The Semiconductor Device Physics of Polyacetylene

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 a range of unipolar devices including Schottky barrier diodes, MIS diodes and MISFET’s. Although carrier mobilities are low, limited by thermally-activated transport between chains, these devices work well, and we find that these structures are remarkably free of surface states and bulk defect states with energy levels within the gap. 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 working in accumulation mode we find the ‘mid-gap’ transition from soliton level to the band edge at energies ranging between 0.55 eV (characteristic of the bulk) for polyacetylene on polymeric insulator layers to 0.8 eV when formed on silicon dioxide. We discuss this spread in energies in terms of the different surface structures at these different interfaces. We also show data for the characteristic vibrational excitations of the soliton, including both the IR-active translation modes and the Raman-active amplitude modes.