Effects of composition of PbTiO₃ on optical properties of (1-x) PbMg[sub ⅓]Nb[sub ⅔]O₃-xPbTiO₃ thin films

Abstract

Thin films of (1-x)PbMg[sub ⅓]Nb[sub ⅔]O₃-xPbTiO₃ (PMN-PT) with x=0, 0.1, 0.3, 0.35, and 0.4 have been fabricated on (001)MgO single-crystal substrates by 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 PMN-PT films. Our results show that these films possess excellent structural properties and are cube-on-cube grown on (001)MgO substrates. Spectroellipsometry (SE) was used to characterize the depth profiles, the microstructural inhomogeneities, including void and surface roughness, refractive indices and extinction coefficients of the films. In the analysis of the measured SE spectra, a double-layer Lorentz model with four oscillators was adopted to represent the optical properties of the PMN-PT films. In this model, the films were assumed to consist of two layers—a bottom bulk PMN-PT layer and a surface layer composed of bulk PMN-PT as well as void. Good agreement was obtained between the measured spectra and the model calculations. The film thickness measured by SEM is consistent with that obtained by SE while the root mean square (rms) surface roughness determined by AFM is also close to our fitted effective surface layer thickness obtained by SE. Our measurements show that the refractive indices of the PMN-PT films increase with PbTiO₃ contents. This dependence is consistent with our optical transmittance measurements which revealed that the energy band gaps of PMN-PT films decrease with increasing PbTiO₃ contents. The correlation between the energy band gap and the refractive index is discussed.