NH3 ON SI(111)7X7 - DISSOCIATION AND SURFACE-REACTIONS

Core-level and valence-band photoelectron spectroscopy on the dissocia tive adsorption of ammonia on Si(111) 7 X 7 is presented. Adsorption a t room temperature produces three nitrogen 1s components, of which two are assigned to NH2 and one to NH, with an initial tendency for doubl e dissociation. These doubly dissociated species can be connected to t he development of silicon atoms coordinated with two nitrogen atoms, n ecessitating silicon-silicon bonds to be broken. The dominating pictur e evolving is thus adatoms saturated by NH2 with a NH group inserted i nto one backbond, and hydrogen preferentially capping restatoms. The d issociation proces is hence much more complex than generally proposed before. This is further accentuated by the fact that not all adatoms a ppear reacted. When annealed above 600 K the dissociation process prog resses and atomic nitrogen appears at 700 K, to be the only remaining specie at 850 K. At 1200 K, further changes in the N 1s core level ind icates true silicon nitride formation.