Some combinations of lattice-mismatched semiconductors can exhibit, under specific growth conditions, a sharp transition from a layer-by-layer growth to the formation of islands. This is the Stranski-Krastanow (SK) growth mode which allows the relaxation of highly strained 2D layers through the free surfaces of 3D islands instead of generating misfit dislocations. These islands are expected to be dislocation free and thus of high structural quality. Usually their typical sizes are on the scale of a few nanometers, so that these self-assembled quantum dots (QDs) are attractive nanostructures for both fundamental physics (interplays of quantum confinement effects, Coulomb blockade effects, etc...) and device applications (QD lasers with higher gain, temperature-insensitive laser threshold, etc...).

To account for the occurrence (or not) of this SK transition during the heteroepitaxial growth, we present a simple equilibrium model taking into account not only the lattice mismatch, but also the dislocation formation energy and the surface energy [1]. It demonstrates the importance of these parameters especially for II-VI systems such as CdTe/ZnTe and CdSe/ZnSe. For II-VIs indeed, as misfit dislocations are easier to form than in III-Vs (such as InAs/GaAs and GaN/AlN ) or IV-IV systems (Ge/Si), the 3D elastic transition is short-circuited by the plastic one. Nevertheless, by increasing the 2D surface energy, telluride and selenide quantum dots can also be grown as predicted by our model and as evidenced experimentally by both reflection high-energy electron diffraction (RHEED) and optical measurements[2].The surface energy variation required to induce the morphology transition is obtained by exposing a given amount of CdTe (CdSe) to an excess of Te (Se) after the growth.

Finally this model has been applied to the low mismatched system GaN/AlN (Δa/a^o = 2.4%). In this case the variation of surface energy has been calculated [3] when the GaN growth front is covered (or not) by a Ga-bilayer. It allows us to understand why, when the GaN is grown under an excess of Ga, the SK transition is completely inhibited, whereas in the opposite case, for a N-rich surface, the formation of SK-coherent QDs is the most favorable growth mode [4]

[1] F. Tinjod, I.-C. Robin, R. Andre, K. Kheng, H. Mariette, invited paper at E-MRS Zakopane 2002.
J. of Alloys and Compounds (2003)
[2] F. Tinjod, B. Gilles, S. Moehl, K. Kheng, H. Mariette, Appl. Phys. Lett. 82 (2003) 4340 ;
J. Cryst. Growth 237-239 (2002) 227.
[3] C.G. Van de Walle et J. Neugebauer, Phys. Rev. Lett. 88 (2002) 066103.
[4] C. Adelmann, J. Brault, D. Jalabert, P. Gentil, H. Mariette, G. Mula, B. Daudin, J. Appl. Phys.91
(2002) 9638 ; C. Adelmann et al. Appl. Phys. Lett. 81 (2002) 3064.

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