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

Abstract

By using scanning tunneling microscopy and spectroscopy (STM and STS), the initial stages of ${\mathrm{NH}}_{3}$ exposure on Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) at different substrate temperatures and dosages have been studied. At room and very high (\ensuremath{\sim}1050 \ifmmode^\circ\else\textdegree\fi{}C) temperatures, the 7\ifmmode\times\else\texttimes\fi{}7 surface structure remains and the nitrided sites appear darker, randomly distributing on the surface. Moreover, we find a constant ratio (\ensuremath{\sim}3.46--3.83) of reacted center adatoms to reacted corner adatoms on the partially nitrided surfaces. At intermediately temperatures (\ensuremath{\sim}900 \ifmmode^\circ\else\textdegree\fi{}C), the majority (>90%) of the reacted surface forms the well-ordered silicon nitride 8\ifmmode\times\else\texttimes\fi{}8 reconstruction. In this regime, hexagonal- and triangular-shaped nitride islands can be observed on the 8\ifmmode\times\else\texttimes\fi{}8 and 7\ifmmode\times\else\texttimes\fi{}7 surfaces, respectively. We have also used STS to investigate the changes of local density of states on the nitrogen-reacted 7\ifmmode\times\else\texttimes\fi{}7 surfaces prepared by different conditions.