Electronic and geometric structure of NH3 on Ge(001) under equilibrium adsorption conditions

The adsorption of NH3 on the Ge(001) surface is studied by angle-resolved UV-photoelectron spectroscopy using synchrotron radiation and by high-resolution low-energy electron diffraction (LEED) measurements. On the clean surface, the phase transition from 2x1 to c(4x2) is observed over a temperature range from 400 to 220 K. Measurements during NH3 admission at varying temperature in adsorption-desorption equilibrium yield a sequential occupation of several states. Up to 1/2 ML (one molecule per reconstruction dimer), NH3 is bound strongly on the dimer down atoms with the Ge-N axis perpendicular to the surface (alpha state). The adsorption occurs via a mobile precursor state. The alpha state is connected with a (2x2) superstructure. Yet, the transition c(4x2)-->(2x2) starts already at very low coverages (0.01 ML) and is completed for about 0.04 ML. We propose that a long-range attractive interaction causes alpha-NH3-island formation and that a short-range repulsive interaction is responsible for a change to 2x2 within the islands by a dimer flip of every second dimer row. At the island edges, this dimer flip continues over the clean part of the surface thus changing its structure to 2x2 long before 1/2 ML saturation. Beyond 1/2 ML, the structure changes again. A good-quality 2x1 structure is seen up to 4 ML indicating pseudomorphic growth up to this coverage. Beyond 4 ML, the adsorbate structure changes irreversibly to NH3-ice without any LEED pattern.