Atomically resolved cross‐sectional scanning tunneling microscope topographic images of heterostructures that include a sequence of 1, 2, 5, and 10 nm AlGaAs and GaAs layers are presented. These layers clearly show alloy fluctuations and interface roughness on an atomic scale. In the thick AlGaAs layers a mottled structure with regions of higher Al content about 2 nm wide and elongated in [1̄12] or [11̄2] directions are observed. Similarly, the interfaces are rough on a 2 nm length scale. These results suggest that, for the conditions used for the epitaxial growth of the ternary layers, Al‐rich regions nucleate and grow anisotropically.

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