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

Lattice mismatch between Si and III-V compounds is main limitation for integration of III-V compounds on Si. A solution is using a thin Ge layer deposited on Si that works as virtual substrate (VS). To be CMOS compatible the growth method should have a low thermal budget. Droplet Epitaxy fulfils such requirement as growth is performed in the 200-350?C range. We report on structural characterization of GaAs/Al0.3Ga0.7As self-assembled QDs grown on metamorphic Ge VS on Si by Droplet Epitaxy. The sample structure was: (100) Si substrate, Ge VS, GaAs, AlGaAs, GaAs QDs (50 nm), AlGaAs, GaAs cap. The QDs were obtained by supplying a total amount of Ga of 3.75 ML at 350?C to form Ga droplets, followed by arsenization by opening an As beam flux of 2.10-4 Torr at 180?C. Characterization was done by TEM using (200) Dark Field (DF), High Angle Annular Dark Field (HAADF), HR-TEM and C-TEM methods. The threading dislocations generated at the Ge VS/Si interface did not propagate to the GaAs layer. Stacking faults and antiphase boundaries seen at the GaAs/Ge VS interface rarely propagated to AlGaAs. (200) DF and HAADF showed that the QDs exhibited composition inhomogeneity at their top where Al composition was less than 30% as estimated from the (200) DF contrast. The QDs contained very tiny defects (few nanometers in size) exhibiting disc-shaped contrast with a line-of-no-contrast, which indicates that they could be precipitates. HR-TEM showed that the atomic lattice planes of such precipitates were coherent with the GaAs ones. This suggests that these precipitates could be a metastable \u27pseudocubic\u27 As phase, i.e. the first nuclei of As agglomeration. Bigger precipitates of hexagonal shape were also observed which should correspond to stable hexagonal As phase that would form when small precipitates increase in size. The precipitate formation mechanisms are discussed