Carrier recombination process and magneto-photoluminescence in Zn_1-xCo_xO layers

High-quality Zn_1-xCo_xO layers were obtained with isolated Co ions in the host (x no more than 0.008). The photoluminescence properties of excitonic and Co-related emissions in the Zn_1-xCo_xO (x=0.0015) layer were mainly investigated in both zero and up to 7 T magnetic fields in order to clear the energy transfer process from excitonic states to Co centers. It was found that Co emissions originated from 3d intraemissions in the Zn_1-xCo_xO (x=0.0015) layer were remarkably enhanced with the suppression of excitonic emissions, indicating the presence of efficient energy transfer to the Co centers. Assignment of absorption and magnetoluminescence spectra revealed that the energy transfer to the Co centers was not derived from the excitonic states but from charge transfer (CT) states formed on the Co centers after a band-to-band excitation. Finally, we consider that the primary role for the energy transfer process of Zn_1-xCo_xO (x=0.0015) layer is related to the phenomenon of impurity Auger recombination acting as a nonradiative energy transfer from the CT states to Co centers.