Energetics and rate constants of Si2H6 and Ge2H6 dissociative adsorption on dimers of SiGe(100)-2 x 1

Dissociative adsorption of Si2H6 and Ge2H6 on various buckled dimers of SiGe(100)-2 x 1 surface has been studied using density functional theory at the B3LYP level. Two paths, individually leading to (I) two chemisorbed SiH3 or GeH3 and (II) surface hydrides plus a gas-phase SiH4 or GeH4, are considered in the energetics calculation. Contrary to the intuitive mechanism involving a four-center transition state (TS) that yields two SiH3(a) or GeH3(a) in one step, the intermolecular bond scission on a dimer is accomplished in two consecutive steps of three-center TS, in which the first step is rate-limiting. The calculated energy barrier of the first step in Si-Si bond scission is higher than that of the H-Si bond scission on Si*-Si dimer by 9 kcal/mol. Therefore, in contrast to the conventional wisdom of favoring Si-Si bond cleavage over H-Si as the first step in chemisorption on Si(100)-2 x 1, the calculation result supports Niwano and co-workers' experiments that concluded that Si2H6 adsorption without breaking the Si-Si bond was preferred. On the other hand, the Ge alloying on the surface reduces the barrier height difference between these two paths. Hence, the energy blockade between dehydrogenation and intermolecular bond scission decreases when Ge2H6 is added in Si2H6 chemical vapor deposition. Rate constants are also calculated and the results are qualitatively in line with the Ge2H6 reactivity on Si(100) measured by Lam et al.