We investigated the thermal stability of Pt/TaSi x /Ni/SiC ohmic contacts, which have been implemented in SiC-based gas sensors developed for applications in diesel engines and power plants. The contacts remained ohmic on lightly doped n-type ($1 9 10 16 cm -3 ) 4H-SiC for over 1000 h in air at 300°C. Although a gradual increase in specific contact resistance from 3.4 9 10 -4 X cm 2 to 2.80 9 10 -3 X cm 2 was observed, the values appeared to stabilize after $800 h of heating in air at 300°C. The contacts heated at 500°C and 600°C, however, showed larger increases in specific contact resistance followed by nonohmic behavior after 240 h and 36 h, respectively. Concentration profiles from Auger electron spectroscopy and electron energy-loss spectroscopy show that loss of ohmic behavior occurs when the entire tantalum silicide layer has oxidized.

The high-temperature stability of a Pt/TaSi 2 /Ni/SiC ohmic contact metallization scheme was characterized using a combination of current-voltage measurements, Auger electron spectroscopy, and transmission electron microscopy imaging and associated analytical techniques. Increasing the thicknesses of the Pt and TaSi 2 layers promoted electrical stability of the contacts, which remained ohmic at 600C in air for the extent of heat treatment; the specific contact resistance showed only a gradual increase from an initial value of 5.2 x 10 -5 -cm 2 . We observed a continuous silicon-oxide layer in the thinner contact structures, which failed after 36 h of heating. Meanwhile, thicker contacts with enhanced stability contained a much lower oxygen concentration that was distributed across the contact layers, precluding the formation of an electrically insulating contact structure.

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