Abstract
The ultraviolet light reflectivity of Al is the most excellent in the applications of flip-chip ultraviolet reflective light-emitting diodes (UV LED). However, because of low work function of Al, it can not form ohmic contact with p-GaN. Therefore, contact metals such as Ni or Pd is needed to obtain the ohmic contact with p-GaN. Ni (1 nm)/Al (160 nm)、Ni (2.5 nm)/Al (160 nm)、Ni (5 nm)/Al (160 nm)、Ni (8 nm)/Al (160 nm), Al48Ni52 (60 nm)/Al (100 nm), Pd (2.5 nm)/Al (160 nm)、Pd (5 nm)/Al (160 nm) and Al25Pd75 (9 nm)/Al (160 nm) were prepared to study the effects of Ni or Pd contact layer thickness and annealing temperature on the photoelectric properties of optical reflective ohmic contacts. In this study, optical reflectivity and contact resistivity of Ni/Al and Pd/Al ohmic contacts to p-GaN were investigated. In contrast to Ni/Al samples, Pd/Al contacts retained their linear Current-Voltage curve after rapid thermal annealing at 500 °C in argon ambient. According to the results of the study, it is suggested that the UV reflectivity decreases with the increase of the thickness of Pd or Ni contact layer. As the annealing temperature increases, Al diffuses to the interface between metal and p-GaN to increase the optical reflectivity, but causes the electrical deterioration. At the same contact layer thickness, the specific contact resistance of Pd/Al is lower than Ni/Al. In Ni-Al system, the maximum annealing temperature for ohmic contact can be 400 ° C, while in Pd-Al system, the maximum annealing temperature is 500 ° C. After 400 ° C rapid thermal annealing, Pd (5 nm) / Al (160 nm) could obtain the lowest specific contact resistance of 3 × 10-3 Ω-cm2 with optical reflectivity about 58%-63% (270 nm-350 nm), and Ni (5 nm)/Al (160 nm) obtains specific contact resistance of 3 × 10-1 Ω-cm2 with optical reflectivity about 70% (270 nm-350 nm). After 500 ° C rapid thermal annealing, Al25Pd75 (9 nm)/Al (160 nm) obtains the specific contact resistance of 2 × 10-2 Ω-cm2 with optical reflectivity about 58%-63% (270 nm-350 nm).