Charge Trapping States at the SiO₂–Oligothiophene Monolayer Interface in Field Effect Transistors Studied by Kelvin Probe Force Microscopy

Using Kelvin probe force microscopy (KPFM) we studied the local charge trapping states at the SiO2-oligothiophene interface in a field effect transistor (FET), where SiO2 is the gate dielectric. KPFM reveals surface potential inhomogeneities within the oligothiophene monolayer, which correlate with its structure. A large peak of trap states with energies in the oligothiophene's band gap due to hydroxyl groups is present at the oxide surface. We show that these states are successfully eliminated by preadsorption of a layer of (3-aminopropyl)triethoxysilane (APTES). Time-resolved surface potential transient measurements further show that the charge carrier injection in the nonpassivated FET contains two exponential transients, due to the charge trapping on the oxide surface and in the bulk oxide, while the APTES-passivated FET has only a single-exponential transient due to the bulk oxide. The results demonstrate that APTES is a good SiO2 surface passivation layer to reduce trap states while maintaining a hydrophilic surface, pointing out the importance of dielectric surface passivation to bridge the gap between soft materials and electronic devices.