Mode of Growth of Ultrathin Topological Insulator Bi₂Te₃ Films on Si (111) Substrates

Layered materials such as graphene, bi-, and multilayer graphene as well as various compounds of topological insulators are currently in the focus of interest due to their extraordinary physical properties related to Dirac surface states. The ability to grow thin films of these complex layered materials is the key to explore their fundamental phenomena giving insights into modern solid-state physics. However, complex materials composed of layers only weakly bonded via van der Waals forces offer unmatched challenges for the deposition of thin epitaxial films. Here, we report on the growth of Bi₂Te₃ ultrathin films on Si (111) substrates using molecular beam epitaxy. Special emphasis is put on the nucleation phenomena and growth dynamics studied in detail by in situ scanning tunnelling microscopy and high-resolution scanning transmission electron microscopy. The morphology of the Bi₂Te₃ surface and the structure of the Si(111)/Bi₂Te₃ interface as well as the formation of threading dislocations and crystal domains are studied at the atomic level. Our data indicate that the film is formed via the nucleation of islands, which float on the substrate; thus, the islands are only weakly bonded to the substrate and rather mobile. Apparently, these floating islands are able to arrange themselves by moving in the <i>x</i>–<i>y</i> direction to perfectly coalesce and form a continuous film. The results present a crucial step toward understanding growth and defect formation in this class of materials and thus pave the avenue to a higher control over both their structural and electronic properties, in order to study the electronic properties of the Dirac surface states