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Self-organized growth of nanometric pyramids in ferrimagnetic epitaxial CoCr2O4 films
Materials Science and Engineering B 126 (2006) 212–216
Self-organized growth of nanometric pyramids in ferrimagnetic epitaxial CoCr2O4 films U. L¨uders, F. S´anchez ∗ , J. Fontcuberta Institut de Ci`encia de Materials de Barcelona, CSIC, Campus U.A.B., 08193 Bellaterra, Spain
Abstract Self-organized pyramidal islands of ferrimagnetic spinel CoCr2 O4 were epitaxially grown on MgAl2 O4 (0 0 1). The surfaces of the islands are {1 1 1} facets, so they can be considered as true three-dimensional objects. Important characteristics of the structures can be tuned by varying growth parameters. Pyramids form above a critical thickness, and the existing granular-like underlying film grow later at a reduced rate. We discuss on the growth mechanism of pyramids and we end comparing the CoCr2 O4 pyramids with the widely investigated Si–Ge dots. © 2005 Elsevier B.V. All rights reserved. Keywords: Oxide nanostructures; Self-organization; Spinels; Ferromagnetic oxides
1. Introduction There is a growing interest in complex oxides due to the functional properties of huge interest they present, for example, high temperature superconductivity, ferroelectricity or colossal magnetoresistance. Whereas epitaxial growth, physical properties, and fabrication of various types of devices are under investigation since decades, little efforts were dedicated to the self-organized fabrication of low-dimensional oxide nanostructures. In contrast, self-organized semiconductor nanostructures receive great efforts since more than 10 years ago [1,2]. Nevertheless, oxide objects having nanometric size are of high interest, for example, to investigate new properties associated to a high specific area [3], or to establish the limits to some properties when size is reduced [4]. Moreover, due to the extreme sensitivity of some properties in complex oxides to slight distortions of crystal lattices, it is of the highest interest to dispose of methods of material combination alternative to multilayer architectures. Of particular relevance is the recent report [5] of coupling of ferromagnetism and ferroelectricity in a nanocomposite film where CoFe2 O4 nanopillars were embedded in a BaTiO3 matrix, but remarkably enough the coupling was absent when the materials were stacked in an epitaxial multilayer. We, recently, found that nanometric objects of CoCr2 O4 (CCO), a ferrimagnetic spinel as CoFe2 O4 , were formed by
∗
Corresponding author. Tel.: +34 935801853; fax: +34 935805729. E-mail address: [email protected] (F. S´anchez).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.09.051
self-organization in epitaxial films deposited on (0 0 1)-oriented MgAl2 O4 (MAO) substrates [6–9]. The objects, {1 1 1}-faceted pyramids, are highly stable and its shape does not change upon further growth or annealing [6], and we proposed [6,7] that its formation is due to the high anisotropy of the surface energy of spinels [10]. We found that CCO pyramids can also be grown on MgO (0 0 1) substrates [8,9]. Here, we report progress on the investigation of the driving forces that promote its formation and we end with a comparison between CCO and Si–Ge objects. 2. Experimental CCO films were deposited on single crystalline MgAl2 O4 (0 0 1) substrates by rf magnetron sputtering. CCO and MAO are isostructural, having a spinel structure with lattice parameters aCCO = 0.833 nm and aMAO = 0.808 nm. The films were prepared at a pressure of 33 Pa (25%O2 –75%Ar), substrate temperatures of 600 and 750 ◦ C, and varied deposition time in the 15–400 min range. We note the difficulties to define accurately the thickness of thicker films because of the important roughness associated with the pyramidal growth, but appropriate calibration of the growth rate was possible with the thinnest film. This film was smooth and thick enough to present interference peaks in X-ray reflectometry measurements. From the determined thickness, a growth rate of 1.7 nm/min was deduced, so the nominal thickness (t) of the films reported are in the t = 42–680 nm range. The crystal structure, epitaxial relationship, and lattice strain state were determined by means of X-ray diffraction (XRD) in a four-circle diffractometer with Cu radiation. The film morphol-
U. L¨uders et al. / Materials Science and Engineering B 126 (2006) 212–216
ogy was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) working in tapping mode and using tips having a nominal radius of less than 10 nm. The ferromagnetic character of the films and the Curie temperature are reported elsewhere [11]. 3. Results and discussion The morphology of pyramidal CCO objects grown at 600 ◦ C is illustrated in Fig. 1. First, the SEM image (Fig. 1a) of a t = 680 nm film shows a high density of pyramidal islands. The shape and the common orientation of their base along the 1 1 0
Fig. 1. Morphology of films deposited at 600 ◦ C: (a) SEM image (3D view, tilt angle ∼70◦ along a [1 1 0] direction) of a t = 680 nm film; (b) AFM topographic image (3D view, tilt angle 70◦ along a [1 1 0] direction); (c) SEM image (2D view, after application of a derivative filter, image sides are [1 0 0] directions) of a t = 340 nm film. Scale bars in all panels correspond to 2 m.
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crystal directions, indicate that islands are faceted. Some islands are quadratic (square base) pyramids and others are hut clusters (rectangular base). Their size is remarkable, the base dimensions of bigger islands exceeding 1 and 100 nm high. Interestingly, faces of all pyramids form the same angle, estimated to be ∼55◦ , with the surface background (i.e., those areas not covered by objects). These findings were also proved by AFM analysis. We note that due to the dense morphology of high aspect ratio islands revealed in the SEM image, the AFM measurement are difficult. Thus, we present in Fig. 1b the AFM image of a t = 340 nm film, which shows islands of lower size and lower density. There are (within the 5 m × 5 m scan area) around 20 islands with lateral size in the 0.6–1 m and height in the 300–700 nm ranges. The angle between base and lateral surfaces of pyramids is confirmed to be ∼55◦ . SEM images were used to get insight on shape (quadratic or rectangular base), size, and spatial distribution of islands. The image in Fig. 3c is a 2D view after application of a derivative filter. The highest emission of secondary electrons occurs in edges between facets, so the derivative filter emphasizes the presence of edges. It evidences the perfect orientational order of islands toward [1 1 0] directions, as well as the coexistence of quadratic pyramids and hut clusters. It is also appreciated that big islands coexist with other of smaller size. Indeed, a statistical analysis revealed a bimodal size distribution [6]. Interestingly, we note the coexistence of quadratic pyramids and hut clusters in both families (small and big islands), proving that pyramids do not evolve necessarily to hut cluster objects above a critical size. Pyramids shown in Fig. 1 are of big size (base dimension even above 1 m), size distribution is bimodal, and a part of the surface remains uncovered by these islands. But morphology can be remarkably modified changing nominal thickness and deposition temperature [6,7], or substrate used [8,9]. The morphology (Fig. 2a) of nanometric objects with uniform shape and size that is found at low nominal thickness (t = 42 nm) is of particular interest. Pyramids are perfectly oriented along [1 1 0] directions, although there is not long range positional order. There are only quadratic pyramids, with a narrow base length distribution centered at around 40 nm. Moreover, XRD reciprocal space maps around asymmetric Bragg peaks indicated [7] that this film is elastically strained, strongly suggesting that nanometric pyramids in Fig. 2a are coherent (dislocation free). Indeed, we proposed that when relaxation in all islands is not simultaneous, bimodal size distributions originate due to the faster growth rate of dislocated islands [6,7]. This explanation is also supported by the single modal size distributions found in films deposited at higher temperature. Interestingly, the uniform growth of pyramids at a deposition rate of 750 ◦ C results in a fully faceted surface when islands coalesce (Fig. 2b). This particular surface, of high specific area and with the low amount of defects expected for faceted crystals, could be used as a template to grow other functional materials. We showed earlier [7] that pyramids were formed above a critical thickness, which was around 30 nm at a deposition temperature of 600 ◦ C. Below the critical thickness, morphology is granular and very similar to that of the background surface after growth of islands. Here, we pay attention to this surface, which
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Fig. 2. SEM images (3D view, tilt angle 45◦ along a [1 0 0] direction) of: (a) t = 42 nm film deposited at 600 ◦ C and (b) t = 340 nm film deposited at 750 ◦ C.
could provide relevant information on why growth of pyramids is triggered above a critical thickness and, once islands formed, why adatoms are massively incorporated to them. Extensive areas of the surface of the t = 340 nm film deposited at 600 ◦ C are
uncovered by big pyramids (Fig. 3a), and a zoom of one of these regions is in Fig. 3b. Clearly, a large number of objects of varied size constitute the background surface. Scan resolution does not allow discerning if their presumably faceted surfaces are (1 1 1) planes. This is an important point to be addressed in a future study: on one hand (1 1 1) faceted surfaces could be expected, since (1 1 1) planes are the lower energy surfaces in spinels and it should minimize island energy; on the other hand, if they are (1 1 1) faceted its chemical potential for adatom incorporation should be identical to that of big islands which are obviously (1 1 1) faceted and thus a high density of such small islands could not be explained. In Fig. 3c, a SEM image of the same film show the presence of big and small pyramids, and it is noticeable that some of the tiny islands that can be also appreciated are very close to the pyramids. We note that in homoepitaxial growth of Al (1 1 0) pyramidal big islands were also found in presence of much more tiny multilayered mounds [12]. Certainly, kinetic formation of mounds in CCO epitaxy cannot be excluded, although the regular shapes revealed by AFM (Fig. 3b) do not support it. Finally, a SEM cross-section analysis (Fig. 3d) of the t = 680 nm film shows that the underlying film, see marked lines, is around 120 nm thick. It indicates its growth continues after formation of big pyramids (critical thickness was estimated around 30 nm) in spite of the preferential attachment of adatoms to the large objects. Now, we discuss on the pyramidal shape of the CCO objects and we will end with a comparison between them and the widely investigated Si–Ge dots. As aforementioned, surface energy in spinels is highly anisotropic, being the (1 1 1) planes of lower energy [10]. XRD -scans of the CCO films on MAO (0 0 1) substrates indicate a cube-on-cube epitaxial relationship (Fig. 4a). Thus, since epitaxial growth is induced by the (0 0 1) plane of
Fig. 3. (a) AFM image (2D view, image sides are [1 0 0] direction) of a t = 340 nm film deposited at 600 ◦ C; (b) zoom of the area in the frame; (c) SEM image (3D view, tilt angle 45◦ along a [1 1 0] direction) of the same film; (d) SEM cross-section of a t = 680 nm film deposited at 600 ◦ C. Solid lines mark the surface of the background layer and its interface with the substrate.
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Fig. 5. A sketch comparing the shapes of Si–Ge dots and CCO objects of equivalent size. Aspect ratio (vertical to lateral dimensions) is at real scale. For simplicity, latter transition of Si–Ge domes to superdomes is not included.
(ii) Si–Ge pyramids bigger (base dimension) than a few 10 nm undergo a shape transition to dome-like objects that, moreover, start to grow faster. It results in coexistence of two families of islands, with different shape and size. In contrast, CCO objects are {1 1 1} faceted irrespective of size and size distribution can be single modal under appropriate growth conditions. Thus, easier controlled fabrication of true three-dimensional CCO objects is expected, which is of high interest considering possible uses of the CCO-nanostructured surfaces. As indicated in Section 1 of this paper, Zheng et al. [5] recently reported biferroicity in BaTiO3 –CoFe2 O4 nanostructures. Regarding this purpose, the peculiar growth of CCO objects could favor controlled fabrication of ferroelectric–ferromagnetism hybrid nanostructures. Fig. 4. (a) XRD -scans of the (1 1 3) reflections of a t = 340 nm film deposited at 600 ◦ C and the substrate and (b) SEM image (3D view, tilt angle 45◦ along a [1 1 0] direction) of the same film. Principal crystals directions are indicated. Edges of a pyramid are highlighted with solid lines, and one of its {1 1 1} facets is indexed.
MAO, the CCO film has a (0 0 1) texture, and therefore, a flat CCO surface should correspond to the high energy (0 0 1) crystalline plane. Although, there is an increase in the total surface area when islands form, the total surface energy can be lowered. Pyramids are {1 1 1} faceted, so their lateral surfaces intersect the (0 0 1) plane along 1 1 0 directions. Thus, the perfect orientational order of islands is a consequence of the epitaxial nature and (0 0 1) texture of the film, which in fact is also the cause of pyramidal growth. Some crystal directions and a (1 1 1) plane are indexed in Fig. 4b. Clearly, the high anisotropy in the surface energy of CCO dominates on other possible factors (total surface of the island, aspect ratio, and total length of edges) that otherwise could have an influence on the islands shape. This is not the case of the Si–Ge dots, where there is first a shape transition with growth from {1 0 5} pyramids to domes, and latter to a transition to superdomes. This is sketched in Fig. 5, where for the sake of simplicity the latter transition to Si–Ge superdomes is not shown. We note that the sketch preserves the aspect ratio of both Si–Ge dots and CCO objects. Two points have to be highlighted: (i) CCO pyramids are true three-dimensional objects as they {1 1 1} lateral surfaces are at 54.7◦ to the base, whereas the {1 0 5} facets of Si–Ge pyramids are at only 11.3◦ .
4. Summary The self-organized growth of ferrimagnetic CCO pyramidal objects has been investigated. The pyramids, which lateral surfaces are low-energy {1 1 1} facets, form when a CCO film is epitaxially grown on a substrate that imposes a (0 0 1) out-ofplane film orientation. Pyramids started to grow on an underlying granular-like layer, which has been found to increase in thickness with further growth, although there is preferential incorporation of adatoms to big pyramids. The high aspect ratio and stability of CCO pyramids favor controlled fabrication of the objects. Acknowledgements Financial support from Ministerio de Educacion y Ciencia of the Spanish Government (projects NAN2004-9094 and MAT2005-5656) and FEDER is acknowledged. References [1] V.A. Shchukin, D. Bimberg, Rev. Mod. Phys. 71 (1999) 1125. [2] B. Voigtl¨ander, Surf. Sci. Rep. 43 (2001) 127. [3] B. Sampedro, P. Crespo, A. Hernando, R. Litr´an, J.C. S´anchez L´opez, C. L´opez Cartes, A. Fern´andez, J. Ram´ırez, J. Gonz´alez Calbet, M. Vallet, Phys. Rev. Lett. 91 (2003) 237203. [4] M.W. Chu, I. Szafraniak, R. Scholz, C. Harnagea, D. Hesse, M. Alexe, U. G¨osele, Nature Mater. 3 (2004) 87.
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[5] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. Salamanca-Riba, S.R. Shinde, S.B. Ogale, F. Bai, D. Viehland, Y. Jia, D.G. Schlom, M. Wuttig, A. Roytburd, R. Ramesh, Science 303 (2004) 601. [6] U. L¨uders, F. S´anchez, J. Fontcuberta, Phys. Rev. B 70 (2004) 045403. [7] U. L¨uders, F. S´anchez, J. Fontcuberta, Appl. Phys. A 79 (2004) 93. [8] U. L¨uders, F. S´anchez, J. Fontcuberta, Appl. Phys. A 81 (2005) 103.
[9] F. S´anchez, U. L¨uders, G. Herranz, I.C. Infante, J. Fontcuberta, M.V. Garc´ıa-Cuenca, C. Ferrater, M. Varela, Nanotechnology 16 (2005) S190. [10] R.K. Mishra, G. Thomas, J. Appl. Phys. 48 (1977) 4576. [11] U. L¨uders, F. S´anchez, J. Fontcuberta, Mat. Sci. Eng. B 109 (2004) 200. [12] F. Buatier de Mongeot, W. Zhu, A. Molle, R. Buzio, C. Boragno, U. Balbusa, E.G. Wang, Z. Zhang, Phys. Rev. Lett. 91 (2003) 016102.
Self-organized growth of nanometric pyramids in ferrimagnetic epitaxial CoCr2O4 films U. L¨uders, F. S´anchez ∗ , J. Fontcuberta Institut de Ci`encia de Materials de Barcelona, CSIC, Campus U.A.B., 08193 Bellaterra, Spain
Abstract Self-organized pyramidal islands of ferrimagnetic spinel CoCr2 O4 were epitaxially grown on MgAl2 O4 (0 0 1). The surfaces of the islands are {1 1 1} facets, so they can be considered as true three-dimensional objects. Important characteristics of the structures can be tuned by varying growth parameters. Pyramids form above a critical thickness, and the existing granular-like underlying film grow later at a reduced rate. We discuss on the growth mechanism of pyramids and we end comparing the CoCr2 O4 pyramids with the widely investigated Si–Ge dots. © 2005 Elsevier B.V. All rights reserved. Keywords: Oxide nanostructures; Self-organization; Spinels; Ferromagnetic oxides
1. Introduction There is a growing interest in complex oxides due to the functional properties of huge interest they present, for example, high temperature superconductivity, ferroelectricity or colossal magnetoresistance. Whereas epitaxial growth, physical properties, and fabrication of various types of devices are under investigation since decades, little efforts were dedicated to the self-organized fabrication of low-dimensional oxide nanostructures. In contrast, self-organized semiconductor nanostructures receive great efforts since more than 10 years ago [1,2]. Nevertheless, oxide objects having nanometric size are of high interest, for example, to investigate new properties associated to a high specific area [3], or to establish the limits to some properties when size is reduced [4]. Moreover, due to the extreme sensitivity of some properties in complex oxides to slight distortions of crystal lattices, it is of the highest interest to dispose of methods of material combination alternative to multilayer architectures. Of particular relevance is the recent report [5] of coupling of ferromagnetism and ferroelectricity in a nanocomposite film where CoFe2 O4 nanopillars were embedded in a BaTiO3 matrix, but remarkably enough the coupling was absent when the materials were stacked in an epitaxial multilayer. We, recently, found that nanometric objects of CoCr2 O4 (CCO), a ferrimagnetic spinel as CoFe2 O4 , were formed by
∗
Corresponding author. Tel.: +34 935801853; fax: +34 935805729. E-mail address: [email protected] (F. S´anchez).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.09.051
self-organization in epitaxial films deposited on (0 0 1)-oriented MgAl2 O4 (MAO) substrates [6–9]. The objects, {1 1 1}-faceted pyramids, are highly stable and its shape does not change upon further growth or annealing [6], and we proposed [6,7] that its formation is due to the high anisotropy of the surface energy of spinels [10]. We found that CCO pyramids can also be grown on MgO (0 0 1) substrates [8,9]. Here, we report progress on the investigation of the driving forces that promote its formation and we end with a comparison between CCO and Si–Ge objects. 2. Experimental CCO films were deposited on single crystalline MgAl2 O4 (0 0 1) substrates by rf magnetron sputtering. CCO and MAO are isostructural, having a spinel structure with lattice parameters aCCO = 0.833 nm and aMAO = 0.808 nm. The films were prepared at a pressure of 33 Pa (25%O2 –75%Ar), substrate temperatures of 600 and 750 ◦ C, and varied deposition time in the 15–400 min range. We note the difficulties to define accurately the thickness of thicker films because of the important roughness associated with the pyramidal growth, but appropriate calibration of the growth rate was possible with the thinnest film. This film was smooth and thick enough to present interference peaks in X-ray reflectometry measurements. From the determined thickness, a growth rate of 1.7 nm/min was deduced, so the nominal thickness (t) of the films reported are in the t = 42–680 nm range. The crystal structure, epitaxial relationship, and lattice strain state were determined by means of X-ray diffraction (XRD) in a four-circle diffractometer with Cu radiation. The film morphol-
U. L¨uders et al. / Materials Science and Engineering B 126 (2006) 212–216
ogy was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) working in tapping mode and using tips having a nominal radius of less than 10 nm. The ferromagnetic character of the films and the Curie temperature are reported elsewhere [11]. 3. Results and discussion The morphology of pyramidal CCO objects grown at 600 ◦ C is illustrated in Fig. 1. First, the SEM image (Fig. 1a) of a t = 680 nm film shows a high density of pyramidal islands. The shape and the common orientation of their base along the 1 1 0
Fig. 1. Morphology of films deposited at 600 ◦ C: (a) SEM image (3D view, tilt angle ∼70◦ along a [1 1 0] direction) of a t = 680 nm film; (b) AFM topographic image (3D view, tilt angle 70◦ along a [1 1 0] direction); (c) SEM image (2D view, after application of a derivative filter, image sides are [1 0 0] directions) of a t = 340 nm film. Scale bars in all panels correspond to 2 m.
213
crystal directions, indicate that islands are faceted. Some islands are quadratic (square base) pyramids and others are hut clusters (rectangular base). Their size is remarkable, the base dimensions of bigger islands exceeding 1 and 100 nm high. Interestingly, faces of all pyramids form the same angle, estimated to be ∼55◦ , with the surface background (i.e., those areas not covered by objects). These findings were also proved by AFM analysis. We note that due to the dense morphology of high aspect ratio islands revealed in the SEM image, the AFM measurement are difficult. Thus, we present in Fig. 1b the AFM image of a t = 340 nm film, which shows islands of lower size and lower density. There are (within the 5 m × 5 m scan area) around 20 islands with lateral size in the 0.6–1 m and height in the 300–700 nm ranges. The angle between base and lateral surfaces of pyramids is confirmed to be ∼55◦ . SEM images were used to get insight on shape (quadratic or rectangular base), size, and spatial distribution of islands. The image in Fig. 3c is a 2D view after application of a derivative filter. The highest emission of secondary electrons occurs in edges between facets, so the derivative filter emphasizes the presence of edges. It evidences the perfect orientational order of islands toward [1 1 0] directions, as well as the coexistence of quadratic pyramids and hut clusters. It is also appreciated that big islands coexist with other of smaller size. Indeed, a statistical analysis revealed a bimodal size distribution [6]. Interestingly, we note the coexistence of quadratic pyramids and hut clusters in both families (small and big islands), proving that pyramids do not evolve necessarily to hut cluster objects above a critical size. Pyramids shown in Fig. 1 are of big size (base dimension even above 1 m), size distribution is bimodal, and a part of the surface remains uncovered by these islands. But morphology can be remarkably modified changing nominal thickness and deposition temperature [6,7], or substrate used [8,9]. The morphology (Fig. 2a) of nanometric objects with uniform shape and size that is found at low nominal thickness (t = 42 nm) is of particular interest. Pyramids are perfectly oriented along [1 1 0] directions, although there is not long range positional order. There are only quadratic pyramids, with a narrow base length distribution centered at around 40 nm. Moreover, XRD reciprocal space maps around asymmetric Bragg peaks indicated [7] that this film is elastically strained, strongly suggesting that nanometric pyramids in Fig. 2a are coherent (dislocation free). Indeed, we proposed that when relaxation in all islands is not simultaneous, bimodal size distributions originate due to the faster growth rate of dislocated islands [6,7]. This explanation is also supported by the single modal size distributions found in films deposited at higher temperature. Interestingly, the uniform growth of pyramids at a deposition rate of 750 ◦ C results in a fully faceted surface when islands coalesce (Fig. 2b). This particular surface, of high specific area and with the low amount of defects expected for faceted crystals, could be used as a template to grow other functional materials. We showed earlier [7] that pyramids were formed above a critical thickness, which was around 30 nm at a deposition temperature of 600 ◦ C. Below the critical thickness, morphology is granular and very similar to that of the background surface after growth of islands. Here, we pay attention to this surface, which
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Fig. 2. SEM images (3D view, tilt angle 45◦ along a [1 0 0] direction) of: (a) t = 42 nm film deposited at 600 ◦ C and (b) t = 340 nm film deposited at 750 ◦ C.
could provide relevant information on why growth of pyramids is triggered above a critical thickness and, once islands formed, why adatoms are massively incorporated to them. Extensive areas of the surface of the t = 340 nm film deposited at 600 ◦ C are
uncovered by big pyramids (Fig. 3a), and a zoom of one of these regions is in Fig. 3b. Clearly, a large number of objects of varied size constitute the background surface. Scan resolution does not allow discerning if their presumably faceted surfaces are (1 1 1) planes. This is an important point to be addressed in a future study: on one hand (1 1 1) faceted surfaces could be expected, since (1 1 1) planes are the lower energy surfaces in spinels and it should minimize island energy; on the other hand, if they are (1 1 1) faceted its chemical potential for adatom incorporation should be identical to that of big islands which are obviously (1 1 1) faceted and thus a high density of such small islands could not be explained. In Fig. 3c, a SEM image of the same film show the presence of big and small pyramids, and it is noticeable that some of the tiny islands that can be also appreciated are very close to the pyramids. We note that in homoepitaxial growth of Al (1 1 0) pyramidal big islands were also found in presence of much more tiny multilayered mounds [12]. Certainly, kinetic formation of mounds in CCO epitaxy cannot be excluded, although the regular shapes revealed by AFM (Fig. 3b) do not support it. Finally, a SEM cross-section analysis (Fig. 3d) of the t = 680 nm film shows that the underlying film, see marked lines, is around 120 nm thick. It indicates its growth continues after formation of big pyramids (critical thickness was estimated around 30 nm) in spite of the preferential attachment of adatoms to the large objects. Now, we discuss on the pyramidal shape of the CCO objects and we will end with a comparison between them and the widely investigated Si–Ge dots. As aforementioned, surface energy in spinels is highly anisotropic, being the (1 1 1) planes of lower energy [10]. XRD -scans of the CCO films on MAO (0 0 1) substrates indicate a cube-on-cube epitaxial relationship (Fig. 4a). Thus, since epitaxial growth is induced by the (0 0 1) plane of
Fig. 3. (a) AFM image (2D view, image sides are [1 0 0] direction) of a t = 340 nm film deposited at 600 ◦ C; (b) zoom of the area in the frame; (c) SEM image (3D view, tilt angle 45◦ along a [1 1 0] direction) of the same film; (d) SEM cross-section of a t = 680 nm film deposited at 600 ◦ C. Solid lines mark the surface of the background layer and its interface with the substrate.
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215
Fig. 5. A sketch comparing the shapes of Si–Ge dots and CCO objects of equivalent size. Aspect ratio (vertical to lateral dimensions) is at real scale. For simplicity, latter transition of Si–Ge domes to superdomes is not included.
(ii) Si–Ge pyramids bigger (base dimension) than a few 10 nm undergo a shape transition to dome-like objects that, moreover, start to grow faster. It results in coexistence of two families of islands, with different shape and size. In contrast, CCO objects are {1 1 1} faceted irrespective of size and size distribution can be single modal under appropriate growth conditions. Thus, easier controlled fabrication of true three-dimensional CCO objects is expected, which is of high interest considering possible uses of the CCO-nanostructured surfaces. As indicated in Section 1 of this paper, Zheng et al. [5] recently reported biferroicity in BaTiO3 –CoFe2 O4 nanostructures. Regarding this purpose, the peculiar growth of CCO objects could favor controlled fabrication of ferroelectric–ferromagnetism hybrid nanostructures. Fig. 4. (a) XRD -scans of the (1 1 3) reflections of a t = 340 nm film deposited at 600 ◦ C and the substrate and (b) SEM image (3D view, tilt angle 45◦ along a [1 1 0] direction) of the same film. Principal crystals directions are indicated. Edges of a pyramid are highlighted with solid lines, and one of its {1 1 1} facets is indexed.
MAO, the CCO film has a (0 0 1) texture, and therefore, a flat CCO surface should correspond to the high energy (0 0 1) crystalline plane. Although, there is an increase in the total surface area when islands form, the total surface energy can be lowered. Pyramids are {1 1 1} faceted, so their lateral surfaces intersect the (0 0 1) plane along 1 1 0 directions. Thus, the perfect orientational order of islands is a consequence of the epitaxial nature and (0 0 1) texture of the film, which in fact is also the cause of pyramidal growth. Some crystal directions and a (1 1 1) plane are indexed in Fig. 4b. Clearly, the high anisotropy in the surface energy of CCO dominates on other possible factors (total surface of the island, aspect ratio, and total length of edges) that otherwise could have an influence on the islands shape. This is not the case of the Si–Ge dots, where there is first a shape transition with growth from {1 0 5} pyramids to domes, and latter to a transition to superdomes. This is sketched in Fig. 5, where for the sake of simplicity the latter transition to Si–Ge superdomes is not shown. We note that the sketch preserves the aspect ratio of both Si–Ge dots and CCO objects. Two points have to be highlighted: (i) CCO pyramids are true three-dimensional objects as they {1 1 1} lateral surfaces are at 54.7◦ to the base, whereas the {1 0 5} facets of Si–Ge pyramids are at only 11.3◦ .
4. Summary The self-organized growth of ferrimagnetic CCO pyramidal objects has been investigated. The pyramids, which lateral surfaces are low-energy {1 1 1} facets, form when a CCO film is epitaxially grown on a substrate that imposes a (0 0 1) out-ofplane film orientation. Pyramids started to grow on an underlying granular-like layer, which has been found to increase in thickness with further growth, although there is preferential incorporation of adatoms to big pyramids. The high aspect ratio and stability of CCO pyramids favor controlled fabrication of the objects. Acknowledgements Financial support from Ministerio de Educacion y Ciencia of the Spanish Government (projects NAN2004-9094 and MAT2005-5656) and FEDER is acknowledged. References [1] V.A. Shchukin, D. Bimberg, Rev. Mod. Phys. 71 (1999) 1125. [2] B. Voigtl¨ander, Surf. Sci. Rep. 43 (2001) 127. [3] B. Sampedro, P. Crespo, A. Hernando, R. Litr´an, J.C. S´anchez L´opez, C. L´opez Cartes, A. Fern´andez, J. Ram´ırez, J. Gonz´alez Calbet, M. Vallet, Phys. Rev. Lett. 91 (2003) 237203. [4] M.W. Chu, I. Szafraniak, R. Scholz, C. Harnagea, D. Hesse, M. Alexe, U. G¨osele, Nature Mater. 3 (2004) 87.
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