Enhancing Vertical Polarization in Aurivillius Phase Ferroelectric Thin Films

Ferroelectric thin films hold significant promise for a wide array of device applications, including synaptic devices based on Ferroelectric Tunnel Junctions (FTJs), non-volatile memory, and data storage. While in-plane polarized devices represent a more recent design development, vertically polarized thin films have undergone over a decade of advancements, particularly in improving compatibility with CMOS technology and working prototypes. However, despite their established industrial use in smart cards and ferroelectric RAMs (FE-RAMs), Aurivillius phase thin films are not ideal for current storage device architectures, such as FTJs, due to their predominantly in-plane polarization. This research aims to address this limitation by enhancing the vertical polarization of Aurivillius phase ferroelectric thin films through the controlled synthesis of growth spirals that lower symmetry in the vertical axis for enhanced ferroelectric response. To achieve this, industrially relevant Direct Liquid Injection Chemical Vapor Deposition (DLI-CVD) processes were developed to optimize vertical polarization in odd-layered Aurivillius films. By carefully controlling supersaturation and oxygen partial pressure, the growth mechanism could be directed towards either 2D nucleation/growth or dislocation-mediated spiral growth. Piezoresponse Force Microscopy and Atomic Resolution Electron Microscopy revealed that films exhibiting growth-spiral morphology, accompanied by associated dislocations, demonstrated significantly enhanced out-of-plane (OOP) or vertical ferroelectric properties. Notably, the minimum vertical switching voltage was reduced from ±20V to ±5V. This enhancement is attributed to the disruption of elastic strain, electrostatic energy, and local symmetry-lowering distortions within the lattice. These findings provide valuable insights into the tailored growth of Aurivillius phase thin films and present new opportunities for precisely tuning their ferroelectric and multiferroic properties, increasing their candidature for a wide range of device applications.