Self-polarization Control Of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃Thin Films

In recent years, with the development of sensor, transducer and microelectronicmechanical system (MEMS), PMN-PT film near morphotropic phase boundary (MPB) hasreceived the most interest. Its high dielectric permittivity, great piezoelectric coefficient andpyroelectric coefficient make it promising in pyroelectric detector, non-volatile semiconductormemory, and MEMS et al. A pyrochlore phase is prone to forming in the preparation process.However, high annealing temperature (>650℃) is still required to avoid the formation of apyrochlore phase, which may hinder the applications of intergrated piezoelectric devices.LNO electrodes and PMN-PT films were prepared by the sol-gel method. The effect ofpretreating temperature on the structure of PMN-PT films was investigated. At the same time,we find that the annealing temperature and the crystallinity of LNO electrodes had effects onthe structure and properties of PMN-PT films. The study contents are listed as follows:1. In this part, the different pretreating temperature was used to fabricate the PMN-PTfilms, and the structure of PMN-PT film was studied. It was observed that a pyrochlore phaseexisted in all the films at the different pretreating temperature. Finally, we choosed300℃asthe pretreating temperature because of the least amount of pyrochlore phase and the weakestintensity of the (110)-oriented peak for the film pretreated at300℃.2. The effect of the annealing temperature on the structure and properties of PMN-PTfilm were investigated. The pyrochlore phase was observed at the annealing temperature of500and550℃, while it become undetectable at600and650℃. It was found that themonoclinic phase coexisted with tetragonal phase in PMN-PT films. With the increase ofannealing temperature, the peaks of tetragonal phase become more evident. With theinvestigations on the electrical properties of films, one can find that the pyrochlore phasedeteriorates the properties. In addition, the self-polarization characteristic can not be observedin the films annealed at500and550℃.3. Increasing the annealing temperature advoids the formation of pyrochlore phase,which hinder its applications of integrated piezoelectric devices based on relaxorferroelectrics. So for this reason, the effect of the crystallinity of LNO electrode on the structure and properties of PMN-PT films was investigated. The crystallization degree ofPMN-PT film had been enhanced by improving that of LNO electrode, and phase-purePMN-PT film with high (001) preferential orientation can be formed at the annealingtemperature as low as500℃. The splitting of the diffraction peak was more evident in thefilms deposited on LNO(100)FWHM=0.29°. It was well known that the piezoelectric coefficient ofthe film without poling was proportional to self-polarization. It can be observed that all thephase-pure PMN-PT films were self-polarized. The Pr value for the PMN-PT films annealedat650℃was about23μC/cm2. The dielectric permittivity the relative tunability were morethan3000and70%, respectively, and the average piezoelectric coefficient d33of the film was~257pm/V in the10μm×10μm detected areas. It was worth noting that the irregular brightstripes in the domain images match well with the protrudent stripes in the surfacemorphologies that have larger piezoresponse compared to the surrounding areas.