- Email: [email protected]
Microstructures of epitaxial La0.7Ca0.3MnO3 thin films grown on SrTiO3 and NdGaO3 substrates
ARTICLE IN PRESS
Journal of Crystal Growth 265 (2004) 241–249
Microstructures of epitaxial La0.7Ca0.3MnO3 thin films grown on SrTiO3 and NdGaO3 substrates J.Q. Hea,b, C.L. Jiab, J. Schubertc, R.H. Wanga,* a
Department of Physics & Center for Electron Microscopy, Wuhan University, Wuhan 430072, China b Institut fur Forschungszentrum Julich GmbH, D-52425 Julich, Germany . Festkorperforschung, . . . c Institut fur Forschungszentrum Julich GmbH, D-52425 Julich, Germany . Schichten und Grenzflachen, . . . Received 21 January 2004; accepted 5 February 2004 Communicated by R. Kern
Abstract Perovskite La0.7Ca0.3MnO3 (LCMO) thin films were grown epitaxially on SrTiO3 (STO) and NdGaO3 (NGO) substrates by pulsed laser deposition. The microstructure of these films was investigated by means of high-resolution and Bragg-diffraction contrast transmission electron microscopy. Due to the small lattice mismatch in the system of LCMO/NGO, the films showed a higher structural perfection than the films on STO substrates. Misfit dislocations were not detected over large areas in the LCMO film grown on NGO. In contrast, two types of misfit dislocations with Burgers vectors a/0 1 0S and a/1 1 0S were frequently observed at the LCMO/STO interface. MnO precipitates were identified in the LCMO films by combining energy-dispersive X-ray spectroscopy with electron diffraction analysis. The MnO precipitates were usually formed in the film away from the film/substrate interface. Their size and density increased with the film thickness. r 2004 Elsevier B.V. All rights reserved. PACS: 68.37.lp; 75.50.Pp Keywords: A1. Transmission electron microscopy; A3. Thin films; B1. La0.7Ca0.3MnO3; B2. Colossal magnetoresistance
1. Introduction A metal–insulator transition and large negative magnetoresistance were discovered 50 years ago in alkaline-earth substituted perovskite-type rareearth manganese oxides of the type A1xBxMnO3 with A=La, Pr, Nd and B=Ca, Sr, Ba [1,2]. In recent years, these materials have regained great *Corresponding author. Tel.: +86-27-87669170; fax: +8627-87654869. E-mail address: [email protected] (R.H. Wang).
interest for research due to their particular microstructure, magnetic and electronic properties [3–8]. Colossal magnetoresistance (CMR) was found to be caused by the magnetic-field-driven shift of a first-order metal–insulator transition in the temperature window around the ferromagnetic ordering of the Mn charges and spins. The CMR materials have basically a perovskite-derived structure with rhombohedral or orthorhombic distortion. In the double exchange mechanism [9] for CMR the Mn–O–Mn bond distance and angle are considered to play an important role in
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.02.004
ARTICLE IN PRESS 242
J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
controlling the CMR properties. The microstructure and the film thickness were also found to have a strong influence on physical properties, such as the resistivity, magnetoresistivity, and Curie temperatures [10–17]. This can be understood on the basis of the fact that the structural response to internal pressure (caused by ion-size mismatch) and lattice distortions induced by substrate constraints and defects can influence the Mn–Mn bonding distance and Mn–O–Mn bond angle, thus leading to changes of electronic and magnetic properties of the materials. In this paper, we report on the investigations of the microstructure of LCMO films grown on STO and NGO substrates by means of high-resolution transmission electron microscopy (HRTEM) combining with Bragg-diffraction analysis. In particular, our efforts focus on the study of the microstructure and defect configuration of the films and their dependence on the type of substrate and on film thickness.
2. Experiments The samples for this study were LCMO films grown under the same conditions on STO and NGO substrates by pulsed laser deposition (PLD). The pulsed laser beam with a wavelength of 248 nm was focused on the ceramic LCMO target with a repetition rate of 10 Hz and an energy density of 5 J/cm2. During film deposition, the substrate was heated to 750 C in an oxygen atmosphere at a pressure of 0.4 mbar. The deposited films were then cooled down to room temperature at 1 bar of O2. Both cross-sectional and plan-view specimens for transmission electron microscopy (TEM) observation were prepared. Cross-sectional specimens were prepared by cutting the film-covered wafer into slices. Two of the slices were glued together face to face and embedded in epoxy resin. After the glue was cured, disks with a diameter of 3 mm were obtained by cutting away excess epoxy. These disks were then ground, dimpled, polished and subsequently Ar-ion milled in a stage cooled with liquid nitrogen. Plan-view specimens were prepared by performing the above thinning
procedure only on the substrate sides. TEM and HRTEM investigations were carried out in a Philips CM20-FEG and JEOL 4000EX microscopes. The compositional homogeneity of the LCMO thin film was investigated by energydispersive X-ray spectroscopy (EDX).
3. Results 3.1. Misfit dislocations and residual misfit strain Based on the basic perovskite unit cell which will be referred to in the following description of the crystallographic features, the lattice mismatch in the system of LCMO (pseudocubic, a ¼ 0:3858 nm taken from Ref. [18]) and STO (a ¼ 0:3905 nm) and the system of LCMO and NGO (pseudocubic, a ¼ 0:3863 nm) is 1.2% and 0.13%, respectively. The lattice mismatch can be accommodated either by straining the lattices or by formation of misfit dislocations at the interface depending on the elastic properties of the materials and the film thickness. Fig. 1(a) and (b) show lowmagnification images of the LCMO films on STO and on NGO, respectively. The interfaces look atomically sharp for both film–substrate systems. In Fig. 1(a), the 70 nm LCMO film has a flat interface with the substrate and a rough top surface. In Fig. 1(b), the LCMO film on NGO shows both flat interface and top surfaces. Misfit dislocations with evident strain contrast are denoted by the vertical arrows at the interface between the LCMO film and the STO substrate, while no dislocations can be detected along the LCMO/NGO interface over large areas for the films with thickness of 70 and 100 nm. Fig. 2(a) shows a [1 0 0] lattice fringe image of a misfit dislocation in the film on STO. Drawing a Burgers circuit surrounding the dislocation core, we obtain a closure failure indicating a projected Burgers vector a [0 1 0]. It was found that these misfit dislocations always located in the film side slightly away from the interface plane. Fig. 2(b) shows a lattice image of an interface area taken along the [1 1 0] direction of STO. The Burgers circuit around the dislocation core yields a closure failure with a projected vector a=2½1 1% 0: Since the vector
ARTICLE IN PRESS J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
243
Fig. 1. Low-magnification images of 70-nm-thick LCMO films on (a) the STO substrate and (b) NGO substrate. White horizontal arrows denote the interfaces. The vertical arrows in (a) mark the contrast induced by misfit dislocations.
is not a lattice translation vector and no other related defects are found in the nearby area, the measured value must be the projected component of a perfect dislocation Burgers vector in the (1 1 0) plane. The relation between a vector b and its projected component p in a plane with the plane normal l can be expressed as: p¼b
bdl l: jlj2
ð1Þ
According to Eq. (1) Burgers vectors which most possibly occur in the system and their components in the (1 0 0) and the (1 1 0) planes are listed in Table 1. From this table it becomes evident that the displacement vector a=2½1 1% 0 observed in Fig. 2b belongs to the (1 1 0) plane component of a dislocation with the Burgers vector of either a½0 1% 0 or a[1 0 0]. Fig. 3(a) shows two misfit dislocations viewed along the [1 1 0] direction. The bottom dislocation has a projected vector of
Fig. 2. (a) [1 0 0] lattice fringe image of a misfit dislocation with a Burgers vector a [0 1 0] in the film on STO. (b) [1 1 0] image of a misfit dislocation with a projected Burgers vector a=2½1 1% 0; which is the projected component of the Burgers vector a[0 1 0].
a=2½1 1% 0 corresponding to the Burgers vector of a[1 0 0] or a½0 1% 0: The top dislocation was measured and found to possess a projected Burgers vector a=2½1 1% 2% ; which is a component of the dislocation with the Burgers vector a½1 0 1% or a½0 1% 1% :
ARTICLE IN PRESS J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
244
Table 1 Burgers vectors of the a/1 0 0S and a/1 1 0S type misfit dislocations and their components in the (1 0 0) and (1 1 0) planes Burgers vectors of dislocation
Projected vector into (1 0 0) plane
Projected vector into (1 1 0) plane
a[1 0 0] a[0 1 0] a[0 0 1] a[0 1 1] a½0 1% 1 a[1 1 0] a½1 1% 0 a[1 0 1] a½1 0 1%
0 a[0 1 0] a[0 0 1] a[0 1 1] a½0 1% 1 a[0 1 0] a½0 1% 0 a[0 0 1] a½0 0 1%
a=2½1 1% 0 a=2½1% 1 0 a[0 0 1] a=2½1% 1 2 a=2½1 1% 2 0 a½1 1% 0 a=2½1 1% 2 a=2½1 1% 2%
The indices are given referring to the notation for the primitive cubic perovskite structure.
In many cases, misfit dislocations of the a a/1 1% 0S-type were found to dissociate into two partial dislocations with Burgers vectors a=2½1 1% 0: a½1 1% 0 ¼ a=2½1 1% 0 þ a=2½1 1% 0:
ð2Þ
This dissociation was observed in the LCMO film side, as shown in Fig. 3(b). The two partials were related by a stacking fault between them. According to Frank’s rule [19], the observed dissociation reaction (2) is energetically favorable since the long-range stress energy of a dislocation is proportional to the square of the modulus of the Burgers vector: jbj2 : For the above dissociation we have, before and after reaction (2), the values of 2a2 and ð2=4 þ 2=4Þa2 ¼ a2 ; respectively. However since a/2/1 1 0S is not a lattice translation vector a stacking fault or anti-phase boundary is unavoidably generated between the partial dislocations. The possible residual mismatch strain in the LCMO film with a thickness of 270 nm on STO was estimated by a consideration of the separation distance and the Burgers vector of these misfit dislocations. With measurements over a large area along the interface we determined an averaged value of 43 nm for the separation distance between two dislocations with the Burgers vector a/1 0 0S. In determining of the averaged distance the small number of the a/1 1 0S dislocations was also
Fig. 3. (a) [1 1 0] image of an interface area including two misfit dislocations stacking along the film normal. The upper dislocation has a projected Burgers vector a=2½1 1% 2% ; which can be the component of the Burgers vector a½1 0 1% or a½0 1% 1% : The lower dislocation shows a projected component a=2½1 1% 0 of either the Burgers vector a[1 0 0] or a½0 1% 0: (b) Two partial dislocations with the Burgers vector a=2½1 1% 0 dissociated from a misfit dislocation with the Burgers vector a½1 1% 0:
considered. Assuming that the misfit strain in the film is completely released by generation of the misfit dislocations the separation distance (S) of
ARTICLE IN PRESS J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
the misfit dislocations can be calculated from the equation S ¼ b=f ;
ð3Þ
where b is the magnitude of the Burgers vector of the misfit dislocations along the basic axis and f is the lattice mismatch in the film–substrate system. Taking the values of the Burgers vector as b ¼ 0:3858 nm, the lattice parameter of LCMO, and f ¼ 0:012; we obtain a dislocation spacing of 32 nm for this system. The experimental value 43 nm is much larger than the value expected for a complete relaxation of misfit strain, indicating that part of mismatch strain still remains in the film or has to be accommodated by another mechanism.
245
connected with a column precipitate. These columnar precipitates usually occur in the film some distance away from the LCMO/STO interface. Fig. 5 shows a cross-sectional image of the columnar precipitates with two different magnifications, taken along the [1 0 0] zone axis. The precipitate column has a width of about 10 nm. The morphology of the boundaries between the
3.2. Secondary phase Fig. 4 shows a low-magnification cross-sectional image of a thick (270 nm) LCMO film on STO. In the film we can observe precipitates embedded in the film matrix. Two types of precipitates can be distinguished with respect to their morphology: large blocks denoted by the letter A in the surface layer of the film and nano-columns denoted by arrow B. The big blocks are identified as the differently oriented grains of LCMO. They are
Fig. 4. Cross-sectional low-magnification images of a 270 nm LCMO film on STO. A differently oriented LCMO grain denoted by the letter A is formed connecting with a columnar precipitate of secondary phase denoted by arrow B.
Fig. 5. Cross-sectional lattice fringe image of a precipitate in film matrix taken along the [1 0 0] direction (a) at a low magnification and (b) at a high magnification.
ARTICLE IN PRESS 246
J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
Fig. 6. Cation concentration of the LCMO film matrix and the MnO phase measured by EDX spectroscopy. S denotes the secondary phase.
precipitate and the film matrix and the dislocations at the boundaries are displayed in the high magnification image. Fig. 6 shows results of EDX spectroscopy analysis cation concentration from several precipitate areas (solid symbols) together with those (hollow symbols) from the film matrix areas calibrated by the spectrum of a standard La0.67Ca0.33MnO3 sample. It is evident that Mn is dominant in the secondary phase precipitates. The signal of Ca and La is on a negligible level. In the film matrix, the atomic ratio of La:Ca:Mn is about 0.65:0.35:1.0, showing the expected stoichiometry. Fig. 7 shows selected-area electron diffraction (SAED) patterns obtained using an aperture simultaneously covering part of the LCMO films, a precipitate and part of the STO substrates along (a) the [1 0 0] and (b) the [1 1 0] zone axis. In these two patterns the strong spots are the reflections of STO which overlap with the basic reflection of LCMO. They are indexed according to the cubic structure of STO. The less strong spots are the reflections of the orthorhombic unit cell of LCMO. For details of the indexing of the LCMO diffraction pattern see Refs. [4–6]. In addition, many weak spots, as marked by arrowheads, are also seen in the patterns. They are the reflections of the precipitate. Using the STO lattice parameter of a ¼ 0:3905 nm as a calibration standard, the weak diffraction spots can be indexed according to the MnO structure. MnO has a rock-salt-type structure with a lattice parameter of a ¼ 0:4445 nm. Therefore,
Fig. 7. SAED patterns taken along (a) the [1 0 0] and (b) [1 1 0] zone axis using an aperture covering the film and part of the STO substrate.
we conclude that the columnar precipitates are MnO. Fig. 8(a) shows a plane-view micrograph of the 270 nm LCMO film on STO substrate. There are two distinguishable structure areas embedded in the film matrix M: a large round block A and small precipitates B in the shape of short bar. The round
ARTICLE IN PRESS J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
247
a½1% 1 1 direction. Fig. 8(c) shows a lattice image of a MnO precipitate in the film matrix viewed along the film normal. The density of the precipitates in the 270 nm film is much higher than in the films with thicknesses of 70 and 100 nm. Fig. 8(d) shows a plane-view micrograph of a 100 nm LCMO film on NGO substrate. Only one type of precipitates is observed in the film matrix, which was identified as the secondary phase MnO.
4. Discussion
Fig. 8. (a) Plan-view low-magnification image of the 270 nm LCMO film on STO. The letter A in the image denotes the LCMO grain with a different orientation from the epitaxial film matrix M. The letter B marks the MnO precipitate. (b) Highmagnification lattice image showing the boundary between a/1 1 1S-oriented grain and the film matrix. (c) High-magnification lattice image of an interface area between the precipitate and the film matrix. (d) Plan-view low-magnification image of the 100 nm LCMO film on NGO. The letter B denotes the MnO precipitates.
block is the differently oriented LCMO grain. The small precipitates are the secondary phase MnO. Fig. 8(b) shows the boundary between the LCMO film matrix and a differently oriented LCMO grain. The normal of the matrix is along the [0 0 1] zone axis while that of the grain is nearly along
Our results show that the structural perfection of the LCMO films depends strongly on the film thickness and the type of substrates employed. For a lattice mismatched system, there is a critical thickness tc of film. If the thickness of the film is smaller than a critical thickness tc ; the mismatch between the substrate and film is accommodated by elastic lattice strain. When the film becomes thicker than the critical thickness tc ; the formation of misfit dislocations would be energetically more favorable than the elastic strain accommodation [20,21]. The critical thickness changes with the level of lattice mismatch and the elastic properties of the system materials. The structural perfection of the LCMO film on NGO is based on the very small lattice mismatch in this system. Since no misfit dislocations were found, the critical thickness of the LCMO for this system can be considered to be larger than the observed film thickness 100 nm. Then the small lattice mismatch is considered to be accommodated by the elastic strain in the film. In the LCMO/STO system, due to a large lattice mismatch the critical thickness of the film is much smaller than the film on NGO substrate. With increasing thickness, the growth mechanism of the film changes from two-dimensional growth to island growth [12]. If the film thickness reaches the critical value, misfit dislocations can nucleate at the edges of these islands and glide or climb to the interface. When the islands coalesce to form a continuous film a rearrangement of the dislocations is expected to minimize the difference in local stress. The nature and density of these misfit dislocations determine the level of the elastic stress
ARTICLE IN PRESS 248
J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
relief. According to the measurement of the density and Burgers vectors of the misfit dislocations, residual strain is expected in the 270 nm LCMO film on STO. MnO precipitates were observed in both films. The secondary phase MnO was reported in the Mn-excess La1xMnO3 films grown on SrTiO3 by Han and Yu-Zhang [15]. Chen et al. [11] observed the secondary phase Mn3O4 in La-deficient La1xMnO3d films on silicon substrates. Gorbenko and coworkers [22] showed that inclusions of MnOx formed in the (La,R)1xAxMnO3 (R=Pr, Nd, A=Ca, Sr, Na ) films with a small excess of Mn, while Mn3O4 exists with a high excess of manganese. In all of these works, the precipitates of Mn-rich secondary phases in the matrix films were directly related to the Mn excess in the target material. In the present study we also observed the secondary phase MnO, in spite of the fact that the target materials are of stoichiometric composition. The formation of the MnO precipitates in the film matrix can be related to a small excess of Mn due to a difference in evaporation rate for the different elements of the target. The excessive Mn is pushed to the growth surfaces of the film or the edges of film islands and accumulates during the film growth. When the concentration of the excessive Mn is large enough MnO precipitates are formed at the boundaries of the growth islands. Since the lattice parameters of MnO are larger than those of LCMO the formation of the columnar precipitates can also contribute to partially relieving the misfit strain which is incompletely accommodated by the misfit dislocation. The differently oriented LCMO grains in the surface layer of the thick film are observed to connect to the MnO precipitates. These grains can nucleate on the surface of the MnO precipitates. Since there are no epitaxial relations between the MnO precipitates and the LCMO film matrix the nucleated grains on the precipitates are randomly oriented.
5. Conclusion LCMO films grown on STO and NGO substrates by the PLD technique were investigated by
means of TEM and HRTEM. The results can be summarized as follows: *
*
*
MnO secondary phase precipitates occurred in both films. The size and density of the precipitate increase with the film thickness. Two types of misfit dislocations occurred in the interface area between LCMO film and STO substrate. The Burgers vector of the first is the a/0 1 0S type, while that of the other is a/1 1 0S. The latter dislocations are dissociated into two partial dislocations with Burgers vectors a/2/1 1 0S. No misfit dislocations occurred along the LCMO/NGO interface. The small lattice mismatch is accommodated by elastic strain in the LCMO film on NGO substrate. Residual strain remains in the 270 nm LCMO film on STO. This residual strain may be partially accommodated by the formation of the MnO precipitates.
References [1] G.H. Jonker, J.H. Van Santen, Physica XVI (1950) 337. [2] J. Vogler, Physica XX (1954) 49. [3] R. Von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [4] R.H. Wang, J.N. Gui, Y.M. Zhu, R. Moodenbaugh, Phys. Rev. B 61 (2000) 11946. [5] R.H. Wang, J.N. Gui, Y.M. Zhu, R. Moodenbaugh, Phys. Rev. B 63 (2001) 144106. [6] J.Q. He, R.H. Wang, J.N. Gui, C. Dong, Phys. Stat. Sol. B 229 (2002) 1145. [7] R.A. Rao, D. Lavric, T.K. Nath, C.B. Eom, L. Wu, F. Tsai, J. Appl. Phys. 85 (1999) 4794. [8] M. Ziese, H.C. Semmelhack, K.H. Han, S.P. Sena, H.J. Blythe, J. Appl. Phys. 91 (2002) 9930. [9] C. Zener, Phys. Rev. 82 (1951) 403. [10] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [11] G.J. Chen, Y.H. Chang, H.W. Hsu, J. Magn. Magn. Mater. 219 (2000) 317. [12] C.J. Lu, Z.L. Wang, C. Kwon, Q.X. Jia, J. Appl. Phys. 88 (2000) 4032. [13] O.I. Lebedev, G. Van Tendeloo, S. Amelinkx, B. Leibold, H.-U. Habermeier, Phys. Rev. B 58 (1998) 8065. [14] H.W. Zandbergen, S. Freisem, T. Nojima, J. Aarts, Phys. Rev. B 60 (1999) 10259. [15] K. Han, K. Yu-Zhang, Philos. Mag. B 79 (1999) 897. [16] S. Pignard, K. Yu-Zhang, Y. Leprince-Wang, K. Han, H. Vincent, J.P. Senateur, Thin Solid Films 391 (2001) 21.
ARTICLE IN PRESS J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249 [17] Y. Xin, K. Han, N. Mateeva, H. Garmestani, P.N. Kalu, K.-H. Dahmen, J. Mater. Res. 16 (2001) 3073. [18] Y.H. Li, K.A. Thomas, P.S.I.P. De Silva, L.F. Cohen, A. Goyal, M. Rajeswari, N.D. Mathur, M.G. Blamire, J.E. Evetts, T. Venkatesan, J.L. MacManus-Driscoll, J. Mater. Res. 13 (1998) 2161.
249
[19] F.C. Frank, Physica 15 (1949) 131. [20] Z.Q. Wu, B. Wang, Thin Film Growth, China Science Press, Beijing, 2001 (In Chinese). [21] J.W. Matthews, J. Vac. Sci. Technol. 12 (1975) 126. [22] Q.Yu. Gorbenko, I.E. Graboy, A.R. Kaul, H.W. Zandbergen, J. Magn. Magn. Mater. 211 (2000) 97.
Journal of Crystal Growth 265 (2004) 241–249
Microstructures of epitaxial La0.7Ca0.3MnO3 thin films grown on SrTiO3 and NdGaO3 substrates J.Q. Hea,b, C.L. Jiab, J. Schubertc, R.H. Wanga,* a
Department of Physics & Center for Electron Microscopy, Wuhan University, Wuhan 430072, China b Institut fur Forschungszentrum Julich GmbH, D-52425 Julich, Germany . Festkorperforschung, . . . c Institut fur Forschungszentrum Julich GmbH, D-52425 Julich, Germany . Schichten und Grenzflachen, . . . Received 21 January 2004; accepted 5 February 2004 Communicated by R. Kern
Abstract Perovskite La0.7Ca0.3MnO3 (LCMO) thin films were grown epitaxially on SrTiO3 (STO) and NdGaO3 (NGO) substrates by pulsed laser deposition. The microstructure of these films was investigated by means of high-resolution and Bragg-diffraction contrast transmission electron microscopy. Due to the small lattice mismatch in the system of LCMO/NGO, the films showed a higher structural perfection than the films on STO substrates. Misfit dislocations were not detected over large areas in the LCMO film grown on NGO. In contrast, two types of misfit dislocations with Burgers vectors a/0 1 0S and a/1 1 0S were frequently observed at the LCMO/STO interface. MnO precipitates were identified in the LCMO films by combining energy-dispersive X-ray spectroscopy with electron diffraction analysis. The MnO precipitates were usually formed in the film away from the film/substrate interface. Their size and density increased with the film thickness. r 2004 Elsevier B.V. All rights reserved. PACS: 68.37.lp; 75.50.Pp Keywords: A1. Transmission electron microscopy; A3. Thin films; B1. La0.7Ca0.3MnO3; B2. Colossal magnetoresistance
1. Introduction A metal–insulator transition and large negative magnetoresistance were discovered 50 years ago in alkaline-earth substituted perovskite-type rareearth manganese oxides of the type A1xBxMnO3 with A=La, Pr, Nd and B=Ca, Sr, Ba [1,2]. In recent years, these materials have regained great *Corresponding author. Tel.: +86-27-87669170; fax: +8627-87654869. E-mail address: [email protected] (R.H. Wang).
interest for research due to their particular microstructure, magnetic and electronic properties [3–8]. Colossal magnetoresistance (CMR) was found to be caused by the magnetic-field-driven shift of a first-order metal–insulator transition in the temperature window around the ferromagnetic ordering of the Mn charges and spins. The CMR materials have basically a perovskite-derived structure with rhombohedral or orthorhombic distortion. In the double exchange mechanism [9] for CMR the Mn–O–Mn bond distance and angle are considered to play an important role in
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.02.004
ARTICLE IN PRESS 242
J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
controlling the CMR properties. The microstructure and the film thickness were also found to have a strong influence on physical properties, such as the resistivity, magnetoresistivity, and Curie temperatures [10–17]. This can be understood on the basis of the fact that the structural response to internal pressure (caused by ion-size mismatch) and lattice distortions induced by substrate constraints and defects can influence the Mn–Mn bonding distance and Mn–O–Mn bond angle, thus leading to changes of electronic and magnetic properties of the materials. In this paper, we report on the investigations of the microstructure of LCMO films grown on STO and NGO substrates by means of high-resolution transmission electron microscopy (HRTEM) combining with Bragg-diffraction analysis. In particular, our efforts focus on the study of the microstructure and defect configuration of the films and their dependence on the type of substrate and on film thickness.
2. Experiments The samples for this study were LCMO films grown under the same conditions on STO and NGO substrates by pulsed laser deposition (PLD). The pulsed laser beam with a wavelength of 248 nm was focused on the ceramic LCMO target with a repetition rate of 10 Hz and an energy density of 5 J/cm2. During film deposition, the substrate was heated to 750 C in an oxygen atmosphere at a pressure of 0.4 mbar. The deposited films were then cooled down to room temperature at 1 bar of O2. Both cross-sectional and plan-view specimens for transmission electron microscopy (TEM) observation were prepared. Cross-sectional specimens were prepared by cutting the film-covered wafer into slices. Two of the slices were glued together face to face and embedded in epoxy resin. After the glue was cured, disks with a diameter of 3 mm were obtained by cutting away excess epoxy. These disks were then ground, dimpled, polished and subsequently Ar-ion milled in a stage cooled with liquid nitrogen. Plan-view specimens were prepared by performing the above thinning
procedure only on the substrate sides. TEM and HRTEM investigations were carried out in a Philips CM20-FEG and JEOL 4000EX microscopes. The compositional homogeneity of the LCMO thin film was investigated by energydispersive X-ray spectroscopy (EDX).
3. Results 3.1. Misfit dislocations and residual misfit strain Based on the basic perovskite unit cell which will be referred to in the following description of the crystallographic features, the lattice mismatch in the system of LCMO (pseudocubic, a ¼ 0:3858 nm taken from Ref. [18]) and STO (a ¼ 0:3905 nm) and the system of LCMO and NGO (pseudocubic, a ¼ 0:3863 nm) is 1.2% and 0.13%, respectively. The lattice mismatch can be accommodated either by straining the lattices or by formation of misfit dislocations at the interface depending on the elastic properties of the materials and the film thickness. Fig. 1(a) and (b) show lowmagnification images of the LCMO films on STO and on NGO, respectively. The interfaces look atomically sharp for both film–substrate systems. In Fig. 1(a), the 70 nm LCMO film has a flat interface with the substrate and a rough top surface. In Fig. 1(b), the LCMO film on NGO shows both flat interface and top surfaces. Misfit dislocations with evident strain contrast are denoted by the vertical arrows at the interface between the LCMO film and the STO substrate, while no dislocations can be detected along the LCMO/NGO interface over large areas for the films with thickness of 70 and 100 nm. Fig. 2(a) shows a [1 0 0] lattice fringe image of a misfit dislocation in the film on STO. Drawing a Burgers circuit surrounding the dislocation core, we obtain a closure failure indicating a projected Burgers vector a [0 1 0]. It was found that these misfit dislocations always located in the film side slightly away from the interface plane. Fig. 2(b) shows a lattice image of an interface area taken along the [1 1 0] direction of STO. The Burgers circuit around the dislocation core yields a closure failure with a projected vector a=2½1 1% 0: Since the vector
ARTICLE IN PRESS J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
243
Fig. 1. Low-magnification images of 70-nm-thick LCMO films on (a) the STO substrate and (b) NGO substrate. White horizontal arrows denote the interfaces. The vertical arrows in (a) mark the contrast induced by misfit dislocations.
is not a lattice translation vector and no other related defects are found in the nearby area, the measured value must be the projected component of a perfect dislocation Burgers vector in the (1 1 0) plane. The relation between a vector b and its projected component p in a plane with the plane normal l can be expressed as: p¼b
bdl l: jlj2
ð1Þ
According to Eq. (1) Burgers vectors which most possibly occur in the system and their components in the (1 0 0) and the (1 1 0) planes are listed in Table 1. From this table it becomes evident that the displacement vector a=2½1 1% 0 observed in Fig. 2b belongs to the (1 1 0) plane component of a dislocation with the Burgers vector of either a½0 1% 0 or a[1 0 0]. Fig. 3(a) shows two misfit dislocations viewed along the [1 1 0] direction. The bottom dislocation has a projected vector of
Fig. 2. (a) [1 0 0] lattice fringe image of a misfit dislocation with a Burgers vector a [0 1 0] in the film on STO. (b) [1 1 0] image of a misfit dislocation with a projected Burgers vector a=2½1 1% 0; which is the projected component of the Burgers vector a[0 1 0].
a=2½1 1% 0 corresponding to the Burgers vector of a[1 0 0] or a½0 1% 0: The top dislocation was measured and found to possess a projected Burgers vector a=2½1 1% 2% ; which is a component of the dislocation with the Burgers vector a½1 0 1% or a½0 1% 1% :
ARTICLE IN PRESS J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
244
Table 1 Burgers vectors of the a/1 0 0S and a/1 1 0S type misfit dislocations and their components in the (1 0 0) and (1 1 0) planes Burgers vectors of dislocation
Projected vector into (1 0 0) plane
Projected vector into (1 1 0) plane
a[1 0 0] a[0 1 0] a[0 0 1] a[0 1 1] a½0 1% 1 a[1 1 0] a½1 1% 0 a[1 0 1] a½1 0 1%
0 a[0 1 0] a[0 0 1] a[0 1 1] a½0 1% 1 a[0 1 0] a½0 1% 0 a[0 0 1] a½0 0 1%
a=2½1 1% 0 a=2½1% 1 0 a[0 0 1] a=2½1% 1 2 a=2½1 1% 2 0 a½1 1% 0 a=2½1 1% 2 a=2½1 1% 2%
The indices are given referring to the notation for the primitive cubic perovskite structure.
In many cases, misfit dislocations of the a a/1 1% 0S-type were found to dissociate into two partial dislocations with Burgers vectors a=2½1 1% 0: a½1 1% 0 ¼ a=2½1 1% 0 þ a=2½1 1% 0:
ð2Þ
This dissociation was observed in the LCMO film side, as shown in Fig. 3(b). The two partials were related by a stacking fault between them. According to Frank’s rule [19], the observed dissociation reaction (2) is energetically favorable since the long-range stress energy of a dislocation is proportional to the square of the modulus of the Burgers vector: jbj2 : For the above dissociation we have, before and after reaction (2), the values of 2a2 and ð2=4 þ 2=4Þa2 ¼ a2 ; respectively. However since a/2/1 1 0S is not a lattice translation vector a stacking fault or anti-phase boundary is unavoidably generated between the partial dislocations. The possible residual mismatch strain in the LCMO film with a thickness of 270 nm on STO was estimated by a consideration of the separation distance and the Burgers vector of these misfit dislocations. With measurements over a large area along the interface we determined an averaged value of 43 nm for the separation distance between two dislocations with the Burgers vector a/1 0 0S. In determining of the averaged distance the small number of the a/1 1 0S dislocations was also
Fig. 3. (a) [1 1 0] image of an interface area including two misfit dislocations stacking along the film normal. The upper dislocation has a projected Burgers vector a=2½1 1% 2% ; which can be the component of the Burgers vector a½1 0 1% or a½0 1% 1% : The lower dislocation shows a projected component a=2½1 1% 0 of either the Burgers vector a[1 0 0] or a½0 1% 0: (b) Two partial dislocations with the Burgers vector a=2½1 1% 0 dissociated from a misfit dislocation with the Burgers vector a½1 1% 0:
considered. Assuming that the misfit strain in the film is completely released by generation of the misfit dislocations the separation distance (S) of
ARTICLE IN PRESS J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
the misfit dislocations can be calculated from the equation S ¼ b=f ;
ð3Þ
where b is the magnitude of the Burgers vector of the misfit dislocations along the basic axis and f is the lattice mismatch in the film–substrate system. Taking the values of the Burgers vector as b ¼ 0:3858 nm, the lattice parameter of LCMO, and f ¼ 0:012; we obtain a dislocation spacing of 32 nm for this system. The experimental value 43 nm is much larger than the value expected for a complete relaxation of misfit strain, indicating that part of mismatch strain still remains in the film or has to be accommodated by another mechanism.
245
connected with a column precipitate. These columnar precipitates usually occur in the film some distance away from the LCMO/STO interface. Fig. 5 shows a cross-sectional image of the columnar precipitates with two different magnifications, taken along the [1 0 0] zone axis. The precipitate column has a width of about 10 nm. The morphology of the boundaries between the
3.2. Secondary phase Fig. 4 shows a low-magnification cross-sectional image of a thick (270 nm) LCMO film on STO. In the film we can observe precipitates embedded in the film matrix. Two types of precipitates can be distinguished with respect to their morphology: large blocks denoted by the letter A in the surface layer of the film and nano-columns denoted by arrow B. The big blocks are identified as the differently oriented grains of LCMO. They are
Fig. 4. Cross-sectional low-magnification images of a 270 nm LCMO film on STO. A differently oriented LCMO grain denoted by the letter A is formed connecting with a columnar precipitate of secondary phase denoted by arrow B.
Fig. 5. Cross-sectional lattice fringe image of a precipitate in film matrix taken along the [1 0 0] direction (a) at a low magnification and (b) at a high magnification.
ARTICLE IN PRESS 246
J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
Fig. 6. Cation concentration of the LCMO film matrix and the MnO phase measured by EDX spectroscopy. S denotes the secondary phase.
precipitate and the film matrix and the dislocations at the boundaries are displayed in the high magnification image. Fig. 6 shows results of EDX spectroscopy analysis cation concentration from several precipitate areas (solid symbols) together with those (hollow symbols) from the film matrix areas calibrated by the spectrum of a standard La0.67Ca0.33MnO3 sample. It is evident that Mn is dominant in the secondary phase precipitates. The signal of Ca and La is on a negligible level. In the film matrix, the atomic ratio of La:Ca:Mn is about 0.65:0.35:1.0, showing the expected stoichiometry. Fig. 7 shows selected-area electron diffraction (SAED) patterns obtained using an aperture simultaneously covering part of the LCMO films, a precipitate and part of the STO substrates along (a) the [1 0 0] and (b) the [1 1 0] zone axis. In these two patterns the strong spots are the reflections of STO which overlap with the basic reflection of LCMO. They are indexed according to the cubic structure of STO. The less strong spots are the reflections of the orthorhombic unit cell of LCMO. For details of the indexing of the LCMO diffraction pattern see Refs. [4–6]. In addition, many weak spots, as marked by arrowheads, are also seen in the patterns. They are the reflections of the precipitate. Using the STO lattice parameter of a ¼ 0:3905 nm as a calibration standard, the weak diffraction spots can be indexed according to the MnO structure. MnO has a rock-salt-type structure with a lattice parameter of a ¼ 0:4445 nm. Therefore,
Fig. 7. SAED patterns taken along (a) the [1 0 0] and (b) [1 1 0] zone axis using an aperture covering the film and part of the STO substrate.
we conclude that the columnar precipitates are MnO. Fig. 8(a) shows a plane-view micrograph of the 270 nm LCMO film on STO substrate. There are two distinguishable structure areas embedded in the film matrix M: a large round block A and small precipitates B in the shape of short bar. The round
ARTICLE IN PRESS J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
247
a½1% 1 1 direction. Fig. 8(c) shows a lattice image of a MnO precipitate in the film matrix viewed along the film normal. The density of the precipitates in the 270 nm film is much higher than in the films with thicknesses of 70 and 100 nm. Fig. 8(d) shows a plane-view micrograph of a 100 nm LCMO film on NGO substrate. Only one type of precipitates is observed in the film matrix, which was identified as the secondary phase MnO.
4. Discussion
Fig. 8. (a) Plan-view low-magnification image of the 270 nm LCMO film on STO. The letter A in the image denotes the LCMO grain with a different orientation from the epitaxial film matrix M. The letter B marks the MnO precipitate. (b) Highmagnification lattice image showing the boundary between a/1 1 1S-oriented grain and the film matrix. (c) High-magnification lattice image of an interface area between the precipitate and the film matrix. (d) Plan-view low-magnification image of the 100 nm LCMO film on NGO. The letter B denotes the MnO precipitates.
block is the differently oriented LCMO grain. The small precipitates are the secondary phase MnO. Fig. 8(b) shows the boundary between the LCMO film matrix and a differently oriented LCMO grain. The normal of the matrix is along the [0 0 1] zone axis while that of the grain is nearly along
Our results show that the structural perfection of the LCMO films depends strongly on the film thickness and the type of substrates employed. For a lattice mismatched system, there is a critical thickness tc of film. If the thickness of the film is smaller than a critical thickness tc ; the mismatch between the substrate and film is accommodated by elastic lattice strain. When the film becomes thicker than the critical thickness tc ; the formation of misfit dislocations would be energetically more favorable than the elastic strain accommodation [20,21]. The critical thickness changes with the level of lattice mismatch and the elastic properties of the system materials. The structural perfection of the LCMO film on NGO is based on the very small lattice mismatch in this system. Since no misfit dislocations were found, the critical thickness of the LCMO for this system can be considered to be larger than the observed film thickness 100 nm. Then the small lattice mismatch is considered to be accommodated by the elastic strain in the film. In the LCMO/STO system, due to a large lattice mismatch the critical thickness of the film is much smaller than the film on NGO substrate. With increasing thickness, the growth mechanism of the film changes from two-dimensional growth to island growth [12]. If the film thickness reaches the critical value, misfit dislocations can nucleate at the edges of these islands and glide or climb to the interface. When the islands coalesce to form a continuous film a rearrangement of the dislocations is expected to minimize the difference in local stress. The nature and density of these misfit dislocations determine the level of the elastic stress
ARTICLE IN PRESS 248
J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249
relief. According to the measurement of the density and Burgers vectors of the misfit dislocations, residual strain is expected in the 270 nm LCMO film on STO. MnO precipitates were observed in both films. The secondary phase MnO was reported in the Mn-excess La1xMnO3 films grown on SrTiO3 by Han and Yu-Zhang [15]. Chen et al. [11] observed the secondary phase Mn3O4 in La-deficient La1xMnO3d films on silicon substrates. Gorbenko and coworkers [22] showed that inclusions of MnOx formed in the (La,R)1xAxMnO3 (R=Pr, Nd, A=Ca, Sr, Na ) films with a small excess of Mn, while Mn3O4 exists with a high excess of manganese. In all of these works, the precipitates of Mn-rich secondary phases in the matrix films were directly related to the Mn excess in the target material. In the present study we also observed the secondary phase MnO, in spite of the fact that the target materials are of stoichiometric composition. The formation of the MnO precipitates in the film matrix can be related to a small excess of Mn due to a difference in evaporation rate for the different elements of the target. The excessive Mn is pushed to the growth surfaces of the film or the edges of film islands and accumulates during the film growth. When the concentration of the excessive Mn is large enough MnO precipitates are formed at the boundaries of the growth islands. Since the lattice parameters of MnO are larger than those of LCMO the formation of the columnar precipitates can also contribute to partially relieving the misfit strain which is incompletely accommodated by the misfit dislocation. The differently oriented LCMO grains in the surface layer of the thick film are observed to connect to the MnO precipitates. These grains can nucleate on the surface of the MnO precipitates. Since there are no epitaxial relations between the MnO precipitates and the LCMO film matrix the nucleated grains on the precipitates are randomly oriented.
5. Conclusion LCMO films grown on STO and NGO substrates by the PLD technique were investigated by
means of TEM and HRTEM. The results can be summarized as follows: *
*
*
MnO secondary phase precipitates occurred in both films. The size and density of the precipitate increase with the film thickness. Two types of misfit dislocations occurred in the interface area between LCMO film and STO substrate. The Burgers vector of the first is the a/0 1 0S type, while that of the other is a/1 1 0S. The latter dislocations are dissociated into two partial dislocations with Burgers vectors a/2/1 1 0S. No misfit dislocations occurred along the LCMO/NGO interface. The small lattice mismatch is accommodated by elastic strain in the LCMO film on NGO substrate. Residual strain remains in the 270 nm LCMO film on STO. This residual strain may be partially accommodated by the formation of the MnO precipitates.
References [1] G.H. Jonker, J.H. Van Santen, Physica XVI (1950) 337. [2] J. Vogler, Physica XX (1954) 49. [3] R. Von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [4] R.H. Wang, J.N. Gui, Y.M. Zhu, R. Moodenbaugh, Phys. Rev. B 61 (2000) 11946. [5] R.H. Wang, J.N. Gui, Y.M. Zhu, R. Moodenbaugh, Phys. Rev. B 63 (2001) 144106. [6] J.Q. He, R.H. Wang, J.N. Gui, C. Dong, Phys. Stat. Sol. B 229 (2002) 1145. [7] R.A. Rao, D. Lavric, T.K. Nath, C.B. Eom, L. Wu, F. Tsai, J. Appl. Phys. 85 (1999) 4794. [8] M. Ziese, H.C. Semmelhack, K.H. Han, S.P. Sena, H.J. Blythe, J. Appl. Phys. 91 (2002) 9930. [9] C. Zener, Phys. Rev. 82 (1951) 403. [10] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [11] G.J. Chen, Y.H. Chang, H.W. Hsu, J. Magn. Magn. Mater. 219 (2000) 317. [12] C.J. Lu, Z.L. Wang, C. Kwon, Q.X. Jia, J. Appl. Phys. 88 (2000) 4032. [13] O.I. Lebedev, G. Van Tendeloo, S. Amelinkx, B. Leibold, H.-U. Habermeier, Phys. Rev. B 58 (1998) 8065. [14] H.W. Zandbergen, S. Freisem, T. Nojima, J. Aarts, Phys. Rev. B 60 (1999) 10259. [15] K. Han, K. Yu-Zhang, Philos. Mag. B 79 (1999) 897. [16] S. Pignard, K. Yu-Zhang, Y. Leprince-Wang, K. Han, H. Vincent, J.P. Senateur, Thin Solid Films 391 (2001) 21.
ARTICLE IN PRESS J.Q. He et al. / Journal of Crystal Growth 265 (2004) 241–249 [17] Y. Xin, K. Han, N. Mateeva, H. Garmestani, P.N. Kalu, K.-H. Dahmen, J. Mater. Res. 16 (2001) 3073. [18] Y.H. Li, K.A. Thomas, P.S.I.P. De Silva, L.F. Cohen, A. Goyal, M. Rajeswari, N.D. Mathur, M.G. Blamire, J.E. Evetts, T. Venkatesan, J.L. MacManus-Driscoll, J. Mater. Res. 13 (1998) 2161.
249
[19] F.C. Frank, Physica 15 (1949) 131. [20] Z.Q. Wu, B. Wang, Thin Film Growth, China Science Press, Beijing, 2001 (In Chinese). [21] J.W. Matthews, J. Vac. Sci. Technol. 12 (1975) 126. [22] Q.Yu. Gorbenko, I.E. Graboy, A.R. Kaul, H.W. Zandbergen, J. Magn. Magn. Mater. 211 (2000) 97.