Gas-source growth of group IV semiconductors: II. Growth regimes and the effect of hydrogen

The crucial difference between gas-source molecular beam epitaxy (MBE) and conventional MBE is the presence of hydrogen on the growth surface. The amount and behaviour of the hydrogen are controlled by a combination of temperature and disilane flux. In situ observations under growth conditions are essential for an accurate understanding of non-equilibrium growth phenomena such as nucleation and coarsening, because once the flux has been cut off the surface material will redistribute itself. We have found that not only does surface hydrogen block silicon diffusion, but also hydrogen saturation of the substrate step edges blocks step-flow growth so that island growth predominates below 700 K, even at low fluxes. The denuded zones seen in MBE are not observed. Above 700 K, the adsorption barrier at step edges is overcome, and a transition from island growth to step-flow growth is observed as the flux is varied. (C) 1997 Elsevier Science B.V.