Transport studies in C-60 and C-60/C-70 thin films using metal-insulator-semiconductor field-effect transistors

We present a study of transport in C-60 and C-60/C-70 (98% C-60) thin films using metal-insulator-semiconductor field-effect transistors (MISFETs). The devices are used to measure the field-effect mobility for charges at a fullerene/insulator interface between 300 and 473 K. We infer from our results that C-60 operates as an n-type semiconductor. We observe that, on exposure to oxygen, the conductivity falls by up to five orders of magnitude but that this process is reversible by annealing the devices in vacuum at 473 K for at least 10 h. We also show that it is possible to obtain working devices in air at elevated temperatures. The highest value measured here for the field-effect mobility, at room temperature, is 2 X 10(-3) cm(2) V-1 s(-1). However, even for films of nominally the same composition, the measured values varied widely. This indicates that sample preparation plays an important role in device performance. All devices remained working at temperatures in excess of 473 K, demonstrating the good thermal stability of the C-60 films. We observed a change in the conductivity and field-effect mobility in one device around room temperature. We discuss this anomaly with reference to the structural phase transition of C-60 single crystals at 260 K.