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

Abstract

The thermal stability, optical reflectivity, and contact resistivity of Pd∕NiO∕Al∕Ti∕Au ohmic contacts to p-type GaN were investigated. In contrast to Pd∕Ni∕Al∕Ti∕Au counterparts, the ohmic contacts Pd∕NiO∕Al∕Ti∕Au retained their specific contact resistivity (<3.3×10−2Ωcm2) 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.