Out‐of‐Plane Bi₂O₂Se Nanoflakes for Sensitive Gate‐Tunable Phototransistors

Abstract Phototransistors based on 2D materials exhibit gate‐voltage‐tunable performance, by which the key device parameters such as the photoresponsivity and photoconductive gain could easily surpass the photodetectors without the photomultiplication effects. Phototransistors with the light‐absorption material of 2D Bi₂O₂Se are rising due to its desirable material properties, such as small effective electron mass, high carrier mobility, and ultrafast intrinsic non‐equilibrium carrier recombination. The in‐plane growth of Bi₂O₂Se is prevailing on mica substrate, where the interfacial electrostatic interaction exists in between. Crystal quality degradation caused by the organic contaminants and corrosive acids, nevertheless, is unavoidable during transfer procedures to other substrates. In this work, different out‐of‐plane growth modes of Bi₂O₂Se are achieved, more specifically, whisker‐like, square‐ and rectangular‐shaped flakes are obtained with optimized growth parameters. The pristine interfaces are preserved in the phototransistors with the clean transfer of Bi₂O₂Se nanoflakes and the electrodes. A photoresponsivity of over 10⁵ A W⁻¹ and a record‐high response speed of 0.32 ms are achieved for the phototransistors fabricated based on the out‐of‐plane Bi₂O₂Se nanoflakes. Furthermore, trap state occupation with photoinduced carriers is modulated with gate voltages and explored under cryogenic conditions for improving the device performance.