ZnO has a large fundamental band gap of 3.37eV

which makes it a promising material for use in ultraviolet light-emitting devices and laser diodes. Apart from higher chemical and thermal stability

ZnO has the advantage of a large exciton binding energy(60meV)

which assures more efficient excitonic emission at higher temperature. Up to now

the visible emission and ultraviolet lasing emission of ZnO have been the subject of much research. The properties of the excitonic luminescence for nanocrystalline ZnO thin films were investigated about the dependence of excitonic photoluminescence(PL) spectra on temperature. High quality nanocrystalline ZnO thin films were prepared by thermal oxidation of ZnS films prepared by low pressure metalorganic chemical vapor deposition(LP MOCVD) technique. The X-ray diffraction(XRD) indicates that the ZnO thin films have a polycrystalline hexagonal wurtzite structure with a preferred (002) orientation when ZnS thin films were oxidized at annealing temperature of 800℃ in an oxygen ambient and the average grain size for all films annealed at 800℃ is about 33nm. The properties of excitonic PL spectra for nanocrystalline ZnO thin films were investigated in the temperature range from 82K to 300K. The photoluminescence(PL) spectra of the ZnO thin films showed that the strong ultraviolet(UV) emission peak at 380nm

while the deep level emission band is barely observable at room temperature. The UV emission is assigned to free excitons and DL emission band is attributed to excitons bound to neutral acceptors. The strength of the exciton longitudinal optical(LO) phonon coupling is deduced from the temperature dependence of the FWHM of the fundamental excitonic peak

the reduce of the exciton longitudinal optical(LO) phonon coupling strength is due to the quantum confinement effect. Because of the quantum confinement effect for nanocrystalline ZnO thin films

the energy separation between 1s and the first excited state(2s) becomes large

and the dissociation efficiency of 1s exciton into the first excited state(2s) or other excited states of the continuum states is largely suppressed. The transition from the ground state(1s) to other excited states including 2s state is reduced and ГLO is effectively reduced.