PRECURSOR AND DIRECT ACTIVATED CHEMISORPTION OF CHLORINE MOLECULES ONTO SI(111) (7X7) AND SI(100) (2X1) SURFACES

The zero-coverage sticking/chemisorption probabilities (S-0) of a mono energetic Cl-2 beam are measured on two faces of silicon: Si(111) (7x7 ) and Si(100) (2x1). The initial sticking probabilities (S-0) are meas ured as a function of the incident translational energy (E(i)), the su rface temperature (T-s), and the angle between the incident beam and t he surface normal (theta(i)). For Cl-2 chemisorption on Si(111) (7X7) at 300 K, there is a moderate increase in S-0 from 54% at 0.038 eV to 75% at 0.66 eV. S-0 is nearly insensitive to the surface temperature f or E(i) > 0.11 eV. At E(i) less than or equal to 0.11 eV there is a dr op in S-0 when the T-s is elevated. These data indicate that the prima ry mechanism for adsorption of Cl-2 onto Si(111) (7x7) is direct activ ated chemisorption with an average barrier approximate to 0.04 eV and that there is a precursor mediated chemisorption channel at low E(i). Conversely, the initial sticking probability is a strong function of t he incident molecular beam energy for Cl-2 onto Si(100) (2x1). For the Si(100) (2x1) surface at 300 K, there is a decrease in S-0 from 58% a t 0.038 eV to 42% at 0.045 eV and then a sharp increase to 72% at 0.16 eV. At the very lowest incident translational energy (0.038 eV), S-0 is a strong function of the surface temperature while for higher incid ent translational energy (greater than or equal to 0.38 eV) S-0 is ind ependent of the surface temperature. These data indicate that Cl-2 ads orbs on Si(100) (2X1) via precursor-mediated chemisorption at low tran slational energies and via direct activated chemisorption with an aver age barrier of approximate to 0.055 +/- 0.010 eV at high translational energies. The S-0 is independent of the incident angle of the molecul ar beam for all incident energies on both surfaces.