Ultrapure C60 field-effect transistors and the effects of oxygen exposure

We report on electrical measurements of ${\mathrm{C}}_{60}$ -based field-effect transistors (FETs) that were fabricated and characterized in an ultrahigh vacuum, and on how their properties are affected by progressive exposure to impurity gases. The in situ experiments demonstrated that oxygen-free devices have unipolar $n$ -type characteristics with an electron field-effect mobility of up to $0.08{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ immediately after fabrication, and up to $0.5{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ after an annealing treatment in a high vacuum. Upon oxygen exposure, the effective electron mobility dramatically decreases in a way that depends on the diffusion time of oxygen into the ${\mathrm{C}}_{60}$ thin film. It is shown that contact with oxygen can lead to ${\mathrm{C}}_{60}$ -FETs with ambipolar characteristics. The real-time measurement of the degradation of the devices subjected to oxygen allows us to derive the diffusion rate for oxygen molecules in ${\mathrm{C}}_{60}$ thin films, yielding a diffusion constant $D=4\times {10}^{-12}{\mathrm{cm}}^2∕\mathrm{s}$ .