The Domain Structure And Evolution Of PZT Ferroelectric Thin Films Studied By Femtosecond Laser Method

Ferroelectric thin films have been widely used in many devices,such as ferroelectric memories,piezoelectric sensors and infrared detectors,due to the excellent ferroelectric,piezoelectric,pyroelectric and electro-optical effect.Domain switching of ferroelectric film is the foundation of ferroelectric memories.Understanding the structures and evolutions of the ferroelectric domains plays an important role in the research,optimization and production of ferroelectric thin film devices.However,the written polarization often suffers from a gradual backswitching,which leads to retention loss.In other words,the polarization tends to relax to lower energy orientations after the removal of the external electric field.In order to clarify and solve this problem,researchers have conducted extensive investigations on the timescale and dynamics of the domain switching.However,the ferroelectric domain switching in the sub-picosecond is still not very clear.Due to non-contact,non-destructive and high time resolution,femtosecond laser technique has become an important method for characterizing materials in recent years.The Terahertz time domain spectrum(THz-TDS)and second harmonic generation(SHG)methods,which are based on the interaction of light and polarization,are effective femtosecond laser methods for studying the polarization properties of ferroelectric thin films.Since the the wireless communication develops continuously to high frequency(the laboratory stage has reached to 300 GHz),the performance of ferroelectric thin films at high frequencies is becoming more and more important and attracts more attentions.Compared with other ferroelectric thin films,Pb(Zr,Ti)O3(PZT)epitaxial films have more excellent ferroelectric properties,such as high remanent polarization,low coercive electric field and good retention performance,and have become the research hotspots.In this paper,the THz-TDS and SHG techniques are used to study the physical properties within THz band and the domain structures of epitaxial PZT thin films.The main contents are given as follows:1.We studied the THz absorption spectrum of epitaxial PZT thin films with different crystal orientations and different Zr/Ti ratios by THz-TDS.The ferroelectric thin films we used in these experiments are PZT thin films prepared by pulse laser deposition(PLD).It was found that the THz absorption spectrum of PZT thin films changed with the crystal orientations and Zr/Ti ratios,which affect the refractive indexes,domain structures and lattice constants.The(110)-oriented PZT thin film has stronger THz absorption spectrum than those of(001)and(111)-oriented PZT thin films,because it has more out-of-plane domains(c domains)and less domain wall densities.The THz absorption spectrums of PZT films decrease with the increase of Zr/Ti ratios.This may because the refractive indexes decrease and the reflectivity increase with the increase of Zr/Ti ratios.2.The ferroelectric domain structures and its evolution of PZT thin films were studied by SHG.Firstly,the SHG signals were analyzed theoretically,and the relationships between domain distributions and SHG signals were obtained.Secondly,we studied SHG of PZT/DSO thin film and PZT/SRO/STO thin film without external electric field,respectively.For PZT/DSO thin film,the SHG signal patterns could be obtained by changing the polarization of the incident light.It was found that the SHG signal patterns did not change obviously when changing the azimuth angle of the sample,indicating that the domains has a uniform distribution in all directions.The PFM test result of PZT/DSO thin film verifies this conclusion.Then the information of the ratios of different domains and the nonlinear coefficients was obtained by fitting the SHG patterns.For PZT/SRO/STO film,we changed the polarization of the incident light to get SHG signal patterns.The information of the ratios of different domains and the nonlinear coefficients was obtained by fitting the SHG patterns,which is also in accordance with the PFM test result.Finally,the evolution of PZT/SRO/STO domain structures under the external electric field was studied by SHG.The bias dependences of the SHG intensity shows a hysteresis loop(a delayed effect),indicating that the SHG signal can reflect the evolution of ferroelectric domains,and is an important means to characterize the ferroelectric domain structures.