Optical reflectivity and electronic structure of layered cadmium halides

Abstract

The dielectric function ɛ^(E), the energy loss function -Im[1/ ɛ^(E)], and plots of $n_{\mathrm{eff}\left(\mathrm{E}\right)}$ and ${\varepsilon }_{\mathrm{eff}\left(\mathrm{E}\right)}$ have been obtained in the region 2–31 eV by Kramers-Kronig analysis of near-normal-incidence reflectance spectra of single crystals of ${\mathrm{CdCl}}_2$, ${\mathrm{CdBr}}_2$, and ${\mathrm{CdI}}_2$. The results have been described in terms of interband transitions and plasma oscillations. The low-temperature spectra of the materials reveal the presence of strong exciton structures. Plasma resonance effects have been identified in the high-energy region (15–22 eV). The optical spectra of Cd$X_2$ (X=Cl, Br, I) were analyzed in terms of the calculated band structures of ${\mathrm{CdCl}}_2$ and ${\mathrm{CdI}}_2$. In ${\mathrm{CdI}}_2$ the smallest energy gap for forbidden (phonon-assisted) direct transitions (at the L point of the Brillouin zone) is 3.8 eV (300 K), while the gap for allowed direct transitions (at Γ) is 4.3 eV (30 K). The fundamental energy gap in ${\mathrm{CdCl}}_2$ and ${\mathrm{CdBr}}_2$ is observed at 6.4 and 5.4 eV, respectively, and corresponds to allowed direct transitions at Γ. The observed excitons $X_0$, $X_1$, and $X_2$ are associated with allowed direct transitions between the halogen np valence band and the Cd 5s conduction band. The excitons A, B, and C are due to transitions of electrons from the halogen valence band to the Cd 5p conduction band.