Nucleation versus instability race in strained films.

Under the generic term Stranski-Krastanov are grouped two different growth mechanisms of SiGe quantum dots. They result from the self-organized Asaro-Tiller-Grinfel'd (ATG) instability at low strain, while at high strain, from a stochastic nucleation. While these regimes are well known, we elucidate the origin of the transition between these two pathways thanks to a joint theoretical and experimental work. Nucleation is described within the master equation framework. By comparing the time scales for ATG instability development and three-dimensional (3D) nucleation onset, we demonstrate that the transition between these two regimes is simply explained by the crossover between their divergent evolutions. Nucleation exhibits a strong exponential deviation at low strain while ATG behaves only algebraically. Consequently, at high (low) strain, nucleation (instability) occurs faster and inhibits the alternate evolution. The crossover between nucleation and ATG instability is found to occur both experimentally and theoretically at a Ge composition around 50%.