Free and Trapped Injected Carriers in C60 Crystals

We report on the conductance from two-contact carrier injection in C60 single crystals. In the nonlinear regime, the current and voltage obey a power law, I \~ V^m, where m can be as high as 10 at room temperature. This nonlinear behavior - the resistance decreases by 6 orders of magnitude without saturation - is among the highest reported for organic systems, and can be explained by injection of free carriers into the trap-filling region. We find that H2 annealing suppresses shallow traps and enhances nonlinearity. Two limiting types of temperature dependence of the nonlinear resistance are observed - decreasing and increasing resistance at the orientational ordering temperature. A simple model incorporating deep traps is presented to understand this behavior and the impact of this model on possible field-effect transistor action is discussed.