STRUCTURE AND COMPOSITION OF GAAS(001) SURFACES

The structure and composition of the GaAs(001) surface was studied with high-resolution medium-energy ion scattering, from As-rich to Ga-rich reconstructions. In contrast to commonly accepted models, we find that first and second layers in the surface may contain both Ga and As atoms. The surfaces are more Ga-rich than previously believed, with Ga atoms occupying As sites. Such mixed compositions are explained by consideration of charge neutrality, as well as Coulomb repulsion between surface electrons. Implications for heteroepitaxial growth on GaAs(001) are discussed.