Microstructure Control And Electric Properties Of PbTiO₃-Based Ferroelectric Thin Films

PbTiO₃(PT)-Based thin film has broad application prospects in pyroelectricdetectors, dynamic random access memory, non-volatile ferroelectric memories andmicro-electro-mechanical systems. However, the PT-based ferroelectric thin films,especially used in practical devices, still have some obstacles, generally includingmaintaining the good electrical properties of bulk materials, reducing processingtemperature, increasing fatigue fatigue resistance and lowing leakage currentcharacteristics. In addition, for PT-based thin films in pyroelectric applications, in orderto increase the figure of merit, it is necessary to maximize pyrocoefficient and lower thepermittivity. In this paper, we use sol-gel technology to prepare Pb(Zr_0.52Ti_0.48)O₃(PZT),Pb(Nb_0.01Zr_0.2Ti_0.8)O₃(PNZT) and Pb_0.8La_0.1Ca_0.1Ti_0.975O₃(PLCT) thin films, and studyorientation control, low-temperature crystallization process, interface optimization,residual stress and electric properties of films.By changing pyrolysis temperature, the PZT films with different orientation typeson Pt(111)/TiO₂/SiO₂/Si substrate are successfully obtained, it is found that the lowerpyrolysis temperature is benefit to the formation of (111)-oriented film; and the higherpyrolysis temperature is benefit to the formation of the (100)-oriented film. For the PZTfilms in the vicinity region of the morphotropic phase boundary(MPB), it is also foundthat the pyrolysis temperature of sol-gel process can change the phase composition ofthin films. R or M phases are main phase compositions in (111)-and mix-oriented PZTfilms, and T is main phase composition in (100)-oriented PZT films. Because ofdifferent orientation types and phase compositions, the PZT films exhibit quite differentpolarization behaviors under external electric field. When the external electric field issmall, the remanent polarization value obtained in the (100)-oriented PZT film with Tphase is lower than that in (111)-and mix-oriented film with R or M phases. Only whenthe external electric field is large, the (100)-oriented film with T phase shows largerremnant polarization.For the (100)-oriented PLCT films, the pyrolysis temperature not only changes thedegree of film orientation, but also change the value and symbols of residual stress inthe films. On this basis, the influence of orientation degree and residual stress on phasetransition temperature in (100) oriented PLCT films is studied,and the experimentalresults are consistent with the theoretical simulation on the bases of Landau-devonshiretheory. At last, because pyroelectric property is closely related to its phase transitiontemperature, the influence of residual stress on pyroelectric property in (100) oriented PLCT films are analyzed.Low temperature growth of highly (100)-oriented PLCT thin film on Pt/Ti/SiO2/Sisubstrate, as low as 450°C, is successfully achieved by a sol-gel route. By using PLCTfilm as seeding layer, we successfully achieved low temperature growth of PNZT thinfilms at 450 oC, and crystalline orientation types and ferroelectric properties of PNZTthin films were strongly influenced by the thickness of PLCT seeding layer, withincreasing the thickness of PLCT seeding layer, the PNZT films changed from (111)-oriented to (100)-oriented obviously. In addition, the PLCT seed layer providesnucleation sites and reduces the activation energy for the crystallization of PNZT films,which was beneficial to obtain superior electrical properties of PNZT thin film. ThePLCT seed layer also helps to attain better PNZT/PLCT/Pt interfaces, which can help toreduce or compensate for the vacancies or other point defects at the interface. Thesereasons may cause the increased fatigue fatigue resistance and low leakage currentcharacteristics.Using the advantages of low temperature crystallization and high orientation inPLCT film, a dense PLCT/porous PLCT/dense PLCT sandwich structure was obtained.It is found that the prosity was introduced by a polymer in the core layer, and thedielectric constant was sharp reduced. In addition, because of high orientation insandwich structure, the pyrocoefficient is still keeping a large value. Due to a largepyrocoefficient, low dielectric constant and prosity-free surface, the sandwich structureresulted in superior pyroelectric figures of merit and low leakage current. It is alsofound the heating rate in the process of crystallization has a very significant impact onorientation, ferroelectric and pyroelectric properties of PLCT sandwich structure films.Based on the advantages of low temperature crystallization of PLCT fiom, the highly(100)-oriented PLCT/PNZT multilayer film is obtained at 450°C, it is shown that theclear boundaries between layers in films after low-temperature crystallization, andinterfacial diffusion are relatively small. Compared to PLCT film, the dielectric constantis reduced effectively and the pyroelectric figure of merit is enhanced. Moreover, theresults indicate that the PLCT/PNZT multilayer film annealed at low temperature(450°C) is a good candidate for integrating ferroelectric thin film directly with a Sibasedreadout integrated circuit, this case has important significance in practicalapplication.