ALKALI ADSORPTION ON GAAS(110) - ATOMIC-STRUCTURE, ELECTRONIC STATES AND SURFACE DIPOLES

During the last six decades the theory of Schottky-barrier formation at metal/semiconductor interfaces has been and still is a matter of scientific controversy. In order to achieve a better understanding we feel that one of the kev systems to be studied is the adsorption of alkali metals. since these adsorbates have a most simple electronic structure. AD additional interesting aspect of such systems has been recently revealed by scanning tunneling microscopy and other experimental studies. which showed that Cs and Na form one-dimensional chains as well as two-dimensional layers. Both structures of the adatom overlayers are nonmetallic. In this paper we review the adsorption of alkali atoms on GaAs(110). We describe the state-of-the-art theory and, in particular, discuss theoretical results in comparison with experiments. We find that the interaction of the alkali metal with the surface is well sketched by that the alkali atom donates its valence electron into a surface Ga dangling orbital. The Ga dangling-bond character determines the key properties of the adsorbate system, such as the surface band structure, the Schottky barrier, and its dependence on the coverage for example, as well as the fact that the adatom-covered surface is insulating. Furthermore, we discuss the adsorbate-induced work-function changes and core-level shifts.