Pathways for hydrogen desorption from during gas-source molecular-beam epitaxy and ultrahigh-vacuum chemical vapor deposition

(Formula presented) temperature-programmed desorption (TPD) has been used to probe pathways for hydrogen desorption from (Formula presented) surfaces. The experiments were performed on Ge-adsorbed Si(001), Si-adsorbed Ge(001), and (Formula presented) alloy layers grown on Si(001). The depositions were done in ultrahigh vacuum using (Formula presented) and (Formula presented) gaseous precursors. Immediately following partial monolayer or alloy film growth (and, in some cases, postdeposition annealing), the samples were quenched to <200 °C, H exchanged for D, and (Formula presented) TPD carried out in situ. All TPD peaks were fit using standard Polanyi-Wigner desorption models. Both the Si and the Ge monodeuteride desorption energies were found to decrease linearly with increasing Ge coverage. In addition, a comparison of adsorbed-layer and alloy film results shows that deuterium desorption energies depend not just upon the surface-layer Ge coverage, but on second-layer Ge concentration as well. Finally, we show that, in contrast to some previous models, hydrogen desorption from Si sites occurs directly, rather than via diffusion to and subsequent desorption from lower-binding energy Ge sites. We briefly discuss the consequences of these results for modeling gas-source (Formula presented) growth kinetics.