Abstract: We report Raman spectroscopy measurements on ZnO crystals grown by the vapor transport method and annealed. Vacuum annealing is found to yield single crystals with ultra low density of defects. We focus on the optical E2 phonon linewidth temperature dependence in the 10?500 K range. The linewidth decrease at low temperature is analyzed and discussed in the light of anharmonic up- and down-conversion processes, unveiling strongly different behaviors for the two E2 phonons.

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