  Topological insulators (TIs) like Bi$_2$Se$_3$ are a class of material with
topologically protected surface states in which spin-momentum locking may
enable spin-polarized and defect-tolerant transport. In this work, we achieved
the epitaxial growth of Bi$_2$Se$_3$ thin films on germanium, which is a key
material for microelectronics. Germanium also exhibits interesting properties
with respect to the electron spin such as a spin diffusion length of several
micrometers at room temperature. By growing Bi$_2$Se$_3$ on germanium, we aim
at combining the long spin diffusion length of Ge with the spin-momentum
locking at the surface of Bi$_2$Se$_3$. We first performed a thorough
structural analysis of Bi$_2$Se$_3$ films using electron and x-ray diffraction
as well as atomic force microscopy. Then, magnetotransport measurements at low
temperature showed the signature of weak antilocalization as a result of
two-dimensional transport in the topologically protected surface states of
Bi$_2$Se$_3$. Interestingly, the magnetotransport measurements also point out
that the conduction channel can be tuned between the Bi$_2$Se$_3$ film and the
Ge layer underneath by means of the bias voltage or the applied magnetic field.
This result suggests that the Bi$_2$Se$_3$/Ge junction is a promising candidate
for tuning spin-related phenomena at interfaces between TIs and semiconductors.
