Ag films on Fe/GaAs(001): From clean surfaces to atomic Ga structures

We prepare 150 nm thick epitaxial Ag buffer layers on Fe-precovered GaAs(001) at 380 K. Post-annealing at 570 K improves the morphology of the films, as observed by scanning tunneling microscopy (STM), without changing the chemical composition at the surface. The main type of defects are screw dislocations. Nevertheless we achieve a mean terrace width of 36 nm. Al temperatures above 620 K, Ga atoms diffuse through the Ag buffer layer to the surface and form an overlayer. Depth profiles and quantitative analysis of X-ray photoemission data yield surface coverages of up to 0.3 ML and Ga concentrations within the Ag bulk Of a few at%. The GaAs substrate acts as a source of Ga atoms even at temperatures much lower than the GaAs vacuum decomposition temperature of 890 K. STM images show that in some areas the overlayer consists of Ga tetramers. Their size and orientation are determined by the separation vector between next-nearest neighbour atoms of the Ag(001) substrate. In other areas the tetramers line up to form parallel rows of Ga pairs. This row structure is reflected in the low-energy electron diffraction (LEED) patterns as a (4 root 2x root 2)R45 degrees superstructure. Based on STM and LEED data we propose a structure model with two Ga atoms per surface unit cell. Finally we predict a long-range interaction between rows and a relaxation of the Ga-Ga separation within the rows.