  We have realized a two-dimensional hole system (2DHS), in which the 2DHS is
induced at an atomically flat hydrogen-terminated Si(111) surface by a negative
gate voltage applied across a vacuum cavity. Hole densities up to
$7.5\times10^{11}$ cm$^{-2}$ are obtained, and the peak hole mobility is about
$10^4$ cm$^2$/Vs at 70 mK. The quantum Hall effect is observed. Shubnikov-de
Haas oscillations show a beating pattern due to the spin-orbit effects, and the
inferred zero-field spin splitting can be tuned by the gate voltage.
