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J. Phys. Chem. C 2010, 114, 17796–17801
Thickness-Dependent Phase Transition and Piezoelectric Response in Textured Nb-Doped Pb(Zr0.52Ti0.48)O3 Thin Films Jing-Feng Li,† Zhi-Xiang Zhu, and Feng-Ping Lai State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: July 10, 2010; ReVised Manuscript ReceiVed: September 4, 2010
[100]-textured Nb-doped Pb(Zr0.52Ti0.48)O3 (PNZT) films with different thicknesses from 80 to 600 nm were fabricated on Pt(111)/Ti/SiO2/Si(100) substrates by a sol-gel process. It was found that the local effective longitudinal piezoelectric coefficient, d33, initially increased with film thickness and reached a peak (∼220 pm/V) for an intermediate thickness (∼350 nm), but then decreased with further increasing thickness. XRD and Raman analyses revealed that, even for an identical Zr/Ti ratio of 52/48, which is near the morphotropic phase boundary of bulk PZT, a pseudophase transition from a tetragonal structure to a rhombohedral structure was induced in [100]-textured PNZT films because of changes in stress with film thickness. This finding revealed a special approach to enhance the piezoelectric properties of PZT-based thin films by combining compositional optimization and the substrate constraint effect. I. Introduction Lead zirconate titanate- (PZT-) based films on conventional platinized silicon substrates have received much attention in terms of both technological and scientific aspects because of their promising applications in microelectromechanical systems (MEMS) devices.1,2 Many factors influencing the phase structure and physical properties of polycrystalline PZT-based films have been studied to improve their performance.3-8 Among them, film thickness is an important factor, and its effects have been studied extensively.7,8 As is well-known, because of substrate constraint effects and limited thicknesses, PZT films usually show inferior piezoelectric properties compared to bulk ceramic materials. Our previous report studied the film thickness dependence of the ferro- and piezoelectric properties in [100]textured tetragonal Nb-doped Pb(Zr0.3Ti0.7)O3 films, for which both the [100] texturing and Nb doping are favorable for piezoelectricity enhancement.7 It was found that the local effective piezoelectric coefficient (d33) of the films increased noticeably and monotonically with the film thickness, and a high d33 value of up to 196 pm/V was obtained when the film was sufficiently thick, around 0.8 µm. As a continuation of that study, the present work focused on a composition (Zr/Ti ratio ≈ 52/48) close to the morphotropic phase boundary (MPB) as found in bulk PZT ceramics, where greatly enhanced piezoelectric properties can be achieved as a consequence of more ferroelectric variants in materials with a MPB composition.9,10 As expected, enhanced ferroelectric and piezoelectric properties were obtained in [100]-textured Nbdoped Pb(Zr0.52Ti0.48)O3 films. However, a different thickness dependence of properties was revealed when the film composition was changed from a ratio of Zr/Ti ) 30/70 to that corresponding to the Zr/Ti ) 52/48 ratio close to the MPB. Interestingly, in the present [100]-textured Nb-doped Pb(Zr0.52Ti0.48)O3 films, both d33 and remnant polarization (Pr) increased initially with film thickness and showed peak values in a film of intermediate thickness (∼350 nm), but then † To whom correspondence should be addressed. E-mail: jingfeng@ mail.tsinghua.edu.cn.
decreased as the film thickness was further increased. Such a thickness dependence can be well explained by the additional effect of substrate constraints on the phase structure of the textured PZT-based thin films. II. Experimental Details Fabrication of the PNZT Films. Most experiments focused on 2 mol % Nb-doped Pb(Zr0.52Ti0.48)O3 (PNZT) films grown on PbO-seeded Pt (155 nm)/Ti (15 nm)/SiO2 (305 nm)/Si(100) (525 µm) substrates by a sol-gel process. A series of PNZT films with the same composition but different thicknesses were fabricated by changing the spin-coating times. The sol-gelderived PbO seeding layers, as reported previously,3 were deposited onto the platinum electrodes to control the crystal orientation of the films. The PNZT precursor solution with a composition of Pb(Zr0.52Ti0.48)0.98Nb0.02O3 was prepared using trihydrate lead acetate [Pb(CH3COO)2 · 3H2O], zirconium npropoxide [Zr(OCH(CH3)2)4)], titanium isopropoxide [Ti(OCH(CH3)2)4)], and niobium ethoxide [Nb(OC2H5)5] as raw materials and 2-methoxyethanol as the solvent. Approximately 20 mol % excess lead was added to the solution to compensate for the lead loss caused by evaporation and to suppress the formation of pyrochlore phases. The Nb elements occupy the B sites in the perovskite structure of PZT, and the amount (2 mol %) of Nb dopant in the present study is an optimized value that is beneficial for the improvement of the electrical properties of PZT materials.7,11 Acetylacetone was added to stabilize the titanium isopropoxide by reducing the hydrolysis speed as a result of its natural sensitivity to moisture when exposed to air. The precursor solution was diluted to 0.5 M and aged for more than 24 h. A 0.5 M lead-containing organic solution as the seeding layer was prepared using a similar procedure. Prior to the deposition of PNZT thin layers, the substrates were overlaid with the lead-containing organic solution and then heated at 500 °C for 2 min to form a thin PbO seeding layer on the platinum electrode. Subsequently, the PNZT films were deposited layer by layer onto the PbO-coated substrates by spincoating at 4000 rpm for 30 s, followed by pyrolysis at 450 °C for 5 min. The PNZT films were finally annealed in air at 650
10.1021/jp106384e 2010 American Chemical Society Published on Web 09/29/2010
Thickness-Dependent Properties of PNZT Thin Films °C for 5 min by a rapid thermal processor (RTP) for densification and crystallization. In addition, PbO-seeded Nb-doped PZT films with Zr/Ti ratios of 30/70 and 70/30 were also prepared by the same procedure, for use as reference samples for the phase structure analysis. Microstructure and Composition Analyses. The crystal structure and preferential orientation of the PNZT films were analyzed by X-ray diffraction (XRD; Rigaku D/max 2500, Tokyo, Japan) with Cu KR radiation. The XRD patterns were recorded at a step scanning rate of 2°/min. For the XRD experiments, silicon powder was used for instrument calibration to reduce the system errors. In addition, the locations of the XRD peaks from the PNZT films were also compared with those from the underlying substrates. The microstructure and nominal film thickness were evaluated by field-emission scanning electron microscopy (FE-SEM; Hitachi S-4800, Tokyo, Japan). Raman measurements were performed in a backscattering geometry on an RM2000 instrument (Renishaw, Gloucestershire, U.K.) with a radiation of Ar+ laser at 514.5 nm focused over a 5-µm-diameter area. The chemical composition of the film was examined by Auger electron spectroscopy (AES; ULVAC-PHI, PHI 700 SAN, Chigasaki, Japan). The Auger spectra were obtained in derivative mode for specific energies ranging from 0 to 2400 eV. The residual gas pressure reached
