disorder in such defective TI-2212 phase are thought to be responsible for the symmetry change. Therefore the resulted phase was regarded as the intermediate state between the T1-2212 and the Tl-1212 phase with fully occupied T I - O layers. The considerable Tl-deficiency in the sample was attributed to the high volatility of T1. Carbon ions are likely to play a role in stabilizing the double T l - O layered T1-2212 structure.
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Acknowledgements This work was partially supported by the National Science Foundation Office of Science and Technology Centers under contract DMR 9 1 - 2 0 ( O and the US Department of Energy, Basic Energy ScienceMaterial Science, Conservation and Renewable Energy - Advanced Utility Concepts - Superconducting Technology Program, under contract #W-31109-ENG-38. References [1] M.A. Subramanian, J.C. Calabrese, C.C. Torardi, J. Gopalakrishnan, T.R. Askew, R.B. Flippen, K.J. Morrissey, U. Chowdhry and A.W. Sleight, Nature 332 (1988) 420. [2] A.W. Hewat, E.A. Hewat, J. Brynestad, H.A. Mook and E.D. Spccht, Physica C 152 (1988) 438. [3] M. Kikuchi, T. Kajitani, T. Suzuki, S. Nakajima, K. Hiraga, N. Kobayashi, H. Iwasaki, Y. Syono and Y. Muto, Jpn. J. Appl. Phys. 28 (1989) L382. [4] M. Verwerft, G. Van Tendeloo and S. Amelinckx, Physica C 156 (1988) 607. [5] T. Kotani, T. Nishikawa, H. Takei and K. Tada, Jpn. J. Appl. Phys. 29 (1990) L902 (see Figs. 7 and 8). [6] H.W. Zandbergen, G. Van Tendeloo, J. Van Landuyt and S. Amelinckx, Appl. Phys. A 46 (1988) 233. [7] S. lijima, T. Ichihashi, Y. Shimakawa, T. Manako and Y. Kubo, Jpn. J. Appl. Phys. 27 (1988) LI061. [8] K. Hiraga, D. Shindo, M. Hirabayashi, M. Kikuchi, N. Kobayashi and Y. Syono, Jpn. J. Appl. Phys. 27 (1988) L1848. [9] I.K. Gopalakrishnan, P.V.P.S.S. Sastry, H. Rajagopal, A. Sequeira, J.V. Yakhmi and R.M. lyer, Physica C 159 (1989) 811. [10] B. Morosin, D.S. Ginley, E.L. Venturini, R.J. Baughman and C.P. Tigges, Physica C 172 (1991) 413. [I 1] S. Sasaki, K. Kawaguchi and M. Nakao, Jpn. J. Appl. Phys. 31 (1992) L467.
[ 12] D.E. Cox, C.C. Torardi, M.A. Subranmanian, J. Gopalakrishnan and A.W. Sleight, Phys. Rev. B 38 (1988) 6624. [13] A. Sequeira, H. Rajagopal, I.K. Gopalakrishnan, P.V.P.S.S. Sastry, G.M. Phatak, J.V. Yakhmi and R.M. lyer, Physica C 156 (1988) 599. [14] B. Morosin, E.L. Venturini and D.S. Ginley, Physica C 183 (1991) 90. [15] W. Dmowksi, B.H. Toby, T. Egami, M.A. Subranmanian, J. Gopalakrishnan and A.W. Sleight, Phys. Rev. Lett. 61 (1988) 2608. [16] W.L. Holstein, L.A. Parisi, C.R. Fincher and P.L. Gai, Physica C 212 (1993) 110. [17] J. Chrzanowski, S. Meng-Burany, W.B. Xing, A.E. Curzon, J.C. Irwin, B. Heinrich, R.A. Cragg, N. Fortier, F. Habib, V. Angus, G. Anderson and A.A. Fife, Supercond. Sci. Technol. 9 (19%) 113. [18] B.J. Hinds, R.J. McNeely, D.L. Studebaker, T.J. Marks, T.P. Hogan, J.L. Schindler, C.R. Kannewurf, X.F. Zhang and D.J. Miller, J. Mater. Res., in press. [19] C.H. Chen, M. Hong, D.J. Werder, J. Kwo, S.H. Liou and D.D. Bacon, Appl. Phys. Lett. 54 (1989) 1579. [20] D.J. Werder and S.H. Liou, Physica C 179 (1991) 430. [21] J.G. Hu, D.J. Miller, D.L. Schulz, B. Han, D.A. Neumayer, B.J. Hinds and T.J. Marks, Physica C 210 (1993) 97. [22] Y. Matsui, H. Maeda, Y. Tanaka and S. Horiuchi, Jpn. J. Appi. Phys. 27 (1988) L372. [23] G. Calestani, G. Salsi, M.G. Francesconi, R. Masini, L. Dimesso, A. Migliori, X.F. Zhang and G. Van Tendeloo, Physica C 206 (1993) 33. [24] A. Yamamoto, M. Onoda, E. Takayama-Muromachi, F. lzumi, T. Ishigaki and H. Asano, Phys. Rev. B 42 (1990) 4228. [25] B.F. Buxton, J.A. Eades, J.W. Steeds and G.M. Rackham, Phil. Trans. Roy. Soc. (London) A 281 (1976) 171. [26] W. Zhou, A. Porch, I.B.M. Van Damme, D.A. Jefferson, W.Y. Liang and P.P. Edwards, J. Solid State Chem. 88 (1990) 193 (see Fig. 2). [27] T.L. Aselage, E.L. Venturini and S.B. Van Duesen, J. Appl. Phys. 75 (1994) 1023. [28] Y.S. Sung, X.F. Zhang and D.J. Miller, Appl. Phys. Lett., 69 (1996) 3420.
Physica C 275 (1997) 146-154
New structural aspects of TlaBaaCaCu2Oy epitaxial thin films grown by MOCVD on LaA10 3 X.F. Zhang a, *, Y.S. Sung a, D.J. Miller a, B.J. Hinds b, R.J. McNeely b, D.L. Studebaker b, T.J. Marks b a Materials Science Division, and the Science and Technology Center for Superconductivity, Argonne National Laboratory, Argonne, IL 60439, USA b Department of Chemistry, and the Science and Technology CenterJbr Superconductivity, Northwestern University, Evanston, IL 60208-3113, USA Received 10 June 1996
Abstract The microstructure of a Tl2Ba2CaCu2Oy (TI-2212) superconducting thin film grown by metal-organic chemical vapor deposition (MOCVD) on a pseudo-cubic (001) LaAIO 3 substrate has been examined by analytical transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Over large regions, the film is epitaxial and T1-2212 phase is found to be the major phase. The film/substrate interface is abrupt and smooth on the atomic scale but a strain-field is induced by the lattice mismatch between the film and the substrate. In addition to the intrinsic modulation structure of the T1-2212 phase, a very different modulation structure has also been found. The space group for the T1-2212 phase in this thin film was determined to be I4mm rather than I4/mmm as usually reported. The loss of the (001) mirror plane is attributed to a defective T1-2212 structure in which a considerable amount of T1 vacancies and TI disorder occurred inhomogeneously in T1-O layers. The reason for the Tl-deficiency is discussed. PACS: 61.16.Bg; 61.50.Em; 68.55.Jk Keywords: TI2Ba2CaCu2Oy superconducting thin film; Microstructure
I. Introduction Although the basic structures for the Tl-based T l 2 B a 2 C a , C u , + l O y (n = 0,1,2...) superconducting cuprates have been known for quite some time [1-3], detailed studies of the microstructure of these materials are still needed to evaluate various defect struc-
* Corresponding author. Fax: + 1 630 252-7777; e-mail: [email protected].~,ov.
tures. For example, homologous intergrown phases (different n values) [4,5]; structural modulations [2,6,7]; non-stoichiometry which is related not only to the oxygen content but also to the inter-layer substitutions [8-11]; mixtures of TI 3+ and TI 1÷ in the T I - O layers [9,12-14]; TI and O disorders [12,14,15] and other defects all contribute to the properties and behavior of these materials. Meanwhile, synthesis and characterization o f TIbased high transition temperature (Tc) superconducting thin films remains attractive due to their great
0921-4534/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S 0 9 2 1 - 4 5 3 4 ( 9 6 ) 0 0 6 9 5 - 8
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potential applications in various electronic and passive microwave devices. Unlike thin films based on other high Tc superconducting compounds, in-situ preparation of Tl-based thin films of high quality is quite difficult due to the high volatility of T120. A two-step ex-situ thallination process has thus been developed. Polycrystalline B a - C a - C u - O precursor films of an appropriate stoichiometric ratio are first deposited onto substrates. The precursor films are then subjected to an annealing process in the presence of TI20 vapor. Thin films consisting of T1-2212 phase or T1-2223 phase have been synthesized by this method [16-18]. According to previous TEM investigations, intergrowth of homologous phases [16], intrinsic modulation [19], granularity based on small misorientations of epitaxial films [18,20] and interfacial layers [21] are present in the Tl-based superconducting thin films. However, detailed TEM characterizations and especially HREM studies on an atomic scale are still valuable in establishing the relationship among the microstructure, the physical properties and the processing conditions of the thin films. This is particularly true for the Tl-based thin films because their microstructure may be affected by many factors as mentioned above. As is shown in this paper, the processing parameters substantially influence the microstructure of the T1-2212 phase formed in the present thin film, which was found to be Tl-deficient as a consequence of annealing under particular conditions.
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ducting transition temperature for zero resistance (To,0) of 105 K and a transport critical current density (Jc) at 77 K of 1.2 × 105 A / c m 2. More details of the processing conditions and physical properties are reported elsewhere [18]. Cross-sectional and plane-view TEM samples were prepared by conventional techniques involving grinding, dimpling, and finally ion milling at liquid nitrogen temperature to final perforation. TEM characterizations were carried out with both a Philips CM30 and a high-resolution Jeol 4000EX transmission electron microscope, operated at 200 and 400 kV, respectively. Energy-dispersive X-ray spectroscopy (EDS) was performed with the Philips CM30 transmission electron microscope equipped with an ultra-thin window.
3. Results Fig. 1 illustrates some typical features observed in the epitaxially oriented film studied. The thickness of the film is about 3500 ,~,. The interface between the substrate and the film appears to be abrupt and straight. Inhomogeneous image contrast can be seen which is usually associated with very small misorientations (one part of the crystal may be tilted a degree or so) but may also be partly due to secondary phases. The selected-area electron diffraction (SAED) patterns obtained from various areas of the film confirm T1-2212 (c = 29.2 ,g,) as the major phase in the film. The c-axis of the film is perpendicular to
2. Experimental Epitaxial Tl2Ba2CaCu2Oy superconducting thin films were grown in a two-step process involving MOCVD growth of B a - C a - C u - O ( F ) thin films on pseudo-cubic (001) LaAIO 3 substrates, followed by a post-annealing in the presence of TI20 vapor. The TI-2212 phase was formed during annealing. This thallination annealing was performed by sealing the precursor film together with a presintered pellet of 2212 stoichiometry in a gold foil pouch to minimize T1 loss. Annealing was carried out at 867°C for 1 hour in a pure oxygen atmosphere. Mixtures of T1-2212/1212 formed when the annealing temperature was beyond a range of (870 + 20)°C. The best films prepared by this method exhibit a supercon-
Fig. I. Morphologyof the TI-2212thin films(denotedas TBCCO) observed along a cross-sectional direction. Interface planes between the film and the LaAIO3 substrate are indicated by white arrows. Arrowheadsindicate grains of differentimage contrast.
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the substrate surface plane in most areas according to electron diffraction. However, in a few places a larger c-axis misorientation can be seen as shown in Fig. 2, in which grain boundaries in the T1-2212 film are easily recognized. A very different orientation was observed for the grain marked by an asterisk, where the [110] axis is close to the substrate surface normal. However, such highly misoriented grains were not common. There is no evidence for a correlation between the misoriented grains in the film and the microstructure (e.g. twinning) or surface morphology of the substrate. The misorientation between grains in the film can be readily seen from the lattice-fringe images shown in Fig. 3. Fig. 3a shows a small angle tilt grain boundary across which the c-lattice fringes in two grains are tilted by 3.5 ° with respect to each other about the incident beam direction (b-axis). Fig. 3b illustrates another small angle tilt grain boundary in the same TEM sample, in this case a 2 ° tilt occurs about the a-axis direction marked in the upper grain. Therefore, the image for the bottom grain is interrupted due to the 2° tilt away from the perfect [010] zone-axis. Fig. 3 indicates that the small angle c-axis misorientation can take place about both a- and b-axes, therefore these c-axis misorientations appear unrelated to the surface morphology, i.e. miscut, of the substrate. We also find that a higher annealing temperature of 890°C causes significant misorientations in the (a-b) plane and the formation of an interfacial phase.
Fig. 3. (a) Lattice-fringe image displaying a 3.5 ° misorientation between the c-axes of two grains. (b) HREM image showing two neighboring grains in the T1-2212 film. The [010] zone-axis of the upper grain is exactly parallel to the incident electron beam direction while the grain at the bottom is rotated by 2 ° about the a-axis. A bold arrow indicates the grain boundary line.
Fig. 2. Bright-field image showing the granularity in the TI-2212 film. c-axes in some misoriented grains are marked. A grain marked by an asterisk has a very different orientation: the [110] direction is parallel to the substrate normal.
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The general orientation relationships between the film and the substrate can be determined by SAED patterns. Fig. 4a shows such an SAED pattern taken along the [010] direction of the T1-2212 with both the film and the substrate included in the selected area. Reflections arising from the LaA103 substrate are marked by black dots. Indexing the substrate based on the pseudo-cubic perovskite structure of the LaA103, the orientation relationships between the substrate and the film can be described as (001)LaAlOs//(001)2212 and [100]LaAIO3//[10012212. In Fig. 4a, satellite reflections appearing around the main reflections associated with the T1-2212 indicate an intrinsic modulation structure in the film. The [010] projected wave vector of the intrinsic modulation is q p = 0 . 1 7 a * + c * giving rise to a 5.9a X c projected super-unit cell in real space, consistent with the intrinsic modulation structure reported for bulk crystals [2,6,7]. This intrinsic modulation also possesses an incommensurate component along the b'-direction, as will be shown later in the SAED patterns from a plane-view sample so that the wave vector should be expressed as q = 0.17a* + 0.17b* + c * . The intrinsic modulation in the present T1-2212 film can be directly imaged by HREM. A prominent feature in these images is the alternately stacked darker and brighter blocks which are reminiscent of the similar image features for the well-characterized
149
modulation in Bi2Sr2CaCu2Oy (Bi-2212) compounds [22,23]. It has been established that atomic disorder is the origin for the modulation of the Bi-2212 phase. Atoms, especially Bi, shift away from the standard crystallographic positions, leading to an uneven atomic distribution in the structure [24]. Those areas containing more cations scatter the incident electrons more strongly resulting in darker contrast, whereas areas containing less condensed cations display a brighter contrast [22,23]. The fact that a similar image contrast is observed for the intrinsic modulation in the T1-2212 phase implies a displacive rather than a compositional character of the modulation. Another type of modulation structure has been found in some areas of the film as shown in Fig. 4b and by the representative drawing in Fig. 4c. The difference in the position of the satellite reflections is obvious compared with that shown in Fig. 4a. The [010] projected modulation wave vector is q p = 0.104a* + 0.52c*. This type of modulation has not been reported previously for the T 1 - B a - C a - C u - O superconductors. The new modulation shown in Fig. 4b is not dominant in the film, therefore the intensity of the satellite reflections is weak, and the coexistence of two types of modulation reflections can be distinguished in SAED patterns obtaining from an area of 0.3 /xm in diameter. According to the SAED pattern in Fig. 4a, a lattice mismatch of 2.1% (T1-2212: a = 3.87 ~k;
Fig. 4. (a) SAED pattern taken along the [010] direction of the film. Some reflections from the substrate are marked by black dots. Small arrows indicate satellite reflections associating with the modulation structure of the T1-2212 phase. (b) [010] SAED pattern taken from a local area (0.2 p~m in diameter) in the T1-2212film. A new modulation is present, as also seen in (c), in which reflections arising from the basic structure of the T1-2212 phase are represented by full circles while open circles correspond to the satellite reflections having a different arrangement from those in (a).
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Fig. 5. Morphology of the film/substrate interfacial area. The periodic change of the image contrast due to the strain field along the interface is indicated.
LaAIO3: a = 3.79 ,&) along the a-axis of the film can be calculated between the substrate and the T1-2212 thin film. This interfacial lattice mismatch is accommodated by misfit dislocations. A periodic distribution of dislocation cores was formed along the interface plane as can be seen in the image in Fig. 5. Points along the interface at which the image contrast is changed due to the dislocation cores are indicated b~¢ arrowheads in Fig. 5. A periodicity of about 200 A (20 nm) is observed which is consistent with the expected value based on the lattice mismatch: L = [ a 2 / ( a I --a2)]a t = 183 ,&, with a I = 3.87 ,& and a 2 = 3.79 A the a-axis lattice parameter in the film and the substrate, respectively. Fig. 6 presents SAED patterns along the [001] and the [101] zone-axes of the T1-2212 film. The 4-fold symmetry is confirmed by Fig. 6a for the basic structure of the TI-2212 phase. The satellite reflec-
1 Fig. 6. SAED pattems taken along (a) the [001] and (b) the [101] directions of the TI-2212 phase. Satellite reflections indicated in (b) reveal the two-dimensional superlattice in the T1-2212 structure.
Fig. 7. (a) [001] CBED pattern of the TI-2212 film. 4mvm' v symmetry can be seen in the whole pattern and the bright-field disk (the central disk), m v and m'v denote the vertical mirror planes parallel to the (100), (010) and the (110), (1~0) planes, respectively.
tions associated with the intrinsic modulation can be observed only when the sample is tilted slightly (e.g. 3.5 °) away from the [001] zone-axis because the satellite reflections locate on the (00/) planes with l = odd integral numbers. Fig. 6b is such an SAED pattern, in which the two-dimensional modulation is visible. The four-fold symmetry is thus not disturbed by imposing the intrinsic modulation on the basic structure of the T1-2212 phase. This is verified by the [001] convergent-beam electron diffraction (CBED) pattern shown in Fig. 7. The 4mvm' v symmetry in the (a *-b* ) plane can be seen in the whole pattern (WP) and the bright-field (BF) disk (zero-beam disk). Here m v and m'v denote the vertical mirror planes parallel to the (100), (010) and the (110), (1~0) lattice planes, respectively. It should be noted that the places used for the CBED studies have been carefully selected. EDS was used to ensure no residual LaA103 was attached to the T1-2212 film. A single crystalline [001] SAED pattern and 4mvm' ~ symmetry of the [001] CBED pattern must be observed on the selected places so that the possible influence of local defects (e.g., c-axis misorientations) on the symmetry determination can be avoided. Although the [001] CBED pattern yields symmetry information in the (a-b) plane, information about the symmetry along the c-axis is still needed to
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Table 1 Summary of symmetries presented by the [001] CBED patterns in Figs. 7 and 8; BF, WP, DF denote the bright-field, whole pattern and dark-field, respectively; m v and m' v denote the mirror planes parallel to the (100), (010) and the (110), (1~0) lattice planes, respectively
Fig. 8. (a) (200) and (b) (110) CBED dark-field patterns of the TI-2212 film. Dark-field disks are circled by dashed lines and the center of the zero-beam disk in each pattern is marked by a cross. m_~ and m'v denote the mirror planes parallel to the (020) and (110) lattice planes, respectively.
determine the space group. Usually this information can be obtained by tilting the sample in the microscope. Based on the reflection conditions obtained in a series of SAED patterns from the T1-2212 film, we were able to deduce that the space group is either I 4 / m m m (no. 139), I4m2 (no. 119), or I4mm (no. 107). An important difference between I 4 / m m m and latter two space groups is the lack of the (001) mirror plane for the I4m2 and I4mm assuming the 4-fold axis is parallel to the [001] axis. In principle, a CBED pattern along any direction perpendicular to the c-axis may be used to examine the (001) mirror symmetry. However, the (001) reflections in reciprocal space of the T1-2212 structure are too closely spaced, leading to severely overlapped (00l) disks. The symmetry information contained in the BF and WP is thus distorted. Therefore, we utilized the [001] dark-field (DF) symmetry to determine the space group. Fig. 8a and 8b show the (200) and the (110) CBED DF patterns, respectively, obtained from the same spot on the sample from which Fig. 7 was obtained. A single mirror plane can be seen in each DF disk marked. This observation excludes the possibility of a 4 / m m m point group for which (200) and (110) DF patterns should present 2mm symmetry rather than m [25]. Combined with the WP and BF symmetries (see Table 1), we can conclude that the point group is 4mm [25] so that the space group for the T1-2212 phase structure studied is I4mm rather than I 4 / m m m as reported for the ideal T1-2212 structure [1-3]. The fact that the I4mm has been found in several randomly selected areas in the film
WP
BF
(200) DF
( l l 0 ) DF
4m v m'v
4m v m'v
mv
m'v
suggests that the symmetry of the T1-2212 phase in the film has been changed globally rather than only locally. Further details of the CBED analyses will be presented elsewhere. As mentioned above, an I4mm space group indicates the loss of the (001) mirror plane compared with I4/mmm. This loss of (001) mirror symmetry can be directly verified by [100] HREM images as shown in Fig. 9. The T1-2212 structure as well as the film/substrate orientation relationships determined by the SAED patterns can be seen. The film/substrate interface is smooth and reasonably abrupt. However, on this atomic scale a disordered interfacial layer with an abnormally bright image contrast
Fig. 9. HREM image showing a film/substrate interfacial area. Black dots correspond to projected columns of cations. Epitaxial growth of the T1-2212 film can be seen from the very first layer. A disordered interfacial layer with an abnormally bright contrast is exhibited in the right part of the figure. Arrowheads and small arrows indicate the T1-O layers showing different image contrast.
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can be seen in some interfacial areas such as the right part of Fig. 9. The thickness of this layer is about 4 - 5 A along the c-axis of the film but the origin and identity of the layer has not yet been established. It has also been observed that the presence of small steps (a few .~, in height) does not disrupt the epitaxial growth of the T1-2212 film. The layer stacking sequence of the T1-2212 structure can be derived from Fig. 9 using the known inter-layer spacing [1-3] and the image contrast of projected atomic columns. In computer simulations, columns associated with cations are projected as black dots in the images taken under our imaging conditions (Scherzer defocus - 4 0 nm and crystal thickness up to 4 nm in the Jeol 4000EX electron microscope). Black dots corresponding to TI columns generally have the larger size than those corresponding to other cations. The two T1-O layers in the ideal T1-2212 structure should appear identical because of their identical structural environments [1-3]. However, a considerable difference in image contrast is exhibited between the neighboring T1-O layers marked by arrowheads and arrows, respectively, and the spacing between each type of T1-O layer is c/2. As a result, the (001) mirror plane of the T1-2212 structure does not exist, verifying the conclusion derived from the CBED results. This change in symmetry is observed throughout the film, from the surface to the film/substrate interface. No new ordering in the T I - O layers has been distinguished by SAED patterns and HREM images along both plane-view and cross-sectional directions. It should be pointed out that the different image contrast between the neighboring T1-O layers can not be induced by varied imaging conditions (various defocus and sample thickness, etc.) due to the spatial proximity of neighboring TI-O layers. Regardless of the imaging conditions, the two neighboring TI-O layers should show identical contrast for the ideal TI-2212 phase structure [1-3]. Therefore, a substantial difference in structure between two neighboring T1-O layers must exist. A similar phenomenon can be observed in HREM images published by other groups although the authors did not address the issue [5,26]. EDS was performed to determine the composition of the T1-2212 phase in the film studied. The results suggest a considerable Tl-deficiency (as much as 30 at% deficient in Tl) and Ca-enrichment (as much as
20 at% enriched in Ca) for the TI-2212 film. Similar results have also been reported by other groups [3,8,10,17]. In spite of some Ca substitution for T1, T1 vacancies can still be expected based on the EDS analysis, leaving a defective structure for the T1-2212 phase formed in the film studied.
4. Discussion
The relative Tl-deficiency in the MOCVD-derived T1-2212 film detected by EDS is likely related to the high volatility of TI. This Tl-deficiency might be a result of the processing conditions used in synthesis of the film or it might arise as an artifact during TEM sample preparation (e.g., during ion milling) or TEM examination (e.g., beam heating). In order to exclude the possibility that the Tl-deficiency arises as an artifact, in situ heating experiments were performed. Bulk crystals of T1-2212 were heated in situ in the TEM (10 -6 torr vacuum) while the composition and crystal structure were monitored by EDS and SAED, respectively. It was found that the composition did not change until the sample temperature reached 400°C at which point a dramatic loss of T1 occurred. Consistent with this behavior, SAED patterns showed no discemible changes until the sample temperature exceeded 400°C. Thus, it appears that the thermallystimulated loss of T1 from the T1-2212 is not a factor at temperatures below about 400°C. It is highly unlikely that the sample reached temperatures approaching 400°C during either sample preparation or TEM imaging, so we speculate that the Tl-deficiency observed is a result of the synthesis conditions. In addition to the annealing temperature which was controlled to form the phase-pure T1-2212 phase rather than mixed TI-2212/T1212 in these experiments [18], the TI20 vapor pressure is also considered a key factor which influences phase formation and T1 content. Generally, a higher T120 vapor pressure stabilizes the double-Tl-layer compounds while lower T120 pressures leads to formation of single-Tl-layer phases. Decreasing the T1 vapor pressure leads to a change from T1-2212 to either T1-1212 or Tl-free phases, depending on the oxygen partial pressure [27]. However, the T1-2212 phase is stable over a range of TI partial pressures spanning nearly
X.F. Zhang et al./Physica C 275 (1997) 146-154
an order of magnitude [27] although the T1-2212 phase with less than 100% occupancy of the TI sites is likely when lower TIzO vapor pressure within the range is applied. In case of the existence of a remarkable Tl-deficiency in the structure, the phase is actually an intermediate state between the T1-2212 and the T1-1212 phase with fully occupied T1-O layers. Our T1-2212 films were synthesized by annealing B a - C a - C u - O ( F ) precursor films in TI20 vapor. The TI20 vapor is provided from a bulk T1-2212 pellet while the pressure is maintained by sealing the film and pellet together in a gold foil pouch. Presumably, the TI20 vapor pressure provided by the bulk pellet cannot be maintained infinitely in this type of system so that TI loss from the film could occur for long annealing times. Chrzanowski et al. have shown that the optimum thallination time in such a semiopen system is related to the mass of the bulk pellet contained in the system [17]. For the 1 g pellet used in the present experiments, the 60 minute thallination time may result in some loss of TI from the film, resulting in less than 100% occupancy of the TI sites (Tl-deficiency) as suggested by the EDS results [17]. Tl-deficiency is thus very likely to occur in our T1-2212 film produced in the similar way. A shorter thallination time may improve the Tl-occupancy but could also have an influence on the oxidation state of the TI in the T1-2212 phase, resulting in a mixture of T11+ and TI 3+ as has been observed in some bulk crystals [9,12,14]. Either the Tl-deficiency measured for this sample or the possible coexistence of TI3+/TI j÷ could lead to the change in space group of the T1-2212 film studied here as long as these defects are inhomogeneously distributed in T I - O layers. Our computer simulations show that a non-uniform distribution of vacant TI sites a n d / o r TI3+/TI l+ could lead to the unusual image contrast noted for Fig. 9. For example, a difference in contrast between the adjacent T I - O layers similar to that observed in Fig. 9 becomes distinct when a T1-O layer has about 20% of the cation sites unoccupied while the adjacent T I - O layer is fully occupied. Details of these simulations will be discussed elsewhere. Since the T! loss is inevitable during processing procedures because of the high volatility of TI, a quite common tendency of Tl-deficiency can be ex-
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pected in the TI-2212 phase. The ideal T1-2212 structure with the space group of I 4 / m m m can be maintained when this Tl-deficiency is not remarkable as in cases of [1-3]. However, considerable amount of TI vacancies could occur under some annealing conditions. As a consequence, the structure should be influenced and is able to be imaged by HREM as shown in Fig. 9. In fact, in addition to Fig. 9 from the T1-2212 thin film studied, image contrast effect which implies the modified T1-2212 structure can also be recognized in HREM images published previously by other groups [5,26]. These observations suggest the possibility that this modified T1-2212 structure is not uncommon. In our samples, although some of the vacant TI sites may be occupied by some Ca, the remaining unoccupied sites (up to 30%) could lead to an unstable structure and the formation of single-Tl-layer (T1-1212) intergrowth. However, carbon which has recently been shown to promote the formation of double-Tl-layer phases [28] may be incorporated around TI vacancies. Thus, it is possible that carbon stabilizes the double-layer phases so that, rather than forming single-Tl-layer intergrowth, a Tl-deficient double-layer phase remains stable. 5. Concluding r e m a r k s Structural characterization of the epitaxial TI-2212 thin film grown by MOCVD on (001) LaAlO 3 substrate has been performed by TEM. The lattice mismatch between the film and the substrate, granularity based on small c-axis misorientation, and two kinds of modulation structures have been observed. The space group of the T1-2212 phase in the film was determined to be I4mm which is different from the 1 4 / m m m reported. The lower symmetry is mainly due to the loss of the (00t) mirror plane which can be directly verified by HREM images. Tl-deficiency a n d / o r Tl-disorder in such defective TI-2212 phase are thought to be responsible for the symmetry change. Therefore the resulted phase was regarded as the intermediate state between the T1-2212 and the Tl-1212 phase with fully occupied T I - O layers. The considerable Tl-deficiency in the sample was attributed to the high volatility of T1. Carbon ions are likely to play a role in stabilizing the double T l - O layered T1-2212 structure.
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