Tailoring UV emission from a regular array of ZnO nanotubes by the geometrical parameters of the array and Al2O3 coating

Self-aligned and equal-spaced zinc oxide (ZnO) nanotube arrays were fabricated with anodic aluminum oxide (AAO)-assisted growth and the ALD technique. The near band-edge (NBE) emission was strongly affected by the nanotube's geometrical parameters, such as a packing density and thickness of the nanotube walls. The NBE emission was further enhanced with Al2O3 coating. The effect was analyzed by X-ray photoelectron spectroscopy (XPS) and ascribed to the surface defect passivation and a ZnAl2O4 spinel formation. The NBE emission enhancement was greater in ZnO nanotubes with thicker walls. A smaller UV enhancement factor was explained by less uniform and integral Al2O3 coverage of the ZnO nanotubes with thinner walls; this, possibly induced a variation of the Al2O3 refractive index along the nanotubes. As a result, the optical conditions at the ZnO/Al2O3/air interfaces was changed and the light extraction efficiency was reduced in the latter samples.