High temperature stable WSix ohmic contacts on GaN

We have sputter-deposited 500-1200 Å thick WSi_0.45 metallization onto 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 1100°C for 30 secs. In the epi samples R_c values of ~10-¹⁴ Ω cm² 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-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, R_c values of <10^-6 Ωcm² were obtained on the implanted samples for 950°C annealing, with values of after 1050°C anneals. The lower R_c values compared to epi samples appear to be a result of the higher peak doping achieved, ~5×10²⁰ cm^-3. We observed wide spreads in R _c values over a wafer surface, with the values on 950°C annealed material ranging from 10^-7 to 10^-4 Ω cm². There appear to be highly nonuniform doping regions in the GaN, perhaps associated with the high defect density (10 ¹⁰ cm^-2) in heteroepitaxial material, and this may contribute to the variations observed. We also believe that near-surface stoichiometry is variable in much of the GaN currently produced due to the relative ease of preferential N₂ loss and the common use of HT containing growth (and cool-down) ambients. Finally the ohmic contact behavior of WSi_x on abrupt and graded composition In _xAl_1-xN layers has been studied as a function of growth temperature, InN mole fraction x=0.5-1) and post WSi_x deposition annealing treatment. R_c values in the range 10^-3-10^-5 Ω cm² are obtained for auto-doped n⁺ alloys, with the n-type background being little affected by growth conditions (n~10²⁰ cm^-3). InN is the least temperature-stable alloy (⩽700°C), and WSi_x contact morphology is found to depend strongly on the epi growth conditions