Cathodoluminescence dependence upon irradiation time

The cathodoluminescence (CL) intensity varies with beam exposure time. In this work, the CL change as a function of irradiation time has been studied using various semiconducting materials: CdS single crystal, CdS evaporated thin films, ZnO ceramics and GaAs single crystal. A current density as low as 60 mu m/cm(2) was used in an electron microprobe analyser. In the case of low excitation level, two stages of the CL variation have been generally observed, i.e. increasing and decreasing parts. In the case of a relatively high excitation, only a decreasing stage can be observed. It is believed that the CL time dependence is closely related to the adsorption-desorption process and the surface contamination which are stimulated by the electron beam excitation.