The structure and optical properties of ZnO nanocrystals embedded in SiO2 fabricated by radio-frequency sputtering

Zinc oxide (ZnO) nanocrystals (NCs) with high crystalline quality were prepared via radio-frequency magnetron sputtering as a SiO2/ZnO/SiO2 trilayer on Si(100) and Al2O3(0001) substrates with an intermediate in situ annealing step. Transmission electron microscopy reveals a uniform dispersion of ZnO NCs in the amorphous SiO2 matrix with typical sizes up to 16 nm with a larger fraction of smaller crystals. The size distribution analysis yields a mean grain size of 5 nm for small particles. Individual ZnO NCs show a well-defined hexagonal close packed wurtzite structure and lattice parameters close to those of bulk ZnO, confirming their high crystalline quality. Mapping of the Zn distribution by means of energy-filtered transmission electron microscopy reveals a strongly non-uniform distribution of Zn within the SiO2 matrix, corroborating the chemical separation of ZnO NCs from surrounding SiO2. Optical transmittance measurements confirm the findings of the electron microscopy analysis. The fabrication technique described opens up new possibilities in the preparation of ZnO NCs with high crystalline quality, including growth in monolithic optical cavities without intermediate ex situ fabrication steps.