Alkali metal/GaAs(110) interfaces: correlation effects and sub-gap electron energy loss spectra

The sub-gap high-resolution electron energy loss spectra (HREELS) of the interfaces between Rb, Cs, K,Na and GaAs(110) are presented. The spectra of the Rb/GaAs(110) interface show that this interface is insulating up to half-monolayer coverage when two strong loss peaks appear at 0.6 and 1.3 eV, in analogy to that already found for Cs and K and confirmed here. By comparing the energies of these two losses in the different interfaces, interesting regularities are found as well as a possible connection with the corresponding bulk metal surface plasmon. This comparison also suggests that the formerly proposed link between the two peaks and the presence of correlation effects is not as direct. The two loss features could be associated with coupled collective excitations of the small clusters originated from the decay, at above half a monolayer, of the multi-row atomic chains formed by Cs, K and presumably also by Rb atoms on the GaAs(110) surface. In the case of the Na/GaAs(110) interface, on the contrary, the formation of bi-polarons seems to prevent the build-up of multi-rows chains and their decay into small bi-dimensional clusters. Accordingly, no sub-gap features are found in the corresponding spectra. (C) 1998 Elsevier Science B.V. All rights reserved.