Optical absorption ofCo2+in ZnO

Abstract

Polarized-optical-absorption spectra of ${\mathrm{Co}}^{2+}$ in ZnO are reported. The crystal-field transitions from the ${}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$ ground state to the ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{} \mathrm{}\left(F\right)\mathrm{}$, ${}_{}{}^4{}_{}{}^{}T_1^{}{}_{}{}^{} \mathrm{}\left(F\right)\mathrm{}$, and ${}_{}{}^2{}_{}{}^{}E \mathrm{}\left(G\right)\mathrm{}$ multiplets are analyzed in detail. These bands consist of sharp zero-phonon lines and their vibronic sidebands, where coupling to 528-${\mathrm{cm}}^{-1\mathrm{}}$ optical phonons and 100-${\mathrm{cm}}^{-1\mathrm{}}$ acoustical phonons from the $M$ point of the ZnO Brillouin zone is shown to be dominant. In order to explain the level scheme derived from the spectra the Hamiltonian describing cubic and trigonal crystal field, Coulomb and spin-orbit interaction has been diagonalized within the full $d^7$ configuration. With $Dq=400\mathrm{}$, $v=120\mathrm{}$, $v^{\prime }=320\mathrm{}$, $B=760\mathrm{}$, $C=3500\mathrm{}$, and $\zeta =430\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ both the band positions and the electronic-fine-structure splittings are satisfactorily accounted for. In addition the $g$ factors of the ${\mathrm{Co}}^{2+}$ ground state, $g_{\parallel }$ and $g_{\perp }$, are correctly predicted.