Gas-surface reactivity in mixed-crystal systems: The reaction of GeH₄ and Ge₂H₆ on Si surfaces

Supersonic molecular beam techniques have been employed to examine the dissociative adsorption of GeH₄ and Ge₂H₆ on the Si(100) and Si(111) surfaces. At sufficiently high incident kinetic energies (≥ 1 eV), both species on both surfaces react via a direct dissociation mechanism. At sufficiently low incident kinetic energies (≤0.6 eV) and substrate temperatures, however, dissociation of Ge₂H₆ on the Si(100) surface proceeds primarily through a trapping, precursor-mediated dissociation channel. Study of this "mixed-crystal" system, i.e., Ge_nH_{2n + 2} on Si, has permitted important comparisons to two related systems: the reactions of these same gas phase species (GeH₄ and Ge₂H₆) on Ge surfaces, and the reactions of silanes (SiH₄ and Si₂H₆) on these same Si surfaces. We find that the Si surfaces are much more reactive (as much as a factor of 10) than their Ge counterparts, whereas, under similar reaction conditions, and on the same Si surface, the Ge hydrides are moderately more reactive (≈factor of 2) than the Si hydrides. The reactivity of ultrathin (≈2 monolayers) Ge(a) epitaxial layers has also been examined. On Si(100), these strained Ge epitaxial layers exhibit a reactivity that is intermediate between those observed on clean Si(100) and on clean (bulk) Ge(100) surfaces. This example illustrates the role that strain can play in gas-surface reactivity, heretofore, an almost unexamined subject. On the other hand, on Si(111), the Ge epitaxial layer is less reactive than both clean elemental surfaces, and this behavior is attributed to intrinsically low reactivity exhibited by the Ge-induced (5 × 5) reconstruction known to form on this surface.