( 20 − 23 ) ZnO thin films grown by pulsed laser deposition on CeO2-buffered r-sapphire substrate

Abstract

Composite ZnO∕CeO2 thin films were grown epitaxially on r-sapphire substrates using the pulsed laser deposition technique. Their crystalline properties were established using x-ray diffraction and showed the ZnO (wurtzite structure) and CeO2 (fluorite structure) layers to be highly textured with the (20−23) and (100) orientations, respectively. ϕ-scan measurements were also carried out and the (20−23)ZnO‖(100CeO2), [1−210]ZnO‖⟨011⟩ CeO2 epitaxial relations established. The rocking curve profiles indicated that the ZnO films grew as four crystallographically equivalent domains. Series of rocking curve and χ−scan measurements at varying ϕ angles, respectively, were used to investigate the domain structure. These showed that the normal to the (20−23) plane in each domain is tilted away from the substrate normal towards one of the four equivalent CeO2 ⟨111⟩ directions by ∼1.60. Atomic force microscopy measurements showed that the ZnO∕CeO2 composite film has a granular microstructure with a rough surface (typical root mean square roughness of 7.9nm). Low temperature photoluminescence spectra showed an intense near-band-edge emission at a photon energy of 3.361eV, with a full width at half maximum of 1.8meV, testifying to the good optical quality of the ZnO material. The optical transmission of the ZnO∕CeO2 composite film was measured in the 200–1000nm spectral domain; it was completely opaque to UV radiation and became transparent with a sharp transition above 380nm. Secondary ion mass spectrometry measurements were used for depth profiling of the ZnO∕CeO2 composite structure. The corresponding data suggest that the CeO2 buffer layer acts as an efficient barrier against the diffusion of aluminum from the sapphire substrate into the ZnO layer.