We demonstrate the effectiveness of controlled surface illumination to enhance the material selectivity at n-GaAs/n-Al0.4Ga0.6As heterojunctions. Selectivities were measured using 785 and 711 nm light in a 5 weight percent aqueous nitric acid etchant at room temperature. The highest selectivity observed was an improvement over the unilluminated case by a factor of 1700. Potential barriers associated with the n-GaAs/n-Al0.4Ga0.6As junction reduced the selectivity of the etch by 50%. A model based on the dissolution of substrate material is presented that relates the selectivity of the etch to the wavelength and intensity of the illuminating light for wavelengths between the cutoff wavelengths of GaAs and Al0.4Ga0.6As. No significant effect on the etch rate was noted for the n-Al0.4Ga0.6As material at wavelengths of 711 and 785 nm.