2D-3D growth transition in metamorphic InAs/InGaAs quantum dots

We study the growth by Molecular Beam Epitaxy of InAs quantum dots (QDs) on InGaAs metamorphic buffers (MBs), allowing independent control of the mismatch f between QDs and MBs (7.2% > f > 4.5%) and the In content x of the surface underlying QDs (0 <= x <= 0.35), taking advantage of the dependence of MB strain relaxation on MB thickness. AFM characterization indicated an enlargement of QD diameters for x >= 0.35, while changing f had negligible effects. The two-dimensional (2D) to three-dimensional (3D) critical thickness theta(c) was measured by RHEED: for fixed x, theta(c) increases with reducing f due to the reduction of the elastic energy of the 2D InAs wetting layer (WL), in agreement with literature model predictions. For fixed f, theta(c) reduces with increasing x, an effect not reported so far that we discuss in terms of: (i) enhanced surface diffusion of In atoms on In-richer surfaces and (ii) In-richer WLs in the picture of 2D-3D transition due to In segregation in WLs. These results indicate that metamorphic QDs provide interesting possibilities to control structure properties and can be a model system to investigate the physical mechanisms of the 2D-3D transition.