Effect of the s,p-d exchange interaction on the excitons in Zn1-xCoxO epilayers

We present a spectroscopic study of Zn1-xCoxO layers grown by molecular beam epitaxy on sapphire substrates. ZnO:Co is commonly considered as a promising candidate for being a diluted magnetic semiconductor ferromagnetic at room temperature. We performed magneto-optical spectroscopy in the Faraday configuration, by applying a magnetic field up to 11 T, at temperatures down to 1.5 K. For very dilute samples (x < 0.5%), the giant Zeeman splitting of the A and B excitons is observed at low temperature. It is proportional to the magnetization of isolated Co ions, as calculated using the anisotropy and g factor deduced from the spectroscopy of the d-d transitions. This demonstrates the existence of spin-carrier coupling. Electron-hole exchange within the exciton has a strong effect on the giant Zeeman splitting observed on the excitons. From the effective spin-exciton coupling < N-0(alpha-beta)>(X)=0.4 eV we estimate the difference of the exchange integrals for free carriers N-0 parallel to alpha-beta parallel to similar or equal to 0.8 eV. The magnetic circular dichroism observed near the energy gap was found to be proportional to the paramagnetic magnetization of anisotropic Co ions even for higher Co contents.