Direct proof of the noncommutativity of GaAs/AlGaAs and AlGaAs/GaAs interfaces of quantum wells (QW's) of width Lz=5nm is obtained from a careful study and mathematical analysis of the structure and shape of photoluminescence (PL) spectra. QW'S were grown by molecular beam epitaxy (MBE) at 620°C with a rate of 1 µm/h and the growth was interrupted at either one, both, or none of the interfaces. A large difference in the microscopic crystallographic and chemical structure of the two types of interfaces is observed. In addition we observe inequivalent incorporation of impurities and traps at the two interfaces. Interruption of growth at the AlGaAs surface deteriorates the quantum efficiency of QW's. Analysis of the PL lineshape gives clear evidence that the AlGaAs growth surfaces are rougher than the GaAs ones. During growth interruption (GRI) a rapid smoothing and the formation of monolayer growth islands with lateral sizes larger than the diameter of the 2D-exciton occurs only for GaAs surfaces in the samples used for this study. AlGaAs surfaces also become smoother upon GRI but large island formation is not found at the present set of experimental and growth parameters