Journal Article Positronium formation at Si surfaces

河裾, 厚男  ,  前川, 雅樹  ,  宮下, 敦巳  ,  和田, 健  ,  海和, 俊亮  ,  長嶋, 泰之

97pp.245303-1 - 245303-8 , 2018-06 , American Physical Society
Positronium formation at Si(111) and Si(001) surfaces has been investigated by changing the doping level systematically over the range 300–1000 K. The temperature dependence of the positronium fraction varied with the doping condition, and there were practically no differences between the two surface orientations. In heavily doped n-type Si (n>1E+18 cm−3), the positronium fraction (I_Ps) increased above 700Kand reached more than 95% at 1000 K. In undoped and lightly doped Si (n, p <1E15 cm−3), I_Ps decreased from 300 to 500 K and increased above 700 K. In heavily doped p-type Si (p >1E18 cm−3), I_Ps increased in two steps: one at 500–600 K and one above 700 K. Overall, the positronium fraction increased with the amount of n-type doping. These phenomena were found to be dominated by two kinds of positronium with energies of 0.6–1.5 eV and 0.1–0.2 eV, which were attributed to the work-function mechanism and the surface-positron-mediated process, respectively, with contributions from conduction electrons. The positron work function was estimated to be positive. This agrees with first-principles calculation. The positive positron work function implies that the formation of excitonic electron-positron bound states begins in the bulk subsurface region and transits to the final positronium state in the vacuum.

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