Lead zirconium titanate, Pb(Zr,Ti)O3 [PZT], have been intensively studied for various ferroelectric applications, and have a renewal interest due to their promising application for microelectromechanical systems (MEMS) because of their outstanding ferroelectric and piezoelectric properties. Remanent polarization (
To achieve this purpose, we switch our idea from growth of the bulky single crystal to epitaxial films with polar axis orientation. In addition, a comprehensive and systematic characterization of ferroelectric properties of PZT films with different volume fraction of polar-axis-oriented domain is investigated.
This chapter investigates the thickness and Zr/(Zr+Ti) ratio dependencies of domain structure and ferroelectric properties, and correlates physical properties, namely lattice parameters and the volume fractions of the domains, as well as the electrical properties such as
PZT thin films were grown on (100)cSrRuO3//(100)SrTiO3 substrates at 540ºC by pulsed-metal organic chemical vapor deposition (MOCVD) from Pb(C11H19O2)2 - Zr(O·t-C4H9)4 -Ti(O·i-C3H7)4 - O2 system (Nagashima et al., 2001). Epitaxial (100)cSrRuO3 thin films used for bottom electrode layers were grown on (100)SrTiO3 substrates by MOCVD (Okuda et al., 2000). The Zr/(Zr+Ti) ratio and the film thickness of PZT films were controlled by the input gas concentration of the source gases and the deposition time, respectively. In this work, we studied PZT films having thickness ranging from 50 to 250 nm.
The orientation of the deposited films was analyzed by high-resolution X-Ray Diffraction (XRD) using a four-axis diffractometer (PANalytical X’Pert MRD). The high-resolution XRD reciprocal space mapping (HRXRD-RSM) was also employed for more detail analysis of crystal structure (orientation, in-plane and out-of-plane lattice parameters, and the internal axial angle) and estimating the relative volume fraction of the
Electron-beam deposition was used to deposit 100 μm
3. Results and discussion
In this section, we demonstrate, first of all, film thickness dependency of the crystal structure of PZT films. We show that polar-axis-oriented films were obtained at very thin films region. Then, we detail the Zr/(Zr+Ti) ratio dependency of the domain structure. For this purpose, we will compare crystal structure evolution as a function of the Zr/(Zr+Ti) ratio at two thicknesses, 50 and 250 nm. This comparative study aims to emphasis the role of the Zr/(Zr+Ti) ratio in PZT film as well as the thickness dependency, discussed in first instance.
Finally, we will cross check the up mentioned results by monitoring the evolution of electrical properties versus thickness and the Zr/(Zr+Ti) ratio in the films. We will synthesis these data by identifying the linear relationship between the square of spontaneous polarization (
Nevertheless, prior to proceeding to this characterization, it is important to check the epitaxial relationship between the bottom electrode and PZT films.
Indeed, it must be kept in mind that the framework of this study is the fundamental understanding of the impact of crystal structure change on the electrical properties, and polycrystalline films might induce measurement artefacts. The epitaxial growth of PZT films on (100)SrRuO3//(100)SrTiO3 substrates was ascertained by High Resolution Transmittance Electron Microscopy (HRTEM) as presented on Fig. 1(a).
Indeed, Fig. 1(a) shows a cross-sectional TEM image of 50 nm thick PZT(35/65) film. It presents smooth interfaces. Fig. 1(b) reveals atomically sharp interface between PZT and SrRuO3 bottom electrode. Moreover, this latter figure shows clearly a coherent epitaxial relationship at PZT/SrRuO3 interface.
3.1. Evolution of domain structure versus film thickness
For this part of our investigation, we chose to characterize PZT films with the Zr/(Zr+Ti) ratio of 0.35 that have a tetragonal symmetry. Fig. 2 presents XRD plots for the 2
Hence, using XRD-RSM technique (Fig. 3), we could monitor
Finally, we checked strain condition when film thickness decrease in the case of PZT(35/65) material. For this purpose, we calculated the both in-plan (
3.2. Domain structure evolution versus film composition
Fig. 6 presents X-ray diffraction diagrams of PZT films having 50 and 250 nm in thickness with various Zr/(Zr+Ti) ratio. All films are found to have (100) and/or (001) orientations regardless of the film thickness and Zr/(Zr+Ti) ratio. Epitaxial relationship of (001)/(100)PZT //(100)
Figures 7(a) - (f) summarize
As shown in Fig. 7, our experimental data are in good agreement with reported data of powders. However, an intermediate region can easily be observed in the 250 nm thick sample. This region could be related to the coexistence of both tetragonal and rhombohedral phases (Morioka et al. 2004a), suggesting a strain relaxation mechanism at this thickness (Morioka et al. 2004b).
The c-domain relative volume fractions,
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,
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.
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 (
Notice that the 250 nm thick film with Zr/(Zr+Ti)=0.19 showed high leakage current level that cannot display
We notice on these figures that fully polar axis oriented [(001)-oriented] films with 50 nm in thickness exhibit larger
To get insight into this issue, we calculated the spontaneous polarization (
On this figure,
We notice that the estimated
A good explanation of this latter result might be given by getting insight into the relationship linking tetragonality (
Indeed, the unit cell distortion inducing
Our data seem to be in agreement with the quadratic form presented in equation (1) linking
We also characterized the relative dielectric constant (
On the other hand, we noticed that squareness in
Epitaxial PZT thin films were grown at 540°C on SrRuO3-coated (001) SrTiO3 substrates by pulsed MOCVD. To characterize the impacts of the Zr/(Zr+Ti) ratio and the film thickness on the volume fraction of
HRXRD characterization showed that 50 nm thick films present a fully polar-axis oriented tetragonal films regardless of Zr/(Zr+Ti) up to 0.54, while 250 nm thick films are tetragonal single phase for the films with the Zr/(Zr+Ti) ratio smaller than 0.45, then, coexistence of tetragonal and rhombohedral phase from Zr/(Zr+Ti)=0.45 to 0.6. Appearing of two symmetry coexistence is related to the stress relaxation process occurring at relatively thick film.
Concerning electrical properties, 50-nm-thick PZT films with fully polar-axis orientation present larger Pr and Psat values than thicker films. On the other hand, we investigated the impact of Zr/(Zr+Ti) ratio on the ferroelectricity of PZT films, showing the linear relationship between Psat2 and the c/a ratio of the films.
This work was partially supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan for the Elements Science and Technology Project (21360316 and 20047004)”
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