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LaNiO3 artificial superlattices
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Journal of Crystal Growth 285 (2005) 345–351 www.elsevier.com/locate/jcrysgro
Effects of substrate temperature on the physical properties of strained BaTiO3/LaNiO3 artificial superlattices Yung-Ching Lianga, Yuan-Chang Liangb, a
National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 300, Taiwan, ROC b Department of Mechanical Engineering, Chienkuo Technology University, Changhua 500, Taiwan, ROC Received 8 July 2005; received in revised form 22 August 2005; accepted 24 August 2005 Communicated by D.P. Norton
Abstract An artificially layered perovskite composed of dielectric BaTiO3 (BTO) and conductive LaNiO3 (LNO) was grown on a LaNiO3-coated SrTiO3 (0 0 1) single crystal substrate by a radio-frequency magnetron sputtering system. The crosssectional image of transmission electron microscopy confirmed the formation of BTO/LNO superlattice structure. Moreover, the clearly main feature and satellite features from the (0 0 L) Bragg reflection of X-ray indicate the high quality of the BTO/LNO superlattice structure was formed on a LaNiO3-coated SrTiO3 substrate. All the superlatttice films deposited at 300–550 1C show a significant enhancement of dielectric constant relative to the BTO single epilayer of the same effective thickness; the higher is the deposition temperature, the larger is the dielectric constant of the films present. r 2005 Elsevier B.V. All rights reserved. PACS: 61.10.Kw; 61.10.Nz; 77.55.+f Keywords: A1. Surface structure; A3. Superlattices; B1. Perovskites; B2. Dielectric materials
1. Introduction Superlattice is a promising approach to control the strain manipulation in the ferroelectric thin films. Recent studies show the property of materials can be improved through structure Corresponding author. Tel.: +886 4 7111111X3132;
fax: +886 4 7111137. E-mail address: [email protected] (Y.-C. Liang).
modification for artificial superlattice of ferroelectric oxides [1–3]. For example, a significant dielectric enhancement in the superlattice of BaTiO3/SrTiO3 (BTO/STO) was found due to the c-axis elongation of the BTO layer induced from the lattice-mismatch strain [1,3]. The dielectric constant in Pb(Zr20Ti80)O3/ Pb(Zr80Ti20)O3 multilayerd thin film is enhanced for five times of that of the single tetragonal phase film [4].
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.08.034
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A variety of deposition techniques including molecular-beam epitaxy (MBE) [5,6], laser MBE [7], atomic layer MOCVD [8], and pulsedlaser deposition [9,10] have been successfully used to grow epitaxial superlattice films of ferroelectric oxides. For these methods, a high temperature (650 1C) is necessary to obtain epitaxial superlattice films of high quality [5–10]. Sputtering is one of the most important methods for epitaxial perovskite oxide film deposition due to high reproducibility in the chemical composition and easy process control [11]. However, radio-frequency (RF) magnetron sputtering deposition has not yet been well established in the growth of high quality of an epitaxial oxide superlattice film. Deposition parameters (such as deposition temperature and plasma power) have a strong influence on the microstructure of the films [12,13]. To obtain an epitaxial superlattice film, one must select a suitable temperature for deposition. An investigation of film structure and its correlation with growth temperature are thus of great significance for the fabrication of superlattice films in this new class. In this work, we fabricated BaTiO3/LaNiO3 (BTO/LNO) superlattice films on LaNiO3-coated SrTiO3 (0 0 1) substrates by RF magnetron sputtering at different deposition temperatures. LNO not only can be a good metallic contact material, but also can act as a lattice-matched substrate for most ferroelectric materials [14,15]. Moreover, an epitaxial LNO film can be easily prepared on various materials by RF magnetron sputtering at temperature as low as 300 1C [16]. The purpose of this work is to investigate the effect of deposition temperature on the physical properties of
BTO/LNO superlattice film by RF magnetron sputtering.
2. Experiment The SrTiO3 substrate was cleaned by supersonic rinsings with acetone and ethanol, and subsequent heating at 750 1C for 20 min in an oxygen atmosphere (2 Pa) to clean the surface [2]. A LNO epilayer (thickness 180 nm) was first deposited on a STO (0 0 1) substrate with RF magnetron sputtering for bottom electrode. The sample temperature was measured with a thermocouple in contact with the substrate. The BTO/LNO superlattice films were deposited at different temperatures (systematically from 300 to 650 1C). The design thickness of BTO and LNO sublayer is fixed at 4 and 2 nm, respectively; the total thickness of the superlattices was fixed about 60 nm, i.e., (BTO4 nm/LNO2 nm) 10. The detailed deposition conditions are summarized in Table 1. The microstructure of prepared superlattice films was characterized by high resolution X-ray scattering; the detailed experimental setup has been described elsewhere [17]. Transmission electron microscopy (TEM) was also employed to observe the cross-sectional structure of the superlattice film. The surface morphology of the films was investigated with an atomic-force microscope (AFM); these observations were conducted with a contact mode on an area 5 5 mm. To measure electrical properties, we sputtered Pt top electrodes onto the surface of superlattice films at room temperature. The dielectric properties of the superlattices were measured (HP 4192 A LF Impendence Analyzer) at room temperature.
Table 1 RF magnetron sputtering conditions for prepared superlattice films, bottom electrode, and top electrode Material
Superlattice film
LaNiO3 bottom electrode
Pt
Substrate temperature (1C) Background pressure (Pa) Working pressure (Pa) Sputtering gas (Ar/O2) Target diameter (cm) Target-substrate distance (cm)
300–650 4 105 4 4/1 5.08 7.0
550 4 105 4 4/1 5.08 7.0
25 4 103 2 Ar 2.54 5.0
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3. Results and discussion The superlattice structure was first examined by TEM. Fig. 1 exhibits the cross-sectional image of the superlattice film in the region near a LNO bottom electrode. A well layered structure is evident. The surface and interface roughness of prepared superlattice films at varied deposition temperatures was investigated by measurements of the specular X-ray reflectivity. To quantify the surface roughness of the superlattice, a best fitting of the reflectivity data were performed with BedeREFS Mercury code [17]. Table 2 shows best fitted results of a set of reflectivity curves of the superlattice
Fig. 1. TEM cross-sectional image of the superlattice film deposited on a LNO-coated SrTiO3 substrate at 550 1C.
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films deposited at varied substrate temperatures. The reflectivity curves of superlattice films deposited at 300–550 1C exhibit a typical shape in which one can find Bragg peaks separated by Kisseig fringes, giving evidence for the vertical periodicity of composition modulation(not shown here) [17,18]. In this work, superlattice film deposited at 550 1C has smooth surface and interface; moreover, quite rough surface and interface were observed with the film deposited at 300 1C. Fig. 2(a) shows crystal truncation rod (CTR) spectra along the [0 0 L] direction of the superlattice films deposited at 300–650 1C. Values of H, K and L given in this paper are expressed in reciprocal lattice units (r.l.u.) referred to the STO lattice parameter (0.3905 nm at room temperature). The intense, sharp feature centered at L ¼ 2 is the SrTiO3 (0 0 2) reflection from the substrate; the broad shoulder on the large L side is ascribed to the (0 0 2) peak of the LNO bottom electrodes. The appearance of satellite peaks beside a main peak confirmed the formation of a superlattice structure with superlattice films deposited at 300–550 1C [2,3]. However, the superlattice film deposited at 650 1C shows poor crystalline quality; there is no superlattice feature with the deposited film. In order to investigate the possible cause of this result, we examined X-ray diffraction patterns of LNO films (200 nm) on STO substrates with different growth temperatures. In Fig. 3 LNO films deposited at 300 and 550 1C exhibit well (1 0 0)-oriented perovskite structure. On the other hand, no clear single LNO phase was formed at the substrate temperature of 650 1C. An increase of substrate temperature to 650 1C yields a X-ray
Table 2 Parameters obtained from the best-fitted results of specular X-ray reflectivity (XRR) curves of BTO/LNO superlattice films deposited at various substrate temperatures Deposition temperature (1C)
s surface (nm) (XRR)
s interface (nm) (XRR)
s surface (nm) (AFM)
FWHM
300 400 500 550
0.862 0.553 0.466 0.442
0.886 0.538 0.427 0.398
0.823 0.548 0.439 0.411
0.0431 0.0281 0.021 0.0181
The relative standard deviation of the fitted data is roughness p8%. The surface roughness determined from AFM measurements was also listed in the table for comparison; The FWHM of the (0 0 2) main peak for superlattice films deposited at different substrate temperatures was also exhibited.
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Fig. 3. X-ray diffraction patterns of LaNiO3 films (200 nm) deposited on STO substrates at various temperatures.
Fig. 2. (a) Crystal truncation rod spectra along the [0 0 L] direction of superlattice films deposited at varied substrate temperatures; the arrows represent the position of mean peak of superlattices. (b) the o-scan rocking curve of the (0 0 2) main peak for the superlttice film deposited at 300 1C and (c) that at 550 1C.
diffraction pattern composed of peaks from La2NiO4 phase. This may be due to the increasing loss of Ni in the deposited film [16,19]. The formation of the LNO phase is depressed by a substitution of forming the La2NiO4 phase [20], which further deteriorates the crystalline quality of the BTO/LNO superlattice film. The modulation length of superlattice films (deposited at 300–550 1C) deduced from the position of satellite peaks by the Schuller formula [2] agrees well with the designed value (6 nm). Rocking curves across the main peak of superlattice films are shown also in Fig. 2(b) and (c). A very small mosaic spread (FWHM ¼ 0.0181) was found in the superlattice film deposited at 550 1C. A relatively larger mosaic spread (FWHM ¼ 0.0431) in the superlattice film deposited at 300 1C clearly reveals an inferior crystalline quality of the superlattice film with lower substrate temperature. The epitaxial relation between the BTO and LNO layers in the superlattice structure is demonstrated by the in-plane orientation with respect to the major axes of the STO substrate. The azimuthal (1 0 2) diffraction patterns of a series of superlattice films in the vicinity of the main feature are shown in Fig. 4. There is no main feature in the (1 0 2) diffraction pattern of the superlattice film deposited at 300 1C revealing a c-axis textured structure of the superlattice film
ARTICLE IN PRESS Y.-C. Liang, Y.-C. Liang / Journal of Crystal Growth 285 (2005) 345–351
Fig. 4. Phi scans of (1 0 2) main peak for superlattice films deposited at various substrate temperatures.
was formed. On the other hand, the clear four features at 901 intervals to each other were found in the superlattice films deposited at 400, 500, and 550 1C indicating that alignment of a- and c-axes of BTO and LNO unit cells along the STO substrate is essentially perfect. This result demonstrates the use of LNO buffer layers can effectively reduce crystalline temperature of BTO epilayers [21]. The mean c-axis lattice constant of the superlattices can be obtained from the position of main peak (Fig. 2(a)). The in-plane lattice parameter of epitaxial superlattice films deposited at 400–550 1C is determined directly from (1 0 2) diffraction patterns with an equation [22], a ¼ 1/ 2 1/2 (d2 1 0 2–d0 0 2) , in which d1 0 2 and d0 0 2 are spacings for planes (1 0 2) and (0 0 2), respectively. Fig. 5 shows the measured mean out-of-plane (c-axis) and in-plane (a-axis) lattice parameters of superlattices as a function of growth temperature. The mean c-axis lattice constant of the superlattice films deposited at 300–550 1C is larger than the weighted mean of the c-axis lattice constant of the BTO and LNO single crystal (the c-axis lattice constant for unstrained BTO is 0.405 nm and that of LNO is 0.3886 nm). Moreover, the in-plane lattice constant of epitaxial superlattice films is also smaller than that of reference BTO epilayer (40 nm). These results clearly demonstrate the
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Fig. 5. Mean out-of-plane (c-axis) and in-plane (a-axis) lattice parameters of superlattices as a function of growth temperature. Out-of-plane and in-plane lattice parameters of the single BTO epilayer (40 nm) deposited at 550 1C are also depicted. The dash represents weighted mean of the c-axis lattice constant of the BTO and LNO single crystal i.e., 0.3995 nm.
elongation of BTO lattice along [0 0 L] by the heteroepitaxial strain in the superlattice film [23]. The BTO layers were in a large in-plane compressive state by means of superlattice structure comparing to the single BTO epilayer (40 nm) [23,24]. To confirm the X-ray reflectivity results and surface morphology of superlattice films, we examined the surface roughness of deposited films with an AFM; from observations conducted with a contact mode on an area 5 5 mm, we calculated the root-mean-square (r.m.s) magnitude of surface roughness. AFM micrographs of the surface of superlattice films deposited at 300 and 550 1C are shown in Fig. 6. For comparison, the surface roughness evaluated from AFM images is listed in Table 2. Obtained from AFM data, the values of surface roughness of superlattice films agree satisfactorily with that evaluated from the best fitted results of specular X-ray reflectivity as shown in Table 2. AFM images (Fig. 6) show that the surface morphology of the superlattice film deposited at 550 1C is mostly planar and smooth, but a quite rough surface consisting of distinct three-dimensional (3-D) islands was found with the superlattice film deposited at 300 1C. The discrepancy of surface morphology may relate to
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Fig. 7. Dielectric constant and dielectric loss (tan d) as a function of measured frequency for superlattice films deposited at various substrate temperatures; dielectric constant of the single BTO epilayer (40 nm) deposited at 550 1C was also displayed for comparison.
Fig. 6. Surface morphology observed with an atomic-force microscope for superlattice films deposited at (a) 550 1C and (b) 300 1C.
the different energy of adatoms with varied substrate temperatures. Moreover, the origin of diffuse scattering of (0 0 2) rocking curves (Fig. 2(b) and (c)) may relate to the formation of 3-D islands of which the crystalline c-axis deviates slightly from the substrate normal direction [25]. The increase of a broad diffuse component i.e., a larger FWHM indicates that the growth of misaligned islands on the growth front is increasing with decreasing the substrate temperature. The observation of AFM images is in agreement with the results of FWHM of (0 0 2) main peaks.
A layer of Pt (80 nm) was subsequently deposited onto the surface of superlattice films at room temperature as a top electrode for dielectric tests. In BTO/LNO superlattice system, the dielectric properties of the superlattice film result mainly from the portion of dielectric layers, i.e., BTO layers because of the conductive property of the LNO material. Fig. 7 shows a set of dielectric constant and dielectric loss (tan d) as a function of measured frequency for superlattice films deposited at 300–550 1C. The dielectric constant of deposited films increases with increasing substrate temperature. Moreover, epitaxial superlattice films exhibit superior dielectric properties relative to that of the c-axis preferred superlattice film deposited at 300 1C. This result indicates that the crystalline quality plays an important factor to the influence on dielectric properties of superlattice films [26]. The reason for the suppression of dielectric constant with lower substrate temperature is possibly caused by cation disorder in the epitaxial ferroelectric film [27]. On the other hand, strained BTO layers of the superlattice film show significant dielectric enhancement relative to the single BTO epilayer 40 nm thick deposited at 550 1C (as shown in Fig. 7). It is reported that the dielectric enhancement is related to the soft-mode coupling with the expansion of average
ARTICLE IN PRESS Y.-C. Liang, Y.-C. Liang / Journal of Crystal Growth 285 (2005) 345–351
out-of-plane lattice parameter in the BTO/STO superlatttice [28]. Neaton et al., calculated the polarization and zone center phonons of tetragonal BTO under in-plane compressive strain by the first principles calculation; they have shown that significant polarization enhancement can be achieved in the BTO layers of BTO/STO superlattice [29]. In BTO/LNO superlattice film, the BTO layer will experience a in-plane compressive strain due to the smaller lattice parameter of the LNO layer. Hence the enhancement of dielectric constant of strained BTO/LNO superlattice films will be expected [1,2,28].
4. Conclusions Strained dielectric superlattice films have been constructed with stacking combinations of BTO and LNO layers on LNO-coated STO substrates at various substrate temperatures. Superlattice films deposited at 300–550 1C show good superlattice feature in (0 0 L) CTR spectra. However, a poor crystalline quality of superlattice film deposited at 650 1C was found due to the formation of La2NiO4 phase which suppressed the formation of LNO phase. From AFM images, the surface of the superlattice film deposited at 550 1C is mostly planar and smooth; a quite rough surface consisting of distinct 3-D islands was found with the superlattice film deposited at 300 1C. An epitaxial BTO/LNO superlattice film can be obtained at the substrate temperature of 400 1C by RF magnetron sputtering. The dielectric constant of deposited superlattice films increases with increasing substrate temperature in the range of 300–550 1C. This result indicates that the crystalline quality of a BTO/LNO superlattice improves with increasing temperature of deposition, although all films were epitaxially grown on the LNO electrode at 400–550 1C. These BTO/LNO superlattice films show a significant enhancement of dielectric constant relative to the BTO single epilayer. Our results clearly show the significant enhancement of dielectric constant of the strained BTO/LNO superlattice film can be achieved with RF magnetron sputtering.
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References [1] H. Tabata, H. Tanaka, T. Kawai, M. Okuyama, Jpn. J. Appl. Phys. Part 1 34 (1995) 544. [2] Y.C. Liang, T. B Wu, H.Y. Lee, Y.W. Hsieh, J. Appl. Phys. 96 (2004) 584. [3] J. Kim, Y. Kim, Y.S. Kim, J. Lee, L. Kim, D. Jung, Appl. Phys. Lett. 80 (2002) 3581. [4] C. Wang, Q.F. Fang, Z.G. Zhu, A.Q. Jiang, S.Y. Wang, B.L. Cheng, Z.H. Chen, Appl. Phys. Lett. 82 (2002) 2880. [5] T. Tsurumi, T. Suzuki, M. Yamane, M. Daimon, Jpn. J. Appl. Phys. 33 (1994) 5192. [6] T. Tsurumi, T. Ichikawa, T. Harigai, H. Kakemoto, S. Wada, J. Appl. Phys. 91 (2002) 2284. [7] H. Tabata, T. Kawai, Appl. Phys. Lett. 70 (1997) 321. [8] Z. Wang, S. Oda, J. Electrochem. Soc. 147 (2000) 4615. [9] G. Koebernik, W. Haessler, R. Pantou, F. Weiss, Thin Solid Films 449 (2004) 80. [10] A. Visinoiu, M. Alexe, H.N. Lee, D.N. Zakharov, A. Pignolet, D. Hesse, U. Gosele, J. Appl. Phys. 91 (2002) 10157. [11] S.O. Park, C.S. Huang, H. Cho, C.S. Kang, H. Kang, S.I. Lee, M.Y. Lee, Jpn. J. Appl. Phys. Part I 35 (1996) 1548. [12] J. Zhang, D. Cui, H. Lu, Z. Chen, Y. Zhou, L. Li, G. Yang, S. Martin, P. Hess, Jpn, J. Appl. Phys. 362 (1997) 76. [13] M. Matsuoka, K. Hoshino, K. Ono, J. Appl. Phys. 76 (1994) 1768. [14] C.M. Wu, T.B. Wu, Mater. Lett. 33 (1997) 97. [15] H. Yamada, M. Kawasaki, Y. Ogawa, Y. Tokura, Appl. Phys. Lett. 81 (2002) 4793. [16] N. Wakiya, T. Azuma, K. Shinozaki, N. Mizutani, Thin Solid Films 410 (2002) 114. [17] Y.C. Liang, T.B. Wu, H.Y. Lee, H.J. Liu, Thin Solid Films 469–470 (2004) 500. [18] Y.C. Liang, H.Y. Lee, H.J. Liu, C.K. Huang, T.B. Wu, J. Crystal Growth 279 (2005) 114. [19] C.C. Yang, M.S. Chen, T.J. Hong, C.M. Wu, J.M. Wu, T.B. Wu, Appl. Phys. Lett. 66 (1995) 2643. [20] H.Y. Lee, T.B. Wu, J. Mater. Res. 12 (1997) 3165. [21] C.H. Lin, B.M. Yen, H.C. Kuo, H. Chen, T.B. Wu, G.E. Stillman, J. Mater. Res. 15 (2000) 115. [22] Y.C. Liang, H.Y. Lee, H.J. Liu, K.F. Wu, T.B. Wu, C.H. Lee, J. Electrochem. Soc. 152 (2005) F129. [23] T. Shimuta, O. Nakagawara, T. Makino, S. Arai, H. Tabata, T. Kawai, J. Appl. Phys. 91 (2002) 2290. [24] Y. Li, Z. Hao, H. Deng, F.G. Chen, Y.R. Li, Appl. Phys. Lett. 97 (2005) 094103. [25] I.J. Lee, J.W. Kim, T.B. Hur, Y.W. Hwang, H.K. Kim, Appl. Phys. Lett. 81 (2002) 475. [26] C.M. Wu, T.B. Wu, Jpn. J. Appl. Phys. 36 (1997) 1164. [27] D. Fuchs, M. Adam, P. Schweiss, R. Schneider, J. Appl. Phys. 91 (2002) 5288. [28] H. Tabata, H. Tanaka, T. Kawai, Appl. Phys. Lett. 65 (1994) 1970. [29] J.B. Neaton, K.M. Rabe, Appl. Phys. Lett. 82 (2003) 1586.
Journal of Crystal Growth 285 (2005) 345–351 www.elsevier.com/locate/jcrysgro
Effects of substrate temperature on the physical properties of strained BaTiO3/LaNiO3 artificial superlattices Yung-Ching Lianga, Yuan-Chang Liangb, a
National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 300, Taiwan, ROC b Department of Mechanical Engineering, Chienkuo Technology University, Changhua 500, Taiwan, ROC Received 8 July 2005; received in revised form 22 August 2005; accepted 24 August 2005 Communicated by D.P. Norton
Abstract An artificially layered perovskite composed of dielectric BaTiO3 (BTO) and conductive LaNiO3 (LNO) was grown on a LaNiO3-coated SrTiO3 (0 0 1) single crystal substrate by a radio-frequency magnetron sputtering system. The crosssectional image of transmission electron microscopy confirmed the formation of BTO/LNO superlattice structure. Moreover, the clearly main feature and satellite features from the (0 0 L) Bragg reflection of X-ray indicate the high quality of the BTO/LNO superlattice structure was formed on a LaNiO3-coated SrTiO3 substrate. All the superlatttice films deposited at 300–550 1C show a significant enhancement of dielectric constant relative to the BTO single epilayer of the same effective thickness; the higher is the deposition temperature, the larger is the dielectric constant of the films present. r 2005 Elsevier B.V. All rights reserved. PACS: 61.10.Kw; 61.10.Nz; 77.55.+f Keywords: A1. Surface structure; A3. Superlattices; B1. Perovskites; B2. Dielectric materials
1. Introduction Superlattice is a promising approach to control the strain manipulation in the ferroelectric thin films. Recent studies show the property of materials can be improved through structure Corresponding author. Tel.: +886 4 7111111X3132;
fax: +886 4 7111137. E-mail address: [email protected] (Y.-C. Liang).
modification for artificial superlattice of ferroelectric oxides [1–3]. For example, a significant dielectric enhancement in the superlattice of BaTiO3/SrTiO3 (BTO/STO) was found due to the c-axis elongation of the BTO layer induced from the lattice-mismatch strain [1,3]. The dielectric constant in Pb(Zr20Ti80)O3/ Pb(Zr80Ti20)O3 multilayerd thin film is enhanced for five times of that of the single tetragonal phase film [4].
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.08.034
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A variety of deposition techniques including molecular-beam epitaxy (MBE) [5,6], laser MBE [7], atomic layer MOCVD [8], and pulsedlaser deposition [9,10] have been successfully used to grow epitaxial superlattice films of ferroelectric oxides. For these methods, a high temperature (650 1C) is necessary to obtain epitaxial superlattice films of high quality [5–10]. Sputtering is one of the most important methods for epitaxial perovskite oxide film deposition due to high reproducibility in the chemical composition and easy process control [11]. However, radio-frequency (RF) magnetron sputtering deposition has not yet been well established in the growth of high quality of an epitaxial oxide superlattice film. Deposition parameters (such as deposition temperature and plasma power) have a strong influence on the microstructure of the films [12,13]. To obtain an epitaxial superlattice film, one must select a suitable temperature for deposition. An investigation of film structure and its correlation with growth temperature are thus of great significance for the fabrication of superlattice films in this new class. In this work, we fabricated BaTiO3/LaNiO3 (BTO/LNO) superlattice films on LaNiO3-coated SrTiO3 (0 0 1) substrates by RF magnetron sputtering at different deposition temperatures. LNO not only can be a good metallic contact material, but also can act as a lattice-matched substrate for most ferroelectric materials [14,15]. Moreover, an epitaxial LNO film can be easily prepared on various materials by RF magnetron sputtering at temperature as low as 300 1C [16]. The purpose of this work is to investigate the effect of deposition temperature on the physical properties of
BTO/LNO superlattice film by RF magnetron sputtering.
2. Experiment The SrTiO3 substrate was cleaned by supersonic rinsings with acetone and ethanol, and subsequent heating at 750 1C for 20 min in an oxygen atmosphere (2 Pa) to clean the surface [2]. A LNO epilayer (thickness 180 nm) was first deposited on a STO (0 0 1) substrate with RF magnetron sputtering for bottom electrode. The sample temperature was measured with a thermocouple in contact with the substrate. The BTO/LNO superlattice films were deposited at different temperatures (systematically from 300 to 650 1C). The design thickness of BTO and LNO sublayer is fixed at 4 and 2 nm, respectively; the total thickness of the superlattices was fixed about 60 nm, i.e., (BTO4 nm/LNO2 nm) 10. The detailed deposition conditions are summarized in Table 1. The microstructure of prepared superlattice films was characterized by high resolution X-ray scattering; the detailed experimental setup has been described elsewhere [17]. Transmission electron microscopy (TEM) was also employed to observe the cross-sectional structure of the superlattice film. The surface morphology of the films was investigated with an atomic-force microscope (AFM); these observations were conducted with a contact mode on an area 5 5 mm. To measure electrical properties, we sputtered Pt top electrodes onto the surface of superlattice films at room temperature. The dielectric properties of the superlattices were measured (HP 4192 A LF Impendence Analyzer) at room temperature.
Table 1 RF magnetron sputtering conditions for prepared superlattice films, bottom electrode, and top electrode Material
Superlattice film
LaNiO3 bottom electrode
Pt
Substrate temperature (1C) Background pressure (Pa) Working pressure (Pa) Sputtering gas (Ar/O2) Target diameter (cm) Target-substrate distance (cm)
300–650 4 105 4 4/1 5.08 7.0
550 4 105 4 4/1 5.08 7.0
25 4 103 2 Ar 2.54 5.0
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3. Results and discussion The superlattice structure was first examined by TEM. Fig. 1 exhibits the cross-sectional image of the superlattice film in the region near a LNO bottom electrode. A well layered structure is evident. The surface and interface roughness of prepared superlattice films at varied deposition temperatures was investigated by measurements of the specular X-ray reflectivity. To quantify the surface roughness of the superlattice, a best fitting of the reflectivity data were performed with BedeREFS Mercury code [17]. Table 2 shows best fitted results of a set of reflectivity curves of the superlattice
Fig. 1. TEM cross-sectional image of the superlattice film deposited on a LNO-coated SrTiO3 substrate at 550 1C.
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films deposited at varied substrate temperatures. The reflectivity curves of superlattice films deposited at 300–550 1C exhibit a typical shape in which one can find Bragg peaks separated by Kisseig fringes, giving evidence for the vertical periodicity of composition modulation(not shown here) [17,18]. In this work, superlattice film deposited at 550 1C has smooth surface and interface; moreover, quite rough surface and interface were observed with the film deposited at 300 1C. Fig. 2(a) shows crystal truncation rod (CTR) spectra along the [0 0 L] direction of the superlattice films deposited at 300–650 1C. Values of H, K and L given in this paper are expressed in reciprocal lattice units (r.l.u.) referred to the STO lattice parameter (0.3905 nm at room temperature). The intense, sharp feature centered at L ¼ 2 is the SrTiO3 (0 0 2) reflection from the substrate; the broad shoulder on the large L side is ascribed to the (0 0 2) peak of the LNO bottom electrodes. The appearance of satellite peaks beside a main peak confirmed the formation of a superlattice structure with superlattice films deposited at 300–550 1C [2,3]. However, the superlattice film deposited at 650 1C shows poor crystalline quality; there is no superlattice feature with the deposited film. In order to investigate the possible cause of this result, we examined X-ray diffraction patterns of LNO films (200 nm) on STO substrates with different growth temperatures. In Fig. 3 LNO films deposited at 300 and 550 1C exhibit well (1 0 0)-oriented perovskite structure. On the other hand, no clear single LNO phase was formed at the substrate temperature of 650 1C. An increase of substrate temperature to 650 1C yields a X-ray
Table 2 Parameters obtained from the best-fitted results of specular X-ray reflectivity (XRR) curves of BTO/LNO superlattice films deposited at various substrate temperatures Deposition temperature (1C)
s surface (nm) (XRR)
s interface (nm) (XRR)
s surface (nm) (AFM)
FWHM
300 400 500 550
0.862 0.553 0.466 0.442
0.886 0.538 0.427 0.398
0.823 0.548 0.439 0.411
0.0431 0.0281 0.021 0.0181
The relative standard deviation of the fitted data is roughness p8%. The surface roughness determined from AFM measurements was also listed in the table for comparison; The FWHM of the (0 0 2) main peak for superlattice films deposited at different substrate temperatures was also exhibited.
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Fig. 3. X-ray diffraction patterns of LaNiO3 films (200 nm) deposited on STO substrates at various temperatures.
Fig. 2. (a) Crystal truncation rod spectra along the [0 0 L] direction of superlattice films deposited at varied substrate temperatures; the arrows represent the position of mean peak of superlattices. (b) the o-scan rocking curve of the (0 0 2) main peak for the superlttice film deposited at 300 1C and (c) that at 550 1C.
diffraction pattern composed of peaks from La2NiO4 phase. This may be due to the increasing loss of Ni in the deposited film [16,19]. The formation of the LNO phase is depressed by a substitution of forming the La2NiO4 phase [20], which further deteriorates the crystalline quality of the BTO/LNO superlattice film. The modulation length of superlattice films (deposited at 300–550 1C) deduced from the position of satellite peaks by the Schuller formula [2] agrees well with the designed value (6 nm). Rocking curves across the main peak of superlattice films are shown also in Fig. 2(b) and (c). A very small mosaic spread (FWHM ¼ 0.0181) was found in the superlattice film deposited at 550 1C. A relatively larger mosaic spread (FWHM ¼ 0.0431) in the superlattice film deposited at 300 1C clearly reveals an inferior crystalline quality of the superlattice film with lower substrate temperature. The epitaxial relation between the BTO and LNO layers in the superlattice structure is demonstrated by the in-plane orientation with respect to the major axes of the STO substrate. The azimuthal (1 0 2) diffraction patterns of a series of superlattice films in the vicinity of the main feature are shown in Fig. 4. There is no main feature in the (1 0 2) diffraction pattern of the superlattice film deposited at 300 1C revealing a c-axis textured structure of the superlattice film
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Fig. 4. Phi scans of (1 0 2) main peak for superlattice films deposited at various substrate temperatures.
was formed. On the other hand, the clear four features at 901 intervals to each other were found in the superlattice films deposited at 400, 500, and 550 1C indicating that alignment of a- and c-axes of BTO and LNO unit cells along the STO substrate is essentially perfect. This result demonstrates the use of LNO buffer layers can effectively reduce crystalline temperature of BTO epilayers [21]. The mean c-axis lattice constant of the superlattices can be obtained from the position of main peak (Fig. 2(a)). The in-plane lattice parameter of epitaxial superlattice films deposited at 400–550 1C is determined directly from (1 0 2) diffraction patterns with an equation [22], a ¼ 1/ 2 1/2 (d2 1 0 2–d0 0 2) , in which d1 0 2 and d0 0 2 are spacings for planes (1 0 2) and (0 0 2), respectively. Fig. 5 shows the measured mean out-of-plane (c-axis) and in-plane (a-axis) lattice parameters of superlattices as a function of growth temperature. The mean c-axis lattice constant of the superlattice films deposited at 300–550 1C is larger than the weighted mean of the c-axis lattice constant of the BTO and LNO single crystal (the c-axis lattice constant for unstrained BTO is 0.405 nm and that of LNO is 0.3886 nm). Moreover, the in-plane lattice constant of epitaxial superlattice films is also smaller than that of reference BTO epilayer (40 nm). These results clearly demonstrate the
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Fig. 5. Mean out-of-plane (c-axis) and in-plane (a-axis) lattice parameters of superlattices as a function of growth temperature. Out-of-plane and in-plane lattice parameters of the single BTO epilayer (40 nm) deposited at 550 1C are also depicted. The dash represents weighted mean of the c-axis lattice constant of the BTO and LNO single crystal i.e., 0.3995 nm.
elongation of BTO lattice along [0 0 L] by the heteroepitaxial strain in the superlattice film [23]. The BTO layers were in a large in-plane compressive state by means of superlattice structure comparing to the single BTO epilayer (40 nm) [23,24]. To confirm the X-ray reflectivity results and surface morphology of superlattice films, we examined the surface roughness of deposited films with an AFM; from observations conducted with a contact mode on an area 5 5 mm, we calculated the root-mean-square (r.m.s) magnitude of surface roughness. AFM micrographs of the surface of superlattice films deposited at 300 and 550 1C are shown in Fig. 6. For comparison, the surface roughness evaluated from AFM images is listed in Table 2. Obtained from AFM data, the values of surface roughness of superlattice films agree satisfactorily with that evaluated from the best fitted results of specular X-ray reflectivity as shown in Table 2. AFM images (Fig. 6) show that the surface morphology of the superlattice film deposited at 550 1C is mostly planar and smooth, but a quite rough surface consisting of distinct three-dimensional (3-D) islands was found with the superlattice film deposited at 300 1C. The discrepancy of surface morphology may relate to
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Fig. 7. Dielectric constant and dielectric loss (tan d) as a function of measured frequency for superlattice films deposited at various substrate temperatures; dielectric constant of the single BTO epilayer (40 nm) deposited at 550 1C was also displayed for comparison.
Fig. 6. Surface morphology observed with an atomic-force microscope for superlattice films deposited at (a) 550 1C and (b) 300 1C.
the different energy of adatoms with varied substrate temperatures. Moreover, the origin of diffuse scattering of (0 0 2) rocking curves (Fig. 2(b) and (c)) may relate to the formation of 3-D islands of which the crystalline c-axis deviates slightly from the substrate normal direction [25]. The increase of a broad diffuse component i.e., a larger FWHM indicates that the growth of misaligned islands on the growth front is increasing with decreasing the substrate temperature. The observation of AFM images is in agreement with the results of FWHM of (0 0 2) main peaks.
A layer of Pt (80 nm) was subsequently deposited onto the surface of superlattice films at room temperature as a top electrode for dielectric tests. In BTO/LNO superlattice system, the dielectric properties of the superlattice film result mainly from the portion of dielectric layers, i.e., BTO layers because of the conductive property of the LNO material. Fig. 7 shows a set of dielectric constant and dielectric loss (tan d) as a function of measured frequency for superlattice films deposited at 300–550 1C. The dielectric constant of deposited films increases with increasing substrate temperature. Moreover, epitaxial superlattice films exhibit superior dielectric properties relative to that of the c-axis preferred superlattice film deposited at 300 1C. This result indicates that the crystalline quality plays an important factor to the influence on dielectric properties of superlattice films [26]. The reason for the suppression of dielectric constant with lower substrate temperature is possibly caused by cation disorder in the epitaxial ferroelectric film [27]. On the other hand, strained BTO layers of the superlattice film show significant dielectric enhancement relative to the single BTO epilayer 40 nm thick deposited at 550 1C (as shown in Fig. 7). It is reported that the dielectric enhancement is related to the soft-mode coupling with the expansion of average
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out-of-plane lattice parameter in the BTO/STO superlatttice [28]. Neaton et al., calculated the polarization and zone center phonons of tetragonal BTO under in-plane compressive strain by the first principles calculation; they have shown that significant polarization enhancement can be achieved in the BTO layers of BTO/STO superlattice [29]. In BTO/LNO superlattice film, the BTO layer will experience a in-plane compressive strain due to the smaller lattice parameter of the LNO layer. Hence the enhancement of dielectric constant of strained BTO/LNO superlattice films will be expected [1,2,28].
4. Conclusions Strained dielectric superlattice films have been constructed with stacking combinations of BTO and LNO layers on LNO-coated STO substrates at various substrate temperatures. Superlattice films deposited at 300–550 1C show good superlattice feature in (0 0 L) CTR spectra. However, a poor crystalline quality of superlattice film deposited at 650 1C was found due to the formation of La2NiO4 phase which suppressed the formation of LNO phase. From AFM images, the surface of the superlattice film deposited at 550 1C is mostly planar and smooth; a quite rough surface consisting of distinct 3-D islands was found with the superlattice film deposited at 300 1C. An epitaxial BTO/LNO superlattice film can be obtained at the substrate temperature of 400 1C by RF magnetron sputtering. The dielectric constant of deposited superlattice films increases with increasing substrate temperature in the range of 300–550 1C. This result indicates that the crystalline quality of a BTO/LNO superlattice improves with increasing temperature of deposition, although all films were epitaxially grown on the LNO electrode at 400–550 1C. These BTO/LNO superlattice films show a significant enhancement of dielectric constant relative to the BTO single epilayer. Our results clearly show the significant enhancement of dielectric constant of the strained BTO/LNO superlattice film can be achieved with RF magnetron sputtering.
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