Helium-atom scattering (HAS) has been used to probe the various temperature regimes of the silane/Cu(111) adsorption system. Adsorption at a surface temperature below 230 K results in the formation of chemisorbed molecular fragments with different structural regimes in the temperature ranges <165 K, 165–207 K, and 207–230 K. Growth below 165 K results in a 3×3 overlayer containing ${\mathrm{SiH}}_3$ moieties together with SiH or Si atoms in a P3m1 structure. All Si atoms are bonded in threefold sites. Ordering into the final 3×3 structure occurs suddenly, near saturation coverage. Further molecular phases are observed at higher temperatures. A 4×4 phase exists between 165 and 207 K, and a second 3×3 structure occurs between 207 and 230 K. Periodicities deduced from the HAS diffraction patterns differ from low-energy electron-diffraction patterns reported previously. A gradual transition from the molecular chemisorbed phase to a surface alloy phase occurs above 230 K. The alloy phase can also be populated directly by adsorption at 400 K, resulting from the complete dissociation of the silane molecule followed by hydrogen desorption and the incorporation of silicon into the surface to give a two-dimensional hexagonal surface alloy (${\mathrm{Cu}}_2$Si).

DOI:https://doi.org/10.1103/PhysRevB.55.10819