Twin-Free Growth of Ultrathin Bi2Te3 Films on CaF2(111)/Si(111)

Abstract

The CaF₂(111) surface has no dangling bond, which allows it to be used as a substrate for the van der Waals epitaxy of layered materials. In addition, a CaF₂(111) thin film can serve as a crystalline insulating buffer on Si(111) due to good lattice matching. We investigated the heteroepitaxy of topological insulator Bi₂Te₃ on a CaF₂(111) thin film by low-energy electron diffraction (LEED), angle-resolved photoelectron spectroscopy, and conductivity measurements. Well-ordered Bi₂Te₃(111) films were obtained by molecular beam epitaxy on the substrate at 370 K. As the thickness increased above one quintuple layer (QL), the lateral domain size rapidly increased, and one domain became dominant. The twin-free structure of the 5 QL film was confirmed by the dynamical LEED calculation. The films with thickness above 2 QL exhibited metallic conduction through the surface band and the bulk-like conduction band. The observed rapid increase in conductivity at 4–5 QL is attributed to the thickness limit of the topological surface state.