Atomic-scale characterization of nitridation processes on Si(100)-2 x 1 surfaces by radical nitrogen

We have investigated the initial nitridation process of Si(100)-2 x I surfaces by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The nitridation was performed by radical nitrogen at a high temperature of 850 degreesC. After the radical-nitrogen exposure of about I Langmuir, linear defects perpendicular to dimer rows were formed. Furthermore, doublet dark lines were frequently observed in the STM image. STS analysis of the dark-line regions clarified that the initial nitridation reaction occurs preferentially at the backbonds of surface Si atoms. Then, the nitridation proceeds via lateral growth of two-dimensional nitride-islands. These islands grow preferentially along the < 011 > direction perpendicular to the Si dimer rows. STS spectra obtained from the 0.5-nm-thick nitride islands distinctly showed a bandgap of about 4.0 eV that is very close to the bandgap of bulk Si3N4.