Research On Preparation Of Compositionally Gradient PZT Ferroelectric Thin Films Fabricated By A Combinatorial Chemistry Process

PZT thin film is a kind of functional materials with excellent piezoelectric and ferroelectric properties, and it has received great interest and been widely studied by many researchers. With the changes of the composition, PZT thin films have different structures and properties, but the current research mainly focuses on the PZT thin films with the composition around MPB, the systematic study on the PZT thin films have been seldom reported. Compositionally gradient PZT thin film is of interest to many researchers for its unconventional properties. However, the cause for the unconventional properties is indeterminate and controversial. The combinatorial synthesis process not only improves the efficiency of the experiment, but also eliminates the systematic errors.In present work, the PT and PZ precursor solutions was prepared without distillation and inert gas shielding at room temperature. Just by using the two precursor solutions, a series of PZT thin films were prepared by a combinatorial process using spin-coating technique. XRD analysis showed that the PZT thin films showed a perovskite structure with (111)-preferred orientation. The relationship between the lattice parameter and the composition is consistent with the PZT phase diagram. XPS result showed that the composition did not deviate significantly from the theoretical composition value for the most samples, except for the sample with composition of x=0.3, which is due to its special thermal chemical reaction behavior. AFM results showed that the films with high Ti content had higher crystallization degree, the surface was uniform and dense, the grain size was large. With the increases of Zr content, the crystallization degree became lower, the surface was uneven, the grain size was smaller, and there was agglomeration. The PZT thin films with composition of x=0.5,0.6showed poor surface morphologies. From x=0.7, as the Zr content increases, the crystallization degree became higher, but the compactness decreased. SEM cross-section morphologies of PZT thin film showed that the film was dense and uniform, attached on the substrate closely, the interface between film and bottom electrode was clear, the thicknesse of the film was around450nm. Ferroelectric hysteresis loops showed that all the PZT thin films had favorable ferroelectric properties. The films with high Ti content showed higher Pr value and Ec value, but the leakage current were larger. With the increases of Zr content, Pr and Ec decreased. The films with composition of x=0.5,0.6showed favorable ferroelectric properties. The films with composition of x=0.7,0.8,0.9showed lower Pr and Ec. The films with composition of x=0.9showed double hysteresis loops. The εr and tanδ increased at first, then decreased with the increases of Zr content. The εr and tanδ of the films with the composition around the MPB were maximum, the εr was about590, the tanδ was about0.05.The structures, compositions, morphologies and electrical properties of the PZT thin films showed that the combinatorial synthesis process could prepared PZT thin films with better qualities. This method could be used for the study of the compositionally gradient PZT thin films.On the base of the variation of the uniform composition PZT thin films, we systematic designed the compositionally gradient PZT thin films variation from the gradient composition, the gradient direction, the gradient span and the number of components. Using a combinatorial synthesis process, a series of compositionally gradient PZT thin films were conveniently prepared by a CSD method. AES result showed that the contents of Zr and Ti change gradually with depth in the direction from substract to the surface. XRD analysis showed that the films with smaller gradient span showed a perovskite structure with (111)-preferred orientation. With the increases of gradient span, the crystallization degree became lower, and presented multiphase. SEM cross-section morphologies of the film showed that the texture of the film was homogeneous and attached on the Pt electrode closely. The thickness of the film was around450nm. AFM results showed that the surface morphologies were significantly different between the up-gradient thin film PZT456and down-gradient thin film PZT654.The gradient composition, the gradient direction and the gradient span could affect the ferroelectric and dielectric properties. No typical offset was observed in the hysteresis loops, but the properties were different between the up-gradient thin films and down-gradient thin films. When the gradient span was smaller, the films with high Ti content showed higher Pr and Ec value; while the films with high Zr content showed higher Pr and Ec value which were different from the uniform composition PZT thin films. PZT456and PZT654showed favorable properties, but their ferroelectric properties were different. The Pr values were equal, while the Ec of PZT654was much lower than PZT456. Comparison of the ferroelectric properties of PZT654and PZT-0.5thin films. Although the total ratio of Zr/Ti were same, but PZT654exhibited better ferroelectric properties. With the increases of gradient span, the ferroelectric properties of the films deteriorated. PZT654exhibited better dielectric properties. Only the films with a reasonable gradient structure could enhance the dielectric properties.Experimental results indicated that design of the composition gradient structure of the thin films could lead us to get a favorite thin film with better properties. In our research, PZT-654showed better ferroelectric and dielectric properties.