NH3onSi(111)7×7:Dissociation and surface reactions

Core-level and valence-band photoelectron spectroscopy on the dissociative adsorption of ammonia on $\mathrm{Si}\mathrm{}\left(111\right)7\times 7$ is presented. Adsorption at room temperature produces three nitrogen $1s$ components, of which two are assigned to ${\mathrm{NH}}_2$ and one to NH, with an initial tendency for double dissociation. These doubly dissociated species can be connected to the development of silicon atoms coordinated with two nitrogen atoms, necessitating silicon-silicon bonds to be broken. The dominating picture evolving is thus adatoms saturated by ${\mathrm{NH}}_2$ with a NH group inserted into one backbond, and hydrogen preferentially capping restatoms. The dissociation proces is hence much more complex than generally proposed before. This is further accentuated by the fact that not all adatoms appear reacted. When annealed above 600 K the dissociation process progresses 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 indicates true silicon nitride formation.