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 x 10(-4) Omega cm(2) and a reflectance of similar to 78% at a wavelength of 460 nm when annealed at 400 degrees 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 degrees C-annealed Ag/Cu contacts exhibit similar to 27% higher output power (at 20 mA) than LEDs with the 400 degrees C-annealed Ag only contacts. X-ray photoemission spectroscopy examinations were carried out to describe the improved electrical behaviour of the Ag/Cu contacts.