HYDROGEN-HALOGEN CHEMISTRY ON SEMICONDUCTOR SURFACES

The chemistry of coadsorbed H and X (X = Cl, Br) on semiconductor surfaces is important in epitaxial growth of silicon from chlorosilanes and of SixGe1-x alloys, in hydrogenating-halogenating cycles in atomic layer epitaxy, and also provides an interesting model system, yet has received little attention to date. We have investigated the interaction of H, HCl, and HBr with Ge(100) by temperature-programmed desorption, and find that H-2, HCl, and HBr each desorb with near-first-order kinetics near 570-590 K, and that GeCl2 and GeBr2 desorb with second-order kinetics near 675 K and 710 K respectively. Trends in the chemistry and kinetics can be rationalized by viewing the dimer atoms on clean Ge(100) - (2 x 1) as being linked by a strained double bond and adsorption, decomposition, and desorption as being analogous to addition, rearrangement, and elimination reactions of molecular germanium compounds. The near-first-order desorption kinetics are attributed to pairing on surface dimers induced by the pi bond on unoccupied dimers. We infer a pairing enthalpy for H + H (approximately the pi bond strength of dimerized Ge(100) surface atoms) of approximately 5 kcal mol-1.