Study On Bistability Of Strained Islands And Mbe Growth Of Topological Insulator Films

Controlled growth and their novel quantum effect of low dimensional nanostructures are two core issues in nanoscience and nanotechnology. In this thesis, we have investigated the molecular beam epitaxy growth of metal nanoscaled islands with regular shape and high quality topological insulator films on Si substrates. Using scanning tunneling microscopy and angle resolved photoemission spectroscopy, we studied the shape transition of strained Ag islands and the thickness-dependent band structure of topological insulator films of Bi_1-xSb_x alloy and Bi₂Te₃ compound.In molecular beam epitaxial growth, the strain in epitaxial films and nanostructures results from the lattice mismatch between the growing material and the substrate, or from the anisotropic surface stress of the substrate. In the stress-induced epitaxial system, there is a close relationship between the morphology and the stress, which is of great fundamental interest in understanding surface thermodynamic properties and is critical for controlling the growth of nanostructures such as quantum dots and quantum wires. Using scanning tunneling microscope, we studied the stress-induced shape transition of Ag islands on the anisotropic Si(111)-(4×1)-In surface. Low temperature growth shows a conventional behavior leading to formation of elongated islands as seen in previous experiments. However, the room temperature growth leads to both elongated and compact islands. The statistical analysis of island sizes and first-principles calculations confirm that Ag islands are under anisotropic stress, satisfying the prerequisite condition of bistability. This work, for the fist time, provides a direct experimental evidence for stress resulted bistability of nanoscale islands on an anisotropic surface, which was theoretically predicted before.Topological insulators are a new state of quantum matter with a bulk insulating gap and metallic surface states. The surface states are protected by bulk topological properties and therefore robust against disorder and contamination. It is predicted that the interaction between topological order and magnetism or superconductivity may lead to great potential for producing exotic physical phenomena as well as future technological applications. Several bismuth-based binary materials in bulk crystal form has been experimentally confirmed as topological insulator. However, very few work has been done with topological thin films although they have the advantage in device fabrication. Using molecular beam epitaxy, we have prepared Bi_1-xSb_x and Bi₂Te₃ thin films with atomically flat terraces at large scale on the Si(111) substrates. The topologically protected surface states with the linear Dirac-like dispersion in these thin films were observed by angle-resolved photoemission spectroscopy, and the dependence of the surface states on film composition and thickness was also studied systematically. The work opens a new avenue for engineering of topological materials.