Critical development stages for the reactive Cr-GaAs(110) interface

High-resolution synchrotron radiation photoemission studies provide a detailed microscopic view of reactions at a refractory-metal/GaAs interface. Core-level and valence-band results indicate weak interaction between the Cr adlayer and the substrate at low coverage, i.e., no reaction products are observed although the Fermi level is pinned by {~}1 A{}. Instead, a metastable Cr overlayer forms and, at a critical coverage of {~}2 A{}, it triggers extensive substrate disruption. A model is presented to account for the reaction onset. Between {~}2 and {~}8 A{} the morphology changes rapidly, corresponding to atomic intermixing and formation of Cr--Ga and Cr--As bonds. Arsenic out-diffuses readily through the intermixed region and is present in chemical states corresponding to a Cr-As phase and surface-segregated As. Gallium out-diffuses very little, but it too forms a Cr-Ga phase. At coverages of {~}50 A{} the valence bands are dominated by Cr d states and resemble those of Cr metal, but core studies show that arsenic is still present in substantial quantities ({~}20% of original level).