PMMA-Ta2O5 bilayer gate dielectric for low operating voltage organic FETs

This paper reports on a study on pentacene field effect transistors (OFETs) with a gate dielectric made of a bilayer PMMA/Ta2O5 where PMMA (poly(methyl methacrylate)) is spin-coated on top of an evaporated layer of Ta2O5. A comparison with devices with only Ta2O5 is presented. These latter exhibit very low operating voltage associated to the high dielectric constant of this oxide but also show some surface trapping and gate leakage. These two drawbacks can be overcome by depositing a PMMA layer on Ta2O5. With such a bilayer gate dielectric, gate leakage current is considerably reduced and the quality of the interface between pentacene and PMMA was much improved compared to that with Ta2O5 as evidenced from the much higher output drain current. The influence of PMMA thickness in the range 15-250 nm is presented. OFETs with field effect mobility, on/off current ratio, and sub-threshold slope of respectively 0.4 cm2 V−1 s−1, 3 × 105 and 1.2 V/decade were obtained.