Investigation of polarity effects on the degradation of Pd/Ti/Pt ohmic contacts to p-type SiC under current stress

Degradation of low resistance Pd/Ti/Pt ohmic contacts to p-SiC under high current density stressing and the effects of polarity are presented. The Pd/Ti/Pt contact is compared to the conventional Ti/Al contact and provides a lower contact resistance, smoother surface morphology, lower required annealing temperature, and better stability under current stressing. A thick Au overlayer is added to both metallizations for continuous direct current (dc) and pulsed dc stressing. A 31% increase in total resistance is measured for the Ti/Al contacts after 20 h of continuous dc stressing at 200 mA for 5 μm radius contacts, compared to an increase of only 2.5% for the Pd/Ti/Pt contacts. Degradation of the entire anode and cathode was observed for the Ti/Al contacts, whereas damage to only the leading edge of the cathode of the Pd/Ti/Pt contacts was found. The degradation behavior of the Pd/Ti/Pt contacts is explained by a polarity effect, where the cathode reached a higher temperature than the anode under continuous dc stressing. When the contacts were instead pulsed with a dc current of 200 mA using 5 μs pulses and a 10% duty cycle, both contact schemes were electrically stable, probably due to the reduced temperature during testing. However, chemical instability of the Ti/Al contacts was still evident, and although the Pd/Ti/Pt ohmic contacts appeared unaltered, voiding between the Au bond pad and the Pd/Ti/Pt metallization occurred.