Nitridation of Si(111) by nitrogen atoms. II

Annealed Si(111) crystals have been reacted at T < 1050°C with nitrogen atoms produced by electron dissociation of N 2 gas near 10 −4 Torr. The initial stage of the reactions at T ≳ 850°C is rate limited by the incident nitrogen flux, has a constant sticking coefficient or order 1, and produces a well ordered “8 × 8” LEED pattern for surfaces free of impurities. The “8 × 8” LEED pattern is shown to arise from a coincidence lattice with real space unit vectors of length 8 11 times the length of the unreconstructed Si(111) unit cell. The previously reported quadruplet pattern is produced for surfaces with a few percent carbon contamination. These LEED patterns become more intense as the Si(111) 7 × 7 spots fade away, and arise from structures of monolayer thickness growing in islands. Auger and electron energy loss (ELS) signals of these monolayer structures are distinct from each other. After the completion of a reacted monolayer, the reaction proceeds more slowly and produces poorly ordered LEED patterns which gradually replace those mentioned above. The kinetics of the reactions following the quadruplet monolayer are faster than those following the “8 × 8”. Thermal desorption measurements indicate that the growth becomes limited by diffusion through the reacted layers for films about 25–30 Å thick. Auger and ELS signals and LEED patterns of thick multilayers formed on either monolayer structure are similar to each other and to those of the quadruplet monolayer. The reactions carried out at lower temperatures generate Auger and ELS signals which are different from those of the T ≳ 850°C reactions, and suggest the presence of thermodynamically unstable configurations produced by slower surface diffusion.