High temperature stable WSi{sub x} ohmic contacts on GaN

Abstract

We have sputter-deposited 500-1200 /spl Aring/ thick WSi/sub 0.45/ metallization onto n/sup +/ GaN (n/spl ges/10/sup 19/ cm/sup -3/) doped either during MOCVD growth or by direct Si/sup +/ ion implantation (5/spl times/10/sup 15/ cm/sup -2/, 100 keV) activated by RTA at 1100/spl deg/C for 30 secs. In the epi samples R/sub c/ values of /spl sim/10-/sup 14/ /spl Omega/ cm/sup 2/ were obtained, and were stable to /spl sim/1000/spl deg/C. The annealing treatments up to 600/spl deg/C had little effect on the WSi/sub x//GaN interface, but the /spl beta/-W/sub 2/N phase formed between 700-800/spl 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/spl deg/C, extending >5000 /spl Aring/ in some cases. This can create junction shorting in bipolar or thyristor devices, R/sub c/ values of <10/sup -6/ /spl Omega/cm/sup 2/ were obtained on the implanted samples for 950/spl deg/C annealing, with values of after 1050/spl deg/C anneals. The lower R/sub c/ values compared to epi samples appear to be a result of the higher peak doping achieved, /spl sim/5/spl times/10/sup 20/ cm/sup -3/. We observed wide spreads in R/sub c/ values over a wafer surface, with the values on 950/spl deg/C annealed material ranging from 10/sup -7/ to 10/sup -4/ /spl Omega/ cm/sup 2/. There appear to be highly nonuniform doping regions in the GaN, perhaps associated with the high defect density (10/sup 10/ cm/sup -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 N/sub 2/ loss and the common use of HT containing growth (and cool-down) ambients. Finally the ohmic contact behavior of WSi/sub x/ on abrupt and graded composition In/sub x/Al/sub 1-x/N layers has been studied as a function of growth temperature, InN mole fraction x=0.5-1) and post WSi/sub x/ deposition annealing treatment. R/sub c/ values in the range 10/sup -3/-10/sup -5/ /spl Omega/ cm/sup 2/ are obtained for auto-doped n/sup +/ alloys, with the n-type background being little affected by growth conditions (n/spl sim/10/sup 20/ cm/sup -3/). InN is the least temperature-stable alloy (/spl les/700/spl deg/C), and WSi/sub x/ contact morphology is found to depend strongly on the epi growth conditions.