The electrical and optical properties of Ag/p-GaN contacts have been investigated as a function of the annealing temperature, oxygen concentration, and annealing time. Specific contact resistance (ρ c) values as low as 1.2 × 10-4 Ω·cm2 were obtained from the Ag/p-GaN contact annealed at 400 °C for 60 s in ambient O2/N2 (1:10). We found that the participation of oxygen improves the formation of ohmic contacts. Oxygen might remove the H in Mg-H complexes to activate the Mg acceptors and enhance Ga out-diffusion to form an Ag-Ga solid solution. We also found that the reflectivity of the Ag layer decreases with increasing annealing temperature in the O2-containing ambient environment. Thus, an optimal annealing condition of Ag/p-GaN for blue and green LEDs is suggested based on these results. We also used the suggested annealing conditions to form ohmic contacts on DUV LEDs and achieved good electrical performance. The forward voltages of UVC LEDs fabricated with annealed Ag contacts were 6.60 V (7.66 V) at a 40 mA (100 mA) injection current.
Keywords: Ag; Circular transmission line model; Ohmic contact; Rapid thermal annealing; p-GaN.
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