Modification of Ammonia Chemisorption on Silicon by Synchrotron Radiation

The passivation of semiconductor surfaces is an issue of physical and practical importance. In this comprehensive study, the passivation of Si(111)2 x 1 surface due to chemisorption of NH_3 was investigated in detail using surface-sensitive techniques. At room temperature, adsorption caused partial dissociation of the NH _3 molecules. Two thirds of the surface dangling bonds was saturated by the NH_ {x} species while the rest presumably was covered by H. Besides the coverage, information about the adsorption geometry, kinetic and thermal stability was also established. In particular, we found that the adsorbates were unstable upon annealing above room temperature. Their gradual removal due to either desorption or dissociation paved the way for sustained surface nitridation. In addition, we have also studied the nonthermal alternative of using synchrotron radiation to activate at room temperature the capped silicon surface. The energetic X-ray photons kept the surface reactive. Consequently silicon nitride was successfully grown via the adsorbed materials, up to thicknesses limited only by diffusion. The resulting nitride overlayer and its interface with the silicon substrate were characterized. Although the reaction mechanism was different from that of the thermal process, it is interesting that they resulted in essentially the same final products, differing only in their hydrogen content.