Optimization and mechanism for Ni/Ag/Ni ohmic contact to p-Al_0.28Ga_0.72N

The optimal conditions and mechanism of ohmic contact formation between Ni/Ag/Ni and p-Al_0.28Ga_0.72N contact layer on deep ultraviolet light-emitting diode (DUV-LED) epitaxial wafers were investigated. By optimizing the metal structures and annealing conditions, the specific contact resistivity between Ni/Ag/Ni (1/50/2 nm) and p-Al_0.28Ga_0.72N was reduced to 1.36 × 10^-3 Ω cm² after annealing at 550 °C for 2 minutes in an air ambient, which was two orders of magnitude lower than that of Ni/Au contact. The DUV-LEDs with Ni/Ag/Ni electrodes annealed at 550 °C for 2 minutes exhibited an external quantum efficiency (EQE) of 8.30% at a current density of 4.7 A/cm². The X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results revealed that gallium (Ga) atoms near the interface diffused outward into the metal, leaving a significant number of Ga vacancies on the p-AlGaN surface after annealing. The diffused Ga atoms combined with silver (Ag) and nickel (Ni) to form Ag-Ga and Ni-Ga solid solutions, respectively. The diffused Ag and Ni combined with oxygen (O) to form AgNiO₂ during the annealing process in an air ambient. The formation of Ga vacancies and the ternary oxide AgNiO₂ may contribute to the reduction of contact resistance. The presence of Ag plays a more critical role than the formation of NiO_x in achieving ohmic contact with p-Al_0.28Ga_0.72N.