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The presence of linear features commonly known as `dark-line defectsÂ¿ is known to have a detrimental effect on the luminescence efficiency of some light-emitting diodes. The paper demonstrates the value of using SEM-CL and SEM-EBIC techniques, both for materials assessment before diode fabrication, and for evaluating the effect of various processing parameters. It is shown that commercial device-grade GaAs0.6 P0.4 material can contain substantial numbers of structural defects (in both the graded and constant-composition layers), many of which cause a marked reduction in the luminescence efficiency of the fabricated diodes. Device processing does not appear to cause further degradation in the operation of the diodes. The nature and origin of the defects are discussed.