  Formation of topological quantum phase on conventional semiconductor surface
is of both scientific and technological interest. Here, we demonstrate
epitaxial growth of 2D topological insulator, i.e. quantum spin Hall (QSH)
state, on Si(111) surface with a large energy gap, based on first-principles
calculations. We show that Si(111) surface functionalized with 1/3 monolayer of
halogen atoms [Si(111)-sqrt(3) x sqrt(3)-X (X=Cl, Br, I)] exhibiting a trigonal
superstructure, provides an ideal template for epitaxial growth of heavy
metals, such as Bi, which self-assemble into a hexagonal lattice with high
kinetic and thermodynamic stability. Most remarkably, the Bi overlayer is
"atomically" bonded to but "electronically" decoupled from the underlying Si
substrate, exhibiting isolated QSH state with an energy gap as large as 0.8 eV.
This surprising phenomenon is originated from an intriguing substrate orbital
filtering effect, which critically select the orbital composition around the
Fermi level leading to different topological phases. Particularly, the
substrate-orbital-filtering effect converts the otherwise topologically trivial
freestanding Bi lattice into a nontrivial phase; while the reverse is true for
Au lattice. The underlying physical mechanism is generally applicable, opening
a new and exciting avenue for exploration of large-gap topological
surface/interface states.
