We report a cathodoluminescence (CL) study at helium temperature of oval defects on quantum well (QW) structures grown by molecular beam epitaxy (MBE). We collected the luminescence emitted both from the facet shaped defect and from the defect free growth region. By comparing the experimental CL peak position with the numerically estimated values, we determined the Al concentration and the thicknesses of the layers in the defect area.