  We use real-time reflection high energy electron diffraction intensity
oscillation to establish the Te-rich growth dynamics of topological insulator
thin films of Bi2Te3 on Si(111) substrate by molecular beam epitaxy. In situ
angle resolved photoemission spectroscopy (ARPES), scanning tunneling
microscopy and ex situ transport measurements reveal that the as-grown Bi2Te3
films without any doping are an intrinsic topological insulator with its Fermi
level intersecting only the metallic surface states. Experimentally, we find
that the single-Dirac-cone surface state develops at a thickness of two
quintuple layers (2 QL). Theoretically, we show that the interaction between
the surface states from both sides of the film, which is determined by the
penetration depth of the topological surface state wavefunctions, sets this
lower thickness limit.
