Ohmic Contacts to Implanted (0001) 4H-SiC

Abstract

The fabrication of low resistance ohmic contacts is a key technology issue for the development of SiC power diodes and transistors. In many cases, contacts are made to implanted regions due to the difficulty of doping SiC by diffusion. In this work, linear transmission line measurements (LTLM) were performed to investigate the relationships between specific contact resistance (rc) and implanted doping concentration (ND) for both N- and Al-implanted samples. Carbon caps were used for all samples during the post-implant thermal activation annealing process. The N- and Al-implanted samples were activated at 1550oC/30min/Ar and 1650oC/30min/Ar, respectively. The alloy NiV7% was used for contacts to N-implanted samples with a contact anneal at 1100oC for 1 min at 10-7 Torr. The alloy Al70%Ti30% was used for Al-implanted samples with a contact anneal at 1000oC for 1 min at 10-7 Torr. The specific contact resistance for a fixed implant concentration was also studied as a function of activation annealing temperature. A second goal of this work has been to determine the activation percentage as a function of the implant concentration for the various activation annealing temperatures. Hall samples were prepared for these measurements. Results for the implanted samples have been compared to the data reported previously for 4H epitaxial layers. Generally, the data show that the specific contact resistances are higher than predicted theoretically at high implant concentrations. This result is consistent with a lower activation percentage due to implant damage, which was demonstrated using Hall measurements. Furthermore, data from activation anneals at different temperatures shows that the specific contact resistance for Al-implanted samples is more sensitive to the activation anneal temperature compared to the contact resistance for N-implanted samples.