Structure-related optical properties of (Pb,La)(Zr,Ti)O3 thin films on indium tin oxide∕quartz substrates

Abstract

To be suitable for integrated optical devices, (Pb,La)(Zr,Ti)O3 (PLZT) ferroelectric thin films require high crystalline quality, low surface roughness, high optical index, and high transparency. In this paper, PLZT thin films have been grown in situ on indium tin oxide (ITO) coated quartz substrates by rf magnetron sputtering. X-ray diffraction, scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to investigate the structural properties of these PLZT films. The results show that the ferroelectric films exhibit satisfying crystallization with the highly (110)-oriented growth from 550°C, and the surface roughness value (∼3.1nm) in studied films is within the optimum range so that a low optical loss can be obtained. High quality PLZT ferroelectric thin films were further investigated by electrical measurements, showing that the remnant polarization Pr and coercive field Ec are approximately 11.3μC∕cm2 and 56.2kV∕cm, respectively. Spectroscopic ellipsometry (SE) was employed to characterize the depth profiles, the microstructural inhomogeneities (void and surface roughness), refractive index n, and extinction coefficient k of the PLZT film. In the analysis of the measured SE spectra, a three-layer Lorentz model with four oscillators was adopted to represent the optical properties of the PLZT film. In this model, the film was assumed to consist of two layers (a bottom bulk PLZT and a surface layer composed of bulk PLZT as well as void). Good agreement was obtained between the measured spectra and the model calculations. The film thickness measured from SEM is consistent with that obtained by SE, while the root mean square roughness determined by AFM is also close to our fitted effective surface layer thickness obtained by SE. The PLZT thin film on ITO-coated quartz substrate is highly transparent in the visible near infrared wavelength region, and the band gap energy Eg is estimated to be 3.54eV. The experimental results above tend to demonstrate the suitability of the PLZT films in situ grown on ITO∕quartz substrates for optical applications.