Intrinsic Photoconductivity in Single Crystals of Cadmium Iodide

Photoconductivity measurements have been made on single crystals of cadmium iodide for illumination over a wide range of photon energies from the fundamental absorption edge to more than 3 eV inside the region of intrinsic absorption. These photoconductivity spectra show all the structure that is normally seen in a reflectivity spectrum. Additional information has been obtained by studying the spectral distribution of photocurrent corrected to a constant number of absorbed photons. The corrected photocurrent is considerably larger than expected near the high energy excitons with absorption peaks at 5.7 and 6.2 eV. This may be accounted for by assuming that these excitons are able to diffuse over significant distances in the crystal before dissociating into free charge carriers. The structure observed in the photoconductivity and optical absorption spectra of CdI 2 is discussed in terms of an empirical model for the energy band structure near k = 0.