Improving the thermal stability of Ag Ohmic contacts for GaN-based vertical light-emitting diodes with a Cu capping layer

Abstract

We investigated the effect of a 20-nm-thick Cu capping layer on the electrical and the optical properties of Ag contacts (200 nm thick) in order to form thermally stable and low-resistance p-type ohmic reflectors for high-performance vertical light-emitting diodes (LEDs). The Ag/Cu contacts give a specific contact resistance of 6.7 × 10 −4 Ωcm 2 and a reflectance of ∼78% at a wavelength of 460 nm when annealed at 400 • C for 1 min in air, which are better than that of Ag only contacts. Blue LEDs fabricated with the Ag/Cu contacts give a forward voltage of 2.90 V at an injection current of 20mA, which is lower than that (2.97 V) of LEDs with Ag only contacts. The LEDs with the 400 • C-annealed Ag/Cu contacts exhibit ∼27% higher output power (at 20 mA) than LEDs with the 400 • C-annealed Ag only contacts. X-ray photoemission spectroscopy examinations were carried out to describe the improved electrical behaviour of the Ag/Cu contacts.