  Quantum spin Hall (QSH) effect is promising for achieving dissipationless
transport devices due to the robust gapless states inside insulating bulk gap.
Here, by using first-principles calculations, we discover group-IV chalcogenide
Si2Te2 film to be a 2D QSH insulator with a fundamental band gap of 0.29 eV,
which is tunable under external strain. This nontrivial topological phase stems
from band inversion between the Si-px,y and Te-px,y orbitals, demonstrated by a
single pair of topologically protected helical edge states with Dirac point
locating in the bulk gap. Notably, the characteristic properties of edge
states, such as the Fermi velocity and edge shape, can be tuned by edge
modifications. Additionally, the h-BN semiconductor is an ideal substrate for
experimental realization of 2D Si2Te2 film, without destroying its nontrivial
topology. Our works open a new route for designing topological spintronics
devices based on 2D silicon-based films.
