STM studies on the structural evolution of N ₂ ⁺ ion induced nitride formation on Si(111)-7×7 surface

The initial stage of N ₂ ⁺ ion induced nitridation of Si(111)-7×7 surface was investigated by using a scanning tunneling microscopy. At room temperature, deposition of N ₂ ⁺ ions on Si(111)-7×7 surface formed an amorphous surface morphology. With submonolayer dose of N ₂ ⁺ ions and post-annealing at temperatures above 950° C, silicon nitride islands appeared preferentially at the step edges implying that those had acted as step pinning sites to result in the rough step edges. Inside the silicon nitride island, three-directional periodicity whose symmetry axes deviated from those of Si(111)-7×7 was observed and the low energy electron diffraction pattern showed the coexistence of 7×7 and quadruplet domains. On the surface area preserving the 7×7 periodicity, Si adatoms which reacted with nitrogen ions appeared dark preferentially on the center adatom sites to corner ones of the dimer-adatom-stacking-fault model. Frequent observation of Si(111)-7×7 phase boundary and the formation of lattice-mismatched silicon nitride islands could be closely related with each other. With higher dose of N ₂ ⁺ ions and post-annealing at temperatures above 950° C, the surface was covered with small patches of silicon nitride islands and Si-7×7 domains. Post-annealing of this surface at temperatures above 1050° C resulted in a very rough morphology and prolonged annealing above 1105° C recovered very flat and clean Si(111)-7×7 surface.