Preparation Of Doped-PZT Thin Films And Its Microstructure And Ferroelectric Properties

Pb(Zr_0.52Ti_0.48)O₃ thin films with Gd, Yb, Gd-Nb dopants (denoted as PGZT,PYZT,PGNZT) and different orientation were prepared on Pt/Ti/SiO2/Si substrates by the sol-gel technique and rapid thermal annealing (RTA) process. Effects of processing parameters including pyrolyzing and post-annealing temperature on the perovskite phase formation and orientation of PZT films were investigated systematically. The results showed that pyrolyzing temperature over 350℃and RTA at 600⁶50℃were the favorable processing for the formation of doped PZT thin films with single perovskite structures. The pyrolyzing temperature has a crucial effect on the orientation of PZT films. (111)-oriented PGZT and PYZT films can be obtained at low pyrolyzing temperature, while (100)-oriented PGZT and PYZT films at a relative high temperature. Gd and Yb dopant contents have little influence on the orientation of the films. Gd and Nb co-doped PZT thin films are polycrystalline prepared by RTA process. Highly (111)-oriented PGZT, PYZT and PGNZT films were obtained by composite thermal annealing. Pyrolyzing temperature has little influence on the orientation of PZT films prepared by composite thermal annealing. With the increase of treating time on hot plate, the (111)-orientation degree of films increased.Mechanism for the growth of oriented doped PZT thin films was discussed. The growth of sol-gel derived PZT thin films is controlled by the nucleation and the nuclei growth. The Pb-rich intermetallic phase formatted in pyrolyzing procedure may affect the selection of texture in PZT films. Pb-rich intermetallic phase (transient compound) in PZT/Pt boundary is veried at different pyrolyzing temperature. When the transient compound in PZT/Pt boundary is well epitaxial with Pt(111), the (111)-oriented PZT films can be obtained by RTA process, in which PZT films grow on the transient compound as a seeding layer. The intermediate phase in PZT/Pt boundary provides a good lattice-matching with PZT (100),the (100)-oriented PZT films can be obtained. During composite thermal annealing, PZT thin films are partially crystallized with (111)-orientation by annealing on the hot plate, and grains grow on the condition of RTA.ω-scan XRD technique was utilized to analyze the texture of PZT films. It showed that (100)-textured PGZT and PYZT films can be formed by pyrolyzing at 450℃and RTA at 650℃. (111)-textured PGZT, PYZT and PGNZT films can be prepared by composite thermal annealing process.Based on the analysis of SEM and AFM, an obvious difference of crystallization properties of PZT thin films with Gd, Yb, Gd-Nb dopants are observed. Compared with undoped PZT thin films, grains with large size and uniform distribution are shown in PZT with 1mol% Gd and 1²mol% Yb dopants, while small sized grains exist in PZT films with more than 1 mol% Gd and 2mol% Yb dopants. Large sized grains could be observed in PGNZT films with the increase of Nb content.Improved electric properties such as increased dielectric constant and remanent polarization, lowered leakage current and coercive field, improved antifatigue properties of PZGT1 and PYZT1 films were obtained. Compared with undoped PZT films, the antifatigue performance of PGZT2 film was improved, the other ferroelectric properties decrease, and there was no obvious improvement of its dielectric properties.The doping content of Gd should be no more than 2mol%. Ferroelectric and dielectric properties of PYZT2 films were improved obviously. The Yb doping level should be no more than 3mol%. Porper composite doping level and ratio of Gd and Nb may promote ferroelectric properties of PZT thin films. Highly (111)-oriented PGNZT films possess improved antifatigue performance in contrast with (100) and (111)-oriented PGZT and PYZT films.Theoretical calculation of tolerance factor, analysis of defect chemistry and Rayleigh law show that Gd and Yb act as donor dopants in low content level and donor effect is dominant. Therefore improved leakage current and polarization fatigue of the films are obtained. With the doping level increased, the acceptor effect could be dominant. Based on this result, electric properties deteriorate. Ferroelectric properties of PGNZT films are better than that of PGZT and PYZT films,which could be attribute to dual-donor effect of Gd-Nb co-doping.