Effect of thermal treatment in oxygen, nitrogen, and air atmospheres on the electrical transport properties of zinc oxide thin films

The effects of thermal treatment in oxygen, air, and nitrogen gas atmospheres, at temperatures ranging from 573 to 1173 K, on the electrical transport properties of thin films of zinc oxide, prepared by sputtering deposition, have been investigated. These experiments have been carried out in preparation for future ion implantation doping studies in zinc oxide. As-prepared samples were slightly oxygen-deficient (Zn: 51%, O: 49%), due to a relatively low concentration of oxygen in the sputtering gas, and exhibited n-type conductivity (sheet resistance ∼3 × 10 2 Ω/square, sheet carrier concentration ∼8 × 10 14 cm − 2 , carrier concentration 4 × 10 19 cm − 3 , and mobility ∼30 cm 2/V s). Heat treatment in oxygen atmosphere led to a decrease in carrier concentration with increasing temperature. Upon annealing at 1173 K the carrier concentration was ∼1 × 10 17 cm − 3 and the mobility was ∼7 cm 2/V s, while heat treatment in nitrogen atmosphere, at the same temperature, resulted in almost no change in carrier concentration but with significant decrease in mobility to ∼1 cm 2/V s. Heat treatment in air led to values of carrier concentration and mobility, intermediate between those observed upon annealing in oxygen or nitrogen gases. These results suggest that the electrical transport properties of zinc oxide thin films are extremely dependent upon deposition conditions and post-deposition treatments and these effects should be carefully considered in any doping attempt by ion implantation.