Stepwise Dissociation of NH3 on the Si(111)-(7x7) Surface: Low Temperature Dissociative Adsorption and Thermal Effects.

摘要

The molecular and dissociative adsorption of NH3 on a clean Si(111)-(7x7) surface has been studied using HREELS, AES and TPD. All NHx (3 > or = x > or = 1) species have been vibrationally observed 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 NH3 is bound as a tilted species to the Si(111)-(7x7) surface. Ammonia exposures above 1.7 x 10 14 NH3/sq cm 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 300 K by an energy loss feature at 1100/cm, 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.