Investigation of Thermal Stability and Degradation Mechanisms in Ni-Based Ohmic Contacts to n-Type SiC for High-Temperature Gas Sensors

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 (similar to 1 x 10(16) cm(-3)) 4H-SiC for over 1000 h in air at 300A degrees C. Although a gradual increase in specific contact resistance from 3.4 x 10(-4) Omega cm(2) to 2.80 x 10(-3) Omega cm(2) was observed, the values appeared to stabilize after similar to 800 h of heating in air at 300A degrees C. The contacts heated at 500A degrees C and 600A degrees 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.