Enhancing Vertical Ferroelectric Polarization Using Growth Spirals

Aurivillius phase materials are valuable ferroelectrics, renowned for their high Curie temperatures (>600 °C) and fatigue resistance, enabling their commercial application in low-power ferroelectric random-access memory (Fe-RAM). Despite their promising attributes, incorporating Aurivillius phases into miniaturized devices presents significant challenges, primarily because their polarization is largely confined to the in-plane direction. This presents a challenge for industry, as conventional vertical devices are designed to operate with out-of-plane (OOP) polarization, making integration with laterally polarized materials difficult. To address this, industrially relevant Direct Liquid Injection Chemical Vapor Deposition (DLI-CVD) processes were developed to enhance OOP polarization and vertical ferroelectric properties in odd-layered Bi4Ti3O12 thin films. By controlling supersaturation and oxygen partial pressure, the growth mechanism is directed to proceed through either two-dimensional nucleation and growth or dislocation-mediated spiral growth. Piezoresponse force microscopy and atomic resolution electron microscopy studies reveal that films exhibiting growth-spiral morphology show enhanced vertical ferroelectric properties. Notably, the minimum vertical switching voltage is reduced from ±20 V to ±5 V. The improvement is attributed to defect-mediated modifications of elastic strain energy, localized electron densities and local symmetry-lowering distortions within the crystal structure. These findings offer valuable insights into the tailored growth of Aurivillius phases, specifically designed for miniaturized, energy-efficient memory and neuromorphic computing applications.