Excitons in layered dielectrics ZnI2 and CdI22:Zn

The fundamental electron absorption spectra in the layered compounds ZnI2 and CdI2, doped with Zn were investigated in a spectral interval 3-5.9 eV. The thin textured films deposited on quartz substrates were used as samples. It is found that ZnI2, unlike CdI2, belongs to direct gap dielectrics in spite of the crystal structure similarity of both compounds. The introduction of the Zn atoms into the cation sublattice of CdI2 with x ≥ 1% causes the absorption associated with indirect transitions to disappear and a strong excitons band to appear at the fundamental absorption band edge. The parameters of excitons bands (spectral position, halfwidth Γ and oscillator strength ρ were measured in a temperature interval 80-330 K. The temperature dependence of Γ in both compounds is found to be typical of 3D-excitons. In ZnI2 the f value decreases with increasing T for Debye-Waller factor but in CdI2:Zn it increases. The latter result suggests that the optical direct transition at the interband absorption edge in CdI2 partially allowed because of exciton-phonon interaction is forbidden.