Stress distribution, strains and energetics of Si-capped Ge quantum dots: an atomistic simulation study

We study Si-capped Ge quantum dots which are very interesting as these nanostructures emit/absorb light in the near infrared and visible. Their preparation proceeds by the deposition of Ge on Si, which forms small islands. By successive deposition of Si and Ge, a periodic structure is formed in three dimensions, of Si-capped Ge (QDs). Experimentally has been found that these QDs have different shapes and sizes, which are strongly depended on their preparation conditions and method. For our study we created atomistic models representing nanostructures consisting from a few hundreds to tenths of thousands atoms of Si where a portion of them replaced with Ge atoms. The replaced atoms were selected in order the Ge atoms to form small islands shaped in pyramids or domes and with different sizes. This nanostructure was repeated periodically in three dimensions, forming finally a periodic QD structure. For these structures we used the Stillinger-Weber interatomic potential models to relax them to the minimum total energy. Then we studied the strains distribution and energetics and their dependence on the substrate composition and the alignment of the QD along the growth direction. It is found that the distribution of the strains differ compared to an equivalent alloy, with the Ge atoms having a broader one, there is a buffer layer composition for which that nanostructure has the lowest energy and the distribution of strains are proportionally divided in the Ge and Si atoms, and that the alignment along the growth axis with the lowest energy corresponds to each QD on top of the next one