Thermal stability of WSi_x and W ohmic contacts on GaN

摘要

We have sputter-deposited 500-1200 A 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 (5x10~(15) cm~(-2), 100 keV) activated by RTA at 1400 deg 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) OMEGA cm~(-2) were obtained and were stable to approx 1000 deg C. The annealing treatments up to 600 deg C had little effect on the WSi_x/GaN interface, but the beta -W_2N phase formed between 700-800 deg 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 deg C, extending >5000 A in some cases. This can create junction shorting in bipolar or thyristor devices. R_c values of approx 10~(-6) OMEGA cm~(-2) were obtained on the implanted samples for 950 deg C annealing, with values of approx 10~(-5) OMEGA cm~(-2) after 1050 deg C anneals. On p-GaN, the contacts are essentially leaky Schottky diodes at 25 deg C, but became ohmic at >=250 deg C, with R_c in the 10~(-2) OMEGA cm~(-2) range. The W-based metallization is much more thermally stable than the more common Ni/Au.