Critical coverage for strain-induced formation of InAs quantum dots

The critical coverage theta (C) for the strain-induced transformation of initial two-dimensional (2D) InAs or InGaAs growth islands into quantum-dot-like three-dimensional (3D) islands is studied with in situ electron diffraction under variation of the flux F, temperature T, and lattice mismatch delta given by the Ga content in InGaAs films. The experimental data are compared to calculations based on a kinetic rate model with layer dependent strain energy inside the islands. With the growth model, the lateral size distribution and the height of the islands is calculated, as well as theta (C). Good agreement between experiments and rate equation results is obtained for the influence of delta and F on theta (C), whereas the T dependencies show a contradictory behavior. This is explained by a temperature-dependent intermixing between the deposited In and Ga from the substrate. Considering this effect in the calculations allows a determination of the temperature-dependent Ga content. Additional calculation results point out that the island diameter shrinks during the 2D to 3D transition and that the island shape is not in equilibrium during growth.