Ti ∕ Au Ohmic Contacts to Al-Doped n-ZnO Grown by Pulsed Laser Deposition

The effect of annealing on the stability of Ti/Au ohmic contacts on heavily Al-doped (n ≈ 10 19 cm -3 ) n-ZnO is reported. Due to the high n-type doping levels in the ZnO, the as-deposited contacts are ohmic with an excellent specific contact resistivity of 2.4 × 10 -7 Ω cm 2 . Subsequent annealing produces a minimum value of 6 × 10 -8 Ω cm 2 after processing at 300°C. The absence of any temperature dependence to the contact resistance indicates that tunneling is the dominant current transport mechanism. Annealing the contact structure at 250°C produces Zn outdiffusion through the Ti/Au, intermixing of Au and Ti, and roughening of the contact surface morphology. The ease of ohmic contact formation to the Al-doped ZnO and extremely low contact resistance achievable even with low-temperature processing makes this metallization scheme attractive in applications where high processing temperatures cannot be employed.