Gated four-probe measurements on pentacene thin-film transistors: Contact resistance as a function of gate voltage and temperature
We describe gated four-probe measurements designed to measure contact resistance in pentacene-based organic thin-film transistors (OTFTs). The devices consisted of metal source and drain electrodes contacting a 300-Å-thick pentacene film thermally deposited on Al2O3 or SiO2 dielectrics with a p-doped Si substrate serving as the gate electrode. Voltage-sensing leads extending into the source-drain channel were used to monitor potentials in the pentacene film while passing current during drain voltage (VD) or gate voltage (VG) sweeps. We investigated the potential profiles as a function of contact metallurgy (Pt, Au, Ag, and Ca), substrate chemistry, VG, and temperature. The contact-corrected linear hole mobilities were as high as 1.75cm2/Vs and the film sheet resistance and specific contact resistance were as low as 600kΩ /◻ and 1.3kΩ-cm, respectively, at high gate voltages. In the temperature range of 50-200K, the pentacene OTFTs displayed an activated behavior with activation energies of 15-30meV. Importantly, the activation energy associated with the contact resistance showed no dependence on contact metal type at high gate voltage. Also, the activation energies of the contact resistance and film resistance were approximately the same. Above approximately 200K and below 50K, the mobility was essentially temperature independent.