(a) (200) DF TEM image of the sample and (b) intensity profile across it along the negative growth direction. In (a), the nominal GaAs QW is the dark stripe between InGaP and AlGaAs and corresponds to the downward peak between InGaP and AlGaAs in (b). The profile (b) clearly shows that it exhibits a contrast darker than the GaAs substrate/buffer (at the right-hand side). (c) High-magnification (200) DF image of the GaAs QW. The image has been treated with Adobe Photoshop to improve the visibility of the extra layer in proximity of the GaAs-on-InGaP interface.