Method for determining optical excitation densities due to photoionization of impurities and to interband transition in thin CdS-type crystals from photoconductivity measurements

Abstract

A method is described for determining optical excitation densities due to photoionization of impurities and to interband transitions in thin CdS‐type crystals from photoconductivity spectra measured using a low‐intensity periodically chopped measuring beam superimposed on a high‐intensity time independent bias illumination at constant photon energy. On the basis of a reaction kinetic model including optical excitation both of electrons and holes from two different types of recombination centres and also band gap excitation it is shown that for high bias illumination intensities the corresponding optical excitation densities of the measuring beam and, under certain assumptions, the corresponding absorption coefficients can be determined from measurements of the degree of modulation of the photocurrent and its phase shift with respect to the light intensity of the measuring beam at four different chopping frequencies. If the intrinsic absorption coefficient is known, it is possible to determine approximately the extrinsic absorption coefficient by measuring only at a single chopping frequency. With this method extrinsic absorption coefficients varying from 10−4 to 10 cm−1 can be determined. Additionally, by considering the phase shift, it is possible to identify the type of the optical transition (electron or hole).