Luminescence and ellipsometry investigations of annealing effects on nano-granular ZnO.

We propose an investigation of nano-granular ZnO very thin layers (<100nm), which morphologies are varied by changing the annealing temperature between 300 and 650° C. A detailed investigation of the optical properties is performed by coupling emission obtained from photoluminescence and dielectric function obtained from spectroscopic ellipsometry. The dielectric function shows a strong excitonic behavior increasing with annealing temperature. The UV emission is optimal at 450° C and the defect related visible bands gradually increase with annealing temperature. A particular attention is paid on i) the blueshift between emission and absorption UV onsets which is analyzed using contribution such as phonon replicas, carriers, bandgap renormalization, band filling and non-parabolicity; ii) the correlated evolution of absorption and emission intensities which is analyzed in terms of simultaneous curing of morphological defects and activation of intrinsic defects with temperature.