Enhanced ferroelectric polarization in epitaxial $(\mathrm{P}{\mathrm{b}}_{1-x}\mathrm{L}{\mathrm{a}}_x)\left(\mathrm{Z}{\mathrm{r}}_{0.52}\mathrm{T}{\mathrm{i}}_{0.48}\right){\mathrm{O}}_3$ thin films due to low La doping

Enhanced polarization is reported in ferroelectric $(\mathrm{P}{\mathrm{b}}_{1-x}\mathrm{L}{\mathrm{a}}_x)\left(\mathrm{Z}{\mathrm{r}}_{0.52}\mathrm{T}{\mathrm{i}}_{0.48}\right){\mathrm{O}}_3$ (PLZT) thin films at low La doping concentrations. Epitaxial PLZT thin films with varying La concentrations were grown in both (001) and (111) crystallographic orientations using the conducting perovskite oxide $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ as the top and bottom electrodes on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates by pulsed laser deposition. Dilute doping of La (0.1–0.5 at. %) was found to enhance the remanent polarizations in the PLZT films irrespective of their orientations. An increase in the remanent polarization by a factor of two is observed in the 0.5 at. % La doped PLZT thin film as compared to the undoped (Pb)$(\mathrm{Z}{\mathrm{r}}_{0.52}\mathrm{T}{\mathrm{i}}_{0.48}){\mathrm{O}}_3$ (PZT) film. The increase in polarization in the PLZT thin films was associated with the tetragonal distortion of the PLZT lattice due to the substitutional doping of La at the Pb sites, as evidenced from x-ray and microstructural analyses. Using the structural and polarization data, a linear correlation between the square of the remanent polarization and crystal-lattice distortion is obtained in the PLZT system, which corroborates with the Landau-Ginsburg-Devonshire thermodynamic model of lattice distortion-induced change in spontaneous polarization. This paper provides a novel route to enhance polarization in PLZT films, which is crucial for the coherent design of high-performance PZT-based ferroelectric and piezoelectric devices.

Figure 1

Crystallinity and orientation in $(\mathrm{P}{\mathrm{b}}_{1-x}\mathrm{L}{\mathrm{a}}_x)\left(\mathrm{Z}{\mathrm{r}}_{0.52}\mathrm{T}{\mathrm{i}}_{0.48}\right){\mathrm{O}}_3$ (PLZT) thin films with varying La concentrations of 0 at. %, 0.1 at. %, 0.5 at. % and 1.0 at. %, denoted as undoped PZT, 0.1 at. % PLZT, 0.5 at. % PLZT and 1.0 at. % PLZT, respectively. The XRD θ-2θ patterns for PLZT thin films grown on (a) STO (100) and (b) STO (111) substrates, respectively. (c) The XRD φ scans performed about the PZT (101) plane and (d) XRD ω scans performed about the PZT (002) plane for PLZT thin films grown on STO (100) substrates, respectively. (e) Room temperature Raman spectra for PLZT thin films grown on STO (100) substrates. Insets to (a) and (b) show the details of the STO and LSMO peaks at small 2θ ranges.

Figure 2

Lattice parameters and tetragonality in $(\mathrm{P}{\mathrm{b}}_{1-x}\mathrm{L}{\mathrm{a}}_x)\left(\mathrm{Z}{\mathrm{r}}_{0.52}\mathrm{T}{\mathrm{i}}_{0.48}\right){\mathrm{O}}_3$ (PLZT) thin films with varying La concentrations of 0 at. %, 0.1 at. %, 0.5 at. %, and 1.0 at. %, denoted as undoped PZT, 0.1 at. % PLZT, 0.5 at. % PLZT, and 1.0 at. % PLZT, respectively. The XRD symmetric θ-2θ scans performed about the PZT (002) and PZT (111) planes for PLZT thin films grown on (a) STO (100) and (b) STO (111) substrates, respectively. (c) The XRD asymmetric 2θ-ω scans performed about the PZT (111) plane for PLZT thin films grown on STO (100) substrates. (d) Lattice parameters ($a$ and $c$) and tetragonality ($c/a$) in PLZT (001), and PLZT (111) thin films. (e) Reciprocal space map performed about STO (103) for 0.5 at. % PLZT thin film grown on LSMO/STO (100) substrate. The LSMO (103), STO (103), and PLZT (103) peaks are labeled. The position of the bulk is indicated with a white dot. Intensities from low to high: blue, green, yellow, red, and brown.

Figure 3

(a) Cross-sectional TEM image of the 0.5 at. % $(\mathrm{P}{\mathrm{b}}_{1-x}\mathrm{L}{\mathrm{a}}_x)\left(\mathrm{Z}{\mathrm{r}}_{0.52}\mathrm{T}{\mathrm{i}}_{0.48}\right){\mathrm{O}}_3$ (PLZT) thin film grown on LSMO bottom electrode on $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) (100) substrate. The HRTEM images of (b) PLZT-LSMO interface and (c) PLZT layer captured from the regions marked with red and blue boxes in (a) of the 0.5 at. % PLZT film. The images in (b) and (c) have been captured with the electron beam parallel to the [011] direction of the PLZT lattice. The crystal planes in both PLZT and LSMO have been marked with arrows. (d), (e) The SAED patterns captured near the PLZT-LSMO interface of the (d) 0.5 at. % PLZT film and (e) the undoped PZT film, respectively. The direction of the $c$ axis and the tetragonal symmetry of the PZT unit cell has been indicated by the white box and arrow. Comparing the white boxes in (c) and (d), a larger tetragonality of the PLZT unit cell can be observed as compared to undoped PZT unit cell. The arrows in (e) indicate the slightly mismatched lattice parameter of PZT and LSMO. (f) Typical AFM image of the PLZT (001) layer exhibiting the smooth particulate-free surface with low roughness ($R_{\mathrm{rms}}\sim 4\mathrm{nm}$) and uniform grains of sizes $\sim 170\pm 10\mathrm{nm}$ captured from the 0.5 at. % PLZT (001) film.

Figure 4

(a), (b) The $P-E$ curves for undoped PZT, 0.1 at. % PLZT ($\mathrm{P}{\mathrm{b}}_{1-x}\mathrm{L}{\mathrm{a}}_x\mathrm{Z}{\mathrm{r}}_{0.52}\mathrm{T}{\mathrm{i}}_{0.48}{\mathrm{O}}_3$), 0.5 at. % PLZT, and 1 at. % PLZT thin films grown along (001) and (111) orientations, respectively, measured at driving voltage of 9 V. Insets to (a) and (b) show the schematic diagrams of the capacitor geometries with LSMO as top and bottom electrodes.

Figure 5

Correlation between lattice distortion and polarization in PLZT thin films. (a) Plots of squares of remanent polarization ($P_r$) and lattice distortion $(c/a-1)$ in the PLZT thin films grown along (001) and (111) orientations, denoted as PLZT (001) and PLZT (111), respectively. The linear fits of the data are also shown by red and blue lines. (b) Schematic illustration depicting the enhancement in tetragonality in the undoped PZT unit cell as a result of substitutional doping of $\mathrm{L}{\mathrm{a}}^{3+}$ in the $\mathrm{P}{\mathrm{b}}^{2+}$ sites in the PLZT unit cell (shown by curved blue arrows) and the creation of defect dipoles between the substituted $\mathrm{L}{\mathrm{a}}^{3+}$ and ${{\mathrm{V}}_{\mathrm{Pb}}}^{2+}$ dipoles within the PLZT unit cell (shown by straight upright violet arrow). The direction of applied field is shown by a red arrow.