SILICON BACKBOND STRAIN EFFECTS ON NH3 SURFACE-CHEMISTRY - SI(111)-(7X7) COMPARED TO SI(100)-(2X1)

The role of surface structure in controlling NH3 surface chemistry has been investigated by HREELS and TPD on Si(111)-(7 X 7) and Si(100)-(2 x 1). Following the initial NH3 dissociative adsorption producing NH2(a) and H(a), the production of NH(a) species is favored on Si(111)-(7 X 7) between 300 and 600 K. This is believed to occur through the insertion of NH(a) into the backbond of the Si adatom on Si(111)-(7 X 7) and to be driven by the relief of bond strain on the Si adatom backbonds. By contrast, this stepwise reaction step is disfavored on Si(100)-(2 X 1). Instead, the surface NH2(a) species remain thermally stable until approximately 600 K, when a new reaction channel opens up leading to the 73% depletion of NH2(a) by recombination with H(a) to form NH3(g).