Thermal stability of WSix and W ohmic contacts on GaN

We have sputter-deposited 500–1200 Å thick WSi0.45 and W metallization onto both n+ GaN (n=1019 cm−3) doped either during MOCVD growth or by direct Si+ ion implantation (5×1015 cm−2, 100 keV) activated by RTA at 1400°C for 10 s and p+(NA=1018 cm−3) GaN. In the n-type epi samples Rc values of 10−4 Ω cm−2 were obtained and were stable to ∼1000°C. The annealing treatments up to 600°C had little effect on the WSix/GaN interface, but the β-W2N phase formed between 700–800°C, concomitant with a strong reduction (approximately a factor of 2) in near-surface crystalline defects in the GaN. Spiking of the metallization down the threading and misfit dislocations was observed at 800°C, extending >5000 Å in some cases. This can create junction shorting in bipolar or thyristor devices. Rc values of ∼10−6 Ω cm−2 were obtained on the implanted samples for 950°C annealing, with values of ∼10−5 Ω cm−2 after 1050°C anneals. On p-GaN, the contacts are essentially leaky Schottky diodes at 25°C, but became ohmic at ≥250°C, with Rc in the 10−2 Ω cm−2 range. The W-based metallization is much more thermally stable than the more common Ni/Au.