Structural characterization of GaAs self-assembled quantum dots grown by Droplet Epitaxy on Ge Virtual Substrates on Si

The structure of self-assembled quantum dots (QDs) grown by Droplet Epitaxy on Ge virtual substrates has been investigated by TEM. The QDs have a pyramidal shape with base and height of 50 nm. By (0 0 2) dark field TEM it was seen that the pyramid top is Ga poor and Al rich most likely because of the higher mobility of Ga along the pyramid sides down to the base. The investigated QDs contain defects identified as As precipitates by Moirè fringes. The smallest ones (3-5 nm) are coherent with the GaAs lattice suggesting that they could be a cubic phase of As precipitation. It seems to be a metastable phase since the hexagonal phase is recovered as the precipitate size increases above ~5 nm.