Carrier concentration dependence of Ti/Al/Pt/Au contact resistance on n-type ZnO

Ti/Al/Pt/Au ohmic contacts on n-type ZnO with a range of carrier concentrations ${\text{(7.5×10}}^{15}{\text{–1.5×10}}^{20}$ ${\mathrm{cm}}^{\text{−3}})$ show as-deposited specific contact resistances in the range from ${\text{3×10}}^{\text{−4}}$ to ${\text{8×10}}^{\text{−7}}\mathrm{\hspace{0.5em}\Omega }{\mathrm{cm}}^2.$ Temperature-dependent measurements showed that the dominant transport mechanisms were tunneling in the contacts in the most highly doped films and thermionic emission in the more lightly doped films. After annealing at 200 °C, the lowest specific contact resistance achieved was ${\text{2.2×10}}^{\text{−8}}\mathrm{\hspace{0.5em}\Omega }{\mathrm{cm}}^2.$ However, the contacts show evidence of reactions between the Ti and the ZnO film even for this low annealing temperature, suggesting that applications requiring good thermal stability will need metallurgy with better thermal stability.