Abstract
High quality topological insulator (TI) Bi2Te3 thin films were grown on Al2O3 (0001) and SrTiO3 (111) substrate by molecular beam epitaxy. During the deposition of Bi2Te3 on Al2O3 (0001) substrate, very sharp reflection high-energy electron diffraction (RHEED) pattern with clear Kikuchi lines was observed at growth temperature of 340°C. The surface morphology was analyzed by atomic force microscopic (AFM), and the size of triangular shaped domains is approaching to 2μm, due to a 2D layer by layer growth mechanism. The normal scans of a 4-circle x-ray diffraction indicated that the c-axis of the epitaxial film was full strain relaxed. We have employed x-ray photoemission spectroscopy (XPS) and synchrotron radiation angle-resolved photoemission spectroscopy (ARPES) to characterize the chemical composition and the electronic structure. The evolution of the band structure with chemical composition is suggesting that Te plays the roles of the donors in the intrinsic doping for this TI materials. Based on the Te/Bi composition ratio calculated from XPS analysis, the ARPES images is explained in following; the higher content of Te, the deeper of the Dirac points (DP) are buried below the Fermi level. Furthermore, the doping level may be tuned by the Te: Bi composition. In varying the Te/Bi flux ratio from 14 to 25, we found the lowest doping level was achieved when the flux ratio was held at 20, according to both ARPES and Hall measurements. On the other hand, high quality Bi2Te3 epilayer prepared on SrTiO3 (111) (STO) was achieved by introducing ~1 nm thick Bi2Se3 seed layer. The TEM image reveals a full epitaxial structure of Bi2Te3 film and Bi2Se3 seed layer down to the first quintuple layer. Furthermore, the crystallinity of the Bi2Te3 film and Bi2Se3 seed layer has been characterized by both RHEED and XRD. The in-plane lattice constant of the Bi2Se3 seed layer was found to be full strain relaxed. The in-plane crystallographic orientation for the Bi2Te3/Bi2Se3 on STO (111) layer-by-layer structure has been well studied and was found with following relationship: 〖[100]〗_(Bi_2 Te_3 )∥〖[110]〗_(Bi_2 Se_3 )∥〖[11 ̅0]〗_STO or 〖[110]〗_(Bi_2 Te_3 )∥〖[110]〗_(Bi_2 Te_3 )∥〖[11 ̅0]〗_STO. The diffraction pattern of Bi2Te3 film was found having an additional set of peaks due to deviation from the original growth orientation by 60° for Bi2Te3 (0 1 5), likely due to the formation of twin boundaries.