纳米ZnO薄膜的激子光致发光特性

Excitonic Luminescence Properties of Nanocrystalline ZnO Films

报道了纳米ZnO薄膜激子光致发光(PL)与温度的关系。首先利用低压 金属有机化学气相沉积(LP MOCVD)技术生长ZnS薄膜,然后将ZnS薄膜在氧气中于800℃下热氧化2h获得纳米ZnO薄膜。X射线衍射(XRD)结果表明,纳米ZnO薄膜具有六角纤锌矿多晶结构且具有择优(002)取向。室温下观察到一束强的紫外(3 26eV)光致发光(PL)和很弱的深能级(DL)发射。根据激子峰的半高宽(FWHM)与温度的关系,确定了激子 纵向光学声子(LO)的耦合强度(ГLO)。

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 ГLO is effectively reduced.