ISSN 163-7745, Crystallography Reports, 213, Vol. 58, No. 3, pp. 488 492. Pleiades Publishing, Inc., 213. Original Russian Text E.A. Stepantsov, F. Lombardi, D. Winkler, 213, published in Kristallografiya, 213, Vol. 58, No. 3, pp. 478 483. SURFACE AND THIN FILMS Growth of YBa 2 Cu 3 O 7 Films with [11] Tilt of CuO Planes to Surface on SrTiO 3 Crystals E. A. Stepantsov a, F. Lombardi b, and D. Winkler b a Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia e-mail: stepantsov@ns.crys.ras.ru b Department of Microtechnology and Nanoscience, Chalmers University of Technology, Kemivagen 9, SE-41296 Gothenburg, Sweden Received May 3, 212 Abstract YBa 2 Cu 3 O 7 films with the CuO plane tilted to the surface have been grown on SrTiO 3 crystalline substrates by pulsed laser deposition. This tilt was obtained by rotating the film lattice with respect to the substrate surface around its [11] axis oriented parallel to the surface. The zero tilt of the CuO plane was implemented at the orientation of the SrTiO 3 crystal surface parallel to the (1) plane. The rotation angles were varied in the range from to 7. It is found that, being tilted at any angle, the CuO planes of the film remain parallel to one or several crystallographic planes of the (1)-type substrate. In the range of tilt angles from to 49, the film has a single-crystal structure. Above 49 the film is transformed into a three-domain texture and its surface roughness sharply increases. DOI: 1.1134/S1637745133243 INTRODUCTION A systematic study of the growth of high-temperature superconductor (HTSC) YBa 2 Cu 3 O 7 films with CuO planes tilted to the surface began when the inclination of these planes in the range from to 5 was found to lead to a higher critical current density and a smoother surface [1 3]. The decrease in the surface roughness results in a lower surface resistance, which is extremely important in the design of passive microwave cryogenic electronics devices [4]. Later a HTSC YBa 2 Cu 3 O 7 film was used as a basis for the design of a bicrystal Josephson junction on an artificial grain boundary that formed due to the symmetric rotation of crystal lattices of both film grains around their common crystallographic [1] axis oriented parallel to the surface and boundary planes in opposite directions [5 7]. This rotation ensured the tilt of CuO planes to the film surface and to each other by 12 and 24, respectively. Junctions of this type were characterized by significantly improved main Josephson characteristics (critical current and characteristic voltage) in comparison with standard bicrystal junctions [8 1] with CuO planes oriented parallel to the surface. This achievement made it possible to significantly improve the parameters of cryogenic electronics devices, for example, microwave generators and detectors based on these junctions. The specific features of bicrystal junctions have provoked researchers to search for ways to grow and study YBa 2 Cu 3 O 7 films with CuO planes tilted to the surface by angles exceeding 12. The purpose of this search was to determine the range of tilt angles ensuring epitaxial film growth. Another urgent problem is to determine the maximally possible angle of rotation of these planes with respect to each other in the formation of an artificial grain boundary for bicrystal Josephson junctions. In context with this, it is important to find out how these limiting angles, as well as structural and morphological features of films, depend on the crystallographic orientation of the axis around which lattice is rotated. The purpose of our study was to analyze the aforementioned problems. An analysis was performed for the case where the film lattice is rotated around the [11] axis oriented parallel to the surface. EXPERIMENTAL Substrates were prepared from Verneuil-grown SrTiO 3 single crystals with cubic symmetry. The crystals were X-ray oriented, cut, and subjected to standard chemical mechanical polishing. As a result, we obtained a set of substrates in which one of the (1) crystallographic planes was tilted to the surface in the angular range from to 7 with a step of 5. The lattice was rotated around the [11] axis and oriented parallel to the surface. HTSC YBa 2 Cu 3 O 7 films were grown on substrates by pulsed laser deposition [11]. Specifically, a rotating YBa 2 Cu 3 O 7 stoichiometric target was irradiated with a UV KrF excimer laser (pulse repetition frequency 1 Hz; wavelength 248 nm). During irradiation, a plasma plume was formed near the target surface. A substrate, glued by silver paste to a resistive heater, was placed near the plume tip. The substrate temperature was maintained at a level of 78 С. Deposition we performed in an oxygen atmo- 488
GROWTH OF YBa 2 Cu 3 O 7 FILMS WITH [11] TILT OF CuO PLANES TO SURFACE 489 O 4 ϕ-scanning O 1 O 2 Ψ O 3 O Ψ-scanning Fig. 1. Scheme of X-ray diffraction ϕ scanning of substrate and film with a change in the tilt angle ψ between the plane of incidence and reflection of X rays and the normal to the surface. sphere at a pressure of.7 mbar. YBa 2 Cu 3 O 7 material was deposited on the substrate surface from the plasma plume at a rate of.5 nm per pulse. After deposition the films were cooled at a rate of 1 K/min in oxygen at a pressure of 1 atm. The HTSC YBa 2 Cu 3 O 7 film thickness in all samples was 2 nm. The surface morphology was investigated with a Solver atomic force microscope (Park Scientific Instrument) in the tapping mode. The structure of the films grown was studied on a four-circle X-ray diffractometer (Philips X Pert) using an X-ray tube with a copper anode (λ =.154 nm); X rays were filtered by a 4 Ge(22) Bartels monochromator. We used two X-ray diffraction techniques. The first one implemented the θ 2θ scanning mode but at different tilt angles ψ (Fig. 1) between the plane of incidence and the reflection of X rays and the normal to the film surface. In the second technique, based on three-dimensional ϕ scanning (Fig. 1), the X-ray tube and detector were oriented in the Bragg position with respect to the (hatched in the figure) (6) crystallographic plane of the film or (1) plane of the substrate. The normal to the film surface (O O 4 ) was tilted by an angle ψ to the plane of incident (O O 1 ) and reflected (O O 2 ) X rays by rotating around the intersection line of this plane and the sample surface. The substrate was also rotated by an angle ϕ around the normal to its surface (O O 4 ). The angle ϕ was counted from the 11 -type crystallographic direction in the substrate oriented parallel to its surface. In the case of three-dimensional ϕ scanning, the angle ϕ was varied from to 36 with a step of 2', while the angle ψ was varied from (at which the О 4 axis was parallel to the plane of incident and reflected X rays) to 7 with a step of 15'. We did not perform ϕ scanning at larger angles ψ because of instrumental limitations. The three-dimensional ϕ-scanning data were presented in the form of a set of curves in threedimensional coordinates, with the reflection intensity plotted on the vertical axis and the radial coordinates (angles ϕ and ψ) plotted in the horizontal plane. The ψ values were plotted on the radial axis, while rotation from the 11 direction in the substrate corresponded to the angle ϕ. To make the graphical pattern less cumbersome, we omitted the ϕ-scanning curves containing no X-ray reflection peaks. RESULTS AND DISCUSSION Figure 2a shows the results of three-dimensional ϕ scanning for a SrTiO 3 substrate with (334) orientation. Scanning was performed over {1} crystallographic planes. To this end, the X-ray tube and detector were oriented in the Bragg position (corresponding to this plane) with respect to the sample; this setting was not changed during measurements. In the (334) substrate, one of three {1} crystallographic planes (specifically, the (1) plane) should be tilted to the surface by an angle of 46.7, and the lattice should be rotated by the same angle around an axis of the 11 type oriented parallel to the surface from the position in which the substrate would be oriented in the cube plane. Two other planes from this set, (1) and (1), are perpendicular to the former and to each other. Therefore, they are tilted with respect to the surface by 59 and I, counts/s (a) 15 1 5 I, counts/s (b) 3 2 1 I, counts/s (c) 8 4 5 5 5 5 5 5 11 5 5 11 5 5 11 5 5 Fig. 2. Three-dimensional plots of ϕ scanning over (a) (1)-type crystallographic planes recorded for the SrTiO 3 substrate with (334) orientation; (b) (6) crystallographic plane recorded for the HTSC YBa 2 Cu 3 O 7 film with (114) orientation; and (c) crystallographic planes of the (1) and (6) types of the substrate and film, respectively, aligned in the parallel azimuthal crystallographic orientation. The curves corresponding to the film and substrate are given in black and gray, respectively. CRYSTALLOGRAPHY REPORTS Vol. 58 No. 3 213
49 STEPANTSOV et al. I, counts/s 3 25 2 15 1 5 (1)YBCO 8 (2)YBCO (1)STO (4)YBCO (5)YBCO 16 24 32 4 48 56 64 2θ, deg symmetrically shifted in azimuth by 124.4 to opposite sides from the (1) plane. It can be seen that the set of curves contains only three peaks. Their angles ψ are, respectively, 46.7, 59, and 59, and the angles of rotation (ϕ) of these peaks with respect to zero direction 11 are, respectively, 9., 34.4, and 145.6. This pattern is in agreement with the angular position of all three planes of the (1) type of the SrTiO 3 cubic crystal on the (334) face. It is noteworthy that the peaks have significantly different intensities; the larger value corresponds to the plane tilted by a smaller angle with respect to the surface. (2)STO (7)YBCO (8)YBCO Fig. 3. X-ray (θ 2θ)-scan curve recorded at a fixed tilt angle ψ (46.7 ) between the plane of incident and reflected X-rays and the surface of YBa 2 Cu 3 O 7 film with (114) orientation. [334] SrTiO 3.5 μm 1. [114] YBa 2 Cu 3 O 7 11 Fig. 4. Surface morphology of YBa 2 Cu 3 O 7 (114) film (AFM image). The HTSC YBa 2 Cu 3 O 7 film grown on a SrTiO 3 substrate with (334) orientation was singlecrystal, with the (114) crystallographic plane lying in the surface plane. This is confirmed by the data of three-dimensional ϕ-scanning (Fig. 2b), in which the X-ray tube and detector are oriented in the Bragg position, corresponding to the reflection from the (6) plane of the film. To increase the scanning efficiency, we chose the sixth-order reflection, because it has the highest intensity among all other reflections from the (1) plane. As can be seen in Fig. 2b, the curve contains only one peak, which is indicative of the single-domain state of the film. This peak corresponds to the angle ψ = 46.7. This means that the (1) basal plane of the film is tilted to the surface exactly like one of the (1)-type planes of the substrate (specifically, the plane tilted by smaller angle). The alignment of both curves in the same plot (Fig. 2c) shows that the curve corresponding to the film (which is shown black in the figure) completely coincides with the curve corresponding to the (1)-type plane of the substrate, tilted to the surface by a smaller angle. Both curves corresponding to the substrate are shown in gray. The complete alignment of the film curves and one of the substrate curves indicates that the (1) basal plane of the films on substrates with this orientation is oriented parallel to the (1) plane of the substrate tilted by a smaller angle to the surface, and the [11] direction in these films is parallel to the [11] direction in the substrate. The structural quality of the films is characterized by the X-ray (θ 2θ)-scan curve (Fig. 3). This scanning was performed at fixed angles ψ and ϕ, at which the three-dimensional ϕ-scan curve of the film (Fig. 2b) contains reflections at ψ = 46.7 and ϕ = 9. As follows from the above (θ-2θ)-scan plot, this curve contains three peaks of three reflection orders from the (1) crystallographic plane of the SrTiO 3 substrate and eight peaks of, correspondingly, eight reflection orders of the plane (1) of the YBa 2 Cu 3 O 7 film. There were no peaks corresponding to reflections from other crystallographic orientations in this material or foreign-phase particles. The FWHM of the (2) peak of the substrate is.7, while this parameter for the nearest (in the θ angle) (5) peak of the film is.16. The small difference in the FWHM values for these peaks indicates that the structural quality of the film is rather high for this substrate material. An AFM study of the surface morphology of the YBa 2 Cu 3 O 7 (114) film revealed a set of nanograins that are not significantly elongated in any direction in the surface plane (Fig. 4). The surface roughness, defined as the arithmetic mean of the heights Ra of surface nanoprotrusions, is 1.6 nm, a value corresponding to the mean roughness of YBa 2 Cu 3 O 7 films grown without tilting CuO planes. CRYSTALLOGRAPHY REPORTS Vol. 58 No. 3 213
GROWTH OF YBa 2 Cu 3 O 7 FILMS WITH [11] TILT OF CuO PLANES TO SURFACE 491 I, counts/s (a) 1 8 6 4 2 5 5 5 5 11 I, counts/s (b) 1 8 6 4 2 5 5 5 5 11 Fig. 5. Three-dimensional plot of ϕ scanning over crystallographic planes of the (6) and (1) types recorded from (a) the HTSC YBa 2 Cu 3 O 7 film with (113) orientation and SrTiO 3 substrate with (111) orientation and (b) from the HTSC YBa 2 Cu 3 O 7 film with (112) orientation and the SrTiO 3 substrate with (332) orientation. The curves corresponding to the film and substrate are shown as black and gray, respectively. The above data are related to the growth of YBa 2 Cu 3 O 7 film on the surface of SrTiO 3 crystal, where the tilt angle for the (1)-type plane least tilted to the surface is 46.7. Similar studies but with other tilt angles yielded exactly the same results in the range of tilt angles from to 49. The pattern radically changes with a further increase in the tilt angle for the (1)-type plane least tilted to the surface to 54.7. This value corresponds to the orientation of the SrTiO 3 substrate parallel to the (111) plane. The three-dimensional ϕ scanning of substrate with this orientation over the (1) crystallographic plane yields three peaks of equal intensity located in one circumference separated by angles ϕ = 12 from each other. Their tilt angle ψ is 54.7. This can be seen in Fig. 5a, where the curve corresponding to the substrate is shown in gray. This arrangement of the peaks from the substrate and the equality of their intensities are explained by the fact that tilt angles to the surface for all three (1)-type planes in substrates with this orientation are the same (54.7 ) because of the crystal symmetry. The curve shown in Fig. 5a in black, which corresponds to three-dimensional ϕ scanning over the (6) plane of the YBa 2 Cu 3 O 7 film grown on a substrate with (111) orientation, exhibits three peaks. They also have equal intensities and coincide in position (i.e., the angles ϕ and ψ) with the substrate peaks. This means that the film is oriented by the (1) basal plane parallel to the (1)-type plane of the substrate during growth. All three (1)-type planes in the SrTiO 3 substrate with (111) orientation are in equivalent positions with respect to the surface, and their tilt angles are identical; therefore, the film is divided into crystalline domains of three equivalent types. At the same time, each of them differs by the orientation of its (1) plane parallel to one of three (1)-type planes equally tilted to the substrate surface. The surface of all these domains corresponds to the (113) crystallographic plane. The equal intensities of all three peaks from the film indicate that the domains of these types have identical total areas. In the range of tilt angles to the surface from 49 to 6 of the (1)-type planes in the SrTiO 3 substrate and, correspondingly, CuO planes of the YBa 2 Cu 3 O 7 film, an intermediate pattern is observed. The film also has a three-domain structure, but its peaks in the three-dimensional ϕ-scan plots have different heights; this difference increases with the difference in the tilt angles. Apparently, a change in the tilt angle above 54.7 increases the total area of only those domains for which the tilt angle of CuO planes to the surface decreases. With a decrease in the tilt angle from 54.7 to 49, as well as with its increase from 54.7 to 6, domains of this type completely displace the other domains. The dominance of the domains with CuO planes less tilted to the surface is explained as follows. During the growth of YBa 2 Cu 3 O 7 film (in the stage of formation and outgrowth of nucleation centers), the centers with the (1) basal plane tilted to the surface by a smaller angle are energetically more favorable in view of the high anisotropy of the surface tension of this material. When the tilt angle exceeds 6, the domains of this type disappear and the film becomes two-domain, composed of domains of two types with identical total areas. As an example, Fig. 5b shows the result of X-ray ϕ scanning of an YBa 2 Cu 3 O 7 film grown on a SrTiO 3 substrate with (332) orientation. The surface of this substrate is formed as a result of lattice rotation around the [11] axis (parallel to the surface) by 64.8 from the position in which the surface plane initially coincided with the (1) plane. The ϕ-scan curves corresponding to the substrate and film are given in black and gray, respectively, in Fig. 5b. The substrate curve contains three peaks. One of them, least in height, corresponds to the reflection from the (1) plane, tilted to the surface by 64.8. The other two peaks have equal ψ values: 5.2. The film peaks (only two in number) coincide with only these substrate peaks, CRYSTALLOGRAPHY REPORTS Vol. 58 No. 3 213
492 STEPANTSOV et al. because, as in the previous cases, the film grows by its basal face (i.e., the CuO plane) parallel to the substrate (1)-type planes, tilted to surface by a smaller angle. The peak heights are identical, because they have equal ψ values. Hence, the film is two-domain, the domains have equal total areas, and their surface is oriented parallel to the (112) crystallographic plane. The latter conclusion follows from the fact that this face forms an angle of 5.2 with the (1) basal plane of the YBa 2 Cu 3 O 7 film at ϕ = 23.1 and 156.9. Since the domains of both types are in crystallographically equivalent positions, they do not compete during growth. CONCLUSIONS The results of this study indicate that HTSC YBa 2 Cu 3 O 7 films with CuO planes tilted up to 49 to the surface can be epitaxially grown on the surface of SrTiO 3 crystals if the film lattice is rotated around the [11] axis (lying in its surface plane). This is implemented by changing the crystallographic orientation of the SrTiO 3 crystal surface by tilting the (1) plane at the same angle. Under these conditions the film roughness is almost the same as for the film with the basal plane untilted. When the tilt angle exceeds 49 the films become textured. ACKNOWLEDGMENTS This study was supported by the Ministry of Education and Science of the Russian Foundation as part of the federal target program Scientific and Scientific- Pedagogical Personnel of Innovative Russia for 29 213 (grant no. 835) and the Swedish agencies VR and SI. REFERENCES 1. M. Mukaida, S. Miyazawa, and M. Sasaura, Jpn. J. Appl. Phys. B 3 (8), L1474 (1991). 2. Y. Y. Divin, U. Poppe, J. W. Seo, et al., Physica C 235 24, 675 (1994). 3. T. Wang, X. Duan, W. Hu, et al., Supercond. Sci. Technol. 15 (8), 1199 (22). 4. O. G. Vendik, I. B. Vendik, and D. V. Kholodniak, Mater. Phys. Mech. 2 (1), 15 (2). 5. Y. Y. Divin, U. Poppe, C. L. Jia, et al., Physica C 372 376, 115 (22). 6. U. Poppe, Y. Y. Divin, M. I. Faley, et al., IEEE Trans. Appl. Supercond. 11 (1), 3768 (21). 7. M. V. Liatti, U. Poppe, and Y. Y. Divin, Appl. Phys. Lett. 88, 15254 (26). 8. D. Dimos, P. Chaudhari, J. Manhart, and K. LeGoues, Phys. Rev. Lett. 61, 219 (1988). 9. D. Dimos, P. Chaudhari, and J. Manhart, Phys. Rev. B 41, 438 (199). 1. R. Gross, P. Chaudhari, M. Kawasaki, et al., Appl. Phys. Lett. 57, 727 (199). 11. G. Brorsson, E. Olsson, Z. G. Ivanov, et al., J. Appl. Phys. 75 (12), 7958 (1994). Translated by Yu. Sin kov CRYSTALLOGRAPHY REPORTS Vol. 58 No. 3 213