On the specific contact resistance of metal contacts to p-type GaN

Extraction of the specific contact resistance, rho(c), using the established circular transmission line measurement (c-TLM) and a series resistance measurement across mesa-isolated diodes is compared for a non-alloyed Pd/Ag contact to p-GaN. The limitations of the c-TLM technique are discussed and it is shown that for rho(c) values below 10(-4)Omega cm(2) both unintentional submicron errors in the actual radii and uncertainty in the resistance measurements can lead to order of magnitude changes in the extracted rho(c). An additional current-voltage measurement across a mesa- isolated diode is proposed. The accuracy of the extracted.c in this case requires consistency of the intrinsic diode characteristic, namely the ideality, at the bias voltages used for extraction, which is in turn related to the carrier transport and recombination properties. From a comparison with the c- TLM, we conclude that the resistance should be extracted at current densities < 10 A cm(-2) as junction heating changes the diode ideality.