ZnO thin films have been grown on Si(100) substrate by plasma enhanced chemical vapor deposition using a zinc organic source [Zn(C2H5)(2)] and carbon dioxide (CO2) gas mixture at 503 K. The dependence of ZnO thin film quality on the gas flow rate ratio of Zn(C2H5)(2) to CO2 (GFRRZC) is studied by x-ray diffraction (XRD), optical absorption (OA) spectra, and photoluminescence (PL) spectra. An excitonic absorption peak is observed in the OA spectra, which closely depends on the GFRRZCs. The NRD spectra show that a c-axis-orientated wurtzite structure ZnO thin film with the full width at half maximum (FWHM) of 0.24degrees has been prepared. The PL spectra show a strong UV emission with a narrow FWHM of 105 meV at 3.289 eV with a weak deep-level defect emission around 2.5 eV, implying the formation of the stoichiometric ZnO thin films. The origin of the UV band is from the free exciton recombination testified by the temperature dependent PL spectra.