Wetting layer evolution in InAs/GaAs(0 0 1) heteroepitaxy: effects of surface reconstruction and strain

InAs heteroepitaxy on the (2×4) and c(4×4) reconstructed surfaces of GaAs(0 0 1) has been studied using scanning tunnelling microscopy, with particular emphasis on the formation and evolution of the two-dimensional (2D) InxGa1−xAs wetting layer. The influence of the initial GaAs reconstruction on the growth behaviour is significant for InAs coverages (θ)<0.8 ML, but both qualitative (surface morphology and reconstruction) and quantitative (step density) surface characteristics converge for θ>1 ML. The effects of surface reconstruction are only important for sub-ML InAs coverages, with strain effects dominating at higher coverages. STM topographs indicate a high and rapid disordering of the WL for θ>0.5 ML, implying that strain-driven alloyed WL formation is important even below θ=1 ML. The 2D→3D growth mode transition and the formation of self-assembled quantum dots (QDs) is delayed at low As fluxes. In this case strain relief occurs first by a reconstruction change from (1×3) to a more In-rich (4×2) structure, with QD formation taking place only after the deposition of an additional amount of InAs (∼0.2 ML).