Investigation of a Reliable Ohmic Contact to n-Type ZnO Thin Films Prepared by Sol–Gel Method

In this letter, we investigated Ti (20 nm)/Al (30 nm)/Au (60 nm) metallic contacts to Al-doped ZnO thin films prepared by the sol-gel method. In order to measure specific contact resistance, circular transfer length method structures were patterned on the samples by a lithography process. The samples were annealed at the different temperatures and electrical characterization (I-V) was performed. The annealing process was optimized to produce ohmic contacts with the lowest specific contact resistance. The results showed that the sample annealed at 350°C had the lowest specific contact resistance of ~3.7 × 10-7Ω · cm2. Surface and cross section energy dispersive X-ray analysis was used to determine the distribution of different elements during the annealing process. The results indicated effective inter-diffusion of Al and Ti as well as outdiffusion of oxygen, resulting in an increase in the n-type carrier concentration close to the surface of the ZnO. Scanning electron microscopy and atomic force microscopy analysis were used to determine the effect of annealing on the surface morphology.