Thermal stability of WSi x and W ohmic contacts on GaN

Abstract

We have sputter-deposited 500–1200 Å thick WSi 0.45 and W metallization onto both n + GaN ( n=10 19 cm −3) doped either during MOCVD growth or by direct Si + ion implantation (5×10 15 cm −2, 100 keV) activated by RTA at 1400°C for 10 s and p +( N A=10 18 cm −3) GaN. In the n-type epi samples R c 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 WSi x /GaN interface, but the β-W 2N 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. R c 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 R c in the 10 −2 Ω cm −2 range. The W-based metallization is much more thermally stable than the more common Ni/Au.