Interface states in polyfluorene-based metal–insulator–semiconductor devices

Abstract

Hybrid metal–insulator–semiconductor structures based on ethyl-hexyl substituted polyfluorene (PF2/6) as the active polymer semiconductor were fabricated on a highly doped p-Si substrate with Al 2 O 3 as the insulating oxide layer. We present detailed frequency-dependent capacitance–voltage ( C – V ) and conductance–voltage characteristics of the semiconductor/insulator interface. PF2/6 undergoes a transition to an ordered crystalline phase upon thermal cycling from its nematic-liquid crystalline phase, confirmed by our atomic force microscope images. Thermal cycling of the PF2/6 films significantly improves the quality of the (PF2/6)/Al 2 O 3 interface, which is identified as a reduced hysteresis in the C – V curve and a decreased interface state density ( D it ) from ∼3.9 × 10 12 eV −1 cm −2 to ∼3.3 × 10 11 eV −1 cm −2 at the flat-band voltage. Interface states give rise to energy levels that are confined to the polymer/insulator interface. A conductance loss peak, observed due to the capture and emission of carriers by the interface states, fits very well with a single time constant model from which the D it values are inferred.