Field-Induced Charges in Polyacetylene MIS and MISFET Devices

Abstract

Conjugated polymers which can be processed to form thin, coherent films can be used as the active layers in semiconductor device structures. We have used the Durham precursor route to polyacetylene to fabricate MIS diodes and MISFETs. These allow injection of charge to form accumulation of inversion layers at the interface between the polyacetylene and insulator layer. The fundamental excitation of the trans-polyacetylene chain is the self-localised soliton-like kink defect in the bond alternation pattern along the chain; the soliton has associated with it an energy state at mid-gap of non-bonding p _z character. We have been particularly concerned to demonstrate the formation of solitons from charges injected into the polyacetylene layer in these device structures, and have measured the changes in the optical properties that accompany soliton formation. For the MIS structures formed on an insulator layer of SiO₂ working in accumulation mode we find the ‘mid-gap’ transition from soliton level to the band edge at energies at 0.8 eV. We present here measurements of the electrical characteristics of MISFETs formed also with SiO₂ as the insulator layer. These devices can operate well, but carrier mobilities are low, being limited by thermally-activated transport between chains. We discuss mechanisms for the charge transport processes under the conditions of operation of these devices.