Oxidation time evolution of 30-nm Ge QDs. (a) Schematic of the SiO2/SiGe/Si3N4 pillar over the Si substrate before oxidation. CTEM images illustrating the time evolution of 30-nm Ge QDs formed after thermal oxidation of Si0.85Ge0.15 pillars of 50-nm diameter for (b) 25, (c) 35, (d), 60, (e) 75, and (f) 90 min, respectively. Arrows in (c) and (d) highlight the presence of stacking faults and twins within the QDs. Micrographs (b) to (f) are all at the same magnification.