Mayuko Nishiwaki

Negative carbon (C−) and oxygen (O−) ions are expected to find wide applications for ion implantation processes. However, systematic experimental data required for their efficient generation and the elucidation of their underlying atomic processes are lacking. In this study, we systematically measured the angular and incident beam energy dependencies of the reflected carbon and oxygen ion properties from Mo, W, and low-work function material 12CaO⋅7Al2O3 electride targets, for low-energy (<1 keV) carbon (C+) and oxygen (O+) ion injections. For all targets, both C+ and C− ions were reflected for the C+ ion injection, whereas only the reflected O− ions were observed for O+ ion injection. The energies of C+, C−, and O− ions reflected from the electride and Mo decreased significantly with increasing reflection angle. The effect of the lower work function was an increase in the intensity of the O− ions reflected at larger-incident and smaller-reflection angles by comparing the reflected ions from Mo and electride targets. On the other hand, the effect of the work function on the production of C− ions was small. When 1 keV-O+ ions were incident on W, the fraction of the reflected O− ions was 10%–30%, and the rest were neutral oxygen particles. Good agreement between the measured and calculated incident angular dependency of reflected O neutral particles was observed.

To efficiently generate H− ions from positive atomic or molecular hydrogen beams injected onto a solid surface, it has been suggested to use a material with a low-work function as the target material. However, it is not clear under what conditions the most efficient H− production is realized for incident beam parameters or reflection angles. Therefore, we studied the interaction between low-energy atomic and molecular hydrogen beams (less than 1 keV/nucleon) with a low-work function electride 12CaO⋅7Al2O3 (C12A7). The production ratio of H− to H+ ions from the C12A7 electride was much higher than Mo targets with higher work functions, especially at smaller incident and smaller reflection angles. The H− to H+ production ratio slightly increased as the incident energies were decreased, but there was no significant difference between the electride and Mo targets. These results indicate that smaller incident angles and lower beam energies of the incident hydrogen beam are favorable for the enhancement of the production ratio of H− to H+ ions in C12A7. The higher production ratio appeared at the vertical beam energies less than on the order of 100 eV, where quantum mechanical processes may become important.