Low specific contact resistance Ti∕Au contacts on ZnO

$\mathrm{Ti}∕\mathrm{Au}$ Ohmic contacts on heavily Al-doped $(n\sim {10}^{19}{\mathrm{cm}}^{-3})$ $n\text{-}\mathrm{ZnO}$ produce low specific contact resistivity of $2.4\times {10}^{-7}\mathrm{\Omega }{\mathrm{cm}}^2$ in the as-deposited condition and extremely low minimum values of $6\times {10}^{-8}\mathrm{\Omega }{\mathrm{cm}}^2$ after annealing at $300°\mathrm{C}$ .The contact resistance is independent of measurement temperature after low temperature anneals, suggesting that tunneling is the dominant transport mechanism in the contacts. The contact morphology roughens after annealing at $150°\mathrm{C}$ and Auger electron spectroscopy depth profiling shows Zn outdiffusion through the metal and intermixing of Au and Ti. However, the morphology does not significantly worsen after anneals at $450°\mathrm{C}$ . This metallization scheme looks very attractive for the $n$ -electrode of $\mathrm{ZnO}$ -based light-emitting diode structures.