Textured organic ferroelectric films from physical vapor deposition and amorphous-to-crystalline transition

TL;DR

This paper presents a low-temperature physical vapor deposition method with restrained crystallization to produce textured organic ferroelectric films, achieving controlled morphology and properties comparable to single crystals.

Contribution

Introduces a novel fabrication process combining vapor deposition and in situ crystallization monitoring for organic ferroelectric films.

Findings

Films exhibit micron-thick disk-shaped morphology with aligned polarization.

Crystallization occurs mainly during post-deposition warming, confirmed by in situ measurements.

Spontaneous polarization and coercive field are comparable to single-crystal values.

Abstract

Crystallization is a key for ferroelectricity which is a collective behavior of microscopic electric dipoles. On the other hand, uncontrolled crystallization leads to uneven morphology and random crystal orientations, which undermines the application potential of ferroelectric thin films. In this work, we introduce a film fabrication method of low-temperature physical vapor deposition followed by restrained crystallization, with electrical properties monitored in real-time by in situ measurements. This method was adopted to fabricate films of 2-methylbenzimidazole (MBI), whose molecule crystals are proton-transfer type biaxial ferroelectrics and tend to grow into a hedgehog-shaped spherulites morphology. The in situ measurements confirm that the crystallization, corresponding to a clear transition of physical properties, occurs dominantly during post-deposition warming. This enables the fabrication of micron-thick films in disk-shaped morphology with one polarization axis aligned along the out-of-plane direction, while the measured spontaneous polarization and coercive field are comparable to the single-crystal values. These results mark an important advancement of film growth that is expected to benefit widely the fabrication of molecular materials films whose functional properties hinge on crystallization to achieve desirable morphology and crystallinity.