Preparation And Properties Of ZAO Transparent Conducting Thin Films

In this paper, the properties, preparation methods and applications of transparent conducting thin films were reviewed. ZnO/Al2O3 compound powders with 2.5wt% Al2O3 were synthesized by coprecipitation method using ZnSO4.7H2O and NH4Al(SO4)2 12H2O as the raw materials and NH4HCO3 as the precipitator. Then the compound powders were reduced by H2 gas at different temperatures. Three kinds of ZnO/Al2O3 ceramic targets were fabricated by hot isostatic pressing (HIP) method with three kinds of powders: reduced compound powder, compound powder and nanometer powders. Aluminum-doped zinc oxide (ZAO) transparent conducting thin films were prepared with ZnO/Al2O3 ceramic targets by r.f. or dc magnetron sputtering method.Phase compositions and morphologies of ZnO/Al2O3 compound powders, pioneer-precipitates of the compound powders, ZnO/Al2O3 ceramic targets and ZAO transparent conducting thin films were investigated by XRD, SEM and TEM. Thermochemistry performances of the pioneer-precipitates of the compound powders were investigated by TG/DSC methods. XPS was applied to analyze the influences of reduction process on the properties of powders. The particles size and size distribution of the compound powders were studied by laser technology. Moreover atomic absorption spectroscopy (AAS) method was used to determine the proportion of impurity elements in the pioneer-precipitate of the compound powders. The hardness Hy and fracture toughness KIC of the targets were determined through microhardness analysis. The resistivity of the targets, the sheet resistance and optical transmittance ofZAO films were measured.The investigation for the compound powders and pioneer-precipitates indicates that particle conglomerations exist in all the compound powders. And the particle size of the compound powder is 12.25 \xm in average. The powder baked at 680C is perfect in properties, and zinc oxide will decompose when the reduction temperature exceeds 700 C,Results of XRD, SEM and TEM for the targets indicate that secondary phase ZnAl2O4 exists in the targets synthesized with reduced or unreduced compound powders, but not in the target synthesized with nanometer powders. The analytical results for the mechanical and electrical properties of the targets indicate that the target synthesized with reduced compound powders has the best general properties, furthermore the relative densities and fracture toughness of all the targets exceed 97% and 1.0 MPa.m1/2 respectively, and the resistivity of the targets attains 10-3.cm. These results indicate that ZAO targets synthesized by HIP can be easily machined by electrical and mechanical machining.Investigation for electrical properties of the films indicates that substrate temperature, sputtering power and oxygen fraction have great impacts on the properties of the films. The resistivity of the films prepared by dc magnetron sputtering under the substrate temperature 330C is 9.3x10-4.cm which is the lowest resistivity of the films in this paper. The lowest resistivity of the films prepared by r.f. magnetron sputtering under the substrate temperature 330C is 1.4x10-3.cmStudy for optical transmittance of the films indicates that the average value of the optical transmittance of the films exceeds 80%. With the substratetemperature increasing the average optical transmittance increases and the absorption edge of the transmission curve of the films moves toward short wavelength.