Nitrogen-polar gallium nitride substrates as solid-state pH-selective potentiometric sensors

This paper reports pH-selective potentiometric responses of epitaxial nitrogen-polar (0001) GaN films on sapphire substrates. The potential of a nitrogen-polar GaN substrate increased with decreasing solution pH from 10 to 2 in a Nernstian manner, whereas it did not significantly change upon increasing the concentrations of the cationic and anionic interfering species. In particular, the negligible responses to anions marked a sharp contrast with previously reported results on gallium-polar GaN that exhibited Nernstian responses to anions. The potentiometric response to pH probably originates from the adsorption of H+ onto oxide-coated nitrogen-polar GaN substrates having polarization-induced negative surface charge.