Ultrafast carrier dynamics of indirect bandgap exciton and band-nesting excitons in mechanically exfoliated  <b> <i>α</i> </b> -In2Se3 flakes

Abstract

Although α-In2Se3 is theoretically predicted to exhibit an indirect bandgap semiconductor, its experimental optoelectronic response resembles that of a direct bandgap material. To provide a possible explanation for this issue, we combined first-principles calculations and femtosecond broad transient absorption spectroscopy—employing a supercontinuum white light probe (450–1700 nm)—to reveal the electronic structure and hot-carrier dynamics in mechanically exfoliated α-In2Se3 flakes with the 2H stacking arrangement. High-energy excitonic states arising from band-nesting regions are found by theoretical calculations, and the hot-carrier cooling and intra-band self-separation of hot holes for indirect bandgap excitons in the Γ valley of 2H-α-In2Se3 is experimentally revealed. The resulting indirect bandgap exciton state possesses long-lifetime free carriers in the nanosecond timescale. These ultrafast processes, including the unique exciton dynamics and band structure evolution in α-In2Se3, are fundamental and crucial for understanding the optoelectronic performance of α-In2Se3-based devices.