High electron mobility and quantum oscillations in non-encapsulated ultrathin semiconducting Bi₂O₂Se

High-mobility semiconducting ultrathin films form the basis of modern electronics, and may lead to the scalable fabrication of highly performing devices. Because the ultrathin limit cannot be reached for traditional semiconductors, identifying new two-dimensional materials with both high carrier mobility and a large electronic bandgap is a pivotal goal of fundamental research. However, air-stable ultrathin semiconducting materials with superior performances remain elusive at present. Here, we report ultrathin films of non-encapsulated layered Bi₂O₂Se, grown by chemical vapour deposition, which demonstrate excellent air stability and high-mobility semiconducting behaviour. We observe bandgap values of ∼0.8 eV, which are strongly dependent on the film thickness due to quantum-confinement effects. An ultrahigh Hall mobility value of >20,000 cm² V^-1 s^-1 is measured in as-grown Bi₂O₂Se nanoflakes at low temperatures. This value is comparable to what is observed in graphene grown by chemical vapour deposition and at the LaAlO₃-SrTiO₃ interface, making the detection of Shubnikov-de Haas quantum oscillations possible. Top-gated field-effect transistors based on Bi₂O₂Se crystals down to the bilayer limit exhibit high Hall mobility values (up to 450 cm² V^-1 s^-1), large current on/off ratios (>10⁶) and near-ideal subthreshold swing values (∼65 mV dec^-1) at room temperature. Our results make Bi₂O₂Se a promising candidate for future high-speed and low-power electronic applications.