  The three-dimensional (3D) topological insulator (TI) is a novel state of
matter as characterized by two-dimensional (2D) metallic Dirac states on its
surface. Bi-based chalcogenides such as Bi2Se3, Bi2Te3, Sb2Te3 and their
combined/mixed compounds like Bi2Se2Te and (Bi1-xSbx)2Te3 are typical members
of 3D-TIs which have been intensively studied in forms of bulk single crystals
and thin films to verify the topological nature of the surface states. Here, we
report the realization of the Quantum Hall effect (QHE) on the surface Dirac
states in (Bi1-xSbx)2Te3 films (x = 0.84 and 0.88). With electrostatic
gate-tuning of the Fermi level in the bulk band gap under magnetic fields, the
quantum Hall states with filling factor \nu = \pm 1 are resolved with quantized
Hall resistance of Ryx = h/e2 and zero longitudinal resistance, owing to chiral
edge modes at top/bottom surface Dirac states. Furthermore, the appearance of a
\nu = 0 state (\sigma xy = 0) reflects a pseudo-spin Hall insulator state when
the Fermi level is tuned in between the energy levels of the non-degenerate top
and bottom surface Dirac points. The observation of the QHE in 3D TI films may
pave a way toward TI-based electronics.
