Optical properties of lead magnesium niobate-lead titanate (PMN-PT) and strontium barium niobate (SBN) thin films

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Optical properties of lead magnesium niobate-lead titanate (PMN-PT) and strontium barium niobate (SBN) thin films


Author: Chan, Ka-yi
Title: Optical properties of lead magnesium niobate-lead titanate (PMN-PT) and strontium barium niobate (SBN) thin films
Year: 2005
Subject: Hong Kong Polytechnic University -- Dissertations
Thin films -- Optical properties
Pulsed laser deposition
Department: Dept. of Applied Physics
Pages: vii, 126 leaves : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1793780
URI: http://theses.lib.polyu.edu.hk/handle/200/1497
Abstract: Epitaxial (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3((1-x)PMN-xPT) films x=0.1,0.3,0.35 and 0.4 grown under different deposition temperatures (630 C to 710 C) were fabricated on MgO substrates using pulsed laser deposition (PLD). X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to characterize the structural properties of these (1-x)PMN-xPT films. Our results show that these films possess excellent structural properties and are cube-on-cube grown on MgO substrates. Spectroscopic ellipsometer (SE) was used to characterize the depth profiles, the microstructural inhomogeneities, including void and surface roughness, refractive indicies and extinction coefficients of the films. In the analysis of the measured SE spectra, a double-layer Lorentz model was adopted to represent the optical properties of the (1-x)PMN-xPT films. On the basis of these results, the effects of PT composition and deposition temperature on the optical properties of (1-x)PMN-xPT were revealed. Our measurements show that the refractive indices of the (1-x)PMN-xPT films increase with PT contents. This dependence is consistent with our optical transmission measurements which revealed decreasing energy band gaps of the (1-x)PMN-xPT films with increasing PT content. Our ellipsometric measurements show that a maximum refractive index of the (1-x)PMN-xPT films is obtained at a deposition temperature of 670 C. No observable change in the band gap energy for various deposition temperatures is detected. This last feature is also confirmed by our independent optical transmission measurements. Thus the change in refractive index due to deposition temperature is attributed to the change in structural properties. Sr0.6Ba0.4Nb2O6 (SBN60) films of different annealing temperature (200 C to 700 C) and durations (0.5 h to 10 h) were fabricated by the sol-gel technique. Our structural characterizations show that these films are amorphous for annealing temperatures <= 500 C, and polycrystalline for annealing temperatures >= 600 C. In the analysis of the measured SE spectra, a triple-layer Lorerntz model was used to interpret the optical properties of the SBN6O films. SE measurements reveal that the refractive indices of SBN6O films increase with annealing temperature. The increase is more significant at around the crystallization temperature of the films, i.e. between 500 C and 600 C. The extinction coefficients also possess a similar trend showing zero values for amorphous films and large values (for example, 0.1 at 2.5 eV) for films annealed above 600 C. Our results demonstrate that the crystallization of the films has an important influence on the refractive index as well as extinction coefficient. Although crystallization can raise the refractive index of the film because of the film densification, it also promotes scattering due to a larger extinction coefficient. Similar phenomena are confirmed by annealing the films for different durations.

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