Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density

We studied temperature dependences of the resistivity, rho(c)(T), of Pd-Ti-Pd-Au ohmic contacts to wide-gap semiconductors n - GaN and n - AlN with a high dislocation density. Both rho(c)(T) curves have portions of exponential decrease, as well as those with very slight rho(c)(T) dependence at higher temperatures. Besides, the Au-Pd-Ti- Pd-n-GaN contacts have a portion of rho(c)(T) flattening out in the low-temperature region. This portion appears only after rapid thermal annealing (RTA). In principle, its appearance may be caused by preliminary heavy doping of the near-contact region with a shallow donor impurity as well as doping in the course of contact formation owing to RTA, if the contact-forming layer involves a material atoms of which serve as shallow donors in III - N compounds. The obtained rho(c)(T) dependences cannot be explained by the existing mechanisms of current transfer. We propose other mechanisms explaining the experimental rho(c)(T) curves for ohmic contacts to n - GaN and n - AIN.