Peculiarities Related to Er Doping of ZnO Nanorods Simultaneously Grown as Particles and Vertically Arranged Arrays

A unique set of undoped and Er-doped ZnO nanorods that are grown by a hydrothermal method under exactly the same conditions in the form of 2D nanoarrays on the SiO2/ZnO substrate or in a free-standing form on random nucleation seeds in solution were investigated. Their optoelectronic properties are characterized by photo-, radio-, and cathodoluminescence in correlation with scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron paramagnetic resonance spectroscopy, resonance Raman spectroscopy, and theoretical computing by using density functional theory. We demonstrate that erbium is incorporated at a regular zinc site in the 2D arrays and as additional nucleation seeds in the free-standing nanorods. The deposited nanorods contain a larger number of shallow donors (by about 2 orders of magnitude) and a larger number of free carriers (by about 1 order of magnitude) as compared to the free-standing ones. It is related to the fact that the nanorods grow about 1 order of magnitude larger and in polycrystalline bunches on the random seeds in solution compared to the deposited arrays. Doping by Er slows the excitonic emission from 465 to 522 ps.