Development of Low Energy Cathodoluminescence System and its Application to the Study of ZnO Powders
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efforts have been done for this purpose such as using thin films [1], operating with lower voltages and applying magnetic field [2]. The first two methods are to reduce the volume of electron-hole pair generation, while the last one to suppress the carrier diffusion. Thin film operation and application of high 10 magnetic field are principally related to CL using transmission electron Si 2 • microscope (TEM). These methods are -Diamon 1 a generally troublesome for specimen 1 i 4preparation or often result in the operation at higher voltages. Operation a 2 GaAs ZnO 0 0.1 at low electron beam voltage is much 0ZnO .2 4 desirable for CL using scanning electron 2 W microscope (SEM). Electron range Re, which is the 0.01 2 4 68 2 1 10 measure of the diameter of electronhole pair generation volume, is given by Electron Energy (keV) [3],
Fig. 1. Dependence of electron range on electron beam energy 75
Mat. Res. Soc. Symp. Proc. Vol. 588 © 2000 Materials Research Society
& = (0.0276A /p Z085 9) Eb'
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(1), where Eb is electron beam voltage in keV, A is the atomic weight in g/mol, p is material density in g/cm 3 . Fig. I shows the variation of electron range with the electron beam energy in typical semiconductors. When we observe ZnO at 30 keV, the electron range is about 4.6 gim. If we reduce the beam energy to 3 keV, the electron range becomes 10 nm, which is enough small to observe nano-structure. Recently, the development of field emission source made it possible to supply enough current at lower electron beam energy. In this paper, we demonstrate our thermal field emission (TFE)-SEM CL system and its application to the study of ZnO nanoparticles.
Fig. 2. Block-diagram of TFE-CL system.
FIELD EMISSION CL SYSTEM A CL system was developed for the operation at low electron beam energies. The block diagram of our system is shown in Fig, 2. A scanning electron microscopy with a thermal field emission gun (TFE-SEM; Hitachi S4200) was used for this purpose. This SEM can supply a current more than I nA under the 1 keV operation, which is enough for most CL experiments. Optical system was designed to realize not only high collection efficiency of luminescence but also uniform collection efficiency,
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Fig.3 Photograph of ellipsoidal mirror.
although such an idea was already introduced in the former CL system for quantitative analysis [4]. Fig. 3 shows the photograph of ellipsoidal mirror. The mirror was located under the objective lens and supported by the arm, in which a bundle of optical fiber was placed. The collected light was guided by this fiber to the monochromator (Jovin-Yvon; Triax-320), which has three gratings for variable resolution and wavelength range. Chargecoupled device (CCD, Jovin-Yvon; Spectrum One) was used for spectrum acquisition. Since CCD can realize the parallel detection of CL spectra, it has markedly reduced the acquisition time. The advantage of the CCD for spectrum acquisition was demonstrated in Ref [5,6]. For CL imaging, on the other hand, photon-counting system was adopted to reduce the b
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