INITIAL LOW-TEMPERATURE NH3 TREATMENT OF A SI(111) 1X1-H SURFACE

Silicon samples have been chemically treated in order to get a Si(111) 1x1: H surface under ultrahigh vacuum. This surface was exposed to do ses of NH3 in the 10(2) to 10(3) Langmuir range, either kept at room t emperature or Joule-heated at 400 degrees C. The Si-2p core level spec tra 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 une xcited NH3, a reaction occurred as soon as the temperature reached abo ut 400 degrees C. The large chemical shift of the Si-2p peak shows tha t nitrogen atoms are imbedded in the silicon surface. The temperature of 400 degrees C at which the reaction occurs is below the lowest one at which H starts leaving the purely hydrogenated surface (over 450 de grees C). It is also lower than the temperature needed to start loosin g hydrogen from a dean 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 nitridatio n in a similar way as alkali metal atoms enhance the oxidation process along Si surfaces.