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 Ga As(001) at 380 K. Post-annealing at 570 K improves the morphology of t he films, as observed by scanning tunneling microscopy (STM), without changing the chemical composition at the surface. The main type of def ects are screw dislocations. Nevertheless we achieve a mean terrace wi dth of 36 nm. Al temperatures above 620 K, Ga atoms diffuse through th e Ag buffer layer to the surface and form an overlayer. Depth profiles and quantitative analysis of X-ray photoemission data yield surface c overages 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 temp eratures 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 separatio n vector between next-nearest neighbour atoms of the Ag(001) substrate . In other areas the tetramers line up to form parallel rows of Ga pai rs. This row structure is reflected in the low-energy electron diffrac tion (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 interacti on between rows and a relaxation of the Ga-Ga separation within the ro ws.