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

Abstract

The adsorption of ${\mathrm{NH}}_{3}$ 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 2\ifmmode\times\else\texttimes\fi{}1 to c(4\ifmmode\times\else\texttimes\fi{}2) is observed over a temperature range from 400 to 220 K. Measurements during ${\mathrm{NH}}_{3}$ 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), ${\mathrm{NH}}_{3}$ is bound strongly on the dimer down atoms with the Ge-N axis perpendicular to the surface (\ensuremath{\alpha} state). The adsorption occurs via a mobile precursor state. The \ensuremath{\alpha} state is connected with a (2\ifmmode\times\else\texttimes\fi{}2) superstructure. Yet, the transition c(4\ifmmode\times\else\texttimes\fi{}2)\ensuremath{\rightarrow}(2\ifmmode\times\else\texttimes\fi{}2) 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 \ensuremath{\alpha}-${\mathrm{NH}}_{3}$-island formation and that a short-range repulsive interaction is responsible for a change to 2\ifmmode\times\else\texttimes\fi{}2 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 2\ifmmode\times\else\texttimes\fi{}2 long before 1/2 ML saturation. Beyond 1/2 ML, the structure changes again. A good-quality 2\ifmmode\times\else\texttimes\fi{}1 structure is seen up to 4 ML indicating pseudomorphic growth up to this coverage. Beyond 4 ML, the adsorbate structure changes irreversibly to ${\mathrm{NH}}_{3}$-ice without any LEED pattern. \textcopyright{} 1996 The American Physical Society.