Phase development, microstructure, and ferroelectric properties of sol-gel derived strontium bismuth tantalate thin films

Abstract

Ferroelectric materials have been studied extensively over the past decade or so as potential candidates for memory applications, since they possess a unique set of properties, including fast polarization switching and non-volatility. The two major candidate materials being studied for integration into ferroelectric random access memories (FeRAMs) are lead zirconate titanate (PZT) and strontium bismuth tantalate (SrBi₂Ta₂O₉ or SBT). Overall, PZT has probably been the most extensively studied of the two materials, however SBT has received a lot of attention over the past few years due to its fatigue resistance, large remanent polarization (Pᵣ), and low coercive field (E(c)) on standard metal electrodes. The purpose of this project was to study the factors that influence the development of ferroelectric properties of sol-gel derived SBT thin films, including composition, heat treatment, phase development, and microstructure. The results show that SBT crystallite size is the single most important factor in determining the ferroelectric properties of SBT compositions with Sr contents ranging from stoichiometric to 20% deficient. SBT volume fraction also plays an important role. Therefore, composition, heat treatment, etc., are only important in that they help establish the SBT crystallite size and SBT area fraction of a particular film. Two strategies for improving the polarization of SBT at lower temperatures, which include the use of highly Sr deficient films, and Bi₂O₃ coatings as a flux, were also studied.