H. Sakaue

H. Tawara

K. Hosaka

K. Tsumori

N. Matsunami

National Institute for Fusion Science (NIFS-Repository)

§13. Secondary Positive Ion Emission from SrCe03 (5 % Yb) Thin Film on Si Matsunami, N. (Dept. Energy Eng. and Sci., Nagoya Univ.), Hosaka, K., Sakaue, H., Tsumori, K., Tawara, H. We have measured the secondary positive ion emission (SPIE) yield y+ from an epitaxial thin film(-100 nm) of 5% Yb doped s~ (SeO) on Si by ion impact. The current due to the positive ions was measured by using a cylindrical Faraday cup with a shield surrounding the cup. The Faraday cup was negatively biased, and the sample and the shield were grounded Thin film on an electrical conductive substrate was employed to minimize the charge accumulation effect. As reported previously, a strong beam current dependence of the secondary electron emission (SEE), i.e., a strong suppression of SEE with increasing the beam current, was observed for thick polycrystalline seo (IrSCQ). We consider that this dependence is partly due to the surface potential modification by the charge accumulation effect. As shown in Fig. 1, it appears that y+ is nearly constant over a wide range of the ion beam current (IB) and saturates at bias of -90 V. It is also found that y- is insensitive to surface condition (clean or unclean surface). A quite similar behavior was observed for Ir impact. The above results of thin SeQ film on Si and of p-SeQ are given in Table 1. Values ofY of thin SeQ film is somewhat smaller than those of p-SeQ. Also included in the Table are the calculated backscattered yield Y B, sputtering yield Y s and corrections Y c associated with the Faraday cup method. The correction means that backscattered ions and sputtered atoms hit the Faraday cup and generates the SEE, resulting in positive currents. Difference between the present Y c and those in ref [1] is due to different choice of Y B, Y s and etc. True SPIE yield, y +, is given by y + = y+ -Y c and the results are given in Table 1. For 2.5 keY W impact, y + divided by the sputtering yield is larger than unity. This indicates that the correction is not adequate, because the sputtering of sea is well described by the elastic collision process for low energy ion impacts, on which the calculation of Y s is based, and hence y + N s should not exceed unity. For Ar+ impact, y +Ns varies from 0.003 to 0.1, and this implies that y + does not scale with the elastic collision and there may be a contribution of electronic excitation effects. To clarify this contnbution, improvement of the correction accuracy is inevitable. Development of other methods to evaluate 'Y + is also under investigation. We would like to thank Prof M. Ishigame and Dr. N. Sata for supplying SeQ films and for their helpful discussions. 20 keV At ->SCO/Si tJ!/iHt 4f. -t646( tcQ-Q",~ 4- ~ -135V >- 0.1 ~ 00 oQ)~oeae + -90V o -22.5V ............ -9V ...... • • • • 10-11 10-10 10-9 10-8 10-7 10-8 CURRENT (A) Fig.l Dependence of secondary positive ion emission yield y- per ion on the beam current for 20 ke V Ar+ impact on SCQ/Si at various bias indicated in the Figure. Reference [1] K. Hosaka, N. Matsunami and H. Tawara, Nucl. Instrum. Meth. B149 (1999)414. Table 1 T , correction Y c with the calculated backscattered yield Y B and sputtering yield Y s for thin sea film on Si. T in the parentheses is the result for thick polycrystalline sea. y+ = T-Yc. Se and Sn are the electronic and nuclear stopping powers in e V /nm, respectively. E(keV) Ion y+ (unclean) YB Ys Yc y+/Ys Se Sn y+ (clean) film (poly) film 2.5 W 0.11 0.18 0.0086 0.065 5.2 47.5 2.08 0.11 2.5 Ar+ 0.04 (0.063) 0.066 1.3 0.036 0.003 76.1 464 0.035 100 Ar+ 0.45 (0.65) 0.025 1.6 0.38 0.04 481 619 150 Ar+ 0.44 (0.5) 0.021 1.35 0.26 0.1 590 561 291 

Secondary Positive Ion Emission from SrCeO3 (5% Yb) Thin Film on Si

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