Charge-transfer doping by fullerenes on oxidized Si surfaces

We have studied the charge transfer between various fullerenes (C60, C78, C84, C60F36, and C60F48) and oxidized Si surfaces in order to evaluate their capability of charge-transfer doping to form ultrashallow junctions in Si. Although the lowest unoccupied molecular orbitals of these fullerene molecules in isolated states are higher in energy than the valence band maximum (VBM) of Si, the fullerenes C60F36 and C60F48, which were deposited on oxidized Si substrates, captured electrons from the Si surfaces; this generated hole inversion layers. C60, C78, and C84 did not induce such electron transfer. From the densities of the generated holes, we estimated the acceptor levels of C60F36 and C60F48, which were condensed on the oxidized Si surface, to be 5.6–5.7 and 5.2eV below the vacuum level, respectively; these values are lower than the VBM of Si.