Scanning tunneling spectroscopic (STS) studies of MBE-grown topological insulators of Bi$_{2}$Se$_{3}$ epitaxial films on Si(111)

We report STS studies of MBE-grown Bi$_{2}$Se$_{3}$ epitaxial films on Si(111) with varying thicknesses. The films were atomically flat on the scale of hundreds of nanometers, with occasional atomic steps of one c-axis lattice constant. In the case of thick Bi$_{2}$Se$_{3}$ films, the tunneling spectra were consistent with those found in single crystalline Bi$_{2}$Se$_{3}$, except that the Dirac point ($E_{Dirac}=-$50 $\sim $ -100 meV) of the MBE-film is generally much closer to the Fermi level ($E$ = 0), in contrast to the large downshift of $E_{Dirac}$ (= -400 $\sim $ -200 meV) commonly found in single crystalline bulk grown Bi$_{2}$Se$_{3}$. The STS spectra of the thinner films deviate from those of the thicker samples, probably the result of strain. Fourier transformed (FT) STS data as a function of energy reveals several quasiparticle scattering interference wave-vectors that are consistent with the topologically protected surface states with chiral spin texture, although the overall FT-STS maps are simpler than those reported on the Bi$_{0.92}$Sb$_{0.08}$ (111) surface due to simpler electronic band-structures of Bi$_{2}$Se$_{3}$. The effect of time reversal symmetry breaking on the FT-STS will be investigated by either magnetic doping or application of magnetic fields. This work was supported by a grant from FENA of FCRP and DARPA.