Investigations on the structural, microstructural, dielectric, ferroelectric, and leakage characteristics of sol-gel derived lanthanum modified lead titanate systems.

摘要

The first part of the work was devoted to understand and optimize the effect of various complex process variables such as the precursor chemistry, effect of thickness, and role of bottom electrodes to grow high quality, stable and reproducible electrical properties of sol-gel derived ferroelectric thin films. The next phase was to systematically study the effect of La doping (0.0 to 30 at.%) in lead titanate thin films on the structural, microstructural evolution, optical, and electrical properties. The choice of precursor materials, have significant impact on the structural and electrical properties of PLT thin films. The process variables are adjusted to get films with good electrical properties. Raman scattering and x-ray diffraction techniques were used to study the correlation between the film thickness and the structural changes in PT films of 21, 64, 128, 210, 310, and 420 nm thicknesses. An exponential decrease in stress with increasing film thickness was observed because of the structural changes in the lattice. Accordingly, a downshift in the transition temperature with decreasing film thickness has been interpreted using the Landau-Devonshire approach. Dielectric, ferroelectric and leakage current characteristics of PLT films deposited on Pt, Pt/Si, RuO₂/Pt/Si and RuO₂/Si bottom electrodes were evaluated. Observed ferroelectric and dielectric properties of the films have been correlated with large interfacial resistance at the film-electrode interface. The La modified lead titanate Pb_1−xLa_xTi_1−x/₄ O ₃ (PLT) (x = 0.0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) thin films were deposited by sol-gel technique. The films were characterized, using various techniques, in terms of their phase formation behavior, composition, microstructure and electrical characteristics. The X-ray diffraction data and micro-Raman analysis show that with the increase in La content the crystal quality of the PLT films undergoes a tetragonal-to-cubic transformation. The optical properties of these films were investigated. The observed variation of band-gap energy and other optical properties with La doping has been correlated with the observed microstructure of these films. The dielectric properties of PLT thin films were studied in the temperature range 80–700 K and frequencies (1kHz–1MHz). Results indicate that PLT films undergo normal-to-relaxor ferroelectric transformation with 30 at% La content in PLT films. The observed behavior is evaluated in terms of diffuseness and Vogel-Fulcher relationship, typical for relaxor ferroelectrics. Novel Graded heterostructure films were synthesized by sol-gel technique on platinum substrates exhibited relatively higher values of P_m and P_r (69 and 38 μC/cm 2, respectively) and excellent dielectric properties with lower loss (K = 1900, tanδ = 0.035 at 100 kHz). The ac electric field dependence of the permittivity at sub-switching fields was analyzed in the framework of the Rayleigh dynamics of domain walls.