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3)O3–0.35PbTiO3 films by pulsed laser deposition
Surface & Coatings Technology 198 (2005) 400 – 405 www.elsevier.com/locate/surfcoat
Growth of highly orientated 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 films by pulsed laser deposition X.L. Zhong, L. Lu*, M.O. Lai Department of Mechanical Engineering, National University of Singapore, 9, Engineering Drive 1, Singapore 117576, Singapore Available online 9 December 2004
Abstract 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 thin films (PMN-PT) on LaNiO3 (LNO) metallic oxide electrode were successfully deposited using pulsed laser deposition technique. LaAlO3 (LAO) and SiO2/Si were employed as substrates. By controlling the operating parameters, high quality with preferred orientation of growth of PMN films on LNO were successfully fabricated. XRD Bragg scan (h–2h) of optimized PMNPT/LNO/SiO2/Si and PMN-PT/LNO/LAO films showed (100) and (200) peaks of the pseudocubic PMN-PT and LNO only indicating the nature of highly orientated out-of-plane texture. In-plan orientations of the films of PMN-PT/LNO/LAO were studied by B scan, which demonstrated cube-on-cube orientation, namely, perovskite [100]tLNO pseudo-cubic [100]tLAO [100]. The crystalline quality of the (100) orientated films were examined by rocking curves of (200) reflections. The full-width at half-maximum (FWHM) value of PMN-PT on LAO substrate is 2.48. Mechanical properties of the PMN-PT film were studied using nano-indentation technique and piezoelectric properties were characterized by a ferroelectric tester. D 2004 Elsevier B.V. All rights reserved. Keywords: Pulsed laser deposition; LaNiO3 (LNO); 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (PMN-PT)
1. Introduction Recent extensive studies on single crystal PMN and its solutions with PbTiO3(PT) have shown that these materials with a composition (75/35) near the morphotropic phase boundary have optimum electromechanical, ferroelectric and electro-optic properties and are excellent candidates for capacitors, actuators, sensors, and micromotors [1]. It is well known that single crystal PMN-PT exhibit high strain, high coupling coefficients and piezoelectric constant [2]. The polycrystalline relaxor ferroelectric thin films present a potential limitation due to their low electromechanical coupling coefficient caused by dielectric losses at the grain boundary where second phases of low permittivity (impurities or pyrochlore) might accumulate [3]. To overcome this problem, it is necessary to deposit single crystal ferroelectric thin films that are expected to have comparable ferroelectric
* Corresponding author. Fax: +65 6779 1459. E-mail address: [email protected] (L. Lu). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.131
and piezoelectric properties to their bulk single crystal counterparts. Since piezoelectric properties depend on crystalline quality of the film, the fast growing interest in thin film form of this material has been on attempting to grow single-crystal-like preferred orientation thin films by different methods such as: sol–gel, sputtering, CVD, spin-coating and PLD on different electrodes [4–8]. A major challenge is to prepare bsingle crystalT epitaxial thin films between electrodes, and integrate them so that their properties can be utilized in piezoelectric devices or MEMS. To date, little success has been achieved in obtaining high quality epitaxial perovskite PMN-PT film on a conductive electrode or substrate due to the appearance of stable pyrochlore phases such as cubic pyrochlore Pb3Nb4O13(P3N4), rhombohedral pyrochlore Pb2Nb2O7(P2N2), and tetragonal pyrochlore Pb3Nb2O8(P3N2) non-ferroelectric [9,10]. Conventional piezoelectric devices usually utilize sintered polycrystalline ceramic layers or bulk single crystals with noble metal Au or Pt electrodes. Metallic oxide LaNiO3(LNO) is of great interest because of its transport properties and its pseudocubic perovskite crystal structure. It
X.L. Zhong et al. / Surface & Coatings Technology 198 (2005) 400–405
has an electrical resistivity of a few hundred AV cm, allowing the use of the material as an electrode. On the other hand, the LaNiO3 metallic oxide electrode is known to have better fatigue property compare to other electrodes and has stronger adhesion to Si substrate compare to metallic electrodes. It is a perovskite-structured paramagnet with the pseudocubic lattice parameter of 3.85A. It is an intrinsic n-type metallic oxide with low lattice mismatch with PMNPT. In addition, since LNO tends to adopt a (100) texture on flat surfaces, it is possible to use the electrode to control the orientation of the ferroelectric films [11,12]. Many investigations have been carried out on the dielectric and piezoelectric properties of the PMN-PT system [13,14]. However, very few investigations have discussed the mechanical properties of PMN-PT ceramics especially thin films even though their main application is for piezoelectric components [15]. The present work may represent the first report on the successful deposition of epitaxial and nearly epitaxial 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (hereafter PMN-PT) thin films by pulsed laser deposition on (100) LaNiO3 metallic oxide electrodes. Pulsed laser deposition is used here to prepare LNO and PMN-PT thin films on top of LAO and SiO2/Si substrates. Properties of the as-deposited PMNPT films are characterized and discussed.
2. Experimental procedures A Lambda Physik KrF excimer pulsed-laser (wavelength 248 nm, pulse width=25 ns) operating at a frequency of 10 Hz and 2.5 J/cm2 fluence was incident on the rotating target at an angle close to 458. The target– substrate distance was 4 cm. A Pb rich PMN-PT disc from Ecertec was used as ablation target. The commercial LNO target was stoichiometric with purity of 99.9%. The deposition chamber was initially evacuated to a pressure of lower than 2105 Torr and then flushed with high
401
Fig. 2. Results from f scan showing epitaxial growth of PMN-PT.
purity oxygen to a pressure ranging from 20 to 400 mTorr. The LAO (100) and Si (100) substrates were cleaned to remove oxide layer and other impurities before deposition. The LNO electrode layers of thickness of 200 nm were grown on Si (100) and LAO (100) substrates from 300 to 650 8C under oxygen partial pressure of 20–100 mTorr. Subsequently, the PMN-PT thin films were deposited under 50–400 mTorr oxygen partial pressure and substrate temperature from 600 to 700 8C. Structures and crystal orientations of the as-deposited films were measured using an X-ray diffractometer with Cu Ka radiation (Shimadzu XRD-6000). Morphology, thickness and roughness of the thin films were analyzed using an FE-SEM (Hitachi FE-4100) and an AFM (Digital Instrument, Multimode). Mechanical properties of the PMN-PT film were studied using nano-indentation (Hysitron Nano-indentor with DI AFM). Commercially available single crystal PMN-PT (MicroFine, Singapore) was used for testing by this nano-indentation method so as to compare its mechanically properties with those of the PMN-PT films. For electrical measurements, Au dots of 3.14105 cm2 area were deposited on the top surface of
Fig. 1. h–2h X-ray diffraction patterns of PMN-PT films on LAO and Si substrates with LNO as seed layer: (a) on LAO (100) substrate, and (b) on Si(100) substrate.
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3. Results and discussion 3.1. Structures and microstructures of As-deposited films
Fig. 3. Rocking curve of (200) diffraction.
the films by sputtering technique through a shadow mask. The polarization hysteresis behavior of the ferroelectric PMN-PT thin films was measured using the Radiant Technologies Precision Workstation Ferroelectric Test System.
LNO film was first deposited on LAO (100) and Si (100) substrates at 600 8C and oxygen pressure of 50 mTorr. PMN-PT film was subsequently deposited on the LNO film. It was found out the optimum deposition conditions for growing epitaxial PMN-PT on LNO were between 600 and 700 8C, oxygen pressure at 175 to 400 mTorr with target-to-substrate distance of 4 cm by 200 mJ KrF laser source at 10 Hz reputation rate. At such conditions, the deposited films with (100) out-of-plane orientation could be obtained. The formation of preferred (100) orientation was believed to be caused by lower surface energy of the films rather than lattice matching [11]. Fig. 1(a) and (b) show the X-ray diffraction of PMN-PT/LNO/LAO(100) and PMN-PT/LNO/Si(100), respectively. It is clear that only (h00) peaks of perovskite PMN-PT and LNO can be observed besides diffraction from the LAO substrate. This observation indicates the arrangement of films and substrates as PMN-PT(100)tLNO(100)tLAO(100) and PMNPT(100)tLNO(100)tSi(100). No extra peaks from pyrochlore phases were detected. The formation of the ferroelectric perovskite phase may be favored by the presence of a
Fig. 4. FE-SEM images of the surface of LNO and PMN-PT: (a) LNO, (b) PMN-PT on LAO substrate with LNO as seed layer, (c) PMN-PT on Si substrate with LNO as seed layer.
X.L. Zhong et al. / Surface & Coatings Technology 198 (2005) 400–405
Fig. 5. AFM surface morphology for the LNO film.
suitable structural and chemical template, i.e., the (100) surface of the LNO bottom electrode [16]. The in-plan orientation of the PMN-PT film on LNO/ LAO(100) was investigated with f scan using (310) diffraction as shown in Fig. 2. The four (310) strong peaks at 908 interval clearly indicate a highly orientated inplane texture. By comparing with the orientation of the LAO substrate, the relationship between (100)PMN-PTt (100)LNOt(100)LAO could be found. The in-plan orientation of the PMN-PT films on LNO/ Si(100) was also studied using f scan. The results showed random in-plane growth of the PMN-PT films on LNO/ Si(100). The crystalline quality of the PMN-PT film of PMNPT/LNO/LAO(001) system was determined by the Rocking curve of the (200) reflection as shown in Fig. 3. The fullwidth at half-maximum (FWHM) value for PMN-PT is 2.48. It is known that the unit cell parameters of pseudocubic LAO and PMN-PT are 3.79 and 4.04 2, respectively, so that the value of lattice-mismatch is 1.84% at room temperature for LNO/LAO. This value is small and acceptable to obtain epitaxial growth. The CTE of LAO and PMNT is 11106 and 8106, respectively [17]. Thus the disorder of the crystal structure indicated by large value of FWHM might be caused by the difference in the thermal expansion coeffi-
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cients, twinned structure near the MPB composition [6] and the non-optimum deposition conditions. The surface morphologies of the LNO and PMN-PT films on LAO and Si substrates are shown in Fig. 4. It is worthwhile to notice from Fig. 4(a) that the surface of the LNO film is extremely smooth, offering a promising property for the subsequent growth of ferroelectric thin films. PMN-PT thin films show equal-axial columnar grains with grain size of about 50–100 nm. They are dense and crack free. More detailed information on the surface morphology was obtained from AFM as shown in Fig. 5. The surface is very uniform and composed of equal-axial grains with surface roughness of about 0.2 nm. This extremely low roughness is unlikely related to a threedimensional island growth. A two-dimensional layer growth mode can occur even with no lattice matching between a film and a substrate [12]. A layer-by-layer growth mode is therefore suggested. The small stable nucleuses are strongly bound to the substrate rather than to each other. Nucleation occurs all over the surface in two dimensions. After the growth of the first layer, the second layer started to grow on top of it and so on. The formation of the planner sheet resulted in a very smooth surface. Fig. 6 reveals the morphologies of the cross-sectional views of LNO and PMN-PT films, showing very dense LNO structure while columnar structure of PMN-PT. The morphological difference between these two films is possibly caused by the different growth mode of the two films. In the case of PMN-PT deposition, in contrast to the LNO film, the deposited atoms or molecules are more strongly bound to each other than to the substrate; thus the formation of islands in three dimension growth resulting in columnar structure. The layer-by-layer growth of LNO leads to very dense and smooth structure. 3.2. Mechanical properties The Meyer’s hardness of the PMN-PT film was measured with nano-indentor. The indentation load–displacement data obtained in the experiment were used to
Fig. 6. Cross-sectional morphologies of (a) LNO and (b) PMN-PT.
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X.L. Zhong et al. / Surface & Coatings Technology 198 (2005) 400–405 Table 2 Comparison of resistivities of LNO films Sample
Resistivity (AV cm)
XRD results
LNO/SiO2/Si LNO/SiO2/Si LNO/LAO
13100F2422 109.64F35.1 79.8F8.3
Amorphous High out-of-plane orientation High our-of-plane and in-plane orientation
elastic modulus, E s of the specimen is found using Eq. (4). 1 1 m2i 1 m2s ¼ þ Er Ei Es Fig. 7. Load–displacement curve from indentation experiment.
determine the Mayer’s hardness, H and Young’s modulus, E, using the method of Oliver and Pharr [18]. Meyer’s hardness can be calculated using Eq. (1). Pmax H¼ ð1Þ Ac where P max is the maximum applied load and A c is the area of contact. It is important to note that the area of contact determined from nano-indentation testing is the projected area, different from the full-contact area used in a standard Vickers hardness test [18]. Fig. 7 schematically shows a typical indentation load– displacement curve during one cycle of loading and unloading. Assuming that the initial unloading is purely elastic, the slope, termed contact stiffness, S is given by dP 2b pffiffiffiffiffi ¼ pffiffiffi Er Ac S¼ ð2Þ dh p where b=1.034 for the Berkovich indenter used in the present study and E r is the reduced modulus. Hence, pffiffiffi p dP ð3Þ Er ¼ pffiffiffiffiffiffiffiffiffi 2 Amax dh
ð4Þ
where E i =1140 GPa (diamond indenter), v i =0.07 (diamond indenter) and v s=0.2 (PMN-PT) [15]. The mechanical properties of the materials are summarized in Table 1. It can be observed that the films with preferred orientation and epitaxial have higher hardness and modulus values which are very close to those of PMN-PT single crystal, indicating that the films with preferred orientation may have excellent mechanical properties similar to those of single crystals. The good mechanical properties of the films with preferred orientation may not only be due to their good crystalline quality but also might be caused by their internal compression stress since the lattice parameter is larger than that of LNO. 3.3. Electrical properties The resistivity of LNO electrode was measured by a 4point Probe (Model 10/C, Four Dimensions). From Table 2, it can be seen that conductivity is directly related to the crystallinity and crystal orientation of the films. Amorphous structure shows low conductivity while crystalline, high conductivity. In addition, preferred orientation reduces electron scattering so that conductivity can be increased. Fig. 8 shows the P–E hysteresis loop of the epitaxial film. The measured hysteresis loop is comparable to
E r, which accounts for deformation of both the sample and the indenter can therefore be determined. The Table 1 Mechanical properties of single crystal, polycrystalline film, amorphous film and films with preferred orientation Sample
Hardness (GPa)
Reduce modules (GPa)
Elastic modules (GPa)
PMN-PT single crystal PMN-PT/SiO2/Si preferred oriented film PMN-PT/LAO epitaxial film PMN-PT/SiO2/Si polycrystalline film PMN-PT/SiO2/Si amorphous film
6.54F0.18
118.46F5.4
126.81F6.4
6.14F0.86
118.05F6.46
126.33F7.68
5.71F0.65
89.03F6.64
92.68F7.46
3.03F0.23
92.53F9.45
96.69F10.58
3.37F0.69
86.00F7.61
89.28F8.51
Fig. 8. Ferroelectric hysteresis loop of the epitaxial PMN-PT(100) film on LAO(100) substrate with LNO(100) as bottom electrode and Au dots as top electrode.
X.L. Zhong et al. / Surface & Coatings Technology 198 (2005) 400–405
published results [19–21] where SRO and Pt electrodes were used. The saturation polarization at 30 V is 149 AC/ cm2, remnant polarization P r, 10.8 AC/cm2 and coercive field E c, 182 kV/cm. The excellent epitaxial quality of the thin film might have contributed to the high polarization values. The relatively large coercive field might be due to the presence of a high density of small-angle grain boundaries. It is can be noted that the hysteresis loop is a little asymmetric in shape. This means an internal bias field has built up in the PMN-PT film due to an asymmetric space-charge distribution.
4. Conclusions Epitaxial PMN-PT(100) films with perovskite structure have been successfully deposited on LAO (100) substrate with (100) LNO as bottom electrode by pulsed laser technique. Although preferred out-of-plane growth of PMN-PT has been obtained on Si(100) substrate, in-plane orientation is random. The LNO promotes the preferred growth of PMN-PT films. The surface of the LNO film is atomically smooth which is favorable in reducing the presence of defects in overlayers and hence improving the ferroelectric properties of the PMN-PT films. The LNO film with preferred orientation exhibits good electrical conductivity, which is a promising characteristic for it to be used as an electrode. The mechanical properties are excellent and are comparable to those of single crystals. High polarization has been obtained at the hysteresis test.
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[3] S.Y. Lee, M.C.C. Custodio, H.J. Lim, R.S. Feigelson, J.-P. Maria, J. Cryst. Growth 226 (2001) 247. [4] D. Lavric, R.A. Rao, Q. Gan, J.J. Krajewski, C._B. Eom, Integr. Ferroelectr. 21 (1998) 499. [5] P.K. Baumann, S.K. Streiffer, G.R. Bai, K. Ghosh, O. Auciello, C. Thompson, S. Stemmer, R.A. Rao, C.-B. Eom, F. Xu, S. TrolierMckinstry, D.-J. Kim, J.-P. Maria, A.I. Kingon, Integr. Ferroelectr. 35 (2001) 151. [6] S.D. Bu, M.K. Lee, C.B. Eom, W. Tian, X.Q. Pan, S.K. Streiffer, J.J. Krajewski, Appl. Phys. Lett. 79 (1) (2001) 3482. [7] J.H. Park, S. Trolier-Mckinstry, J. Mater. Res. 16 (1) (2001) 268. [8] J.T. Wang, F. Tang, W.D. Brown, J.D. Fan, K. Lian, Int. J. Mod. Phys. B 13 (29) (1999) 30. [9] S.-Y. Chen, C.-M. Wang, S.-Y. Cheng, J. Am. Ceram. Soc. 74 (10) (1991) 2506. [10] N. Wakiya, K. Shinozaki, N. Mizutani, Thin Solid Films 409 (2) (2002) 248. [11] B.G. Chae, Y.S. Yang, S.H. Jang, S.J. Lee, S.H. Kim, W.S. Baek, S.C. Kwon, Thin Solid Films 401 (2002) 107. [12] S.S. Kim, T.S. Kang, J.H. Je, Thin Solid Films 405 (2002) 117. [13] L. Eric, Cross, Active Control: Actuators (1998 Jul. 6–9). [14] J. Kelly, M. Leonard, C. Tantigate, A. Safari, J. Am. Ceram. Soc. 80 (4) (1997) 957. [15] M.-S. Kim, J.G. Fisher, H.-Y. Lee, S.-J.L. Kang, J. Am. Ceram. Soc. 86 (11) (2003) 211. [16] T.J. Zhu, L. Lu, Appl. Phys., A (in press). [17] J.-P. Maria, W. Hackenberger, S. Trolier-McKinstry, J. Appl. Phys. 84 (9) (1998) 5147. [18] W.C. Olive, G.M. Phar, J. Mater. Res. 7 (6) (1992) 1564. [19] E. Fribourg-Blanc, E. Cattan, D. Remiens, M. Dupont, D. Osmont, J. Phys., IV France 11 (2001) 145. [20] J.-P. Maria, W. Hackenberger, S. Trolier-McKinstry, J. Appl. Phys. 84 (9) (1998) 5147. [21] J.-P. Maria, W. Hackenberger, S. Trolier-McKinstry, Mater. Res. Symp. Proc. 493 (1998) 421.
Further reading References [1] J. Levoska, M. Tyunina, A. Sternberg, S. Leppavuori, Appl. Phys., A 70 (2000) 269. [2] A.R. James, S. Priya, K. Uchino, K. Srinivas, J. Appl. Phys. 90 (7) (2001) 3504.
[1] A.C. Fischer-Cripps, Nano-indentation, Springer-Verlag, New York, 2002.
Growth of highly orientated 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 films by pulsed laser deposition X.L. Zhong, L. Lu*, M.O. Lai Department of Mechanical Engineering, National University of Singapore, 9, Engineering Drive 1, Singapore 117576, Singapore Available online 9 December 2004
Abstract 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 thin films (PMN-PT) on LaNiO3 (LNO) metallic oxide electrode were successfully deposited using pulsed laser deposition technique. LaAlO3 (LAO) and SiO2/Si were employed as substrates. By controlling the operating parameters, high quality with preferred orientation of growth of PMN films on LNO were successfully fabricated. XRD Bragg scan (h–2h) of optimized PMNPT/LNO/SiO2/Si and PMN-PT/LNO/LAO films showed (100) and (200) peaks of the pseudocubic PMN-PT and LNO only indicating the nature of highly orientated out-of-plane texture. In-plan orientations of the films of PMN-PT/LNO/LAO were studied by B scan, which demonstrated cube-on-cube orientation, namely, perovskite [100]tLNO pseudo-cubic [100]tLAO [100]. The crystalline quality of the (100) orientated films were examined by rocking curves of (200) reflections. The full-width at half-maximum (FWHM) value of PMN-PT on LAO substrate is 2.48. Mechanical properties of the PMN-PT film were studied using nano-indentation technique and piezoelectric properties were characterized by a ferroelectric tester. D 2004 Elsevier B.V. All rights reserved. Keywords: Pulsed laser deposition; LaNiO3 (LNO); 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (PMN-PT)
1. Introduction Recent extensive studies on single crystal PMN and its solutions with PbTiO3(PT) have shown that these materials with a composition (75/35) near the morphotropic phase boundary have optimum electromechanical, ferroelectric and electro-optic properties and are excellent candidates for capacitors, actuators, sensors, and micromotors [1]. It is well known that single crystal PMN-PT exhibit high strain, high coupling coefficients and piezoelectric constant [2]. The polycrystalline relaxor ferroelectric thin films present a potential limitation due to their low electromechanical coupling coefficient caused by dielectric losses at the grain boundary where second phases of low permittivity (impurities or pyrochlore) might accumulate [3]. To overcome this problem, it is necessary to deposit single crystal ferroelectric thin films that are expected to have comparable ferroelectric
* Corresponding author. Fax: +65 6779 1459. E-mail address: [email protected] (L. Lu). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.131
and piezoelectric properties to their bulk single crystal counterparts. Since piezoelectric properties depend on crystalline quality of the film, the fast growing interest in thin film form of this material has been on attempting to grow single-crystal-like preferred orientation thin films by different methods such as: sol–gel, sputtering, CVD, spin-coating and PLD on different electrodes [4–8]. A major challenge is to prepare bsingle crystalT epitaxial thin films between electrodes, and integrate them so that their properties can be utilized in piezoelectric devices or MEMS. To date, little success has been achieved in obtaining high quality epitaxial perovskite PMN-PT film on a conductive electrode or substrate due to the appearance of stable pyrochlore phases such as cubic pyrochlore Pb3Nb4O13(P3N4), rhombohedral pyrochlore Pb2Nb2O7(P2N2), and tetragonal pyrochlore Pb3Nb2O8(P3N2) non-ferroelectric [9,10]. Conventional piezoelectric devices usually utilize sintered polycrystalline ceramic layers or bulk single crystals with noble metal Au or Pt electrodes. Metallic oxide LaNiO3(LNO) is of great interest because of its transport properties and its pseudocubic perovskite crystal structure. It
X.L. Zhong et al. / Surface & Coatings Technology 198 (2005) 400–405
has an electrical resistivity of a few hundred AV cm, allowing the use of the material as an electrode. On the other hand, the LaNiO3 metallic oxide electrode is known to have better fatigue property compare to other electrodes and has stronger adhesion to Si substrate compare to metallic electrodes. It is a perovskite-structured paramagnet with the pseudocubic lattice parameter of 3.85A. It is an intrinsic n-type metallic oxide with low lattice mismatch with PMNPT. In addition, since LNO tends to adopt a (100) texture on flat surfaces, it is possible to use the electrode to control the orientation of the ferroelectric films [11,12]. Many investigations have been carried out on the dielectric and piezoelectric properties of the PMN-PT system [13,14]. However, very few investigations have discussed the mechanical properties of PMN-PT ceramics especially thin films even though their main application is for piezoelectric components [15]. The present work may represent the first report on the successful deposition of epitaxial and nearly epitaxial 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (hereafter PMN-PT) thin films by pulsed laser deposition on (100) LaNiO3 metallic oxide electrodes. Pulsed laser deposition is used here to prepare LNO and PMN-PT thin films on top of LAO and SiO2/Si substrates. Properties of the as-deposited PMNPT films are characterized and discussed.
2. Experimental procedures A Lambda Physik KrF excimer pulsed-laser (wavelength 248 nm, pulse width=25 ns) operating at a frequency of 10 Hz and 2.5 J/cm2 fluence was incident on the rotating target at an angle close to 458. The target– substrate distance was 4 cm. A Pb rich PMN-PT disc from Ecertec was used as ablation target. The commercial LNO target was stoichiometric with purity of 99.9%. The deposition chamber was initially evacuated to a pressure of lower than 2105 Torr and then flushed with high
401
Fig. 2. Results from f scan showing epitaxial growth of PMN-PT.
purity oxygen to a pressure ranging from 20 to 400 mTorr. The LAO (100) and Si (100) substrates were cleaned to remove oxide layer and other impurities before deposition. The LNO electrode layers of thickness of 200 nm were grown on Si (100) and LAO (100) substrates from 300 to 650 8C under oxygen partial pressure of 20–100 mTorr. Subsequently, the PMN-PT thin films were deposited under 50–400 mTorr oxygen partial pressure and substrate temperature from 600 to 700 8C. Structures and crystal orientations of the as-deposited films were measured using an X-ray diffractometer with Cu Ka radiation (Shimadzu XRD-6000). Morphology, thickness and roughness of the thin films were analyzed using an FE-SEM (Hitachi FE-4100) and an AFM (Digital Instrument, Multimode). Mechanical properties of the PMN-PT film were studied using nano-indentation (Hysitron Nano-indentor with DI AFM). Commercially available single crystal PMN-PT (MicroFine, Singapore) was used for testing by this nano-indentation method so as to compare its mechanically properties with those of the PMN-PT films. For electrical measurements, Au dots of 3.14105 cm2 area were deposited on the top surface of
Fig. 1. h–2h X-ray diffraction patterns of PMN-PT films on LAO and Si substrates with LNO as seed layer: (a) on LAO (100) substrate, and (b) on Si(100) substrate.
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3. Results and discussion 3.1. Structures and microstructures of As-deposited films
Fig. 3. Rocking curve of (200) diffraction.
the films by sputtering technique through a shadow mask. The polarization hysteresis behavior of the ferroelectric PMN-PT thin films was measured using the Radiant Technologies Precision Workstation Ferroelectric Test System.
LNO film was first deposited on LAO (100) and Si (100) substrates at 600 8C and oxygen pressure of 50 mTorr. PMN-PT film was subsequently deposited on the LNO film. It was found out the optimum deposition conditions for growing epitaxial PMN-PT on LNO were between 600 and 700 8C, oxygen pressure at 175 to 400 mTorr with target-to-substrate distance of 4 cm by 200 mJ KrF laser source at 10 Hz reputation rate. At such conditions, the deposited films with (100) out-of-plane orientation could be obtained. The formation of preferred (100) orientation was believed to be caused by lower surface energy of the films rather than lattice matching [11]. Fig. 1(a) and (b) show the X-ray diffraction of PMN-PT/LNO/LAO(100) and PMN-PT/LNO/Si(100), respectively. It is clear that only (h00) peaks of perovskite PMN-PT and LNO can be observed besides diffraction from the LAO substrate. This observation indicates the arrangement of films and substrates as PMN-PT(100)tLNO(100)tLAO(100) and PMNPT(100)tLNO(100)tSi(100). No extra peaks from pyrochlore phases were detected. The formation of the ferroelectric perovskite phase may be favored by the presence of a
Fig. 4. FE-SEM images of the surface of LNO and PMN-PT: (a) LNO, (b) PMN-PT on LAO substrate with LNO as seed layer, (c) PMN-PT on Si substrate with LNO as seed layer.
X.L. Zhong et al. / Surface & Coatings Technology 198 (2005) 400–405
Fig. 5. AFM surface morphology for the LNO film.
suitable structural and chemical template, i.e., the (100) surface of the LNO bottom electrode [16]. The in-plan orientation of the PMN-PT film on LNO/ LAO(100) was investigated with f scan using (310) diffraction as shown in Fig. 2. The four (310) strong peaks at 908 interval clearly indicate a highly orientated inplane texture. By comparing with the orientation of the LAO substrate, the relationship between (100)PMN-PTt (100)LNOt(100)LAO could be found. The in-plan orientation of the PMN-PT films on LNO/ Si(100) was also studied using f scan. The results showed random in-plane growth of the PMN-PT films on LNO/ Si(100). The crystalline quality of the PMN-PT film of PMNPT/LNO/LAO(001) system was determined by the Rocking curve of the (200) reflection as shown in Fig. 3. The fullwidth at half-maximum (FWHM) value for PMN-PT is 2.48. It is known that the unit cell parameters of pseudocubic LAO and PMN-PT are 3.79 and 4.04 2, respectively, so that the value of lattice-mismatch is 1.84% at room temperature for LNO/LAO. This value is small and acceptable to obtain epitaxial growth. The CTE of LAO and PMNT is 11106 and 8106, respectively [17]. Thus the disorder of the crystal structure indicated by large value of FWHM might be caused by the difference in the thermal expansion coeffi-
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cients, twinned structure near the MPB composition [6] and the non-optimum deposition conditions. The surface morphologies of the LNO and PMN-PT films on LAO and Si substrates are shown in Fig. 4. It is worthwhile to notice from Fig. 4(a) that the surface of the LNO film is extremely smooth, offering a promising property for the subsequent growth of ferroelectric thin films. PMN-PT thin films show equal-axial columnar grains with grain size of about 50–100 nm. They are dense and crack free. More detailed information on the surface morphology was obtained from AFM as shown in Fig. 5. The surface is very uniform and composed of equal-axial grains with surface roughness of about 0.2 nm. This extremely low roughness is unlikely related to a threedimensional island growth. A two-dimensional layer growth mode can occur even with no lattice matching between a film and a substrate [12]. A layer-by-layer growth mode is therefore suggested. The small stable nucleuses are strongly bound to the substrate rather than to each other. Nucleation occurs all over the surface in two dimensions. After the growth of the first layer, the second layer started to grow on top of it and so on. The formation of the planner sheet resulted in a very smooth surface. Fig. 6 reveals the morphologies of the cross-sectional views of LNO and PMN-PT films, showing very dense LNO structure while columnar structure of PMN-PT. The morphological difference between these two films is possibly caused by the different growth mode of the two films. In the case of PMN-PT deposition, in contrast to the LNO film, the deposited atoms or molecules are more strongly bound to each other than to the substrate; thus the formation of islands in three dimension growth resulting in columnar structure. The layer-by-layer growth of LNO leads to very dense and smooth structure. 3.2. Mechanical properties The Meyer’s hardness of the PMN-PT film was measured with nano-indentor. The indentation load–displacement data obtained in the experiment were used to
Fig. 6. Cross-sectional morphologies of (a) LNO and (b) PMN-PT.
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X.L. Zhong et al. / Surface & Coatings Technology 198 (2005) 400–405 Table 2 Comparison of resistivities of LNO films Sample
Resistivity (AV cm)
XRD results
LNO/SiO2/Si LNO/SiO2/Si LNO/LAO
13100F2422 109.64F35.1 79.8F8.3
Amorphous High out-of-plane orientation High our-of-plane and in-plane orientation
elastic modulus, E s of the specimen is found using Eq. (4). 1 1 m2i 1 m2s ¼ þ Er Ei Es Fig. 7. Load–displacement curve from indentation experiment.
determine the Mayer’s hardness, H and Young’s modulus, E, using the method of Oliver and Pharr [18]. Meyer’s hardness can be calculated using Eq. (1). Pmax H¼ ð1Þ Ac where P max is the maximum applied load and A c is the area of contact. It is important to note that the area of contact determined from nano-indentation testing is the projected area, different from the full-contact area used in a standard Vickers hardness test [18]. Fig. 7 schematically shows a typical indentation load– displacement curve during one cycle of loading and unloading. Assuming that the initial unloading is purely elastic, the slope, termed contact stiffness, S is given by dP 2b pffiffiffiffiffi ¼ pffiffiffi Er Ac S¼ ð2Þ dh p where b=1.034 for the Berkovich indenter used in the present study and E r is the reduced modulus. Hence, pffiffiffi p dP ð3Þ Er ¼ pffiffiffiffiffiffiffiffiffi 2 Amax dh
ð4Þ
where E i =1140 GPa (diamond indenter), v i =0.07 (diamond indenter) and v s=0.2 (PMN-PT) [15]. The mechanical properties of the materials are summarized in Table 1. It can be observed that the films with preferred orientation and epitaxial have higher hardness and modulus values which are very close to those of PMN-PT single crystal, indicating that the films with preferred orientation may have excellent mechanical properties similar to those of single crystals. The good mechanical properties of the films with preferred orientation may not only be due to their good crystalline quality but also might be caused by their internal compression stress since the lattice parameter is larger than that of LNO. 3.3. Electrical properties The resistivity of LNO electrode was measured by a 4point Probe (Model 10/C, Four Dimensions). From Table 2, it can be seen that conductivity is directly related to the crystallinity and crystal orientation of the films. Amorphous structure shows low conductivity while crystalline, high conductivity. In addition, preferred orientation reduces electron scattering so that conductivity can be increased. Fig. 8 shows the P–E hysteresis loop of the epitaxial film. The measured hysteresis loop is comparable to
E r, which accounts for deformation of both the sample and the indenter can therefore be determined. The Table 1 Mechanical properties of single crystal, polycrystalline film, amorphous film and films with preferred orientation Sample
Hardness (GPa)
Reduce modules (GPa)
Elastic modules (GPa)
PMN-PT single crystal PMN-PT/SiO2/Si preferred oriented film PMN-PT/LAO epitaxial film PMN-PT/SiO2/Si polycrystalline film PMN-PT/SiO2/Si amorphous film
6.54F0.18
118.46F5.4
126.81F6.4
6.14F0.86
118.05F6.46
126.33F7.68
5.71F0.65
89.03F6.64
92.68F7.46
3.03F0.23
92.53F9.45
96.69F10.58
3.37F0.69
86.00F7.61
89.28F8.51
Fig. 8. Ferroelectric hysteresis loop of the epitaxial PMN-PT(100) film on LAO(100) substrate with LNO(100) as bottom electrode and Au dots as top electrode.
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published results [19–21] where SRO and Pt electrodes were used. The saturation polarization at 30 V is 149 AC/ cm2, remnant polarization P r, 10.8 AC/cm2 and coercive field E c, 182 kV/cm. The excellent epitaxial quality of the thin film might have contributed to the high polarization values. The relatively large coercive field might be due to the presence of a high density of small-angle grain boundaries. It is can be noted that the hysteresis loop is a little asymmetric in shape. This means an internal bias field has built up in the PMN-PT film due to an asymmetric space-charge distribution.
4. Conclusions Epitaxial PMN-PT(100) films with perovskite structure have been successfully deposited on LAO (100) substrate with (100) LNO as bottom electrode by pulsed laser technique. Although preferred out-of-plane growth of PMN-PT has been obtained on Si(100) substrate, in-plane orientation is random. The LNO promotes the preferred growth of PMN-PT films. The surface of the LNO film is atomically smooth which is favorable in reducing the presence of defects in overlayers and hence improving the ferroelectric properties of the PMN-PT films. The LNO film with preferred orientation exhibits good electrical conductivity, which is a promising characteristic for it to be used as an electrode. The mechanical properties are excellent and are comparable to those of single crystals. High polarization has been obtained at the hysteresis test.
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Further reading References [1] J. Levoska, M. Tyunina, A. Sternberg, S. Leppavuori, Appl. Phys., A 70 (2000) 269. [2] A.R. James, S. Priya, K. Uchino, K. Srinivas, J. Appl. Phys. 90 (7) (2001) 3504.
[1] A.C. Fischer-Cripps, Nano-indentation, Springer-Verlag, New York, 2002.