Field-effect studies of C60 thin films before and after implantation with potassium

We report measurements of the field-effect mobility in thin films of the fullerene C60, before and after implantation with potassium ions at 30 keV. Potassium ion implantation is known to produce reductive doping in C60 at low implantation doses, and was used to raise the bulk conductivity of the C60 layer. Devices were fabricated on doped silicon wafers, using thermally grown oxide to provide the insulator layer, and gold source and drain contacts. C60 was deposited by vacuum sublimation. Field-effect mobilities prior to implantation of about 7 × 10−7 cm2 V−1 s−1 were obtained, and the lowest implantation dose (1010 ions cm−2), this was raised by about 50%. We attribute this improved mobility to the presence of extra bulk charge carriers. At higher implantation levels, however, the mobility decreased, falling to 1.4 × 10−7 cm2 V−1 s−1 at a dose of 2 × 1011 ions cm−2. We propose that this is due to an increase in disorder in the C60 lattice caused by the implantation process.