Influence of nanoparticle seeding on the phase formation kinetics of sol-gel-derived Sr_0.7Bi_2.4Ta₂O₉thin films

Abstract

Sr 0.7 Bi 2.4 Ta 2 O 9 (SBT) thin films were deposited on unseeded and SBT nanoparticle (approximately 60-80 nm) seeded Pt/Ti/SiO 2 /Si substrates via sol-gel and spin-coating techniques. The SBT thin films were heated at 600°C for 1 h to form the fluorite phase, and these fluorite films were further heated at 730-760°C for fluorite-to-Aurivillius phase transformation. The volume fractions of Aurivillius phase formation obtained through quantitative x-ray diffraction analyses showed highly enhanced kinetics in seeded SBT thin films. Johnson-Mehl-Avrami isothermal kinetic analyses were performed for the characterization of Aurivillius phase formation in unseeded and seeded SBT thin films using the volume fraction values. The Avrami exponents were determined as approximately 1.4 and approximately 0.9 for unseeded and seeded SBT films, respectively, which reveals different nucleation modes. By using Arrhenius-type plots, the activation energy values for the phase transformation of unseeded and seeded SBT thin films were determined to be approximately 264 and approximately 168 kJ/mol, respectively. This gives a key reason for the enhanced kinetics in seeded films. Microstructural analyses on unseeded SBT thin films showed formation of randomly oriented needlelike crystals, while those on seeded ones showed formation of domains comprising directionally grown wormlike crystals. On the basis of the phase formation kinetics and microstructural development, a model representing different nucleation and crystal growth mechanisms for the unseeded and seeded SBT thin films was proposed.

Comparing macroscopic and microscopic properties of seeded ferroelectric thin films

“…Literature reported that SBT films may be synthesized at temperatures ranging from 700°C (when using Rapid Thermal Annealing) to 780-800°C (when using conventional annealing) [5,6,[8][9][10][11]. The presence of fluorite as an intermediate phase that can be converted to layered perovskite was commonly observed.…”

“…This conversion was improved by the introduction of seeds. Previous studies on seeded SBT thin films reported that the use of seeds lowers the incubation time and temperature of the fluorite-to-Aurivillius phase transformation, while enhancing the transformation kinetics due to a decrease of the activation energy [5,6].…”

“…More recently, the modification of the microstructural and electrical properties of ferroelectric thin films prepared via this methodology have been reported [4][5][6]. The overall activation energy for the perovskite crystallization was reduced, and the pure crystalline phase was obtained at low temperatures [7].…”

Strong thickness dependence of aurivillius phase formation in SrBi2Ta2O9 thin films

“…The detail preparation route of Sr-Bi-Ta methoxyethoxide stock solution was described in the previous publications [6,7,15]. The SBT thin films were prepared on the Pt/Ti/SiO 2 /Si wafers with 4 × 4 cm in size using spin coating (Headway Research.…”

“…Strontium bismuth tantalate (SBT) has been identified for potential applications, such as a ferroelectric layer in capacitors for non-volatile ferroelectric memories (NvFRAMs) [5][6][7][8][9]. The Pt/SBT/Pt capacitors exhibit almost polarization fatigue free operation for up to ∼10 12 switching cycles, low operation voltage, little tendency to imprint, and relatively low leakage, making SBT capacitors one of the strongest candidates for application in the first generation of NvFRAMs.…”