2枚グリッドLEEDシステムと光電子増倍管とによるオージェ電子分光

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For the purpose of the detection of atomic species in a few atomic layers of the outermost surface of solids, the electron-bombardment excited Auger electron spectroscopy (AES) using 3-grid LEED system as a retarding field electron energy analyzer is now widely used due to the relative simplicity of the apparatus to observe LEED pattern and Auger spectrum for the same surface under “in situ” conditions. In the case of an earlier 2-grid LEED system, however, a capacitance neutralizer should be used to avoid capacitance coupling and to improve resolution and S/N ratio. The another possibility to eliminate the capacitance coupling between a retarding grid and a fluorescent screen as an electron collector, would be the use of photons emitted from the fluorescent screen. In the case of the multi-grid LEED-AES system, however, the use of a photoelectron multiplier as a detector of Auger electron peaks contained in the secondary electrons from the solid surface, has not yet been reported. Only few investigators suggested the use of the photomultiplier to obtain the energy distribution N (E) of secondary electrons from the solid surface by means of the 3-grid LEED system. It is the purpose of this work to show the capability of photoelectron multiplier combined with the 2-grid LEED system as a retarding field analyzer for detecting Auger electrons. A Fe (100) single crystal ribbon was mounted, as a sample, in the diffraction chamber of the 2-cylindrical grid LEED system (Japan Vacuum Eng. Co., EDL-3) and all measurements were done in the UHV region. The sample surface was bombarded by the primary electron beam (600 eV, 0.7 μA) generated from the LEED gun and the incident angle was approximately 70° from the surface normal. The lower electron current was preferable to decrease the perturbation effects against adsorbing or adsorbed molecules. The energy distribution of secondary electrons emitted from the surface was measured by modulating the retarding grid and scanning its dc potential over the required energy range. The modulation frequency was 20 Hz with an amplitude of about 14 Vp-p, and the time contant used was 3 sec with a scan speed of 10 V/min. The emitted light from the central part of the fluorescent screen held at 3 kV, was then detected by the photomultiplier (Hamamatsu TV Co., R-430, 11 stages, gain of 106), and the ac component of the output was amplified and differentiated by the PAR Model-121 lock-in amplifier to obtain the first and second derivative spectra. The Auger spectrum, in the second derivative form, obtained by the present method for the Fe (100) surface having p (1×1) LEED pattern after ion-bombardment-anneal cleaning, showed only Fe Auger peaks and no traces of other impurities have been detected. The c (2×2) surface structure appeared after heating the clean Fe (100) surface was determined to be due to impurity sulphur and due not to carbon. The minimum detectable sensitivity of about 0.5 percent of a monolayer of the stable adatoms, for example sulphur atoms in the Fe (100) c (2×2) -S structure, was obtained by the present method. When the Fe (100) surface was covered by the weakly bound admolecules such as H2S or CO in the disordered state, the reduced sensitivity of about 5 to 10 percent of a monolayer was obtained because of the electron-bombardment induced desorption. Although the resolution was relatively poor due to the field penetration from the high voltage screen to the retarding grid, the very high sensitivity of this method using low primary electron current has been demonstrated.
日本真空協会の論文

2枚グリッドLEEDシステムと光電子増倍管とによるオージェ電子分光

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