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

The temperature dependences of the contact resistance ρ c ( 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 ρ c ( T ) for both contacts contain portions of exponential decrease ρ c ( T ) and very weak dependence ρ c ( T ) at higher temperatures. Furthermore, a plateau portion ρ c ( 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 ρ c ( T ) for ohmic contacts to n -GaN and n -AlN are proposed.