The mechanisms responsible for the transparency of Ni/Au and Ni/Au/ZnO on p-GaN has been investigated. It was found that the optical transmission of Ni/Au contacts is dominated by the film thickness and morphology. A change in film thickness, induced by thermal annealing, results in a more transparent contact material. In addition a ZnO film was used as an antireflection layer on top of a Ni/Au contact. The Ni/Au/ZnO film was found to have an increased light transmission of 15% compared with an annealed Ni/Au contact. The maximum optical transmission measured through the Ni/Au/ZnO contact was 90%.
Tseng, C. L., Youh, M. J., Moore, G. P., Hopkins, M. A., Stevens, R., & Wang, W. N. (2003). Mechanism for the increased light transmission through Ni/Au/ZnO contacts on p-GaN for high power optoelectronic devices. Applied Physics Letters, 83(18), 3677-3679. https://doi.org/10.1063/1.1619558