I've some ZnO films grown on sapphire substrate. The ZnO film, however, is a highly-oriented polycrystalline film. The annealing was carried out at 800'C at different ambient, (N2, O2, etc.) After annealing, as expected, the crystallinity improved (narrowing FWHM of XRD). Resistivity increased (due to the well-developed grain boundaries after annealing and annihilation of some shallow defects). However, the PL results puzzled me. I integrated the PL spectrum (radiative emission efficiency) of the as-grown and annealed samples. I found that the efficiency degraded after the annealing. This is abnormal phenomenon. Generally, at this annealing temperature, the sample has better crystallinity and PL efficiency. Of course, i know that the effect of annealing depends much on the ZnO films such as stoichiometry, etc. But it is generally accepted that 800'C annealing will improved stuctureal and optical properties of ZnO. I'm wondering why my batch of samples show different characteristic. Here is my speculation, the degrade of PL efficiency is obviously due to the increase of non-radiative recombination centers (defects). Since the FWHM of the XRD narrowed after the annealing, most of the non-radiative defects are generated at or near the grain boundaries. Hence, i get better crystalliinity of the films after annealing, while degrade in PL efficiency due to the creation of non-radiative defects. Am I right to say so? Is there anyway to prove my speculation?