ZnO thin films with different thicknesses were grown using pulsed laser deposition by varying the deposition time. The PL spectra of these films showed that the green emission decreases with the film thickness. The deposited single film was thicker at the center than at the edge and the PL spectra from different positions from the center to the edge showed the same behavior. X-ray diffraction analysis show that the thicker film with less green emission has better crystallinity. The film's composition, analyzed using energy dispersive X-ray spectroscopy, revealed that the oxygen-to-zinc ratio was larger in the sample with stronger green emission. This result does not favor the previous suggestions that oxygen vacancies or zinc interstitials are the origins of the green emission. Based on the film thickness dependence, we suggest that the green emission is related with the strain due to lattice mismatch between the film and the substrate.
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