Emission, Er ion defects and structure of ZnO nanocrystal films prepared by ultrasonic spray pyrolysis

This work focuses on the investigation of near band edge (NBE) and Er ion related emissions, morphology and structure of Er-doped ZnO nanocrystal (NC) films prepared by ultrasonic spray pyrolysis with the different Er contents. The scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) methods have been used. It is revealed that the crystal structure of ZnO:Er films can be improved at low Er-doping (≤ 2 at%), when Er ions occupy the Zn vacancies with the formation of the substitutional Er _Zn defects. Simultaneously, the PL intensity of NBE emission enlarges and the PL intensity of green PL band, connected with native deep defects, decreases. Meanwhile, the IR emission via the 4f intra-shell optical transitions ( ⁴ I _13/2 – ⁴ I _15/2 ) in Er ions at this doping is insignificant. The PL intensities of NBE emission, all visible and IR emission bands, connected with the 4f inner-shell optical transitions in Er ions, have enlarged essentially at higher Er-doping (<5 at%). It is revealed at XPS study that the process of Er ion oxidizing is realized at this Er content. The Er concentration of <5 at% can be considered as an optimal for the bright Er ion emission owing to oxidizing and the formation of a low symmetry oxygen environment around the Er ions. Further Er content enlarging provokes PL intensity decreasing and ZnO crystallinity falling down together with the native defect generation in ZnO NCs. It is expected that deep understanding the Er-doping process will be useful for the strategy development to obtaining ZnO:Er NC films with tailored parameters.