Studies of photoconductivity and associated phenomena in cadmium iodide

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Abstract

The photoconductivity of cadmium iodide is described in this dissertation. Measurements have been made at temperatures between 300oK and 1OoK on high quality single crystals grown from solution. A sensitive electrometer, with a noise level of less than 10-16 amps enabled low currents to be measured and low illumination intensities to be used; the photocurrent was then proportional to the intensity. The photoresponse has been determined at energies up to ~6.5eV; at high energies, near 5.6eV, where exciton effects become important, the photoconductivity is comparable to that in the adjacent band to band transition regions.

Reflectivity measurements have been made in the same apparatus, many of the reflectivity features appeared in the photoconductivity spectrum in particular the high energy exciton reflectivity peaks appeared as photoresponse dips. The reflectivity spectra enabled the photoresponse to be corrected for photons lost by reflection. When the photoresponse for absorbed photons was plotted against absorption coefficient .a smooth curve was obtained. The points corresponding ,to the exciton lines at 5.7eV and 6.2eV did not fall on the curve, however, the exciton photoresponse was some 100% greater than that predicted on the basis of absorption coefficient. The smooth curve may be interpreted in terms of the De Vore model and the increased photoresponse at the exciton lines in terms of an exciton diffusion process.

A semi-empirical electron energy level scheme has been given, this is based upon the correspondence between the optical spectra obtained from many metallic iodides. The scheme is consistent with the available
optical information,and agrees with the photoconductivity results.

The measurements of photoconductivity, particularly those made at low temperatures, were involved with the polarisation of the sample caused by the trapping of charge carriers. This polarisation was found to be both intensity and energy dependent, so that it could influence the shape of the photoresponse spectrum. An explanation has been given. for these effects, in particular the energy dependence was explained in terms of the variation of the absorption coefficient. The thermally stimulated current curve method has been used in an investigation of trapping levels. Illumination took place at 10oK, subsequent warming indicated that many different trapping levels were active. The trapping level energies were.determined and glow peaks compared in two different electrode geometries. Some peaks were enhanced when a volume sensitive electrode geometry was used; it was therefore possible to distinguish volume and surface distributed traps. These conclusions were substantiated when a thin film was used, since only surface traps would then be important. Spectrographic analysis of typical crystals showed that the impurity content was low, the chief impurity was iron with 3 ppm.

A discussion of the effects of intense ultra-violet illumination, i.e. photodecomposition, on cadmium iodide is given in Chapter Seven. These experiments indicated that decomposition occurred most readily at crystal imperfections. After visible decomposition the surface resistance of typical crystals fell by many orders of magnitude to a value of ~106[omega]. This resistance was not strongly temperature dependent, the current was thought to be carried along filaments of decomposition products. Part of the photodecomposition investigation lead to the discovery of the diffusion of silver in CdI2 at temperatures near 250 degrees C. This result
indicated that silver electrodes were not suitable for photoconductivity or ionic conductivity measurements at high temperature and explained why previous high temperature measurements were unreliable. Evaporated gold has been shown to give ohmic contacts and to be stable at high temperatures even after prolonged use.

The electronic and optical properties of CdI2 were summarized in Chapter Eight. Proposals have been made for future experiments to investigate these properties. Amongst these a new technique for the measurement of photoconductivity and simultaneous correction for reflectivity loss was described.

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