Electrical properties of Bi4Ti3O12 (BiT) films on Si(100) were improved due to insertion of silicon oxynitride (SiON) buffer layers with thicknesses of 1–2 nm. Capacitance–voltage measurements indicated that the improvement was largely attributable to better Si interface properties rather than to the difference of the BiT film quality. By means of x-ray photoelectron spectroscopy and high-resolution transmission microscopy, the Si interfaces of the specimens with and without the SiON buffer layers were investigated. Consequently, we found that a postannealing treatment at 680 °C inevitably resulted in nonuniform growth of Si oxide layers at the Si interface of the specimen without the SiON buffer layer, and that the layer thickness mounted to approximately 10 nm. In contrast, 1–2-nm-thick SiON buffer layers terminated the growth of the additional oxide layer of less than about 3 nm, and the resulting Si oxide layers were quite uniform. © 2001 American Institute of Physics.