The Study Of Imaging Mechanism Of Piezoresponse Force Microscopy And Domain Dynamics In Electro-optical Crystal

Ferroelectric materials are widely used in modern technologies in order to exploit their ferroelectric, piezoelectric, and pyroelectric properties. Standard devices generally use microscopic patterned electrodes on ferroelectric ceramics, single crystals, or films to produce actuators, filters, resonators, sensors, and memories. However, the continuing demand for miniaturization in acoustic applications, in ultrahigh density information storage, and in micro-electromechanical systems (MEMS) has made alternative solutions necessary.In the late 20th century, the rapidly developed Piezoresponse Force Microscope (PFM), which can be used for static domain structure characterization and real-time dynamic domain writing as a new experiment tool, has been widely used in ferroelectric domain structures and ferroelectric properties characterization, low-voltage high-density ferroelectric memory research in thin film, ferroelectric domain inversion in bulk materials, static and dynamic analysis of ferroelectric domain structures at nano-scale. PFM has become an important tool for experimental study of domain dynamics.In this thesis, we studied the influence of feedback parameters on PFM imaging and the phase imaging in high frequency range in the contact resonance regime in lithium niobate (LN) crystal. We also fabricated the dot, strip and planar domain structures based on nanolithography technology, in which the physical mechanism of the growth and relaxation process was studied. The innovative achievements are as follows:(1) Influence of feedback parameters on PFM imaging and high-frequency phase imagingwe systemically analyzed the impacts of the feedback parameters on ferroelectric domain imaging with PFM, it is found that:(1) the PR amplitude and phase differences decrease with the increasing drive frequency below cantilever resonance for the electrostatic interactions, while its fluctuations are induced by the resonance of sample; (2) The drive phase in PFM imaging can be considered as a constant adding to the PR phase signal, which does not alter the PR amplitude and phase contrast; (3) The PR amplitude is proportional to the drive amplitude while the PR phase is drive amplitude independent. The larger piezoelectric vibration amplitude and fitting PCs calculated by vectorial analysis compared with the known value are originated from the sample resonance, and the local electrostatic force can lead to a nonlinear shift of the measured PCs; (4) The large contact force determined by the deflection setpoint can effectively suppress the piezoelectric displacement and PCs, which could further influence the PR image contrast. Besides the frequency dependent PFM image contrast, the impacts of the electrostatic force and contact force inevitability make the quantitative analysis of PFM image and measurements of PCs difficult. Thus, the drive amplitude and setpoint should be carefully chosen in PFM imaging.The drive frequency dependent PR phase and the corresponding PR phase difference of the antiparallel domains are carefully investigated by frequency sweep method. It is found that, whether the sample is ferroelectric or not, the system-inherent background that is determined by universally existed local and nonlocal electrostatic interaction has a significant impact on the PR phase signal in any cantilever and tip-sample interaction system. The PR phase difference can be boosted and achieve the peak values at contact resonances. The frequency shift of PR phase difference at high eigenmodes is induced by the change of tip-sample stiffness, which is sensitive to the modulation voltage magnitude related indentation force.(2) Domain dynamics in dot, strip and planar domain structuresWe fabricated one-dimensional strip, two-dimensional dot and non-regular domain structures by scanning probe patterning technique and the radius of the fabricated dot domain is in the nano-scale range of 200 nm.We fabricated dot domain of different size using scanning probe microscopy with plus magnitude of 80-120 V and plus width of 0.1-200 s. It is found that switched domains growth process consists of two stages with plus width in the range of 0.1 s<τ<1 s and τ>1 s. We introduced a parameter a in domain dynamic equations, which describes the compensation of the polarization charges by free charges. By modifying the existing dynamic theory, we estimated the domain size in both of the stages and gave a theoretical description of the dynamic processes:the domain growth starts with nucleation, and then domain expands in a non-activated type under strong polarization field, during which the domain size reaches a critical size and penetrates the sample, which results in lower the depolarization energy, and then domain further creeps horizontally in the next stage in a activated type.Strip domain structures of different width were fabricated by scanning probe microscope using different polarization magnitudes and scanning rates. Roughness exponent was obtained by fitting the function of correlation length, which is extracted from the PR phase images. It is found that the roughness exponent ζ decreases from 0.8 to about 0.6 with the increase of domain width, which indicates that the random disorder in the system changes form the combined effects of the defect related random bond disorder and the charge related random field disorder to the single effect of random bond with the growth of domain width. Furthermore, as the increase of the domain width, the domain wall can optimize the elastic energy to equilibrium state in much smaller length scale because of the weakening of the internal disorder.Strip domain structures of different width were fabricated by scanning probe microscope using different polarization magnitudes and scanning rates in LN, it is found:(1) switched domain width increases linearly with the increasing bias strength, while decreases exponentially with the increasing scan rate; (2) after the polarization bias removed, the switched domains decay started at the narrow region of the strip domain, where the surface energy is larger, the process is comparable with domains relaxation theory in LN crystal proposed by Molotskii et al.;(3) The minimum stable sizes of both the strip domains fabricated by polar scan method and dot domains tailored by fixed point poling method increase exponentially with sample thickness, but the strip domain has much smaller minimum stable size due to its small average surface energy originated from its smaller surface area.Planar domain structures were fabricated by polar scan method; the domain switching and growth process were studied. It is found:(1) The shape of reversed domain is hexagonal with small scan area and the domain boundary is constructed by Y domain wall only; (2) while with increase scan area, the shape of reversed domain transformed from hexagon to square, the domain boundary transformed to the composition of both X and Y domain walls; (3) Y domain wall formed in priority under strong polarization electric field, which suggests that Y domain wall has a lower free energy comparing to X domain wall; (4) the growth rate of the reversed domain area decreases with the increase of scanning area, which is induced by the fixed scan line numbers in the polar scan method. The decrease of the average reversed area over the perimeter of the setting area is also related with the formation of the Y domain wall; (5) although the reversed domain area decreases with the increase of the scan rate, the reversed domain area is not sensitive to the scan rate.