Effect of an indium-tin-oxide overlayer on transparent Ni/Au ohmic contact on p-type GaN

We report a low-resistant, thermally stable, and transparent ohmic contact on p-type GaN using an indium-tin-oxide (ITO) overlayer on Ni/Au contact. Ni (20 Å)/Au (30 Å)/ITO (600 Å) contact with preannealing at 500 °C before ITO deposition showed lower contact resistivity by one order of magnitude than the contact without the preannealing. The preannealing produced NiO, acting in the role of diffusion barrier for outdiffusion of N and Ga atoms and indiffusion of In during the subsequent post-annealing. Thus, the formation of Au–In solid solution was effectively suppressed, resulting in the decrease of contact resistivity and enhancement in thermal stability.