  Strain can induce a topological phase transition in bismuth dichalcogenides.
We present the phase diagram for 3D topological insulators Bi$_2$Te$_3$,
Sb$_2$Te$_3$, Bi$_2$Se$_3$ and Sb$_2$Se$_3$ with uniaxial and biaxal strain,
which show metallic and insulating phases, both topologically trivial and
non-trivial. In particular, uniaxial tension can drive Sb$_2$Te$_3$ into a
topologically trivial insulating phase. Thus, we propose a
Sb$_2$Te$_3$/Bi$_2$Te$_3$ heterojunction in which a topological interface state
arises in the common gap of this topological insulator-normal insulator
heterojunction that can be switched on or off by means of uniaxial strain. This
interface state is confined in the Sb$_2$Te$_3$ subsystem and is physically
protected from ambient impurities. Therefore, Sb$_2$Te$_3$/Bi$_2$Te$_3$
heterojunctions can host robust helical interface states with promising
spintronic applications.
