SURFACE-PROPERTIES OF SI(111)7 X 7 UPON NH3 ADSORPTION AND VACUUM ANNEALING

The adsorption of NH3 on a clean Si(111)7 x 7 reconstructed surface and the effect of vacuum annealings of the NH3-saturated surface have been studied by low energy electron diffraction, Auger electron spectroscopy and photoemission yield spectroscopy. The analysis of the surface order of the nitrogen coverage and of the workfunction, ionization energy and filled surface state band changes upon NH3 exposure or annealing temperatures gives the following results: the adsorption is dissociative into NH2 + H and occurs in a two-step process, a fast one with an initial sticking coefficient equal to one up to half saturation, then a much slower one. At saturation, 10 +/- 1 NH3 molecules per 7 x 7 unit cell are adsorbed, the NH2 radicals occupying the 7 rest atom dangling bonds plus three sites on the average which are atributed to the presence of steps or missing adatoms. Annealings lead to a two-step process, first a hydrogen desorption from 450-degrees-C to 700-degrees-C then a surface nitridation beyond 800-degrees-C.