Chemical understanding and utility of H3PO4 etching of group-III- nitrides

Abstract

The etch mechanism of group-III-nitrides in hot H3PO4 has been studied and compared to KOH. The etched surfaces were investigated by AFM, SEM, TEM, EDX and the etch solutions by NMR. AlGaN is shown to be selectively etched by low temperature H3PO4 or molten KF·2H2O. The material dependency causes much flatter etch pit angles for AlGaN over GaN as well as H3PO4 over molten KOH. 27Al-NMR investigations prove a fast ligand exchange as well as homogenous ligands. 31P-NMR confirms that the dissolved metal is present as AlH2PO42+ and no hydration takes place nor does water play a role in the chemical reaction, opposed to KOH etching. Hence we propose a dissolution limited etch mechanism which can explain difference etch pit angles and material dependencies, as they are the result of material dependent dissolution, assuming that the vertical etch rate is constant.