Relaxor ferroelectric $Pb(Sc_{0.5} Nb_{0.5})O_3$ and its solid solutions with ferroelectric $PbTiO_3(PT)$ have been the focus of intense researche interests in light of their application potential and interesting scientific phenomena. In recent years, fabrication and characterization of relaxor ferroelectrics and their solid solution thin films with PT are much desired in device oriented application fields such as micro-actuators, micro-sensors, image transducers and MEMS. Basic studies to distinguish the physical properties of thin films from those of bulk materials may shed light on relaxor’s behavioral phenomena. Relaxor-based thin films are of interest because they exhibit large electrostrictive strains and field induced piezoelectric coefficients that are comparable to those of lead zirconate titanate(PZT) films, but favorably with much smaller hysteresis loss.
For the last few years, attempts were made to prepare and investigate the properties of these materials in thin film form. However, due to difficulties in preparation of the pure perovskite PSN and PSN-PT thin films without precipitating pyrochlore phase, there have been scarce reports on the preparation and properties of these thin films. The objectives of this thesis are to prepare pure PSN and PSN-PT thin films near the MPB composition without any second phases present and to investigate the crystal structure and microstructure of relaxor-ferroelectric thin films which critically determines the dielectric and electromechanical properties in comparison to these of the bulk materials. The studies on these thin films were performed along the four major directions: the precursor synthesis of PSN and PSN-PT chemical solutions, preparation of thin films and characterization of micro-structural morphology, dielectric and electromechanical properties. For submicron observation, PSN ceramics were prepared first and then their microstructural change evolution with temperature were looked into by TEM.
Stoichiometric but disordered PSN ceramics has shown the spontaneous phase transition from the relaxor to normal ferroelectric phase. Morphological change of domain patterns during heating and cooling were investigated by TEM. It has been observed that ferroelectric domains change continuously from macrosize domains to nanosize domains.
The synthesis of chemical precursors prior to the preparation of PSN and PSN-PT thin films near MPB compositions will be explained in detail. Developing the homogeneous and stable precursor solutions requires strict control and the following purification-processing is very important to obtain perfect pervskite PSN, PSN-PT thin films without second phases precipitation.
In developing defect free thin films, special efforts were made such as implementing new heat treatment methods (2-step pyrolysis and 2-step annealing), tveating substrates with oxide seed layers with $TiO_2$, $(La,Sr)CoO_3$, optimally controlling excess PbO and $Sc_02O_3$. As crystallization of pure PSN and PSN-PT thin films generally takes place at high temperatures (over 680℃) on silicon wafers, a new heat-treatment method has been devised to stimulate easy nucleation and to facilitate high densification of thin films at the preferably low optimum temperature. Preparation of the substrates that induce the nucleation of pure perovskite phase at low temperatures and homogeneous grain growth with preferred orientation are also very much desired. Introduction of oxide seeding layers such as $TiO_2$, $(La,Sr)CoO_3$ causes the optimization of the narrow processing window. Effects of the prepared substrates with different seeding layers have been investigated. It has been found that the crystallographic orientation and microstructure can be modified by controlling the nucleation and growth of thin films by seeding layers on different substrates. In lead based compounds, optimizing the amount of excess PbO is very important parameter. From the conclusion drawn from the results of TEM investigations, the amount of excess PbO and $Sc_2O_3$ was adjusted.
PSN thin films can show typical relaxor behaviors: frequency-temperature dependence of the dielectric permittivity and slim ferroelectric hysteresis loops at room temperature. However, several differences exist between the properties of PSN thin films and bulk materials, the maximum transition temperature shifts less and the overall permittivity as value is smaller in thin film form than in bulk form. The reason for these differences is still not clear yet. It may result from: (1)the small thickness of the films(size effect); (2)undetected small amounts of a residual pyrochlore phase or defects within films that can reduce the permittivity, even if they are not detected by XRD and TEM; (3)clamping of the thin films by that substrate(strain effect); (4)a dead layer between the ferroelectric film and the substrate; (5)the lower sintering temperature (700℃) compared to that of the bulk materials (1,500℃).
Especially, the ac and dc field dependences of the dielectric response are investigated. The change of the relative permittivity as a function of temperature at different frequencies for different ac applied fields exhibits the same behavior as in single crystals and bulk ceramics that show the increasing amplitude of the applied field to shift the maximum in “ε’ vs. T” to lower temperatures. Due to the thin thickness of films, it is possible to measure the relative permittivity versus temperature response under much higher ac driving field than reported in bulk or singe crystal materials. Relaxation behaviors and frequency dispersions increase with the increasing ac field amplitude. The change of the relative dielectric permittivity and loss as a function of temperature at different applied dc fields during cooling is investigated. Contrary to what happened in bulk ceramics, no anomaly associated with field-induced transition into a ferroelectric phase is observed in field cooling. Disappearance of the frequency dispersion and the stable relative permittivity have been observed over a wide temperature range under dc field. The dielectric losses are also reduced by dc field.
Electrostrictive coefficient, M and Q, are determined by measuring strain S and polarization P as a function of the electric field $(E_ac)$. At large fields $(>2.6×10^6 V/m)$, S vs. $P_2$ appears to deviate from the linear behavior suggesting that the electrostrictive coefficient Q becomes nonlinear in this field range. PSN thin films show the piezoelectric response in absence of the dc electric field $(d_33=60pm/V)$.
In epitaxial PSN films deposited on $SrTiO_3(100)$ single crystal substrate, the difference of morphology and size of polar regions between PSN ceramics and thin films is also investigated by TEM.
PSN-PT thin films with the (111) texture crystallized on $TiO_2/Pt/TiO_2/SiO_2/Si$ substrate has been investigated. They show normal ferroelectric phase transition behaviors. However, their maximum permittivity value is still lower than that of PSN-PT ceramics. It can be related to the properties of the substrates and electrodes used, or the processing conditions. Even, with the current dielectric and electromechanical constant values, PSN and PSN-PT thin films are still useful at temperatures near the permittivity maximum. They can become alternative candidates to replace PZT thin films for precise micro-positioning actuators for instance. The electrostrictive strain level is comparable to the piezoelectric strain in PZT ferroelectrics having smaller hysteresis. In the temperature vs. permittivity curve, under dc-field application, the temperature of the maximum dielectric constant of thin films does not shift much with the field. Except that, the general feature of the thin film curve maintains that of the bulk samples.
