Fabrication and characterization of sol-gel derived (KxNa1-x)0.4(Sr0.6Ba0.4)0.8Nb2O6 (KNSBN)

Abstract

High quality potassium sodium strontium barium niobate powders and films with composition of K0.2Na0.2Sr0.48Ba0.32Nb2O6 (KNSBN) were prepared by sol-gel technique using methoxyethanol as the solvent. In the film fabrication, dip coating method was used and single crystal (100)Si wafers were employed as substrates. The effect of different annealing temperature on the degree of crystallization of KNSBN was characterized by differential thermal analysis (DTA), thermogravity analysis (TGA), X-ray diffraction (XRD) and Raman spectroscopy. As a result, powders started to crystallize at temperatures as low as 600 C and fully converted into tetragonal tungsten bronze (TTB) phase at temperatures higher than 1000 C. On the other hand, films can be crystallized at a lower temperature of 500 C and its orthorhombic tetragonal phase transition temperature occurred at 600 C. These temperatures are lower than those of sol-gel derived strontium barium niobate (SBN). The effects of sol concentration and thickness on the surface morphology of the films were measured by atomic force microscope (AFM). Small and spherical grains of uniform size distribution with no apparent grain agglomeration were observed at low sol concentration or for thin films. Increasing sol concentration or film thickness would enhance surface roughness. At the same time, the grain became bigger with a larger size distribution. As the sol concentration used is higher than 0.18 M. micro-cracks were observed on the film surfaces. Optical properties of single layer KNSBN films were studied by ellipsometric measurements. The single layer films consisted of two different sub-layers. The upper layer of the films, composed of KNSBN and void, had a smaller refractive index and extinction coefficient than that of the lower layer which composed of KNSBN only. This indicated that the bottom layer had a higher degree of crystallinity. Besides, the refractive indices and extinction coefficients of the films were increased with the sol concentration increased. However, all the values obtained in our films were smaller then that of KNSBN single crystal. That means the density of the films were still less than that of single crystal.