THE STEPWISE DISSOCIATION OF NH3 ON THE SI(111)-(7X7) SURFACE - LOW-TEMPERATURE DISSOCIATIVE ADSORPTION AND THERMAL EFFECTS

The molecular and dissociative adsorption of NH3 on a clean Si(111)-(7 X 7) surface has been studied using high-resolution electron-energy-loss spectroscopy (HREELS), Auger electron spectroscopy, and temperature-programmed desorption (TPD). All NH(x) (3 greater-than-or-equal-to x greater-than-or-equal-to 1) species have been observed vibrationally under varying experimental conditions. Adsorbed molecular ammonia is observed by both HREELS and TPD below 200 K. The absence of a strong delta(s), (NH3) mode in our vibrational spectra suggests that NH3 is bound as a tilted species to the Si(111)-(7 X 7) surface. Ammonia exposures above 1.7 X 10(14) NH3/cm2 at 110 K populate a weakly bound NH3 (a) state which desorbs upon heating with a peak desorption temperature of 115 K. The NH2 (a) species is observed to form at 80 K at all coverages and exhibits an enhanced thermal stability at higher ammonia exposures. The NH(a) species is detected above 200 K by an energy-loss feature at 1100 cm-1, which we assign to the delta(NH) mode. NH(a) is stable to 750 K on high coverage layers. Finally, we observe an increase in the Si-H stretching frequency as the concentration of NH(a) and N(a) species is increased by heating.