Electro-Optical Properties of the MIS

We have shown in section 5.3 that the sub-gap optical absorption of the depletion layo formed in the polyacetylene Schottky diodes shows the removal of the mid-gap soliton states associate with the extrinsic charges present in the undepleted material. The MIS structures operate in both charge accumulation and depletion, and diere is particular int st in the charge accumulation layer in that the charges injected are present without any associated dopant We have therefore carried out an extensive sales of expoiments on the electro-opticd properties of these MIS structures, covering both the electronic excitations of the solitons at mid-gap and also the IR and Raman vibrational excitations of the soliton. All the MIS structures which have been electrically characterised as described in section 6.2 have, been constructed so that they are semitransparent, either over the IR (silicon substrate) or over the IR and visible (polymer insulators, glass substrates). 

We find a value for a at the peak of the mid-gap absorption band of 1.2 x 10 5 cm, somewhat lower than the value found for the solitons due to the extrinsic doping in the Schottky diodes, and we consider that the differences are due to the high level of disord in the surface layer of polyacetylene at the interface with the silicon dioxide in this MIS device. It is important to note that the value of a is constant throughout the region of accumulation, and that it reaches this value at the onset of accumulation. It provides, therefore, very clear evidence that almost all the charge that is injected into the structure, which is measured by the diffoential capacitance, is stored in states which give rise to the new optical behaviour which is characteristic of the formation of a charged soliton-like excitation on the polymw chain. We have no evidence for the presence of trap states, which if present would have reduced the value of a, particularly at the onset of accumulation. 

The size of the charge-induced optical response can be quite large for these MIS structures. For the device shown here, the peak value in the fractional change in absorption is some 0.7%, and we note that the limit, set by equation 30 and the dielectric strength of the insulator is here about 2.4%. 

We have made measurements of the electro-optical properties of the MIS devices fabricated with PMMA and with polyimide. We show in figure 34 the spectrally-resolved response for the device with PMMA previously characterised for its electrical response in figure 29. The charge-induced mid-gap absorption is seen here at 0.55 eV this is lower than the 0.8 eV found for the Si( 2-based devices, but at the same energy as the 

At higher photon energies there is additional structure some of this is as expected, other features less so. There is a relatively weak feature at 1.15 eV which is of the same sign as the 0.SS eV response, with absorption in accumulation and bleaching in depletion. This feature is not directly related to die fmnation of the charge accumulation layer as it shows a very different dependence on bias voltage to both the differential capacitance and the electro-optical response at 0.55 eV, as shown in figure 37, with a peak in response at around +5 V. We do not have an explanation for this feature. 

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Poly(methylmethacrylate), PMMA, is soluble in the solvents used for the Durham polyacetylene precursor, so that deposition of the polyacetylene was made first, followed by the PMMA layer. Note that in this case, the insulator/semiconductor interface is made at the tq>, free surface of the polyacetylene, and we can expect that there are differences in the structure of top and the bottom surfaces. We return to this later in connection with the electro-optical properties of the MIS devices in section 6.3. The variation of the differential capacitance with bias voltage for a MIS device built in this way is shown in figure 29. It shows a very similar form to that seen for the other MIS devices, and standard analysis of the capacitance in the depletion regime gives values of Na = 2.1 x 10 6 cm 3 and Vf = -1 V. 

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