High-Reflectivity Mg/Al Ohmic Contacts on n-GaN

This work reports a Mg (130 nm)/Al (50 nm) bilayer to realize high-reflectivity ohmic contact for n-GaN. The contact resistivity of 3.75 x 10(-4)Omega center dot cm(2) with Mg/Al contact was achieved after annealing at 250 degrees C in ambient Ar for 1 min. The specific contact resistivity was found to change moderately when the annealing temperature was below 400 degrees C, but the ohmic characteristics deteriorated significantly when the annealing temperature was above 450 degrees C. X-ray photoemission spectroscopy revealed that the Ga 3d (Ga-N) peak decreased by 0.28 eV with the increase of the annealing temperature from 250 to 450 degrees C, which can increase the metal-semiconductor contact barrier height, leading to deteriorated current-voltage characteristics. The reflectivity of the samples was found to decrease with the increase of the annealing temperature, and decreased greatly when the temperature exceeded 350 degrees C. When the annealing temperature is below 300 degrees C, the reflectivities of the samples are greater than 90% for wavelengths from 350 to 550 nm. Mg/Al is therefore a promising candidate to serve as a reflective n-GaN electrode for GaN-based flip-chip LEDs to improve the light extraction efficiency.