Thermal annealing effects on Ni/Au contacts to p type GaN in different ambient

Current–voltage (I–V) characteristics, contact resistance, and optical transmittance measurements are performed to investigate the effects of different annealing ambient conditions on properties of contacts of Ni/Au/p-GaN. With increasing annealing temperature in the range from room temperature to 600°C in air, I–V characteristics become linear, and the specific contact resistance (ρc) reduces continually. ρc of Ni (20 nm)/Au (50 nm) contact to p-GaN annealed in air at 600°C is obtained as 2.76×10−2 Ω cm2. However, in N2 ambient, ρc is higher and shows a more complex evolution with increasing annealing temperature. The optical transmittance of the specimens increases and maintains a high value with the annealing temperature in air while it increases first, and then decreases monotonically in N2 atmosphere. We suggest that formation of low-resistance and high optical transmittance p-type Ohmic contact may be attributed to the oxidized Ni layer. The mechanism of Ohmic contact formation under different ambient conditions is also discussed.