Optical reflectivity and electronic structure of layered cadmium halides.

Abstract

The dielectric function \ensuremath{\varepsilon}^(E), the energy loss function -Im[1/ \ensuremath{\varepsilon}^(E)], and plots of ${n}_{\mathrm{eff}(\mathrm{E})}$ and ${\ensuremath{\varepsilon}}_{\mathrm{eff}(\mathrm{E})}$ 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 \ensuremath{\Gamma}) 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 \ensuremath{\Gamma}. 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.