Study Of Polarization-controlled Carrier Relaxation Behavior In Perovskite Ferroelectrics By Ultrafast Spectroscopic

The inherent spontaneous polarization characteristics of ferroelectric materials can show many physical properties（such as photoelectric,piezoelectric,pyroelectric and other effects）through the coupling with external excitations such as optical field,electric field,force field,and temperature field,making ferroelectric materials have a wide range of applications in different fields such as random access memory,photodetector,piezoelectric sensor,optical modulator,acousto-optic deflector,optical holographic memory,and optical frequency multiplier.In recent years,with the rise of hot spots such as the ferroelectric photovoltaic effect,the research and application of traditional ferroelectric materials in the field of optoelectronics has gained more attention.The photodetection,photoelectric sensing,and photovoltaic properties of ferroelectric materials are particularly closely related to the motion behavior of photogenerated carriers in ferroelectric polarization fields.The ferroelectric polarization behavior has microphysical processes on the time scale of nanoseconds（ns）to femtoseconds（fs）,and the existing electrical testing techniques can only provide the means to study the changes of physical processes with time scales below nanoseconds（ns）.Especially in the study of optoelectronic behavior（electron excitation,carrier lifetime and relaxation process）in ferroelectric materials.There is a need to provide research methods that can work on the picosecond（ps）to femtosecond（fs）time scale.Femtosecond time-scale ultrafast spectroscopy has extremely high time resolution,which is very important for the study of the micro-relaxation process of photo-generated carriers and photo-generated carrier lifetimes by ferroelectric polarization in ferroelectric materials on the ps～fs time scale.The main work of this thesis is to use femtosecond time-resolved pump-probe spectroscopy to study the effect of ferroelectric polarization on the microdynamics of photo-generated carriers at the picosecond to femtosecond time scale.In this paper,three representative ferroelectric systems:Ba Ti O₃,Ni²⁺:Pb Ti O₃ and Sr²⁺:Pb Ti O₃ are selected as the research objects.The change of ferroelectric polarization are achieved through temperature changes,applied electric fields,and doping,with particular attention to crystal structure,phase transitions,polarization orientation（applied electric field）,and changes in electron-hole concentration（equivalent and unequivalent doping）.And the femtosecond pump-probe reflection spectroscopy is used to analyze and study the regulation behavior and mechanism of the above-mentioned ferroelectric polarization change on the relaxation process of photo-generated carriers.The main research work and the results obtained are as follows:1.Ba Ti O₃ ceramic polycrystalline samples are prepared by solid-phase sintering method.Using variable temperature XRD,raman spectroscopy and dielectric spectroscopy characterization methods,it is confirmed that the lattice structure of Ba Ti O₃ ceramics can transform from tetragonal to cubic with temperature.Using ultrafast femtosecond time-resolved reflectance spectroscopy,the change of photogenerated carrier lifetime of Ba Ti O₃ ceramics under different polarization intensities and the influence of polarization direction on the carrier relaxation process are investigated by methods such as temperature change and pre-polarization.The study found that:1)When the temperature is lower than the phase transition temperature（400 K）,the polarization strength of Ba Ti O₃ decreases with the increase of temperature.According to the second-order fitting analysis of the ultrafast reflectivity change process,it is found that the photogenerated carriers mainly relax from the conduction band to the valence band in the form of electron-hole recombination.When the temperature increases,the polarization intensity decreases,and the lifetime of photogenerated carriers also shortens.According to the band-bending model induced by the depolarization field,the polarization strength decreases with increasing temperature,the band-bending decreases,and the energy barrier for electron and hole recombination decreases.As a result,the recombination rate of photogenerated carriers is increased,and the lifetime of photo-generated electrons/holes is shortened.2)Ultrafast time-resolved pump-probe reflectance spectroscopy measurements of femtosecond pulses are performed on surfaces with different polarization directions,respectively.It is found that there is a faster carrier relaxation process（ps time scale）when the sample surface is in an upwardly polarized state（separated photogenerated electrons accumulate on the surface）.The main reason for this phenomenon is that the mobility of electrons in the domain is often larger than that of holes,and electrons can form a shield against the original depolarization field before holes.This results in a significantly faster electron relaxation process on the upwardly polarized surface on the picosecond scale（～1 ps）.2.The Pb Ti_1-xNi_xO₃ series ferroelectric thin films with highly preferential orientation are prepared by sol-gel method.By changing the polarization intensity of the film with different doping amounts,the photogenerated carrier dynamics of PTNO ferroelectric films with different polarization intensity was studied by using the time-resolved reflectance spectroscopy of ultrafast femtosecond pulsed laser.The study found that:1)With the increase of doping amount,the lattice structure of PTNO ferroelectric thin film gradually changed from the tetragonal phase to the more symmetrical cubic phase.At the same time,the proportion of the a-domain structure in the film gradually decreases,and the polarization strength of the PTNO film also weakens.In PTNO,the polarization intensity is the dominant factor for the separation degree of the carriers in the photocurrent,and the magnitude of the photocurrent is closely related to the polarization intensity;2)On the basis of the ultrafast time-resolved reflectance spectral data,the quasi-first-order formula fitting analysis of the photo-generated carrier dynamics of the equivalent doped PTNO system was carried out.In the case of excluding the influence of diffusion,Auger recombination and electron-phonon coupling,it is determined that the relaxation mechanism of the bottom electrons in the conduction band to the valence band in PTNO is mainly carried out in the form of electron-hole recombination.And the photogenerated carrier lifetime is positively correlated with the polarization intensity;3)Comparing the three physical parameters of photogenerated carrier lifetime,photocurrent and remanent polarization,it is found that the greater the ferroelectric polarization,the stronger the corresponding depolarization field,which is more effective for the separation of photogenerated electrons and holes.The electron-hole recombination can be better suppressed,thereby prolonging the lifetime of photogenerated carriers and increasing the size of the photocurrent.3.The Pb_1-xSr_xTi O₃ ferroelectric thin films with highly preferential orientation and high quality are prepared by sol-gel method.Using ultrafast femtosecond time-resolved reflectance spectroscopy,the photogenerated carrier dynamics of PSTO ferroelectric films with different polarizations and different phase structures are investigated.The research found that:1)With the increase of Sr doping amount,the curie temperature of PSTO gradually decreased,and the polarization intensity at room temperature weakened accordingly.When the doping amount reaches 0.5（x=0.5）,the tetragonal to cubic phase transition occurs.The magnitude of the PSTO photocurrent is positively correlated with the polarization intensity.And the photocurrent at the transition of the phase structure changes abruptly,which indicates that the influence of ferroelectric polarization plays a leading role in this process.2)The photogenerated carrier dynamics process of the equivalently doped substituted PSTO system is analyzed by the second-order formula,and it is determined that the relaxation process is mainly the electron-hole recombination mechanism at the bottom of the conduction band.Comparing the lifetimes of photogenerated carriers under different polarization intensities,it is found that there are two time stages in the relaxation process of photogenerated carriers:less than 1 ps and greater than1 ps.The reason for this phenomenon is the rapid establishment（～1ps）of the shielding electric field opposite to the depolarization field.3)The lifetime of photogenerated carriers is positively correlated with the polarization in the time scale of 1 ps after the relaxation.When the time exceeds 1 ps,the rapid establishment of the shielding electric field weakens the separation ability of the ferroelectric polarization（depolarization field）for photogenerated carriers.At this time,the control of the lifetime of the photogenerated carriers by the change of the depolarization field intensity becomes no longer obvious.From the calculation of the average drift velocity of electrons in the ferroelectric domain,it can be known that,the establishment time of the shielding electric field formed by the separation of the depolarization field is in the ps scale,which is consistent with the experimental results.