Modification of Si(100) Substrate Bonding by Adsorbed Ge or Si Dimer Islands

High-resolution scanning tunneling microscopy studies of the $\mathrm{Si}\left(100\right)-(2\times 1)$ surface show a heretofore unrecognized distortion of the substrate structure when islands form during the initial stage of growth of either Si or Ge. The distortion, reflecting the influence of strain, extends at least three dimers away from the adsorption sites. We present a realistic structural model.