Theory of vacancy-stabilized (3 × 3) displacive reconstruction of the clean Si(111) surface

We show that a (3 × 3) buckling distortion with one raised and two lowered surface atoms per unit cell lowers by as much as 0.18 eV/(surface atom) the energy of the clean, unreconstructed Si(111) surface. On the defect-free surface this state is not a true energy minimum, and is unstable against conversion into the lower energy (2×1) -bonded chain reconstruction. A low concentration of added surface vacancies, however, is found to yield a buckled (3 × 3) state which is a stable local minimum. Predictions are made for possible scanning-tunneling-microscopy observation of this displacive reconstruction.