Thermal stability of Ag _XCu _{1 - X} alloys and Pt capping layers for GaN vertical light emitting diodes

In this paper, alloy metals of Ag-Cu (4.2-5.4 at.%) are investigated to achieve both ohmic contact and high reflectivity for vertical light emitting diode (VLED) applications. As the capping metal, platinum (Pt) is used to reduce the agglomeration of the metal alloy structures. At a high temperature of over 500°C, pure Ag and Ag-Cu alloy metals without Pt show serious agglomeration, resulting in poor reflectance. It is found that the introduction of Cu to Ag and Pt capping layers could inhibit the agglomeration of the Ag-based alloy metals and hence sustain high reflectivity during the annealing process while making ohmic contact to p-GaN as low as 5 × 10 ^{- 3} Ω-cm ². The Ag-Cu 4.2 at.% alloy with a Pt capping layer annealed at 450°C shows the highest level of reflectivity; 88% at the wavelength of 460 nm. The optical output power of the corresponding VLED is 14% higher than that of the VLED using pure Ag with a Pt layer.