Stability of the step distribution and MBE growth mechanisms on vicinal GaAs ((111)over-bar) substrates

The surface morphologies of thick GaAs layers grown by molecular beam epitaxy on vicinal GaAs ((111) over bar) substrates strongly depend on the growth parameters, but also on the magnitude and the direction of the substrate tilt. Atomic force microscopy study shows directly without ambiguity that the stable steps are aligned along the [110] directions and step downwards toward [<2(11)over bar>], [(1) over bar 2 (1) over bar] and [<(11)over bar 2>] equivalent directions. An explanation is given for the surface morphologies observed which involves the nature of the equilibrium distribution of steps and the surface diffusion length of Ga adatoms. In order to obtain mirror-like surfaces, step flow growth is required for substrates tilted toward [<2(11)over bar>] whereas cluster nucleation on the terraces is required for substrates tilted toward the opposite direction, i.e. [(2) over bar 11]. This interpretation reconciles the various contradictory results found in the literature. Our results also suggest that for a given temperature, higher As overpressure is favorable to the seep flow mode. (C) 1998 Elsevier Science B.V.