  Homojunctions between Bi$_2$Se$_3$ and its Mn-doped phase are investigated as
a sample geometry to study the influence of spin degrees of freedom on
topological insulator properties. $n$ quintuple layers (QLs) of Bi$_2$Se$_3$
are grown ontop of Mn-doped Bi$_2$Se$_3$ by molecular beam epitaxy for $0\le n
\le 30\,$QLs, allowing to unhamperedly monitor the development of electronic
and topological properties by surface sensitive techniques like angle resolved
photoemission spectroscopy. With increasing $n$, a Mn-induced gap at the Dirac
point is gradually filled in an "hourglass" fashion to reestablish a
topological surface state at $n \sim 9\,$QLs. Our results suggest a competition
of upwards and downwards band bending effects due to the presence of an n-p
type interface, which can be used to tailor topological and quantum well states
independently.
