CONTRASTED BEHAVIOR OF SI(001) AND SI(111) SURFACES WITH RESPECT TO NH3 ADSORPTION AND THERMAL NITRIDATION - A N 1S AND SI 2P CORE-LEVEL STUDY WITH SYNCHROTRON-RADIATION

The role of surface structure in controlling NH3 surface chemistry has been investigated by N 1s and Si 2p core level photoemission on Si(00 1)-2 x 1 and Si(111)-7 x 7, taking advantage of the surface sensitivit y provided by synchrotron radiation tunability. Sequential treatments, i.e. 90 K adsorption/300 K annealing/1193 K nitridation, have been ca rried out. A common feature of low-temperature NH3 adsorption on Si(00 1) and Si(111) is the growth of a solid NH3 layer over a decomposed am monia interlayer where, among other species, atomic nitrogen is presen t. Increasing the temperature causes this solid NH3 layer to desorb. A 300 K annealing of the Si(001) surface eliminates also the adsorbed a tomic nitrogen species to reach a situation in which only H atoms and NH2 fragments decorate the silicon dimer broken bonds. In contrast to the Si(001) case, a 300 K annealing of the rougher Si(111) surface doe s not lead to a unique adsorption site/NH3 species: in particular atom ic N remains. The formation of higher subnitride states (already at 90 K) is also evidenced on Si(111) with respect to Si(001). The situatio n of greater complexity (due to the 7 x 7 reconstruction) of the Si(11 1) surface with respect to Si(001), when one considers low- and room-t emperature adsorption processes, is strikingly reversed when one deals with thermal nitridation. The Si(111) subnitride distribution is comp atible with an ideal abrupt interface, when the Si3N4/Si(001) interfac e appears as rougher. Using the clean and 300 K annealed surfaces as t emplates, the amounts (per unit area) of subnitrides at the Si3N4/Si i nterface are estimated.