Does luminescence show semiconductor interfaces to be atomically smooth?

Luminescence spectra from quantum wells are routinely interpreted in terms of atomically smooth and atomically abrupt interfaces. Here we show that this interpretation is inconsistent with photoluminescence, photoluminescence excitation, and quantitative microscopic (chemical lattice imaging) results. We argue that the discussion of interfacial roughness in terms of ‘‘an island size’’ is too naive. A full characterization of an interface requires the description of a ‘‘roughness spectrum,’’ specifying the amplitude of the interfacial corrugation versus corrugation wavelength over the relevant length scale.