  Despite many years of efforts, attempts to reach the quantum regime of
topological surface states (TSS) on an electrically tunable topological
insulator (TI) platform have so far failed on binary TI compounds such as
Bi2Se3 due to high density of interfacial defects. Here, utilizing an optimal
buffer layer on a gatable substrate, we demonstrate the first electrically
tunable quantum Hall effects (QHE) on TSS of Bi2Se3. On the n-side,
well-defined QHE shows up, but it diminishes near the charge neutrality point
(CNP) and completely disappears on the p-side. Furthermore, around the CNP the
system transitions from a metallic to a highly resistive state as the magnetic
field is increased, whose temperature dependence indicates presence of an
insulating ground state at high magnetic fields.
