Structural, Morphological and Optical Characteristics of Low Temperature Oxidized Metallic Zinc Films

This work presents the effect of low temperature- below Zn melting point oxidation of Zn film on the optical, morphological and structural properties of thermally oxidized Zn films. Zinc oxide (ZnO) thin films were synthesised from thermal vacuum deposited metallic zinc thin films by low temperature thermal oxidation of the Zn films between 200ºC and 400ºC. The ZnO films exhibited preferred (011) orientation wurtzite structure, with presence of Zn peaks that diminishes with increase in the oxidation temperature. The films show modification in morphology exhibiting different nanostructures at different Zn film oxidation temperature. The stoichiometric ratio between Zn and O atoms of the ZnO films synthesized at oxidation temperature of 400ºC show much closer value to the ideal 1:1 ratio giving ZnO0.94 while at 200 and 300ºC oxidation temperature the deviation from ideal was more with stoichiometry of ZnO0.74. The films had optical transmittance of 64-71% in the visible region with the transmission edge becoming sharper and the optical band gap improving from 2.90 to 3.30 eV with increase in oxidation temperature. The lattice parameter c/a ratio was ≈1.732 for all the samples, showing a deviation of 0.099 from the ideal 1.633 of wurtzite structure belonging to the space group P63mc by 0.099. The Zn-O bond length were ≈1.834 Å for each samples, which is also less than ideal 1.993 Å along c-axis an indication of the planes being more closely packed than the ideal. The refractive index dispersion of the ZnO films below the interband absorption edge were analyzed based on the single-oscillator model.