Hidden phase uncovered by ultrafast carrier dynamics in thin Bi2O2Se

TL;DR

This study investigates ultrafast carrier dynamics in thin Bi2O2Se films, revealing a ferroelectric transition in films thinner than 8 nm influenced by strain and non-equilibrium states, advancing understanding of their phase behavior.

Contribution

It provides a systematic analysis of thickness and fluence effects on carrier dynamics and uncovers a ferroelectric transition in ultrathin Bi2O2Se films, which was not previously well understood.

Findings

Ferroelectric transition occurs in Bi2O2Se films thinner than 8 nm.

Strain and non-equilibrium states influence the ferroelectric phase.

Ultrafast spectroscopy reveals thickness-dependent carrier dynamics.

Abstract

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.