Morphology of porous n-GaP anodically formed in different mineral acids

n-type GaP(100) was anodized in H₂SO₄, HF, HCl, HBr, HI, HNO₃ and H₃PO₄ in order to obtain porous structures. Remarkable differences in the morphology were found when anodisation was carried out under comparable electrochemical conditions. Etching in HF led to a statistically porous structure, but no evidence for higher ordering was obtained. Etching in HCl solutions caused a localized attack of the surface leading to porous insular areas. In HBr highly defined rectangular pores grow perpendicular to the (100)-surface forming several μm thick well defined porous layers. In HI pores are aligned parallel to equipotential lines along defects. Pore formation in H₃PO₄ and HNO₃ can lead to highly complex ordered structures. Clearly the work shows that the formation of porous n-GaP is strongly dependent on the anion present in the electrolyte.