Insulator–metal phase transitions of alkali atoms on GaAs(0 0 1)

The electronic properties of alkali metals (Cs and Na) deposited on the gallium-rich GaAs(0 0 1 surface), have been investigated using synchrotron-radiation photoemission spectroscopy, at liquid nitrogen temperature. From the analysis of the alkali core-level spectra and using usual criteria for metallicity (namely: appearance of the Fermi-level edge, presence of plasmon losses in core-level spectra, and disappearance of surface photovoltage) we find evidence of a surface metallic phase, both for Cs and Na. In agreement with previous determinations of the phase diagram for such system, we find that the metallic phase coexists with an insulating phase. We also observe a precursor of the surface metallic phase, for which only one of the above criteria is satisfied (plasmon losses). The formation of a Cs metallic phase leads mainly to the rising in the Cs4d core-level spectrum of a narrow component at higher binding energy. In contrast, for Na adsorption, the metallic-related component in the Na2p core-level spectrum lies at lower binding energy. These findings are discussed and tentatively explained in terms of the classical Born–Haber cycle as well as with a tight-binding description of alkali metal adsorption. The core-level photoemission results suggest a morphological model in which part of alkali atoms are bonded to the substrate (first layer; insulating phase) and part are bonded to alkali atoms (either second layer or clusters; metallic phase).