The stepwise dissociation of NH3 on the Si(111)–(7×7) surface: Low-temperature dissociative adsorption and thermal effects

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Abstract
The molecular and dissociative adsorption of NH3 on a clean Si(111)–(7×7) surface has been studied using high‐resolution electron‐energy‐loss spectroscopy (HREELS), Auger electron spectroscopy, and temperature‐programmed desorption (TPD). All NHx (3≥x≥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 δs(NH3) mode in our vibrational spectra suggests that NH3 is bound as a tilted species to the Si(111)–(7×7) surface. Ammonia exposures above 1.7×1014 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 δ(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.