Luminescence properties of ZnO whiskers obtained by chemical vapor deposition

Luminescence properties of highly (0001) oriented ZnO whiskers obtained by atmospheric chemical vapor deposition have been investigated over wide temperature range of 12 K to 300 K. An ultraviolet emission associated with bound-exciton recombination has been observed at 12 K under the low power photoexcitation. The emission exhibited temperature quenching with an activation energy of similar to9 meV, which is treated as the binding energy of the bound exciton. The shape of the emission was considerably modified at high-power UV laser excitation. The principal feature of this modification is an occurrence of a series of equally spaced, narrow emission components, decreasing in intensity at longer wavelength. The separation between these components is found to correspond to the energy of a longitudinal optical phonon (similar to70 meV) determined by Raman scattering measurements. The activation energy of the temperature quenching of the emission considerably enhanced up to similar to58 meV under the laser excitation. This enhancement of the activation energy is discussed as being due to the domination of the free-exciton emission in the photoluminescense spectra under the high-power excitation. The balance between free- and bound-exciton states is discussed in terms of exciton generation and the annihilation scheme.