Ferroelectric Thin Films by Metal Organic Chemical Vapour Deposition

ABSTRACTThere is a growing interest in thin films of ferroelectric oxides because of their electronic and optoelectronic applications. Various growth processes are being explored, but here, we review progress with chemical vapour deposition in a purpose built low pressure reactor. The two ferroelectric perovskites selected for our initial studies were lead titanate and lead scandium tantalate which have necessitated the synthesis of proprietary precursors. These compounds are based on metal alkoxides and β-diketonates, and are suitably modified to exhibit the required volatility and necessary thermal and hydrolytic stabilities.Deposition has been studied over the temperature range 400–800°C and, in general, amorphous films result which can be converted by subsequent annealing to crystalline perovskites. However, the inclusion of hydroxyl group compounds (H2O or alcohols) in the vapour train catalyses the crystallisation process and enhances the growth rates at temperatures in excess of 600°C. In order to deposit the perovskite phase, it is important to maintain the gas phase composition throughout the growth. Deposition rates of up to 10μm/hour can be achieved, but the best thin films, in terms of density and morphology, are formed at lower deposition rates. The crystallite size of the deposit may vary from 0.1μm up to 2μm, depending on temperature. The choice of precursors, gas compositions and the growth conditions will be discussed in conjunction with the electrical and structural properties of the layers grown.