Electrical properties of Ti/Al ohmic contacts to sulfur-passivated N-Face n-type GaN for vertical-structure light-emitting diodes

We investigate the electrical properties of Ti/Al ohmic contacts on (NH₄)₂S_x-passivated N-face n-GaN:Si (4.2× 10¹⁸ cm^-3) grown by molecular beam epitaxy. It is shown that the passivation results in an increase in the photoluminescence intensity of n-GaN. Current-voltage (I-V) measurements show that the passivated samples experience a slight degradation in the electrical properties upon annealing at 300°C, while the untreated samples show some improvement although still nonohmic. Based on the I-V and X-ray photoemission spectroscopy results, we describe the possible mechanisms for the passivation and annealing dependence of the electrical properties of the Ti/Al contacts to the N-face n-GaN.