Lead Titanate Ferroelectric Thin Films Research Articles

Polarization direction and stability in ferroelectric lead titanate thin films

In this article, we examine the initial polarization of PbTiO3 thin films grown epitaxially on SrRuO3 electrodes. It is found that the as-grown predominant polarization is directed toward the SrRuO3 bottom electrode in films thinner than 20 nm and directed toward the top surface in thicker films. The data is interpreted in terms of a Landau–Ginzburg–Devonshire model for a semiconducting ferroelectric with asymmetric boundary conditions. Based on the measured hysteresis loops and the stability of the two polarization directions with time, it is concluded that charged defects serve to impose a preferential downward polarization in very thin films.

The effect of different electrode structures on the dielectric properties of lanthanum-doped lead titanate ferroelectric thin films

Lanthanum-doped lead titanate [(Pb0.9,La0.1)TiO3, PLT10] ferroelectric thin films were grown on Si(100) and Pt/Ti/SiO2/Si(100) substrates by radio frequency (RF) magnetron sputtering. The crystalline properties of PLT10 films were studied by X-ray diffractometry (XRD). Photolithographic technique was applied to fabricate the interdigital electrodes on PLT10 thin films on Si(100) substrates. The dielectric properties of PLT10 thin films with different electrodes were measured. At room temperature and 1 kHz testing frequency, the dielectric constant of the PLT10 thin film with interdigital electrodes is 386. The dielectric constant of the PLT10 thin film fabricated under the same technological conditions with parallel plate electrodes structure is 365, while the dielectric constant and loss of the PLT10 thin film with interdigital electrodes are decreased faster than those of the film with parallel plate electrodes with increasing frequency. This is because more influences of interface state are introduced due to the interdigital electrode configuration.

Nanoscale banded 90° domain patterns in (111) oriented (Pb,La)TiO3 thin films

Lanthanum doped lead titanate ferroelectric thin films were grown on Pt∕Ti∕SiO2∕Si(100) substrates using rf magnetron sputtering. X-ray diffraction patterns show that the main phase of the films is tetragonal perovskite with preferred orientation of (111) perpendicular to the substrate. We investigated the ferroelectric domain patterns and the corresponding polarization arrangements of the films using piezoresponse force microscopy. Head-to-tail polarization configurations were often found to exist, indicating that the low electrical energy configurations are quite common in these thin films. Nanoscale banded 90° a-a domain patterns as small as 30–60nm in width were observed. The banded domains alternated with adjacent lamellae can be attributed to mechanical strains at the surface and at the interface between the substrate and the film.

Morphology control of ferroelectric lead titanate thin films prepared by electrostatic spray deposition

Ferroelectric PbTiO3 thin films of various surface morphologies (dense or highly porous) were prepared on Si(111) and Pt/Ti/Si(111) substrates by electrostatic spray deposition (ESD). Substrate temperature was the key factor affecting the film morphology, and the appropriate substrate temperature was a little higher than the boiling point of the solvent in precursor. The PbTiO3 films prepared by repeated deposition/pyrolysis cycles were porous due to the preferential deposition of charged droplets. Modified ESD was developed to densify the porous films by repairing the cracks and pores in the porous films. The dielectric constant and the dissipation factor of the PbTiO3 film at 100 kHz were 177 and 0.025, respectively. A model was proposed to illustrate the deposition process of the films.

Morphology control of ferroelectric lead titanate thin films prepared by electrostatic spray deposition

Ferroelectric PbTiO 3 thin films of various surface morphologies (dense or highly porous) were prepared on Si(111) and Pt/Ti/Si(111) substrates by electrostatic spray deposition (ESD). Substrate temperature was the key factor affecting the film morphology, and the appropriate substrate temperature was a little higher than the boiling point of the solvent in precursor. The PbTiO 3 films prepared by repeated deposition/pyrolysis cycles were porous due to the preferential deposition of charged droplets. Modified ESD was developed to densify the porous films by repairing the cracks and pores in the porous films. The dielectric constant and the dissipation factor of the PbTiO 3 film at 100 kHz were 177 and 0.025, respectively. A model was proposed to illustrate the deposition process of the films.

Low temperature preparation of piezoelectric thin films by ultraviolet-assisted rapid thermal processing

Ca- and La-modified lead titanate ferroelectric thin films were prepared by a sol–gel method from photosensitive solutions and with an ultraviolet (UV)-assisted rapid thermal processor, including an arrangement of excimer lamps. The diol-based route was used in the preparation of the precursor solutions, whose UV absorption and thermal decomposition were determined. Multiple deposition and crystallization steps of the deposited solution were used to promote preferential orientation. A comparative ferro and piezoelectric study of films prepared by conventional rapid thermal processing at 650°C and films prepared at 550°C with an intermediate step of UV irradiation at 250°C will be presented to assess the value of these films for their use in integrated piezoelectric sensors and microelectromechanical systems. Piezoelectric d 33 and e 31 coefficients were determined by double-beam laser interferometry and by the direct piezoelectric effect on cantilevers, respectively. The effect of the substrate and processing method on the preferential crystalline orientation of the films and the corresponding piezoelectric properties will be reported. The role of the composition will also be discussed.

Ca and La-Modified Lead Titanate Sol-Gel Thin Films by UV-Assisted Processing for Piezoelectric Sensors

A diol-based sol-gel method was used to prepare UV sensitive precursor solutions in the processing of modified lead titanate ferroelectric thin films. Films were deposited by spin-coating onto platinized Si-based substrates. A <111> preferential orientation was promoted by appropriated choice of the substrate and multiple deposition and crystallization steps. Processing under UV radiation was carried out in a novel RTP equipment including an arrangement of high intensity UV excimer lamps. To obtain the desired ferroelectric perovskite structure, moderate temperatures and times were required, with respect to those of the processing without UV irradiation. XRD measurements were carried out to determine the degree of orientation of the films. Ferroelectric characterization was made to investigate the optimum poling conditions of the films. Piezoelectric coefficients, e 31 and d 33 , were determined in order to evaluate the feasibility of the use of the films in piezoelectric sensors.

Stress-induced depolarization of (Pb, La)TiO3 ferroelectric thin films by nanoindentation

Electrical depolarization has been observed in lanthanum-modified lead titanate ferroelectric thin films stressed by nanoindentation. A spherical metallic indenter was used as a top electrode to locally pole the films and then to measure the depolarization current intensity. The current intensity had distinctive maxima at given indentation forces. These are related to the stress thresholds for the depolarization mechanism, which is probably 90° domain wall movements. Knowledge of the depolarization stresses is necessary for the design of microelectromechanical systems that include a ferroelectric layer.

Mechanical characterisation of ferroelectric thin films for MEMS

The optimum design of MEMS based on ferroelectric thin films requires knowledge of the mechanical properties of the films. This means not only of their elastic coefficients, but also of the stability of the ferroelastic domains under stresses, and of their plastic and fracture behaviour. We present here a procedure using nanoindentation with a spherical indenter geometry capable of providing the elastic, anelastic and plastic contributions to the deformation of a film as a function of the indentation force. The procedure was tested on two lanthanum modified lead titanate ferroelectric thin films. The indentation stiffness coefficient of the films was found to be 130 GPa for both. The probable ferroelastic origin of the anelastic deformation is discussed. Pore compaction was found to be a major source of permanent deformation.

Raman Studies of the Effects of Nb Dopant on the Ferroelectric Properties in Lead Titanate Thin Film

Raman spectroscopy was used to study the effects of Nb additives in ferroelectric lead titanate thin films. Raman spectra were measured from lead titanate thin films with Nb concentrations between 0 and 5%. The Nb additive was found to significantly influence the profile of the A1(2TO) mode, which is the relative vibration between the Ti atom and other atoms along the c-axis, indicating that the doped Nb occupied a Ti position. A decrease of all transverse modes followed the increase in Nb. The observed mode frequencies were used to extract the dependence on Nb addition of the clamped dielectric constants. The dielectric constants along both the a- and c-axes increase with increasing Nb concentration.

Measurement of Electrical Charge During Nanoindentation of Ferroelectric Thin Films

Electrical charge transients have been measured during the nanoindentation of lanthanum modified lead titanate ferroelectric thin films. Using the spherical indenter as the top electrode, the films were locally poled. The films were then reindented at the exact same positions. The origin of the charge released is discussed in terms of piezoelectricity and depolarisation. The depolarisation must have been produced by a stress induced ferroelastic domain reorientation. Current intensity transients recorded during indentation showed a distinct maximum. This maximum corresponded to a force at which the depolarisation rate was a maximum. This provides the basis to define a coercive indentation force, and then a coercive indentation stress.

Lead titanate ferroelectric thin films modified with 20% lanthanum

Lanthanum modified lead titanate ferroelectric thin films with a composition Pb 0.7 La 0.2 TiO 3 have been prepared by a diol based sol-gel technique. The effect on the film structure, microstructure and composition of the initial PbO content, from the nominal value to an excess of 20 mole%, and of the heating rate of the thermal treatment of crystallisation at 650°C, from 10°C min -1 to more than 500°C min -1 , has been studied. It is not possible to obtain single phase films with the treatment at 10°C min - 1 , even when PbO volatilisation is compensated with an initial excess of PbO of 20 mole%. It is possible with the treatment at more than 500°C min - and the latter excess of PbO. These films present a relative dielectric permittivity at room temperature of e= 700, and a variation between room temperature and 100°C of Δe= 75, being both parameters very interesting for DRAMs.

Pyroelectric properties of sol-gel processed lanthanum modified lead titanate ferroelectric thin films

Textured lanthanum modified lead titanate ferroelectric thin films have been prepared on Pt/Si substrates by a diol-based sol-gel procedure. Precursor solutions including a growing excess of PbO, and crystallization with a heatin rate higher than 500°C min -1 , are used. Pb/Ti ratio in films is measured by EDXS, and is found to increase systematically with the excess of PbO included in solutions. Such increase in Pb/Ti is likely related to the disappearance of a Pb deficient second phase, reaching the nominal value of the perovskite in those films prepared from solutions including a 20% excess of PbO Films dielectric constant, e, remanent polarization, P R , and pyroelectric coefficient, γ, measured in poled samples, increase as the Pb/Ti ratio, reaching values of -730, ∼25 μC cm -2 and ∼ 1.4x10 -2 μC cm -2 K - in those with the nominal ratio of the perovskite. Infrared sensor figures of merit for voltage responsivity, F v γ/ e, and specific detectivity, F m γ/ (e) 1/2 , are calculated. The possibility of using films containing small amounts of second phase, with a smaller pyroelectric coefficient, but with a smaller dielectric constant, is discussed.

Study of the surface layer of lead titanate thin film by x-ray diffraction

Lead titanate ferroelectric thin films are prepared by a modified sol-gel process and the differences between the surface and bulk structure are studied. It is experimentally investigated with glancing angle x-ray diffraction (GAXRD) that the lattice constants of the surface layer of lead titanate thin film are differing from those of the inner part, crystallite size and lattice strain increase towards the substrate.

SOL-GEL derived ferroelectric lead titanate thin films on doped silicon substrates

Abstract PbTiO3 thin films were fabricated by the spinning method in the sol-gel process. Films with thickness of 0.15 μm to 4.0μm were coated on doped silicon substrates, using a multiple deposition process. The processing yields highly reliable crack-free films with easy control of crystal structure and composition at firing temperature of 550 to 750 °C. The very fine grain size in PbTiO3 films was studied as a function of firing temoperature. The ferroelectric and dielectric properties of the films were investigated using metal-ferroelectric-semiconductor configaration, and were strongly dependent on the film thickness and the firing temperature. Good ferroelectric properties can be obtained in the films of thickness more than 2μm fired at 600-650 °C.

Strain relaxation by domain formation in epitaxial ferroelectric thin films.

The origin of strain-induced, modulated domain structures observed in epitaxial ferroelectric lead titanate thin films is discussed using a phenomenological total-energy calculation. Linear elasticity is used to account for the substrate contribution while a free-energy functional of the Landau-Ginzburg-Devonshire type is used to calculate the domain-wall and the polarization contributions from the film. Good agreement between the predictions of this model and the experimental results is found for thickness-dependent properties such as the relative domain population and spontaneous strain.

Chemical vapour deposition of PbTiO3 films onto TiO2-Si

Ferroelectric lead titanate thin films were deposited on the TiO2 coated Si(100) substrate by chemical vapour deposition under atmospheric pressure. We have used Pb powder and tetraethyle-orthotitanate [Ti(C2H5O)4] as the source material of Pb and Ti vapour. We investigated the variations of the phase of deposited films with the Ti(C2H5O)4 input fractions. When the mole fraction was 0.021, PbTiO3 single phase was obtained. For the other Ti(C2H5O)4 input fractions, we could find PbO solid solutions phase in addition to the PbTiO3 phase. The results of AES analysis revealed that the TiO2 layer on the Si substrate acted as diffusion barriers between Si substrate and PbTiO3 films.