(a) STEM-HAADF image of the whole structure. The nominal GaAs QW is the bright stripe between the InGaP and AlGaAs barriers. (b,c) HAADF intensity profile across (a) and only across the nominal GaAs QW at higher magnification, respectively. Intensity scan along the negative growth direction. In (c), 1 and 2 indicate the two sublayers replacing the nominal GaAs QW (see text).