Inclined Ultrathin Bi₂O₂Se Films: A Building Block for Functional van der Waals Heterostructures

As an emerging ultrathin semiconductor material, Bi₂O₂Se exhibits prominent performances in electronics, optoelectronics, ultrafast optics, etc. However, until now, the in-plane growth of Bi₂O₂Se thin films is mostly fulfilled on atomically flat mica substrates with interfacial electrostatic forces setting obstacles for Bi₂O₂Se transfer to fabricate functional van der Waals heterostructures. In this work, controlled growth of inclined Bi₂O₂Se ultrathin films is realized with apparently reduced interfacial contact areas upon mica flakes. Consequently, the transfer of Bi₂O₂Se could be facile by overcoming weaker electrostatic interactions. From cross-sectional characterizations at the Bi₂O₂Se/mica interfaces, it is found that there are no oxide buffer layers in existence for both in-plane and inclined growths, while the un-neutralized charge density is apparently decreased for inclined films. By mechanical pressing, inclined Bi₂O₂Se could be transferred onto SiO₂/Si substrates, and back-gated Bi₂O₂Se field effect transistors are fabricated, outperforming previously reported in-plane Bi₂O₂Se devices transferred with the assistance of corrosive acids and adhesive polymers. Furthermore, Bi₂O₂Se/graphene heterostructures are fulfilled by a probe tip to fabricate hybrid phototransistors with pristine interfaces, exhibiting highly efficient photoresponses. The results in this work demonstrate the potential of inclined Bi₂O₂Se to act as a building block for prospective van der Waals heterostructures.