Low Resistance Non-Alloyed Ohmic Contacts to High Al Composition n-type AlGaN

Ohmic contacts to high (>70\%) Al content n-type Al$_x$Ga$_{1-x}$N ultra-wide bandgap semiconductor layers in nitride electronic and photonic devices are typically fabricated by a lift-off process and high temperature ($>700^\circ$C) thermal alloying. These conditions often result in significant structural deformations of the fabricated structures and impose a harsh thermal budget on all other aspects of the device. Here, we report the fabrication of \textit{non-alloyed} \textit{as-deposited} ohmic contacts to 71\% n+AlGaN ($E_\text{g}\sim5.4$~eV) with a free carrier concentration of roughly $7\times 10^{19}$~cm$^{-3}$ and a resistivity of 4 - 5.5 m$\Omega$cm (among the lowest reported for Al$_{0.71}$Ga$_{0.29}$N) with linear $I-V$ characteristics and a contact resistivity of $\rho_\text{c}=(4.4\pm1.0)\times10^{-4}$~$\Omega$cm$^2$ (measured at zero voltage). Contacts with this quality are formed by two separate fabrication schemes: (i) metal-first patterning, and (ii) lift-off with an oxygen asher descum prior to metal deposition. Given the low threading dislocation density in the single-crystal AlN substrate used for epitaxy, the smooth morphology of the contacted epitaxial surface, and the non-alloyed nature of the contacts, this contact resistivity is attributed purely to thermionic field emission through the metal-semiconductor junction. Contact resistivity extraction at low current injection enables us to model these results using a thermionic field-emission model of contact resistivity, yielding a barrier height for Ti/Al$_{0.71}$Ga$_{0.29}$N of $(0.81\pm0.02)$ eV.