Low-Threshold pentacene OTFT by using NdTaON gate dielectric and ITO gate electrode

Ta-doped Nd oxynitrides (Nd_xTa_(1-x)ON) are adopted as gate dielectrics in pentacene organic thin-film transistors on ITO glass for transparent applications. The device with Nd_0.76Ta_0.24ON can achieve the optimal performance with a high carrier mobility of 2.32 cm²/V·s and a small threshold voltage of −0.21 V. The capacitance–voltage results indicate that besides producing a high permittivity (high-k) for the gate dielectric, Ta incorporation in NdON can generate negative fixed charges in the oxide to assist the external gate voltage to produce a small threshold voltage. The threshold voltage is further reduced by the ITO gate electrode because ITO with higher work function can decrease the energy barrier for the charge-carrier injection at the source/channel interface. XPS and AFM reveal that the moisture resistance of pure Nd oxynitride can be enhanced by Ta doping to improve the smoothness of the dielectric surface, which results in weaker surface-roughness and grain-boundary scatterings, and consequently higher carrier mobility. The carrier mobility is further enhanced by the ITO gate electrode because similar to metal-gate/high-k stack, metallic ITO gate with higher electron concentration than Si gate can offer stronger screening effect on the carrier scattering induced by the severe atomic oscillation in the high-k NdTaON gate dielectric.