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Chemical etchants for this sort of thing are more an art than a science. Somebody probably tried mixturs of things until they worked.The HF supplies fluoride ions which will dissolve the gallium. The H2O2 oxidises the nitride- probably to nitrate and thus makes it soluble. It also oxidises the gallium.The alcohol might simply be a solvent, or it might have some strange surfactant effect that produces the porous product.Please note that some of the witch's brews used for etching are potentially explosive as well as toxic and corrosive.