Strain and electronic structure studies of bismuth selenide and antimony selenide grown by molecular beam epitaxy

Abstract

Symmetry protected topological state has currently been a very intriguing and prosperous field in condensed matter physics. Topological insulators are a class of the representative materials that host such novel quantum state. They hold the promise of a variety of exciting predictions including Non-Abelian Fermions, magnetic monopole and dissipationless electron transportation, etc. As candidates of three-dimensional (3D) topological insulators, V(Bi, Sb)-VI(Se, Te) compounds have been studied extensively due to the readiness in their syntheses and the simplicity in their band structures. Three of them have been confirmed experimentally to be non-trivial topological insulators while only Sb2Se3 is of the trivial phase. However, it is also predicted that Sb2Se3 can be tuned into topological non-trivial phase via strain or proximity effect. On the other hand, the reversed topological phase transition has also been suggested to occur in Bi2Se3 via strain. Motivated by these theoretical predictions, this thesis will focus on strain and electronic properties of epitaxial Sb2Se3 and Bi2Se3. Known as a versatile crystal growth method, molecular beam epitaxy (MBE) plays the central role in the studies of this thesis. In addition, various characterization techniques have also been employed to probe the structural and electronic properties of the materials. By comparing strain relaxation profiles of Bi2Se3(111) grown on van der Waals (vdW) and non-vdW substrates, it is found that the strain relaxation process in epitaxial Bi2Se3(111) on the non-vdW substrate is gradual, contrasting that on the vdW substrate where strain is fully relaxed at the initial stage of sample growth. Besides, 2x2 surface structure on Bi2Se3(111) surface has been uncovered. For Bi2Se3 films grown along the high-index, say [221], direction on In2Se3 buffered GaAs (001) substrate, even more retarded strain relaxation process has been observed. Strong covalent chemical bonding at the heterointerface has been revealed by the first-principle calculations. Additionally, growth of Bi2Se3(221) on Ga2Se3 is also experimented and electronic state of such high-index surface has been studied. Preparation of Sb2Se3 samples has encountered the obstacle that the thermally stable phase of Sb2Se3 is the orthorhombic structure instead of the rhombohedral one. It is latter that has been referred to in the theoretical predictions, however. Thanks to the kinetics of MBE growth, the rhombohedral phase of Sb2Se3 can be induced when grown on substrates with the rhombohedral lattice structure, such as Bi2Se3, in ultrahigh vacuum environment. After the successful preparation of Sb2Se3 samples, topological state has been probed, showing its existence in the Sb2Se3-on-Bi2Se3 sample. The emergence of the topological state has been interpreted as being induced by the proximity effect at the interface of two materials. Encouraged by the successful synthesis of rhombohedral Sb2Se3 and strain in high-index Bi2Se3, growth of high-index Sb2Se3 has also been attempted. The grown samples have been characterized by reflection high-energy electron diffraction (RHEED) and the results evidence successful fabrication of high-index rhombohedral Sb2Se3. The above findings have raised hope of manipulations of the topological state in Bi2Se3 and Sb2Se3 by strain or proximity effect, which is of interests in both fundamental physics and applications.