NH3-ADSORPTION ON GE USING A CYLINDRICALLY SHAPED CRYSTAL

The low-temperature adsorption of ammonia was studied on a cylindrically shaped Ge crystal in the orientation range (001)-(113)-(111)-(110) using the methods of low-energy electron diffraction (LEED and SPALEED) and photoelectron spectroscopy (UPS, ARUPS and XPS). Below 100 K, ammonia condenses. At 110 < T < 200 K adsorption is molecular on all orientations. At 110 < T < 130 K, adsorption proceeds via a mobile precursor with constant sticking coefficient up to ML-saturation with the ML coverage depending slightly on orientation. The ionisation energy decreases by 2.3 eV indicating nitrogen end-down adsorption and a change transfer to the surface. Up to 0.5 ML on (001), corresponding to one molecule per dimer, the adsorbate is orientated perpendicularly. Above 0.5 ML, a transition to a titled and more weakly bound configuration occurs. On (111) and all other orientations, only this weakly bound titled form is observed. On (001), the surface order and reconstruction remains whereas it decreases elsewhere. On (111), the intensity of the Ge 3d surface core level component decreases upon adsorption because a part of it is shifted to the bulk position. On (001), the clean Ge 3d surface component is only slightly shifted by adsorption and an additional component of equal intensity is formed. We assign them to the up-atom and the down-atom, respectively, of the dimers after adsorption. At 200 K, adsorption occurs preferentially on (001) and its vicinals confirming that the stronger bond is related with the surface dimers. Adsorbed ammonia is dissociated under the influence of electron and high-energy photon radiation.