Ferroelectrics Characterization and Modeling Part 8 doc

Changes of Crystal Structure and Electrical Properties with Film Thickness and Zr/(Zr+Ti) Ratio for Epitaxial Pb(Zr,Ti)O 3 films Grown on (100) c SrRuO 3 //(100)SrTiO 3 Substrates… 235 001 , 100 002 200 SrRuO 3 100 c SrTiO 3 100 SrRuO 3 200 c SrTiO 3 200 Pt 111 [Top electrodes] 100 200 Log [intensity (arb. units)] 20 30 40 50 2 θ , CuKα 1 (deg) (a) (b) (c) (d) 0.19 Zr/(Zr+Ti) ; 0.13 0. 30 0. 35 0. 54 0. 53 0. 67 0. 63 250nm 50nm 250nm 50nm 250nm 50nm 250nm 50nm Fig. 6. XRD ω-2θ diagrams of PZT films having 50 and 250 nm in thickness and different Zr/(Zr+Ti) ratio: from small Zr/(Zr+Ti) ratio (a) to large Zr/(Zr+Ti) ratio (d). Ferroelectrics - Characterization and Modeling 236 0.390 0.395 0.400 0.405 0.410 0.415 Lattice parameter (nm) Shirane et al. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 0.0 0.1 0.2 0.3 0.4 c/a ratio 90– α (°) 0.0 0.2 0.4 0.6 0.8 1.0 62 64 66 68 70 Unit cell volume (10 -3 nm 3 ) Zr/(Zr+Ti) ratio 0.390 0.395 0.400 0.405 0.410 0.415 Shirane et al. Lattice parameter (nm) 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 0.0 0.1 0.2 0.3 0.4 c/a ratio 90– α (°) 0.0 0.2 0.4 0.6 0.8 1.0 62 64 66 68 70 Zr/(Zr+Ti) ratio Unit cell volume (10 -3 nm 3 ) (a) (b) (c) (d) (e) (f) Thickness : 50nm Thickness : 250nm Fig. 7. Lattice parameters (a, d), tetragonality, c/a ratio, and the internal angles and (b, e), and unit cell volume (c, f) as a function of Zr/(Zr+Ti) ratio for (a)-(c) 50 and (d)-(f) 250 nm thick films. Dashed lines are powder data reported by Shirane et al (Shirane & Suzuki, 1952). The c-domain relative volume fractions, V c , are shown in Fig. 8 as a function of the Zr/(Zr+Ti) ratio. These values are obtained from HRXRD-RSM characterization reported elsewhere (Morioka et al. 2004a). On this figure we notice that the 50 nm thick Films are fully polar axis-oriented films, (001) orientation, regardless of the Zr/(Zr+Ti) ratio up to 54% (Fig. 8(a)). On the other hand, V c decreased with increasing PZT film thickness. Indeed, we notice for the 250 nm thick films (Fig. 8(b)) that V c is about 70% up to Zr/(Zr+Ti) = 0.45. In the intermediate region, V c fluctuates between 55 and 75% due to the experimental errors induced by the tetragonal and rhombohedral duplicated peaks (Saito et al., 2003b). This result is totally coherent with our previous results showing the domain structure simplification with decreasing PZT film thickness. The structure domain simplification from coexisting a- and c-domains to fully polar axis orientation is supported by the compressive stress appearing at very thin deposited films (Morioka et al., 2003; Morioka et al., 2009). This compressive stress is induced by the lattice misfit stress and thermal stress due to the mismatches of lattice parameters and thermal expansion coefficients between PZT films and SrTiO 3 substrates, respectively. Changes of Crystal Structure and Electrical Properties with Film Thickness and Zr/(Zr+Ti) Ratio for Epitaxial Pb(Zr,Ti)O 3 films Grown on (100) c SrRuO 3 //(100)SrTiO 3 Substrates… 237 0.0 0.2 0.4 0.6 0.8 1.0 V(001)/[V(100)+V(001)] 0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 1.0 Zr/(Zr+Ti) ratio Tetra. Rhombo. Tetra. Rhombo. Mixed (a) (b) Fig. 8. c-domain volume fraction, V c , measured from HRXRD-RSM data (Morioka et al. 2004a) for (a) 50 and (b) 250 nm thick PZT films. 3.3 Electrical characterization Fig. 9 shows the leakage current density as a function of applied electric field for 50 and 250 nm thick PZT films with various Zr/(Zr+Ti) ratio. We notice that PZT thickness and Zr/(Zr+Ti) ratio influences leakage current density. Indeed, below 20% of Zr/(Zr+Ti) ratio, the 250 nm thick films show higher current density than 50 nm thick sample [see Fig. 9(a)]. Increasing Zr/(Zr+Ti) ratio in films lead to a decrease of the leakage current density level in the 250 nm thick PZT films from above 10 -3 A/cm² to 10 -6 A/cm² at an electric field of 100 kV/cm for Zr/(Zr+Ti) ratio ranging from 0.19 and 0.63 respectively. Ferroelectrics - Characterization and Modeling 238 50nm 250nm (a) (b) (c) (d) 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 Leakage current density (A/cm 2 ) 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 Zr/(Zr+Ti) ; 0.13 0.19 0.30 0.35 0.54 0.53 0.67 0.63 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 -300 -200 -100 0 100 200 300 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 Electric field (kV/cm) Fig. 9. Leakage current density as a function of electric filed for 50 nm thick (plain line) and 250 nm thick (dotted line) PZT thin films with different Zr/(Zr+Ti) ratio: from small Zr/(Zr+Ti) ratio (a) to large Zr/(Zr+Ti) ratio (d). Changes of Crystal Structure and Electrical Properties with Film Thickness and Zr/(Zr+Ti) Ratio for Epitaxial Pb(Zr,Ti)O 3 films Grown on (100) c SrRuO 3 //(100)SrTiO 3 Substrates… 239 On the other hand, the 50 nm thick PZT film show a relatively low leakage current level oscillating between 10 -7 and 10 -5 A/cm² at an electric field of 100 kV/cm, independently from the Zr/(Zr+Ti) ratio. These results are coherent with reported data (Shiosaki, 1995; Oikawa et al., 2002). Indeed, it has been shown that PZT films with low Zr/(Zr+Ti) ratio present typically larger leakage current density compared to that of films with large Zr/(Zr+Ti) ratio (Shiosaki, 1995). While it has been revealed (Oikawa et al., 2002) that Sr and/or Ru diffusion into PZT might create a conductive path, which is in good agreement with our results because a longer deposition time could induce a large amount of Sr and/or Ru diffusion into the bottom electrode. Fig. 10 summarizes the polarization - electric field (P - E) relationships for films with various Zr/(Zr+Ti) ratio and film thickness. -120 -80 -40 0 40 80 120 -120 -80 -40 0 40 80 120 Polarization (µC/cm 2 ) -120 -80 -40 0 40 80 120 -400 -200 0 200 400 -120 -80 -40 0 40 80 120 Electric field (kV/cm) -120 -80 -40 0 40 80 120 -120 -80 -40 0 40 80 120 Polarization (µC/cm 2 ) -120 -80 -40 0 40 80 120 -400 -200 0 200 400 -120 -80 -40 0 40 80 120 Electric field (kV/cm) Zr/(Zr+Ti) ; 0.13 0.30 0.54 0.67 0.35 0.53 0.63 (a) (b) (c) (d) (e) (f) (g) (h) Thickness : 50nm Thickness : 250nm Fig. 10 Polarization - electric field (P - E) relationships for films with various Zr/(Zr+Ti) ratio and film thickness. from 50 nm thick films [(a)-(d)] and 250 nm thick films [(e) - (h)]. Ferroelectrics - Characterization and Modeling 240 0 40 80 120 P r (µC/cm 2 ) Zr/(Zr+Ti) ; 0.13 0.30 0.54 0.67 0 100 200 300 400 0 50 100 150 200 Electric field (kV/cm) E c (kV/cm) Zr/(Zr+Ti) ; 0.13 0.30 0.54 0.67 0 40 80 120 P r (µC/cm 2 ) Zr/(Zr+Ti) ; 0.35 0.53 0.63 0 100 200 300 400 0 50 100 150 200 Electric field (kV/cm) E c (kV/cm) Zr/(Zr+tI) ; 0.35 0.53 0.63 (a) (b) (c) (d) Thickness : 50nm Thickness : 250nm Fig. 11. Saturation properties of P r and E c values against the maximum applied electric filed for 50 nm [(a), (b)] and 250nm [(c), (d)] PZT films having various Zr/(Zr+Ti) ratio content. Notice that the 250 nm thick film with Zr/(Zr+Ti)=0.19 showed high leakage current level that cannot display P - E hysterisis loops from the PZT films [Fig. 9(a)]. However, others showed P - E hysteresis loops originated to the ferroelectricity. Good saturation properties of Pr and the coercive field (Ec) against the maximum electric filed are confirmed for both of 50 nm and 250 nm thick films. We notice on these figures that fully polar axis oriented [(001)-oriented] films with 50 nm in thickness exhibit larger P r values than 250 nm PZT films with the coexistence of (100) and (001) orientations. This result is coherent if we consider the difference in the relative volume fraction of the c-domain in the latter samples. However, the differences in domain structure between the two thickness of the present samples remains and issue to fix for a good comparative analysis. To get insight into this issue, we calculated the spontaneous polarization (P sat ) from P r /V c , calibrated P r value by the c-domain volume fraction, V c , assuming that the 90° a-domain do not switch under an external electric field. The results are summarized in Fig. 12. On this figure, P r values extracted from 50 nm-thick films are presented with closed diamonds, while open diamonds represents P r values of 250 nm-thick films. By way of comparison, we included on Fig. 12, reported data for the 100% c-axis-oriented Pb(Zr 0.5 Ti 0.5 )O 3 film by Ishida et al. (square plot) (Ishida et al., 2002) and also theoretical calculated data of P sat against the Zr/(Zr+Ti) ratio reported by Haun et al. (dashed line) (Haune et al., 1989). We notice that the estimated P sat value in the present study is larger than reported data. However, our results have the same trend as predicted by theoretical calculations. A good explanation of this latter result might be given by getting insight into the relationship linking tetragonality (c/a) to spontaneous polarization (P sat ). Changes of Crystal Structure and Electrical Properties with Film Thickness and Zr/(Zr+Ti) Ratio for Epitaxial Pb(Zr,Ti)O 3 films Grown on (100) c SrRuO 3 //(100)SrTiO 3 Substrates… 241 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0 20 40 60 80 100 120 Zr/(Zr+Ti) ratio P sat (µC/cm 2 ) Haun et al. Ishida et al. 50nm 250nm Fig. 12. Estimated P sat as a function of Zr/(Zr+Ti) ratio in films. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 0 20 40 60 80 100 120 c/a ratio P sat (µC/cm 2 ) Fig. 13. Relationship between P sat and the c/a ratio of PZT films. Ferroelectrics - Characterization and Modeling 242 Indeed, the unit cell distortion inducing P sat might be related to cell parameters by the following equation (Joan & Shirane, 1992): 2 1 sat c QP a ⋅=− (1) where, Q represents the apparent electrostrictive coefficient. To highlight this relationship, we gathered data presented on Figures 7(b) and (e) and Fig. 12 on the same chart, as shown in Fig. 13: Our data seem to be in agreement with the quadratic form presented in equation (1) linking P sat to unit cell tetragonality, c/a ratio. In a previous article we could emphasis that our estimated electrostrictive coefficient, Q, is Q = 0.049 m 4 /C 2 We also characterized the relative dielectric constant (ε r ) versus Zr/(Zr+Ti) ratio as well as thickness dependency. These results are presented on Fig. 14. On this figure, we notice that both thicknesses considered in this study respect the same ε r tendency with regards to Zr content. Indeed, ε r increases with increasing Zr/(Zr+Ti) ratio. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0 200 400 600 800 Zr/(Zr+Ti) ratio Relative dielectric constant , ε r 50 nm 250 nm Fig. 14. Evolution of ε r measured at 1 kHz as function of Zr/(Zr+Ti) ratio for 50 nm ( ) and 250 nm ( ) thick films. On the other hand, we noticed that squareness in P - E hysteresis loops defined by the P r /P sat ratio, is also influenced by the Zr/(Zr+Ti) ratio of the films regardless of V c in the film. [see Fig. 15 (a)]. Indeed, as it can be observed on this figure, both 50 nm and 250 nm thick films present the same decreasing trend when Zr/(Zr+Ti) ratio increases. This result is in a good agreement with results presented on Fig. 14 since ε r can be extracted from the saturated branch of P - E hysteresis loops. Changes of Crystal Structure and Electrical Properties with Film Thickness and Zr/(Zr+Ti) Ratio for Epitaxial Pb(Zr,Ti)O 3 films Grown on (100) c SrRuO 3 //(100)SrTiO 3 Substrates… 243 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.5 0.6 0.7 0.8 0.9 1.0 Zr/(Zr+Ti) ratio P r /P sat ratio 1.00 1.01 1.02 1.03 1.04 1.05 1.06 0.5 0.6 0.7 0.8 0.9 1.0 c/a ratio P r /P sat ratio (a) (b) 50nm 250nm Fig. 15. Evolution of P r /P sat ratio as function of Zr/(Zr+Ti) ratio (a) and c/a ratio (b) 4. Summary Epitaxial PZT thin films were grown at 540°C on SrRuO 3 -coated (001) SrTiO 3 substrates by pulsed MOCVD. To characterize the impacts of the Zr/(Zr+Ti) ratio and the film thickness on the volume fraction of c-domain, 50 nm and 250 nm thick films have been grown with different Zr/(Zr+Ti) ratio ranging from 0.1 to 0.7. [...]... cm-1 bands, and Ti-O bonds are closely related to the formation of the 257 256 Ferroelectrics - Characterization and Modeling and 307 cm-1 bands in BaTi1-yCayO3 Fig.9 shows the bands 299, 520 and 723 cm -1 were independent of the formation of CaBa and CaTi in Bi-BCT and Bi-BTC From these results we can conclude that Ba-O bonds are closely related to the formation of the 520 and 723 cm-1 bands, and Ti-O... bands, and Ti-O bonds to the formation of the 262 and 299 cm-1 bands We also find the development of a weak new Raman band at 82 7cm-1 for Bi-BTC and Bi-BCT (see Fig 9) Almost the same Raman bands (83 2 cm-1) have been observed in Ba(Ti0. 985 Ca0.005Nb0.01)O3 and BaTi1-yCayO3 (y=0.005 and 0.015) ceramics (Chang & Yu, 2000) This higher frequency Raman band has resulted from the formation of CaTi defects... Raman bands related to the phonon vibration of the Ba-O bonds shift to higher frequency (512 and 719 cm1 for x = 0.005, 521 and 730 cm–1 for x = 0.20) while the Raman bands caused by the phonon vibration of the Ti-O bonds shift to lower frequency (259 and 306 cm -1 for x = 0.005, 2 48 and 2 98 cm–1 for x = 0.20) For B-site substitution, Ba-O bonds are closely related to the formation of the 517 and 7 18 cm-1... defined peaks and highest intensity line (101/110) without intermediate phase 80 0 ∗ (101) 70 0 10 0 ∗ (301) 20 0 ∗ (202) ∗ (220) 30 0 ∗ (111) ∗ (110) 40 0 ∗ (002) ∗ (200) ∗ (210) ∗ (201) ∗ (211) ∗ (112) 50 0 ∗ (001) ∗ (100) Intensity (a.u) 60 0 0 10 20 30 40 50 60 2 θ (d e g ree s) Fig 1 XRD pattern of Ba0.8Pb0.2 Zr0 .8 Ti0.2O3 ferroelectric phase 70 80 2 68 Ferroelectrics - Characterization and Modeling. .. Ca)TiO3 and Bi doped (Ba, Sr, Ca)TiO3 ferroelectric ceramics Some emphasis will be on the roles of acceptor-doping and donor-doping in understanding the physics of these materials 2 Experimental procedures 2.1 Ceramics synthesis A conventional solid reaction route was employed to synthesize ceramics samples Reagent grade BaCO3 (99 .8% ), Bi2O3 (99 .8% ), SrCO3 (99 .8% ), CaCO3 (99 .8% ) and TiO2 ( 98% ) as the... Bi-BCT and at 280 K for Bi-BSCT, with a nearly linear P-E relationship at the mid -part of the hysteresis loop With further increased temperature, hysteresis could not be detected, the P-E loops were slim and showed dielectric quasilinearity over a wide electric field range 250 Ferroelectrics - Characterization and Modeling 15 Bi-BCT (x=0.10) 15 10 10 5 5 -2 ) 0 Polarization ( μC / cm Bi-BSCT (x=0.10) 280 ... No.10, (October 1 987 ), pp 1329-1335 Mitsui, T & Westphal, W B (1961) Dielectric and X-ray Studies of CaxBa1-xTiO3 and CaxSr1xTiO3 Physical Review, Vol.124, No.5, (July 1961), pp 1354-1359 Baskara, N & Chang, H (2003).Thermo-Raman and Dielectric Constant Studies of CaxBa1−xTiO3 Ceramics Materials Chemistry and Physics, Vol.77, No.3, (January 2003), pp 88 9 -89 4 Karl, K & Hardtl, K H (19 78) Electrical After-Effects... Electrical After-Effects in Pb(Ti, Zr)TiO3 Ceramics Ferroelectrics, Vol.17, No.1, (January 19 78) , pp 473- 486 Postnikov, V S.; Pavlov, V S & Turkov, S K (1970) Internal Friction in Ferroelectrics Due to Interaction of Domain Boundaries and Point Defects Journal of Physics and Chemistry of Solids, Vol.31, No .8, (August 1970), pp 1 785 -1791 Lambeck, P V & Jonker, G H (1 986 ) The Nature of Domain Stabilization in... PZT’s and hard PZT’s The soft PZT’s have higher piezoelectric coefficient and are easy to pole and depole, compared to hard PZT’s Barium doped PZT’s show good piezoelectric properties, dielectric properties and better ME properties, while lead and zirconium doped BaTiO3 (BPZT) is used to enhance the piezoelectric and mechanical properties Barium lead zirconate titanate (BPZT - Ba0.8Pb0.2Zr0.8Ti0.2O3) and. .. (21360316 and 20047004)” 6 References K Nagashima, M Aratani, and H Funakubo, J Appl Phys 89 , 4517 (2001) N Okuda, K Saito, and H Funakubo, Jpn J Appl Phys., Part 1 39, 572 (2000) K Saito, T Kurosawa, T Akai, T Oikawa, and H Funakubo, J Appl Phys 93, 545 (2003) G Shirane and K Suzuki, J Phys Soc Jpn 7, 333 (1952) H Morioka, S Yokoyama, T Oikawa, K Saito and H Funakubo Mat Res Soc Symp Proc 784 , C6.2.1 . Zr/(Zr+Ti) ratio and film thickness. -120 -80 -40 0 40 80 120 -120 -80 -40 0 40 80 120 Polarization (µC/cm 2 ) -120 -80 -40 0 40 80 120 -400 -200 0 200 400 -120 -80 -40 0 40 80 120 Electric. 400 -120 -80 -40 0 40 80 120 Electric field (kV/cm) -120 -80 -40 0 40 80 120 -120 -80 -40 0 40 80 120 Polarization (µC/cm 2 ) -120 -80 -40 0 40 80 120 -400 -200 0 200 400 -120 -80 -40 0 40 80 120 Electric field (kV/cm) Zr/(Zr+Ti). for Zr/(Zr+Ti) ratio ranging from 0.19 and 0.63 respectively. Ferroelectrics - Characterization and Modeling 2 38 50nm 250nm (a) (b) (c) (d) 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 Leakage