Thermal stability of WSix and W ohmic contacts on GaN

We have sputter-deposited 500-1200 Angstrom thick WSi0.45 and W metallization onto both n(+) GaN (n = 10(19) cm(-3)) doped either during MOCVD growth or by direct Si+ ion implantation (5 x 10(15) cm(-2), 100 keV) activated by RTA at 1400 degrees 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) Ohm cm(-2) were obtained and were stable to similar to 1000 degrees C. The annealing treatments up to 600 degrees C had little effect on the WSix/GaN interface, but the beta-W2N phase formed between 700-800 degrees 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 degrees C, extending > 5000 Angstrom in some cases. This can create junction shorting in bipolar or thyristor devices. R-c values of similar to 10(-6) Ohm cm(-2) were obtained on the implanted samples for 950 degrees C annealing, with values of similar to 10(-5) Ohm cm(-2) after 1050 degrees C anneals. On p-GaN, the contacts are essentially leaky Schottky diodes at 25 degrees C. but became ohmic at greater than or equal to 250 degrees C, with R-c in the 10(-3) Ohm cm(-2) range. The W-based metallization is much more thermally stable than the more common Ni/Au. (C) 1999 Elsevier Science S.A. All rights reserved.