Maxwell-Wagner limit of copper contacts on DNTT transistors

The contact resistance of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) thin-film transistors with copper source/drain electrodes was extracted by the transmission-line method between 200 and 400 K on two gate-dielectric treatments (OTS and HMDS). Resolved interval by interval and at constant gate overdrive, the activation energy of the contact resistance vanishes below 260 K and above 300 K, and all of the activation is concentrated in the crossover between these two plateaus, which yields an injection barrier of 0.63 eV. The contact is therefore limited by a succession of mechanisms rather than by one: tunnelling at low temperature, where the available fields lie one to two orders of magnitude below the Fowler-Nordheim scale so that the barrier remains trapezoidal; thermionic emission through the crossover; and, once injection ceases to limit, a temperature-independent floor of 3-6 kΩ·cm, voltage independent once current crowding is removed, consistent with the Maxwell–Wagner space-charge limit for a DNTT conductivity of about 10⁻⁷ S/cm. The conventional plot of activation energy against the square root of field is linear and would alone indicate Schottky barrier lowering, but it fails the thermal-scaling test that barrier lowering equally imposes; current crowding, which accounts for the high-temperature field exponent, cannot generate the crossover. Both surface treatments give the same physics and imply the same conductivity to within a factor of two, indicating a floor set by the dielectric relaxation of the DNTT itself rather than by the metal–organic energetics.