  Recent experimental studies have shown that well-annealed, unstrained Si(105)
surfaces appear disordered and atomically rough when imaged using scanning
tunnelling microscopy (STM). We construct new models for the Si(105) surface
that are based on single- and double-height steps separated by Si(001)
terraces, and propose that the observed surface disorder of Si(105) originates
from the presence of several structural models with different atomic-scale
features but similar energies. This degeneracy can be removed by applying
compressive strains, a result that is consistent with recent observations of
the structure of the Ge/Si(105) surface.
