⇦Chinese Physics Letters, 2019, Vol. 36, No. 10, Article code 107701 Roles of Nano-Domain Switching and Non-180$^{\circ}$ Domains in Enhancing Local Piezoelectric Responses of Highly (100)-Oriented Pb(Zr$_{0.60}$Ti$_{0.40}$)O$_{3}$ Thin Films * Chen-Fei Jin (靳辰飞)1, Si-Qi Zhang (张思琦)1**, Zhi-Qiang Shen (沈志强)1, Wei-Li Li (李伟力)2 Affiliations 1School of Physics, Harbin Institute of Technology, Harbin 150001 2School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 Received 5 July 2019, online 21 September 2019 *Supported by the National Natural Science Foundation of China under Grant No 11504071, and the Key Laboratory of Micro-optics and Photonic Technology of Heilongjiang Province for PFM Measurement during the Research.
**Corresponding author. Email: zhangsiqi@hit.edu.cn Citation Text: Jin C F, Zhang S Q, Chen Z Q and Li W L 2019 Chin. Phys. Lett. 36 107701    Abstract Ferroelectric Pb(Zr$_{0.60}$Ti$_{0.40}$)O$_{3}$ thin films deposited on the niobium-doped SrTiO$_{3}$ and Pt (111)/Ti/SiO$_{2}$/Si substrates are fabricated by a sol-gel method. X-ray diffraction indicates that the films have a 'cube-on-cube' growth with highly (100) preferred orientation and good surface qualities. Using piezoelectric force microscopy, we investigate domain structures and butterfly amplitude loops of ferroelectric thin films. The results indicate that the film deposited on Nb:SrTiO$_{3}$ has both kinds of 180$^{\circ}$ polarizations perpendicular or parallel to the surface while the film deposited on Pt/Ti/SiO$_{2}$/Si has irregular phase differences. Excellent piezoelectric polarization are observed in the films on niobium-doped SrTiO$_{3}$ with local $d_{33}^{\ast}$ values around 45 pm/V three times more than that of the films around 13 pm/V deposited on Pt (111)/Ti/SiO$_{2}$/Si. Our findings emphasize that nano-domain switching ability and non-180$^{\circ}$ domains will contribute significantly to enhance piezoelectric responses of ferroelectric thin films. DOI:10.1088/0256-307X/36/10/107701 PACS:77.55.fg, 77.65.-j, 77.55.-g © 2019 Chinese Physics Society Article Text Ferroelectric thin films like lead zirconate titanate [Pb(Zr$_{1- x}$Ti$_{x})$O$_{3}$, PZT] have been extensively studied not only because of their technological importance for piezoelectric transducer and actuator applications but also for their fundamental aspects.[1] Previously, many researchers revealed that crystal structures and electromechanical properties of ferroelectric PZT thin films can be substantially modified under strain states.[2–4] Typically, misfit strain can shift the MPB composition of PZT bulk away from conventional Zr/Ti composition near 52/48 where excellent dielectric/piezoelectric response is located.[5,6] Recent progress on piezoelectric responses and domain structure has been made for ferroelectric thin films like PZT on single crystals,[7–9] La-doped Bi$_{3.25}$ La$_{0.75}$Ti$_{3}$O$_{12}$ thin Films on Pt/Ti/SiO$_{2}$/Si,[10] BiInO$_{3}$-PbTiO$_{3}$ thin films with lower symmetry distortion and higher Curie temperature[11] and BiFeO$_{3}$ thin films on GdScO$_{3}$ (110) substrates.[12] These studies emphasized that excellent piezoelectricity is closely related to the existence of nanodomains. Domain structures can be essentially changed by film orientations for ferroelectric films deposited on Pt/Ti/SiO$_{2}$/Si.[13] Additionally, studies revealed that the growth of polycrystalline thin films can be controlled by thermal treatment,[14] seeding/or buffer layer[15,16] and the selection of substrates.[17] Our studies have shown that ferroelectric polarization of PZT-based thin films on Pt/Ti/SiO$_{2}$/Si can be mainly related with monoclinic phase and (100)-preferred orientation.[18,19] Yet, within our knowledge, issues of optimizing piezoelectric and polarization response of ferroelectric thin films will still require further investigation if we consider external contributions including phase components, nano-domains switching and surface qualities. In this work, we focus on the domain structures and piezoelectric responses of Pb(Zr$_{0.60}$Ti$_{0.40})$O$_{3}$ thin films deposited on Nb-doped SrTiO$_{3}$ (with 0.007 wt.%) single crystal. Traditional Pt/Ti/SiO$_{2}$/Si substrates are selected considering their different thermal expansion coefficients relative to the films.[20,21] Using piezoelectric force microscopy (PFM), we comparatively study their domain structures and butterfly amplitude loops for the sake of establishing an internal relation between microstructure and piezoelectric properties of ferroelectric thin films. Ferroelectric PZT thin films were prepared by a sol-gel method and details were described elsewhere.[22] Structure and orientation of the films were characterized by X'pert XRD (PANalytical) using Cu K$\alpha$ radiation. Film thicknesses were measured (around 250 nm) by a surface profile meter (Alpha-Step D-100, KLA-Tencor). Morphology, domain structure and 'butterfly' shaped displacement were carried out by AFM and PFM (NT-MDT). The frequency and amplitude of the applied ac signal were 368 kHz and 3 Vpp and the dc bias voltage was swept from $-$5 to 5 V. Figure 1 shows the $\theta$–$2\theta$ scan XRD curves of as-crystallized PZT thin films. In Fig. 1(a), (100)$_{\rm pc}$ reflection of the film on the Nb:SrTiO$_{3}$ substrate is visible with high (100) reflection intensities indicating that the film is well crystallized and has a 'cube-on-cube' growth. In contrast, intensities of (110) diffraction are relatively higher for the film deposited on the Pt/Ti/SiO$_{2}$/Si substrate, which suggests a mixture of (100) and (111)-oriented fiber growth. Film (200)$_{\rm pc}$ diffraction peaks are shown in Fig. 1(b). For the film deposited on Pt/Ti/SiO$_{2}$/Si, an asymmetry of (200)$_{\rm pc}$ diffraction is observed, which can be considered as a feature of low-asymmetry monoclinic phase ($M_{a}$ with $Cm$ space group).[18] Obviously, the (200)$_{\rm pc}$ reflection deposited on the Nb:SrTiO$_{3}$ is singlet predicting hitherto believed 'rhombohedral' phase or monoclinic $M_{a}$ phase.[23] Additionally, (200)$_{\rm pc}$ diffractions are located at lower and higher 2$\theta$ angles for the films deposited on Nb:SrTiO$_{3}$ and Pt/Ti/SiO$_{2}$/Si than that of strain-free Pb(Zr$_{0.60}$Ti$_{0.40})$O$_{3}$ bulk ceramic (2$\theta \sim 44.4^{\circ})$. For the (100)-oriented films, out-of-plane $a$-axis is generally consistent with $z$-direction. Based on Bragg's law, the films are under tensile strain with small out-of-plane lattice constant. Similarly, the film on Nb:SrTiO$_{3}$ located at low (100)$_{\rm pc}$ peak location will be under compressive strain. Generally, tensile stress for sol-gel-derived PZT films on Pt/Ti/SiO$_{2}$/Si will be favorably developed due to thermal mismatches of the crystal lattice parameters between the film and the substrate.[24,25] Compressive stress will be formed considering that thermal expansion coefficient of the PZT film is larger than that of Si and lower than that of Nb:SrTiO$_{3}$, with all their values around $6\times 10^{-6}$ K$^{-1}$, $3.4\times 10^{-6}$ K$^{-1}$ and $10.4\times 10^{-6}$ K$^{-1}$, respectively. Additionally, as seen in Fig. 1(c), the Pb(Zr$_{0.60}$Ti$_{0.40}$)O$_{3}$ film on Nb:SrTiO$_{3}$ exhibits good surface qualities with a dense and uniform surface in contrast with the coarse surfaces of the film on Pt/Ti/SiO$_{2}$/Si. Further, the roughness of the films in turn can be calculated from the AFM data with average roughness $R_{\rm a}$ around 1.1 nm and 4.0 nm, respectively.
cpl-36-10-107701-fig1.png
Fig. 1. (a) The $\theta$–$2\theta$ scan XRD curves of the films deposited on Nb:SrTiO$_{3}$ and Pt/Ti/SiO$_{2}$/Si substrates, respectively, (b) typical (200)$_{\rm pc}$ diffraction peaks, among which pseudo (200)$_{\rm pc}$ diffraction of Pb(Zr$_{0.60}$Ti$_{0.40}$)O$_{3}$ bulk is marked, and (c) surface topography of AFM scanned with 'tapping' mode.
Film orientation distribution can be calculated from the full width at half maximum (FWHM) of $\omega$-scan XRD rocking curves.[26] Compared with other methods such as pole figure, inverse pole figure, orientation distribution function (ODF) and the intensity ratio of XRD peaks, it is an effective and simple method to qualitatively and quantitatively characterize film fiber texture. The relative intensity curve of $\omega$-scan XRD for a certain crystal plane is proportional to the pole density of the plane. In Fig. 2, the film deposited on Nb:SrTiO$_{3}$ has a quite small FWHM value about 0.68$^{\circ}$, which is comparable to that of pure Nb:SrTiO$_{3}$ (0.5$^{\circ}$ commercially available), thus the film is of highly (100)-oriented growth. In contrast, the film deposited on Pt/Ti/SiO$_{2}$/Si has a (100)$_{\rm pc}$ FWHM value around 6.23$^{\circ}$, indicating that (111)-preferred orientation is favored.[27]
cpl-36-10-107701-fig2.png
Fig. 2. The $\omega$-scan XRD curves on the (100) diffraction of PZT films. Inset: fiber growth of the PZT film on Pt/Ti/SiO$_{2}$/Si.
Figure 3 shows typical domain structure and polarization of the films at the same region around 12 µm $\times$ 12 µm. In Figs. 3(a) and 3(d), height images of the films are in accordance with their surface qualities as scanned by AFM. Changes of domain structures can be noticed from Figs. 3(b) and 3(e). Typically, there is an obvious grayscale domain contrast without applying dc bias for the films on Pt/Ti/SiO$_{2}$/Si, which may be related to the arrangement of domains in grains and to the orientation of the grains in the films.[28] In contrast, the majority of the nanodomains in the films on Nb:SrTiO$_{3}$ exhibit a sharp domain boundary as well as fine spherical nanoscale grains. By applying a dc bias, domain structures and polarization behavior of the films are comparatively studied. The complementarity of the VPFM and LPFM images is obvious. This feature can be observed either from 2 µm $\times$ 2 µm (light or deep yellow within white box line) poling areas at $+$10 V or from 6 µm $\times$ 6 µm poling areas at a dc bias voltage of negative 10 V to contrast enhancement. Thus the film deposited on Nb:SrTiO$_{3}$ has sharp domain boundaries between different polarized areas, suggesting the existence of large non-180$^{\circ}$ domains perpendicular or parallel to the surface. In contrast, poling areas of the film on Pt/Ti/SiO$_{2}$/Si can be characterized by alternating light and dark poling areas through the whole poling region in Figs. 3(e) and 3(f). As is seen, slight contrast (typically within white box line) is observed. Phase differences of the domains marked by green cross-section line are around 150$^{\circ}$ and 5$^{\circ}$, respectively, while the film deposited on Nb:SrTiO$_{3}$ has phases around 180$^{\circ}$ and 80$^{\circ}$ in Figs. 3(b) and 3(c). These domains perpendicular or parallel to the surface may account for the presence of low-asymmetry $M_{a}$ phase of the films on Pt/Ti/SiO$_{2}$/Si. These irregular phase differences or slight grayscale domain contrast may be caused by the existence of unevenly distributed laminate domains and strong defect pinning to the movement of domain walls.[29]
cpl-36-10-107701-fig3.png
Fig. 3. (a)–(c) PFM images of the films deposited on the Nb:SrTiO$_{3}$ substrate with height, out-of-plane and in-plane phase images, respectively, and (d)–(f) are the corresponding images of the film deposited on Pt/Ti/SiO$_{2}$/Si.
In Fig. 4, we present the typical butterfly shaped displacement-voltage curves of the films measured at 5 V bias voltage. In the case of PZT films deposited on Nb:SrTiO$_{3}$, the saturated curve is observed and the amplitude reaches its maximum around 140 pm in contrast with an unsaturated curve obtained in the PZT film on Pt/Ti/SiO2/Si. In Fig. 4(b), displacement reaches a maximum around 45 pm at 1.5 V and then decreases as dc bias voltage is increased. The calculated local piezoelectric coefficient ($d_{33}^{\ast}$) values of the films are shown in Fig. 4(c). It is emphasized that the film on Nb:SrTiO$_{3}$ has excellent $d_{33}^{\ast}$ values around 45 pm/V, which is three times more than 13 pm/V of the film deposited on Pt/Ti/SiO$_{2}$/Si. Here excellent piezoelectric responses of highly (100)-oriented films deposited on Nb:SrTiO$_{3}$ can be mainly associated with large and reversible non-180$^{\circ}$ ferroelectric domains. Additionally, dense surface qualities of the films will be beneficial to reduce the domain pinning effect and to enhance the ability of non-180$^{\circ}$ domain switching. Thus these non-180$^{\circ}$ ferroelectric domains switching under external electric field will offer distinctive strain responses in polycrystalline PZT.[30]
cpl-36-10-107701-fig4.png
Fig. 4. VPFM amplitude and phase dependencies on bias voltage for the films with (a) deposited on Nb:SrTiO$_{3}$ and (b) on Pt/Ti/SiO$_{2}$/Si, and (c) calculated $d_{33}^{\ast}$ values of the films deposited on different substrates.
In summary, Pb(Zr$_{0.60}$Ti$_{0.40})$O$_{3}$ thin films deposited on Nb:SrTiO$_{3}$ and Pt/Ti/SiO2/Si have been fabricated using a sol-gel method. The film on Nb:SrTiO$_{3}$ is of highly (100)-preferred orientation and possesses good surface qualities. The results evidence that excellent piezoelectric properties can be reached in (100)-oriented PZT films deposited on Nb:SrTiO$_{3}$ with its local $d_{33}^{\ast}$ value three times larger than that of the film on Pt/Ti/SiO$_{2}$/Si. In addition to the roles of non-180$^{\circ}$ domains, an enhanced poling ability of out-of-plane polarization can directly influence domain switching due to the release of strain limitation. Furthermore, super (100)-oriented growth and good qualities will make poling ability better by reducing the defect pinning effect to the movement of domain walls. Our study will provide important help in optimizing piezoelectric properties of ferroelectric thin films on electromechanical systems. References Ferroelectricity in Ultrathin Perovskite FilmsEnhanced thermodynamic stability of tetragonal-phase field in epitaxial Pb(Zr,Ti)O3 thin films under a two-dimensional compressive stressFerroelectric Domain Structures in Epitaxial PZT Thin Films: A ReviewStudy of strain effect on in-plane polarization in epitaxial BiFeO 3 thin films using planar electrodesShifting of the morphotropic phase boundary and superior piezoelectric response in Nb-doped Pb(Zr,Ti)O3 epitaxial thin filmsMisfit strain modulated phase structures of epitaxial Pb(Zr1−xTix)O3 thin films: The effect of substrate and film thicknessNanodomains in morphotropic lead zirconate titanate ceramics: On the origin of the strong piezoelectric effectFerroelectric Properties of Bi 3.25 La 0.75 Ti 3 O 12 Thin Films Crystallized in Different N 2 /O 2 AmbientsTunable ferroelectric domain orientation in polycrystalline and highly oriented Na0.5Bi0.5TiO3 Thin FilmsPiezoresponse Force Microscopy Imaging of Ferroelectric Domains in Bi(Zn 1/2 Ti 1/2 )O 3 -Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 Piezoelectric CeramicsImproved Polarization Retention of BiFeO 3 Thin Films Using GdScO 3 (110) SubstratesCharacterization of La-doped x BiInO 3 (1 − x )PbTiO 3 Piezoelectric Films Deposited by the Radio-Frequency Magnetron Sputtering MethodEffects of texture and residual stress on the transition of ferroelectric perovskite thin films with C-axis polarizationThe effect of crystallographic orientation and solution aging on the electrical properties of sol–gel derived Pb(Zr0.45Ti0.55)O3 thin filmsOrientation control of SrRuO3 thin film on a Si substrate by chemical solution deposition for an electrode of lead zirconate titanate thin filmsSeeding effect on micro- and domain structure of sol–gel-derived PZT thin filmsSubstrate effects on domain structures of PZT 30/70 sol-gel films via PiezoAFMCombined contributions of phase structure and preferred orientation on the piezoelectric properties of polycrystalline (Pb0.94La0.04) Zr0.6Ti0.4O3 thin filmsThickness-Dependent Monoclinic Phases and Piezoelectric Properties Observed in Polycrystalline (Pb 0.94 La 0.04 )(Z r0.60 Ti 0.40 )O 3 Thin FilmsStresses in Pt/Pb(Zr,Ti)O 3 /Pt thin‐film stacks for integrated ferroelectric capacitorsStrain and in-plane orientation effects on the ferroelectricity of (111)-oriented tetragonal Pb(Zr0.35Ti0.65)O3 thin films prepared by metal organic chemical vapor depositionFerroelectric properties of highly (111)-oriented Pb(Zr1−x, Tix)O3 thin films with different Zr/Ti ratiosStability of ferroic phases in the highly piezoelectric Pb(Zr x Ti 1−x )O 3 ceramicsScience and Technology of Ferroelectric Films and Heterostructures for Non-Volatile Ferroelectric MemoriesStress-induced shifting of the morphotropic phase boundary in sol–gel derived Pb(Zr0.5Ti0.5)O3 thin filmsCharacterization of fiber texture by ω-scan x-ray diffractionSol-Gel Processing of PbTiO3, PbZrO3, PZT and PLZT Thin filmsSFM investigation of nanoscale domain structure in ferroelectric PZT thin filmsFerroelectric domain structures in <001>-oriented K 0.15 Na 0.85 NbO 3 lead-free single crystalThe effect of strain on the domain switching of ferroelectric polycrystals
[1] Fong D D et al 2004 Science 304 1650
[2] Oh S H and Jang H M 1998 Appl. Phys. Lett. 72 1457
[3] Lee K, Lee K S, Kim Y K et al 2002 Integr. Ferroelectr. 46 307
[4] Chen Z, Zou X, Ren W et al 2012 Phys. Rev. B 86 235125
[5] Zhu Z X, Li J F, Liu Y Y et al 2009 Acta Mater. 57 4288
[6] Liu Y Y, Zhu Z X, Li J F et al 2010 Mech. Mater. 42 816
[7] Theissmann R, Schmitt L A, Kling J et al 2007 J. Appl. Phys. 102 024111
[8] Li J J, Yu J, Li J et al 2009 Chin. Phys. Lett. 26 047702
[9] Miriyala K and Ramadurai R 2016 Mater. Lett. 178 23
[10] Liu L M, Zeng H R, Chao Z Z et al 2011 Chin. Phys. Lett. 28 087701
[11] Xu S Q, Zhang Y, Guo H Z et al 2017 Chin. Phys. Lett. 34 027701
[12] Sun K X, Zhang S Y, Kiyotaka W et al 2016 Chin. Phys. Lett. 33 064301
[13] Liu C Q, Fei W D and Li W L 2008 Thin Solid Films 516 1265
[14] Alkoy E M, Alkoy S and Shiosaki T 2007 Ceram. Int. 33 1455
[15] Ohno T, Fukumitsu K, Honda T et al 2016 Mater. Lett. 181 74
[16] Kholkin A L, Gruverman A, Wu A et al 2001 Mater. Lett. 50 219
[17] Dunn S and Whatmore R W 2002 J. Eur. Ceram. Soc. 22 825
[18] Zhang S Q, Li W L, Jin C F et al 2015 Mater. Lett. 160 347
[19] Zhang S Q, Zheng F, Jin C F et al 2015 J. Phys. Chem. C 119 17487
[20] Spierings G A C M, Dormans G J M, Moors W G J et al 1995 J. Appl. Phys. 78 1926
[21] Kuwabara H, Menou N and Funakubo H 2007 Appl. Phys. Lett. 90 222901
[22] Zhang S Q, Fei W D, Li W L et al 2009 J. Alloys Compd. 487 703
[23] Pandey D, Singh A K and Baik S 2008 Acta Crystallogr. Sect. A: Found. Crystallogr. 64 192
[24] Brennecka G L, Huebner W, Tuttle B A et al 2004 J. Am. Ceram. Soc. 87 1459
[25] Zhang S Q, Wang L D, Li W L et al 2011 Mater. Res. Bull. 4 6 1237
[26] Fei W D, Liu C Q, Ding M H et al 2009 Rev. Sci. Instrum. 80 093903
[27] Chen S Y and Chen I W 2005 J. Am. Ceram. Soc. 81 97
[28] Zeng H R, Li G R, Yin Q R and Tang X G 2003 Acta Phys. Sin. 52 1783 (in Chinese)
[29] Chen Y, Wong C M, Deng H et al 2015 AIP Adv. 5 037117
[30] Tong Y, Liu M, Chen H M, Li G P et al 2015 J. Appl. Phys. 117 074102