Enhanced light output of GaN-based power LEDs with transparent Al-doped ZnO current spreading layer

In this study, Al-doped ZnO (AZO) Ni-AZO and NiO_x--AZO films were deposited on p-type GaN films. The depositions were followed by thermal annealing to form Ohmic contacts. The p-GaN-AZO contacts exhibited a non-Ohmic electrical characteristic. However, electrical characteristic could be greatly improved by insertion of Ni or NiO_x between AZO film and p-GaN layer. In case of 1×1 mm² ultraviolet light-emitting diodes (LEDs) with Ni-AZO contacts, the light output approached saturation point when the injection current was about 400 mA. However, the saturation point was as high as 500 mA for the LEDs with NiO_x--AZO contacts. The lower saturation point could be due to the fact that the resistivity of Ni-AZO films was higher than that of NiO_x--AZO films, thus leading to a severe current crowding effect. The increased resistivity of the Ni-AZO films could be attributed to the interdiffusion between Ni and AZO films. When compared to the LEDs with Ni-Au Ohmic contacts, the light output of the LEDs with Ni-AZO and NiO_x--AZO contacts was higher by 38.2% and 60.6% at 350 mA, respectively.