MATERlAM SCIEMCE & ENGINEERIWG ELSEVIER B Materials Science and Engineering B42 (1996) 289-292 Cathodoluminescence dependence upon irradiation time S. A&our*, A. Harabi, N. Tabetl Ceramics Laboratory, Reseasch Unit of Materials Physics and Applications, University of Constantine, 25000 Constantine, Algeria Abstract 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 ym/cm’ 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-desorptionprocess and the surface contamination which are stimulated by the electron beam excitation. Keywords: Cathodoluminescence; CdS single crystal; Surface contamination 1. Introduction The cathodoluminescence (CL) kinetics of semicon- ducting materials, when investigated at 300 K, have more or lessa fast initial period of increase of the l!ght emission, followed by a relatively slow decreasing stage. This variation is usually interpreted as a result of a stimulated interaction between the irradiated surface and the residual vacuum, as well as different induced processeswithin the sample. This is an embarrassing problem during CL intensity measurement. Therefore, an understanding of the CL intensity change with the time is necessary to obtain a correct information about the CL mechanisms. The present paper deals with the CL intensity change as a function of time, taking into account the accelerating voltage, the beam diameter, the sample nature and the surface treatment. 2. Experimental Various semiconducting materials namely, CdS single crystals, CdS evaporated films, ZnO ceramics and GaAs single crystal were used. The above materials were studied in various forms: as prepared, cleaved, vacuum annealed, mechanically polished and chemi- * Corresponding author. ’ Present address: KFUPM, Physics Department, Dhahran, Saudi Arabia. 0921-5107/96/$15.00 0 1996 - Elsevier Science S.A. All rights reserved PII SO921-5107(96)01723-O tally polished. A current density as low as 60 PA/cm2 was used in a microprobe analyser. The CL detection system which has been used (type S20R) covers the wavelength range between 0.2 and 0.9 pm. The acceler- ating voltage was taken between 10 and 35 keV. Both the edge and the defect CL emission were recorded as a function of the irradiation time at room temperature. 3. Results We have investigated in a systematic way the effect of the observation conditions (energy and size of the elec- tron beam) as well as that of the surface preparation mode on the CL kinetics in different materials. 3.1. Effect of the size and electron beam energy Figs. 1 and 2 display the variation of the CL intensity versus time which has been obtained in CdS single crystals for different beam sizes and two beam energies (10 and 35 keV). Two stages can be distinguished. In the very first stage, the CL intensity increases,reaches a maximum value and then, rapidly decreases during the second stage. It can be noticed that the maximum is reached faster as the beam diameter decreases, i.e. as the current density increases. In addition, for a given electron beam size, d = 40 pm for instance, the maxi- mum CL value is reached much faster at low beam energy (after 1 min for 10 keV, and 3 min for 3.5keV).