Thermal stability improvement by using Pd/NiO/Al/Ti/Au reflective ohmic contacts to p-GaN for flip-chip ultraviolet light-emitting diodes

The thermal stability, optical reflectivity, and contact resistivity of PdNiOAlTiAu ohmic contacts to p -type GaN were investigated. In contrast to PdNiAlTiAu counterparts, the ohmic contacts PdNiOAlTiAu retained their specific contact resistivity (<3.3× 10-2 Ω cm2) and high reflectivity (>75% @ 370 nm) after a long thermal aging at 200 °C for 100 h in nitrogen ambient. According to the results of the secondary ion mass spectroscopy in-depth profiles study, it is found that the NiO layer is more transparent and a better diffusion barrier than Ni to prevent the penetration of upper metals into p -type GaN during thermal treatment.