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Nature of textured growth of laser-deposited YBaCuO thin films on (100) MgO
Materials Science and Engineering, B7 ( 1991 ) 287-296
287
Nature of Textured Growth of Laser-deposited Y - B a - C u - O Thin Films on (100) MgO R. K. S1NGH*, K. JAGANNADHAM and J. NARAYAN Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695- 7916 (U.S.A.) (Received June 28, 1990; in revised form August 3, 1990)
Abstract
We have studied, using X-ray and transmission electron microscopy techniques, the texturing of YBa 2Cu307 superconducting thin films with c axis perpendicular to the lattice-mismatched (100) magnesium oxide substrate. The results were compared with epitaxial growth on the (100) SrTiO~ where the c axis of the film is either perpendicular or parallel to the substrate. Texturing with c axis perpendicular to the substrate occurs as a result of preferential grain growth in the "a" and "b" directions, whereas epitaxial growth involves lattice matching with the underlying substrate. Thin films of YBa2Cu~O7_ x were deposited using a pulsed-laser evaporation technique and were further annealed in oxygen to recover the superconducting properties, and to study the nature of textured growth. The grain growth in films was investigated as a function of annealing treatments. The high-temperature annealed films exhibited large textured grains (about 5-10/am) with c axis perpendicular to the substrate, but the (001) planes were found to be rotated randomly in the plane parallel to the substrate. The low-temperature annealed films showed small grains (-~ 100 nm) with no preferred texturing. From the microstructural variations between the high- and low-temperature annealed films, the process of grain growth in high-T, superconducting films was analysed. We propose a model based upon higher mobility of the a-b grain boundaries to explain the texturing of thin films with c axis perpendicular to the substrate. 1. Introduction
There has been a concentrated effort in the past two years to fabricate high-quality epitaxial *Present address: Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 (U.S.A.). 0921-5107/91/$3.50
and textured high-Tc superconducting thin films on various substrates [1-12]. Detailed experiments have conclusively shown the importance of texturing and epitaxial growth of grains in controlling the critical current densities of new highTc oxide superconductors. Critical current densities exceeding 106 A cm- 2 at 77 K (and zero field) have been obtained in textured films de'posited on (100) MgO and epitaxial films grown on (100) SrTiO3 substrates [1, 2, 8, 10]. There are two different ways by which superconducting thin films can exhibit a preferred orientation: (a) epitaxial growth where the atoms at the substrate-film interface form a coherent or semicoherent interface; and (b) textured growth where the grains are preferentially oriented along a particular direction after annealing at high temperatures (700-950 °C). The primary goal of this paper is to understand the texturing of thin films after oxygen annealing treatments needed to recover the superconducting properties. In our experiments, epitaxial and textured YBazCu307 films were grown on lattice-matched ((100) SrTiO3) and lattice-mismatched ((100) MgO) substrates, respectively. The orientation of epitaxial YBa2Cu307 film growth on (100) SrTiO 3 is controlled by the lattice matching of the substrate with either the "a" or "c" parameter of YBa2Cu307. Consequently, both films with the c axis parallel and perpendicular to the substrate are observed, depending on the processing conditions. In contrast to epitaxial growth on (100) SrTiO 3 substrates, the large lattice parameter mismatch (8.8%) of YBa2fu307 with MgO may prevent the formation of a coherent or semi. coherent interface. Thus, the primary mechanism for preferred orientation should occur by the anisotropic grain growth process. As the grain growth is a thermally activated process, high temperatures (about 900 °C) are required to induce high growth rates and the formation of textured © Elsevier Sequoia/Printed in The Netherlands
288 films. To understand the preferred texturing of superconducting grains with the c axis perpendicular to the substrate, we have investigated the anisotropic grain growth in YBazCu307 thin films as a function of annealing treatment. The results were compared with epitaxial growth on (100) SrTiO3 substrates.
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2. Experimental procedures Thin films of Y - B a - C u - O were deposited on (100) MgO and (100) SrTiO 3 substrates using the pulsed-laser evaporation technique [13]. This technique involves evaporation of material from a bulk YBa2Cu307 target and deposition of the evaporated material on to a substrate, that is placed parallel to the target. The nature of the deposition process and the factors affecting the spatial thickness and composition variations have been discussed elsewhere [14, 15]. The deposition of films was performed at pressures ranging from 10 5 to 5 x 10 -3 Torr and substrate temperatures of about 400-450 °C. Laser evaporation leads to deposition of atomically mixed chemical species with stoichiometric composition, and therefore the nucleation of the superconducting phases does not require significant diffusion of the species. The as-deposited films were non-superconducting and were further annealed both at low and high temperatures to understand the nature of the grain growth process, and to recover the superconducting properties. In the low-temperature annealing cycle, the films were heated at 650 °C for 20-40 min in oxygen, while in the high-temperature cycle the films were heated at 920 °C for 2-20 min in helium and oxygen. The cooling rates used in both of the cycles were 3 °C min ~ in an oxygen atmosphere. It may be noted that an in situ asdeposited superconducting thin film was formed at oxygen partial pressures (100-200 mTorr) and higher temperatures (500-650 °C), which require no post annealing [2].
3. Results The films annealed in a high-temperature cycle exhibited sharp reproducible superconducting transitions with zero resistance at about 85 K. Figure 1 shows a typical plot of resistivity against temperature with Tc = 85 K and the room temperature resistivity of 350 /~ff~ cm. The X-ray
0 Z
0.2
0.0
,
I 100
,
I
200
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, 300
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Fig. 1. Temperature dependence of normalized electrical resistance of superconductingfilm on (100) MgO substrate. The films were textured with c axis perpendicular to the substrate and showedzero resistanceat about 85 K.
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Y-Ba-Cu-O on (100)MgO
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Fig. 2. X-ray diffractionof the preferentiallyoriented film formed after high-temperature annealing with its c axis normal to the (100) MgO substrate.
diffraction data revealed that the films were preferentially oriented with c axis normal to the MgO substrate surface. This strong texture can be observed in the X-ray diffraction data shown in Fig. 2, where peaks corresponding to the (00/) planes ( / = 3 , 4, 5, 6, 7) of the orthorhombic superconducting YBazCu307_x phase are only observed. The X-ray diffraction data clearly show that the [001] direction of the superconducting phase is perpendicular to the (100) MgO substrate. The relative orientation of the [100] and [010] axes of the film cannot be determined from these data. Figure 3(a) shows an ordered surface topography of an epitaxial film with the c axis parallel
289
Fig. 3. (a) SEM micrograph of an epitaxial film on SrTiO 3 substrate with its c axis parallel to the substrate. (b) SEM micrograph of textured superconducting film with c axis normal to the (100) MgO substrate. Both films were obtained after annealing at high temperatures.
to the SrTiO 3 substrate. These films were formed after deposition at 450°C and subsequent annealing at 900 °C for 20 min in oxygen. The ordered grain structure results from lattice matching of the c/3 and b axes with the SrTiO 3 substrate. As the SrTiO3 and superconducting phase possess an isotropic cubic perovskite and orthorhombic structure, respectively, the superconducting grains with either the a or c axis perpendicular to the substrate can have two mutually perpendicular epitaxial growth orientations [16, 17]. Planar transmission electron microscopy [18] revealed that the shorter dimensions of each of the rectangular grains lie along the c axis with an average "a" by "c" length ratio of the grains being 3.0. In contrast, textured superconducting grains with c axis perpendicular to the (100) MgO substrate do not exhibit an ordered surface topography. Figure 3(b) shows a typical example of the
textured films with c axis perpendicular to the (100) MgO substrate formed after high-temperature annealing at 920 °C for 20 min. This SEM micrograph shows large superconducting grains with grain size varying from 5-10 ktm, and a random-plate-like morphology indicating random orientation of the "a" and "b" axes of the grain. Figure 4(a) is a TEM micrograph from a typical region of a superconducting film with its c axis perpendicular to the (100) MgO substrate. This micrograph shows a similar shape of grains as exhibited by the SEM micrograph. All the grains are copiously twinned on the (110) plane. The high number density of 45 ° (110) twins observed in the previous figure are characteristic of films with their c axis perpendicular to the substrate [18]. As all the grains have their c axis parallel to the electron beam, the diffraction pattern from the grains correspond to the [001] zone axis as shown in Fig. 4(b). From the relative orientation of (110) twins in different grains, it can be inferred that the (001) planes of different grains that are parallel to the substrate are rotated about the [001] axis. However, it must be noted that some preferred orientation relationship between the film and the substrate has also been observed [4, 19]. In contrast to the textured nature of the grains present in high-temperature annealed samples, the low-temperature annealed grains exhibit a random orientation. Figure 5(a) shows a planar TEM micrograph with (Fig. 5(b)) its corresponding diffraction pattern for low-temperature annealed films. The films were deposited at 450 °C and further annealed in oxygen at 650 °C for 30 min. The X-ray diffraction of the sample showed the random orientation of the 1-2-3 grains after the low-temperature annealing. The X-ray results revealed the presence of a secondphase material resulting from the incomplete transformation of the deposited material into the 1-2-3 phase at low temperatures. These films exhibit a transition temperature onset at 90 K with zero resistance at 40 K. A small grain size (about 100 nm) with no preferred texturing orientation is observed in these films. 4. Discussion
From the above results it is concluded that the superconducting grains are initially small and exhibit a random orientation. Furthermore, high-
290
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Fig. 4. (a) TEM micrograph of c-axis textured film on MgO substrate showing the extensive twinning of the grains. (b) Typical diffraction pattern from each of the grains showing the preferred texturing of the film.
temperature annealing leads to the formation of large grains with c axis perpendicular to the substrate. This preferential texturing of the grains with c axis perpendicular to the substrate can be explained by the process of grain growth, which implies high mobilities associated with the a-b (composed of (100) and (010) planes) boundaries. We have observed (Fig. 2) the higher mobility of the b axis compared to the c axis resulting in long elongated grains on films deposited on strontium titanate substrates. In addition, the textured superconducting thin films on MgO after high-temperature annealing exhibit a thin plate-
like morphology similar to the platelets of 1-2-3 crystals formed during crystal growth [20-21]. The crystals grown are generally in the form of rectangular prisms or platelets, the smallest dimension being parallel to the c axis. The preferred orientations of the growth ledges in the (100) and (010) directions may be indicative of the anisotropy in the surface energy in these materials. The process of grain growth, which occurs during high-temperature annealing, is an important factor affecting texturing in thin films. In the following, we analyse the grain growth processes,
291
Fig. 5. (a) Planar TEM micrograph of a low-temperature processed film showing the random orientation of the grain. (b) The corresponding selected area diffraction pattern.
which occur because of the difference in free energy and which provide a driving force for the migration of atoms across the grain boundary [22, 23]. This free-energy difference arises from the curved nature of the grain boundary and can be
expressed as
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(1)
r
where ), is the grain boundary surface energy, Vm
292
transferred across the grain boundary with the aid of thermal fluctuations. A gradient in the chemical potential biases the probability of transfer such that grain boundary migration occurs so as to lower the free energy of the system. As our system contains grain growth in a thin film where the dimension perpendicular to the substrate is small (shown in Fig. 6(b)), we assume growth of a cylindrical grain whose growth rate can be expressed as the difference between the forward and reverse fluxes. The growth rate V of the cylindrical grain can be expressed as
J
Substrate
(a)
V =A2nlvl
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0
Grain I
(b)
Grain 2
o,stQnce
Fig. 6. (a) Schematic free-energy diagram of an atom during the process of jumping from one grain to the other. (b) Schematic diagram to show the radial growth of a grain present in the superconducting thin film.
Azrtlv I
AG~/ exp - RT J
(2)
where A 2 refers to the sticking probability of an atom in grain 2, nl is the number of atoms in a favorable position to make a jump and v I is the vibration frequency of the atom 1. The flux in the reverse direction is similar but as the grain 2 has a lower free energy than grain 1 because of the grain boundary curvature, the exponential term is different. The reverse direction flux is given by A1 n2 Vz exp
AG~ + AG] ~-
]
(3)
The rate of grain boundary migration can be determined by assuming that at dynamic equilibrium equal and opposite fluxes of atoms are
RT
exP/RT
]
(4)
where h and r are the thickness and radius of the cylindrical grain, respectively. Assuming that AG ~ R T and substituting the value of A G, we can simplify the above equation into
V=Kr is the molar volume and r is the radius of curvature of the grain. Let us consider the effect of the driving force on the kinetics of grain boundary migration. As the motion of grain boundaries is a thermally activated process, the atoms must acquire thermal activation energy A G a to break away from grain 1. This is shown schematically in Fig. 6(a). The grain boundary flux from grain 1 to grain 2 can be expressed as an Arrhenius-type equation given by
1-exp
~
exp/~
]
(5)
where all the pre-exponential terms in the previous equation have been incorporated into the factor K. The above equation shows a linear relationship between the driving force A G and the velocity of the boundary. The variation in the grain boundary mobility with the grain boundary crystallography can be expressed in the anisotropic variation in the exponential term of the above expression. The variation in the activation energy with respect to the structure was analysed by Turnbull and Glieter [23, 24] for cubic crystals. In f.c.c, lattices, it has been observed that grain boundaries have least mobility when two lattices show a definite relationship with each other where the atoms of both lattices occupy common positions called coincidence sites. Higher values of the ratio of sites in the main lattice site per site in a coincident lattice results in higher grain boundary mobility. In superconducting thin films, we envisage that the anisotropy in the grain boundary energy and structure gives rise to different jump frequencies, atomic jump distances, etc. Though the specific reason for the anisotropy cannot be determined, experimental observations have clearly revealed the higher mobility grain boundaries in the [100] and [010] directions compared with the [001] direction.
293
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Fig. 7. Atomic arrangement of (010) planes in YBa2Cu307 compound, which are parallel to the substrate for a-axisperpendicular films.
The higher mobility in different growth directions can be readily understood by analysing the '(010) and (001)lattice planes, which are parallel to the substrate for the "a"- and "c"-axis-perpendicular films respectively. As the superconducting thin films have very small dimensions perpendicular to the substrate, the grain growth processes primarily occur in planes parallel to the substrate. The schematic atomic structure of the (010) planes that are parallel to the substrate for "a"-axis-perpendicular films is shown in Fig. 7. This figure shows that the stacking sequence along the [001] direction is made of four separate repeating layers of Cu, Ba-O, Cu-O and Y. In contrast, the stacking sequence in the [100] direction consists of alternate layers of Cu-O and Ba-O-Y. As the grain growth processes occur by the addition of atoms to each layer, it is expected that the simpler stacking sequence in the [100] direction would result in higher grain boundary mobility in that direction. This is most readily observed in superconducting c-axisparallel films on SrTiO 3 substrates where the grains are rectangular in shape and the a/c dimensional ratio corresponds to about 3.0. The anisotropy in the grain boundary mobility is incorporated in the exponential factor in eqn. (5). Furthermore, for "c"-axis-perpendicular films, both the [100] and [010] directions have similar stacking sequences comprising alternate layers of Cu-O and Ba-O-Y. Thus no anisotropy in the grain boundary mobility is expected, which gives rise to grains having similar dimensions in
the [100] and [010] directions. In addition, as both [100] and [010] directions possess very high mobility, the average grain size should be larger for c-axis-parallel films for the same growth condition which is corroborated by experimental observations. It must be noted that the grain growth process is affected by the presence of a second phase including precipitates, which can pin down the movement of grain boundaries. The effect of these factors requires further consideration in these materials. Another important method of understanding the higher mobility of certain grain boundaries is to examine the grain boundary atomic structure associated with small-angle and large-angle grain boundaries in different orientations [25]. The primary orientations of the coincident-site lattice boundaries satisfy the angular relation, tan
=Mav
where ap is the misorientation angle associated with the boundary, a t the magnitude of the Burgers vector of the lattice dislocation, and av the direction perpendicular to the Burgers vector of the crystal lattice dislocations. In the a - b boundaries, their magnitudes are equal so that a~ = av. Similarly, if the boundaries are made of a and c lattice parameters, c = 3 a so that a, = c/3 = av. The different primary coincidence boundaries are obtained by giving different values to M with N = 1. Here, we show different grain boundaries in the YBa2Cu307 superconductors, which belong to a special class of boundaries known as "primary coincidence boundaries", whose energies correspond to the lowest values of the cusps in the energy against misorientation angles. Figure 8 is a symmetric primary coincidence a - b tilt boundary of misorientation angle 12.6 ° formed between two superconducting grains with N = 1 and M = 9. The (a-b) atomic planes misoriented with respect to each other are shown along with the coincident lattice points at B and C, occupied by oxygen ions. The coincident site lattice (CSL) unit cell belonging to the two grains, shown by the dashed lines ABCD (BEFC in grain 2), contains 82 lattice points with 41 occupied by oxygen ions, 16 by copper ions and the remaining are vacant sites. The relaxations of the atom positions and the coalescence of the boundary surfaces takes place to reduce the total energy
294 at
Fig. 8. Primarycoincidencesymmetrictilt boundary of the a-b type with a misorientationof 12.6°. The CSL unit cell is shown by the dashed line.
Fig. 9. Primarycoincidencesymmetrictill boundary of the a-c type with a misorientationof 12.6°.
of the configuration. The total energy consists of the strain energy associated with the boundary distortions and the surface energy of the boundary surfaces. Whereas two polygonal configurations repeat
along the a - b boundaries, those in the a-c type repeat after every n intervals, where n > 1, depending on the misorientation angle. The primary coincidence tilt boundary of misorientation, 12.6 °, with N= 1, M = 9 is shown in Fig. 9. The corners of the unit cell are occupied by oxygen ions. The grain boundary polygons repeat after four (n = 4) intervals. A comparison of the polygons of the a - b and a-c boundaries in Figs. 8 and 9 shows that copper and oxygen ions are present in the first, whereas copper, oxygen, barium or yttrium ions are present in the second. Also, a different void volume is associated with each polygon since the Burgers vector of the grain boundary dislocations is different. The grain growth responsible for the formation of textures is determined by the mobility of the grain boundaries, which in turn is influenced by their atomic structure. For the a-c or b-c boundaries, the mobility is impeded because extensive rearrangement of unlike ions is needed for diffusion and mass transport. For the same misorientation, the CSL unit cell of a - b boundaries is found to contain a smaller number of lattice points than the a-c boundaries. From the above geometrical description of the primary CSL boundaries in the layered Y - B a - C u - O oxides, the following salient features can be summarized: (a) the CSL unit cell is much larger in the a-c boundary than in the a - b type for the same misorientation and coincidence point; (b) the a - b type boundaries are characterized by only the presence of copper or oxygen ions, whereas in the a-c-type boundaries, copper, yttrium, barium and oxygen ions are present; (c) unlike a-b boundaries, a - c and b-c grain boundaries require extensive rearrangement of ions for the grain boundary mobility. Thus, a higher activation energy for mobility of the a - c boundary agrees with the experimental results shown earlier. The lower activation energy and higher mobility of specially oriented grain boundaries play an important role in the development of textured recrystallized structures. The process of grain growth is shown schematically in Fig. 10. Initially as observed in annealing at low temperatures, the grains possess a random orientation. During hightemperature annealing, not all the grains will grow at the same rate; those grains that are oriented with c-axis perpendicular to the substrate will have a higher mobility and outgrow the boundaries of the randomly oriented grains. As
295
~
a
THINFILM
SUBSTRATE (MgO)
THIN FILM
l
SUBSTRATE (MgO)
Fig. 10. Schematic diagram illustrating the process of grain growth and texturing of superconducting films on (100) MgO substrate. (a) After low-temperature annealing the grains are small and possess a random orientation; (b) after hightemperature annealing large grains with c axis perpendicular to the substrate are formed.
observed in the grain growth on SrTiO 3 substrates, the growth rate of grains with their a - b plane parallel to the substrate surface will be the fastest, that is maximum growth rates are obtained in grains which have their c axis perpendicular to the substrate. This criterion, which maximizes the grain growth rate, does not depend on the orientation of the "a" and "b" axes. Also, as the interface between the superconducting thin film and the susbtrate is incoherent, there are no interfacial restraints of ordering of the [100] and [010] directions. Thus, high-temperature annealed films should show grains with their (001) planes parallel to the substrate surface, though randomly rotated with respect to each other. It should also be noted that preferentially textured films with c axis parallel to the substrate can not be formed on lattice mismatched substrates because of restrictions in grain growth in this orientation.
5. Conclusions Superconducting Y-Ba-Cu-O thin films grown on lattice-mismatched (100) MgO substrates exhibit strong texturing with the c axis of the grains perpendicular to the substrate, but with the (001) planes randomly rotated about each other. This is in contrast to epitaxial film growth on a
lattice-matched (100) SrTiO 3 substrate, where the c axis of the film is either parallel or perpendicular to the substrate depending on the lattice matching at the processing temperature. A theory of oriented growth of superconducting thin films on lattice-mismatched substrates is proposed in which the grain boundary mobility depends on the orientation of the crystallites. During the growth process, a selection process occurs where only the crystallites in a particular orientation grow and swamp out the other crystallites. The preferred texturing of the grains with c axis perpendicular to the substrate results from the anisotropic grain growth properties; when the grains are oriented with c axis normal to the substrate, the grain growth rates are maximum. The above results have been verified by detailed planar TEM and X-ray diffraction studies as a function of annealing treatments of films on (100) MgO substrates. Atomic geometrical modelling of the grain boundary structures in the 1-2-3 oxides shows that the a - b boundary will have a higher mobility than the a - c or b - c boundaries, which are restricted by extensive rearrangement of the different ions needed for diffusion and mass transport phenomena.
Acknowledgments Part of this research is sponsored by the Office of Energy Systems Research, Division of Energy Conversion and Utilization Technologies (ECUT) programs under Subcontract 19X4337C, U.S. Department of Energy, Martin Marietta Energy Systems, Inc., Oak Ridge National Laboratories, and National Science Foundation Project 8618735.
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296 7 H. Adachi, K. Hirochi, K. Setsune, M. Kitabatake and M. Wasa, Appl. Phys. Lett., 51 (1988) 2263. 8 T. Terashima, K. Iijima, K. Yamamoto, Y. Bando and H. Mazaki, Jpn. J. Appl. Phys., 27 (1988) 191. 9 H. Okhuma, T. Mochiku, Y. Kanke, Z. Wen, S. Yokoyama, H. Asano, I. Iguchi and E. Yamaka, Jpn. J. AppL Phys., 26 (1987) L1484. 10 J. Fujita, T. Yoshitake, A. A. Kamijo, T. Satoh and I. Igarashi, J. Appl. Phys., 64 (1988) 1292. 11 H. Myoren, Y. Nishiyama, H. Nasu, T. lmura, Y. Osaka and S. Yamanaka, Jpn. J. Appl. Phys., 27 (1988) L 1068. 12 B. Oh, M. Naito, S. Arnason, P. Rosenthal, R. Barton, M. R. Beasley, T. H. Geballe, R. H. Hammond and A. Kapitulnik, Appl. Phys. Lett., 51 (1987)852. 13 J. Narayan, N. Biunno, R. Singh, O. Auchiello and O. W. Holland, Appl. Phys. Lett., 51 (1987) 234. 14 R. K. Singh, N. Biunno and J. Narayan, Appl. Phys. Lett., 53 (1988) 1013. 15 R. K. Singh and J. Narayan, Phys. Rev. B, 41 (1990) 8843. 16 A. Mogro-Campero, L. G. Turner and E. H. Hall,
J. Appl. Phys., 65 (1989) 4951. 17 S. Chan, D. M. Huang and L. Nazar, J. Appl. Phys., 65 (1989) 4719. 18 R. K. Singh, N. Biunno and J. Narayan, J. Appl. Phys., 65 (1989) 3785. 19 L. A. Tietz, C. B. Carter, D. K. Lathrop, S. E. Russek, R. E. Buhrman and J. R. Michael, J. Mater. Res., 4 (1989) 1072. 20 D. L. Kaiser, F. Holtzberg, M. E Chisholm and T. K. Worthington, J. Cryst. Growth, 85 (1988) 593. 21 B. N. Das, L. E. Toth, A. K. Singh, B. Bender, M. Osofsky, C. S. Pande, N. C. Koon and S. Wolf, J. Cryst. Growth, 85 (1987) 588. 22 D. A. Smith, C. M. E Rae and C. R. M. Grovernor in R. M. Baluffi led.), Grain Boundary Structure and Kinetics, American Society for Metals. Metals Park, OH, 1979. 23 D. Turnbull, Trans. Metall. Soc. A1ME, 191 ( 1951 ) 661. 24 H. Glieter, Acta Metall., 17 (1969) 565. 25 K. Jagannadham and J. Narayan, Philos. Mag., 59 (1989) 917.
287
Nature of Textured Growth of Laser-deposited Y - B a - C u - O Thin Films on (100) MgO R. K. S1NGH*, K. JAGANNADHAM and J. NARAYAN Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695- 7916 (U.S.A.) (Received June 28, 1990; in revised form August 3, 1990)
Abstract
We have studied, using X-ray and transmission electron microscopy techniques, the texturing of YBa 2Cu307 superconducting thin films with c axis perpendicular to the lattice-mismatched (100) magnesium oxide substrate. The results were compared with epitaxial growth on the (100) SrTiO~ where the c axis of the film is either perpendicular or parallel to the substrate. Texturing with c axis perpendicular to the substrate occurs as a result of preferential grain growth in the "a" and "b" directions, whereas epitaxial growth involves lattice matching with the underlying substrate. Thin films of YBa2Cu~O7_ x were deposited using a pulsed-laser evaporation technique and were further annealed in oxygen to recover the superconducting properties, and to study the nature of textured growth. The grain growth in films was investigated as a function of annealing treatments. The high-temperature annealed films exhibited large textured grains (about 5-10/am) with c axis perpendicular to the substrate, but the (001) planes were found to be rotated randomly in the plane parallel to the substrate. The low-temperature annealed films showed small grains (-~ 100 nm) with no preferred texturing. From the microstructural variations between the high- and low-temperature annealed films, the process of grain growth in high-T, superconducting films was analysed. We propose a model based upon higher mobility of the a-b grain boundaries to explain the texturing of thin films with c axis perpendicular to the substrate. 1. Introduction
There has been a concentrated effort in the past two years to fabricate high-quality epitaxial *Present address: Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 (U.S.A.). 0921-5107/91/$3.50
and textured high-Tc superconducting thin films on various substrates [1-12]. Detailed experiments have conclusively shown the importance of texturing and epitaxial growth of grains in controlling the critical current densities of new highTc oxide superconductors. Critical current densities exceeding 106 A cm- 2 at 77 K (and zero field) have been obtained in textured films de'posited on (100) MgO and epitaxial films grown on (100) SrTiO3 substrates [1, 2, 8, 10]. There are two different ways by which superconducting thin films can exhibit a preferred orientation: (a) epitaxial growth where the atoms at the substrate-film interface form a coherent or semicoherent interface; and (b) textured growth where the grains are preferentially oriented along a particular direction after annealing at high temperatures (700-950 °C). The primary goal of this paper is to understand the texturing of thin films after oxygen annealing treatments needed to recover the superconducting properties. In our experiments, epitaxial and textured YBazCu307 films were grown on lattice-matched ((100) SrTiO3) and lattice-mismatched ((100) MgO) substrates, respectively. The orientation of epitaxial YBa2Cu307 film growth on (100) SrTiO 3 is controlled by the lattice matching of the substrate with either the "a" or "c" parameter of YBa2Cu307. Consequently, both films with the c axis parallel and perpendicular to the substrate are observed, depending on the processing conditions. In contrast to epitaxial growth on (100) SrTiO 3 substrates, the large lattice parameter mismatch (8.8%) of YBa2fu307 with MgO may prevent the formation of a coherent or semi. coherent interface. Thus, the primary mechanism for preferred orientation should occur by the anisotropic grain growth process. As the grain growth is a thermally activated process, high temperatures (about 900 °C) are required to induce high growth rates and the formation of textured © Elsevier Sequoia/Printed in The Netherlands
288 films. To understand the preferred texturing of superconducting grains with the c axis perpendicular to the substrate, we have investigated the anisotropic grain growth in YBazCu307 thin films as a function of annealing treatment. The results were compared with epitaxial growth on (100) SrTiO3 substrates.
1.0 o o
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s I1: •D
0.6
11.4
2. Experimental procedures Thin films of Y - B a - C u - O were deposited on (100) MgO and (100) SrTiO 3 substrates using the pulsed-laser evaporation technique [13]. This technique involves evaporation of material from a bulk YBa2Cu307 target and deposition of the evaporated material on to a substrate, that is placed parallel to the target. The nature of the deposition process and the factors affecting the spatial thickness and composition variations have been discussed elsewhere [14, 15]. The deposition of films was performed at pressures ranging from 10 5 to 5 x 10 -3 Torr and substrate temperatures of about 400-450 °C. Laser evaporation leads to deposition of atomically mixed chemical species with stoichiometric composition, and therefore the nucleation of the superconducting phases does not require significant diffusion of the species. The as-deposited films were non-superconducting and were further annealed both at low and high temperatures to understand the nature of the grain growth process, and to recover the superconducting properties. In the low-temperature annealing cycle, the films were heated at 650 °C for 20-40 min in oxygen, while in the high-temperature cycle the films were heated at 920 °C for 2-20 min in helium and oxygen. The cooling rates used in both of the cycles were 3 °C min ~ in an oxygen atmosphere. It may be noted that an in situ asdeposited superconducting thin film was formed at oxygen partial pressures (100-200 mTorr) and higher temperatures (500-650 °C), which require no post annealing [2].
3. Results The films annealed in a high-temperature cycle exhibited sharp reproducible superconducting transitions with zero resistance at about 85 K. Figure 1 shows a typical plot of resistivity against temperature with Tc = 85 K and the room temperature resistivity of 350 /~ff~ cm. The X-ray
0 Z
0.2
0.0
,
I 100
,
I
200
Temperature
, 300
(K)
Fig. 1. Temperature dependence of normalized electrical resistance of superconductingfilm on (100) MgO substrate. The films were textured with c axis perpendicular to the substrate and showedzero resistanceat about 85 K.
(200) M g O ~
Y-Ba-Cu-O on (100)MgO
(O06)
(005) (O03)
(007)
60
L
i
L
50
40 20
30
20
Fig. 2. X-ray diffractionof the preferentiallyoriented film formed after high-temperature annealing with its c axis normal to the (100) MgO substrate.
diffraction data revealed that the films were preferentially oriented with c axis normal to the MgO substrate surface. This strong texture can be observed in the X-ray diffraction data shown in Fig. 2, where peaks corresponding to the (00/) planes ( / = 3 , 4, 5, 6, 7) of the orthorhombic superconducting YBazCu307_x phase are only observed. The X-ray diffraction data clearly show that the [001] direction of the superconducting phase is perpendicular to the (100) MgO substrate. The relative orientation of the [100] and [010] axes of the film cannot be determined from these data. Figure 3(a) shows an ordered surface topography of an epitaxial film with the c axis parallel
289
Fig. 3. (a) SEM micrograph of an epitaxial film on SrTiO 3 substrate with its c axis parallel to the substrate. (b) SEM micrograph of textured superconducting film with c axis normal to the (100) MgO substrate. Both films were obtained after annealing at high temperatures.
to the SrTiO 3 substrate. These films were formed after deposition at 450°C and subsequent annealing at 900 °C for 20 min in oxygen. The ordered grain structure results from lattice matching of the c/3 and b axes with the SrTiO 3 substrate. As the SrTiO3 and superconducting phase possess an isotropic cubic perovskite and orthorhombic structure, respectively, the superconducting grains with either the a or c axis perpendicular to the substrate can have two mutually perpendicular epitaxial growth orientations [16, 17]. Planar transmission electron microscopy [18] revealed that the shorter dimensions of each of the rectangular grains lie along the c axis with an average "a" by "c" length ratio of the grains being 3.0. In contrast, textured superconducting grains with c axis perpendicular to the (100) MgO substrate do not exhibit an ordered surface topography. Figure 3(b) shows a typical example of the
textured films with c axis perpendicular to the (100) MgO substrate formed after high-temperature annealing at 920 °C for 20 min. This SEM micrograph shows large superconducting grains with grain size varying from 5-10 ktm, and a random-plate-like morphology indicating random orientation of the "a" and "b" axes of the grain. Figure 4(a) is a TEM micrograph from a typical region of a superconducting film with its c axis perpendicular to the (100) MgO substrate. This micrograph shows a similar shape of grains as exhibited by the SEM micrograph. All the grains are copiously twinned on the (110) plane. The high number density of 45 ° (110) twins observed in the previous figure are characteristic of films with their c axis perpendicular to the substrate [18]. As all the grains have their c axis parallel to the electron beam, the diffraction pattern from the grains correspond to the [001] zone axis as shown in Fig. 4(b). From the relative orientation of (110) twins in different grains, it can be inferred that the (001) planes of different grains that are parallel to the substrate are rotated about the [001] axis. However, it must be noted that some preferred orientation relationship between the film and the substrate has also been observed [4, 19]. In contrast to the textured nature of the grains present in high-temperature annealed samples, the low-temperature annealed grains exhibit a random orientation. Figure 5(a) shows a planar TEM micrograph with (Fig. 5(b)) its corresponding diffraction pattern for low-temperature annealed films. The films were deposited at 450 °C and further annealed in oxygen at 650 °C for 30 min. The X-ray diffraction of the sample showed the random orientation of the 1-2-3 grains after the low-temperature annealing. The X-ray results revealed the presence of a secondphase material resulting from the incomplete transformation of the deposited material into the 1-2-3 phase at low temperatures. These films exhibit a transition temperature onset at 90 K with zero resistance at 40 K. A small grain size (about 100 nm) with no preferred texturing orientation is observed in these films. 4. Discussion
From the above results it is concluded that the superconducting grains are initially small and exhibit a random orientation. Furthermore, high-
290
F
Fig. 4. (a) TEM micrograph of c-axis textured film on MgO substrate showing the extensive twinning of the grains. (b) Typical diffraction pattern from each of the grains showing the preferred texturing of the film.
temperature annealing leads to the formation of large grains with c axis perpendicular to the substrate. This preferential texturing of the grains with c axis perpendicular to the substrate can be explained by the process of grain growth, which implies high mobilities associated with the a-b (composed of (100) and (010) planes) boundaries. We have observed (Fig. 2) the higher mobility of the b axis compared to the c axis resulting in long elongated grains on films deposited on strontium titanate substrates. In addition, the textured superconducting thin films on MgO after high-temperature annealing exhibit a thin plate-
like morphology similar to the platelets of 1-2-3 crystals formed during crystal growth [20-21]. The crystals grown are generally in the form of rectangular prisms or platelets, the smallest dimension being parallel to the c axis. The preferred orientations of the growth ledges in the (100) and (010) directions may be indicative of the anisotropy in the surface energy in these materials. The process of grain growth, which occurs during high-temperature annealing, is an important factor affecting texturing in thin films. In the following, we analyse the grain growth processes,
291
Fig. 5. (a) Planar TEM micrograph of a low-temperature processed film showing the random orientation of the grain. (b) The corresponding selected area diffraction pattern.
which occur because of the difference in free energy and which provide a driving force for the migration of atoms across the grain boundary [22, 23]. This free-energy difference arises from the curved nature of the grain boundary and can be
expressed as
AG =2yVm
(1)
r
where ), is the grain boundary surface energy, Vm
292
transferred across the grain boundary with the aid of thermal fluctuations. A gradient in the chemical potential biases the probability of transfer such that grain boundary migration occurs so as to lower the free energy of the system. As our system contains grain growth in a thin film where the dimension perpendicular to the substrate is small (shown in Fig. 6(b)), we assume growth of a cylindrical grain whose growth rate can be expressed as the difference between the forward and reverse fluxes. The growth rate V of the cylindrical grain can be expressed as
J
Substrate
(a)
V =A2nlvl
2:rr~
0
Grain I
(b)
Grain 2
o,stQnce
Fig. 6. (a) Schematic free-energy diagram of an atom during the process of jumping from one grain to the other. (b) Schematic diagram to show the radial growth of a grain present in the superconducting thin film.
Azrtlv I
AG~/ exp - RT J
(2)
where A 2 refers to the sticking probability of an atom in grain 2, nl is the number of atoms in a favorable position to make a jump and v I is the vibration frequency of the atom 1. The flux in the reverse direction is similar but as the grain 2 has a lower free energy than grain 1 because of the grain boundary curvature, the exponential term is different. The reverse direction flux is given by A1 n2 Vz exp
AG~ + AG] ~-
]
(3)
The rate of grain boundary migration can be determined by assuming that at dynamic equilibrium equal and opposite fluxes of atoms are
RT
exP/RT
]
(4)
where h and r are the thickness and radius of the cylindrical grain, respectively. Assuming that AG ~ R T and substituting the value of A G, we can simplify the above equation into
V=Kr is the molar volume and r is the radius of curvature of the grain. Let us consider the effect of the driving force on the kinetics of grain boundary migration. As the motion of grain boundaries is a thermally activated process, the atoms must acquire thermal activation energy A G a to break away from grain 1. This is shown schematically in Fig. 6(a). The grain boundary flux from grain 1 to grain 2 can be expressed as an Arrhenius-type equation given by
1-exp
~
exp/~
]
(5)
where all the pre-exponential terms in the previous equation have been incorporated into the factor K. The above equation shows a linear relationship between the driving force A G and the velocity of the boundary. The variation in the grain boundary mobility with the grain boundary crystallography can be expressed in the anisotropic variation in the exponential term of the above expression. The variation in the activation energy with respect to the structure was analysed by Turnbull and Glieter [23, 24] for cubic crystals. In f.c.c, lattices, it has been observed that grain boundaries have least mobility when two lattices show a definite relationship with each other where the atoms of both lattices occupy common positions called coincidence sites. Higher values of the ratio of sites in the main lattice site per site in a coincident lattice results in higher grain boundary mobility. In superconducting thin films, we envisage that the anisotropy in the grain boundary energy and structure gives rise to different jump frequencies, atomic jump distances, etc. Though the specific reason for the anisotropy cannot be determined, experimental observations have clearly revealed the higher mobility grain boundaries in the [100] and [010] directions compared with the [001] direction.
293
a
~
1'1.~ A
.: ~
~3~s~
1.935 A
z~2~
~e~
z =c
1. ~ 6 A
•
A
•
,
•
•
•
•
,,
•
•
•
•
,,
•
•Ba
•
•
•
~'
•
'liP' Y
iV ,Ao
• Contributlonsto ~ theUnllCefl t
CU 112 B a l O 1/20 1
Cu 1 O 2
1 Cu O
¢u
Ba 1 Cu 112 O 10 1/2
Fig. 7. Atomic arrangement of (010) planes in YBa2Cu307 compound, which are parallel to the substrate for a-axisperpendicular films.
The higher mobility in different growth directions can be readily understood by analysing the '(010) and (001)lattice planes, which are parallel to the substrate for the "a"- and "c"-axis-perpendicular films respectively. As the superconducting thin films have very small dimensions perpendicular to the substrate, the grain growth processes primarily occur in planes parallel to the substrate. The schematic atomic structure of the (010) planes that are parallel to the substrate for "a"-axis-perpendicular films is shown in Fig. 7. This figure shows that the stacking sequence along the [001] direction is made of four separate repeating layers of Cu, Ba-O, Cu-O and Y. In contrast, the stacking sequence in the [100] direction consists of alternate layers of Cu-O and Ba-O-Y. As the grain growth processes occur by the addition of atoms to each layer, it is expected that the simpler stacking sequence in the [100] direction would result in higher grain boundary mobility in that direction. This is most readily observed in superconducting c-axisparallel films on SrTiO 3 substrates where the grains are rectangular in shape and the a/c dimensional ratio corresponds to about 3.0. The anisotropy in the grain boundary mobility is incorporated in the exponential factor in eqn. (5). Furthermore, for "c"-axis-perpendicular films, both the [100] and [010] directions have similar stacking sequences comprising alternate layers of Cu-O and Ba-O-Y. Thus no anisotropy in the grain boundary mobility is expected, which gives rise to grains having similar dimensions in
the [100] and [010] directions. In addition, as both [100] and [010] directions possess very high mobility, the average grain size should be larger for c-axis-parallel films for the same growth condition which is corroborated by experimental observations. It must be noted that the grain growth process is affected by the presence of a second phase including precipitates, which can pin down the movement of grain boundaries. The effect of these factors requires further consideration in these materials. Another important method of understanding the higher mobility of certain grain boundaries is to examine the grain boundary atomic structure associated with small-angle and large-angle grain boundaries in different orientations [25]. The primary orientations of the coincident-site lattice boundaries satisfy the angular relation, tan
=Mav
where ap is the misorientation angle associated with the boundary, a t the magnitude of the Burgers vector of the lattice dislocation, and av the direction perpendicular to the Burgers vector of the crystal lattice dislocations. In the a - b boundaries, their magnitudes are equal so that a~ = av. Similarly, if the boundaries are made of a and c lattice parameters, c = 3 a so that a, = c/3 = av. The different primary coincidence boundaries are obtained by giving different values to M with N = 1. Here, we show different grain boundaries in the YBa2Cu307 superconductors, which belong to a special class of boundaries known as "primary coincidence boundaries", whose energies correspond to the lowest values of the cusps in the energy against misorientation angles. Figure 8 is a symmetric primary coincidence a - b tilt boundary of misorientation angle 12.6 ° formed between two superconducting grains with N = 1 and M = 9. The (a-b) atomic planes misoriented with respect to each other are shown along with the coincident lattice points at B and C, occupied by oxygen ions. The coincident site lattice (CSL) unit cell belonging to the two grains, shown by the dashed lines ABCD (BEFC in grain 2), contains 82 lattice points with 41 occupied by oxygen ions, 16 by copper ions and the remaining are vacant sites. The relaxations of the atom positions and the coalescence of the boundary surfaces takes place to reduce the total energy
294 at
Fig. 8. Primarycoincidencesymmetrictilt boundary of the a-b type with a misorientationof 12.6°. The CSL unit cell is shown by the dashed line.
Fig. 9. Primarycoincidencesymmetrictill boundary of the a-c type with a misorientationof 12.6°.
of the configuration. The total energy consists of the strain energy associated with the boundary distortions and the surface energy of the boundary surfaces. Whereas two polygonal configurations repeat
along the a - b boundaries, those in the a-c type repeat after every n intervals, where n > 1, depending on the misorientation angle. The primary coincidence tilt boundary of misorientation, 12.6 °, with N= 1, M = 9 is shown in Fig. 9. The corners of the unit cell are occupied by oxygen ions. The grain boundary polygons repeat after four (n = 4) intervals. A comparison of the polygons of the a - b and a-c boundaries in Figs. 8 and 9 shows that copper and oxygen ions are present in the first, whereas copper, oxygen, barium or yttrium ions are present in the second. Also, a different void volume is associated with each polygon since the Burgers vector of the grain boundary dislocations is different. The grain growth responsible for the formation of textures is determined by the mobility of the grain boundaries, which in turn is influenced by their atomic structure. For the a-c or b-c boundaries, the mobility is impeded because extensive rearrangement of unlike ions is needed for diffusion and mass transport. For the same misorientation, the CSL unit cell of a - b boundaries is found to contain a smaller number of lattice points than the a-c boundaries. From the above geometrical description of the primary CSL boundaries in the layered Y - B a - C u - O oxides, the following salient features can be summarized: (a) the CSL unit cell is much larger in the a-c boundary than in the a - b type for the same misorientation and coincidence point; (b) the a - b type boundaries are characterized by only the presence of copper or oxygen ions, whereas in the a-c-type boundaries, copper, yttrium, barium and oxygen ions are present; (c) unlike a-b boundaries, a - c and b-c grain boundaries require extensive rearrangement of ions for the grain boundary mobility. Thus, a higher activation energy for mobility of the a - c boundary agrees with the experimental results shown earlier. The lower activation energy and higher mobility of specially oriented grain boundaries play an important role in the development of textured recrystallized structures. The process of grain growth is shown schematically in Fig. 10. Initially as observed in annealing at low temperatures, the grains possess a random orientation. During hightemperature annealing, not all the grains will grow at the same rate; those grains that are oriented with c-axis perpendicular to the substrate will have a higher mobility and outgrow the boundaries of the randomly oriented grains. As
295
~
a
THINFILM
SUBSTRATE (MgO)
THIN FILM
l
SUBSTRATE (MgO)
Fig. 10. Schematic diagram illustrating the process of grain growth and texturing of superconducting films on (100) MgO substrate. (a) After low-temperature annealing the grains are small and possess a random orientation; (b) after hightemperature annealing large grains with c axis perpendicular to the substrate are formed.
observed in the grain growth on SrTiO 3 substrates, the growth rate of grains with their a - b plane parallel to the substrate surface will be the fastest, that is maximum growth rates are obtained in grains which have their c axis perpendicular to the substrate. This criterion, which maximizes the grain growth rate, does not depend on the orientation of the "a" and "b" axes. Also, as the interface between the superconducting thin film and the susbtrate is incoherent, there are no interfacial restraints of ordering of the [100] and [010] directions. Thus, high-temperature annealed films should show grains with their (001) planes parallel to the substrate surface, though randomly rotated with respect to each other. It should also be noted that preferentially textured films with c axis parallel to the substrate can not be formed on lattice mismatched substrates because of restrictions in grain growth in this orientation.
5. Conclusions Superconducting Y-Ba-Cu-O thin films grown on lattice-mismatched (100) MgO substrates exhibit strong texturing with the c axis of the grains perpendicular to the substrate, but with the (001) planes randomly rotated about each other. This is in contrast to epitaxial film growth on a
lattice-matched (100) SrTiO 3 substrate, where the c axis of the film is either parallel or perpendicular to the substrate depending on the lattice matching at the processing temperature. A theory of oriented growth of superconducting thin films on lattice-mismatched substrates is proposed in which the grain boundary mobility depends on the orientation of the crystallites. During the growth process, a selection process occurs where only the crystallites in a particular orientation grow and swamp out the other crystallites. The preferred texturing of the grains with c axis perpendicular to the substrate results from the anisotropic grain growth properties; when the grains are oriented with c axis normal to the substrate, the grain growth rates are maximum. The above results have been verified by detailed planar TEM and X-ray diffraction studies as a function of annealing treatments of films on (100) MgO substrates. Atomic geometrical modelling of the grain boundary structures in the 1-2-3 oxides shows that the a - b boundary will have a higher mobility than the a - c or b - c boundaries, which are restricted by extensive rearrangement of the different ions needed for diffusion and mass transport phenomena.
Acknowledgments Part of this research is sponsored by the Office of Energy Systems Research, Division of Energy Conversion and Utilization Technologies (ECUT) programs under Subcontract 19X4337C, U.S. Department of Energy, Martin Marietta Energy Systems, Inc., Oak Ridge National Laboratories, and National Science Foundation Project 8618735.
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