Energy-Transfer
Efficiency in Eu-Doped ZnO Thin Films:
The Effects of Oxidative Annealing on the Dynamics and the Intermediate
Defect States
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Abstract
We have studied ultrafast dynamics
in thin films of Eu-doped zinc
oxide (ZnO), prepared by radio-frequency sputtering onto sapphire
substrates. Following UV excitation of ZnO, a red emission is observed.
Postdeposition annealing in an oxygen atmosphere improves the crystallinity
and emission intensity of the films, which are highly sensitive to
the dopant concentration. Transient-absorption spectroscopy shows
that the excited semiconductor host transfers energy to rare-earth
ions on a time scale of only a few picoseconds. The dynamics as a
function of the probe wavelength change dramatically after annealing,
with annealed films showing the fastest dynamics at much lower wavelengths.
Our results show that annealing greatly affects the defect energy
levels of the films and the dynamics of the trapped carriers. Unannealed
films show dynamics consistent with energy transfer from O vacancies
to the dopant, while energy transfer in annealed samples involves
acceptor-type defects such as Zn vacancies as intermediates