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

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