Electrical and structural properties of Ti/Au ohniic contacts to n-ZnO

We have investigated Ti/Au (30/50 nm) ohmic contacts to n-ZnO:Al. The samples are annealed at temperatures of 300 and 500°C for 60 s in a flowing N₂ atmosphere. Current-voltage measurements show that the as-deposited sample is ohmic with a specific contact resistance of 2 × 10^-2 Ω cm². However, annealing of the sample at 300°C results in much better ohmic behavior with a contact resistance of 2 × 10^-4 Ω cm². Further increase in annealing temperature (500°C) causes the degradation of the ohmic property. Glancing angle X-ray diffraction and Auger electron spectroscopy are used to investigate interfacial reactions between the Ti/Au and ZnO layers. It is shown that both rutile and srilankite TiO₂ phases are formed in the as-deposited and annealed samples. It is further shown that annealing at 500°C results in the formation of new phases such as Ti₃Au and TiAu₂. A possible explanation is given to describe the annealing temperature dependence of the specific contact resistance.