Origin of the 3.331 eV emission in ZnO nanorods: Comparison of vapour phase transport and pulsed laser deposition grown nanorods

Abstract

In this work, we report the growth of vertically aligned ZnO nanorods with excellent optical quality by both catalyst free vapour phase transport (VPT) and catalyst free pulsed laser deposition (PLD). We compare the near band edge emission of such deposits, with a focus on the identification of the origin of the 3.331eV emission feature. X-ray diffraction (XRD), scanning electron microscopy (SEM) and low-temperature (13K) photoluminescence (PL) were used to characterise these nanorod deposits. XRD and SEM data reveal that both techniques lead to highly textured ZnO nanorod arrays with uniform c-axis orientation normal to the substrate surface. The VPT-grown nanorods are well separated and show smooth, facetted surfaces whereas the PLD-grown nanorods are more closely packed and display comparatively rougher surfaces. The optical quality of the samples obtained by both growth methods was very good and low-temperature PL spectra were dominated in both cases by a strong I6 bound exciton (BX) emission (3.36eV), and also showed emission from the surface exciton and the free exciton. A comparatively weak visible emission was also observed in samples deposited by both techniques. The main difference between the PLD- and VPT-grown nanorod samples is the presence of the 3.331eV emission in the former, and its complete absence in the latter (as well as in continuous PLD-grown seed layers) which is discussed in light of the differing surface morphologies mentioned above and which provides strong support for our previous assignment of the origin of this defect to structural defects in the inhomogeneous sub-surface region close to the rough nanorod surface.