Reaction and barrier formation at metal-GaP(110) interfaces

The interfacial chemistry of Cu, Ag, Au, Ni, Pd, Al, Ga, In, Sn, and Bi deposited on cleaved GaP(110) surfaces has been studied using soft-x-ray photoemission spectroscopy. Of the noble metals, Cu and Au tend to disassociate the GaP, whereas the Ag shows little sign of reactivity with the substrate. The transition metals Ni and Pd react strongly with the GaP to form phosphides in which Ga is segregated. Of the column-III overlayers, Ga and In display strong clustering and little reactivity with the substrate, but the interface with Al shows a cation replacement reaction similar to that seen at Al/GaAs(110). Sn and Bi exhibit Stranski-Krastanov growth, with the establishment of metallic islands on a laminar first monolayer. Despite the differences in reactivity, the barrier heights for all the metals but In are observed to stabilize in the range of 1.14 to 1.46 eV. Hence the n-type GaP barrier height shows a much weaker dependence on the overlayer work function than reported previously. As with GaAs(110) this Fermi-level stabilization position is in the range of both the defect levels as created by irradiation, and the theoretical charge neutrality level toward which metal-induced gap states should move the surface Fermi level. The influence of overlayer reactivity, morphology, and metallicity on the barrier development is discussed in relation to these two potential sources of interface states.