Observation of Surface Dirac Cone in High-Quality Ultrathin Epitaxial Bi₂Se₃ Topological Insulator on AlN(0001) Dielectric

Bi₂Se₃ topological insulators (TIs) are grown on AlN(0001)/Si(111) substrates by molecular beam epitaxy. In a one-step growth at optimum temperature of 300 °C, Bi₂Se₃ bonds strongly with AlN without forming interfacial reaction layers. This produces high epitaxial quality Bi₂Se₃ single crystals with a perfect registry with the substrate and abrupt interfaces, allowing thickness scaling down to three quintuple layers (QL) without jeopardizing film quality. It is found by angle-resolved photoelectron spectroscopy that, remarkably, Bi₂Se₃ films maintain the 3D TI properties at very low thickness of 3QL (∼2.88 nm), exhibiting top surface gapless metallic states in the form of a Dirac cone