Improved thermally stable ohmic contacts on p-GaN based on W ₂B

The annealing temperature (25-800°C) dependence of ohmic contact characteristics on p-GaN using a W ₂B/Ti/Au metallization scheme deposited by sputtering are reported. The contacts are rectifying in the as-deposited condition but become ohmic for annealing at ≥500 °C. A minimum specific contact resistivity of 1.7×10 ^-3 Ω cm ^-2 was obtained after annealing at 800°C for 60 s. Higher annealing temperatures produced sharp increases in the resistivity of the GaN and irreproducible contact properties. However, the contact morphology was similar over the entire annealing range used here. Auger electron spectroscopy profiling showed the onset of Ti out-diffusion through the Au at 500°C. By 800°C the Ti was almost completely removed to the surface, where it became oxidized. These boride-based contacts have superior thermal stability to the more common Ni/Au, whose morphology degrades significantly above 500°C.