Positronium Formation At Si Surfaces

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 greater than or similar to 10(18) cm(-3)), the positronium fraction (I-Ps) increased above 700 K and reached more than 95% at 1000 K. In undoped and lightly doped Si (n, p greater than or similar to 10(15) cm(-3)), IPs decreased from 300 to 500 K and increased above 700 K. In heavily doped p-type Si (p greater than or similar to 10(18) cm(-3)), IPs 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.