Nitruration initiale par NH₃ à basse température de Si(111) 1x1-H

Abstract
Silicon samples have been chemically treated in order to get a Si(111) 1x1 : H surface under ultrahigh vacuum. This surface was exposed to doses of NH₃ in the 10² to 10³ Langmuir range, either kept at room temperature or Joule-heated at 400° C. The Si_2p core level spectra were recorded upon synchrotron light beam excitation at a surface - sensitive photon energy wavelength of 128 eV. The N_2s nitrogen peak binding energy at 20 eV was also recorded. While at room temperature the hydrogenated surface remained unperturbed whatever the dose of unexcited NH₃, a reaction occurred as soon as the temperature reached about 400° C . The large chemical shift of the Si_2p peak shows that nitrogen atoms are imbedded in the silicon surface. The temperature of 400° C at which the reaction occurs is below the lowest one at which H starts leaving the purely hydrogenated surface (over 450° C) . It is also lower than the temperature needed to start loosing hydrogen from a clean Si(111) surface which has been saturated by NH₃ : these results suggest that the H-induced 1x1 relaxed reconstruction of the Si(111) surface facilitates the surface reaction of nitridation in a similar way as alkali metal atoms enhance the oxidation process along Si surfaces.