Short-Range order in submonolayer Ni on GaAS(110) by XPS Forward Scattering.

Angle-resolved x-ray photoelectron spectroscopy (XPS) has been used to examine the structural arrangement of atoms in submonolayer coverages of Ni on GaAS(110). Insights into the short-range order around a lattice atom are possible because the outgoing photoelectron diffracts off overlaying lattice atoms. This diffraction contains a strong component of forward scattering that yields enhanced intensities along axes connecting the emitting atom to its overlying nearest and next-nearest neighbors. We have found that these enhanced intensities are observed in the Ni 2p core level for 0.5ML Ni on GaAS(110) after annealing to 300C but are much weaker before annealing. The angles at which these enhanced intensities appear suggest that upon annealing the Ni atoms replace Ga and As atoms in the third layer from the surface and perhaps in the second layer. Replacement may also occur in the top layer but this would produce no forward scattering signature. Thus our structural results are consistent with this being a reactive interface.