  Silicon has long been synonymous with semiconductor technology. This unique
role is due largely to the remarkable properties of the Si-SiO_2 interface,
especially the (001)-oriented interface used in most devices. Although Si is
crystalline and the oxide is amorphous, the interface is essentially perfect,
with an extremely low density of dangling bonds or other electrically active
defects. With the continual decrease of device size, the nanoscale structure of
the silicon/oxide interface becomes more and more important. Yet despite its
essential role, the atomic structure of this interface is still unclear. Using
a novel Monte Carlo approach, we identify low-energy structures for the
interface. The optimal structure found consists of Si-O-Si "bridges" ordered in
a stripe pattern, with very low energy. This structure explains several
puzzling experimental observations.
