Thermal stability and contact degradation mechanisms of TaC ohmic contacts with W/WC overlayers to n-type 6H SiC

Ohmic contacts comprised of the layer sequence of W/WC/TaC/SiC showed excellent thermal stability after testing at 600 degreesC for 1000 h. No degradation of the specific contact resistance nor a reaction of the film with the SiC substrate was observed. From Auger electron spectroscopy depth profiles, it was found that W and WC reacted to form W2C on TaC after annealing. After annealing for several hundred hours at 1000 degreesC, the specific contact resistance of the W/WC/TaC/SiC contacts displayed noticeable degradation. According to secondary ion mass spectrometry (SIMS) analysis, after annealing at 1000 degreesC for 600 h, small but measurable changes in the electrical characteristics were associated with O incorporation at the interface between TaC and SiC. Investigation of the W/WC/TaC/SiC interface by transmission electron microscopy (TEM) indicated that a reaction between the W and WC had occurred, but there was no observed reaction with the SiC substrate. After annealing for 1000 h, substantial changes in the chemistry, the microstructure, the specific contact resistance, and the spreading resistance of the contacts were observed. To understand the mechanisms associated with the degradation of the electrical properties, SIMS and TEM analyses were performed on samples annealed for 600, 700, 800, 900, and 1000 h at 1000 degreesC. These findings indicated that both oxidation and metallurgical reactions played important roles in the degradation of the electrical properties. (C) 2001 American Institute of Physics.