Optical Spectroscopy of the Charge Accumulation Layer in Mis Structures with Polymeric Insulator and Semiconductor Layer
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OPTICAL SPECTROSCOPY OF THE CHARGE ACCUMULATION LAYER IN MIS STRUCTURES WITH POLYMERIC INSULATOR AND SEMICONDUCTOR LAYERS J. H. BURROUGHES and R. H. FRIEND, Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, UK.
ABSTRACT Metal-Insulator-Semiconductor (MIS) and MISFET structures constructed with polyacetylene prepared by the Durham precursor route provide convenient systems for the investigation of charge storage and transport in this polymer. The charge accumulation layer is particularly easy to form, and is of particular interest because charge is introduced into the polymer without compensation by chemical dopants. Charge is stored in soliton-like excitations of the chain, and we are able to characterise these from optical measurements of their electronic excitations. We find that the nature of the soliton-like states is very sensitive to the structure of the polyacetylene at the interface between the insulator and polyacetylene, and we report here the properties of devices formed with various organic polymers as the insulator layers which we contrast with those formed with silicon dioxide. INTRODUCTION The use of conjugated polymers as the active semiconductors in device structures has been limited by the difficulties of processing the polymer to a suitable form. It is with the development of polymers which can be solution-processed, either in the form of a precursor polymer as is the case with Durham-route polyacetylene [1], or which are intrinsically soluble as is the case with poly(3-alkyl thienylenes), that fabrication of these devices has become feasible. We have used Durham route polyacetylene for this work [2,3]; others have used poly(3-alkyl thienylenes) and similar materials [4-9]. The device physics of conjugated polymers is intrinsically different to that of threedimensionally bonded inorganic semiconductors because the fundamental electronic excitations of the chain are self-localised excitations which take the form of solitons in the trans isomer of polyacetylene, and more generally as polarons. Thus, charge injected onto the polymer chains causes local chain relaxation to form these excitations, and the description of all aspects of the operation of such devices is controlled by their excitations and motion. These self-localised excitations of the chain pull energy levels away from the band edges to create gap states, which for the limiting case of the soliton in polyacetylene are of the character of non-bonding Pz states and lie at mid-gap [10]. Two clear consequences of this behaviour are, firstly, that charge mobilities are often low for materials where charge transport across the device requires many inter-chain transfers, and secondly, that there are novel electro-optical properties associated with the electronic and vibrational excitation of the self-localised states. In our work with Durham polyacetylene we have been able to characterise the 'mid-gap' optical transition from the soliton level to the band edge, the 'translation' IR active modes, and the Raman-active amplitude mode of t
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