Cross-sectional scanning tunneling microscopy was used to identify individual Al atoms on cleaved surfaces of two sets of (AlGa)As heterostructure samples grown with metalorganic vapor-phase epitaxy and molecular-beam epitaxy. We determined the average Al concentration profile perpendicular to the GaAs–(AlGa)As interfaces. Based on former investigations of short-range ordering in (AlGa)As bulk material grown with metalorganic vapor-phase epitaxy, we conclude that short-range ordering during growth of the interfacial layers contributes significantly to the observed interface roughness.