Mechanism of initial adsorption of NO on the Si(100) surface

High-resolution photoemission study of the adsorption of NO molecules on the Si(100)2×1 surface at 150 K is performed using undulator synchrotron radiation. N 1s, O 1s, as well as Si 2p core-level spectra are measured for the series of NO exposures from the very initial adsorption to the full monolayer coverage. The metastable adsorption species are observed with N 1s binding energies of 396.4 and 395.8 eV, along with the stable major adsorption species of N≡Si₃ at 397.4 eV. Both of the metastable species are ascribed to the intermediate reaction species of N=Si₂ with a dangling bond. In the O 1s spectra, the major O species at 532.4 eV is found to shift toward high binding energies with increasing NO exposures or increasing substrate temperature. This shift is explained by the changes in bonding configurations induced by the active oxygen agglomeration. From these results, it is concluded that NO adsorption above 150 K is completely dissociative without any metastable molecular adsorption species. Instead, it is deduced that the dissociated N atoms metastably form twofold coordinated N=Si₂ species through multiple dissociation paths involving one or two Si dimer atoms.