Morphology-controlled growth of ZnO nanorods by chemical bath deposition and seed layer dependence on their structural and optical properties

Abstract

Zinc oxide (ZnO) nanorods (NRs) were grown on ion-plated ZnO:Ga (GZO)/glass, sputtered Au/SiO2/Si(100) and commercial Au/Ti/Si(100) substrates by chemical bath deposition (CBD) using the aqueous solution of zinc nitrate hexahydrate [Zn(NO3)2·6H2O] and hexamethylenetetramine (C6H12N4) at the different growth times (tg). Regardless of the difference in the substrate materials, ZnO NRs were highly oriented in the direction perpendicular to the substrate surface. Up to a tg of 30 min, for all the NRs grown on the three different types of substrates, both the average width and length of the NRs increased rapidly with increasing tg. With further increasing tg, the average widths of the NRs grown on the GZO/glass, Au/SiO2/Si(100) and Au/Ti/Si(100) substrates tended to be saturated at the widths of ~200 nm, ~400 nm and ~1000 nm, respectively. The maximum average lengths grown on the GZO/glass, Au/SiO2/Si(100) and Au/Ti/Si(100) substrates were ~1100, ~1600 and ~2000 nm, respectively. The tg dependence of the residual stress acting on the NRs grown on the GZO/glass exhibited a typical compressive-tensile-compressive evolution, which is characteristic to the films grown by the Volmer-Weber type growth mechanism. For the NRs grown on the Au/SiO2/Si(100) or Au/Ti/Si(100) substrates, however, the stresses acting on the NRs were tensile over the entire tg range. Photoluminescence (PL) spectra of the NRs grown on the Au/Ti/Si(100) substrates were dominated by an orange band (OB) emission associated with oxygen interstitials over the entire tg range. The NRs grown on the GZO/glass or Au/SiO2/Si(100) substrates at longer tgs exhibited a near-band-edge (NBE) emission at a wavelength of ~380 nm, and the PL intensity of the NBE emission relative to that of the OB emission became larger with increasing tg, indicating a decrease in surface state density with increasing tg. PL excitation spectra revealed that the OB emission is effectively excited by the generation of the excitons and/or the electron-hole pairs associated with the NBE emission. Photoacoustic spectra results yield that the selection of the substrate material or the precursor should be effective in the suppression of the formation of the deep-level defect associated with the in-gap photoacoustic band.