Recombination dynamics of electron-hole pairs in impurity band of ZnO nanowires

Doping concentration is a fundamental parameter characterizing the optoelectronic properties of ZnO films and nanostructures. Defect and impurity states contribute to the n-type conductivity and are also invoked as origin of ZnO below-bandgap photoluminescence (particularly strong in ZnO nanostructures). Such emission band, centred in the green region (500-550 nm), shows features useful for applications: its efficiency can be tuned by changing the defectivity and/or the nanostructure sizes and it also present interesting sensitivity to gas adsorption. We present a study of recombination processes characterizing the green band in ZnO nanowires by means of time-resolved photoluminescence measurements (TRPL) and photoluminescenceexcitation (PLE) experiments. We show that the radiative processes are due to the superposition of an unimolecular and of a bimolecular recombination channel, thus indicating the presence, before optical excitation, of occupied electrons or hole levels which pin the Fermi level at about 0.9 eV below the conduction band. The numerical analysis allows to determine the doping concentration density associated to such levels and to observe saturation of optical absorption. It is worth noting that determination of these parameters, having a major importance for applications in optoelectronics or for use of ZnO as active laser materials, would be difficult to be obtained by electrical contacts due to disordered morphology of nanowire ensembles.