The control of quantum well shapes in GaAs/AlGaAs material after growth has been investigated both theoretically and experimentally. Double quantum well samples capped either by SiO2 or fluorides of the group IIA elements were annealed, and energy gap shifts were measured by photoluminescence. These experimental energy shifts were compared to a theoretical model to obtain the diffusion coefficient of aluminum into the quantum wells. Fluorides were found to inhibit the intermixing process almost completely, whereas SiO2 is known to enhance it. The aluminum diffusion coefficients for samples annealed at 920 °C for 30 s are 4.0×10-17 cm2/s and 2.1×10-15 cm2/s for SrF2 and SiO2 caps, respectively. The activation energies found were 4.09 and 6.40 eV for the same two caps.