Trapping-mediated chemisorption of disilane on Si(100)-2×1

Abstract

Disilane adsorption probabilities have been measured on Si(100)-2×1 over a wide range of incident kinetic energies, incident angles, and surface temperatures using supersonic molecular beam techniques. The trapping-mediated chemisorption mechanism is shown to be the dominant adsorption pathway under the conditions investigated. The first step in such a mechanism, namely trapping into the physical adsorption well, has been studied directly via measurements at a surface temperature of 77 K. As expected, the trapping probability drops with increasing kinetic energy, but nearly 50% of incident molecules trap at 1 eV incident energy, indicating that trapping is quite efficient over a wide range of translational energies. Chemisorption probability values measured at higher surface temperatures are fit to a simple trapping-mediated chemisorption model that can be used to predict adsorption probabilities over a wide range of conditions. Measurements of the chemisorption probability at 500 K are independent of incident angle at kinetic energies of 0.75 eV and below. However, trapping probabilities measured at 77 K are shown to decrease with increasing angle of incidence at kinetic energies of 0.6 eV and above. This unusual effect is discussed in terms of molecular scattering during parallel momentum accommodation. In order to investigate the effect of surface hydrogen formed as a result of disilane decomposition, adsorption probabilities were measured as a function of monohydride coverage as well. On a monohydride-saturated surface the trapping probability is found to be lower than on a bare surface, most likely due to a decreased disilane physical adsorption binding energy compared to the bare surface. Also, the trapping probability varies linearly with hydrogen coverage between bare-surface and monohydride-saturated values. On the other hand, the hydrogen coverage dependence of the chemisorption probability is found to follow a simple second-order kinetic scheme based on chemisorption occurring at two vacant surface sites.