Synthesis and Laser Processing of ZnO Nanocrystalline Thin Films
Résumé
We present the results on pulsed laser deposition of high quality nanocrystalline zinc oxide films. Using mass and optical spectroscopy methods we show that ablation into the gas (O2 or O2+He) leads to the very efficient formation of nanoclusters in gas phase. Oxygen is reactive agent and helium helps to cool and to slow down the nanoclusters during their condensation and deposition. The size of the clusters can be varied from 5 to 20 nm and depends on the preparation conditions. Thin films of nanostructured ZnO have wurtzite crystalline structure with c-axis perpendicular to the substrate surface. Optical and lasing properties of the films were studied by conventional photoluminescence and excitation spectroscopy using femtosecond laser. The photoluminescence spectra consist of wide green defect-related band and narrow UV excitonic emission band. Optimisation of conditions of synthesis and post-growth laser annealing of the films leads to increase in the intensity of the excitonic luminescence and to reduction of the green emission. Under femtosecond laser excitation we observe appearance of the UV emission bands related to exciton-exciton collisions (P-band) and to electron-hole plasma recombination. The spectral position of the last one shifts to lower energy side with increasing excitation density. Finally, we demonstrate laser emission from ZnO nanoclusters at room temperature.