Thermal nitridation of the Si(111)-(7×7) surface studied by scanning tunneling microscopy and spectroscopy

By using scanning tunneling microscopy and spectroscopy (STM and STS), the initial stages of ${\mathrm{NH}}_3$ exposure on Si(111)-(7×7) at different substrate temperatures and dosages have been studied. At room and very high (∼1050 °C) temperatures, the 7×7 surface structure remains and the nitrided sites appear darker, randomly distributing on the surface. Moreover, we find a constant ratio (∼3.46–3.83) of reacted center adatoms to reacted corner adatoms on the partially nitrided surfaces. At intermediately temperatures (∼900 °C), the majority (>90%) of the reacted surface forms the well-ordered silicon nitride 8×8 reconstruction. In this regime, hexagonal- and triangular-shaped nitride islands can be observed on the 8×8 and 7×7 surfaces, respectively. We have also used STS to investigate the changes of local density of states on the nitrogen-reacted 7×7 surfaces prepared by different conditions.