structure of superconducting thin films of … · department of materials science and engineering...

JOURNAL OF ELECTRON MICROSCOPY TECHNIQUE 8:263-272 (1988)

Structure of Superconducting Thin Films of YBa2C~307-x Grown on SrTi03 and Cubic Zirconia

L.A. TIETZ, B.C. DE COOMAN, C.B. CARTER, D.K. LATHROP, S.E. RUSSEK, AND R.A. BUHRMAN Department of Materials Science and Engineering (L.A. T, B. C.D. C., C.B.C.) and School ofApplied and Engineering Physics (D.K.L.. S.E.R.. R.A.B.). Cornell Uniuersity, Ithacq New York 14853

KEY WORDS High T, superconductors, Electron microscopy, HREM image stimulations

ABSTRACT Thin films of the superconductive oxide YBa2Cu307., have been made by electron-beam coevaporation of the metals in an oxygen atmosphere onto single-crystal { 001)-oriented SrTiO3 and yttria-stabilized zirconia (YSZ) substrates. The oxide films were superconducting in the as-deposited state (T, = 81-83K, J, = lo6 A/cm2 at 4.2K). Bright-field imaging, selected- area diffraction (SAD), and high-resolution imaging in the transmission electron microscope were used to characterize the microstructure of these films. All of the films were polycrystalline. On SrTi03 the films were oriented, for the most part, with (110) parallel to the substrate surface. On YSZ, two microstructures were observed: one with smaller rectangular grains oriented with (100) or (010) parallel to the substrate surface and the other with (001) parallel to the surface (i.e., c-axis up).

INTRODUCTION

The discovery of the superconducting oxide YBa2C~307.~ with a critical temperature above 90°K (Chu et al., 1987; Wu et al., 1987) has made the use of superconducting materials at liquid-nitrogen temperatures possible. Although the compound is easily formed by a solid-state reaction of the component oxides, many of the applications envisioned will require other fabrication techniques to obtain the required product specifications. Some of these applications-for example, in the area of microelectronics-will require thin films of the superconducting oxide. Sev- eral groups have reported success in growing such films on ceramic substrates by a variety of techniques including electron-beam evaporation (Laibowitz et al., 1987; Naito et al., 1987) and dc magnetron sputtering (Jin et al., 1987; Moriwaki et al., 1987; Somekh et al., 1987). YBa2C~307-x thin films formed by deposition and subsequent annealing in an oxygen atmosphere have had critical temperatures in excess of 80°K and critical current densities better than lo5 A/cm2 at 77°K (Chaudhari et al., 1987a,b).

The structure of the superconducting phase has been shown to be an orthorhombic lay- ered perovskite structure with the c-axis ap-

proximately three times the length of the a- or b-axis (Beyers et al., 1987; Cava et al., 1987). Recently, neutron scattering has been used to refine this structure so that the cor- rect designation for the compound is YBa2Cq07., (space g o u p = Pmmm, a = 3.8231 A , b = 3.8864 A , c = 11.6807 A) (see Fig. 1). The copper and oxygen ions are arranged as Cu-0 chains along the b-direction between the Ba-0 layers and as rumpled CuO2 planes between the Ba-0 and Y layers (Beno et al., 1987).

Studies of single crystals (Dinger et al., 1987) have shown that the properties of these materials are quite anisotropic. For example, critical densities in the (001) plane are ten times greater than that measured perpendicular to this plane. Because of this anisotropy, high-quality thin films for device applications will require growth of correctly oriented YBa2C~307.~. The choice of substrate is expected to exert a strong influence on the

Received August 28, 1987; accepted November 17, 1987. Address reprint requests to Prof. C. Barry Carter, Department

of Materials Science and Engineering, Bard Hall, Cornell Uni- versity, Ithaca, NY 14853.

0 1988 ALAN R. LISS, INC.

264 L.A. TIETZ ET AL.

TABLE 1. Misfit calculations and linear coefficients of thermal expansion for YBaZCu307-x on various

substrates.'

A d a , along [loo] [OlO] %[001] CX(OK-')~

Substrate

SrTi0s3 0.021 0.0048 0.0030 0 . 8 2 ~ 1 0 - ~ YSZ4 0.294 0.278 0.276 1.04 x

'All values calculated for 25°C. 'For YBa2CU307-o a1=a2 = 1 . 1 ~ 1 0 - ~ , a3 = 2 . 5 ~ 1 0 - ~ (Yukino et al., 1987). 3JCPDS powder diffraction file; Krishnan, Srinivasan, and Devanaravanan. 1979. 4JCPDS p"owder' diffraction file; Adams, Nakamura, Ingel, and Rice, 1985.

quality of the films through epitaxy and in- teraction or reaction with the film. The two substrates chosen for this study have very different lattice mismatches with the superconducting phase. SrTiO3(ST) has a perovskite-type structure which closely matches the YBazCu307., phase in lattice parameter in thermal expansion. Misfit calculations listed in Table 1 suggest that the (100) plane of the superconducting phase is the favorable interface with a (001)ST surface; they also suggest that the c-axis can be oriented equally well with either [100]s~ or [olO]sT in this plane. On the other hand, yttria-stabilized cubic zirconia (YSZ) has a fluorite-type structure and a large lattice parameter mismatch with respect to the superconducting phase. It is less clear for this substrate what orientation would be expected for the film.

In this study, the microstructure YBa2- C U ~ O ~ . ~ films has been characterized by using bright-field imaging, selected-area diffraction (SAD), and high-resolution electron microscopy (HREM) to determine the influence of the substrate on their epitactic growth. Computer stimulation of the high- resolution images has been used to confirm that HREM is able to verify this structure by locating the oxygen structural vacancies and lattice distortions and can thus supplement the information obtained by SAD.

MATERIALS AND METHODS

Thin films of the superconducting Y-Ba- Cu-0 phase 1 pm thick were deposited on single-crystal substrates by electron beam coevaporation of the metals, Y, Ba, and Cu from three separate sources. During deposition, the oxygen partial pressure in the chamber was approximately 1 mtorr and the substrates were held at 700°C. Mechanically

polished, (001)-oriented, single-crystal substrates of both SrTiO3 and YSZ were used in this study. Following deposition, the chamber was backfilled with 100 mtorrs oxygen while the film cooled. The as-deposited oxide films were black, tough, and exhibited excel- lent adhesion to the substrates. The films were superconducting in the as-deposited state with T, = 8143°K and a transition width ATc = 4°K. Films on both substrates had critical current densities of lo6 Mcm2 at 4.2"K (Lathrop et al., 1987).

Plan-view sections of the films were prepared for examination by transmission electron microscopy. Discs, 3 mm in diameter, were cut with an ultrasonic drill. The discs were then ground, polished, and dimpled from the substrate side to less than 20 pm. All of the cutting and polishing steps were performed in nonaqueous media (e.g. ethyl- ene glycol). In order to preserve the information on the crystallographic relationship between the film and the substrate, it was necessary to thin the specimens by ion-milling. The specimens were ion-milled to perfo- ration from the substrate side only with 5- kV Ar+ ions. The specimen was not cooled during ion-milling; this procedure may cause an increase in lattice damage over that pro- duced by milling while cooling the sample, but the different crystallographic orienta- tions could still be easily identified by each technique used. The foils did not require car- bon coating for examination in the electron microscope. Bright-field imaging and SAD studies were performed on a JEOL 1200EX 120-kV or a Siemens Elmiskop 102 125-kV transmission electron microscope. High-resolution lattice-imaging was performed on a JEOk 4000EX a t 400 kV with better than 1.8- A point-to-point resolution.

Computer simulation of high-resolution images

An extensive analysis of simulated images of the superconducting phase and related structures has been carried out to assess the suitability of HREM for direct study of the structure described by Beno et al. (1987). The simulated images were all obtained by using the SHRLI program (M.A. O'Keefe, private communication) and show that HREM can provide two important pieces of information in this study. First, by using either a 125-kV or a 400-kV machine, the orientation of the c-axis can be identified in grains which are

THIN FILMS OF YBazCu@-, GROWN ON SrTiO3 AND YSZ 265

O Y 0 Ba 0 cu . O

Structural vacancy

" Y B a Z C u 3 0 7 "

B Fig. 1. A "Perfect" YBazCu309 structure. B: YBaz

Cu307 structure described by Beno et al. (1987). Lattice parameters: a = 3.823 if, b = 3.8886 if, c = 11.681 A . Not drawn to scale.

much too small for SAD analysis. Second, a t the higher voltage, the simulations show that both the presence of oxygen structural vacancies at [%,O,O] and [O,O,%] sites and the distortion ("rumpling") of the Cu02 planes should be detectable by the observation of image symmetry with the electron beam di- rected parallel to either [loo] or [OlO]. In the present paper, only a limited summary of the results of the computer simulations will be presented to illustrate these uses and to show how high-resolution imaging can supplement the SAD information regarding the epitactic alignment of the epilayer and the substrate. Projected-potential diagrams and simulated images have been calculated for both the "perfect" YBazCu309 and YBazCu307 structures and are shown in Fig- ure 1A and B, respectively. YBa2Cu309 contains no oxygen structural vacancies; all of the possible oxygen sites are filled. Although the lattice is orthorhombic, it can be considered to be essentially cubic since the lattice parameters a-b-cI3 and all of the cubic subcells are identical except for the central atom, which can be either Ba or Y. The [loo] projected-potential diagram for this structure is shown in Figure 2A. The YBazCu307 structure, however, contains oxygen structural vacancies arranged in rows along the [loo] and [OlO] directions and distortions in the Cu02 planes, so the structure cannot be considered to be nearly cubic. Viewed along [ 1001, only those vacancies in the Y-plane are visible, while vacancies in both the Cu-0- and Y-planes are visible by viewing the structure along [OlO]. This is shown by the projected potentials in Figures 3A and 4A respectively.

A series of three simulated images, all calculated at Scherzer defocus for the 400-kV machine, is shown for each of the three cases-namely, [100]-YBa2Cu309, [loo]- YBazCu307, and [010]-YBa2Cu307 (see cap- tions to Figs. 2B-D, 3B-D, and 4B-D for more details). In each of the series of image simulations, the image contrast varies dra- matically with crystal thickness, but the overall symmetry of the image remains the same. The thin crystal images (Figs. 2,3,4B) show the least amount of structural detail; the detail increases with crystal thickne%s and contrast reversal occurs between 30 A and 66 A. All three simulated images of the "perfect" structure exhibit a pseudofourfold symmetry as would be expected from the essentially cubic symmetry of the YBa2Cu309 structure. In the YBa2Cu307 structure, the pseudofourfold symmetry is reduced to two- fold symmetry by the presence of the structural vacancies and lattice distortions. This change is reflected in the simulated images in Figures 3 and 4. Image simulation has also been performed for the YBazCu309 and YBa2Cu307 structures for a [110] beam direction; examples are shown, for the same set of imaging conditions, in Figures 5 and 6, respectively. Notice that none of the rows of oxygen structural vacancies is visible in YBa2Cu307 when viewed along [110], although the distortions in the Cu02 plane are visible, as can be seen in the projected-potential diagram (Fig. 6A). A comparison of these two cases shows that the contrast in the images is very different particularly in the thicker crystals; however, the overall symmetry of the two serigs is the same and the presence of the 11.7-A periodicity can be di- rectly recognized. The latter result. also ap- plies to images obtained by using the 125-kV machine. In order to distinguish between the YBa2Cu309 and YBa2Cu307 structures, then, the crystal thickness and defocus values for the image must be known so that the position of the structural features can be lo- cated. However, if a [loo] or [OlO] beam direction is used, the presence of the structural vacancies and lattice distortions can be de- termined simply from changes in the symmetry of the 400-kV images; it is not necessary to know the thickness of the crystal. HREM can therefore be used to identify the composition of a particular grain by using the more precise structural information available from x-ray studies. When the grains are large, SAD is adequate for identifying

THIN FILMS OF YBazCu@-, GROWN ON SrTiO3 AND YSZ 267

the orientation of the c-axis; for small grains, HREM provides this information.

OBSERVATIONS SrTiO3

Figure 7 shows a lattice image of the as- deposited superconducting Y B ~ ~ C U ~ O ~ - ~ film. The planes have an 11.5-A spacing, which corresponds well with that measured by Beno et al. (1987) for the (001) planes in YBazCu307,. Thus, in this grain, the c-axis is parallel to the (001) substrate surface. The film is polycrystalline with small areas of different orientation lying within the larger single-crystal regions. The apparent porosity of this film is believed to be due to surface roughness which would lead to the formation of holes when the specimen was ion-milled from the substrate side only.

The major portion of the film was oriented with respect to the substrate as shown by the SAD pattern in Figure 8a which was re- corded from an area similar to that shown in Figure 7. It shows that the YBa2Cu307., film has grown with (110) parallel to ( 0 0 1 ) s ~ ~ although it was not possible to determine from the pattern whether the plane is (€10) or (110). This pattern also confirms that the grain size was -1 pm, the diameter of the area included in the SAD aperture. These observations differ from those of Naito et al. (1987), who reported seeing randomly oriented grains, grains with a-axes perpendicular to the substrate, and a mixture of grains with a- and c-axes perpendicular to the surface in their films depending on the film’s composition and the substrate temperature during deposition.

Figure 8b is a SAD pattern taken on an area such as the small area arrowed in Fig- ure 7. It shows that these small areas are oriented with their c-axes approximately per-

Fig. 2. “Perfect” YBazCu309 structure, [loo] beam direction. A Projected potential (unit cell is outlined): heavy circle = Ba, light circle = Y, square = CdO, closed circle = 0. Image simulations (Scherzer defocus) at (B) 7.65-A, (C) 30.58-A, (D) 61.17-A thickness.

Fig. 3. YBazCu307 structure [loo] direction: A Pro- jected potential (unit cell is outlined): heavy circle = Ba, light circle = Y, square = C d O or CdO-vacancy, closed circle = 0, star = missing row of 0. Image simulations (Scherzer defocus) at (B) 7.65-A, (C) 30.58-A, (D) 61.17- A thickness.

YBazCu307 structure [OlO] beam direction. A: Projected potential (unit cell is outlined): heavy circle = Ba, light circle = Y, square = CdO, closed circle = 0, star = missing row of 0. Image simulations (Scherzer defocus) a t (B) 7.77-A, (C) 31.09-A, (D) 62.18-A thickness.

Fig, 4.

Fig. 5. “Perfect” YBazCu309 structure, [ l l O ] beam direction. A Projected potential (unit cell is outlined): heavy circle = BdO, light circle = Y/O, square = Cu, closed circle = 0. Image simulations (Scherzer defocus) at: (B) lO.9O-A, (C) 32.71-A, (D) 65.42-A thickness.

Fig. 6. YBazCu307 structure, [110] beam direction. A: Projected potential (unit cell is outlined): heavy circle = BdO, light circle = Y/O-vacancy, square = Cu, closed circle = O/O-vacancy. Image simulations (Scherzer defocus) at: (B) lO.9O-A, (C) 32.71-A, 0) 65.42-A thickness.

pendicular to that of the larger surrounding area, but still parallel to the substrate surface. This 90” microstructure is to be expected for { 100)-oriented cubic substrates because of the fourfold rotational symmetry about < 100 > as discussed previously.

High-resolution imaging was also performed on these films at 400 kV. The YBazCu307, film appeared to be heavily faulted parallel to the (001) planes; these defects may have been caused by damage during observation in the transmission electron microscope or by the ion-milling rather than be intrinsic to the structure.

Yttria-stabilized zirconia The microstructure of the thin film depos-

ited on YSZ was different from those grown on SrTiO3. Figure 9 is a general view of one of the as-deposited films. Two distinct types of microstructure are visible: (1) mosaic structures (Ml, M2) and (2) nonmosaic structures (N). The mosaic regions consist of rectangular grains 200-300 nm in length. Individual grains within these regions are oriented with either (100) or (010) parallel to


Fig. 7. Plan-view bright-field images of a YB?&u307., film deposited on SrTiO,. A small region of different orientation arrow and - 11.5-A spacing of the (001) planes.

Fig. 8. SAD patterns of YBazCu307., film on SrTiO3. A Main orientation of the film: [%lo] or [110] pole. B: Two grains with c-axes rotated - 90" to each other.

THIN FILMS OF YBazCu307-, GROWN ON SrTiO3 AND YSZ 269

the ( 0 0 1 ) ~ ~ ~ substrate surface so that the c- axis is parallel to the interface as shown by the SAD pattern in Figure 10a and b taken from a similar area of the same specimen. Within a mosaic region (e.g., Ml), adjoining grains are oriented with either their c-axes parallel or rotated by 90°C to each other. Adjacent mosaic structures that are rotated 45" with respect to each other (e.g., M1 and M2 in Fig. 9) are often found in these films.

Large areas of the films exhibit the nonmosaic-type structure such as region N indi- cated in Figure 9. These areas consist of larger, irregularly shaped grains up to 1 pm in diameter. The grains have their c-axes perpendicular to the substrate surface as shown in the SAD pattern in Figure 1Oc and d taken from a similar region of the same specimen.

DISCUSSION Specimen preparation and its

relationship to defects The planar stacking defects observed by

high-resolution microscopy in this study may

be the result of damage to the YBazC~307.~ structure from ion-milling or from the electron beam in the microscope during observation. Reports of planar defects in specimens prepared by ion-thinning have been made by Bentley et al. (19871, who reported that such defects could by avoided by ion-thinning with the benefit of LN2 cooling of the specimen. Furthermore, Kelly et al. (1987) reported that Lal.8Sro,2Cu04 foils experienced oxygen loss when heated in vacuum (e.g., by ion or electron beams) and Ar implantation during ion- milling. Clark et al. (1987) have observed that YBazCu307., phase become amorphous during irradiation with 300-keV electrons in the microscope, although in the present study, a stable image could be obtained by using a 400-kV microscope.

Damage by ion and electron irradiation which leads to planar defects has been observed in other related materials. For example, in p"'-alumina, it has been shown (h4orrissey et al., 1984) that cations (e.g., Na) can be removed from an area of interest during electron-beam irradiation. This displace-

Fig. 9. Plan-view bright-field image of YBaZCu307., film deposited on YSZ. General view shows mosaic structure (MI, Mz) and nonmosaic structure (N).


ment can lead to the collapse of the conduction planes which appear as planar stacking defects in the high-resolution image. Ar+ ions can also replace alkali-metal cations during ion-thinning, as shown by EDS. These effects are ascribed to the very open structure of the P"' phase. YBa2Cu307., also has an open structure due to the oxygen structural vacancies. Thus, YBa2Cu307., may be a candidate for similar damage.

In thin film studies such as that presented in this paper, it is necessary to use ion-thinning in order to preserve the information on the fildsubstrate interface. Therefore, care should be used in ascribing the defects observed here to the deposition process, as they

may simply be artifacts of the specimen preparation technique and observation in the electron microscope.

Comparison of observations on thin film microstructure with those reported

in the literature Based on the lattice mismatch calculation

listed in Table 1, certain orientation relation- ships were expected for epitactic film growth. In the case of SrTi03 substrate, the c-axis was found to lie parallel to the (001)srr SU- face, as expected from the very small mismatch for this direction. Also, 90" rotation of the c-axes in different grains was observed as expected from the substrate symmetry.

Fig. 10. SAD patterns of YBazCu307., film on YSZ. a: SAD on grain mosaic region showing orientation with substrate. b: Schematic of a showing pole of grain is [loo] or [OlO]. Stars indicate spots from c-axis of an adjoining grain. c: SAD on grain in nonmosaic region. d: Schematic of c showing pole of grain is [OOl].

THIN FILMS OF YBazCu307-, GROWN ON SrTiO3 AND YSZ 271

However, it is unclear why the films adopted a < 100> growth direction. With the plan- view specimens it was not possible to determine which directions were aligned in the filmhubstrate interface. Naito et al. (1987) have suggested that the occurrence of orien- tations not favored by lattice mismatch may be stabilized by the presence of a second phase.

In other studies of YBa2C~307.~ films formed by electron-beam coevaporation (Chaudhari et al., 1987a; Naito et al., 19871, the authors report that the as-deposited films are not superconducting and exhibit highly disordered or amorphouos structures and that subsequent annealing is required to obtain the superconducting phase. In this study, all of the films discussed showed highly crystal- lized microstructures and were superconducting at 80°K in the as-deposited state.

Finally, it should be noted that the results presented here for plan-view specimens give no information on the distribution of the microstructure in the direction parallel to the electron beam. Naito et al. (1987) have reported that the YBa2C~307.~ grains in their thin films exhibit a columnar microstructure, while Chaudhari et al. (198710) have described their films as having a “bilayer” microstructure. In the latter study, single- crystal growth occurs near the substratelepi- layer interface with the c-axis perpendicular to the interface, and a polycrystalline microstructure of grains with c-axes in the plane of the film lies on top of this such that the c- axes align with the a- or b-axis of the under- lying YBa2C~307.~ film. However, in that case, the reported structure was only obtained after postdeposition annealing in oxygen. The as-deposited YBa2Cu307., films grown on YSZ and described in this paper could also have this “bilayer” microstructure. It has been observed that the grains in the mosaic regions are often aligned with their c-axes parallel to the a- or b-axis of the grains in the adjacent nonmosaic region. Cross-section TEM samples are being prepared in order to clarify this point.

ACKNOWLEDGMENTS

The authors would like to thank Mr. R. Coles for maintenance of the TEM facility and Ms. M. Fabrizio for photographic work. The Materials Science Facility for Electron Microscopy is supported, in part, by NSF (DMR-85-16616). This research was supported by NSF under grant No. DMR-85- 21834 and through use of the National Nan- ofabrication Facility a t Cornell University

(ECS-82-00312). Additional support was pro- vided by the Office of Naval Research under grant No. N00014-K-0296.

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