Energetics and Rate Constants of Si₂H₆ and Ge₂H₆ Dissociative Adsorption on Dimers of SiGe(100)-2 × 1

Abstract

Dissociative adsorption of Si2H6 and Ge2H6 on various buckled dimers of SiGe(100)-2 × 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 × 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.

Digermane Deposition on Si(100) and Ge(100): from Adsorption Mechanism to Epitaxial Growth

“…Although Si–Si bond scission was originally proposed for disilane − adsorption on Si(100) and Ge(100) surfaces, it was recently shown that a beta-hydride elimination mechanism ,,, is operative for disilane on both surfaces, as predicted by theoretical calculations. − The situation with digermane was less clear. Experiments had suggested that digermane adsorption involves Ge–Ge bond scission while theoretical calculations showed a large barrier associated with such a chemisorption pathway. Cheng et al recently showed that chemisorption of digermane on Si(100) through the β-HEP has a lower chemisorption barrier than chemisorption through Ge–Ge bond scission.…”

“…17−19 The situation with digermane was less clear. Experiments had suggested that digermane adsorption involves Ge−Ge bond scission 14 while theoretical calculations 26 showed a large barrier associated with such a chemisorption pathway. Cheng et al 26 recently showed that chemisorption of digermane on Si(100) through the β-HEP has a lower chemisorption barrier than chemisorption through Ge− Ge bond scission.…”

Extended Compositional Range for the Synthesis of SWIR and LWIR Ge1–ySny Alloys and Device Structures via CVD of SnH4 and Ge3H8

“…A similar mechanism has also been proposed for disilane. 20 Both reactions require two open sites at the surface, which seem to contradict the experimental observation of weak dependence on the H-coverage. 21 However, a possible variation of the above mechanism is Ge 2 H 6 (g) + H(a) → Ge 2 H 5 (a) + H 2 (g), which does not require open sites because the digermane in the gas phase attacks a H-covered site and the reaction eliminates H 2 to the gas phase.…”

“…However, more recent experimental and theoretical work suggest that the preferred pathway is the hydrogen-elimination process Ge 2 H 6 (g) → Ge 2 H 5 (a) + H­(a). A similar mechanism has also been proposed for disilane . Both reactions require two open sites at the surface, which seem to contradict the experimental observation of weak dependence on the H-coverage .…”

“…18 This was justified on the basis that the Ge−Ge bond in Ge 2 H 6 is weaker than the Ge−H bonds. However, more recent experimental 19 and theoretical 20 work suggest that the preferred pathway is the hydrogen-elimination process Ge 2 H 6 (g) → Ge 2 H 5 (a) + H(a). A similar mechanism has also been proposed for disilane.…”