A general study of electroabsorption on semi-insulating solids

Abstract

The electroabsorption technique has been used to study the properties of a wide range of semi - insulating materials including CdTerCl single crystals, solid solutions o f CdS-ZnS, amorphous silicon and polymerized diacetylene Langmuir films. The small change in a sample's optical transmission consequent upon the application of large electric fields has been carefully recorded using phase-sensitive detection equipment. Measurements on CdTe:Cl samples have established an experimentalprocedure to cope with the complications of photoconductivity and space charge effects. The electro absorption spectra in this material are interpreted in terms of the Franz-Keldysh effect, thus allowing the calculation o f the reduced effective mass. The result is in good agreement with published data obtained using other methods. In solid solutions of CdS-ZnS, the observed exciton electroabsorption has been analysed within the context of the Stark theory. This enables the lowest bound exciton states to be located precisely and results the determination of the reduced effective mass of the exciton which is found tovary in early with the band gap as predicted theoretically. Electro absorption spectra for a-Si in dicate that only transitionsinvolving localized states respond to electric field modulat .on. These spectra, which are preparation dependent, have also been interpreted in terms of the Stark effect ; An interesting result obtained for glow dis-charge produced samples has been interpreted in terms of an effective change in the mobility edge with temperature. The final material discussed in the thesis is the diacetylenemolecule; the films were prepared using the Langmuir trough technique developed in Durham. Interpretation of the electro absorption data for the red polymer form has led to our calculating the static polarizability change between the excited and ground states in this molecule. The thesis concludes with a summary of possible applications ofthe electroabsorption technique and suggestions as to how the presentwork could be extended.