Formation process of high reflective Ni/Ag/Au Ohmic contact for GaN flip-chip light-emitting diodes

The combinations of Ni, Ag, and Au which form both Ohmic and reflective multilayer contacts for flip-chip light-emitting diode applications are examined. A strong interdiffusion of Ohmic metals and GaN during the annealing process is found to result in poor reflectance (63% at the wavelength of 465 nm). The authors propose a two step metallization method, the Ni/Ag double layer is first deposited/annealed followed by the deposition of Au, to improve the correspondent reflectivity (as high as 92%). This method is different from the conventional one step Ni/Ag/Au formation method in which these three metals are deposited together and then annealed. The secondary ion mass spectrometry depth profiles indicate that a wide interdiffusion region existed only in the conventional one step formation samples; thus the low reflectivity of Ni/Ag/Au-annealed contacts can be attributed to the strong interdiffusion of Ohmic metal into GaN. Their proposed two step metallization method avoids the strong interdiffusion and improves the reflectivity effectively.