Shape transition of self-assembled InAs quantum dots on the GaAs(114)A

InAs quantum dots (QD's) grown by molecular beam epitaxy on $\mathrm{Ga}\mathrm{As}(114)A$ substrates were studied by atomically resolved in situ scanning tunneling microscopy. Two frozen-in QD distributions prepared at different temperatures are analyzed under the assumption that QD's are depicted, which exhibit some variation in evolution mainly due to the statistics in nucleation. After nuclei formation, the QD's were found to grow in a flat form with {137} oriented facets and an aspect ratio of only 0.10. Shapes of presumably different stability were observed. During the following growth, a shape transition from flat to more steep occurs, the latter being characterized by {110} and $(111)A$ facets and an aspect ratio of 0.20. The shape transition occurs at a critical size of the diameter which was found to be temperature dependent, i.e., $12.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at $380\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at $430\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, respectively. The steep shape stays for a large size range until it is changed again. At this point both edge and screw dislocations were incorporated, which could be depicted here on top of the facets. Arguments are given that the change in critical diameter is related to In and Ga alloying and the related change in induced strain. An according growth model is proposed. Overall we conclude that the island shape derives mainly from thermodynamics rather than from kinetics.