Published online by Cambridge University Press: 26 February 2011
We constructed an ultrahigh vacuum (UHV) system with which in-situ conductivity and Seebeck coefficient measurements can be performed, including the detailed examination of the oxygen effect. It can also carry out the deposition of metal electrodes and organic films under UHV condition through measurements. The conductive characteristics of fullerene (C60) films were examined by the measurement at conductivity and Seebeck coefficient under UHV. We observed that the conductivity at room temperature was higher than 1×10−3 S cm−1. This value is two orders of magnitude in conductivity higher than the Hamed report. [A. Hamed et al., Phys. Rev. B 47, 10873 (1993).] Seebeck coefficient S was also measured. Its sign was negative, indicating electron conduction for C60 film. After the measurements under UHV, oxygen was introduced into the chamber by a variable leak valve. The conductivity decreased drastically by increasing the pressure of oxygen. It decreased more than five orders from under UHV to 1 atom oxygen. The temperature dependence of the conductivity was also examined. The conductivity was thermally activated. Activation energies of the conductivity increase by increase oxygen pressure. This means that the oxygen act as carrier traps. As an n-type dopant, acridine orange base (AOB) was co-deposited into C60 films. The magnitude in the conductivity increased but not drastic. The conductivity of doped film also decreased by increasing the pressure of oxygen. The conductivity even under 6.6×10−2 Pa, however, was high enough. Though AOB was not effective dopant against C60 film in UHV condition, AOB doped C60 film became resistant to oxygen.
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