Microstructure of superconducting laser ablated thin films of Y1Ba2Cu3O7−δ on single crystal MgO substrate

Mat. Res. Bull., Vol. 26, pp. 171-177, 1991. Printed in the U S A . 0025-5408/91 $3.00 + .00 Copyright (c) 1991 Pergamon Press ple.

MICROSTRUCTURE OF SUPERCONDUCTING LASER ABLATED THIN FILMS OF Y1Ba2Cu307.a ON SINGLE CRYSTAL MgO SUBSTRATE C. Vignolle and A. Gervais Laboratoire de Minrralogie-Cristallographie, associ6 au CNRS, Universitrs Paris VI et Paris VII, 4 Place Jussieu, 75252 Paris Cedex 05, FRANCE D. Chambonnet and C. Belouet Laboratoires de Marcoussis, route de Nozay, 91460 Marcoussis, FRANCE (Recived September 3, 1990; Refereed) ABSTRACT: Superconducting thin films of Y 1 B a 2 C u 3 0 7 - a grown on (001) MgO substrate by the laser ablation technique have been studied by transmission electron microscopy (TEM). The films studied were superconducting with a critical temperature Tc(R=0) at 82K and a total transition width of 5K. The films were oriented mainly with the c-axis perpendicular to the substrate surface (c±) and presented a dilatation of the c parameter. The c± orientation allows both, the minimisation of the thin film surface energie and the fit of the two oxygen sublattices at the substrate/film interface. Materials Index: Yttrium, Barium, Copper, Oxides. INTRODUCTION There has been considerable effort during the past three years to d e v e l o p high Tc s u p e r c o n d u c t i n g thin films. The formation of superconducting thin films with critical temperatures exceeding 77K is expected to result in advances in electronic and microwave devices. The performances of these materials are dependent, among other things, on the control of the microstructure. This is a complex problem involving growth conditions, film composition and thickness, the substrate nature and the oxygen stoichiometry. All these factors play an important role on the key caracteristics as the critical temperature Tc, the critical current 171

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density Jc, and the surface resistance at microwave frequencies Rs.

In this paper, we present a study by Transmission Electron Microscopy (TEM), of the microstucture of Y 1 B a 2 C u 3 0 7 - s t h i n films prepared by laser deposition technique on (001) MgO substrate. EXPERIMENTAL PROCEDURES Thin films about 180nm thick were prepared from stoichiometric Y B a 2 C u 3 0 7 - s t a r g e t s by a laser ablation technique 1. The laser was a SOPRA pulsed Excimer laser (k=308nm, x=30ns) operated at a 1-2Hz frequency. The energy density at the target surface was 2 to 3 J/cm 2. The temperature of the substrates, positionned at about five centimeters above the target in the vertical direction, was estimated to be 670°C and deposition was made under a pressure of 10 Pa (Ar 50% and 02 50%). The deposition rate was typically 0.03 nm/s. After deposition, films were cooled down to room temperature in a 104 P a oxygen pressure. A slow cooling protocole including three steps, namely: cool down to 475°C at l°C/mn, plateau at 475°C for four hours and cool down to room temperature for four hours, was used in these experiments. Films r e a d y from growth were s u p e r c o n d u c t i n g with a critical temperature Tc(R=0) at 82K and a total transition width AT of 5K. For TEM observations, plan-view and cross-section specimens were prepared. The plan-view samples of 2x2 mm2 square were cut and the substrate side was removed first by mechanical thinning down to a thickness of 50 ~tm, then by ion milling until perforation. For the preparation of cross-section specimens , the sample was cut into 2x4 mm 2 lamellas. These lamellas were sticked together, with epoxy. The block obtained was cut into 500 ~tm plates. They were mechanically thinned and ion-milled, as described above. TEM studies were carried out with a JEOL 2000EX operating at 200 kV. Two complementary techniques were used: Rutherford Backscattering Spectrometry (RBS) to determine the profile distribution of elements through the thickness with a depth resolution of about 10nm until a l~tm t h i c k n e s s 2 and X-ray diffraction measurements, obtained with a Phillips 0 - 2 0 diffractometer with CuKa radiation, to characterize the grain orientations and the cell parameters of YBaCuO.

RESULTS Plan-view observations The film can be described by two components: 1) An epitaxial layer with likely-square grains having their c-axis perpendicular to the (001) MgO substrate (noted c±) and 2) inclusions of

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grains, more or less misoriented with their c-axis parallel to the substrate surface (noted c//).

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FIG 1 (a) Plan-view of Y1Ba2Cu3OT-8 superconducting thin film with c//and c± grains. (b) Selected area diffraction pattern with a schematic representation.

Figure la illustrates the distribution of c// grains in the c± layer. The c// grains present nearly square shape, their size varies from 100 to 150nm and their proportion ranges between 20 and 50 per cent of the projected surface. The selected area diffraction in Figure l b indicates the epitaxialgrowth nature of the c± film on the substrate. The orientation relationships are given below, with the index s for the substrate. (001) // (001)s and [100] // [100]s or [010] // [100]s (1) For the c// grains, the corresponding relations are: (100) // (001)s and [001] // [100Is or [001]// [010]s (2)

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The layer structure seems to be homogeneous and the surface to be smooth in comparison with other observations of thin films grown on (001) MgO by the laser ablation technique. CuO particles, crystalline Y2BaCuO5 or amorphous.phases were not observed. The square shape of c// grains observed here, is in contrast with numerous reports showing the anisotropic growth of YBaCuO crystals, the c-axis being the direction of slowest growth. The isotropic shape may be related to the particular growth of these grains. Their images show fringes suggesting that they do not nucleate directly on the substrate but closer to the free surface, on c± grains. We have observed two types of contrast -regularly spaced fringes (figure 2a) indicating that the orientation relation follows the mode (2). -Moir6 "wavy like" fringes (figure 2b). These grains are rather misoriented with respect to the relation (2)

FIG 2 Two types of contrast in c// grains (a) regularly spaced fringes

(b) Moir6 "wavy-like"

Cross-sectional observations Cross-sectional micrographs corroborate the plan-views. Figure 3a, emphasizes the presence of a well oriented ca_ layer (or tabulated grains) and c// grains closer to the surface. The film/substrate interface is abrupt. The YBaCuO epitaxy is evident on the diffraction pattern in the selected area mode.(Figure 3b) Defects A very low density of twins i s another characteristic of these films. Twins appear on the (110) YBCO plane in (001)YBCO projection. The twins density decreases in the course of TEM work until it vanishes. This phenomenon has already been observed 3. Diffraction contrast on small domains, frequently limited by short defect lines appear on the ca_ f i l m s ;

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(a) Cross-sectional TEM micrography (b) The corresponding selected area diffraction pattern and a schematic representation.

however, sub-grain boundaries are not visible. In this respect the in-situ grown films depart from high temperatu+re annealed or bulk specimens where well organized sub-boundaries are observed. Microcracks, imaged by contrasted and sinuous limits are present. RBS spectrum reveals a nearly 123 cationic composition on a multilayer sequence but this composition is not homogeneous. Consistent with the cross-section observations, RBS analysis shows no interdiffusion between the substrate and the film. X-Ray Diffraction indicates that the films are textured with two orientations (100) and (001) in respective proportions of 30 and 70 per cent, supporting the TEM observations. It

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also shows that c parameter is equal to 1.174nm. DISCUSSION. It is well known that the lattice parameters of YBaCuO depend strongly and anisotropically on the temperature, as well as the oxygen content incorporated in the 1-2-3 phase. Our experimental c parameter is larger than the values corresponding to materials with a critical temperature Tc(R=0)= 82K. In bulk materials this result is usually related to an oxygen deficiency 4,5. In thin films it can be due also to the presence of defects introduced during in-situ and low-temperature growth6,7, 8. Using the relation c= 12.771-0.1557x determined 7 by a least-squares fitting of bulk values we find that the corresponding value associated to c=l,174nm is x= 6.65. This value is in the vicinity of the phase transition orthorhombic/tetragonal stoichiometry 4 for bulk materials. Considering the growth conditions ( ambient pressure= 10 -1 Torr; substrate temperature= 670°C; atmosphere during the c o o l i n g stage100 Torr) the orthorhombic/tetragonal transition temperature, in bulk material should be close to 610°C. If the cooling rate is slow enough to allow the oxygen indiffusion until 475°C, the oxygen stoichiometry, should reach x=6.9. This oxygen replenishment (x= 6.9) may not be achieved. Looking now at information published on film layers, our experimental values of c and Tc fit on a curve c vs Tc drawn for in-situ grown films 8. The To= 82K value shows that the film is in the orthorhombic phase, and the deposition conditions were probably close to the transition temperature. The larger c value may be interpreted as the presence of structural defects not affecting drastically the superconductivity state(ie oxygen vacancies.). Concerning the orientation, the films, in-situ deposited at 670°C are shown to be epitaxial on (001) MgO substrate through TEM, electron diffraction and X-Ray diffraction. The intergrowth of isolated c// grains into the c± film is mainly located in the external layer. However the volume fraction of material with this c// orientation is rather important in these samples. Such a double orientation has been studied on SrTiO3 substrate and the prefered c± orientation has been explained to occur on the basis of minimization of the film elastic energy combining the temperature dependence of the parameter mismatch with the variation of the c lattice parameter on the oxygen content6, 9. Such an elastic model is not relevant to the (001) MgO plane, since the a-substrate parameter is larger than c/3 instead of being midway between c/3 and a for SrTiO3. Considering the surface energies in epitaxial growth, the condition for the film to wet the substrate layer by layer (Frank-Van der Merwe growth) is expressed through the inequality between free-energies terms as > ~f + 6 i , neglecting the strain energy in the film 10 (as, af, ai are respectively the substrate surface,the film free surface and the interface free energies). If we consider the fit between the oxygen sublattices of the two oxides, we

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can predict the favoured orientation c±. On the other hand, the c-axis being the direction of the slowest growth, it means of is lower for (001) plans. So t~f + t~i is minimum for a c± growth. This model do not include an explanation for c//grains to develop. This c//growth may be controlled by the presence of defects in the film during the deposition. CONCLUSIONS Both, plan views and cross-sections show an homogeneous c± layer with some c// grains close to the free surface. The main c± orientation corresponds to a minim±sat±on of the surface free energies and also to a good fit of the oxygen sublattices at the film/substrate interface. A dilatation of the c parameter has also been observed. This dilatation is correlated with the presence of planar defects in this layer.

A ~ O ~ r ~ ~ The authors gratefully acknowledge A. Cheenne and Dr.J. Perri~re for performing the RBS analysis. REFERENCES .

2. 3.

,

5. 6 7. 8 9. 10.

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