NH3-INDUCED C(4X2) TO (2X2) TRANSITION ON THE GE(001) SURFACE BY A DOMINO-LIKE DIMER FLIP

Under the conditions of thermodynamic adsorption-desorption equilibriu m, the first strongly bound molecular adsorption state of ammonia on G e(001) saturates at one molecule per Ge reconstruction dimer (1/2 ML). High-resolution electron diffraction studies show that this adsorptio n is accompanied by a structural transition from c(4 x 2) on the clean surface to a (2 x 2) structure which is already completed for a cover age of about 0.04 ML, far below saturation. We propose a model implyin g the formation of NH, islands locally covered with 1/2 ML and a (2 x 2) periodicity caused by a flip of the dimer tilt direction of every s econd dimer row. Beyond the edge of these islands, the dimer flip cont inues domino-like along the dimer rows over the clean parts of the sur face. Elongated (2 x 2) domains about 280-330 Angstrom long and 30-65 Angstrom wide are formed, depending on coverage.