Various thicknesses of AlGaAs are grown on GaAs substrates by MOCVD. Low temperature photoluminescence of the substrate is observed even for layers of AlGaAs 24μm thick. Direct excitation by the 488.0 nm pumping radiation and excitation by reradiation from the AlGaAs are eliminated as causes. From photoluminescence and EBIC studies, evidence is given to show that the substrate luminescence is caused by a much larger than expected electron diffusion length. A small trace of GaAs luminescence may be due to alloy segregation in the AlGaAs films themselves.