Energy bands of the cadmium halides from electron energy loss spectroscopy

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Abstract
The wavevector and frequency dependent dielectric function epsilon (q, omega ) was determined from electron energy loss spectroscopy for the layered solids CdI2, CdBr2 and CdCl2. Electron energy loss measurements were carried out in the energy range 0-90 eV and at wavevector magnitudes up to the Brillouin zone dimension. For each value of the wavevector, epsilon (q, omega ) was determined from the energy loss probability using Kramers-Kronig analysis. The data analysis which includes a method for removing the effect of multiply scattered electrons from the raw data is described. Thin polycrystalline films were used and these were found to be highly oriented with the basal planes of the crystallites parallel to the substrate. The energy loss experiments were performed with the electron beam direction perpendicular to the basal planes. Based on the dielectric function obtained for these materials an electron energy band scheme is proposed. It was found that on going from CdI2 to CdBr2 to CdCl2 the upper valence band narrowed and the conduction bands became more separated. In general, with increasing q, both epsilon 1 and epsilon 2 become broader and decrease in peak height. At the same time the main weight of epsilon 2 moves to higher energies and the negative portion of epsilon 1 diminishes and tends to remain positive for all omega . This behaviour is expected to be similar for all semiconductors.

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