Evolution of nanostructure, defect-free photoluminescence and enhanced photoconductivity of oxidized Zn films

Nanostructured zinc oxide thin films were prepared by the oxidation of nanostructured zinc films deposited on glass substrates by thermal evaporation of metallic zinc in an atmosphere of nitrogen. The films were oxidized at different temperatures. X-ray diffraction was used to study the structural transformations of the films with oxidation temperature. Atomic force microscopy images of surface morphology of the films revealed the kinetics of aggregation of the grains as the oxidation temperature was increased. Raman spectra of the samples contained a surface mode at ∼482 cm−1 showing characteristics of fine grain size. UV-Visible absorption spectra of the samples showed a blueshift of bandgap in comparison with that of bulk ZnO crystals. The photoluminescence emission spectra of the samples were free from defect related emissions. The increase in oxidation temperature of these samples caused a narrowing of the photoluminescent emission band in the UV region and an increase in the UV photocurrent. The ratio of maximum photocurrent to dark current was found to increase from 100 to 2000 as the oxidation temperature of the films increased from 350 to 500 showing an enhanced photoconductivity. The increased photocurrent was attributed to the interconnected grain structure of the ZnO samples oxidized at higher temperature.