  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.
