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(001)-2 × 1 and Si(111)-7 × 7, taking advantage of the surface sensitivity provided by synchrotron radiation tunability. Sequential treatments, i.e. 90 K adsorption/300 K annealing/1193 K nitridation, have been carried out. A common feature of low-temperature NH₃ adsorption on Si(001) and Si(111) is the growth of a solid NH₃ layer over a decomposed ammonia interlayer where, among other species, atomic nitrogen is present. Increasing the temperature causes this solid NH₃ layer to desorb. A 300 K annealing of the Si(001) surface eliminates also the adsorbed atomic 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 does not lead to a unique adsorption site/NH₃ species: in particular atomic N remains. The formation of higher subnitride states (already at 90 K) is also evidenced on Si(111) with respect to Si(001). The situation of greater complexity (due to the 7 × 7 reconstruction) of the Si(111) surface with respect to Si(001), when one considers low-and room-temperature adsorption processes, is strikingly reversed when one deals with thermal nitridation. The Si(111) subnitride distribution is compatible with an ideal abrupt interface, when the Si ₃N₄ Si(001) interface appears as rougher. Using the clean and 300 K annealed surfaces as templates, the amounts (per unit area) of subnitrides at the Si₃N₄ Si interface are estimated.