  Three dimensional (3D) topological insulators (TIs) are an important class of
materials with applications in electronics, spintronics and quantum computing.
With the recent development of truly bulk insulating 3D TIs, it has become
possible to realize surface dominated phenomena in electrical transport
measurements e.g. the quantum Hall (QH) effect of massless Dirac fermions in
topological surface states (TSS). However, to realize more advanced devices and
phenomena, there is a need for a platform to tune the TSS or modify them e.g.
gap them by proximity with magnetic insulators, in a clean manner. Here we
introduce van der Waals (vdW) heterostructures in the form of topological
insulator/insulator/graphite to effectively control chemical potential of the
TSS. Two types of gate dielectrics, normal insulator hexagonal boron nitride
(hBN) and ferromagnetic insulator Cr2Ge2Te6 (CGT) are utilized to tune charge
density of TSS in the quaternary TI BiSbTeSe2. hBN/graphite gating in the QH
regime shows improved quantization of TSS by suppression of magnetoconductivity
of massless Dirac fermions. CGT/graphite gating of massive Dirac fermions in
the QH regime yields half-quantized Hall conductance steps and a measure of the
Dirac gap. Our work shows the promise of the vdW platform in creating advanced
high-quality TI-based devices.
