  Bi2O2Se has attracted intensive attention due to its potential in
electronics, optoelectronics, as well as ferroelectric applications. Despite
that, there have only been a handful of experimental studies based on ultrafast
spectroscopy to elucidate the carrier dynamics in Bi2O2Se thin films, Different
groups have reported various ultrafast timescales and associated mechanisms
across films of different thicknesses. A comprehensive understanding in
relation to thickness and fluence is still lacking. In this work, we have
systematically explored the thickness-dependent Raman spectroscopy and
ultrafast carrier dynamics in chemical vapor deposition (CVD)-grown Bi2O2Se
thin films on mica substrate with thicknesses varying from 22.44 nm down to
4.62 nm at both low and high pump fluence regions. Combining the thickness
dependence and fluence dependence of the slow decay time, we demonstrate a
ferroelectric transition in the thinner (< 8 nm) Bi2O2Se films, influenced by
substrate-induced compressive strain and non-equilibrium states. Moreover, this
transition can be manifested under highly non-equilibrium states. Our results
deepen the understanding of the interplay between the ferroelectric phase and
semiconducting characteristics of Bi2O2Se thin films, providing a new route to
manipulate the ferroelectric transition.

Materials Science