Formation mechanism of contact resistance to III–N heterostructures with a high dislocation density

The temperature dependences of the contact resistance ρc(T) of ohmic Pd-Ti-Pd-Au contacts to n-GaN and n-AlN wide-gap semiconductors with a high dislocation density are studied. The dependences ρc(T) for both contacts contain portions of exponential decrease ρc(T) and very weak dependence ρc(T) at higher temperatures. Furthermore, a plateau portion ρc(T) is observed in the low-temperature region for the Au-Pd-Ti-Pd-n-GaN contact. This portion appears only after rapid thermal annealing (RTA). In principle, the appearance of the plateau portion can be associated with preliminary heavy doping of the near-contact region with a shallow donor impurity and with doping during contact fabrication as a result of RTA, if the contact-forming layer contains a material that is a shallow donor in III–N. The dependences obtained are not explained by existing charge-transport mechanisms. Probable mechanisms explaining the experimental dependences ρc(T) for ohmic contacts to n-GaN and n-AlN are proposed.