Si adatoms deposited on Si(100)-(2 X 1) below 200'C assume metastable binding configurations, each distinct from the dimer string topology ordinarily observed following deposition at higher temperatures.Isolated ad-dimers which nucleate predominantly on substrate dimer rows at room temperature do not constitute the precursors for dimer strings. Instead, they transform upon annealing at =120 C to metastable, dimerized lines assembled end to end, parallel to the substrate's dimer bonds. PACS numbers: 68.55.Jk, 68.35.BsThe need to control dopant incorporation and diffusion in epitaxial Si has contributed to recent efforts to develop silicon molecular beam epitaxy near room temperature [1 -4]. While it has already been recognized that Si deposited on Si(100)-(2 X 1) near room temperature adopts metastable adsorbate structures, the associated atomic topologies have not yet been identified [5].It is therefore critical to characterize the configurations and stability of those adatom structures which mediate the evolution of the morphology of Si(100) during homoepitaxial growth near room temperature.

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The position of gallium atoms on a silicon (111)surface has been completely determined using the tunneling microscope and x-ray standing-wave methods. The (&3X&3)R30' electron diffraction pattern observed with -, '-monolayer coverages is shown to result from a simple adatom gallium lat-0

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