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

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 Al2O3 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)/Al2O3 interface, which is identified as a reduced hysteresis in the C-V curve and a decreased interface state density (D-it) from similar to 3.9 x 10(12) eV(-1) cm(-2) to similar to 3.3 x 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. (C) 2007 Elsevier B.V. All rights reserved.