In this work we investigate the problem in detail using as a case study the highly mismatched (In,Ga)N/GaN system. A stack of six nominally identical (In,Ga)N quantum disks (QDs) were grown by MBE on GaN nanowires. The samples showed a systematic decrease in the aspect ratio of the QDs along the stacking sequence, High-resolution transmission electron microscopy and geometrical phase analysis reveal a correlation between the strain state of each GaN spacer and the aspect ratio of the corresponding disk above, evidencing the role of the strain in the observed evolution of the shape of the QDs along the stack.
The experimental observation is explained by assuming that the final shape of each QD is defined by the dynamic equilibrium between its surface tension and misfit stress. If the misfit stress varies in a systematic way from one QD to the next, their equilibrium shape will change correspondingly. Calculations performed by finite element method show that while the stress induced by each QD does not pile up over distances larger than one GaN spacer, the change in morphology of one QD is sufficient to change the stress state of the next QD, leading to the systematic shape variation.