Structural, optical and cathodoluminescence characteristics of sprayed undoped and fluorine-doped ZnO thin films

Abstract

Undoped ZnO and F-doped ZnFxO(1-x) (x = 5 at%) films prepared by the spray pyrolysis (SP) technique have been studied using cathodoluminescence imaging and spectroscopy, x-ray diffraction and spectrophotometry. These films, deposited at the optimal substrate temperature (Ts = 450°C), present a hexagonal close packed structure. The doping of this material with fluorine keeps their optical absorption threshold unchanged. At room temperature, the cathodoluminescence (CL) spectra of the undoped and the doped ZnO films exhibit the common near-ultra-violet (UV) band gap peak at λ = 382 nm but they differ as regards their visible emissions. The undoped ZnO emits an intensive blue-green light (λ = 520 nm) and a red emission (λ = 672 nm). The presence of fluorine gives rise to a new light emission corresponding to λ = 454 nm and the disappearance of the blue-green emission. The CL imaging of undoped ZnO films shows that the luminescence is located at defined sites giving rise to a grain-like structure inherent to the sample surface morphology. The presence of fluorine leads to a more homogeneous re-partition of the emitting centres.