Extended Data Fig. 2: STEM characterization of quantum dot layers.

a, Low-magnification STEM image of SiGe QD superlattice. Due to the Z-contrast property of HAADF imaging, the bright regions are SiGe, while the darker regions are interlayer Si. The crystal orientations are indicated as [\(001\)] for the growth direction and \([1\bar{1}0\)] for the lateral, planar direction. A HAADF image of the first 10 layers from the Si substrate is superimposed with an elemental EELS map. Red and blue indicate silicon and germanium concentration, respectively. Histogram plot to the right displays Ge composition fraction of the corresponding 10 layers. b, HAADF image of a single QD from the blue region in a showing dome-like structure and dislocation-free interfaces. The green box denotes the region in Fig. 1a. c, Core-loss EEL spectra of interlayer Si and SiGe QD. d, Low-magnification HAADF image with associated thickness mapping and horizontally averaged thickness profile. QDs studied in this work were chosen in the 40–50 nm thickness range. Scale bar is 100 nm. e, Strain mapping of QD in b and interlayer Si using geometrical phase analysis (GPA) with associated line profile of the horizontally integrated region marked by the white box. Reference strain of 0% was chosen to be that of interlayer Si. Scale bar is 10 nm. f, Low-magnification HAADF image of several planar-view QDs with the associated crystal directions. g, High magnification HAADF image of a single QD showing a slight elongation of 8.8%. h, i, High resolution EELS Ge composition maps showing 15.5% (h) and 2.68% (i) elongation, respectively. The variances in the lateral direction were obtained via 2D Gaussian fitting. The white and black arrows denote long and short axes, respectively. The scale bars in g–i are 20 nm.