  The naturally existing chalcogenide Bi2Se3 is topologically nontrivial due to
the band inversion caused by strong spin-orbit coupling inside the bulk of the
material. The surface states are spin polarized, protected by the
time-inversion symmetry, and thus robust to the scattering caused by
non-magnetic defects. A high purity topological insulator thin film can be
easily grown via molecular beam epitaxy (MBE) on various substrates to enable
novel electronics, optics, and spintronics applications. However, the unique
surface state properties have historically been limited by the film quality,
which is evaluated by crystallinity, surface morphology, and transport data.
Here we propose and investigate different MBE growth strategies to improve the
quality of Bi2Se3 thin films grown by MBE. In addition, growths of topological
trivial insulator (Bi0.5In0.5)2Se3 (BIS) are also investigated. BIS is often
used as a buffer layer or separation layer for topological insulator
heterostructures. Based on the surface passivation status, we have classified
the substrates into two categories, self-passivated or unpassivated, and
determine the optimal growth mechanisms on the representative sapphire and
GaAs, respectively. Growth temperature is a crucial control parameter for the
van der Waals epitaxy for both types of substrates. For Bi2Se3 on GaAs, the
surface passivation status determines the dominant growth mechanism.
